Evidence for Transient Ylide Intermediates of the Type ^tBu₂P(H)=CHZ That Are Prototropic Tautomers of ^tBu₂PCH₂Z

The mechanism of the formation of ^tBu₂PCH₂P(1,1'-bi-2-naphtholate) (3) from the P-Si exchange reaction between ^tBu₂PCH₂SiMe₃ (1) and ClP(1,1'-bi-2-naphtholate) (2) is explored. Through an iterative process of kinetic measurements and mechanistic hypotheses, the evidence supports the intermediacy of the ylide ^tBu₂PH═CHSiMe₃. The reaction of 1 with 2 is efficiently catalyzed by HCl. In the absence of 2, HCl reacts with 1 to give [^tBu₂PHCH₃]Cl, and it is suggested that this protodesilylation proceeds via the ylide ^tBu₂PH═CH₂ (Y1), the prototropic tautomer of ^tBu₂PCH₃. The products observed upon treatment of 1 with DCl, support the formation of ^tBu₂PD═CH₂ (d-Y1) but, in addition, they support the reversible formation of ^tBu₂PD═CHSiMe₃ (d-Y2). When 3 was treated with the chlorophosphite ClP(3,3'-diphenyl-1,1'-bi-2-naphtholate) (2a), an equilibrium mixture of 2, 2a, 3 and ^tBu₂PCH₂P(3,3'-diphenyl-1,1'-bi-2-naphtholate) (3a) is formed. Kinetic modeling of the proposed mechanism of the formation of 3 from 1 and 2 via the ylide intermediate Y2 correlates well with the experimental data obtained under a variety of conditions and provides strong support for the veracity of the mechanism.