Reactions of Some Rhodium and lridium Hydrides

Abstract

<p>Reactions of the complexes [C₅Me₅MCl₂]₂ (M=Rh and Ir) with alcoholic base gave the bridged hydrides (C₅Me₅M)₂HCl₃.</p> <p>The hydrides reacted readily at room temperature with cyclic and acyclic diolefins to give π-allylic complexes, the cyn- isomers being the ones obtained when a choice was available. When both terminal and internal double bonds were present, addition of M-H occurred preferentially to the terminal double bond.</p> <p>The allylic complexes showed varying degrees of stability towards reductive elimination (by lossof HCl) and the formation of 1,3-diene complexes. Allyis with only syn-substituents were stable towards diene complex formation. Those with an additional C-2 methyl substituent were less stable and the diene complexes could be obtained under moderately drastic conditions, presumably via G-allyl intermediates. Allyls with an anti-methyl or -methylene substituent underwent reductive elimination of HCI to give diene complexes spontaneously and these reactions were accelerated by the presence of triethylamine.</p> <p>Kinetic studies on these hydride-diene reactions indicated that the reactions were first-order with respect to the hydride and independent of the nature and concentration of the diene. Except for 2,5-dimethyl-2,4-hexadiene, all dienes studied reacted at the same rate with the same hydride. The deuterides (C₅Me₅M)₂DCl₃ did not show any significant kinetic isotope effect. These results suggested that the rate-determining step of these hydride-diene reactions was the cleaving of the chloride bridge to create a vacant site. Mechanisms for the formation and decompositions of the allyls are proposed.</p> <p>Reactions of [C₅Me₅MCl₂]₂ with dibenzylideneacetone, allyl cyanide and some β-diketones are described.</p>