Stereoselectivity and proton binding in some transition‐metal complexes of N‐(o‐hydroxybenzyl)alanine

Stability constants and stereoselective effects are presented for some transition‐metal complexes of the new legand, N‐(o‐hydroxybenzyl)alanine, H2(oHBAl). The preparation of DLH2(oHBAl) and optically pure LH2(oHBAl), by hydrogenation of the salicylaldimine of alanine, is described. The M‐oHBAl systems studied are complicated by the presence of protonated species [M = Co(II), Ni(II), Cu(II) and Zn(II)] or a hydroxylated species [M = Zn(II)]. From comparison of competition constants it is concluded that the proton in the protonated complexes is bound to the phenoxy atom (CoHA2, NiHA2, NiHA and CuHA) or, possibly, to the secondary nitrogen atom (CuHA2, ZnHA2) of (oHBAl)2−, A. Appreciable stereoselective effects, in favour of the “racemic complex” {M(LA)2 + M(DA)2}, are found for the cobalt and nickel bis‐complexes. In the copper bis‐complexes no stereoselectivity was found.