Salt-free betaine intermediate

Currently accepted mechanism of the Wittig reaction of aldehydes with non-stabilized ylides involves the formation of oxaphosphetanes through a [2-I-2]-cycloaddition-like reaction . The oxaphosphetanes are thermally unstable and collapse to alkene and phosphine oxide below room temperature. Under salt-free conditions there is no formation of betaine intermediates. The salt-free ylides can be prepared by the reaction of phosphines with carbenes generated in situ. Vedejs etal proposed a puckered 4-centre cyclic transition state I for sy -oxaphosphetane and planar structure J for anff-oxaphosphetane. In general, the flnfi-oxaphosphetane J is more stable than the syn-oxaphosphetane I, and under equilibrium conditions (when stabilized ylides are used) the E-alkene product is favoured (Scheme 4.24). However, kinetic control conditions, which appear to dominate when non-stabilized ylides are used, would lead to Z-alkene. 

The (Z)-stereoselectivity of salt-free Wittig olefmations leading to the thermodynamically less stable cA-alkenes has long been the subject of intense investigations. At one time, the olefmation was thought to proceed via an ionic stepwise process involving a zwitterionic betaine and a 1,2-oxaphosphetane intermediate. However, NMR spectroscopy studies revealed the oxaphosphetane as the only observed intermediate. ... 

Starting material, is significantly affected by the inclusion of, or protocols to exclude, soluble salts, in particular lithium. Coordination of lithium to the betaine (11 and 12)/oxaphosphetane (13 and 14) intermediates retards the rate of loss of phosphine oxide 8, thus allowing more time for equilibration. For example, consider the reaction of benzaldehyde 15 with the ylide 16 to generate alkene The addition of lithium salts had a dramatic effect on the overall yield of the reaction. Not only was the yield negatively impacted, the product distribution was affected. Under salt free conditions, the product distribution favored the cis over trans. However, the addition of various forms of lithium salts resulted in an erosion of the selectivity to 50 to 50. 

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