Insertions of electrophiles into metal hydride bonds. Reactions of (C5Me5)Rh(PMe3)H2 with activated alkynes to produce η2-alkene complexes

The dihydride complex (C5Me5)Rh(PMe3)H2 (1) reacts with the electron deficient alkynes phenylacetylene (PhCCH), diphenylacetylene (PhCCPh), methyl propiolate (HCCE, E=CO2Me), dimethylacetylenedicarboxylate (ECCE, DMAD) and hexafluoro-2-butyne (F3CCCCF3) to produce in all cases η2-olefin complexes of the hydrogenated alkyne (2a–j). The complexes (C5Me5)Rh(PMe3)(η2-methacrylate) (2a, 2b), (C5Me5)Rh(PMe3)(η2-styrene) (2c, 2d) and (C5Me5)Rh(PMe3)(η2-cis-stilbene) (2e, 2f) exist as pairs of rotamers that do not interconvert. The reaction of 1 with PhCCPh also produces (C5Me5)Rh(PMe3)(η2-trans-stilbene) (2g) at higher temperatures. Insertion of F3CCCCF3 into 1 occurs stereoselectively to yield (C5Me5)Rh(PMe3)(η2-trans-hexafluoro-2-butene) (2h). A mixture of cis- and trans- insertion products is observed in the reaction of 1 with ECCE to yield (C5Me5)Rh(PMe3)(η2-fumarate) (2i) and (C5Me5)Rh(PMe3)(η2-maleate) (2j) complexes. A crossover experiment using 1 and 1-d2 with DMAD shows only d0 and d2 η2-olefin products, demonstrating that both hydrogens come from the same molecule of 1. A second crossover experiment using (C5Me5)Rh(PMe3)(η2-fumarate)-d2 and free dimethylfumarate shows no evidence for incorporation of the free olefin. The vinyl hydride intermediates (C5Me5)Rh(PMe3)(CRCHR)H are observed below −20°C for R=E and at room temperature for R=CF3. Cyclic voltammetry of the metal complex 1 and of the substituted alkynes shows irreversible behavior and half-wave redox potentials E1/2 are reported. CHCl3 quenching of the vinyl hydride products from the reaction of 1 with F3CCCCF3 yields crystals of the complex (C5Me5)Rh(PMe3)[trans-C(CF3)CH(CF3)]Cl (5a) which was established as the trans complex by X-ray crystallography.