Development of PCP pincer complexes as catalysts for organic transformations involving the activation of "unreactive" bonds

Abstract

The attempted hydroxylation of cyclooctane catalyzed by the hydridohydroxo complex, IrH(OH){C6H3-2,6-(CH2PBuᵗ2)2} (2), gives only cyclooctene and no oxygenated products such as alcohol or ketone. This observation reveals that a dissociative mechanism is involved in the dehydrogenation reaction. After comparing the kinetic aspects for the dehydrogenation of alkanes catalyzed by the dihydrido complexes, IrH2{C6H3-2,6-(CH2PBuᵗ2)2} (1), and complex 2, it is very clear that both complexes 1 and 2 undergo a dissociative pathway to form the same 14-electron Ir(I) intermediate (PCP)Ir through which the dehydrogenation of alkanes occurs. As an extension of our studies on the functionalization of the C-H bond, pincer-ligated platinum complexes, PtCl{C6H3-2,6-(CH2NEt2)2} (5), and the novel complex, PtCl{C6H3-2,6-(OPPrͥ2)2} (6), have been explored as C-H bond hydroxylation catalysts. Both complexes have been found to be much more active and selective catalysts than the original Shilov catalyst, [PtCl4]²ˉ for the conversion of 1-propanol to I,3-diol. Moreover, the pincer complexes catalyze this transformation even upon replacement of Pt(IV) salts by more economical CuCl2 as an oxidant. An intermediate complex from the catalytic sequence has been isolated and characterized. The mechanistic studies indicate that after replacing the Pt(IV) salt with CuCl2 the pincer complex-mediated hydroxylation process does not involve a classic Shilov-type mechanism. In addition to the activation of the C-H bond, the functionalization of the aryl-Cl bond was also explored. An effective one-pot catalytic system has been developed to apply the PCP pincer palladium complex, PdCl{C6H3-2,6-(OPPrͥ2)2} (3), as an active catalyst to mediate the cross-coupling reaction between phenylacetylene and a wide array of electronically deactivated, non-activated and activated as well as hindered aryl chlorides with modest to excellent yields. The presence of ZnCl2 is found to be essential to enhance the catalytic activity of complex 3.