Phosphonium ylides four membered

Olefination Reactions Involving Phosphonium Ylides. The synthetic potential of phosphonium ylides was developed initially by G. Wittig and his associates at the University of Heidelberg. The reaction of a phosphonium ylide with an aldehyde or ketone introduces a carbon-carbon double bond in place of the carbonyl bond. The mechanism originally proposed involves an addition of the nucleophilic ylide carbon to the carbonyl group to form a dipolar intermediate (a betaine), followed by elimination of a phosphine oxide. The elimination is presumed to occur after formation of a four-membered oxaphosphetane intermediate. An alternative mechanism proposes direct formation of the oxaphosphetane by a cycloaddition reaction.236 There have been several computational studies that find the oxaphosphetane structure to be an intermediate.237 Oxaphosphetane intermediates have been observed by NMR studies at low temperature.238 Betaine intermediates have been observed only under special conditions that retard the cyclization and elimination steps.239... 

For the reaction of labile ylides with aldehydes, the formation of cis-oxaphosphetanes is favored, unless lithium ion-containing bases are used. The four-membered ring system of cis-39 dissociates in a [2+2]-cycloreversion and forms triphenyl phosphonium oxide and the desired olefin Z-13. Using labile ylides in Wittig reactions that contain no electron-withdrawing group except the phosphonium moiety, leads predominantly to the formation of c/ -oxaphosphetanes. Hence, Z-olefins are typically formed with > 90 % selectivity. ... 

Mesomeric phosphonium dications. 4 Reaction with nucleophiles to form ylides. 4 Mesomeric phosphonium inner salts. 5 A stable four-membered ring ylid-ketone adduct. 5... 

Stabilized ylides possess EWG on an a-C atom to the phosphonium group. Unstable ylides are avoided in EWG, and the carbanion is only stabilized by the phosphonium unit. E/Z stereoselectivity resides in a transitory formation of a four-membered oxaphosphetane ring whose structure is dependent on the stability of ylides (Schemes 2.16 and 2.17) [18]. 

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