Barber, Whitesides I
Redut'tiLleClearage of Carhon-Halogen Bonds
239
Bondsby
of Carbon-Halogen
ReductiveCleavage
T ri-n-butyltinHydride,Lithium
KetYl,
4,4'-DimethylbenzoPhenone
a n d M e t a l l i cM a g n e s i u m l
James J. Barber and Georgc N{. Whitesides*
(ont ribut i on J ront t he Departntentrl' Chem i.st r)', M a.ssat'h uset t'sI nst i t u t e rtJ' T echnoI o 91"
t's0 2 I 39. R e c e i r e dM a t ' 1 4 , l q 7 9
Cantbridge, M ct.ssut'husel
A b s t r a c t :R a t c - s t r u c t u r cp r o fi l c sa r c c o m p a r c df o r f o u r r c a c t i o n s r: e d u c t i t t no f a l k y l c h l o r i d e sa n d b r o r n i d c sw i t h t r i - n - b u t y l t i n
o C a n d 0 o C - .r c s p c c i i v c l l ) . r c d u c r i o no i a l k y ' l b r o r n i d e sw i t h l i t h r u m ' 1 , 4 ' - d i n r c t h y l b e n z o p h e n o kn c t } ' l
hydridc (Et:O. -15
( t : t : O , 0 ' C ' ) . A l l i o u r r a t c s t r u c t u r ep r o f i l e sa r e l i n t i H f . , 2 4 . a ) , a n d r c a c r i s no l ' a l k l l c h l o r i d c i w i t h m c t a l l i cn r a g n c s i u n r
bond is
c a r l y ' c u r r c l a t c d( c o r r c l a t i o nc o c l . l ' i c i c nrt ) 0 . t t O ) .T h c s i n r i l a r i t yo f t h c s e p r o f i l c s s u g g c s t st h a t t h e c a r b o n ' h a l o g c n
n
unlbcr
l
i
n
l
i
t
c
d
p
o
t
c
n
t
i
a
l
s
o
f
a
r
c
d
u
c
t
i
o
n
[
:
l
c
c
t
r
o
c
h
c
r
n
i
c
a
l
o
f
'
r
c
a
c
t
i
o
n
.
t
y
p
c
s
t
a
t
c
l
o
r
e
a
c
h
t
n
r
n
s
i
t
i
o
n
i
n
t
h
e
b
r
o
k
c
n
signil'ic1ntl1
halidcs
o l : a l k y l h l l i d c s a l s o c o r r c l a t el i n c a r l l ' w i t h t h c s c r a t c s .T h e s i n r i l a r i t ; ' o l t h c r a t c s t r u c t u r cp r o fi l e l ' o r r e a c t i o no f a l k y ' l
s u r f a c ei s
a t a n t a g n c s i u n sr u r f a c c t o t h o s c o f h o r n o g e n e o ursc a c t i o n ss u p p o r t st h c i n l ' c r c n c ct h a t r c a c t i o na t t h c n r a g n e s i u m
n o t c o m p l i c a t c db 1 ' a d s o r p t i o no r o t h c r h c t c r o g e n e o ups h e n o n l e n a .
Introduction
The formation of alkyl Grignard rcagentsfrom alkyl halides
a n d m c t a l l i c m a g n e s i u mo c c u r s b y a m e c h a n i s mw h i c h i n v o l v c s , a t l e a s t i n p a r t , r e d u c t i v e c l c a v a r g co f t h e c a r b o n e
h a l o g e nb o n d a n d f o r m a t i o n o l f r c c a l k l l r a d i c a l s . l S t r u c ture-reactivity profiles obtained from nonmasstransport
l i m i t e d r c a c t i o n s o f a l k y l c h l o r i d e sa n d a l k y l b r o m i d e s a r e
c o n r p a t i b l c w i t h t w o l i m i t i n g m e c h a n i s m sf o r t h e c a r b o n halogenbond-breakingstep.The first is transferof an electron
l ' r o r nm a g n e s i u mm e t a l t o t h e a l k y l h a l i d e ,a n d g e n e r a t i o no f
an alkyl halideradicalanionand a (formally) magnesium(l)
s p e c i e s( e q l : i n t h i s e q u a t i o n ,M g . d e n o t e sa s u r f a c em a g n e s i u m a t o m ; M g ( l ) m i g h t b e e i t h e r a s u r f a c eo r a s o l u b l e
spccics).The secondis halogenatom abstractionfrom thc alkyl
h a l i d e b y a n r a g n e s i u ma t o m ( e q 2 ) .
R -X * Ms,
c
sfcr
[R-X]-.
t"'llt: (l)
+ M e ( l-) R . +x - rcelCtlOn
R-X
* Mg, + ft' + X-Mg(l)
-
furthcrreaction (2)
Thc first reactionis an outcr sphereelectrontransfcr reaction;
the sccondis an inner sphereelectrontransfer.Although both
pathwaysultimatcly generatean alkyl radical and a halide ion,
. i r s t ,a n o u t e r
t h e y d i f f e r , a t l e a s tf o r m a l l y , i n t h r e e r c s p e c t s F
sphereelcctron transfer mechanisminvolveselectrondonation
t o t h c C - X a n t i b o n d i n go r b i t a l . A l t h o u g h p o p u l a t i o no f t h i s
ox orbital will increasethc C-X bond length to someunknown
extent, it may not change the hybridization at the carbon
bondcd to halogen significantly.By contrast,inner sphere
clectron transfer breaks the carbon-halogcn bond in the
t r a n s i t i o n s t a t e , a n d s h o u l d b e a c c o m p a n i e db y c h a n g e si n
hybridization at carbon. Second,outer sphereelectron transfer
does not requirc interaction between magnesium and the
halogenatom in thc transitionstate,while inner spheretransfer
i n v o l v e st h e f o r m a t i o n o f a h a l o g e n - m a g n e s i u mb o n d .T h i r d ,
requiresthat the alkyl
inncr sphereelectrontransfernecessarily
r a d i c a l s b e g e n e r a t e di n i t i a l l y v e r y c l o s e t o t h e m a g n e s i u m
surfacc; outer sphereelectron transfer resulting in formation
of an intermediatealkyl halideradicalanion with finite lifetime
would allow initial gcnerationof alkyl radicalsto occur at some
s c p a r a t i o nf r o m t h e m a g n e s i u ms u r f a c e .
The question relevant to the mechanism of formation of
Grignard reagentsposedby theselimiting alternativesconcerns
t h e i n t e r m e d i a c yo f a l k y l h a l i d e r a d i c a l a n i o n s :A r e t h e s e
s p e c i e sd i s c r e t ei n t e r m e d i a t c sa l o n g t h e r c a c t i o n c o o r d i n a t e
leading from alkyl halide reactant to organomagnesium
p r o d u c t ? T h e i n t e r m e d i a c yo f t h e s es p e c i e si n t h e f o r m a t i o n
o f G r i g n a r d r e a g e n t si s p a r t o f a b r o a d e r a n d u n r e s o l v e d
q u c s t i o n c o n c c r n i n gt h e i r e x i s t e n c ei n a n y o r g a n i cr e a c t i o n .
