Indirect Wittig

Whittlesey, Williams and co-workers fnrther developed the catalytic indirect Wittig reaction and fonnd that the more electron-rich NHC present in complex 18 provided a more reactive catalyst [8]. Catalyst 18 was used to convert benzyl alcohol 8 and phosphoninm ylide 19 into the product 20 under slightly milder reaction conditions and in a shorter time than in previous work (Scheme 11.4). Other C-C bond-forming reactions from alcohols using a borrowing hydrogen approach have been reported, with Peris and co-workers using Ir-NHC complexes for the C-3 alkylation of indoles with alcohols [9]. 

Scheme 11.4 An improved catalyst for indirect Wittig reactions on an alcohol...

Another reported example of the carbon-carbon bond-forming reaction from alcohols is that of the indirect Wittig reaction, which utilizes an [IrCl(cod)]2/dppp/ CsCOs catalyst system and leads to the production of alkanes (Equation 10.24) [47]. 

Mechanisms of competing reactions of Wittig reagents with substituted 2-amino- 1,4-naphthoquinones have been discussed73 and a study of the stereoselectivity of the indirect Wittig reaction of a 1,2-hydroxyphosphonium salt has led to the conclusion... 

Scheme 3.21 Catalytic cycle of an indirect Wittig reaction.

Phosphoranide anions 1 - i.e. species based on pentacoordina-ted phosphorus having a lone pair as one of its five substituents -were proposed by Wittig and Maercker as early as 1967, to act as intermediates or transition states in nucleophilic substitutions at tricoordinated phosphorus ( p. Shortly afterwards in 1969, Hellwinkel brought indirect evidence for the formation of phosphoranide 3 in equilibrium with the carbanion 4, since the action of a base on phosphorane 2 gave, after acidic treatment, a mixture of 5 and 6 in proportions that depend on the experimental conditions (i). 

More recently, Wilson et have used ESI-MS to detect transient intermediates in the Wittig, Mitsunobu and Staudinger reactions directly from solution. Ionic intermediates were directly detected while zwitterionic intermediates were indirectly detected following acid quenching. The systems studied were those which had been previously investigated via P NMR. This work demonstrated that the progress of these reactions can be... 

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