Abstract

Abstract

Abstract

Abstract

Abstract

Alcohols can be temporarily converted into carbonyl compounds by the metal-catalysed removal of hydrogen. The carbonyl compounds are reactive in a wider range of transformations than the precursor alcohols and can react in situ to give imines, alkenes, and α-functionalised carbonyl compounds. The metal catalyst, which had borrowed the hydrogen, then returns it to the transformed carbonyl compound, leading to an overall process in which alcohols can be converted into amines, compounds containing CC bonds and β-functionalised alcohols.

The successful development of an indirect three-step domino sequence for the formation of C–C bonds from alcohol substrates is described. An iridium-catalysed dehydrogenation of alcohol 1 affords the intermediate aldehyde 2. The desired C–C bond can then be formed by a facile Wittig olefination, yielding the intermediate alkene 3. In the final step the alkene is hydrogenated to afford the indirect Wittig product, the alkane 4. The key to this process is the concept of borrowing hydrogen; hydrogen removed in the initial dehydrogenation step is simply borrowed by the iridium catalyst. Functioning as a hydrogen reservoir, the catalyst facilitates C–C bond formation before subsequently returning the borrowed hydrogen in the final step. Herein we present full details of our examination into both the substrate and reaction scope and the limitations of the catalytic cycle. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

The dihydride ruthenium N-heterocyclic carbene complex Ru(IMes)(PPh₃)₂CO(H)₂ (1) (IMes=1,3-dimesityl-1,3-dihydro-2H-imidazol-2-ylidene) is an efficient catalyst for both direct hydrogenation and transfer hydrogenation of ketones and imines, in the absence of base.

This paper describes how enantiomerically enriched Evans auxiliaries can be successfully prepared by either an enzymatic desymmetrisation strategy or an asymmetric synthesis using racemic auxiliaries and an enzymatic resolution. Desymmetrisation of N-Boc-protected serinol has been achieved in good yield and high enantiomeric excess using porcine pancreas lipase. This has been exploited in different ways to prepare enantiomerically enriched (4R)- and (4S)-substituted 2-oxazolidinones. In another approach to asymmetric synthesis, starting from a racemic Evans auxiliary, by means of a diastereoselective aldol reaction coupled with a lipase-catalysed resolution, we achieved the preparation of enantiomerically enriched β-hydroxy acids and enantiomerically enriched 2-oxazolidinones.

Enantiomerically pure α-amino esters were applied in palladium catalysed allylic substitution reactions with moderate diastereoselectivity (up to 70% d.e.) using achiral ligands on symmetrical allylic acetates. Nucleophilic substitution of amino esters with enantiomerically pure ligands enabled diastereoselective ratios of 95:5 to be obtained. Chirality 15:190–195, 2003. © 2003 Wiley-Liss, Inc.

The synthesis of the core unit of cycloaraneosene and ophiobolin M has been investigated, following a general strategy applicable to both 5−8 bicyclic systems. The synthetic strategy includes a ring-closing metathesis reaction to generate the central eight-membered ring as well as a palladium-mediated coupling of a Grignard reagent to introduce the exocyclic side-chain. The stereochemistry of the ring junction is also discussed and moderate diastereoselectivity has been achieved. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

Ausgeliehene Wasserstoffatome, die durch Dehydrierung des Eduktalkohols zum entsprechenden Aldehyd abgegeben werden, werden nach In-situ-Wittig-Olefinierung dieses Aldehyds in einem Hydrierungsschritt wieder zurückgegeben. Dies ermöglicht eine indirekte Wittig-Reaktion von Alkoholen ohne Gesamtoxidation und bietet eine Alternative zu herkömmlichen Verfahren, bei denen ein Alkohol zunächst in ein Alkylhalogenid überführt wird.

Hydrogen atoms are borrowed during the in situ oxidation of the starting alcohol to the corresponding aldehyde and are subsequently returned in the hydrogenation of the alkene intermediate, which is formed by Wittig olefination of the aldehyde. This process permits an indirect Wittig reaction of alcohols without overall oxidation and offers an alternative to traditional methods that involve, for example, conversion of an alcohol into an alkyl halide.

The use of tricyclohexylphosphine (and related phosphines) in palladium-catalysed allylic substitution reactions enables the selective conversion of branched allylic acetates into the branched substitution products (up to 120:1 regioselectivity). Linear allylic acetates do not show the same selectivity for the branched substitution products, thereby demonstrating a memory effect.

Nucleophilic addition to cycloalkenols has been achieved by catalytic electronic activation of the substrate. Catalyzed reversible conversion of the alcohol into a ketone causes electronic activation of the alkene, allowing nucleophilic addition to occur (see scheme).

Nucleophile Addition an Cycloalkenole durch katalytische elektronische Aktivierung des Substrats: Die katalysierte reversible Umwandlung des Alkohols in ein Keton erzeugt eine elektronisch aktivierte Alkenfunktion und erleichtert damit die Addition eines Nucleophils (siehe Schema).

Platinum complexes of phosphino-oxazoline ligands catalyse allylic alkylations with good yields and enantioselectivity (see diagram), but show interesting differences compared to similar palladium complexes. Highly regioselective alkylation of mono(alkyl)-substituted allylic acetates is observed when Cy₃P is used as ligand.