Evidence for single electron transfer (SET) pathway in the reaction of primary alkylcadmium reagents with p-benzoquinone

Abstract

The reaction of primary alkylcadmium reagents with p-benzoquinone at various conditions was studied. On the basis of our
results, reaction proceeds through a SET mechanism that forms loose and tight intermediates, which produce quinole (1) and
substituted hydroquinone (2). In both cases, hydroquinone (3) is obtained in different yields. © 2001 Elsevier Science B.V. All
rights reserved.

Introduction

The organocadmium reagents are known as a very
mild and regioselective reagent in organic synthesis [1a].
The most important synthetic application of
organocadmium reagents has been in the preparation of
ketones in reaction with acyl chlorides [1b,c]. Although
this reagent does not react with ketones and esters [2],
it reacts with a,b-unsaturated ketones mainly through
1,4-addition [3]. Results obtained from the reaction of
Grignard reagents with some ketones and benzoquinone
have shown that it proceeds via SET mechanism
[4].
On the basis of recent reported results [5], and our
studies on the reaction of several organocadmium
reagents with benzoquinone [6], we decided to investigate
the mechanism of these reactions. We are glad to
report the operation of a SET mechanism in the reaction
of organocadmium reagents with benzoquinone.

Results and discussion
The reaction of several organocadmium reagents
with p-benzoquinone was carried out at low and high temperatures. Products were identified by the comparison
of their spectroscopic data with authentic samples.
The results are shown in Tables 1 and 2.
On the basis of our results (Table 1), it seems likely
that the quinol formation has arisen via a SET mechanism
through the formation of a-complex between the
substrate and reagent and subsequent single electron
transfer from the reagent to benzoquinone which leads
to the formation of cross-conjugated tight intermediate
(CCTI) in solvent cage (Scheme 1). The appearance of
the deep blue color during the addition of the reagent
to benzoquinone supports the SET mechanism [4b].
Moreover, when we carried out the scavenging studies
by addition of p-dinitrobenzene to the reaction contents,
the starting material benzoquinone was recovered

Conclusions

From this investigation, it can be concluded that the
reaction of the primary alkylcadmium reagent with
p-benzoquinone proceeds through a SET mechanism.
The products and yields are different and depend on
the temperature, solvent and the nature of the alkyl
group. Reaction at low and high temperature leads to
1,2 and 1,4-addition products, respectively. The yield of
the 1,4-addition product is less than the 1,2-addition
product, since radical ion pairs diffusing out of the
solvent cage is enhanced at higher temperatures. The
formation of high percentage yield of hydroquinone at
high temperature and in the case of all alkyl groups
supports this proposal, as well. Diffusing out of the
solvent cage also depends on the viscosity and basicity
of the solvent. Comparing the yields of hydroquinone
in diethyl ether (DEE) and THF at high temperature
shows the increasing yield in DEE, which has, lower
basicity and viscosity. The nature of the primary alkyl
group is also an important factor in reaction mechanism,
since unlike the other primary alkylcadmium
reagents, the benzyl cadmium reagent leads to 1,4-addition
product even at low temperature.

adapted from:
Mansour Shahidzadeh, Mehdi Ghandi *
Department of Chemistry, Uni6ersity of Tehran, PO Box 13145 -143, Tehran, Iran
Received 18 September 2000; accepted 15 November 2000