P u l s e r a d i o l y s i so f a l k y l h a l i d e si n c y c l o h e x a n eh a s b e e n i n t e r p r e t c d a s i d e n t i f y i n g a m e t h y l c h l o r i d er a d i c a l a n i o n ( r 1 7 2
r y 3 x l 0 - E s ) , r 0a n d a r y l h a l i d e r a d i c a l a n i o n sh a v eb e e nr e l a l i d e sw i t h d i s o d i u mt e t p o r t e d . r rR e a c t i o n so f 5 - h e x e n y h
|2
i a p h e n y l e t h y l e n ed i a n i o n i n m e t h y l t e t r a h y d r o f u r a n a n d o f
F
r
3
h
a v eb e e n
i
n
T
H
a
n
i
o
n
s
r
a
d
i
c
a
l
w
i
t
h
a
r
o
m
a
t
i
c
h
a
l
i
d
e
s
alkyl
consideredto suggestalkyl halide radical anions.The chemical
s t u d i e sa r e a m e n a b l et o d i f f e r e n t i n t e r p r e t a t i o n s , l ' l ' luan d t h s
e x i s t c n c eo f a l k y l h a l i d e r a d i c a l a n i o n sa s r e a c t i o n i n t e r m e d i a t e si s n o t e s t a b l i s h e d .
This paper compares rate-structure profiles for three rea c t i o n s i n c t h e r e a ls o l v e n t s r: e a c t i o n so f a l k y l c h l o r i d e sw i t h
n r c t a l l i c m a g n e s i u m ,r e d u c t i o no f a l k y l c h l o r i d e sw i t h t r i - n butyltin hydride under free radical conditions(an inner sphere
r c a c t i o n ) , r 5a n d r e d u c t i o n o f a l k y l b r o m i d e s w i t h l i t h i u m
4 , 4 ' - d i m e t h y l b c n z o p h e n o nkee t y l ( a s i n g l e - e l e c t r o nt r a n s f e r
r e a c t i o n ) .r 6 '| 7 T h e c h o i c eo f r e d u c t i o no f a l k y l c h l o r i d e sw i t h
tri-n-butyltin hydride and oi alkyl bromides with lithium
ketyl to model plausibleprocesses
4,4'-dimethylbenzophenonc
i n v o l v e d i n r e a c t i o n o f a l k y l c h l o r i d e sw i t h m a g n e s i u mw a s
basedon severalconsiderations.Tri-n-butyltin radical, rather
than chromousionls or phenyl radical,lewas usedas a reagent
to effect halogen abstraction for two reasons:first the abs t r a c t i n g c e n t e r i s a n e l e c t r i c a l l yn e u t r a l , s t e r i c a l l yh i n d e r e d
m e t a l a t o m a n d m a y r e s e m b l ea m a g n e s i u ms u r f a c e a t o m .
Second, the large difference in electronegativitybetweentin
, p r e v i o u se x a m i n a t i o ne s a n d m a g n e s i u mn o t w i t h s t a n d i n g a
t a b l i s h e ds i m i l a r H a m m e t p v a l u e sf o r t h e r e a c t i o n so f s u b stituted aryl halides with tri-n-butyltin hydride and magnes i u m a n d s u g g e s t e ds i m i l a r c h a r g e d i s t r i b u t i o n si n t h e r a t e l i m i t i n g t r a n s i t i o n s t a t e s . sT h e c h o i c e o f l i t h i u m 4 , 4 ' - d i methylbenzophenoneketyl reduction of alkyl bromides as a
model for electron transfer was based on the experimental
d e m o n s t r a t i o nb y G a r s t e t a l . t h a t t h e a n a l o g o u sr e a c t i o no f
lithium benzophenoneketyl with alkyl iodides is as free of Sp2
componentsas any homogeneouselectron transfer system so
f a r e x a m i n e d . r 6T h e r a t e o f r e a c t i o no f t h i s k e t y l w i t h a l k y l
(f
[Reprinted f'rom the .Journal of the American Chemical Society, 102,239 980).]
Copyright qc)1980 by the American Chemical Society and reprinted by permission of the copyright owner
140
Journal oJ the Anterican C'henticalSot'iett' / 102;l f Januarl, 2, lg80
'l'able
l . R e l a t i v e R a t e s ( 4 , . t = k n x / r t r - c 5 H e X )f o r R e d u c t i o no f A l k y l H a l i d e s a n t l R c l a r e dE l e c t r o c h e m i c aR
l c d u c t i o nP o t e n t i a l su
An x / 4 ,- c . r
B ur S n
RCI
(c ll rct6lJr)l
RBr
T T JF . 2 4 " C
oC
E r r O ,3 _ 5
c-C-sHq
c - C 6 H 11
c-C7H;.j
c - C 3 H1 . 5
r r - C sF l 11
(CHr)rCCH:
R'CH2CHCHT''
R'CH:C(CH r):/
I
I
03r
1.3
t0
023
0 12
0.67
4.3
0.05
8
0.61
1.2
0.Otr2
0.00-s6
0 .r 9
1.1
Bu rSnH
Rllr
l,'r :
Il g '
C.,
RBr
R(-l
DM tj.2_5
D M F , 2 50 C
"C
lV{g
R('l
[ : t r O ,0 o (
F . t 1 O . 0o C
I
0.60
I.-5
1.1
0.69
0..19
0 . 9r
2.9
-1.19
I
0 -5.5
-
_.:t
1.1
,-t
- - . -|
-1.-5
0.ll
0.053
0 . 9t
t.9
-2.).6
-2.31
-2.26
-2 19
1
f ?
-2.19
-2.83
- 2.66
-2.60
unccrtlintyis cslimalcdto bc ca + l0E fof all rclativcrdlc datt. /' Rcfcrcncc6. , Rcfcrcncc20. / Relcrcnce21. . R, =
,- " EJ(pcrinlcntal
C H.rCllr for rcrctionof RCI and Bursntl and for rcdlclionof RCI ar a glasslcarbonelcctrodc;R' = CHr for reactionof RCI with Mg and
R Br with aryl kctyland BursnH, and for rcductionof RBr at a Hg cathodc.I R' = CH,cH, fo. ,.aclionof R Br with afyl kctyl; R, = Ct tj
Ior rcactionsof RCI with M8 rnd BursnH: R' = H for rcactionoi RBr and BurSnHand for reductionof RCI at ghssy;arbonand RBr
rt
llg cxthodcs.
)
'8
o
o
,
na
co
.E
O.8
o..
\
0.6
CO
0.4
d
v.a
a
v'-
J.(
A
,L
l-e
o.o
o.o qz
0.4 0.6 o'8
LO
- ros [9-co\Cr],/[q-crxrcr]"
F i g u r el . c o m p e t i t i v ck i n e t i c sc x p e r i m e n t su s e dt o e s t i m a t cr c l a t i v cr a t c s
o f r e d u c t i o no i a l k y l c h l o r i d e sb y r e a c r i o nw i r h r r i - n - b u r y l t i nh y d r i d ci n
d i c t h y lc t h c r a t 3 5 o C ( f i 2 , a z o b i s i s o b u r y r o n i t r) i.l e
bromides at 25 oC was, however, inconveniently slow. By
substituting methyl groups in the aromatic rings of the ketyl,
its reactivity was increased by approximately a factor of ten,
a n d ra t esof r educ t iono f a l k y l b ro mi d e s(b u t n o t o f th e l ess
reactive
alkylchlorides)
couldbefollowedconvenienrly.
ldeally
we mi ght hav epr ef er re d
a re a g e nw
t h i c hw o u l dre d u c eal kyl
chlorides,
sinceour mostextensive
(nonmass
transportlimited)
ra te se r iesf or r eac t ionw i th m a g n e s i u m
i n v o l v ea
s l k y l chl ori d e s;n onet helestshe
, a v a i l a b l ee v i d e n c e
i n d i c a te th
s a t the
ra te so f r eac t ionof alk y l c h l o ri d e a
s n d b ro mi d e sw i th magn csi u mar e linear lyc orre l a te w
d h e nth e l a tte ra re n o t m asstra n sp orItim it ed,and t h e e ffo rtre q u i rc dto d e v e l o p
a c o nvenientsingle-electron
reducingagentfor alkylchlorides
did not
se e mj u s t if ied.
o.0
o.o o.2 0.4 0.6 0"8 t.o
- log
I c - C . H r B r/] , [ q - C r H r B r ] .
