STOICHIOMETRIC AND CATALYTIC HYDROAMINATION OF ALKYNES AND ALLENE BY ZIRCONIUM BISAMIDES CP2ZR(NHR)2

We have found that the bisamides Cp2Zr(NHR)2 (1) catalyze the hydroamination of alkynes and allene and have investigated the scope and mechanism of this process. Examining both catalytic and stoichiometric additions of 2,6-dimethylaniline to diphenylacetylene in the presence of the corresponding zirconium bisamide 1a at 95-degrees-C indicates that the reaction is first order in the concentration of bisamide and alkyne and inverse first order in amine. These results are consistent with a reversible rate determining alpha-elimination of amine and generation of the transient imido complex Cp2Zr = NR. Alkyne and amine then compete for this reactive intermediate. While reaction with amine regenerates bisamide, cycloaddition with alkyne provides zirconium azametallacyclobutenes such as 3a. The metallacycles are rapidly protonated by ArNH2 at the zirconium-carbon bond forming enamide amides (e.g., 6a) which then undergo alpha-elimination of enamine to regenerate Cp2Zr = NAr and continue the cycle. Support for the mechanism of the zirconium-catalyzed hydroamination of alkynes (illustrated in Scheme III) was obtained by examining the individual steps of the catalytic reaction in a stoichiometric manner. Several proposed intermediates were independently synthesized and shown to undergo the steps postulated in the catalytic reaction. The ability of bisamides Cp2Zr(NHR)2 1a-e to undergo this process is critically dependent on the size of the zirconium-bound amide ZrNHR. Although all attempts to hydroaminate unstrained or moderately strained olefins have been unsuccessful, hydroamination of the C-C double bond of allene in the presence of 1a and 2,6-dimethylaniline led to catalytic production of the 2,6-dimethylphenylimine of acetone.