CIDNP Studies of Hydrogen Abstraction
by Aroyl Radicals in Norrish Type I Processes
C. B a k and K . P r a e f c k e
In stitu t für Organische Chemie der Technischen U niversität Berlin
K . A. M u s z k a t and M . W e i n s t e i n
D epartm ent o f Structural C hem istry, The W eizm ann In stitu te o f Science, R eh ovot, Israel
(Z. N aturforsch.
32 b, 674-677 [1977]; received N ovem ber 22. 1976)
C ID N P Studies, Aroyl R adicals
The abstraction o f hydrogen atom s by benzoyl radicals in the photolyses o f 2,2'dihalogeno-benzil d im eth ylk etals and o f S-p-tolyl esters o f su b stitu ted thio benzoic acids
was in vestigated b y th e C ID N P technique. T he present stu d y supports previous conclu­
sions th a t in th e absence o f scavengers th e form yl H atom s o f the benzaldehydes thus
form ed originate from cycloh exad ien yl-lik e free radical precursors.
Benzaldehydes are generally obtained as one of
the main products of the photocleavage of asubstituted phenyl ketones ArCO-X-Ar' (1). The
formation of benzaldehydes through hydrogen ab­
straction by the initially formed benzoyl radical
(or its derivatives) was observed in benzoins2-3,
X = CROH, benzoin ethers, X = CROR' 2-3, ben­
zoin esters, X = CROCOR' 2, benzil-dimethylketal,
X = C(OR)2 4, desoxybenzoin2, X = CH 2, and also
in S-esters of thiobenzoic acids, X = S lb-5. In these
systems photocleavage proceeds through a Norrish
type I mechanism,
1
— ► si
ArCO + Ar'X‘.
B. D im ethylketals of benzils (3), X = C(OCH 3 )2 ,
Ar = A r' = 2-ClC 6 H 4, 2-BrC 6 H 4.
Fig. 1 shows typical *H 90 MHz C ID N P results
obtained when a CD 3 CN solution of 3 (Ar = A r' =
2-BrCeH4) was irradiated w ith a 2500 w X e-H g
lam p+.
(1)
Step (1) being followed by dimerizations of ArCO
and of Ar'X', recombination, rearrangement, as
well as by the hydrogen abstraction step,
ArCO + HZ -* ArCHO + Z.
(2)
We have carried out a photoCIDNP study
intended to provide information on the hydrogen
donor species HZ' in step 2 for the following two
series of a-phenylketone derivatives ArCOXAr',
A. S-p-Tolyl esters of thiobenzoic acids (2), X = S,
Ar' = 4-CH3C6H4, Ar = 4-CH3OC6H4,
2,3-(CH30 ) 2C6H3, 2,4-(CH30)2C6H3,
3,5-(CH30)2C6H3.
R equ ests for reprints should be sen t to Dr. K . A.
The W eizm ann In stitu te o f Science, D ep a rt­
m ent of Structural Chem istry, R ehovot, Israel.
6 (ppm vs T M S )
F ig . 1. 90 M Hz 1H NMR spectra o f CD3CN solution
o f 3 (Ar — A r '= 2-BrCeH4) recorded at 40 H z/cm .
A - no U V irrad iation ; B - during U V irradiation w ith
2500 w H g /X e lam p, a - proton traces o f CD3CN;
b - H D O im p u r ity ; c - CH3O protons o f 3: d - protons
o f photoproduct; e - CH4 protons.
M u s z k a t,
+ For experim ental details see ref. 6 b.
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C. B ak et al. • C ID N P S tu d ies o f H ydrogen A bstraction
675
O ur observations can be sum m arized as follows:
engers th e aldehyde CHO protons originate from
(a)
Em ission (E) from th e aldehyde CHO protons free radical species HZ' to be described below.
The effect of thiophenol indicates th a t in its
is observed in both series A an d B upon UV
absence
th e polarization in system s A and B
irradiatio n of th eir solutions in CeDö, CD3CN or
originates
from ‘o u t of cage’ (escape) recom bination
CDCI3 . These signals are weak in series A b u t m uch
processes. The sm all ex ten t of polarization observed
stronger in B. In series A (Fig. 2) th is is th e only
in (a) for series A m ay be th e outcom e of several
polarization observed.
processes having different polarizations all of which
100 Hz
result in one p ro d u ct (benzaldehyde).
