Iron(II) Metal-Ligand Cooperative Catalysts for Endo-Selective Intramolecular Hydrofunctionalization

Benjamin Bridge, The University of Western Ontario

Abstract

Two new cationic Fe(II) P^R₂N^R'₂ complexes, [Fe(Cp*)(P^R₂N^Ph₂)(MeCN)]PF₆ (R = cyclohexyl, 3a) and [Fe(Cp*)(P^R₂N^Ph₂)(MeCN)]PF₆ (R = phenyl, 3b), were synthesized and characterized. The catalytic activity for intramolecular hydrofunctionalization was tested using the benchmark substrate 2-ethynylaniline (EA) and both catalysts effectively produced indole (Ind). The selectivity of catalysis was also tested using 2-ethynylbenzyl alcohol (EBA) and both catalysts formed only the endo product isochromene (IC). These complexes represent the first well-defined Fe(II) catalysts that are endo selective for the hydrofunctionalization of alkynes. An Fe(II) vinylidene complex, [Fe(Cp*)(P^R₂N^Ph₂)(=C=CHPh)]PF₆ (R = cyclohexyl, 7), was also characterized that suggests an analogous intermediate is responsible for catalytic selectivity. Catalyst decomposition was also probed and a likely off-cycle deactivation product was identified. On-cycle intermediates were potentially identified and proposed as resting-state species in the catalytic cycle. Preliminary studies were attempted to assess activity and selectivity on a small scope of substrates, but more optimization will be required.