Selective dehydrogenation of alcohols and diols catalyzed by a dihydrido iridium PCP pincer complex

The PCP pincer complex, IrH2{C6H3-2,6-(CH2P-t-Bu-2)(2)} (1) catalyzes the t ransfer dehydrogenation of primary and secondary alcohols. Dehydrogenation occurs across the C-O bond rather than the C-C bonds and the corresponding aldehydes or ketones are obtained as the sole products arising from the deh ydrogenation reactions. Methanol is an exception to this pattern of reactiv ity and undergoes only stoichiometric dehydrogenation with 1 to give the ca rbonyl complex, Ir(CO){C6H3-2,6-(CH2P-t-Bu-2)(2)} (2). The products are obt ained in nearly quantitative yields when the reactions are carried out in t oluene solutions. Under the same conditions, 2,5-hexanediol is converted to the annulated product, 3-methyl-2-cyclopenten-1-one which has been isolate d in 91% yield in a preparative scale reaction.