A new catalytic system for beta-alkylation of secondary alcohols has been developed. In the presence of [CpIrCl(2)](2) (Cp = pentamethylcyclopentadienyl) catalyst and base, the reactions of various secondary alcohols with primary alcohols give beta-alkylated higher alcohols in good to excellent yields without any hydrogen acceptor or hydrogen donor. This reaction proceeds via successive hydrogen-transfer reactions and aldol condensation. [reaction: see text]
Several new cationic Cp*Ir N-heterocyclic complexes have been synthesized and their catalytic activities in the Oppenauer-type oxidation have been investigated in order to improve the catalytic activity of [Cp*IrCl(µ-Cl)] 2 . The reactions of [Cp*IrCl(µ-Cl)] 2 (1) with N-heterocyclic carbene ligands afforded Cp*Ir(L)Cl 2 (3a-d; L ) N-heterocyclic carbene ligands). The cationic complexes [Cp*Ir(L)(MeCN) 2 ] 2+ (5a-d) were obtained by the treatment of 3a-d with 2 equiv of AgOTf followed by addition of CH 3 CN. Structures of complexes 3a-d and 5a-d were determined by X-ray crystallographic studies. Complex 5a (L ) 1,3,4,5tetramethylimidazol-2-ylidene) catalyzed the Oppenauer-type oxidation of primary and secondary alcohols very selectively under mild conditions. In the oxidation of 1-phenylethanol and cyclopentanol using 5a as a catalyst, turnover numbers reached 3200 and 6640, respectively. These results demonstrate that, to the best of our knowledge, the cationic carbene complex 5a is the most effective catalyst in homogeneous oxidation of alcohols in terms of its high catalytic activity and wide applicability to the oxidation of primary and secondary alcohols. In this catalytic system, the stronger electron-donating ability of the N-heterocyclic carbene ligand than the phosphine ligand is more favorable for acceleration of the hydride transfer to acetone as a hydrogen acceptor. Additionally, dihydrido carbene complex Cp*Ir(L)(H) 2 (6) and dinuclear iridium carbene complex [Cp*Ir(L)(µ-H)] 2 2+ (7) were prepared to investigate the catalytically active species and fate of the catalyst. Thus, it is highly probable that an iridium-monohydride complex is the catalytically active species and that 7, which could be generated by dimerization of the iridium-monohydride complex in the catalytic system, is inactive.
All reactions and manipulations were carried out under an atmosphere of argon by means of standard Schlenk techniques. 1 H and 13 C{ 1 H} NMR spectra were recorded on JEOL A-500 and EX-270 spectrometers. Gas chromatography analyses were performed on a Shimadzu GC-14A gas chromatograph with capillary column (Shimadzu CBP1-M25-025) or GL-Sciences GC353B gas chromatograph with capillary column (GL-Sciences TC-17). Melting points were determined on a Yanagimoto micro melting point apparatus. Elemental analyses were carried out at the
The new iridium N-heterocyclic carbene complexes Cp N Ir(Ii-PrMe)I 2 and CpIr(Ii-PrMe)I 2 (Cp N ) (2-(dimethylamino)ethyl)cyclopentadienyl; Ii-PrMe ) 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene) have been synthesized along with Ir(cod)(Ii-PrMe)Cl, Cp N Ir(Ii-PrMe)(η 2 -cod), and CpIr(Ii-PrMe)(η 2 -cod). Facile intramolecular alkyl C-H bond activation reactions of Cp*Ir(Ii-PrMe)Cl 2 (Cp* ) η 5 -pentamethylcyclopentadienyl) and Cp N Ir(Ii-PrMe)I 2 have occurred by treatment with MeONa and AgOTf, respectively.
The new iridium N-heterocyclic carbene complexes Cp* N Ir(IMeMe)Cl 2 and Cp* N Ir(IMe)Cl 2 (Cp* N ) η 5 -1-[2-(dimethylamino)ethyl]-2,3,4,5-tetramethylcyclopentadienyl; IMeMe ) 1,3,4,5-tetramethylimidazol-2-ylidene; IMe ) 1,3-dimethylimidazol-2-ylidene) have been synthesized by reaction of η 5 :η 1 -Cp* N -IrCl 2 with silver(I) carbene complexes or a free carbene. The catalytic systems using new iridium carbene complexes as a catalyst precursor and AgOTf showed high catalytic activities in Oppenauer-type oxidation of alcohol and were applicable to oxidations of acid-sensitive alcohols that had been hard to oxidize by using [Cp*Ir(IMeMe)(MeCN) 2 ][OTf] 2 .
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