Ionic Hydrogenations

Abstract

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....

Catalytic Ionic Hydrogenation of Ketones by {[Cp*Ru(CO)2]2(μ-H)}+

“…We propose that the catalytic hydrogenation of CO bonds occurs from the dihydrogen complex [Cp*Ru(CO) 2 (η 2 -H 2 )] + OTf − through an ionic hydrogenation mechanism similar to that proposed in a series of Mo and W complexes that we previously reported. , (Scheme ). Heinekey and co-workers reported that protonation of the neutral Ru hydride Cp*Ru(CO) 2 H by HBF 4 ·Et 2 O at low temperature generates [Cp*Ru(CO) 2 (η 2 -H 2 )] + BF 4 − .…”

Hydrogen Transfer Catalysis beyond the Primary Coordination Sphere

“…For transition-metal-mediated ionic hydrogenations, H 2 is activated toward heterolytic cleavage through the formation of a M–(η 2 -H 2 ) complex. Sufficient electrophilic activation of the coordinated H 2 ligand facilitates heterolysis and transfer of H + and H – to a polar double bond (e.g., R 2 CO, R 2 CNR), which may occur in a stepwise or concerted fashion. ,− Importantly, the hydridic and protic character of the catalyst and reaction components must be well-matched near equilibrium values for efficient catalytic turnover (Figure , left). , …”

Thermodynamic Hydricity of Transition Metal Hydrides

“…Several other mechanistic studies of hydride transfer have been carried out, , including analysis of the ionic hydrogenation of CO bonds under stoichiometric , and catalytic ,− conditions. In ionic hydrogenation, the key product-forming step involves hydride transfer from a neutral metal hydride to a protonated ketone or aldehyde.…”