Cationic rhodium(I)/BINAP complex-catalyzed isomerization of secondary propargylic alcohols to alpha,beta-enones

We have developed a cationic rhodium(I)/BINAP complex-catalyzed isomerization of secondary propargylic alcohols to alpha,beta-enones. The asymmetric variant of this reaction, a kinetic resolution of secondary propargylic alcohols, was also developed with good selectivity. The mechanistic study revealed that the isomerization proceeds through intramolecular 1,3- and 1,2-hydrogen migration pathways. [reaction: see text]     

CpRu(PN) complex-catalyzed isomerization of allylic alcohols and its application to the asymmetric synthesis of muscone

Highly efficient isomerization of allylic alcohols into saturated carbonyls is accomplished using the catalyst system of Cp*RuCl[Ph2P(CH2)2NH2-kappa2-P,N]-KOt-Bu (Cp* = eta5-C5(CH3)5) under mild conditions. Mechanistic consideration based on isotope-labeling experiments indicated the present reaction is applicable to the asymmetric isomerization of racemic sec-allylic alcohols with a prochiral olefin via dynamic kinetic resolution. A concise asymmetric synthesis of muscone has been achieved, where the asymmetric isomerization using an optically active ligand is a key reaction.     

Oxazaborolidine-catalyzed enantioselective reduction of alpha-methylene ketones to allylic alcohols

Oxazaborolidine-catalyzed enantioselective reduction of alpha-methylene ketones was efficiently carried out by using borane-diethylaniline as a stoichiometric reducing agent. The combination of this method and subsequent hydrogenation of thus-formed allylic alcohol improved stereoselectivity in the reduction of 24-oxocholesteryl ester to 24-(R)-hydroxycholesteryl ester.     

Direct, enantioselective iridium-catalyzed allylic amination of racemic allylic alcohols

The direct route: Iridium-catalyzed direct conversion of branched allylic alcohols into enantioenriched branched primary allylic amines is highly regio- and enantioselective (see scheme; coe=cyclooctene).     

Synthesis of aliphatic ketones from allylic alcohols through consecutive isomerization and chelation-assisted hydroacylation by a rhodium catalyst

An allylic alcohol, utilized as a precursor for an aliphatic aldehyde, reacted with olefins to afford aliphatic ketones in the presence of RhCl(PPh(3))(3), 2-amino-4-picoline, aniline, and benzoic acid through a tandem reaction of an isomerization and a chelation-assisted hydroacylation.     

A versatile new catalyst for the enantioselective isomerization of allylic alcohols to aldehydes: scope and mechanistic studies.

A new planar-chiral bidentate phosphaferrocene ligand (2) has been synthesized and structurally characterized. The derived rhodium complex, [Rh(cod)(2)]BF(4), serves as an effective catalyst for asymmetric isomerizations of allylic alcohols to aldehydes, furnishing improved yields, scope, and enantioselectivities relative to previously reported methods. The catalyst is air-stable and can be recovered at the end of the reaction. Mechanistic studies establish that the isomerization proceeds via an intramolecular 1,3-hydrogen migration and that the catalyst differentiates between the enantiotopic C1 hydrogens.     

Iridium-catalyzed isomerization of primary allylic alcohols under mild reaction conditions

The isomerization of primary allylic alcohols into the corresponding aldehydes has been accomplished using an analogue of Crabtree’s iridium hydrogenation catalyst and by adequately tuning the experimental conditions. A wide range of substrates is converted quantitatively into the desired aldehyde at room temperature in expedient reaction times by using catalyst loading as low as 0.25 mol %.     

Ruthenium-catalyzed reduction of allylic alcohols: an efficient isomerization/transfer hydrogenation tandem process

A simple and highly efficient method for the selective reduction of the C=C bond in allylic alcohols has been developed using the ruthenium(II) catalyst [{RuCl(mu-Cl)(eta(6)-C(6)Me(6)}2].     

Cp2TiCl-promoted isomerization of trisubstituted epoxides to exo-methylene allylic alcohols on carvone derivatives

The ring-opening reaction of trisubstituted epoxides promoted by Cp2TiCl led to exo-methylene allylic alcohols as major compounds when 0.5 M solutions of the epoxides were added to 0.1 M solutions of the reagent at room temperature in THF. In most cases, the allylic alcohols were contaminated with saturated alcohols. Normal and reverse addition modes led to the alternate product being favored. The different stereochemical outcome of cis- and trans-epoxy acetates is rationalized in terms of mechanistically biased elimination processes.     

