Cation Charge Density and Precatalyst Selection in Group 2-Catalyzed Aminoalkene Hydroamination

The magnesium methyl and the calcium and strontium silylamide beta-diketiminate derivatives [{ArNC(Me)CHC(Me)NAr}MX-(THF)(n)] (M = Mg, X = CH3, n = 0; M = Ca, X = N(SiMe3)(2), n = 0 or 1; M = Sr, X = N(SiMe3)(2), n = 1, Ar = 2,6-diisopropylphenyl), the silylamides [M{N(SiMe3)(2)}(2)](2) and [M{N(SiMe3)(2)}(2)(THF)(2)] (M = Mg, Ca, Sr, Ba), and the alkyl species [M{CH(SiMe3)(2)}(2)(THF)(2)] (M = Mg, Ca, Sr, Ba) have been studied as precatalysts for the hydroamination/cyclization of aminoalkenes. Hydroamination afforded a series of five- and six-membered pyrrolidine and piperidine derivatives in near quantitative yields with all precatalysts, apart from the barium species, which were apparently limited to a maximum of two turnovers with even the favorable 1-amino-2,2-diphenyl-4-pentene substrate. Significant formation of hexahydroazepines was observed only with the magnesium amide species, while the group 2 dialkyl derivatives decomposed at high reaction temperatures and proved to be more limited in scope. In general, the calcium precatalysts proved to be more reactive than their strontium analogues, which, in turn, proved to be far more reactive than the magnesium species. Among the beta-diketiminate derivatives, the greater coordinathe unsaturation of the THF-free calcium derivative provided increased activity for the cyclization of primary aminoalkene substrates in comparison to its THF-solvated counterpart. In contrast, the unsolvated bis(amides) displayed lower activity with these substrates than their THF-solvated analogues. Use of silylamide precatalysts provides potentially reversible entry into the catalytic manifold. The position of the equilibrium is perturbed by both the nature of the substrate and the identity of the group 2 element, highlighted by the reaction of [{ArNC(Me)CHC(Me)NAr}Sr[N(SiMe3)(2)(THF)] with 2-methoxyethylamine, which results in equilibration between the starting silylamide, an isolable amine adduct, and the product of amine/silylamide transamination. Performing the same reaction with the calcium precatalyst provided the analogue of the latter product as the only observable species. A kinetic study of the cyclization of (1-allylcyclohexyl)methanamine with each of the calcium and strontium silylamide precatalysts provided an apparent first-order dependence in [catalyst], while determination of the activation barriers and Eyring analyses provided quantitative evidence that the group 2 catalysts reported herein provide activities at least commensurate with previously reported lanthanide-based catalyses. In the particular cases of systems based upon calcium and strontium, an enhanced catalytic performance is proposed to arise from a tangible entropic advantage resulting from the reduced charge density of the larger divalent alkaline earth cations and consequentially less constrained rate-determining alkene insertion transition states. The rate of cyclization was also found to decrease with increasing substrate concentration. This latter observation, along with the observation of large kinetic isotope effects (> 4), proposed to be a result of a beneficial and concerted proton transfer step associated with rate-determining alkene insertion, are reasoned to be consistent with Michaelis-Menten-type kinetics.