Mechanism and stereoselectivity of the aza-Wittig reaction between phosphazenes and aldehydes.
The mechanism of the aza-Wittig reaction between phosphazenes and aldehydes has been studied computationally, using DFT methods (B3LYP/6-31G level), and experimentally. It has been found that the reaction consists of a tandem [2+2] cycloaddition-cycloreversion sequence in which pi and sigma orbitals as well as lone pairs are involved. Both [2+2] processes take place via thermally allowed supra-supra mechanisms. P-trimethyl-lambda(5)-phosphazenes are predicted to be more reactive than their P-triphenyl analogues. The stereochemical outcome of the whole reaction depends only on the second step, because conformational changes in the intermediate 1,3,2-lambda(5)-oxazaphosphazetidines have a much lower activation energy than the second [2+2] cycloreversion reaction. Preferential or exclusive formation of the corresponding (E)-imines is predicted.
Cossío
Fernando P
FP
Departamento de Química Organica I, Facultad de Química, Universidad del País Vasco (UPV/EHU), P. K. 1072, 20080 San Sebastian/Donostia, Spain. fp.cossio@ehu.es
Alonso
Concepción
C
Lecea
Begoña
B
Ayerbe
Mirari
M
Rubiales
Gloria
G
Palacios
Francisco
F