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

Abstract

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.

Highly Active and Recyclable Heterogeneous Iridium Pincer Catalysts for Transfer Dehydrogenation of Alkanes

“…[12] Dimethylation of the commercially available dimethyl 5-aminoisophthalate (6) was achieved by adaptation of a method developed by Borch and Hassid. [13] The diester 7 was then reduced to 5-dimethylamino-1,3-benzenedimethanol (8) by lithium aluminum hydride in THF. [14] Diol 8 was then treated with PBr 3 in acetonitrile and the resulting dibromide 9 was recrystallized in 77% yield from acetonitrile/water [15] before conversion to Me 2 N-PCP-H (10) by an adaptation of the method of Moulton and Shaw.…”

“…The resulting solution was stirred at room temperature for 8 h and the solvent was removed under reduced pressure. The wet-solid obtained thereby was washed thoroughly with distilled water and air-dried to give 7 as a light yellowish-white solid; yield: 5.39 g (97% Synthesis of 5-Dimethylamino-1,3-benzenedimethanol (8) To a stirred suspension of 2.69 g (67.4 mmol) of lithium aluminum hydride (95%) in THF (50 mL) at 0 8C under an argon atmosphere was slowly added a THF (100 mL) solution of 7 (5 g, 21.1 mmol). After the addition was complete, the resultant suspension was refluxed for 18 h, diluted with 100 mL tetrahydrofuran and cooled to 0 8C.…”

“…Even more notably, these complexes have been reported to catalyze dehydrogenation with high regioselectivity for the terminal positions of n-alkanes or n-alkyl groups. Moreover, in addition to simple al-A C H T U N G T R E N N U N G kanes, iridium pincer complexes have been reported to catalyze dehydrogenation of numerous other substrates including amines, [7] alcohols, [8] and polyolefins. [9] Mechanistic details of the dehydrogenations by both iridium PCP and POCOP pincer catalysts have been disclosed.…”

Structures, relative energies, and ligand dissociation energies of iridium carbonyl phosphine clusters

“…Iridium complexes play important roles in oxidation (e.g., of alcohols [4][5][6], phenols [7][8][9], and amines [10,11]), hydrogenation [12,13], C-H activation [14][15][16], cycloaddition (e.g., [2 + 2 + 2] [17][18][19], [2 + 2 + 1] [20,21], and [4 + 2] [22,23]), cycloisomerization [1,2], and ring-opening reactions [24]. The richness of this chemistry explains the broad interest in the properties of the family of iridium complexes and clusters [25].…”

Reactivity of Pincer Complexes Toward Carbon Monoxide

“…Recently, the reactivity of these complexes has been extended to the activation of other DÀH (D ¼ donor atom) bonds including the dehydrogenation of alcohols [31] and amines [32]. In this respect, authors have gained valuable information from the reactions of some of these complexes with carbon monoxide.…”