Oxaphosphetanes theoretical studies

The mechanism proposed is an addition of the nucleophilic ylide carbon to the carbonyl group to yield a dipolar intermediate (a betaine), followed by elimination of a phosphine oxide. The elimination is presumed to occur after formation of a four-membered oxapho-sphetane intermediate. An alternative mechanism might involve direct formation of the oxaphosphetane.137 There have been several theoretical studies of these intermediates.138 Oxaphosphetane intermediates have been observed by NMR studies at low temperature.139 Betaine intermediates have been observed only under special conditions that retard the... 

The number of publications reporting theoretical studies and those reporting mechanistic studies have increased following the reduction in these numbers last year. One of these reports includes the isolation and separate decomposition of certain oxaphosphetanes and this has allowed the first kinetic study of the second step of the Wittig reaction, albeit for a rather special system. Complex phosphonate carbanions and ylides continue to be widely used in synthesis and the number of reports of the use of the aza-Wittig and related reactions in heterocyclic synthesis remain at last year s high level. 

Other theoretical insights into the second step of the Wittig reaction have also been reported. These studies included isomerization and thermal decomposition of isolable spiro-l,2-oxaphosphetanes (OPAs). Key features of these investigations were (1) the synthesis of enantio-pure derivatives, (2) the identification of three mechanisms of OPA... 

---