The initial sequence of steps th a t could explain
th e nuclear polarization of the CHO protons (in the
absence of radical scavengers) sta rts w ith (1). This
step is followed by th e addition of ArCO to the
(intact) su b strate molecule (step 3). In th e case of
para addition to e.g. 2 (Ar = 2 -CH 3 OC 6 H 4 ) the
following cyclohexadienyl free radical hydrogen
donor would be obtained.
A rC O S A r’ + ArC O ’ ------ ►
< ^ ^ -C 0 ^ ^ -C 0 -S -^ ^ -C H 3
( A r = 2 -C H 3OC6HA)
6
0C H3
OCH3
(p p m v s TM S )
F ig . 2. 90 M H z *H N M R spectra o f a CßD6 solution o f
S-p-tolyl-4-m ethoxythiobenzoate
(2, A r '= 4-CH3OC6H 4).
A —before U V irradiation, B - during U V irradiation,
C - sam e as B , but on a tenfold expanded scale,
a - CHO proton, b - C6D 5H .
(b) The polarization of these protons is reversed
(enhanced absorption, A) on addition o f either thiop h e n o l2 or of 2,6-ditert.-butylphenols know n as
efficient hydrogen atom donors6 an d free radical
scavengers.
(c) Continued UV irradiation in series B changes
th e sign of th e C ID N P effect in th e corresponding
protons from an initial emission, as in (a) to a b ­
sorption.
In m ost cases only scarce evidence is available
ab o u t th e origin of CHO hydrogen in benzaldehydes
form ed upon photolysis of a-phenylketones. One
exception however is benzoin m ethyl eth er where an
isotopic substitution - mass spectrom etric stu d y
was carried out by A d a m , G ü s t e n , S t e e n k e n and
S c h u l t e - F k o h l i n d e . 3 They show th a t th e form yl
hydrogen atom originates m ostly from th e cor­
responding benzoyl m oiety and to a sm aller ex te n t
from th e X = CHOCH3 g ro u p .3 Photolysis in CöDe
also indicates th a t no incorporation of hydrogen
atom s from this solvent occurred115. Assuming these
results to be valid for the present system s we m ay
conclude th a t in th e absence of free radical scav­
(3 )
u
Species of ty p e 4 have been recently invoked as
possible interm ediates in th e form ation of ‘poly­
m eric’ products in th e photolysis of benzoin alkyl
ethers, CöHs-COCHR-CeHs (R = OCH 3 ).3 A dduct 4
would th e n undergo loss of th e hydrogen atom s
((H)) as in (4).
♦ <Q-co- — ^ ^ - c o —<^ rp i
och3
och3
co- s-
^ -
och3
ch3 +
^"^- cho*
och3
In th e absence of scavengers th e spin correlated
radical pair R P 1 is probably form ed by encounter.
Howrever due to th e relative lack of reactivity of the
com ponents only a m inor p a rt of the correlated
radical pairs R P 1 give polarized products by
transfer of th e polarized atom H to ArCO in a
gem inate recom bination step (collapse). Most of
R P I pairs dissociate (escape), th e polarized benz­
aldehyde being form ed by out of cage recom bination
of polarized 4 w ith unpolarized ArCO+.
The addition of thiophenol, in (b), prevents the
form ation of polarized products through the a tta ck
of benzoyl radicals on th e substrate. This effect is
due to th e com petition between scavenger and
su b strate for th e benzoyl radicals and m akes evident
th e polarization resulting from other processes (see
below).
+ W e are in d eb ted to Dr. J.
th is p ossib ility.
L ib m a n
for suggesting
676
C. B ak et al. • C ID N P Studies o f H ydrogen A bstraction
The sign of th e polarization due to th e n e t effect,
in (a), m ay be obtained by applying K a p t e i n ’s
ru les7 in a straightforw ard wray, th e signs of th e
param eters being: // - positive (F-pair), e - negative
(escape, m ajor p a th ) ; positive (collapse, lesser path),
A i - positive (hydrogen ß to cyclohexadienyl sy­
stem ), A g - positive (gHz- > gArco).
