Organothorium-catalyzed hydroalkoxylation/cyclization of alkynyl alcohols. Scope, mechanism, and ancillary ligand effects

Organothorium complexes bearing amide or alkyl proligands are active toward the highly selective hydroalkoxylation/cyclization of alkynyl alcohols. Substrates include primary and secondary alcohols, as well as terminal and internal alkynes. Catalysts with strongly binding ligation such as pentamethylcyclopentadienyl (Cp* = C₅Me₅) or "constrained geometry catalysts" (CGC = Me₂Si(η ⁵-Me₄C₅)(^tBuN)) remain soluble throughout the reaction, with the more sterically open (CGC)Th(NMe ₂)₂ (1) exhibiting higher activity than Cp*₂Th(CH₂TMS)₂ (2). The use of precatalyst [(Me₃Si)₂N]₂Th[κ²- (N,C)-CH₂Si(CH₃)₂N(SiMe₃)] (3) leads to precipitation upon the addition of alcohol substrates, although catalytic activity is retained. The substrate scope for 1 includes primary and secondary alcohols as well as terminal and internal alkynes. In situ¹H NMR spectroscopic monitoring indicates that the rate law is zero-order in [substrate] and first-order in [catalyst]. The rates of primary alcohols and terminal alkynes are significantly more rapid than their more sterically hindered counterparts, suggesting that steric demands dominate the hydroalkoxylation/cyclization transition state. Turnover frequencies as high as 49 h^-1 at 60 C are observed, producing exclusively the exo-methylene products. For internal alkyne substrates, alkenes with E-orientation are formed with complete selectivity. Activation parameters ΔH^‡ = 27.9(0.4) kcal/mol, ΔS^‡ = -3.0(1.1) eu, and E _a = 28.6(0.4) kcal/mol are largely in accord with observations for other f-element-mediated insertive hydroelementation processes, and an ROH/ROD kinetic isotope effect of 0.97(0.02) is observed. The reactivity patterns, kinetics, and activation parameters are consistent with a pathway proceeding via turnover-limiting alkyne insertion into the Th-O bond, with subsequent, rapid Th-C protonolysis, regenerating the initial Th-OR species.