Reactions of alkynes and olefins with tantalumn hydrides containing aryloxide ancillary ligation: Relevance to catalytic hydrogenation

The reactivity of the three hydride compounds [Ta(OC6H3Ph2-2,6)(2)(H)(2)Cl( PMe3)(2)] (1), [Ta(OC6H3Pr2i-2,6)(2)(H)(2)Cl(PMe2Ph)(2)] (2), and [Ta(OC6H3 Bu2t-2,6)(2)(H)(2)Cl(PMePh2)] (8) toward olefins and alkynes has been inves tigated. The reactivity observed is highly dependent on the nature of the a ncillary aryloxide ligands. The 2,6-diphenylphenoxide 1 reacts with styrene to produce 1 equiv of ethylbenzene and the styrene adduct [Ta(OC6H3Ph2-2,6 )(2)(eta(2)CH(2)=CHPh)Cl(PMe3)] (6). In contrast, I reacts with 5-hexyne to eliminate H-2 along with formation of the analogous alkyne complex 6. Stru ctural studies of 5 and 6 show a square-pyramidal geometry with an axial ol efin (alkyne) unit lying along the Cl-Ta-P) axis. Structural parameters sup port a tantalacyclopropane (tantalacyclopropene) bonding picture for these molecules. Compound 5 is converted back into 1 under H-2 along with formati on of PhEt. The dihydride 2 reacts with styrene to form 1 equiv of PhEt, H- 2, and the dehydrogenation product Ta(OC6H3Pri-eta(2)-CMe=CH2)(OC6H3Pr2i-2, 6)Cl(PMe2Ph)(2)] (7). The related adduct [Ta(OC6H3Pri-eta(2)-CMe=CH2)(OC6H3 Pr2i-2,6)Cl(PEt3)(2)] (9) was isolated by treatment of [Ta(OC6H3Pr2i-2,6)(2 )Cl-3] with PEt3/Bu3SnH and was structurally characterized. Labeling studie s show that the H-2 generated comes exclusively from the aryloxide o-Pr-i g roup which was dehydrogenated. Both hydrides initially attached to the meta l are transferred to the olefin substrate. In the case of the 2,6-di-tert-b utylphenoxide compound 3, reaction with styrene generates the mono-cyclomet alated compound [Ta((OC6H3BuCMe2CH2)-C-t)(OC6H3Bu2t-2,6)(CH2CH2Ph)Cl] (9). Structural studies of 9 confirm the presence of a phenethyl group. The rela ted trans-phenylvinyl compound 10 is produced when 3 is reacted with phenyl acetylene. Addition of 2,6-dimethylphenyl isocyanide (xyNC) to 10 produces the bis-(iminoacyl) derivative 11, in which xyNC has inserted into the cycl ometalated carbon as well as the Ta-CH=CHPh bond in 10. Structural studies of 11 confirmed the trans arrangement of the hydrogen atoms in the phenylvi nyl group. Mechanistic studies of the formation of 10 and 11 show the prese nce of two competing pathways. The first involves direct elimination of H-2 from the dihydride and formation of an intermediate olefin/alkyne adduct. The product then arises by CH bond activation of the aryloxide with the hyd rogen transferring to a carbon atom of the tantalacyclopropane (tantalacycl opropene) ring. The second pathway involves insertion of olefin/alkyne into a Ta-H bond followed by CH bond activation by the remaining hydride.