Ylide formation polarization effects

The fZj-selectivity has been attributed to be due to a propeller-like conformation of the stationary phenyl groups which, in the transition state, favours cw-oxaphosphetane formation [51]. Introducing 6>rr/io-substituents in the phenyl groups increases the steric pressure in the transition state and thus the fZj-selectivity. However, the sterically less restricting tris-furyl substituted phosphorus ylides also lead to high fZj-selectivities (see Section C.2.a) obviously, in addition to purely steric effects, polarity effects are also important. 

The role of steric effects in explaining the prevalent formation of Z olefin from non-stabilized and keto-stabilized ylides has been highlighted. New insights into the second step of the Wittig reaction have been reported oxaphosphetane decomposition was found to take place in a single step via a polar transition state. 

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