Mechanism of Ruthenium-Catalyzed Hydrogen Transfer Reactions. Concerted Transfer of OH and CH Hydrogens from an Alcohol to a (Cyclopentadienone)ruthenium Complex

Abstract

Kinetic studies of the ruthenium-catalyzed dehydrogenation of 1-(4-fluorophenyl)ethanol (4) by tetrafluorobenzoquinone (7) using the Shvo catalyst 1 at 70 degrees C show that the dehydrogenation by catalytic intermediate 2 is rate-determining with the rate = k[4][1](1/2) and with deltaH++ = 17.7 kcal mol(-1) and deltaS++ = -13.0 eu. The use of specifically deuterated derivative 4-CHOD and 4-CDOH gave individual isotope effects of k(CHOH)/k(CHOD) = 1.87 +/- 0.17 and k(CHOH)/k(CDOH) = 2.57 +/- 0.26, respectively. Dideuterated derivative 4-CDOD gave a combined isotope effect of k(CHOH)/k(CDOD) = 4.61 +/- 0.37. These isotope effects are consistent with a concerted transfer of both hydrogens of the alcohol to ruthenium species 2.

Catalysis at the Interface of Ruthenium Carbene and Ruthenium Hydride Chemistry: Organometallic Aspects and Applications to Organic Synthesis

“…[76] More recently, cyclopentadienyl complexes of ruthenium have attracted considerable interest as precatalysts. [77,78] Catalyzed transfer hydrogenation and dehydrogenation have been reviewed, [79] and especially enantioselective variants have attracted considerable attention. [80] The simplified mechanistic picture outlined in Scheme 20 is sufficient to highlight the basic principles of these catalytic transformations.…”

Racemization of Secondary Alcohols Catalyzed by Cyclopentadienylruthenium Complexes: Evidence for an Alkoxide Pathway by Fast β‐Hydride Elimination–Readdition

“…[6a, c, 24] We have previously proposed that the most likely pathway for the racemization of alcohols catalyzed by 3 a and 3 b involves b-hydride elimination from an alkoxide complex to give a h 3 -Ph 5 C 5 hydride ketone ruthenium complex (7) (step ii, Scheme 2) that undergoes reversible insertion (step iii, Scheme 2). [5,26] Thus, the ketone stays coordinated to the metal center and does not leave the coordination sphere.…”

Alcohol Dehydrogenation with a Dual Site Ruthenium, Boron Catalyst Occurs at Ruthenium

“…Our general strategy of devising a catalyst with a built-in Lewis acid to direct the metal to a particular substrate is analogous to these. Along these lines, the Shvo mechanism is similar to the Noyori systems and has been studied previously by Shvo [23], Casey [24,25], Bäckvall [6,26,27] us [28], and others. Alcohol oxidation with 1 has isotope effects on both C-H (k H /k D = 2.6) and O-H(k H /k D = 1.8), which indicates that both of these hydrogen atoms are transferred in (or possibly before) the rate-determining transition state ( Figure 1A) [27].…”

“…Along these lines, the Shvo mechanism is similar to the Noyori systems and has been studied previously by Shvo [23], Casey [24,25], Bäckvall [6,26,27] us [28], and others. Alcohol oxidation with 1 has isotope effects on both C-H (k H /k D = 2.6) and O-H(k H /k D = 1.8), which indicates that both of these hydrogen atoms are transferred in (or possibly before) the rate-determining transition state ( Figure 1A) [27]. Further, 1-catalyzed oxidation of 1-phenylethanol has a Hammett ρ value of −0.9, which is larger than expected for a transition state involving β-hydride elimination (ρ = −0.43) [29] or free-radical hydrogen atom transfer (ρ = −0.3) [30].…”