A series of neutral and cationic rhodium and iridium(I) complexes based on hemilabile O-donor-and N-donor-functionalized NHC ligands having methoxy, dimethylamino, and pyridine as donor functions have been synthesized. The hemilabile fragment is coordinated to the iridium center in the cationic complexes [Ir(cod] has been determined by X-ray diffraction. The iridium complexes are efficient precatalysts for the transfer hydrogenation of cyclohexanone in 2-propanol/KOH. A comparative study has shown that cationic complexes are more efficient than the neutral and also that complexes having O-functionalized NHC ligands provide much more active systems than the corresponding N-functionalized ligands with TOFs up to 4600 h À1 . The complexes [Ir(NCCH 3 )(cod)(MeImR)] + (R = 2-methoxyethyl and 2-methoxybenzyl) have been successfully applied to the reduction of several unsaturated substrates as ketones, aldehydes, α,β-unsaturated ketones, and imines. The investigation of the reaction mechanism by NMR and MS has allowed the identification of relevant alkoxo intermediates [Ir(OR)(cod)(MeImR)] and the unsaturated hydride species [IrH(cod)(MeImR)]. The β-H elimination in the alkoxo complex [Ir(OiPr)(cod)(MeIm(2-methoxybenzyl))] leading to hydrido species has been studied by DFT calculations. An interaction between the β-H on the alkoxo ligand and the oxygen atom of the methoxy fragment of the NHC ligand, which results in a net destabilization of the alkoxo intermediate by a free energy of +1.0 kcal/mol, has been identified. This destabilization facilitates the β-H elimination step in the catalytic process and could explain the positive effect of the methoxy group of the functionalized NHC ligands on the catalytic activity.
The elongated dihydrogen complex [Os{C6H4C(O)CH3}(η 2 -H2)(H2O)(P i Pr3)2]BF4 (1) reacts with 1,1-diphenyl-2-propyn-1-ol and 2-methyl-3-butyn-2-ol to give the hydride-hydroxyvinylidene-π-alkynol derivatives [OsH{dCdCHC(OH)R2}{η 2 -HCtCC(OH)R2}(P i Pr3)2]BF4 (R ) Ph (2), Me (3)), where the π-alkynols act as four-electron donor ligands. Treatment of 2 and 3 with HBF4 and coordinating solvents leads to the dicationic hydride-alkenylcarbyne compounds [OsH(tCCHdCR2)S2( 6)), which in acetonitrile evolve into the alkenylcarbene complexes [Os(dCHCHdCR2)(CH3CN)3(P i Pr3)2][BF4]2 (R ) Ph (7), Me (8)) by means of a concerted 1,2hydrogen shift from the osmium to the carbyne carbon atom. Treatment of 2-propanol solutions of 5 with NaCl affords OsHCl2(tCCHdCPh2)(P i Pr3)2 (10), which reacts with AgBF4 and acetonitrile to give [OsHCl-(tCCHdCPh2)(CH3CN)(P i Pr3)2]BF4 (11). In this solvent complex 11 is converted to [OsCl(dCHCHd CPh2)(CH3CN)2(P i Pr3)2]BF4 (12). Complex 5 reacts with CO to give [Os(dCHCHdCPh2)(CO)(CH3CN)2-(P i Pr3)2][BF4]2 (15). DFT calculations and kinetic studies for the hydride-alkenylcarbyne to alkenylcarbene transformation show that the difference of energy between the starting compounds and the transition states, which can be described as η 2 -carbene species (OsdC(R)H), increases with the basicity of the metallic center. The X-ray structures of 4 and 7 and the rotational barriers for the carbene ligands of 7, 8, and 12 are also reported.
The borrowing hydrogen methodology allows for the use of alcohols as alkylating agents for CC bond forming processes offering significant environmental benefits over traditional approaches. Iridium(I)-cyclooctadiene complexes having a NHC ligand with a O- or N-functionalised wingtip efficiently catalysed the oxidation and β-alkylation of secondary alcohols with primary alcohols in the presence of a base. The cationic complex [Ir(NCCH3 )(cod)(MeIm(2- methoxybenzyl))][BF4 ] (cod=1,5-cyclooctadiene, MeIm=1-methylimidazolyl) having a rigid O-functionalised wingtip, shows the best catalyst performance in the dehydrogenation of benzyl alcohol in acetone, with an initial turnover frequency (TOF0 ) of 1283 h(-1) , and also in the β-alkylation of 2-propanol with butan-1-ol, which gives a conversion of 94 % in 10 h with a selectivity of 99 % for heptan-2-ol. We have investigated the full reaction mechanism including the dehydrogenation, the cross-aldol condensation and the hydrogenation step by DFT calculations. Interestingly, these studies revealed the participation of the iridium catalyst in the key step leading to the formation of the new CC bond that involves the reaction of an O-bound enolate generated in the basic medium with the electrophilic aldehyde.
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