Selective reduction of acyl chlorides to aldehydes by anionic Group VIB transition metal hydrides

Kao, Shih-Han; Gaus, Paul L.; Youngdahl, Kay A.; Darensbourg, Marcetta Y.

doi:10.1021/om00088a031

Cited by 33 publications

(17 citation statements)

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“…Darensbourg et al have conducted extensive studies of the nucleophilic reactivity of a series of anionic metal carbonyl hydrides [24], which have been used for the reduction of alkyl halides [25], acyl chlorides [26], and ketones [27]. The kinetics of bromide displacement from n-BuBr by a series of anionic hydrides (Eq.…”

Section: Transition Metal Hydrides As Hydride Donorsmentioning

confidence: 99%

Ionic Hydrogenations

Bullock¹

2006

The Handbook of Homogeneous Hydrogenation

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7 Ionic Hydrogenations 7 Ionic Hydrogenations 154 Scheme 7.1sis documents the early progress in this field; a book gives further details [2]. As shown in Eq. (2), proton transfer to the alkene generates a carbenium ion, and hydride transfer from the hydrosilane generates the product. 2Ionic hydrogenations of C=C bonds generally work well only in cases where a tertiary or aryl-substituted carbenium ion can be formed through protonation of the C=C bond. Alkenes that give a tertiary carbenium ion upon protonation include 1,1-disubstituted, tri-substituted and tetra-substituted alkenes, and each of these are usually hydrogenated by ionic hydrogenation methods in high yields.The success of stoichiometric ionic hydrogenations is due to achieving a fine balance that favors the intended reactivity rather than any of several possible alternative reactions. The acid must be strong enough to protonate the unsaturated substrate, yet the reaction of the acid and the hydride should avoid producing H 2 too quickly under the reaction conditions. The commonly used pair of CF 3 CO 2 H and HSiEt 3 meets all these criteria.The very strong acid, CF 3 SO 3 H (triflic acid, abbreviated as HOTf) can be used in conjunction with HSiEt 3 for the hydrogenation of certain alkenes [3]. These reactions proceed cleanly in 5 minutes at -50 8C. This discovery was surprising, since a review of the use of CF 3 CO 2 H and HSiEt 3 had stated that "F F F stronger acids cannot be used in conjunction with silanes because they react" [1]. Indeed, rapid evolution of H 2 does occur when HOTf is added to HSiEt 3 in the absence of an alkene. The order of addition is important in the use of HOTf/HSiEt 3 for hydrogenation of C=C bonds, to ensure that acid-induced formation of H 2 is minimized. The addition of HOTf to a solution containing the alkene and the hydrosilane results in rapid and clean hydrogenation, but the reaction is still subject to the limitation of forming a tertiary carbenium ion.Another potential mechanistic complication is capture of the intermediate carbenium ion by the conjugate base of the acid. When CF 3 CO 2 H is used as the acid, this would lead to trifluoroacetate esters. Kursanov et al. showed that, under the reaction conditions for ionic hydrogenations, trifluoroacetate esters can be converted to the hydrocarbon product (Eq. (3)). 7.2 Stoichiometric Ionic Hydrogenations 155 7 Ionic Hydrogenations 7.2 Stoichiometric Ionic Hydrogenations 161 Scheme 7.3 Thermodynamic cycle for determination of the hydricity using heterolytic cleavage of hydrogen. 38 7.3.6 Catalytic Hydrogenation of Iminium Cations by Ru ComplexesNorton and coworkers found that catalytic enantioselective hydrogenation of the C=N bond of iminium cations can be accomplished using a series of Ru complexes with chiral diphosphine ligands such as Chiraphos and Norphos [68]. Even tetra-alkyl-substituted iminium cations can be hydrogenated by this method. These reactions were carried out with 2 mol.% Ru catalyst and 3.4-3.8 bar H 2 at room temperature in CH 2 Cl 2 solvent (Eq....

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Section: Transition Metal Hydrides As Hydride Donorsmentioning

confidence: 99%

Ionic Hydrogenations

Bullock¹

2006

The Handbook of Homogeneous Hydrogenation

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“…[42] Such selective reductions have previously been observed using [HFe(CO) 4 ] Ϫ [43] and [CpV(CO) 3 H] Ϫ . [44] (20)…”

Section: Reaction With Acyl Chloridesmentioning

confidence: 56%

“…[42] Further studies allowed a determination of the scope and limitations of the stoichiometric reduction of aldehydes and ketones. [47] [PPN] ϩ [HCr(CO) 5 ] Ϫ reacts rapidly with paraformaldehyde in THF (as shown by IR analysis) to yield methanol (98%, 0.5 h) after hydrolysis with a Brønsted acid (AcOH or H 2 O).…”

Section: Reaction With Aldehydes and Ketonesmentioning

confidence: 99%

Pentacarbonylhydridochromates M+[HCr(CO)5]−: Reactivity in Organic Synthesis and Homogeneous Catalysis

Brunet

2000

Eur. J. Inorg. Chem.

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“…viously investigated by Darensbourg et al [6Ϫ8] This reagent Treating cyclohexanone with 1 in THF at room temperahas been shown to reduce selectively acid chlorides into al-ture for 24 h resulted in 50% reduction (GC analysis) to dehydes without further reduction to alcohols. [7] cyclohexanol after hydrolysis of the reaction medium. [PPN] ϩ [HCr(CO) 5 ] Ϫ itself is also inactive towards ketones, but becomes an efficient reducing agent in the presence of one equivalent of acetic acid (H ϩ assistance and in situ protonolysis).…”

Section: Resultsmentioning

confidence: 99%