F i g u r e2 . c o m p e t i t i v ek i n c t i c sd a t a u s c dt o e s t i m a t er e l a t i v er a t c so f ' r e d u c t i o no i a l k y l b r o m i d e sb v l i t h i u m 4 , 4 ' - d i m e t h l ' l b c n z o p h c n oknecn , l
( T H I . .2 4 + I o C ) .
reduction of alkyl bromidcs with tri-n-butyltin hydride (Et:O,
0 oC. hy, AI BN ) and ratesol'reaction oi alkyl chlorides with
magnesium (E,t:O, 0 oC), both taken from previous work.6 In
addition, this table includes half-wave potentials taken from
t h e l i t e r a t u r e f o r r e d u c t i o n o f a l k y l h a l i d e sa t m e r c u r ) ' a n d
g l a s s yc a r b o n c a t h o d e s . l o .t l
For comparisonwith prcviouswork, absoluterate constants
w e r c a l s od e t e r n t i n c df o r t h e r e a c t i o n so f c y c l o p e n t y lb r o m i d e
w i t h k e t y l s i n T l - l t ' . \ \ ' i t h a n c x c e s so f k e t y l . t h e r e a c t i o n s
f o l l o w e dp s e u d o - f i r s t - o r d exr i n e t i c s .
- d l R X )I a r = k I R X ]
[Ar2COLi]
(4)
Rate constanls (deiined for eq 4) were: for lpC H r C r , H + ) : C O - . L i +k, = 1 . 5X l 0 - r L m o l - r s - r ( 2 4 " C .
T H F ) ; f o r ( C r , H . s ) : C O - . L i +k, = 9 . 5 X l 0 - 5 L m o l - t . - t ( 2 0
oC, THF). The datum
f o r l i t h i u m b e n z o p h e n o n ek e t y l i s
Results
comparable to reported literature data.22Tie order of rcacRa te - s t r uc t ur cpr of i l e sfo r b o th th e re d u c ti o no f al kyl
t i v i t y o f a l k y l b r o m i d e sw i t h 4 , 4 ' - d i m e t h y l b e n z o p h e n o n
k cet l ' l
ch l o ri des
by t r i- n- but y l ti h
n y d ri d ei n d i e th y e
l th e r,a n dfor the
( t e r t i a r y ) s e c o n d a r y) p r i m a r y ) a n d t h e a b s e n c eo i O - a l re d u ctionol alk y l br om i d e sw i th l i th i u m 4 ,4 ,-d i m e th yl ben- k y l a t e d p r o d u c t i n t h e
r e d u c t i o no f n - p e n t y l b r o m i d c s u g g c s r
zo p h e noxkeet y lin T H F , w e rco b ta i n e db y u s i n gc o mp eti ti ve t h a t n u c l e o p h i l i cd i s p l a c e m e n t s
by the ketyl oxygenare unkinctics.5
The disappearance
of two alkyl halidesreactingwith
i m p o r t a n t ,i n a n a l o g yw i t h G a r s t ' sw o r k . l 6
rrncxcess
of reducingagentwasfollowedby G LC: plotsof log
Thc major objectivesof this work are to dctermine whether
[ ( R r X ) , / ( R r X ) oa
] g a i n slto g [ ( R 2 X ) , / ( R : X ) ow] c r et i n e a r t h e h o m o g e n e o u rse d u c t i o n so i a l k y l h a l i d e sb y t r i - n- b u t y l t i n
to g re at ert hanT jv ac on s u m p ti oonf th c mo rere a c ti v e
h al i de. r a d i c a l s a n d b y l i t h i u m 4 , 4 ' - d i m e t h y l b e n z o p h e n o nkee t y l
a n d e valuat ionof t he s l o p e ,c mp l o y i n gl i n c a r re g ressi on r a d i c a la n i o n sa r c d i s t i n g u i s h a b l e
o n t h e b a s i so f r a t e - s t r u c t u r e
a n a l ys isy,ieldedk , " c=r k t l k t d i re c tl y(e q 3 )
c o r r e l a t i o n sa, n d t o u s ei n f o r m a t i o nd e r i v e df r o m t h i s a n a l y s i s
t o c l a s s i f yt h c t r a n s i t i o n s t a t e f o r t h e h e t e r o g e n o u sr e a c t i o n
l o g[ ( R r X ) , / ( R r X ) o=] k , l k : l o g[ ( R : X ) , / ( R : X ) s ] ( 3 )
o f a l k y l c h l o r i d e sw i t h m a g n e s i u mA
. n i m m e d i a t cc o n c l u s i o n
R cp re s ent at ivdat
c a ( r e l a ti v cto c y c l o p e n tycl h l o ri d e )are
from thc data summarizedin Table I which is pertinentto these
sh o wnin F igur eI f or t r i -n -b u ty l ti nh y d ri d erc d u c ti o n s;
d ata
objcctivesis that the rate-stru(,ture prrfiles for all three re(rclativcto cyclopentyl
bromide)areshownfor reductions
with
at'tion,gare linearl), relatetl. Figure 3 gives plbts oi particular
l i th i u m4. 4' - dim et hy l b e n z o p h e nk o
en
ryeli n F i g u re2 . R e sul ts r e l e v a n c ct o t h i s c o n c l u s i o n .T a b l e l l s u m m a r i z e st h e c o r r e a resu mm ar iz ed
in T abl eI, to g e th ew
r i th c o m p a ra b lra
e tesof
lation coefficientsand least-squares
slopesfor theseplots,and
241
Barber, WhitesidesI ReductiueCleatageof Carbon-HalogenBonds
E n e r g yC
- o r r e l a t i o n sb e t u ' e c nR c a c t i o n sW h i c h
T : r b l e l l . C o r r c l a t i o n C l o c l i i c i c n t s( a n d l - e a s t - S q u a r e sS l o p c s ) C h a r a c t e r i z i n g L i n e a r F r e e
R c d u c eA I k y l [ l a l i d c s "
R C I+ M g
( E r r O . 0 ' C ) R C I + B u r S n HR B r * B u r S n H R B r * ( C H r C a H + ) : C O L i R C I( t r l z )
'C)
R C I + B u r S n H( E t : O . 3 5
(
E
t
:
O
,
0
B
u
r
S
n
H
*
RBr
"C)
i HF.24"C)
R B R + ( C H r C o H r ) r C O L( T
R C I( E r 7 : C
, , D M F , 2 5" C )
R B r ( [ r r : ,H g ,D M F , 2 5" C )
o.9o(r.5)b
0.9'7( 2 . 0 ) '
0 . 8 2( 3 . 2h)
0 . 9 3 ( 0 . 7 8) D
0 . e e( 6 . - s ) ,
0 . 7 4( 9 . s/)
0 . 9 4( 0 . 6 2h)
0 . 9 6( 6 . 3) '
0 . 73 ( 4 . 3) , /
0 . t t 7( 0 . 3 1 ) 6
0 . 9 1( 3 . 1 ) ,
0 . 8r ( I J . 4 ) /
0 . 9 3( 9 . 5) ,
0 . 9 3( 1 3 ) /
0 . 8 5( 0 . 6 9 ) '
b
of cigh!rlkyl halides
(
,, Thcseparamctcrs
arc derivcdfrornplotsof log I vs.log l'. or fronrlog vs.C 1,1. Thc rclttivc ratesof reaction-hab- I -rlky^lethaneand
I
n-penryl,
cycloocryl.
cycrop"ntyi,.y.to-rl"*yt,.y-cloheptyt.
are includcdin rhiscorrelarion:
i,h"l:-?-":l'Iblc'l:
'/
(all
listed
(cf. foornores
? and/ol Tablcl). . o"ry ,rrJi#l"v"rii
lre correlated
in footnoteb cxccptcyclooctyl)
"tt
of those
yi r,rtiil"t l;.i"4 in f*tnore b aiecomparcd Scvenalkylhalidcs
for plots from each of the rcmaining pairwise sets of data
s u m m a r i z e di n T a b l e I .
