Scope and Limitations of the Base-Catalyzed Phospha-Peterson PC Bond-Forming Reaction

Phosphaalkenes (MesPCRR‘:  R = R‘ = Ph (1a); R = R‘ = 4-FC6H4 (1b); R = Ph, R‘ = 4-FC6H4 (1c); R =
R‘ = 4-OMeC6H4 (1d); R = Ph, R‘ = 4-OMeC6H4 (1e); R = Ph, R‘ = 2-pyridyl (1f)) are prepared from the
reaction of MesP(SiMe3)2 and OCRR‘ in the presence of a trace of KOH or NaOH. The base-catalyzed phospha-Peterson reaction is quantitated by NMR spectroscopy, and isolated yields of phosphaalkene between 40 and 70%
are obtained after vacuum distillation and/or recrystallization. The asymmetrically substituted phosphaalkenes (1c,
1e, 1f) form as 1:1 mixtures of E and Z isomers; however, X-ray crystallography reveals that the E isomers crystallize
preferentially. Interestingly, E-1e and E-1f readily isomerize in solution in the dark, although the rate of isomerization
is much faster when samples are exposed to light. X-ray crystal structures of 1b, E-1e, and E-1f reveal that the
PC bond lengths (average of 1.70 Å) are in the long end of the range typically found in phosphaalkenes (1.61−1.71 Å). Attempts to prepare isolable P-adamantyl phosphaalkenes following this route were unsuccessful. Although
AdPCPh2 (2a) is detected by 31P NMR spectroscopy, attempts to isolate this species afforded the 1,2-diphosphetane
(AdPCPh2)2 (3a), which was characterized by X-ray crystallography.