Low-temperature characterization of the intermediates in the Wittig reaction

Article abstract of DOI:10.1021/ja00400a055

Nonstabilized salt-free ylides react with aldehydes and nonhindered or strained ketones at -78°C to give oxaphosphetanes. The Wittig intermediates can be observed by ³¹P and ¹H NMR techniques. In the presence of LiBr, betaine-lithium bromide adducts often precipitate from solution. The oxaphosphetane from PhCHO + CH₂=PPh₃ reacts rapidly with LiBr to give a betaine·LiBr adduct, and the corresponding salt Ph₃P⁺CH₂CHOHPh Br^- reacts with KH at -40°C to form the oxaphosphetane. No salt-free betaine has been detected. Lithium bromide is shown to decrease cis selectivity (CH₃CH=PPh₃ + PhCH₂CH₂CHO) in the condensation step and not by oxaphosphetane equilibration. Oxaphosphetane reversal to ylide + aldehyde is confirmed for aryl aldehydes but not for aliphatic aldehydes or ketones according to three types of crossover experiments. Rationales for cis selectivity of aldehyde-ylide reactions are discussed. A "crisscrossed" cycloaddition rationale is proposed, aldehyde and ylide planes tilted toward an orthogonal arrangement to minimize steric interactions, to explain cis-alkene formation. Other transition-state geometries having carbonyl and ylide planes roughly parallel are considered more likely for trans-olefin formation or for Wittig reactions of ketones.