1.2- Oxaphosphetane Wittig transition state

Calculations on two Wittig reactants, alkylidenetriphenylphosphorane (a non-stabilized ylid) and its benzylidene analogue (a semi-stabilized one), have been used to identify the origin of the product selectivities for the two classes. A planar transition state gives a trani-oxaphosphetane intermediate, while a puckered one leads to cis-. These two transition states were favoured by the semi- and un-stabilized reactants, respectively. 

How can the Z selectivity in Wittig reactions of unstabilized ylids be explained We have a more complex situation in this reaction than we had for the other eliminations we considered, because we have two separate processes to consider formation of the oxaphosphetane and decomposition of the oxaphosphetane to the alkene. The elimination step is the easier one to explain—it is stereospecific, with the oxygen and phosphorus departing in a syn-periplanar transition state (as in the base-catalysed Peterson reaction). Addition of the ylid to the aldehyde can, in principle, produce two diastere-omers of the intermediate oxaphosphetane. Provided that this step is irreversible, then the stereospecificity of the elimination step means that the ratio of the final alkene geometrical isomers will reflect the stereoselectivity of this addition step. This is almost certainly the case when R is not conjugating or anion-stabilizing the syn diastereoisomer of the oxaphosphetane is formed preferentially, and the predominantly Z-alkene that results reflects this. The Z selective Wittig reaction therefore consists of a kinetically controlled stereoselective first step followed by a stereospecific elimination from this 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 intermediacy of the betaines used in the mechanistic discussions of the Wittig reaction has been questioned and Vedejs has proposed an alternative explanation. The Wittig reaction is subject to solvent effects that indicate a nonpolar transition state for stabilized ylids.There appears to be no direct evidence for the presence of betaines, and none have been isolated. Alternatively, Vedejs and Snoble detected oxaphosphetanes as the only observable intermediates in several Wittig reactions of nonstabilized ylids, using 3Ip NMR. In more recent work, Vedejs devised a test for the betaine mechanism based on changes in phosphorus stereochemistry in the proposed intermediates (betaine vs. oxaphosphetane). The results of this test suggested that "the conventional betaine mechanism l can play at most a minor role in the Wittig reaction".Vedejs points out that the "stereochemical test does not necessarily disprove mechanisms via intermediates with lifetimes that are short compared to the time scale of bond rotation. "494... 

The detection of oxaphosphetanes as relatively stable Wittig intermediates led to revision of the reaction mechanism theory. An asynchronous cycloaddition between ylide and carbonyl component was proposed for the oxaphosphetane formation a transition state resembling the starting material in the case of reactive ylides and a transition state resembling oxaphosphetane in the case of moderate and stable ylides can explain the different (E/Z)-selectivities of the different ylide types [44]. 

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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