Detection and Reaction of Oxaphosphetanes Derived from Benzaldehyde and 1-Adamantylmethylidene Ylide

Publisher Website
Google Scholar
Abstract
The Wittig reaction of (1-adamantylmethylidene)triphenylphosphorane (Ph3PCH(1-Ad)) with benzaldehyde was investigated, and the results were compared with those of other ylides. The substituent effect in the reaction of the ylide with benzaldehydes was determined by competition experiments, which gave a Hammett ρ value of 3.2. The ρ value is much larger than those reported for analogous reactions of Ph3PCH(CH2)2CH3 (ρ = 0.20) and Ph3PCH(CH3)2 (ρ = 0.59), indicating that the reaction mechanism differs for Ph3PCH(1-Ad) and the other ylides. The cis/trans ratio of the product alkene is 74/26 for the reaction with the parent benzaldehyde and highly depends on the position of the substituent; ortho substituted benzaldehydes gave the trans alkenes up to 90%. Monitoring the reaction by means of 31P NMR revealed that both cis and trans oxaphosphetane intermediates were formed and that the formation and decomposition of the cis oxaphosphetane are 7−12 times faster than those of the trans oxaphosphetane. From the comparison of the reaction of Ph3PCH(1-Ad) + benzaldehyde with those of Ph3PCH(CH2)2CH3 + benzaldehyde and benzophenone, and Ph3PCH(CH3)2 + benzophenone, it was concluded that all the reactions with these nonstabilized ylides proceed via an electron-transfer mechanism and that the rate-determining step changes from the electron transfer step to that of radical combination when the substrate or ylide becomes more sterically demanding.

This publication has 14 references indexed in Scilit: