Modification of aminoallenylidene complexes of chromium via exchange of the C3-bound amino group

The aminoallenylidene(pentacarbonyl)chromium complexes [(CO)5Crdouble bond; length as m-dashCdouble bond; length as m-dashCdouble bond; length as m-dashC(NR1R2)Ph] (1a–c) react with dimethylamine by addition of the amine to the C1double bond; length as m-dashC2 bond of the allenylidene ligand to give alkenyl(amino)carbene complexes [(CO)5Crdouble bond; length as m-dashC(NMe2)CHdouble bond; length as m-dashC(NR1R2)Ph] (2a–c) (R1 = Me: R2 = Me (a), Ph (b); R1 = Et: R2 = Ph (c)). In contrast, addition of a large excess (usually 20 equivalents) of ammonia or primary amines, H2NR, to solutions of [(CO)5Crdouble bond; length as m-dashCdouble bond; length as m-dashCdouble bond; length as m-dashC(NMe2)Ph] (1a) affords the aminoallenylidene complexes [(CO)5Crdouble bond; length as m-dashCdouble bond; length as m-dashCdouble bond; length as m-dashC(NHR)Ph] (1d–w) in which the dimethylamino group is replaced by NH2 or NHR, respectively. In addition to simple amines such as methylamine, butylamine, and aniline, amines carrying a functional group (allylamine, propargylamine) and amino acid esters as well as amino terpenes and amino sugars can be used to displace the NMe2 substituent. Usually the Z isomer (with respect to the partial C3–N double bond) is formed exclusively. Products derived from addition of H2NR to the C1double bond; length as m-dashC2 bond of 1a are not observed. The amino group in 1d–w is rapidly deprotonated by excess of amine to form iminium alkynyl chromates [1d–w]−, thus protecting 1d–w from addition of free amine to either C3 or across the C1double bond; length as m-dashC2 bond. The iminium alkynyl chromates are readily reprotonated by acids or by chromatography on wet SiO2 to reform 1d–w.