VARIABLE REGIOCHEMISTRY IN THE STOICHIOMETRIC AND CATALYTIC HYDROAMINATION OF ALKYNES BY IMIDOZIRCONIUM COMPLEXES CAUSED BY AN UNUSUAL DEPENDENCE OF THE RATE LAW ON ALKYNE STRUCTURE AND TEMPERATURE

We have investigated the regiochemistry of the stoichiometric and catalytic hydroamination of disubstituted alkynes by imidozirconium complexes. The addition of alkynes to Cp2Zr=NR occurred regioselectively to give metallacycles 2, with the larger alkyne substituent RL located a to the metal center. Hydrolysis of the metallacycles then gave enamines and their tautomeric imines which were the net result of anti-Markovnikov addition to the alkyne. The size of the R group on the imido ligand and the size of the alkyne were influential in determining the degree of regioselectivity. By utilizing Cp2(THF)Zr=NR to generate Cp2Zr=NR at room temperature and Cp2Zr(R')(NHR) to generate Cp2Zr=NR at high temperature, it was determined that the thermodynamic and kinetic regioselectivities were nearly identical for dialkylacetylenes. In contrast, for 1-phenylpropyne, the thermodynamic regioselectivity was found to be greater than the kinetic regioselectivity. The regioselectivity was found to be invariant from -6 to 45-degrees-C in the addition of both 4-methyl-2-pentyne and 2-hexyne to Cp2(THF)Zr=NAr. However, a significant erosion of regioselectivity was observed when 2-hexyne and 4-methyl-2-pentyne were catalytically hydroaminated by Cp2Zr(NHAr) 2 at 120-degrees-C. A kinetic study of the catalytic reaction suggested that the reason for this erosion was that the protonation step in the catalytic cycle (k3[amine]) was slower than the cycloreversion of the stereoisomeric metallacycles to alkyne and Cp2Zr=NAr(k-2)(the step that leads to regioequilibration). Because the protonation is selective for the metallacycle in which the smaller substituent is located at the position adjacent to the metal center, it counters the regioselectivity of the cycloaddition. This was an unexpected result because (1) an earlier study showed that k3[amine] was larger than k-2 for the addition of diphenylacetylene to Cp2Zr=NR at both 25-degrees-C and the catalytic reaction temperature (95-degrees-C in this case) and (2) in the present work, it was demonstrated that k3[amine] was also larger than k-2 for dialkylacetylenes at 25-degrees-C. It was concluded that the relative magnitudes of the rate constants k3 (protonation) and k-2 (reversion) must vary with temperature more dramatically in the dialkylacetylene + Cp2Zr=NR reaction than in the diarylacetylene + Cp2Zr=NR reaction. This was confirmed by direct competition experiments carried out at 25, 60, and 100-degrees-C. We believe that the cycloreversion step k-2 is characterized by both a larger DELTAH(double dagger) and a larger DELTAS(double dagger) than protonation because the former is a unimolecular and the latter a bimolecular reaction.