Two featurcsof thescdata are relcvant to their accuracy and
d e s e r v eb r i e f c o m m e n t . F i r s t , t h e r a t e s o f r e a c t i o n o f a l k y l
chloridesand alkyl bromides with tri-n-butyltin hydride were
o b t a i n e d i n i n d e p e n d e n ts e t so f e x p e r i m e n t sb y d i f f e r e n t i n d i v i d u a l s .T h e e x c e l l e n tc o r r c l a t i o nb e t w e e nt h e s es e t so f r e l a t i v e r a t e se s t a b l i s h e tsh e i r i n t e r n a l c o n s i s t e n c yT. h c r a t e so f
r e a c t i o no f t r i - n - b u t y l t i n r a d i c a l w i t h a l k y l c h l o r i d e sa r e a p p r o x i m a t e l y 1 0 2 m o r e s e n s i t i v et o c h a n g e s i n a l k y l g r o u p
s t r u c t u r et h a n t h o s ew i t h a l k y l b r o m i d e s .T h i s o b s e r v a t i o ni s
consistenw
t i t h g r e a t e r b o n d r e o r g a n i z a t i o ni n t h e t r a n s i t i o n
state for cleavageof the carbon-chlorine bond than for
c l e a v a g eo f t h e w e a k e rc a r b o n b r o m i n e b o n d . I n p r i n c i p l e ,a
similar check on the accuracyof the rate data for reductionof
a l k y l b r o m i d e sb y l i t h i u m 4 , 4 ' - d i m e t h y l b e n z o p h e n o nkee t y l
m i g h t h a v e b e e n o b t a i n e d b y c o m p a r i s o nw i t h r e l a t i v e r a t c s
o f r e a c t i o no f a l k y l i o d i d e sw i t h l i t h i u m b e n z o p h e n o n ek e t y l ,
but this comparisonhas not beenmade. Second,in correlations
of the ratesof reactionof alkyl chlorideswith magnesiumwith
r a t e s o f r e a c t i o n o f a l k y l c h l o r i d e sa n d a l k y l b r o m i d e s w i t h
t r i - n - b u t y l t i nr a d i c a l ,t h e p o i n t 8 f o r t e r t i a r y a l k y l h a l i d e sf a l l s
s i g n i f i c a n t l yo f f t h e b e s t l c a s t - s q u a r e lsi n e . I t i s n o t p o s s i b l e
t o e s t a b l i s ht h e i n t e r n a l c o n s i s t e n c ya n d r e l i a b i l i t y o f t h e r e l a t i v e r e a c t i o n o f a l k y l h a l i d e sw i t h m a g n c s i u m b y t h e s a m e
t e c h n i q u eu s c d w i t h t r i - n - b u t y l t i n h y d r i d c t h a t i s . b y c s t a b l i s h i n g a g o o d l i n c a r c o r r e l a t i o n b e t w e e nd a t a o b t a i n e d
independentlyfor alkyl bromidesand alkyl chlorides because
only a limited number of alkyl bromidesreact with magnesium
r a t e s .T h c t e r t i a r y a l k y ' l
a t l e s st h a n m a s s - t r a n s p o r t - l i m i t e d
p o i n t h a s , h o w e v e r ,b e e n r e c h e c k e dc a r e f u l l y a n d i s r e p r o d u c i b l e .T h c a p p a r e n t l ya n o m a l o u s l ys l o w r a t c o f ' r e a c t i o no f
t c r t i a r y a l k y l c h l o r i d e w i t h m a g n c s i u mi s t h u s n o t a n c x p c r i m c n t a l a r t i f a c t a n d m a y r c f l e c t s t e r i c r e q u i r e m c n t sf o r a p proach of a hinderedcarbon centcr to the magnesiumsurface'
T h e d a t a f o r t e r t i a r y a l k y l h a l i d e sh a v e b e e n i n c l u d e d i n c a l c u l a t i n g t h c c o r r e l a t i o nc o e f f i c i e n t si n T a b l e l l . S i g n i f i c a n t l y
i m p r o v e dc o r r e l a t i o n sa r e o b t a i n e d l o r c o m p a r i s o no f t h e r e a c t i o n o f a l k y l c h l o r i d e sw i t h m a g n e s i u ma n d t h e r e d u c t i o n
o f a l k y l c h l o r i d e sa n d b r o m i d e sb y t r i - n - b u t y l t i n r a d i c a l i i t h e
d a t a f o r r e a c t i o no f t e r t i a r y a l k y l h a l i d e sa r e d c l c t c d ( r = 0 . 9 7
a n d 0 . 9 2 , r e s p e c t i v e l y ) I. n a n y c a s e ,t h c s c c o n s i d e r a t i o n sd o
n o t s u b s t a n t i a l l ya l t c r t h e m e c h a n i s t i cp i c t u r c .
Discussion
Thc rates of reaction of the alkyl halidesused in these
structurc reactivity studies would be expectedto reflect,
a m o n g o t h e r s t r u c t u r a lf c a t u r e s c, h a n g e si n t h e h y b r i d i z a t i o n
of'thc carbon atom attachedto halogcnin going from the
g r o u n d s t a t c t o t h e r a t e - l i m i t i n g t r a n s i t i o ns t a t e . l l ' r l T h e o b scrvationof a lincar correlationbetwccnrcactionof thcsealkyl
h a l i d e sw i t h t r i - n - b u t y l t i n r a d i c a l ,w i t h d i a r y l k e t y l , a n d w i t h
rnagnesiumindicatesthat the carbon-halogenbond is broken
a /
9,1
i t
.
,
il
I
a/
't/."
a,
a,/
'a'/
|
.. |
q / t
t"
"/
RCr.
tl
t"t
Hsngc!
R&.
A'2COL,
R&.
