4
874
J . Org. Chem. 1997, 62, 4874-4876
Rea ction s of Alk yllith iu m a n d Gr ign a r d
Sch em e 1
Rea gen ts w ith Ben zoqu in on e: Evid en ce for
a n Electr on -Tr a n sfer Mech a n ism
J ason McKinley, Aaron Aponick, J effrey C. Raber,
Christine Fritz, David Montgomery, and Carl T. Wigal*
Department of Chemistry, Lebanon Valley College,
Annville, Pennsylvania 17003
Received February 4, 1997
Quinols have been used as synthons in several syn-
1
-3
thetic methodologies.
Preparation of these compounds
has been accomplished by addition of alkyllithium or
4
Grignard reagents to quinone monoketals, quinone
silylcyanohydrins,5 and quinones directly, the latter
method being complicated by hydroquinone formation.
To date, little mechanistic information has been reported
on the competitive nature of addition versus reduction
for these reactions. In this work we investigate the
mechanistic dichotomy of quinone addition versus reduc-
tion by organometallic reagents and the factors that
influence the product distribution.
gioselectivity11 and stereoselectivity12 of addition are
influenced by quinone substituents, organometallic re-
agent, and solvent effects. Hydroquinone formation
during these reactions has been reported with inference
to a SET mechanism;3 however, mechanistic evidence
of such a pathway is lacking. Our investigation focuses
on the competitive nature of addition and reduction, be
it through competing or concerted SET and polar path-
ways.
,12
Resu lts a n d Discu ssion
Electron transfer from electron rich donors to electron
deficient acceptors is a well-established chemical process
producing intermediate radical ion pairs. The subse-
quent coupling of such radical ion pairs provides an
indirect mechanism for bond formation between a donor
and an acceptor. This mechanism, usually referred to
as the ET or SET (single-electron transfer) mechanism,
represents a plausible general alternative to direct, or
polar, bond formation.6
We studied the reaction between benzoquinone (1) and
various Grignard reagents as summarized in Scheme 1.
The ratio of addition to reduction products is directly
related to the steric effects of R in the organometallic
reagent. Addition is most favored in the case of methyl
giving 95% 2a . The amount of addition decreases for
primary Grignard reagents (R ) ethyl, n-propyl, and
n-butyl) giving 78% 2b, 63% 2c, and 60% 2d , respectively.
Reduction products are favored over addition for both
secondary (R ) isopropyl and sec-butyl) and tertiary (R
) tert-butyl) Grignard reagents giving greater than 90%
hydroquinone 3 in all cases as determined by GC-MS
and 1H NMR. It is noteworthy to mention that com-
pounds 2e,f slowly decompose upon standing for several
days at room temperature; thus, combustion analysis
proved impossible. Similar observations have been pre-
It is known that carbanion equivalents, such as alkyl-
lithium and Grignard reagents, are susceptible to ET
7
processes as well as conventional polar mechanisms.
Consequently, in the presence of a good electron acceptor,
carbanion equivalents pose a mechanistic dilemma of
polar versus SET processes. Previous reports have
indicated that quinones are potential ET acceptors as
8
13
well as electrophiles in polar additions. This, coupled
viously reported for compound 2g.
with our continuing interest in quinone chemistry,9
prompted our study of the mechanistic nature of alkyl-
lithium and Grignard additions to 1,4-quinones.
Since alkyllithiums are better reducing agents than
analogous Grignard reagents,6 one might assume that
quinone reduction products would be formed in greater
abundance if addition and reduction occurred through
competing polar and SET pathways, respectively. As
shown in Scheme 2, variation of the metal has no
significant effect on product distributions. Methyl, n-
butyl, sec-butyl, and tert-butyl alkyllithiums gave similar
product ratios upon reacting with 1 as compared to
analogous Grignard reagents in Scheme 1. This is a
significant observation which is inconsistent with com-
peting polar and SET mechanisms. Consequently, our
efforts focused on demonstrating a concerted mechanism
with products forming through a common intermediate.
To invoke a concerted polar mechanism leading to 3,
the quinol 2 would have to degrade to 3 through dealky-
c,14
The reactions of alkyllithium and Grignard reagents
with 1,4-quinones have been the subject of numerous
investigations.1 Previous reports indicate that the re-
0
(1) Evans, D. A.; Hart, D. J .; Koelsch, P. M.; Cain, P. A. Pure Appl.
Chem. 1979, 51, 1285.
(
(
(
(
(
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6) For reviews of electron-transfer mechanisms, see: (a) Eberson,
L. Adv. Phys. Org. Chem. 1982, 18, 79. (b) Pross, A. Acc. Chem. Res.
985, 18, 212. (c) Eberson, L. Electron Transfer Reactions in Organic
Chemistry; Springer-Verlag: Berlin, 1987.
7) Liotta, D.; Saindane, M.; Waykole, L. J . Am. Chem. Soc. 1983,
05, 2922.
8) Patai, S. The Chemistry of Quinoid Compounds; Wiley: New
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1
(
1
(
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3369.
(9) (a) Wigal, C. T.; Grunwell, J . R.; Hershberger, J . J . Org. Chem.
1
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(13) Stern, A. J .; Swenton, J . S. J . Org. Chem. 1988, 53, 2465.
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Org. Chem. 1992, 57, 4304. (c) Wigal, C. T.; McKinley, J . D.; Coyle, J .;
Porter, D. J .; Lehman, D. E. J . Org. Chem. 1995, 60, 8421.
(10) Reiker, A.; Henes, G. Tetrahedron Lett. 1968, 3775.
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