ELECTROPHILIC AND NUCLEOPHILIC ATTACKS ON ALKYLF NITRILE, AND ISONITRILE COMPLEXES OF Pd(II) AND Pt(II)

In this paper are reported the results of a comprehensive mechanistic study of nucleophilic attack by amines on coordinated isocyanides in square—planar Pd(II) complexes, leading to amino—carbene complexes. The reactivity pattern that has emerged displays several facets: (i) whatever the details, in this stepwise mechanism the most prominent step always involves direct attack of the amine nitrogen on the isocyanide carbon, followed by the required rearrangements; (ii) this fundamental primary step is affected by the donor ability of the entering amine, the electro— philic character of the isocyanide carbon, and the steric crowdiness around the reacting centers, with the solvent playing an important role; (iii) the conversion of the intermediate originating from the primary step into the final carbene species is often aided by the entering amine itself which behaves as a bifunctional catalyst; (iv) the relative magnitude of the elementary step rates can be changed with a fair degree of versatility by properly tailoring the reaction system through the choice of the sub— stituents on the amine and isocyanide aromatic rings and of the ancillary ligands in the metal complex; (v) the variety of steric and electronic parameters that can be changed at will makes the system ideally suited as a mechanistic model for elucidating the reactivity of coordinated CO— isoelectronic ligands as precursors of carbene moieties. First examples of stable transition metal carbene complexes were reported by E.O. Fisher et al. (ref.1): OLi OH OMe +LiR / +H / +CH2N2 / M(CO) (Go) M—C —4 (co) M—C b (co) M—C 6 5 5 \ 5 R R + Me3OBF4 (M = Cr, Mo, W; R = Me, Ph) Carbene complexes were also prepared in which the heteroatom (X) in the carbene moiety, M—C( , can be N, S or Se. The main preparative routes for amino—carbene derivatives, are: (i) nucleophilic attacks on metal coordinated or on preformed carbene groups; (ii) electrophilic attacks on imino—complexes; (iii) reactions of metal complexes with organic carbene precursors, such as electron—rich olefins, gem—dichlorides, imidazolium salts, etc.; (iv) electrophilic attacks on coordinated isocyanides. The following types of carbene ligands are known: = NHR, OR, SR, SeR; /X /R /X /H Y = NHR, OR; M—C M—C M—C M—C R = alkyl, aryl; Y R R x M = transition metal The products have been alternatively described as metal—carbenes, stabilized carbonium ions or metallated amidinium ions: NHR JNHR NHR NHR /i _ ec/ _ _ M—C M—C M—C NNHR, NNHR NNHR,