Insertions of electrophiles into metal hydride bonds. Reactions of (C5Me5)Rh(PMe3)H-2 with activated alkynes to produce eta(2)-alkene complexes

The dihydride complex (C5Me5)Rh(PMe3)H-2 (1) reacts with the electron defic ient alkynes phenylacetylene (PhC drop CH), diphenylacetylene (PhC drop CPh ), methyl propiolate (HC drop CE, E = CO2Me), dimethylacetylenedicarboxylat e (EC drop CE, DMAD) and hexafluofo-2-butyne (F3CC drop CCF3) to produce in all cases eta(2)-olefin complexes of the hydrogenated alkyne (2a-j). The c omplexes (C5Me5)Rh(PMe3)(eta(2)-methacrylate) (2a, 2b), (C5Me5)Rh(PMe3)(eta (2)-styrene) (2c, 2d) and (C5Me5)Rh(PMe3)(eta(2)-cis-stilbene) (2e, 2f) exi st as pairs of rotamers that do not interconvert. The reaction of 1 with Ph C drop CPh also produces (C5Me5)Rh(PMe3)(eta(2)-trans-stilbene) (2g) at hig her temperatures. Insertion of F3CC drop CCF3 into 1 occurs stereoselective ly to yield (C5Me5)Rh(PMe3)(eta(2)-trans-hexafluoro-2-butene) (2h). A mixtu re of cis- and trans- insertion products is observed in the reaction of 1 w ith EC drop CE to yield (C5Me5)Rh(PMe3)(eta(2)-fumarate) (2i) and (C5Me5)Rh (PMe3)(eta(2)-maleate) (2j) complexes. A crossover experiment using 1 and 1 -d(2) with DMAD shows only d(0) and d(2) eta(2)-olefin products, demonstrat ing that both hydrogens come from the same molecule of 1. A second crossove r experiment using (C5Me5)Rh(PMe3)(eta(2)-fumarate)-d(2) and free dimethylf umarate shows no evidence for incorporation of the free olefin. The vinyl h ydride intermediates (C5Me5)Rh(PMe3)(CR=HR)H are observed below - 20 degree s C for R = E and at room temperature for R = CF3. Cyclic voltammetry of th e metal complex 1 and of the substituted alkynes shows irreversible behavio r and half-wave redox potentials E-1/2 are reported. CHCl3 quenching of the vinyl hydride products from the reaction of 1 with F3CC=CCF3 yields crysta ls of the complex (C5Me5)Rh(PMe3)[trans-C(CF3)=CH(CF3)]Cl (5a) which was es tablished as the trans complex by X-ray crystallography. (C) 2000 Elsevier Science S.A. All rights reserved.