The PCP pincer complex, IrH 2 {C 6 H 3 -2,6-(CH 2 P-t-Bu 2 ) 2 } (1) catalyzes the transfer 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 dehydrogenation reactions. Methanol is an exception to this pattern of reactivity and undergoes only stoichiometric dehydrogenation with 1 to give the carbonyl complex, Ir(CO){C 6 H 3 -2,6-(CH 2 P-t-Bu 2 ) 2 } (2). The products are obtained in nearly quantitative yields when the reactions are carried out in toluene solutions. Under the same conditions, 2,5-hexanediol is converted to the annulated product, 3-methyl-2-cyclopenten-1-one which has been isolated in 91% yield in a preparative scale reaction.Résumé : Le complexe PCP en forme de pince, IrH 2 {C 6 H 3 -2,6-(CH 2 P-t-Bu 2 ) 2 } (1) catalyse la réaction de déshydrogé-nation par transfert des alcools primaires et secondaires. La déshydrogénation se fait à travers la liaison C-O plutôt qu'à travers les liaisons C-C et on n'obtient que les aldéhydes et les cétones comme seuls produits de ces réactions de déshydrogénation. Le méthanol est une exception à ce mode de réactivité et il ne subit qu'une déshydrogénation stoechiométrique avec 1 pour conduire à la formation d'un complexe de carbonyle, Ir(CO){C 6 H 3 -2,6-(CH 2 P-t-Bu 2 ) 2 } (2). Lorsqu'on effectue les réactions en solution dans le toluène, les produits sont obtenus en rendements pratiquement quantitatifs. Dans les mêmes conditions, l'hexane-2,5-diol est transformé en produit cyclique, la 3-méthylcyclopent-2-én-1-one qui a été isolée avec un rendement de 91% au cours d'une réaction à l'échelle préparative.
A rationally designed and selective synthesis of hybrid phosphine−phosphaalkene ligands E-1a (Cy 2 PCH 2 CH PMes*, Mes* = 2,4,6-tri-tert-butylphenyl) and E-1b (Ph 2 PCH 2 CHPMes*) was developed using phospha-Wittig methodology. The new hybrid ligands E-1a and E-1b were used to prepare the Pd and Pt dichloride complexes Pd(Cy 2 PCH 2 CHPMes*)Cl 2 (2a), Pd(Ph 2 PCH 2 CHPMes*)Cl 2 (2b), Pt(Cy 2 PCH 2 CHPMes*)Cl 2 (3a), and Pt(Ph 2 PCH 2 CHPMes*)Cl 2 (3b). The crystal structures of E-1a, E-1b, 2a•1.33CHCl 3 , 3a•CH 3 CN, and 3b were determined. DFT calculations (M06/LACV3P**) on 2a revealed that the π* orbital located on the PC unit is low-lying and accessible. An NBO analysis concluded that the phosphaalkene ligand is a significantly poorer σ donor and a slightly better π acceptor than its tertiary phosphine counterpart, due to the presence of the PC double bond.
Selective Dehydrogenation of Alcohols and Diols Catalyzed by a Dihydrido Iridium PCP Pincer Complex.-The title complex catalyzes the transfer dehydrogenation of primary and secondary alcohols to afford the corresponding aldehydes and ketones in most cases as sole products in quantitative GC yields. Diol (V) gives an unexpected cyclopentenone under this iridium catalysis. With methanol the dihydridoiridium complex forms a carbonyl complex.
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