Lanthanide-Catalyst-Mediated Tandem Double Intramolecular Hydroalkoxylation/Cyclization of Dialkynyl Dialcohols: Scope and Mechanism

Lanthanide-organic complexes of the general type [Ln{N-(SiMe ₃)₂}₃] (Ln = La, Sm, Y, Lu) serve as effective precatalysts for the rapid, exo-selective, and highly regioselective tandem double intramolecular hydroalkoxylation/cyclization of primary and secondary dialkynyl dialcohols to yield the corresponding bi-exocyclic enol ethers. Conversions are highly selective with products distinctly different from those generally produced by conventional transition metal or other catalysts, and the turnover frequencies with some substrates are too large to determine accurately. The rates of terminal alkynl alcohol hydroalkoxylation/cyclization are significantly more rapid than those of internal alkynyl alcohols, arguing that steric demands dominate the cyclization transition state. The hydroalkoxylation/cyclizations of internal dialkynyl dialcohols afford excellent E selectivity. The rate law for dialkynyl dialcohol hydroalkoxylation/ cyclization is first-order in [catalyst] and zero-order in [alkynyl alcohol], as is observed for the organolanthanide-catalyzed hydroamination/cyclization of aminoalkenes, aminoalkynes, and aminoallenes, and the intramolecular single-step hydroalkoxylation/cyclization of alkynyl alcohols. An ROH/ROD kinetic isotope effect of 0.82(0.02) is observed for the tandem double hydroalkoxylation/ cyclization. These mechanistic data implicate turnover-limiting insertion of C-C unsaturation into the Ln-O bond, involving a highly organized transition state, with subsequent, rapid Ln-C protonolysis.