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Reactions Involving Electron Transfer. I. Reduction of
2,2,6, 6-Tetrarnethyl-4-hepten-3-one1 Kerry W. Bowers, Roger W. GieseYza James Grimshaw, Herbert 0. Nancy H. Kolodny,2aKarlheinz Kronberger, and D. K. Roe
Contribution f r o m the Department of Chemistry, and the Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139. Received September 9, 1969 Abstract: The polarographic reduction potentials and the half-lives of the initial radical anion reduction products
have been measured for a representative group of a,@-unsaturatedketones. In dipolar aprotic media (DMF, HMP) the radical anion 16 derived from 2,2,6,6-tetramethyl-trans-4-hepten-3-one(11)has a sufficiently long halflife (ca. 10 min) to permit measurement of its properties. The lifetime of this radical anion 16 is markedly lowered by the presence of proton donors, lithium cation, or oxygen in the reaction medium. In the presence of proton donors or metal cations, this radical anion 16 is rapidly converted to a racemic dihydrodimer 24. Evidence is presented for two geometrically isomeric radical anions 17 and 16 being formed by reduction of the cis (14)- and trans (11)-enones. However, one of the isomeric anion radicals is rapidly converted to the other more stable isomer believed to be 16. Epr measurements have demonstrated the occurrence of electron exchange between the radical anion and the corresponding unreduced enone. The epr measurements also suggest that 40-50% of the unpaired electron density of the radical anion 16 is located at the @-carbonatom with little or no unpaired spin density at the a-carbon atom. The properties of solutions of sodium in hexamethylphosphoramide (HMP) have been examined and reduction of the geometrically isomeric tetramethylheptenones 11 and 14 with sodium in HMP have been studied. In HMP solution either the monomeric (25) or dimeric (24) reduction product could be made to predominate by the choice of reaction conditions. Under appropriate reaction conditions the dimerization process was at least partially stereospecific with the racemic dimer 24 being formed from the trans-enone 11 and mainly the meso-dimer 30 being formed from the cis-enone 14. useful classification divides organic redox reactions into either (1) reactions in which one or more atoms is transferred in each step of the reaction path (e.g., metal hydride reductions) or ( 2 ) reactions in which one or more steps in the reaction path involves only the transfer of an electron (e.g., reductions with dissolving metals). In this paper and subsequent publications in this series we will describe our investigations of redox reactions involving at least one electrontransfer step. The purpose of our investigations is twofold; we wish both to gain a better understanding of the reaction paths and the properties of intermediates in these redox reactions and also to explore the possible use of reaction intermediates formed by electron transfer (e.g., anion radicals or cation radicals) as reactive intermediates in other synthetically useful processes such as C-C bond formation. The program has been initiated3 with a study of the reduction of a,P-unsaturated ketones by the addition of electrons from a suspension of alkali metal, from a solution of an alkali metal, or from a mercury cathode in an electrolysis cell. As a result of these studies we believe the principal reaction paths followed with aliphatic enones under the conditions of our work are illustrated in Scheme I. The results of our work and other studies4 leave little doubt that the initial
A
(1) This research has been supported by Public Health Service Grant No. 1-R01-CA10933 from the National Cancer Institute, by Research Grant. No. AFOSR-68-1518 from the Directorate of Chemical Sciences, Air Force Office of Scientific Research, and by the Joint Services Electronics Programs (U. S . Army, U. S. Navy, and U. S . Air Force) under Contract D A 28-043-AMC-O2536(E). (2) (a) National Institutes of Health Predoctoral Fellow, 1966-1969 ; (b) author to whom inquiries should be addressed. (3) For Part I1 see H. 0. House, R. W. Giese, I
