Electroreduction stereoselectivity

The anion radical species formed by the electroreduction of aliphatic esters show interesting reactivities, and the reduction of olefinic esters gives bicyclic products with high regio- and stereoselectivity. The electroreduction of the ester in the presence of chlorotrimethylsilane affords a tricyclic product (Scheme 21) [35, 40]. The mechanism of this cyclization reaction seems to be the addition of anion radical species, formed by the reduction of the ester group, to the carbon-carbon double bond. 

Imines can be stereoselectively hy-drodimerized to 1,2-diamines with rac/ meso rahos of 0.9 to 1.1 similar to the cathodic coupling of carbonyl compounds to pinacols in an acidic medium [304]. With 4,6-dimethylpyrimid-2-one only the meso-diamine was obtained [305]. Electroreduction of diimines prepared from... 

Amputch MA, Little RD (1991) Tetrahedron 47 383. Utley JHP, (1987) Electrogenerated bases. In Steckhan E (ed) Topics in current chemistry. Springer, Berlin Heidelberg New York, p 133 Little RD, Sowell CG (1991) Stereoselection in electroreductive cyclization. Construction of a Corey lactone precurser. In Little RD, Weinberg NL (eds) Electroorganic synthesis, festschrift for Manuel M. Baizer. Dekker, New York, p 323 Shono T, Mitani M (1971) J. Am. Chem. Soc. 93 5284... 

The electroreductive cyclization reaction of 6-heptene-2-one 166, producing CIS-1,2-dimethylcyclopentanol 169, was discovered more than twenty years ago [166]. In agreement with Baldwin s rules, the 5-exo product is obtained in a good yield. Since that time, the mechanism of this remarkable regio- and stereoselective reaction has been elucidated by Kariv-Miller et al. [167-169]. Reversible cyclization of the initially formed ketyl radical anion 167 provides either the cis or the trans distonic radical anion. Subsequent electron transfer and protonation from the kinetically preferred 168 leads to the major cis product 169. The thermodynamically preferred 170 is considered as a source of the trace amounts of the trans by-product 171 (Scheme 32). 

A series of bicyclo[3.3.0]octanols are accessible by electroreductive tandem cyclization of linear allyl pentenyl ketones 189, as shown by Kariv-Miller et al. [189]. The electrolyses are carried out with an Hg-pool cathode and a Pt-flag anode. As electrolyte, tetrabutylammonium tetrafluororborate is used. The reaction is stereoselective, yielding only two isomers 192 and 193. In a competing reaction, a small amount of the monocyclic alcohol is formed. Since all the monocycles have the 1-allyl and the 2-methyl group in trans geometry it is assumed that this terminates the reaction. The formation of a bicyclic product requires that the first cyclization provides the cis radical anion which leads to cis-ring juncture [190] (Scheme 37). 

A stereoselective electroreductive cyclization has been used in the synthesis of /-sterpurene, an isolactarane-type sesquiterpene 95). 

Nedelec and coworkers [56] reported highly stereoselective and stereospecific crosscoupling of activated olefins with stereoisomeric vinyl halides via nickel-catalyzed electroreduction. 

The reductive cyclization of unsaturated carbonyl compounds can also be achieved by using chemical reducing reagents, such as Al-amalgam, MgTiCl4, and NaNH3, but in general not only chemical yields of cyclized products but also stereoselectivity are lower than those of the electroreductive cyclizations. 

Shono and coworkers [176] found that 1,3-dimethanesulfonates of 1,3-diols underwent electroreductive cyclization in dime thy Iformamide (DMF) containing R4N" ", giving stereoselectively the corresponding cyclopropanes in a good yield ... 

Indirect electroreductive cyclization of y-bromo-Q, yS-unsaturated esters using Co(III) or NI(II) redox mediator provides six-membered ring products stereoselectively, as in Eq. (33) [227]. 

Stereoselective electroreduction of some organometal complexes has been reported. The reduction of o -oxotetramethylene-cyclopentadienyl-iron - 7 -benzenes in an acidic solution occurs specifically from the exo side to give only the en /o-alcohol, as in Eq. (38) in alkaline solution the corresponding stereoisomeric (d, 1 and mso) pinacols are formed but the stereochemistry has not been determined [257]. 

Bode, H.E., Sowell, C.G., and Little, R.D., Electrolyte-assisted stereoselection and control of cyclization vs. saturation in electroreductive cyclizations. Tetrahedron Lett., 31, 2525, 1990. 

Shono and coworkers achieved electroreductive intra- and intermolecular couplings of ketones and nitriles in 2-propanol solutions containing Et4NTos using Sn cathodes at controlled potential (-2.8 V SCE). Intramolecular coupling of cyclic y- and (5-cyano ketones, besides good to excellent yields, proved to be cis stereoselective when a-hydroxy ketones with bicyclo[3.3.0] or [4.3.0] skeletons were formed. When the reactions were carried out at 65 °C instead of 25 °C, dehydration of hydroxy ketones occurred and the corresponding a,jS-unsaturated ketones were obtained. The presence of alkyl or 2-ethoxycarbonyl substituents did not hinder the cyclization. In Table 13 some representative examples are shown. 

In the case of phosphonium salts (entries 3-6) once the individual epimers had been separated, stereoselective alkaline hydrolysis or electroreductive cleavage of one of the groups afforded the phosphine oxide or the free phosphine respectively. 

Stereoselectivity in the cycllzatidfl Of d -alkyl j3 pheiiylpr0pionic acids in polyphosphoric acid to give exo and endo diastereomers of [Cr(CO)3(7j -indanone)] appears to be under steric control. A method using optically active complexes has allowed the determination of the kinetic product in a thermodynamically controlled reaction. In another stereochemical study, electroreduction of ketones such as 41 (R = = Me R = Me, R = Ph)... 

The formation of 1,5-dienes by dimerization of allylic ammonium or phos-phonium salts during electroreduction has been examined, and convenient stereoselective syntheses of the three isomeric octa-2,6-dienes have been described. ... 

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