Wittig versus Corey–Chaykovsky reaction?

Question

The Wittig and Corey–Chaykovsky reaction both involve an ylid, phosphonium ylid in the former versus a sulfonium ylid in the latter, attacking a carbonyl compound. I am looking for an answer that explains the differences between the outcomes of these two reactions, and their respective intermediates.

Answer 1

Short Answer: Witting will produce an epoxide while Corey-Chaykovsky will produce an alkene.

Long Answer: If you are asking why we observe oxasphosphetanes but not oxathietanes.... that question can be answered. It involves looking at the MO diagram and how Sulfur and Phosphate "behave" when bonded.