Intermolecular insertion of an N,N-heterocyclic carbene into a nonacidic C-H bond: Kinetics, mechanism and catalysis by (K-HMDS)2 (HMDS = Hexamethyldisilazide).
The reaction of 2-[13C]-1-ethyl-3-isopropyl-3,4,5,6-tetrahydropyrimidin-1-ium hexafluorophosphate ([13C1]-1-PF6) with a slight excess (1.03 equiv) of dimeric potassium hexamethyldisilazide ("(K-HMDS)2") in toluene generates 2-[13C]-3-ethyl-1-isopropyl-3,4,5,6-tetrahydropyrimid-2-ylidene ([13C1]-2). The hindered meta-stable N,N-heterocyclic carbene [13C1]-2 thus generated undergoes a slow but quantitative reaction with toluene (the solvent) to generate the aminal 2-[13C]-2-benzyl-3-ethyl-1-isopropylhexahydropyrimidine ([13C1]-14) through formal C-H insertion of C2 (the "carbene carbon") at the toluene methyl group. Despite a significant pKa mismatch (Delta pKa 1+ and toluene estimated to be ca. 16 in DMSO) the reaction shows all the characteristics of a deprotonation mechanism, the reaction rate being strongly dependent on the toluene para substituent (rho = 4.8(+/-0.3)), and displaying substantial and rate-limiting primary (k(H)/k(D) = 4.2(+/-0.6)) and secondary (k(H)/k(D) = 1.18(+/-0.08)) kinetic isotope effects on the deuteration of the toluene methyl group. The reaction is catalysed by K-HMDS, but proceeds without cross over between toluene methyl protons and does not involve an HMDS anion acting as base to generate a benzyl anion. Detailed analysis of the reaction kinetics/kinetic isotope effects demonstrates that a pseudo-first-order decay in 2 arises from a first-order dependence on 2, a first-order dependence on toluene (in large excess) and, in the catalytic manifold, a complex noninteger dependence on the K-HMDS dimer. The rate is not satisfactorily predicted by equations based on the Brønsted salt-effect catalysis law. However, the rate can be satisfactorily predicted by a mole-fraction-weighted net rate constant: -d[2]/dt = ({x2 k(uncat)} + {(1-x2) k(cat)})[2]1[toluene]1, in which x2 is determined by a standard bimolecular complexation equilibrium term. The association constant (Ka) for rapid equilibrium-complexation of 2 with (K-HMDS)2 to form [2(K-HMDS)2] is extracted by nonlinear regression of the 13C NMR shift of C2 in [13C1]-2 versus [(K-HMDS)2] yielding: Ka = 62(+/-7) M(-1); delta(C(2)) in 2=237.0 ppm; delta(C(2)) in [2(K-HMDS)2] = 226.8 ppm. It is thus concluded that there is discrete, albeit inefficient, molecular catalysis through the 1:1 carbene/(K-HMDS)2 complex [2(K-HMDS)2], which is found to react with toluene more rapidly than free 2 by a factor of 3.4 (=k(cat)/k(uncat)). The greater reactivity of the complex [2(K-HMDS)2] over the free carbene (2) may arise from local Brønsted salt-effect catalysis by the (K-HMDS)2 liberated in the solvent cage upon reaction with toluene.
Lloyd-Jones
Guy C
GC
The Bristol Centre for Organometallic Catalysis, School of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 1TS, UK. guy.lloyd-jones@bris.ac.uk
Alder
Roger W
RW
Owen-Smith
Gareth J J
GJ