Oxaphosphetanes trans-disubstituted

One has a detailed conception of the mechanism of the Wittig reaction (Figure 9.7). It starts with a one-step [2+2]-cycloaddition of the ylide to the aldehyde. This leads to a heterocycle called an oxaphosphetane. The oxaphosphetane decomposes in the second step—which is a one-step [2+2]-cycloreversion—to give triphenylphosphine oxide and an olefin. This decomposition takes place stereoselectively (cf. Figure 4.39) A cfs-disubstituted oxaphosphetane reacts exclusively to give a cw-olefin, whereas a trans-disubstituted oxaphosphetane gives only a trans-olefin. In other words, stereospecificity occurs in a pair of decomposition reactions of this type. 

Further insight into structural details of possible TS depends on the correlation of stereochemical trends tabulated earlier and summarized in Table 22. Stabilized ylide reactions with simple aldehydes favor the trans-selective pathway. As discussed in connection with Table , the trans-disubstituted oxaphosphetanes are thermodynamically more stable than the cisdiastereomers, and the same preference should be felt in the late TS of... 

Only a few results are available in the case of ylides L3P—CHX that are structurally biased toward planar four-center geometries (similar to 126 or 138 DBP or RPhjP phosphorus environments). Allylic or benzylic ylides of both types show the expected trend for trans-disubstituted oxaphosphetanes and the corresponding ( )-alkenes, as do allylic ylides of the Bu3P=CHX family. To date, no ylides with a-heteroatom substituents have been studied in any of the trans-selective phosphorus environments. 

Several tests are available to determine whether equilibration of stereochemistry occurs in the course of oxaphosphetane decomposition (methods A-E, Scheme 7), but each method has some limitations. In method A, oxaphosphetane diastereomers are prepared independently by deprotonation of the )S-hydroxyphosphonium salts 27 or 28 with base (NaHMDS, NaNHj, KO-tert-Bu, etc.) (20). If each isomer affords a distinct oxaphosphetane 31 or 32 according to NMR analysis (usually, or H), then the solutions are warmed up to the decomposition temperature. Kinetic control is established if stereospecific conversion to the alkenes can be demonstrated from each diastereomer. A less rigorous version of this test is to perform the experiment only with isomer 27, the precursor of the cis-disubstituted oxaphosphetane 31 (21c). All known examples of significant (> 5%) stereochemical equilibration involve 31 and not the trans-disubstituted isomer 32 (20, 21c). A negative equilibration result with the cis diastereomer 31 can be assumed to apply to 32 as well. 

Schlosser Modification. Almost pure tran -olefins are obtained from nonstabilized ylides by the Schlosser modification of the Wittig reaction (Wittig-Schlosser reaction). For example, treatment of the (cij )-oxaphosphetane intermediate A with n-BuLi or PhLi at -78 °C results in lithiation of the acidic proton adjacent to phosphoras to produce the P"Oxido phosphonium ylide B. Protonation of B with f-BuOH leads to the trans-1,2-disubstituted alkene C. 

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