ROLE OF THROUGH-SPACE 2p–3d OVERLAP EFFECTS IN LITHIUM-ION CATALYZED WITTIG REACTIONS

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Abstract
The reactions of benzylidene-bis-(o-methoxyphenyl)methylphosphorane(17) with either banzaldehyde or trimethylacetaldehyde in the presence of lithium ion in tetrahydrofuran solution give higher ratios of cis: trans alkenes than do the corresponding reactions with benzylidenediphenylmethylphosphorane(16). However, when the intermediate oxaphosphetanes, 11 and 22 are produced by the action of lithium diphenylphosphide and lithium bis-(o-methoxyphenyl)phosphide (5), respectively, on trans-stilbene oxide (6), with subsequent addition of methyl iodide, the reactions are stereospecific, and only cis-stilbene is produced in each case. The results themselves and also attempted crossover experiments indicate that the oxephosphetanes do not revert to ylide plus aldehyde in the presence of lithium ion. The above observations, when combined with our knowledge of the preferred geometry of the through space 2p–3d overlap effect have enabled us to suggest a detailed mechanism for the lithium ion-catalyzed Wittig reaction of benzylidene-bis-(o-methoxyphenyl)methylphosphorane (17) with an aldehyde.