Phosphadiarsiranes

Abstract

ment process in the trigonal-bipyramidal structure (3), which brings the rupturing C ~~P bond into the apical position. We postulate that bond rupture occurs during or just after formation of (4); a betaine (5) is formed.The stereochemistry of the olefin being formed is determined by the electronic character of the residue R 2 on the original ylide and of the stationary ligands R'. If R2 is an electron donor and the ligands R3 are phenyl groups a very rapid elimination from (5) of the triphenylphosphane oxide (S), R = C6HS, takes place and Z-olefins (9) are obtained with high stereoselectivity. If R2 is an electron acceptor the lifetime of (5) is increased. Rotation is possible, resulting in the thermodynamically more stable conformation (7), from which the E-olefins (8) are formed with cleavage of (6), R = C6H5. Substituents R' with donor character (e. g. alkyl or cycloalkyl groups) retard the phosphane oxide cleavage, and the probability of the conformational change (5)+ (7) becomes greater. As a result, a sharp increase in E-olefin formation is observed on going from R3 = C6H5 to R3 = alkyl or cycloalkyl['l.Reaction of the cyclic phosphaallene ylide (lo)['] with car-bony1 compounds affords stable crystalline primary products, to which, on the basis of the mechanistic postulates discussed above, we assign a trigonal-bipyramidal structure (3) [']. The X-ray structure analysis[41 of the adduct obtained from fluorenone and (10) now confirms this assignment. 0 Verlag Chemie, CmbH, 6940 Weinheim, 1979 0570-0833/79/1111-0878 $ 02.50/0 Angew. Chem. Int. Ed. Engl I 8 (IY7Y) No. I1