Organolanthanide-Catalyzed Hydroamination/Cyclization Reactions of Aminoalkynes. Computational Investigation of Mechanism, Lanthanide Identity, and Substituent Effects for a Very Exothermic C−N Bond-Forming Process

This contribution focuses on organolanthanide-mediated catalytic hydroamination processes and analyzes the exothermic hydroamination/cyclization of a prototypical aminoalkyne, H2N(CH2)3C⋮CR, mediated by Cp2Sm− complexes, using density functional theory. The reaction is found to proceed in two discrete steps, namely, cyclization with concerted Ln−C and C−N bond formation and subsequent Ln−C protonolysis. Dissociation of the cyclized amine then follows to regenerate the active catalyst. Analysis is carried out for (i) insertion of the triple bond moiety into the Sm−N bond via a four-center transition state, (ii) subsequent Sm−C protonolysis by a second substrate molecule, (iii) the effects of other Ln+3 ions and aminoalkyne R substituents on the reaction energetics, and (iv) comparison to the analogous, essentially thermoneutral process for aminoalkenes. DFT energetic profiles are computed for the turnover-limiting aminoalkyne C⋮C triple bond insertion into the Ln−NH− linkage, and the geometries and stabilities of reactants, intermediates, and products are analyzed. The picture that emerges involves concerted, rate-limiting, exothermic insertion of the alkyne fragment into the Ln−N(amido) bond via a highly organized, seven-membered chairlike cyclic transition state (ΔHcalcd = 4.6 kcal/mol, ΔScalcd = −11.9 eu). The resulting cyclized complex then undergoes exergonic protonolysis to yield an amine−amido complex, the likely resting state of the catalyst. Large rate accelerations effected by smaller lanthanide ions and certain alkyne substituents can be understood in terms of approach distances and charge buildup in the cyclization transition state.