Nucleophilic Wittig reagents

The Wittig reaction consists in the replacement of carbonyl oxygen of aldehydes and ketones by a methylene group with the aid of phosphine-methylenes resulting in the formation of cis or trans olefines. The reaction proceeds through the nucleophilic addition of Wittig reagent (phosphine methylene) across the > C = O bond and formation of an intermediate cyclic. 

C H3MgBr, CIljLi, or related methyl nucleophiles. It also does not react with the Wittig reagent in DMSO at 70". Addition does take place if the ketone is added to an excess of reagent generated in a minimum of solvent (toluene) preheated to about 92". 

Acyl silanes can display disparate behaviour when treated with carbon nucleophiles, even of related types5,61149. For example, when aroyl silanes were treated with a Wittig reagent, none of the expected alkenes was obtained, and the only reaction products isolated were silyl enol ether and triphenylphosphine (Scheme 73)182,183. When alkanoyl silanes were treated with Wittig reagents, however, only the normal olefinated vinyl silane products were isolated (Scheme 74)182-184 Under soluble lithium salt conditions, Z-vinyl silanes were produced with very high selectivities the reaction was used to prepare a pheromone component (50) of the sweet potato leaf folder moth (Scheme 75)183. 

The currently accepted mechanism of the Wittig reaction involves two steps. Like other nucleophiles, the Wittig reagent attacks an electrophilic carbonyl carbon, but then the initial addition adduct undergoes elimination to form an alkene. Mechanism 21.4 is drawn using Ph3P=CH2. 

A vast number of vinylcyclopropanes have been synthesized by performing condensation reactions between nucleophilic reagents and cyclopropanecarbaldehydes and cyclopropyl ketones. A range of reaction conditions have been employed, but in most syntheses the carbonyl function has been converted to an alkene moiety by using a Wittig reagent. 

In a study towards the synthesis of the tricarbonyliron complex of 8-methylenespiro[2.5]octa-4,6-diene 52 starting from spiro[2.5]octa-4,6-diene (49), conversion of its tricarbonyliron complex 50 via hydride abstraction, nucleophilic addition of hydroxide and oxidation gave the spiro[2.5]octa-5,7-dien-4-one-tricarbonyliron complex 51, which, however, could not be converted to the target complex 52 via Wittig olefination. An alternative approach to generate a Wittig reagent from spiro[2.5]octa-4,6-diene (49) via conversion to a phosphonium system 53 was likewise unsuccessful. ... 

Phosphorus ylides are metal-free carbon nucleophiles that provide irreversible reactions and have been shown to react with support-bound aldehydes. DoUe et al. have reported such a transformation using 4 equiv. of the Wittig reagent in THF [382]. 

Many Wittig reagents do not possess electron-withdrawing substituents on the carbanion carbon. Such alkyl-substituted phosphonium ylides are referred to as non-stabilized and react readily with carbonyl and other polar groups. Addition of the ylide to the carbonyl group takes place rapidly with aldehydes or ketones, both of which usually react equally well with these reagents. The number and nature of the alkyl substituents on the carbanion carbon normally has little influence on the extent of nucleophilic character of the phosphonium ylide. 

Introduction. Sodium bis(trimethylsilyl)amide is a synthetically useful reagent in that it combines both high basicity and nucleophilicity, each of which may be exploited for useful organic transformations such as selective formation of enolates, preparation of Wittig reagents, formation of acyl anion equivalents, and the generation of carbenoid species. As a nucleophile, it has been used as a nitrogen source for the preparation of... 

Hard bases attack silacyclobutanes at the heteroatom exclusively, whereas soft nucleophiles such as the Wittig reagents cause rupture of these molecules at a C—C bond (38). 

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