AI2COL,
:
oi
R& . x$iar!
l.o t
I
I
- t . 0 II
II
-a o
I
I
r
ct
tl
.
ra//
I
o |
,/
,/a
al
"4./
to
5t
;l
c r
y'"
./
.y
'2'
, /
r/'
t r/2 (ry)
/(
av 2 t c )
r _ - - T - - - _ . r r _ - - - - T _(H9)
---'--_..1
- z.9o - ?:ro
-2.4o - ?2o - 2 og Erzz
-.[---T-tros
(c)
- 2.59E',,
hrr,
('orrcllrtitln c()clF i g u r e 3 . ( ' r r r r c l l r l i r ) n \( ) l d : r t l r s u r t t r t t l t r i z c di n T l r b l c l .
l i c ' r c n t sa r c l i s t c d i n T r r b l c I l . O r d i n a t c s a r c i n a l l c a s c sl o g 4 r " 1 .A b s c i s s r t s
a r c 1 , 1 , : ( V ) l b r c l c c t r g c h c r n i c a lr c d u c t i o n s a n d a r c l t l g l r . 1 t b r o t h c r r c l r c ti o n s .
t o a n a p p r e c i a b l ea n d s i m i l a r c x t e n t i n a l l t h r e ep r o c e s s e sT.h e
i n l c r c n c eo f a p p r e c i a b l ec a r b o n - h a l o g e nb o n d b r e a k i n gi n t h e
t r a n s i t i o ns t a t e f o r t r i - n - b u t y l t i n r a d i c a l i s e x p e c t e d . l s ' rT6 h e
o b s e r v a t i o no f r e h y b r i d i z a t i o na t c a r b o ni n t h e t r a n s i t i o ns t a t e
l b r r c d u c t i o nb y l i t h i u m d i a r y l k e t y l i s c o m p a t i b l ew i t h a r e a c t i o n i n w h i c h c a r b o n - h a l o g e nb o n d b r e a k i n g i s c o n c e r t e d
242
Journal of the Americon Chemical Societ.v-/ 102:l / Januarl, 2, 1980
with clectrontransfer.It may,however,
with
alsobecompatible
fo rmat ionof an int er me d i a te
a l k y l h a l i d era d i c a a
l n i on,provi d e dt hat lengt heni nagn dw e a k e n i nogf th ec a rb o nh a l ogen
b o n d oc c ur sc onc ur re n tl w
y i th e l e c tro ntra n s fe r.In ei ther
e ve nt .t hes et hr eet y p e so f re a c ti o nas re e s s e n ti a l li yn di sti ng u i shable,
whenex am i n e db y u s i n ga c o mp a ri s o o
n f ratestru ct ur epr of ilesder i v e dfro m th i s s e t o f a l i p h a ti ch al i des.
work with aryl halidesindicateda similarsensitivity
Previous
i n ra tc sof r eac t ionwi th b o th ma g n e s i u m
a n d tri -r-b u tyl ti n
ra d i c alt o s ubs t it uen o
tsn th e a ry l ri n g .E
profilefor electrochemical
The potential-structure
reduction
of alkyl bromidesat a mercurycathodeshowsonly a general
co rrelat ionwit h t he ra te -s tru c tu rep ro fi l e sc h a ra c t eri zi ng
re a ct ions
of alk y l hali d e sw i th m a g n e s i u m
a n dtri -n -b utyl ti n
radicals,whileelectrochemical
reductionof alkyl chloridesat
a glassycarboncathodeis characterized
by an apparently
(TableII).27Thedatafor reduction
highercorrelation
at glassy
carbonincludeonly lbur acyclicalkyl halides,however.If the
rcd u c t ionof onlyt heco rre s p o n d i nagc y c l i ca l k y lb ro mi dcsat
mercuryis considered,
correlations
comparableto thosefor
re d u c t ionat c ar bqnareo b ta i n e dT. h u s ,a l th o u g hth e l imi ted
data availablefor electrochemical
reductionsmake quantil"ativemechanistic
comparison
of the reductionof alkyl halides
by homogeneous
reagentsand electrochemical
methodsdifi ta ti v e lth
y a t re l a ti v ere a c ti v i ti es
fi cu l t ,it appear qual
s
of organichalidesarecloselysimilarin reactions
with tri-n-butyltin
ra d i c als diar
,
y l k et y l s ,ma g n e s i u mme ta l ,a n d c a rb o nand
me rc ur yc at hodes . 2 s
Th e bear ingof t hisw o rk o n th e l o n g -s ta n d i nqgu e s ti on
of
the existenceof alkyl halide radical anionsdeservesbrief
comment.The linearcorrelationbetweenrate-structureprof i l e sfor t he t hr eer ed u c ti o nos f a l k y l h a l i d e sw i th k e tyl ,ti n
h yd ri de,and m agnesi u m
i s c o m p a ti b l ew i th p a th si n w hi ch
th e re ar e no int er m e d i a tebse tw e e ns ta rti n gm a te ri al sand
productsfor each,but cannotrigorouslyexcludeeithera path
involvingan intermediate
after the rate-determining
transition
state,or one in which superficiallydifferentreactions(e.g.,
halogenabstractionby Bu3Sn-and formationof a short-lived
RX-. by electrontransfer)coincidentally
showsimilarchanges
in hybridization
at the carbonoriginallybondedto halogenin
th e i r r es pec t ivter ans i ti o ns ta te s T
. h e ro u g h l yc o mparabl e
influenceof changesin structureon the rateof eachreaction
suggests,however,that carbon-halogenbond breakingis
si g n i f ic ant
ands im ila rfo r e a c h a, n da rg u e sa g a i n sat re acti on
verysimilarin geometry
involvingany intermediate
coordinate
to th es t ar t ingm at er i a lT. h u s ,i f a n a l k y lh a l i d era d i c a ani
l on
is formedasan intcrmediate
in reactionof an alkylhalidewith
4 ,4 '-d im et hy lbc nz o p h c nkoenty
e l o r ma g n e s i u m,
a n d i f the
stru ct ur eof t his r adic a la n i o nre s e mb l eth
s e tra n s i ti onstate
lor its lbrmationor decomposition,
itsstructuremustbesimilar
i n th e hy br idiz at ion
an dc h a rg eo n c a rb o nb o n d e dto h al ogen
to that fbr thetransitionstatefor abstraction
of a haloge
n atom
b y tri -n- but y lt in
r adic a lA
. c l o s cs i m i l a ri tyb c tw e c nth esetw o
stru ct ur es( l and 2) ca n n o tb e e x c l u d e db u t w o u l d be surprising;an intermediate
of structure3ramight be expected
to
rcscm blct r ans it ions ta te2 i n h y b ri d i z a ti o n
a n d c h a rgeon
ca rb on.
\\\
--\\
[;)C-Hr'J-'
t-C "' F]r"'SnR,i t;i('rrrrHr
t23
Th eor iginalques t ioonf w h e th eth
r ere a c ti oonf a l k y lh al i des
wi th m agnc s ium
pr oce e dbsy a n i n n e rs p h e reo r o u tc rsphere
p a th w ayt hushasnot b e e nrc s o l v e b
d y th i sw o rk . Mo reover,
i t p ro bablyc annotbe re s o l v c d
b y a n e x p e ri me n taalp p roach
b a se don qualit at iv cco mp a ri s o nosf ra te -s tru c tu re
p r ofi l cs
with thosefor reactions
whichmightbeexpcctcdto excmpliiy
th e semec hanis t ic
lir n i ts .s i n c ca l l o f th e s ep ro fi l c sa p p earto
b c l i n e a r l yc o r r e l a t cw
d i t h o n ea n o t h e rI.t i s p o s s i b ltch a t a n
i ncompl ete
w asexami ned
i n thi swor k,
setof modelreacti ons
and that noneprovi desan exampl eof, e.g.,an outers pher e
generati ng
e
el ectrontransferrezrcti on
an i ntermedi a t alkyl
hal i deradi calani on.In thi sevent.exami nati on
of someot her
pr of ile
e
reacti onmi ght yi el da di sti ngui shablrate-structure
characteri stiof
c thi stypeof reacti on.E veni f sucha react ion
t hat
exi sts,how ever.
thew orkof E ndi cottet a1.26
emphasizes
well-defined
innerandoutersphereelectrontransferprocesses
profi l e s.Thus
may exhi bi tl i nearl ycorrcl atedrate-structure
profi l esfor rcduct ions
i t i s not evi dentthat rate-structure
ol'
to thi sdi sti ncti on.
al kyl hal i desw i l l bc sensi ti ve
ExperimentalSection
G e n e r a lM e t h o d s . M o s t a l k y l h a l i d e sw e r e o b t a i n e d f r o n r , \ i d r r c h .