A versatile route to silylsubstituted ketones by rhodium catalyzed isomerization of silylated allyl alcohols

The rhodium catalyzed isomerization of α-, β-, and γ-silylated allyl alcohols has been successfully applied to the selective synthesis of acylsilane, α-silyl ketones, and β-silyl ketones, respectively.     

Pentamethylcyclopentadienyl ruthenium: an efficient catalyst for the redox isomerization of functionalized allylic alcohols into carbonyl compounds

The catalytic activity of the ruthenium(II) complex [RuCp^∗(CH₃CN)₃][PF₆] 1 in the transposition of allylic alcohols into carbonyl compounds, in acetonitrile, is reported. This catalyst has proven to be able to catalyze the transformation of poorly reactive and/or functionalized substrates under smooth conditions.     

Nickel-catalyzed enantioselective hydrovinylation of silyl-protected allylic alcohols: An efficient access to homoallylic alcohols with a chiral quaternary center

Asymmetric hydrovinylation of silyl-protected allylic alcohols catalyzed by nickel complexes of chiral spiro phosphoramidite ligands was developed. A series of homoallylic alcohols with a chiral quaternary center were produced in high yields (up to 97%) and high enantioselectivities (up to 95% ee). The reaction provides an efficient method for preparing bifunctional compounds with a chiral quaternary carbon center.     

Highly efficient rhodium/monodentate phosphoramidite catalyst and its application in the enantioselective hydrogenation of enamides and alpha-dehydroamino acid derivatives

An easily prepared and highly efficient monodentate phosphoramidite ligand derived from BINOL, (S)-2,2'-O,O-(1,1'-binaphthyl)-dioxo-N,N-diethylphospholidine, was examined in the hydrogenation of both enamides and alpha-dehydroamino acid derivatives. The catalyst provided remarkably high enantioselectivities (up to 99.6% ee for enamides and >99.9% ee for alpha-dehydroamino acid derivatives).     

Chemoenzymatic dynamic kinetic resolution of allylic alcohols: a highly enantioselective route to acyloin acetates

Dynamic kinetic resolution (DKR) of a series of sterically hindered allylic alcohols has been conducted with Candida antarctica lipase B (CALB) and ruthenium catalyst 1. The optically pure allylic acetates obtained were subjected to oxidative cleavage to give the corresponding acylated acyloins in high yields without loss of chiral information.     

Theoretical study on the mechanism of iron carbonyls mediated isomerization of allylic alcohols to saturated carbonyls

The conversion of allylic alcohols to enols mediated by Fe(CO)(3) has been studied through density functional theoretical calculations. From the results obtained a complete catalytic cycle has been proposed in which the first intermediate is the [(allyl alcohol)Fe(CO)(3)] complex. This intermediate evolves to the [(enol)Fe(CO)(3)] complex through two consecutive 1,3-hydrogen shifts involving a pi-allyl hydride intermediate. The highest Gibbs energy transition state corresponds to the partial decoordination ot the enol ligand prior to the coordination of a new allyl alcohol molecule that regenerates the first intermediate. Alternative processes for the [(enol)Fe(CO)(3)] complex such as [Fe(CO)(3)]-mediated enol-aldehyde transformation and enol isomerization have also been considered. The results obtained show that the former process is unfavourable, whereas the enol isomerization may compete with the enol decoordination step of the catalytic cycle.     

Highly efficient redox isomerization of allylic alcohols at ambient temperature catalyzed by novel ruthenium-cyclopentadienyl complexes--new insight into the mechanism

A range of ruthenium cyclopentadienyl (Cp) complexes have been prepared and used for isomerization of allylic alcohols to the corresponding saturated carbonyl compounds. Complexes bearing CO ligands show higher activity than those with PPh3 ligands. The isomerization rate is highly affected by the substituents on the Cp ring. Tetra(phenyl)methyl-substituted catalysts rapidly isomerize allylic alcohols under very mild reaction conditions (ambient temperature) with short reaction times. Substituted allylic alcohols have been isomerized by employing Ru-Cp complexes. A study of the isomerization catalyzed by [Ru(Ph5Cp)(CO)2H] (14) indicates that the isomerization catalyzed by ruthenium hydrides partly follows a different mechanism than that of ruthenium halides activated by KOtBu. Furthermore, the lack of ketone exchange when the isomerization was performed in the presence of an unsaturated ketone (1 equiv), different from that obtained by dehydrogenation of the starting allylic alcohol, supports a mechanism in which the isomerization takes place within the coordination sphere of the ruthenium catalyst.