Competing nucleophilic displacement and radical-chain reduction in reactions of transition-metal hydride anions with alkyl bromides

Ash, Carlton E.; Hurd, P. W.; Darensbourg, Marcetta Y.; Newcomb, Martin

doi:10.1021/ja00245a021

Cited by 35 publications

(15 citation statements)

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“…Whilst direct displacement of the bromide by hydride is the prevalent mechanism in the reaction of this primary alkyl bromide, a radical chain mechanism involving hydrogen atom transfer from the metal hydride is also operative. This radical chain mechanism (S H 2 pathway) is the predominant pathway in reactions of these same anionic hydrides with sterically encumbered alkyl halides, where the S N 2 hydride displacement pathway is disfavored [28].…”

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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“…1.2 × 10 4 HMo(CO) 3 Cp * 6.5 × 10 3 HRe(CO) 5 2.0 × 10 3 HW(CO) 3 Cp * 1.9 × 10 3 HMo(CO) 3 Cp 3.8 × 10 2 HW(CO) 3 (C 5 H 4 Me) 2.5 × 10 2 cis -HMn(CO) 4 (PPh 3 ) 2.3 × 10 2 HSiEt 3 1.5 × 10 2 HW(CO) 3 Cp 7.6 × 10 1 HMn(CO) 5 5.0 × 10 1 HW(CO) 3 (C 5 H 4 CO 2 Me) 7.2 × 10 − 1 provided mechanistic insight that helped to explain this observation [27] . While the reaction of the metal hydrides with the primary alkyl halide proceeded mostly by nucleophilic displacement of the halide by the hydride, an alternate mechanism was found to be operative in the reactions with the hindered alkyl halides.…”

Section: Metal Hydridementioning

confidence: 93%

Molybdenum and Tungsten Catalysts for Hydrogenation, Hydrosilylation and Hydrolysis

Bullock

2010

Catalysis Without Precious Metals

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“…[52] (28) (29) A still more interesting quantitative assessment of the competing pathways (ionic H Ϫ transfer, S N 2, vs. radical chain mechanism, S H 2) was achieved by studying the reaction of [HM(CO) 5 ] Ϫ complexes with two mechanistic probes, namely 6-bromo-1-hexene and 4-bromo-3,3-dimethyl-1-butene (Scheme 4). [54] Scheme 4. Possible reduction products from mechanistic probes Computation of the results indicated the relative contributions of the S N 2 and S H 2 mechanisms given in Table 3.…”

Section: Reaction With Organic Halidesmentioning

confidence: 99%

“…For the above reaction, the reactivity order of comparable hydrides {[HCr(CO) 5 ] Ϫ Ͻ [HW(CO) 5 ] Ϫ Ͻ [HW(CO) 4 P(OMe) 3 ] Ϫ } is the same as that found for the hydride/halide displacement reactions with organic halides (vide supra). [54] The reaction of KHCr(CO) 5 with stoichiometric amounts of α,β-unsaturated ketones and esters has recently been studied. [56] The reaction with mesityl oxide was found to proceed rapidly (within 30 min) at room temperature to selectively afford the corresponding saturated ketone without significant formation of high molecular weight products [Equation (31)].…”

Section: Reactivity Towards C‫؍‬c Double Bondsmentioning

confidence: 99%

“…[51] The hydrogenation of ketones was found to be catalyzed by Cr(CO) 6 in the presence of NaOMe in methanol under 100 bar H 2 at 100Ϫ120°C [Equation (54)]. (54) Both aliphatic and aromatic ketones could be hydrogenated in Ͼ 98% yield within 3 h (Table 4). Aromatic aldehydes were also hydrogenated, whereas aliphatic ones proved to be prone to secondary reactions.…”

Section: Catalytic Hydrogenations Of Aldehydes and Ketonesmentioning

confidence: 99%

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