Betaine lithium halide complexes

Reversal correlates with the presence of lithium ion and also with the involvement of betaine species. These two risk factors are interrelated because lithium halides rapidly cleave oxaphosphetane 31 or 32 (Scheme 8) at — 70°C resulting in the reversible formation of the betaine lithium halide complexes 40 or 41, respectively (18b). Donor solvents shift the equilibrium toward the oxaphosphetane by coordinating the lithium halides and thereby promote stereospecific decomposition to the alkenes. If the solvent is not an effective lithium coordinating agent, then 40 and 41 decompose slowly, and the risk of... 

The reaction of nonstabilized ylides with aldehydes can be induced to yield trans 3 kenes with high stereoselectivity by a procedure known as the Schlosser modification of the Wittig reaction." In this procedure, the ylide is generated as a lithium halide complex and allowed to react with an aldehyde at low temperature, presumably forming a mixture of diastereomeric betaine-lithium halide complexes. At the temperatures under which the addition is carried out, fragmentation to an alkene and triphenylphosphine oxide does not occur. This complex is then treated... 

---