Wittig reagents stereoselectivity

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... 

A brominated polystyrene is reacted with sodium diphenylphosphan to form the polymeric Wittig reagent. Reaction with a halide and a base form the ylid which reacts with the carbonyl compound to the olefin. Wittig olefination can be made stereoselective (33). The formation of cis-olefins is accomplished in salt-free solu-... 

The reagent is useful because the Wittig reagent (C6H5)3P=CHCHO and the usual phosphonate reagent (C2 H 5 0)2 P OCH 2 C H (O C2 H.-,), do not react with ketones. Moreover, the reaction proceeds stereoselectively to give the trans olefin thus benzaldehyde affords cinnamaldehyde in 11% yield. 

Some trifluoromethyl compounds such as 2-trifluoromethylaniline are converted to alkenes." Trifluoroacetyl-stabilized Wittig reagents undergo Grignard reactions, but the products decompose on acid treatment to give alkenes. Noteworthy is the dependency of the stereoselectivity on the acid strength. 

Isopropylidene-D-glyceraldehyde (from D-mannitol) reacts stereoselectively with organometallic or Wittig reagents leading to either erythro- or threo- -substituted glyconic acids and hence... 

Lithiated phosphine oxides, the so-called Horner-Wittig reagents, are important intermediates in organic syntheses, for example in stereoselective syntheses of alkenes and asymmetric syntheses of chiral compounds [61, 62], Despite the importance of these reagents very little is known about their structures. The first lithiated phosphine oxide containing Li-C bonds has been reported recently. The self-assembled... 

V. Samano and M.J. Robins, Stereoselective addition of a Wittig reagent to give a single nucleoside... 

Alkenes are formed with high stereoselectivity from PS-Wittig reagents under Li-salt and Li-free conditions, similar to results with soluble Wittig reagents.A triphase method is convenient for reactions of PS-benzyl- and PS-allyl-phosphonium salts with aldehydes using solid potassium carbonate as base and THF as solvent. 

Trialkyl phosphono[ CJacetates ((EtOlzPl0) CH2 COOR) The use of trialkyl phospho-no[ C] acetates 159 in olefination reactions (Homer-Wadsworth-Emmons or HWE reactions) is generally superior to the use of the related Wittig reagents. The carbanions, generated by a suitable base, are more nucleophilic than the latter, so that they react even with ketones. Furthermore, they often show an improved stereoselectivity. They provide 100% -configured acrylates when they react with linear aldehydes, and ( )-alkenes... 

A useful application of the silver-mediated additions is 1,3 -diene synthesis by three-carbon elongation of aldehydes [48,51,53]. The bimetallic reagent 3-trimethylsilyl-l-propenylzirco-nocene chloride (A Scheme 8.23) reacts with aldehydes under the influence of a catalytic amount of Ag+ to give the intermediate zirconocene-alkoxide B, which then undergoes a Peterson-type 1,4-elimination of TMS alkoxide to stereoselectively afford ( )-dienes (fc/Z > 96 4) (Scheme 8.23). A Wittig reaction yields the same products without stereoselectivity (ca. 1 1 mixtures of E- and Z-isomers). 

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