Evidence for Transient Ylide Intermediates of the Type ^t Bu ₂ P(H)=CHZ That Are Prototropic Tautomers of ^t Bu ₂ PCH ₂ Z

High Resolution Image Download MS PowerPoint Slide The mechanism of the formation of t Bu 2 PCH 2 P(1,1′-bi-2-naphtholate) ( 3 ) from the P–Si exchange reaction between t Bu 2 PCH 2 SiMe 3 ( 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 t Bu 2 PH═CHSiMe 3 . The reaction of 1 with 2 is efficiently catalyzed by HCl. In the absence of 2, HCl reacts with 1 to give [ t Bu 2 PHCH 3 ]Cl, and it is suggested that this protodesilylation proceeds via the ylide t Bu 2 PH═CH 2 ( Y1 ), the prototropic tautomer of t Bu 2 PCH 3 . The products observed upon treatment of 1 with DCl, support the formation of t Bu 2 PD═CH 2 ( d- Y1 ) but, in addition, they support the reversible formation of t Bu 2 PD═CHSiMe 3 ( 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 t Bu 2 PCH 2 P(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.