Coordinatively saturated To(M)MgMe (1; To(M) = tris(4,4-dimethyl-2-oxazolinyl)phenylborate) is an active precatalyst for intramolecular hydroamination/cyclization at 50 °C. The empirical rate law of -d[substrate]/dt = k'(obs)[Mg](1)[substrate](1) and Michaelis-Menten-type kinetics are consistent with a mechanism involving reversible catalyst-substrate association prior to cyclization. The resting state of the catalyst, To(M)MgNHCH(2)CR(2)CH(2)CH═CH(2) [R = Ph, Me, -(CH(2))(5)-], is isolable, but isolated magnesium amidoalkene does not undergo unimolecular cyclization at 50 °C. However, addition of trace amounts of substrate allows cyclization to occur. Therefore, we propose a two-substrate, six-center transition state involving concerted C-N bond formation and N-H bond cleavage as the turnover-limiting step of the catalytic cycle.