E a s t m a n ,o r C h e m i c a l S a m p l e sC o . ; 2 - b r o m o - 2 - m e t h y l p c n t u n\c\ i r \
e lkyl hulidc' * crc
s y n t h es i z e db y s t a n d a r dl i t e r a l u r e p r o c e d u r e s . : A
c h e c k e df o r p u r i t y b y G L C b e f o r e u s e ,a n d i m p u r e s a n r p l c ' * s 1 g
p a s s e dt h r o u g h a c o l u n r no f 'W u c l n r A c t i v i t y - | a l u m i n a a n d d i : t r l l ed
T r i - r - b u t y ' l t i n h l ' d r i d e w a s p r c p a r e db y l i t e r a t u r e m e t h o d s r. ' )d i s t iI l c d .
a n d s t o r c d i n a S c h l c n k f l : t s ku n d e r a r g o n i n t h e d a r k . A z o b i s i ' , r b u . y r o n i t r i l c ( A I B N ) u ' a s r e c r v s t a l l i z c df r o m 9 - 5 % c t h a n o l r . . rn l p
1 0 3 1 0 4o C ( l i t . r r m p 1 0 2 1 0 3 ' C ) a n d s t o r e da t I o C i n t h c d . r r k
B c n z o p i n a c ow
l a s u s c d a s r e c e i v e df r o m A l d r i c h , a n d ; 1 . . 1 ' . 1 " . 1 " ' t e t r a m e t h y l b e n z o p i n a c owl a s s y n t h e s i z e df r o m c o m m c r c i r r l J . J ' d i m e t h y l b e n z o p h c n o n ac n
r ld h a d m p 1 7 3 l l 4 o C ( l i t . r 2m p l l 5 l r o C 4 , 4 ' - D i m c t h ll b c n z h ld r 1| p c n t y l c t h c r a n d I , I - d i - p - t o l rl h c r r rn o l
).
r cthl llithrurl
w c r c s y n t h c s i z c db r s t r r n d u r dl i t er a t u r c p r o c e d u r e s . l M
r r u su s e dl r sr c c c i r c dl r t l r r r , ' \ l l ' u - \ ' c n t r tC
t no r p . a n d w a s t i t r u t c d L r c l i r r c
u s c b - r t h c ( j i l n t i r n d o u b l c t i t r r r t i o nm c t h o d , u s i n g l . l - d i b r , r n t t l et h a n e . r r1 ) i et h l l c t h e r a n d T l l F w c r c d i s t i l l e df r o n r d i s o d i u r nb c n z o p h e n o n ed i a n i o n u n d e r a r g o n . E x c e p tw h e r e n o t e d .u n c x c c p t i ( ) n l r l
i n e r t a t m o s p h e r es y r i n g ea n d c a n n u l a t e c h n i q u e sw e r e e n r p l o r c d . r '
A n U l t r a v i o l c t P r t ' d u c t s I. n c . . M o d e l U V S L - 2 5 l o n g w a v c l c n g t hU V
l a m p w a s u s c d t o i r r a d i a t e t r i - n - b u t y l t i n h y d r i d e r e d u c t i o n s .( i [ . ( a n a l y s e sw e r e c a r r i c d o u t b y u s i n g f l a m c i o n i z a t i o n i n s t r u m c n t s
c q u i p p e dw i t h c l c c t r o n i ei n t c g r l t o r s .
R e d u c t i o n s o f A l k v l C ' h l o r i d e sw i t h T r i - n - b u t y l t i n H y d r i d e . A l l
r c d u c t i o n sw er e p c r l i r r r n c di n a s i r n i l a rm a n n e r .R e a c t i o na l i q u o t sw e r e
q u e n c h c dc i t h c r u i t h C C I + .o r b 1 c m l i n g t o 0 o C ( c o n t r o lc x p e r i m c n t s
c o n f i r m e d t h a t t h e r e d u c t r o no i a l k y l c h l o r i d e sw i t h t r i - n - b u t l l t i n
h y d r i d ei s n c g l i g i b l ea t 0 o C i n d i e t h y lc t h c r ) .T y p i c a l l y ,t o a 2 5 - r n l .
r o u n d - b o t t o m e df l a s k e q u i p p c dw i t h a m a g n e t i cs t i r r i n g b a r a n d r c l l u x
c o n d c n s e ra n d l ' l u s h c dw i t h a r g o n w c r e a d d e d e t h e r ( 8 . 0 m [ - ) a n d
t r i - n - b u t y l t i nh l d r i d c ( l . l l n r l - . 6 . 8m m o l ) . T h e s o l u t i o nw a s b r o u g h t
t o r e f l u r . a n d a s o l u t i o no f ', \ I B N ( 0 . 0 17 g , 0 . I I m m o l ) , a h y d r o c a r b o n
G L C s t a n d a r d1 + 0 l l ) . u n d n i o d i f fc r e n t a l k y l c h l o r i d e s( 4 0 i r L e a c h .
approximatel0
y . - 1 0n r n r o l )i n c t h c r ( l n r L ) w a sa d d e d .A n a l i q u o t( 0 . - 5
m L ) w a s r c n r o v c d ,a n d t h e l - l a s kw a s i l l u n r i n a t c dw i t h a l t l n g w ' u v c length UV lamp. Additional aliquots were rcmoved pcrrodicalll.
Rclativc ratcs were obtaincd by cvaluating the slopc ol'log
( [ R r C l I , i I R ' ( ' l l o ) v s . l o g ( [ R : C ' l l r /[ R r C l l 0 ) .
Determinationof the Rate of Reductionof Cyclopentyl Bromide with
L i t h i u m B e n z o p h e n o nK
e e t r l . T h c r r r t cc o n s t a n tw a s d c t e r m i n e du n d c r
p s e u d o - f i r s t - o r d ecro n d i ti o n s e r np l o l i n g e x c e s sl i t h i u m b c n z o p h c n o n c
k e t y l . A r g o n w a s s c r u b b e db y b u b b l i n g t h r o u g h a p u r p l e s o l u t i o no l '
d i s o d i u mb c n z o p h c n o ndc i a n i o ni n T H I - -T. o a I 5 - m l - c c n t r i f ' u g ct u b c
w h i c h h a d b c c n s t o p p c r e dw i t h a N o - A i r s c p t u m a n d f l a r n c d r i c d
u n d c r a n u r g o n s t r c l n r r v r r sr r d d c dT l l F ( 7 m t - ) a n d b c n z o p i n r r c o l
( 0 . 7 5 g , 2 . 1 r l r n o l ) . r \ l ' 1 c rd i s s o l u t i o no f ' t h c b e n z o p i n a c o li .t w a s
. i g h - p r c s s u r ev i a l c q u i p p c d w i t h
t r a n s f c r r c dt o a J - i - n r 1 . .f l a n r e - d r i c d h
a m a g n c t i c s t i r r r n g b a r a n d s c a l c dw i t h a b u t y l r u b b c r s c p t u n ra n d
c r o w n c a p . T h c t c n r p c r l r t u r ue u s n r e i n t a i n c da t 2 0 o C w i t h a w a t c r
b a t h . T o t h i s v i g o r o u s l ys t i r r c d s o l u t i o n w e r c a d d c d n r c t h y l l i t h i u n r
( 1 . 7 M i n c t h c r .1 . 0 n r l - ,- 1 . -nl r n r o l )l n d 5 0 p l - c a c ho f n - o c t a n c( i n t c r n a l( i | . ( ' s t an d a r d) l n d o c l o p c n t l "bl r o m i d e( 0 . 4 7 m m o l ) . A l i q u o t s
( 0 . 5 0 . u n r l - ) u e r c r en r o v e du f ' t c r0 . 5 . l u , 3 9 , 6 0 , 9 0 , 1 2 0 . l l J 0 .a n d
2 5 3 n r n i t n d q u c n c h c du ' i t h w a t c r . A p s c u d o - l ' i r s t - o r d eprl o t o l ' l o g
( [ r ' - p c n t y l l ] r l , / [ r ' - p c n t y ' l B r lv1s;.)t i r n c w a s l i n e a ro v c r t h c i i r s t ] h
( - j 0 7 or c a c t i o n ) [. : r ' a l u a t i o on 1 ' t h i sp l o t g a v cA = 9 . 5 X l 0 - ' [ - n r o l - r
,.-|
R e d u c t i o no f n - P c n t v l B r o m i d e w i t h L i t h i u m 4 , 4 ' - I ) i m e t h r l b e n z o p h e n o n eT. o r r v i g o r o u s l vs t i r r c d s o l u l i o no l ' - 1 . - 1 ' . , 1 " . - 1 " ' - t c t r r r r nl -c t h r
-IHlr
( 17 n r [ - ) u ' r r sl d d c d \ l t r l - i ( 1 .]