Thus in th e case of th e escape p a th of polarized 4,
a negative / ne is obtained,
r
33
(Ar
= 2 -B rC 6 H4 ) ^
+ < ^ - C ( O C H 3 )2*
NEj
(E).
In th e presence of scavengers (case b) th e trip let
excited su b stitu ted benzaldehyde reversibly a b ­
stracts a hydrogen atom from a ground state
aldehyde molecule
■Tne =
ju eA iA g
=
—
3ArCHO + ArCHO
ArCO + ArCHOH* (5).
RP2
Following th e escape step th e polarized hydroxybenzyl radicals ArCHOH undergo hydrogen ex ­
change w ith unpolarized benzaldehyde or su b strate
molecules,
ArCHOH* + A r C H O A r C H O * - f A rCH OH (6).
N uclear polarization due to steps such as (5) and
(6) has been widely studied both in benzaldehydes
and in benzoin.8 F o r another possible process
explaining th e polarization of th e benzaldehydes see
step (10) below.
The C ID N P effect on continued irrad iatio n in th e
absence of scavengers (case c) is clearly due to a
secondary photolysis of th e su b stitu ted benzalde­
hyde formed in step (2), th e explanation for the
C ID N P effect being exactly th e same as for case (b).
The polarization in cases (b) and (c) is opposite to
th a t found for (a) since th e hyperfine coupling
constant for th e polarized proton H* in the
ArCHOH* radical is negative (a p roton of a benzyl
free radical).
Closely related to (1) and (4) are th e processes
leading to nuclear polarization of aldehyde CHO
protons wiien diisopropylketone9 or phenyl-aphenylethylketone (1, X = CHCH 3 ) 8 undergo UV
photolysis. In both instances the polarization in the
CHO groups is due to th e transfer of polarized H
atom s to acyl radicals.
The dim ethylketals of benzils (group B) also showr
polarization of other protons. The emission signal a t
<5 = 0.2 ppm is probably due to m ethane protons,
th e m ethane being form ed by decom position of
escaped (and polarized) Ar'C(OCH 3 )2 * radicals
which undergo spin selection in step (7), e.g.,
Br
C(OCH3 )2* — *■
q ^ C 0 0 C H 3* +
Br
CH3*
(7 )
Br
CH3*
(8 )
Br
+ R -H
R' + C H *
— *-
(9 )
Polarized 2-brom om ethylbenzoate, < 5och3 = ~ 4 p p m
is form ed by th e sam e process (step 8). Application
of K a p t e i n ’ s rules for polarization of m ethyl
protons in
Q - C ( O C H 3)2
Br
proceeds as follow s: fi - positive; s - negative,
escape p ro d u cts; Ai - positive, and Ag - positive
(gAr'c(ocH3)2 — gAröo > 0) resulting in emission,
r ne =
h------- 1— b = — •
Thus th e collapse of an encounter pair (F) of type
RP3
A rCHO H + Ar'C(OMe)2 ->
R P 3 (F)
(10)
ArCHO + ArCOOMe + CH4.
can be ruled o ut as source of the polarization in
m ethyl benzoate and in methane.
6 (p pm v s TMS)
F ig. 3. 270 MHz XH spectrum of a CDCI3 solution of 3
(Ar = Ar' = 2-BrC6H4).
The p artia l assignm ent of the polarized arom atic
protons of 3 (Br) rests largely on the 270 MHz XH
spectrum (Fig. 3) of this compound. The arom atic
proton region of Fig. 1 is redisplayed on an expanded
scale (4 Hz/cm) in Fig. 4. Most of the signals of the
polarized protons (labelled a -i in spectrum B,
Fig. 4) clearly correspond to distinct signals of 3
(Br), cf. Fig. 3.
C. B ak et al. • C ID N P Studies o f H yd rogen A bstraction
F ig. 4. As in F ig. 1 but on expanded scale (4 H z/cm ).
Only arom atic protons region is shown.
