Stereochemistry and Mechanism in the Wittig Reaction

The modern interpretation of the Wittig reaction mechanism

“…Several excellent reviews on the topic have previously been written that contain extensive detail on the Wittig reaction mechanism. 2,3,4,5 M ore recent reviews do 25 not focus on the details of the mechanism. 6,7,8 The reaction (see Scheme 1) occurs between a carbonyl compound (aldehyde or ketone in general, 2) and a phosphonium ylide (1) to give alkene (3) with phosphine oxide (4) as the by-product.…”

“…These show very consistently high Z-selectivity in Li-salt free reactions with a broad range of aldehydes. 4 The only known exceptions involve reactions of ethylidenetriphenylphosphorane with aromatic aldehydes (and these reactions have been shown not to proceed Very high E Chart 1. Summary of observed selectivity trends for reactions of representative ylides.…”

“…Thus allylidene or benzylidene triphenylphosphoranes (Chart 1 (e)) generally react to give approximately equal proportions of E and 15 Z alkene, while semi-stabilised alkyldiphenylphosphine-derived ylides (Chart 1 (f)) show moderate and in some cases very high E-selectivity. 4 Semi-stabilised ylides with two or more of the Pphenyl groups replaced by alkyl groups (Chart 1 (g)) 4 and also those derived from methyldibenzophosphole (Chart 1 (h)) 60 show 20 exceptionally high E-selectivity. As with non-stabilised ylides, the general observation applies that for a given semi-stabilised ylide Z-selectivity is highest for reactions with tertiary aldehydes, and E-selectivity is highest for reactions with primary aldehydes.…”

“…Wittig reactions of stabilised ylides derived from both triphenylphosphine (Chart 1 (i)) and trialkylphosphine (Chart 1 (j)) generally show very high selectivity for E-alkene in polar 30 aprotic solvents. 4 M ethyldiphenylphosphine-derived ester stabilised ylides (Chart 1 (k)) show decreased although still predominant E-selectivity.…”

“…82 70 Evidence against this mechanism: A number of tests for radical involvement in Wittig reactions are described in the review of Vedejs and Peterson. 4 One of these is reproduced here. The unimolecular rate constants for ring opening of cyclopropylcarbinyl radicals are extremely large (Scheme 75 10(b)).…”