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Nucl ear MagneticResonanceSpectroscopy.
Gamlns,gamma,Dimethylallylmagnesium
Bromide
By GeorgeM. Whitesides,J. Eric Nordlander and John D. Roberts
[Reprinted from the Journal of the American Chemical Society, S{,2010 (1962).]
Copyrigbt l9ti2 by the American Chemical Society and repriuted by permission of the copyright owoer.
NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY.
BROMIDE'
Sir:","-DIMETI{TLALLYLMAGNESIIIM
The nuclear magnetic resonance (n.m.r.) spectrum of allylmagnesium bromide2 is characterized
by a simplicity inconsistent with anv single classical
structure; notablv, the a and 7 protons occupy
magnetically equivalent positions. This observation may be accommodated by formulating the
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. ^<
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F i g . 2 . - ' V e t h 1 ' l p r o t o n r e s o n a n c eo f 7 , 7 - d i m e t h y l a l l y l m a g nesium bromide as a function of temperature.
The Grignard reagent of 7,7-dimethyiailyl
bromide rvas prepared by reaction of a highly dilute
solution of halide in diethyl ether with a column of
amalgamated magnesium turnings.3 The reaction
product was seriouslv contaminated witJr coupling
products; these rvere convenientlv removed by
concentration of the solution and extraction with
dry isooctane.
At room temperature, the n.m.r. spectrum of
FiS. l.-Nuclear
7,7-dimethylallyl Grignard reagent (FiS. 1) conmagnetic resonance spectrum of l,^tsists of a triplet in the vinvl proton region, a high
tlirnethl'lall1'lmagnesium bromide in diethl'l ether at 33o and
field doublet, and a sharp singlet at slightly lower
Li() r\{c.7'sec. rvith tetramethylsilane (0 cps. ) as internal
-lhe
field than the ether metlyl quartet. These resostanrjard.
intense triplet and quartet centered at -73
nances have areas approximateiy in the ratic)
c p s . a n d - 2 1 9 c p s . a r e t h e r e s o n a n c e sr . ' f t h e d i e t h l . l e t h e r
I :2:6 and can be assigned,respectively,to the single
CHr and CH,r. The cornplex of lines at - 55 cps. is due to
isodctane. Residuai coupling products from the preparation
d proton, the trvo a protons, and the six methvl
proton. of 7,7-dimetir-"*lallylmagnesium
bromide,
o f t h e G r i g n a r d r e a g e n t g i v e r i s e t o t h e r e s o n a n c ca t - 1 0 4
on the assumption that th.e cis and trans meth,"*l
cps The a-CH2 doubiet of the Grignard reagent comes at
- i i { c p s . ; t h e p - C H t r i p i e t a t - : j 3 5 c p s . ; t h e ^ , , - C H :rle s o n - groups experience averaging of their chemical
shifts through rvhat amounts to rotation around the
r u n c ea t - 9 l J c p s .
clouble bond as the result of interconversionswith
reagent as either a rapicilv equiiibrating
Grignarrl
a,a-dirnethylallylmagnesiumbromide (presentonl1'
mixture of two equivalent classicalstructures (I)
i n verv l ow concentrati ons).
or a bridged structure with magneticallvequivalent
Upon lowering the temperature of the sample,
p r o t o n s( I I ) .
the methvl proton singlet broadens and then spiits
syrnmetricallv into two equal components (Fig. 2).
'tr
-cHs
'UHrMgBr
,, n4""\n
Neither the vinvl proton resonancenor the high
: ItIgBrCHf
HzL'
CH,
fielcl
doublet significantll' changesform or position.
I
Clearlv, :rt ihe lorver temperatures, the iifetime of a
H
rrrethvl group in a cis or trans position is increased,
(-I
and
the rnagnetic environment of protons on the
//,/,"\ '\
methyl groups is no longeraveraged.5
HrC.,
The obsen-ation of two methvl resonances at
\'lagn'
/ ,gH,
low temperatures excludesa symmetrical structure
analogous to II for this Grignard reagent. The
II
methyl groups in a symmetricai structure would
Available data for allylmagnesium brornide2'3 be static; hence a decreasein temperature would
and for butenvlmagnesium bromide3'{ appear to not be expected to produce the observed separafavor the formuiation of these Griglard reagents ti on.
as rapidly equilibrating mixtures even at -80o.
Application of the procedure of Gutowsly and
We now wish to report evidencervhich we interpret
Holm6 to the variation of tJre line shapes of the
as excluding ttr.e symmetrical structure for 7,Imethyl resonancesfrom -55 to +33' gave AH* :
dimethylallylmagnesium bromide, and by' analogy 7 * 3 kcal .r/mol e,and J : 103 to 108.sec. - rf or
for allyl- and butenylmagnesium bromides.
(4)
(l)
Supported
in part
by the Oflice of Naval
Researcb.
(2) J. E. Nordlander
and J. D. Roberts, J. An. Chtm. Soc., E1r 1769
( l9;19).
(3) J. ft. Nordlander,
Ph.D. Tbesis, California Institute of Tecbnology,
l9ti0.
J . E . N o r d l a n d e r , W . G . Y o u n g a n d J . D . R o b e r t s ,J . A m - C h c m .
S o c . ,E 3 , 1 9 4 ( 1 9 0 1 ) .
(5) J. D. Roberts, "Nudear \Iagnetic Resonance," trIcGraw-Hill
Book Company, Inc., New York, N. Y., 1959,Chap. -1.
(6) H. S. Gutowsky and C. H. Holru, J. Chem. Phys.,26, L228
( 19 5 6 ) .
20,1962
ConuoxrcATroNs To rHE Eorron
the process leading to interchange of cis and trans
methyl groups.T
If an appreciable amount of. a,a-dimethylalll'1magnesium bromide were present in the solutions,
a decrease in temperature would be expected to
result in an increase in concentration of the more
stabie 7,7-dimetlylallyl
isomer. The tempera(7)
culties
metbyl
The
magnitude
in obtaiuing
protoss.
of the
uncertainty
an accurate
in these values
value for the relaxatioo
reflects difhtime
Ir
of tbe
20ii
ture independenceof this doublet and of the highfield doublet of the related butenyl Grignarcl riagent indicates tiat tJrese Grignard reagents exist
almost exclusively as the primary isomers.
CoNrRrsurroN No. 2822
Gronce NI. \\'rrrreslues
Gares axo CnpllrN LaaonaroRIES or CneursrRy
or
CalrnoRx-ra lNsrrrute
TBcnxolocv
J. Euc NonoLaxpBn
P,rs.roeNe, Car.rronNra
Joux D. RonBnrs
RBcBrvRo Mancs
7. 1962