b c n z o p i n u c o( l0 . 6 g . l . - l r l n r o l ) i n
M i n d r c t h t l c t h c r .1 . 0 n r l . l . ( r n r n t o l ) /. r - n ( ) n r r n( c- 1 0! 1 . . ( i l - C s t r r n -
I
Barber, Whitesides f Reductiue Cleatage of Carbon Halogen Bonds
243
dard),and n-pentylbromide(120,rL,0.97mmol).AIiquots(ImL)
werercmovedevery8 h, quenchedwith mcthanol,and analyzcdby
(17)carst,J.F.Acc.Chen.Res.1971,4,400-406.
Holy.N. L. Chem.
Rev
- 1974'74.243277
from that of 4.4/-dimcthylbenzhydryl
pentylcthcr.
K.-J.
iloi G.o"rt, F. L.: Kobayashi,
5328.
GLC.Asthereactionprogiessed.acompoundwa,rormedpo,sc,.ini
lY j # 3EI f"g;;,,i1";iiiil.*
l]315ffll.r,,,Iti?:::l
arctentiontimeequaltothatofl,l,-di-p-tolyhexanol
:rnddiffcrcnt
Am.
1sao,
R e d u c t i o n so f A l k y l B r o m i d e s w i t h L i t h i u m 4 , 4 , - D i m e t h y l b e n z o p h e n o n eK e t y l . A l l r e d u c t i o n sw e r e p e r f o r m e di n a s i m i l a r n t a n n c r .
A r g o n w a s s c r u b b e db y b u b b l i n g t h r o u g h a p u r p l e s o l u r i o no l d i s o d i u n - rb c n z o p h c n o n cd i a n i o n i n T H F . R e a c t i o n sw c r c p c r f b r r n c di n
- l - 5 - n r Lh i g h - p r c s s u r cg l a s sv i a l s e q u i p p c dw i t h n t a g n e t i cs t i r r i n g b u r s .
s e a l e dw i t h b u t y l r u b b c r s e p t aa n d c r o w n c a p s ,a n d l l a m c d r i c d u n d c r
a n a r g o n s t r e a m . T y p i c a l l y , a s o l u t i o no f 4 , 4 ' , 4 " , 4 " ' - t c t r a m e t h y l b c n z o p i n a c o(l0 . 5 0 g , 1 . 2 m m o l ) i n T t { F ( 2 0 m L ) w a s p u r g e df b r 0 . - 5
h w i t h a r g o n a n d t r a n s f e r r e dt o a s e a l e d h i g h - p r c s s u r ev i a l . T h e
t c n r p e r a t u r ew a s m a i n t a i n e da t 2 4 + | o C w i t h a w a t e r b a t h . T o t h i s
v i g o r o u s l ys t i r r e ds o l u t i o nw a s a d d e d M e l - i ( 1 . 6 M i n c r h e r . 1 . 3m L .
2 . 1 r n r n o l )a n d 2 - 5p L e a c ho f n - a l k a n e( i n t e r n a l G L C s t a n d a r d ) a n d
t w o a l k y l b r o m i d e s( a p p r o x i r n a t ley 0 . 2 m m o l o f e a c h ) .A l i q u o t s ( 0 . 5
n r [ - ) w c r c p e r i o d i c a l l yr e m o v e d ,q u c n c h c dw i t h m c t h a n o l o r w a t c r ,
r u n du n u l y z c db y G L C . T h e a b s o l u t er a t c c o n s l l l n tl b r t h e r e d u c t i t t n
o l ' c y c l o p c n t y lb r o r n i d eb y l i t h i u m 4 , 4 ' - d im e rh y l b c n z o p h e n o n ke e t y l
w a s o b t a i n e df r o m p l o t so f l o g ( [ r ' - p e n t y l B r l l / [ c - p e n t y l B r ] 6v)s . r i m e
u n d c r p s e u d o - f i r s t - o r d e rc o n d i t i o n s .
References and Notes
( 1 ) S u p p o r t eb
d y t h e N a t i o n a l s c i e n cFeo u n d a t i o7n7 i i 2 8 2 C H E .
( 2 ) W a l b o r s k yH, . M . ; A r o n o f f ,M . S . J . O r g a n o m e tC. h e m . 1 9 7 3 ,5 1 , 3 1 -
6?
(3) Bodewitz, H. W. H. J.; Blomberg, C.; Bickelhaupt, F. Tetrahedron 1973,
29, 7 19-726. tbid. 1975, 3 7, 1053_1063.
(4) Schaart, B. J.; Bodewitz, H. W. H. J.; Blomberg, C.; Bickelhaupt,F. J. Am.
Chem. Soc. 1976. 98. 37 12-37 13.
( 5 ) L a w l e r , R . G . ; L i v a n t ,P . J . A m . C h e m . S o c . 1 9 7 6 , 9 A , 3 7 1 0 - 3 7 1 2 .
( 6 ) R o g e r s , H . R . ; H i l l , C . L . ; F u j i w a r a ,Y . ; R o g e r s , R . J . ; M i t c h e t l ,H . L . ; W h i tesides, G. M. J. Am. Chem. Soc., accompanying paper in this issue.
(7) Rogers, H. R.; Deutch, J.; Whitesides, G. M. J. Am. Chem. Soc., accompanying paper in this issue.
(8) Rogers, H. R.; Rogers, R. J.; Mitchell, H. L.;Whitesides, c. M. J. Am. Chem.
Soc., accompanying paper in this issue.
(9) Hill, C. L.; Vander Sande, J. B.;Whitesides, G. M. J. Org. Chem Submitted
for publication.
( 1 0 ) l n l e l t a ,P . P . ; S c h u l e r ,R . H . J . P h y s .C h e m . 1 9 7 Z , 2 6 , 9 8 7 - 9 9 9 . A t k y t b r o mide radical anions were not unambiguously detected. For other related
studies, cf . Anbar, M.; Hart, E. J. lbid 1967, 71,3700-3702 reference '14,
this paper.
