Oxaphosphetanes cis-disubstituted

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. 

Thermal Equilibration of Salt-free cis-Disubstituted Oxaphosphetanes. Only three examples of this process are known. The reaction appears to occur spontaneously when certain oxaphosphetanes are warmed to temperatures near the decomposition point. Spontaneous equilibration is restricted to oxaphosphetanes derived from P-trialkyl ylides and aromatic or tertiary aliphatic aldehydes. A catalyzed process via betaine derivatives is not ruled out, but there is no direct evidence to implicate catalysis. 

Nonplanar TS geometries in the Ph3P=CHR reactions are favored by a-branching in the aldehyde substituent R because the 1,3-interactions become increasingly important. However, the steric constraints decrease carbonyl reactivity and work against an early TS. The result is a consistent but relatively small trend toward the cis-selective pathway for the reaction of tertiary aldehydes with a variety of ylides. Heteroatom branching at the a-carbon has a similar effect on selectivity. In this case, the steric constraints may be smaller, but there are new constraints due to the need to minimize lone-pair or dipole-dipole interactions. There are fewer conformational options for the carbonyl component in the TS, and the result is an increase in the importance of 1,3-interactions. Since a-heteroatom substituents in the carbonyl reactant will also increase carbonyl reactivity, there will be a trend toward an earlier, more puckered TS. The combination of conformational constraints and increased reactivity results in higher selectivity for the cis-disubstituted oxaphosphetane. 

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