Investigations on the Coupling of Ethylene and Alkynes in Tp Me2 Ir Compounds: Water as an Effective Trapping Agent

Abstract

The reaction of the bis(ethylene) complex [TpMe2Ir(C2H4)2] (1) (TpMe: hydrotris(3,5-dimethylpyrazolyl)borate) with two equivalents of dimethyl acetylenedicarboxylate (DMAD) in CH2Cl2 at 25 °C gives the hydride–alkenyl species [TpMe2IrH{C(R)=C(R)C(R)=C(R)CH=CH2}] (2, R: CO2Me) in high yield. A careful study of this system has established the active role of a number of intermediates en route to producing 2. The first of these is the iridium(I) complex [TpMe2Ir(C2H4)(DMAD)] (4) formed by substitution of one of the ethylene ligands in 1 by a molecule of DMAD. Complex 4 reacts further with another equivalent of the alkyne to give the unsaturated metallacyclopentadiene [TpMe2Ir{C(R)=C(R)C(R)=C(R)}], which can be trapped by added water to give adduct 7, or can react with the C2H4 present in solution generating complex 2. This last step has been shown to proceed by insertion of ethylene into one of the Ir-C bonds of the metallacyclopentadiene and subsequent β-H elimination. Complex 1 reacts sequentially with one equivalent of DMAD and one equivalent of methyl propiolate (MP) in the presence of water, with regioselective formation of the nonsymmetric iridacyclopentadiene [TpMe2Ir{C(R)=C(R)C(H)=C(R)}(H2O)] (9). Complex 9 reacts with ethylene giving a hydride–alkenyl complex 10, related to 2, in which the C2H4 has inserted regiospecifically into the Ir-C(R) bond that bears the CH functionality. Heating solutions of either 2 or 10 in CH2Cl2 allows the formation of the allyl species 3 or 11, respectively, by simple stereoselective migration of the hydride ligand to the Cα alkenyl carbon atom and concomitant bond reorganization of the resulting organic chain. All the compounds described herein have been characterized by microanalysis, IR and NMR spectroscopy, and for the case of 3, 7, 7⋅CO, 8⋅NCMe, 9, 9⋅NCMe, and 10, also by single-crystal X-ray diffraction studies.