Synthesis of alkenes from carbonyl compounds and carbanions alpha to silicon. III. Full report and a synthesis of the sex pheromone of gypsy moth

Abstract

The synthesis of alkenes from carbonyl compounds and carbanions a to silicon is described. Methylenation of carbonyl compounds was accomplished by their reactions with trimethylsilylmethyl carbanion followed by treatment with thionyl chloride or acetyl chloride. Additions of organolithiums to triphenylvinylsilane were used as a method to generate a-silylalkyllithiums which reacted with carbonyl compounds to give the corresponding alkenes. A simple synthesis of the sex pheromone of gypsy moth by this method is reported.A general method for the synthesis of alkenes can be represented by eq 1. The Wittig reaction3 (Y = PR3+) and its many modifications are based on the key role of phosphorus in the carbanion formation as well as the elimination steps. An interesting alternative is Corey's olefin synthesis from sulfinamide derivatives (Y = SONRZ).~ The use of other sulfur derivatives has subsequently been reported.5~6 This paper describes our studies in the synthesis of alkenes from reaction of carbonyl compounds with carbanions a toIn 1962, Gilman and Tomasi7 studied the reaction of trimethylsilylmethylenetriphenylphosphorane (111) with benzophenone. They obtained tetraphenylallene (IV), a prod-uct which they considered as rather unexpected. I t was in 1968 that Peterson,* in extending his work on the generation of a-silyl carbanion^,^ demonstrated the alkene synthesis based on eq 1 (Y = SiR3). Thus, the formation of IV from the reaction of I11 with benzophenone can be viewed as a special case of alkene synthesis uia a-silyl carbanion. At about the same time, we initiated our studies on the use of silicon compounds for a variety of organic syntheses.10-12 This general interest led to a study of the olefination reaction using a-silyl carbanions. We have subsequently reported our preliminary results1 on this reaction. Results and DiscussionIn order for the reaction (eq 1, Y = SiR3) to be a viable method of forming the carbon-carbon double bond, two conditions must be met: first, a facile method of generating carbanions a to silicon must exist; and second, the elimination of the siloxy group from the p-silyl oxyanion adduct (11, Y = SiR3) must occur readily. We shall now examine these two conditions.Methods of Generating Carbanions a to Silicon. One of the simplest ways of preparing a-silyl carbanion is the reaction of a-halosilanes with magnesium13J4 or lithium. The synthetic utility of this reaction is considerably enhanced by the fact that a-halosilanes can in turn be prepared by free-radical halogenation of a1kyl~ilanes.l~ Alternatively, metal-halogen exchange reactions between ahalosilanes and alkyllithiums can lead also to the formation of a-silylalkyllithium compounds. l5 L1M = MgBr or Li I t has recently been shown by Peterson9@ that the highly reactive n-butyllithium-tetramethylethylenediamine complex (n-BuLi-TMEDA) can metalate a number of weakly acidic heteroatom-substituted methanes. This provides a useful method for preparing a-silylakyllithium compounds, especially when the alkyl group is further activated...

Reagents for Carbonyl Methylenation in Organic Synthesis

“…However, by treating the initial magnesium or lithium alkoxide with either thionyl or acetyl chloride, methylenecyclohexane can be isolated in quantitative yield (Scheme 29) [57]. However, by treating the initial magnesium or lithium alkoxide with either thionyl or acetyl chloride, methylenecyclohexane can be isolated in quantitative yield (Scheme 29) [57].…”

Aza-Peterson Olefinations: Rapid Synthesis of (E)-Alkenes

“…A long-standing issue for direct Peterson olefinations -using stabilised α-silyl carbanions -is the lack of control over the stereoselectivity of the resulting olefin. [3][4][5]12,13 O'Shea and coworkers have addressed this problem through replacement of The Peterson olefination has been applied to a wide-variety of alkenes, yet the synthesis of 1,3-dienes, an important building block within the synthetic community, 14 is relatively underexplored [15][16][17] . Common routes to access 1,3-dienes include olefination reactions (Wittig and Horner-Wadsworth-Emmons), [18][19][20] enyne metathesis, 21,22 and transition metalcatalyzed cross coupling reactions.…”

“…2 -Silyl carbanions are commonly accessed through deprotonation using a strong base or metalhalogen exchange from the corresponding -halosilanes (Scheme 1a). 3,4 Bench stable bis(trimethylsilane) reagents have emerged as a milder method to access -silyl carbanions by activation with catalytic or stoichiometric amounts of Lewis bases (nBu 4 NF, nBu 4 NSiF 2 Ph 3 , CsF, KOSiMe 3 / nBu 4 NCl). [5][6][7][8][9][10] In most examples, strong bases are still required to synthesise these reagents.…”

“…A long-standing issue for direct Peterson olefinations -using stabilised -silyl carbanions -is the lack of control over the stereoselectivity of the resulting olefin. [3][4][5]12,13 O'Shea and co-workers have addressed this problem through replacement of carbonyl compounds with N-aryl or N-sulfinyl imines to give (E)-and (Z)-alkenes, respectively (Scheme 1c). 8,10 Scheme 1 Controlling E/Z selectivity of direct Peterson olefinations The Peterson olefination has been applied to a wide variety of alkenes, yet the synthesis of 1,3-dienes, an important building block within the synthetic community, 14 is relatively underexplored.…”

Trimethylsilylmethyllithium

“…1a,b,9 Acetyl Chloride or Thionyl Chloride can also be used to accomplish this elimination. 10 A wide variety of aldehydes and ketones have been used as substrates in this reaction. 1b The use of Cerium(III) Chloride has been advocated with reagent (1) to favor nucleophilic addition with enolizable carbonyl compounds.…”