On th e basis of th e previous discussion we suggest
th a t polarization originates in th e trip le t gem inate
pair
♦
Br
< Q > - C ( O M e ) 2*
Br
(fi = + ) . This radical pair separates com pletely and
th e polarized 3 (Br) results from secondary re ­
com bination of escaped radicals (e = — ). The E S R
d a ta for both radicals + suggest th a t Ai = — for
ortho and para protons and Ai = -f- for m eta
p ro to n s3-10. The g values for closely related ra d i­
cals3-10 strongly suggests th a t gArc(OMe)2 > gArco+ The hyperfine coupling con stan ts for th e benzoyl
protons seem to depend strongly on th e p osition o f
th e CO group 10. The discussion th a t follow s assum es
a linear geom etry.
1 a P a r t X V in th e series ‘O rganic P h o to c h e m is tr y ’
(for p a r t X IV , see R . L ü d e r s d o r f , J. M a r t e n s ,
B . P a k z a d , an d K . P r a e f c k e , L ieb ig s A nn. C h em .,
in p ress) an d p art X IV in th e series ‘S p ectro sco p ic
I n v e s tig a tio n s ’ (for p art X I I I , se e J . M a r t e n s , K .
P r a e f c k e , U . S c h u l z e , H . S c h w a r z , an d H . S im o n ,
Z. N a tu rfo rsch . 32b, [1977], in p ress).
b J. M a r t e n s and K . P r a e f c k e , Chem. Ber. 107,
2319 [1974].
2 For a review o f th e literature see: F. D . L e w is ,
R . T. L a u t e r b a c h , H .-G . H e i n e , W . H a r t m a n n ,
and H . R u d o lp h , J. Amer. Chem. Soc. 97, 1519
[1975].
3 S. A d am , H . G ü s t e n , S. S t e e n k e n , an d D . S c h u l t e F r o h l i n d e , Liebigs A n n . Chem. 1974, 1831.
677
Thus for protons originating in the ortho and para
positions of ArCO as A g = — we have F — — and
for th e meta protons r = + . For protons originating
from ArC(OMe)2 , as A g = + , we have F = — for
th e meta protons and r = + for th e ortho and para
protons.
The assignm ents of Fig. 3 taken together w ith
these considerations perm it a seemingly firm conclu­
sion to be reached concerning peaks a, b, h and i in
Fig. 4 . These peaks originated respectively from
th e ortho (a, b) and para (h, i) protons of the aroyl
radical. T hus we a ttrib u te these peaks to protons 6
and 4 of 3 (Br) (see Fig. 3 for num bering). Though
we could sim ilarly a ttrib u te some of th e c -f signals
to protons 3 and 5 th e correlation here seems less
certain.
F inally we would like to emphasize th a t more
definite conclusions m ust aw ait th e results of an
isotopic stu d y concerning the origins of the form yl
hydrogens in these molecules. Similarly, the detailed
in terp retatio n of th e other polarizations, noticeably
those of th e arom atic protons of 3 m ust be deferred
till 270 MHz C ID N P d a ta become available.
We are greatly indebted to Mr. M. G r in b e r g of
th e D ep artm en t of S tructural Chemistry, The Weizm ann In stitu te of Science, Rehovot, for the NMR
m easurem ents.
K . P r a e f c k e th an k s th e D eutsche Forschungs­
gem einschaft for financial support and the M inerva
Com m ittee for a scholarship. C. B a k thanks th e
Fritz-Ter-M eer-Stiftung for a scholarship.
4 M. R . S a n d n e r and C. L. O s b o r n , Tetrahedron
L etters 1 974, 415.
5 Y . O g a t a , K . T a k a g i , and Y . T a k a y a n a g i , J. C . S.
Perkin I, 1973, 1244.
6 K . A. M u s z k a t and M . W e i n s t e i n ;
a Chem. Commun. 1975, 143;
b J . C. S. Perkin I I 1976, 1072;
c Z. P h y s. Chem. N . F ., 101, 105 [1976].
7 R . K a p t e i n , Chem. Commun. 1 971, 732.
8 For a review see: G. L. C l o s s , in A. R . L e p l e y and
G. L. C l o s s (eds): Chem ically Induced M agnetic
P olarization, p. 96, W iley, N ew Y ork 1973.
9 J . d e n H o l l a n d e r , R . K a p t e i n , and P. A. T. M.
B r a n d , Chem. P h ys. L etters 10, 430 [1971].
10 P. J . K r u s i c and T . A. R e t t i g , J. Amer. Chem. Soc.
92, 722 [1970].