Proto-demetallation

The nudeophile is activated by the formation of a titanium(IV)-imido complex 19. The next step is a [2 + 2] cydoaddition with one of the jt-bonds of the allene, depending on the regioselectivity leading to either 20 or 22. Compound 20 then delivers 21 by twofold stepwise proto-demetallation and the latter enamine tau-tomerizes to the imine 24 (Scheme 15.3). Compound 22, on the other hand, should provide allylamines 23, but as we shall see, there are no examples of that mode of reaction known so far. 

Compared with the direct use of strong acids, the benefit of the metallic electrophile, which is exchanged for a proton later by a proto-demetallation step, is higher selectivities of the reactions [13]. Efforts to circumvent the disadvantages of mercury then led to the use of silver(I). After Bertrand et al. initially observed that silver is active [14], this was developed to a useful synthetic protocol shown for the transformation of 28 to 29 by Olsson and Claesson (Scheme 15.5) [15]. 

A catalytic addition of acidic alcohols or phenols to hexafluoropropene is induced by the complex Pd(PPh3)4 [110]. Catalytic activity is increased in the presence of cocatalytic l,4-bis-(diphenylphosphino)butane (dppb) (Scheme 19). The authors propose a mechanism involving external protonation of a Pd(0)-coordinated olefin and reductive elimination to the ether product, but both steps appear improbable. There is literature precedence for insertion of tetrafluoroethylene into the Pt-O bond of (dppe)PtMe(OMe) to give (dppe)PtMe(CF2CF20Me), but proto-demetallation of the resulting complex has not been reported [111, 112]. 

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