( 1 1 ) J o r d a n ,K D . ; M i c h e j d a ,J . A . ; B u r r o w ,P . D . J . A m . C h e m . ) o c . 1 9 7 6 , 9 8 .
7189-7191. Warman, J. M.; Sauer, M. C.; Johnson,G R A. J. Chem. phys.
1969, 51, 3477-3484. Andrieux,C. P.; Blocman, C.;Dumas-Bouchiat.J -M ;
S a v e a n t ,J . - M . J . A m . C h e m . S o c . 1 9 7 9 , 1 0 1 , 3 4 3 1 r - 3 4 4 1 T s o u , T T ;
Kochi, J. K. lbid. Submitted for publication.
( 1 2 ) G a r s t ,J . F ; R o b e r t s , R . D . ; P a c i f i c i ,J . A . J . A m . C h e m . S o c . . t 9 l 7 . 9 9 .
3528-3529.
(13) Bank, S.;Juckett, D. A. J. Am. Chem. Soc. 1976. 99,7742-7746.
(14) Garstlistsseveral alternalivestothepostulationofalkyl halideradicalanions
as intermediates as rationalizations of his experimental product yields. The
possibility of complexation of alkyl radicals with sodium halide, in particular,
has experimental precedent (Mishra, S P ; Symons, M C. R. J. Chem. Soc.,
Perkin Trans. 2 1973,391-395. Symons, M. C. R. J. Chem. Fes. (S) 1978,
360 361).
. ( 1 5 ) K u i v i l a ,H . G A c c . C h e m . R e s . 1 9 6 8 , 7 , 2 9 9 - 3 0 5 . C a r l s s o n ,D . J . ; I n g o l d ,
K. U. J. Am. Chem. Soc. 1968. 90.7047-7O55.
( 1 6 ) G a r s t ,J . F . ; S m i t h , C . D . J . A m . C h e m . S o c . 1 9 7 6 . 9 9 . | 5 2 0 * 1 5 2 6 .
chen. soc.
62.5324-
(21) Lambert, F. L.; Ingall, G. B. Tetrahedron Lett. 1974,3231-3234.
(221 For the reduction of n-propyl bromide by lithium benzophenone ketyl in
oC, k =
dioxaneat 20
6 . 8 X 1 0 - 5 L m o l - 1 s - 1 . C a r t e r ,H . V . ; M c C l e l l a n d ,
B. J.: Warhurst, E. Irans. Faraday Soc. 1960, 56,343-347.
(23) Overberger, C. J.; Biletch, H.; Finestone, A. B.; Lilker, J.; Herbert, J. J. Am.
Chem. Soc. 1953, 75,2078-2082.
(24) Ruchardt. C. Angew Chem., lnt. Ed. Engl. 1970, 9, 830-843. Applequist,
D. E.; Klug. J. H. J. Org. Chem. 1978, 43, 1729-1733.
(25) Bond energy data indicate that the abstraction of a halogen atom from alkyl
halides by tri-n-butyltin radical is only moderately exothermic (AH = -2
to -7 kcal/mol; Nash, G. A.; Skinner, H. A.; Stack, W. F. Trans. Faraday
Soc. 1965, 6 7, 640-648, 2122-2125). This and the similarity of structural
sensitivity to that observed in several studies known to involve radical
character in the transition state2a suggest that significant carbon-halogen
bond breaking has occurred in the transition state for halogen abstraction
by tri-n-butyltin radical.
(26) We note explicitly that the common assumptions that reaction of tri-n
butyltin radical with an alkyl halide proceeds by inner sphere electron
transfer with halogen bridging and that ketyl reductions occur by outer
sphere electron transfer are not rigorously proved. For example, a pathway
for reduction of RX with Bu3Sn. involving initial electron transfer and formation of an ion pair with structure RX-.Bu3Sn+ or R.X-Bu3Sn+ followed
by rapid Sn-X bond formation is difficult to exclude. Conversely, reaction
of RX with Ar2CO-.M+ might involve inner sphere transfer through bridging
halogen and metal ion (Ar2CO-. . . . M+ ' . . XR). Present work suggests
that rate structure profiles may not be very useful in making distinctions
of this type. Work in better defined inorganic electron-transfer reactions
has also established that rate-structure profiles for inner and outer sphere
electron transfer reactions may be linearly correlated and reinforces this
c o n c l u s i o n ( D u r h a m ,B . ; E n d i c o t t ,J . F . ; W o n g , C . - L . ; R i l l e m a , D . P . J . A m .
Chem. Soc. 1979. 101,847-857\.
(27)' A number of electrochemical studies suggest that reduction potentials of
organic halides at glassy carbon are less influenced by adsorption effects
than those at other electrodes: cf. Bard, A. J.; Merz, A. J. Am. Chem. Soc.
1979, 10 t, 2959-2965; and references cited therein.
(28) The failure of the rate-potential profiles to consistently follow eq i,
(29)
(30)
(31)
(32)
(33)
(34)
(35)
(i)
togK=u.5t'fa
derived for weak overlap electron transfer reactions, provides weak evidence that simple electron transfer is not occurring in the one-electron
reductions of alkyl halides (Marcus, R. A. J. Chem. Phys. 1965, 43,
0 7 9 - 7 0 1 ) . T h e k i n e t i c i r r e v e r i s i b i l i t yo f t h e s e r e a c t i o n s m a k e s a n y e f f o r t
to apply electron-transfer theory to these reactions suspect. For reductions
o f o r g a n i c s u b s t r a t e si n w h i c h e q i d o e s a p p l y , c l . N g , F . T . T . ; H e n r y , P .
M. J. Am. Chem. Soc.'1976,98,3606-3611. Tsou,T. T.; Kochi,J. K.
Reference 11.
2-Pentanonewas treated with MeLi in ether to afford 2-methyl-2-pentanol.
Treatment of the alcohol with HBr in ether gave 2-bromo-2-methylpentane,
p o s s e s s i n gp h y s i c a l p r o p e r t i e s i d e n t i c a l w i t h t h o s e r e p o r t e d : B r o w n , H .
C . ; N a k a g a w a .M J . A m . C h e m . S o c . 1 9 5 5 , Z Z , 3 6 1 0 - 3 6 1 3
v a n d e r K e r k , G . J . M . ; N o l t e s ,J . G . ; L u i t j e n ,J . G . A . J . A p p l . C h e m . 1 9 5 7 ,
z, 366-369.
O v e r b e r g e r ,C . G . ; H u a n g , P . ; B e r e n b a u m , M . B . " O r g a n i c S y n t h e s e s " ,
Collect.Vol. lV; Wiley: New York, 1963; pp 66-67.
Gomberg,M.; Bachmann,W. E. J. Am.Chem. Soc. 1927,49,236-257.
4,4'-Dimethylbenzhydryl pentyl ether was synthesized by the acid-catalyzed
s o l v o l y s i so f 4 , 4 ' - d i m e t h y l b e n z h y d r oi ln 1 - p e n t a n o l ,a n d p u r i fi e d b y d i s t i l l a t i o n ( b p 1 2 0 o C a t ' 0 . 0 2 T o r r ) . 1 , 1 - D i - p t o l y l h e x a n o lw a s o b t a i n e d o n
treatment of ethyl hexanoate with 2 equiv of p.tolyl magnesium bromide
and purified by chromatograohy on alumina.
Gilman, H.; Cartledge, F. K J. Organomet. Chem. 1964, 2,447-454.
B r o w n , H . C . " O r g a n i c S y n t h e s e sv i a B o r a n e s " , W i l e y : N e w Y o r k , 1 9 7 5 ;
Chapter 9.