Aldehyde-Assisted Hydrogen Transfer during the Formation of Hydride-Iridafurans from Alkynes and Aldehydes

Abstract

The bis(ethylene) IrI complex [Tpmath imageIr(C2H4)2] (1; Tpmath image=hydrotris(3,5-dimethylpyrazolyl)borate) reacts with two equivalents of aromatic or aliphatic aldehydes in the presence of one equivalent of dimethyl acetylenedicarboxylate (DMAD) with ultimate formation of hydride iridafurans of the formula [Tpmath imageIr(H){C(R1)[DOUBLE BOND]C(R2)C(R3)O}] (R1=R2=CO2Me; R3=alkyl, aryl; 3). Several intermediates have been observed in the course of the reaction. It is proposed that the key step of metallacycle formation is a C[BOND]C coupling process in the undetected IrI species [Tpmath imageIr{η1-O-R3C([DOUBLE BOND]O)H}(DMAD)] (A) to give the trigonal-bipyramidal 16 e− IrIII intermediates [Tpmath imageIr{C(CO2Me)[DOUBLE BOND]C(CO2Me)C(R3)(H)O}] (C), which have been trapped by NCMe to afford the adducts 11 (R3=Ar). If a second aldehyde acts as the trapping reagent for these species, this ligand acts as a shuttle in transfering a hydrogen atom from the γ- to the α-carbon atom of the iridacycle through the formation of an alkoxide group. Methyl propiolate (MP) can be used instead of DMAD to regioselectively afford the related iridafurans. These reactions have also been studied by DFT calculations.