Sulfoxide complexes hydrogen transfer

Henbest and Mitchell [78] have shown that water can be used as hydrogen source with chloroiridic acid (6) as the catalyst through oxidation of phosphorous acid (59) to phosphoric acid (60) in aqueous 2-propanol. Under these conditions, no hydrogen transfer occurs from 2-propanol. However, iridium complexes with sulfoxide or phosphine ligands show the usual transfer from 2-pro-panol [79-81]. 

Kinetic resolutions. A chiral alcohol is obtained on. selective removal of one enantiomer by acetylation using a chiral analog 1 of DMAP, or by oxidation based on hydrogen transfer to acetone mediated by a Ru complex 2. Benzylic secondary alcohols are resolved by selective pivaloylation with optically activeA-pivaloyl-4-t-butylthiazolidine-2-thione. A kinetic resolution of sulfoxides is based on asymmetric oxidation with (i-PrO)4Ti-cumyl hydroperoxide in the presence of a tartrate ester. Kinetic resolution of 1,3-diarylallenes is realized by selective oxidation with NaClO catalyzed by a chiral (salen)manganese(III) complex, whereas asymmetric hydrolysis of terminal epoxides with the aid of a chiral (salen)cobalt(II) catalyst solves the problem of their accessibility. 

Hydrogen transfer reactions are catalyzed by several iridium complexes, including the dimethyl sulfoxide (DMSO) complexes cis- and trans-[Ir(Cl)4(DMSO)2]", [Ir(Cl)3(DMSO)3] and [lr(H)-(Cl)2(DMSO)3], as well as the cyclooctadiene (cod) complexes [Ir(Cl)(cod)]2 and [Ir(3,4,7,8-Me4phen)(cod)], and tra 5-[Ir(Cl)(CO)(PPh3)2]. Vaska s complex catalyzes the conversion of p-methoxybenzoyl chloride to the corresponding aldehyde. The dimethyl sulfoxide iridium(III) complexes catalyze hydrogen transfer from propan-2-ol to unhindered cyclohexanones to yield cyclohexanols, while the cod complexes serve as catalysts in the transfer of hydrogen from propan-2-ol to alkenes, ketones and a,/3-unsaturated ketones. ... 

In non-HBD solvents such as n-heptane, tetrachloromethane, diethyl ether, deuterio-tri-chloromethane, and dimethyl sulfoxide, tropolone transfers its proton to triethylamine to give an ion pair, which is in equilibrium with the non-associated reactants. There is no formation of a hydrogen-bonded complex between tropolone and triethylamine because of the fact that tropolone itself is intramolecularly hydrogen-bonded. The extent of the ion pair formation increases with solvent polarity. In polar HBD solvents such as ethanol, methanol, and water, this proton-transfer equilibrium is shifted completely towards the formation of triethylammonium tropolonate [171]. 

Having demonstrated the potential of artificial metalloenzymes for the reduction of V-protected dehydroaminoacids, we turned our attention towards organometallic-catalyzed reactions where the enantiodiscrimination step occurs without coordination of one of the reactants to the metal centre. We anticipated that incorporation of the metal complex within a protein enviromnent may steer the enantioselection without requiring transient coordination to the metal. In this context, we selected the palladium-catalyzed asymmetric allylic alkylation, the ruthenium-catalyzed transfer hydrogenation as well as the vanadyl-catalyzed sulfoxidation reaction. Indeed, these reactions are believed to proceed without prior coordination of the soft nucleophile, the prochiral ketone or the prochiral sulfide respectively. Figure 13.5. 

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