Pseudo-tetrahedral Rhodium and Iridium Complexes: Catalytic Synthesis of E-Enynes

The reactions of the rhodium(I) and iridium(I) complexes [M(PhBP3 )(C2 H4 )(NCMe)] (PhBP3 =PhB(CH2 PPh2 )3 - ) with alkynes have resulted in the synthesis of a new family of pseudo-tetrahedral complexes, [M(PhBP3 )(RC≡CR')] (M=Rh, Ir), which contain the alkyne as a four-electron donor. The reactions of these unusual compounds with two-electron donors (L=PMe3 , CNtBu) produced a change in the "donicity" of the alkyne from a 4e- to a 2e- donor to give five-coordinate complexes. These were the final products with the iridium complexes, whereas further reactions took place with the rhodium complexes. In particular, C(sp)-H bond activation of the alkyne occurred leading to hydrido alkynyl complexes. This process is essential for the further reactivity of the alkynes, and if the alkyne itself was used as reagent, E-enyne complexes were obtained. As a consequence of this chemistry, we show that the complex [Rh(PhBP3 )(C2 H4 )(NCMe)] is a very efficient pre-catalyst for the regioselective di- and trimerization of terminal alkynes to E-enynes and benzene derivatives, respectively. Interestingly, acetonitrile significantly enhanced the catalytic activity by facilitating the C(sp)-H bond activation step. A hydrometalation mechanism to account for these experimental observations is proposed.