Incubation of the apoB2 subunit of Escherichia coli ribonucleotide reductase with Fe2+ and O2 produces native B2, which contains the tyrosyl radical-dinuclear iron cluster cofactor required for nucleotide reduction. The chemical mechanism of this reconstitution reaction was investigated by stopped-flow absorption spectroscopy and by rapid freeze-quench EPR (electron paramagnetic resonance) spectroscopy. Two novel intermediates have been detected in the reaction. The first exhibits a broad absorption band centered at 565 nanometers. Based on known model chemistry, this intermediate is proposed to be a mu-peroxodiferric complex. The second intermediate exhibits a broad absorption band centered at 360 nanometers and a sharp, isotropic EPR signal with g = 2.00. When the reaction is carried out with 57Fe2+, this EPR signal is broadened, demonstrating that the intermediate is an iron-coupled radical. Variation of the ratio of Fe2+ to B2 in the reaction and comparison of the rates of formation and decay of the intermediates to the rate of formation of the tyrosyl radical (.Y122) suggest that both intermediates can generate .Y122. This conclusion is supported by the fact that both intermediates exhibit an increased lifetime in a mutant B2 subunit (B2-Y122F) lacking the oxidizable Y122. Based on these kinetic and spectroscopic data, a mechanism for the reaction is proposed. Unlike reactions catalyzed by heme-iron peroxidases, oxygenases, and model complexes, the reconstitution reaction appears not to involve high-valent iron intermediates.

Equilibrium studies have been performed to determine the Brønsted acidity of [(C 6 F 5 ) 3 B(OH 2 )]‚ H 2 O, the aqua species that exists in acetonitrile solutions of B(C 6 F 5 ) 3 in the presence of water. NMR spectroscopic analysis of the deprotonation of [(C 6 F 5 ) 3 B(OH 2 )]‚H 2 O with 2,6-Bu t 2 C 5 H 3 N in acetonitrile allows a pK value of 8.6 to be determined for the equilibrium [(C 6 F 5 ) 3 B(OH 2 )]‚H 2 O a [(C 6 F 5 ) 3 B(OH)] -+ [H 3 O] + . On the basis of a calculated value for the hydrogen bond interaction in [(C 6 F 5 ) 3 B(OH 2 )]‚H 2 O, the pK a for (C 6 F 5 ) 3 B(OH 2 ) is estimated to be 8.4 in acetonitrile. Such a value indicates that (C 6 F 5 ) 3 B(OH 2 ) must be regarded as a strong acid, with a strength comparable to that of HCl in acetonitrile. Dynamic NMR spectroscopic studies indicate that the aqua and acetonitrile ligands in (C 6 F 5 ) 3 B(OH 2 ) and (C 6 F 5 ) 3 B(NCMe) are labile, with dissociation of H 2 O being substantially more facile than that of MeCN, by a factor of ca. 200 in rate constant at 300 K. Ab initio calculations were performed in the gas phase and with a dielectric solvent model to determine the strength of B-L bonds (L ) H 2 O, ROH, MeCN) and hydrogen bonds involving B-OH 2 and B-O(H)R derivatives.

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Many reactions of transition‐metal hydrides involve H* transfer. With olefins such transfer gives a radical cage, from which escape and collapse lead to product formation. Inverse isotope effects, second‐order kinetics independent of ligand concentration, and CIDNP are diagnostic for this mechanism. Many other reactions of transition‐metal hydrides occur by radical chain mechanisms, in which H* is abstracted by carbon‐centered radicals or by metal radicals. Reasonably accurate values are now available for the M‐H bond strengths of most of the common hydrides, and these values help rationalize the known H* transfer reactions of these hydrides. While the rates of certain H* transfer reactions have been measured by radical clock methods, the measurement of H* transfer rates to a substituted trityl radical has provided the first general comparison of the H* donor abilities of the various hydrides. These relative H* transfer rates are significantly affected by steric factors.

The thermodynamics and kinetics of all three cleavage modes for Rh-H, the transfer of H(-), H(+), or H•, have been studied for the Rh(III) hydride complex Cp*Rh(2-(2-pyridyl)phenyl)H (1a). The thermodynamic hydricity, ΔG°H(-), for 1a has been measured (49.5(1) kcal/mol) by heterolytic cleavage of H2 with Et3N in CH3CN. The transfer of H(-) from 1a to 1-(1-phenylethylidene)pyrrolidinium is remarkably fast (kH(-) = 3.5(1) × 10(5) M(-1) s(-1)), making 1a a very efficient catalyst for the ionic hydrogenation of iminium cations. The pKa of 1a in CH3CN has been measured as 30.3(2) with (tert-butylimino)tris(pyrrolidino)phosphorane (12), and the rate constant for H(+) transfer from 1a to 12 has been estimated (kH(+) = 5(1) × 10(-4) M(-1) s(-1)) from the half-life of the equilibration. Thus, 1a is a poor H(+) donor both thermodynamically and kinetically. However, 1a transfers H• to TEMPO smoothly, forming a stable Rh(II) radical Cp*Rh(2-(2-pyridyl)phenyl)• (14a) that can activate H2 at room temperature and 1 atm. The metalloradical 14a has a g value of 2.0704 and undergoes reversible one-electron reduction at -1.85 V vs Fc(+)/Fc in benzonitrile, implying a bond-dissociation enthalpy for the Rh-H bond of 1a of 58.2(3) kcal/mol--among the weakest Rh(III)-H bonds reported. The transfer of H• from 1a to Ar3C• (Ar = p-(t)BuC6H4) is fast, with kH• = 1.17(3) × 10(3) M(-1) s(-1). Thus, 1a is a good H(-) and H• donor but a poor H(+) donor, a combination that reflects the high energy of the Rh(I) anion [Cp*Rh(2-(2-pyridyl)phenyl)](-).

Catalysis by CpRu(P-P)H (where P-P is a chelating diphosphine) of the ionic hydrogenation of an iminium cation inolves (1) the transfer of H(-) to form an amine, (2) the coordination of H(2) to the resulting Ru cation, and (3) the transfer of H(+) from the coordinated dihydrogen to the amine formed in (1). With CpRu(dppe)H the principal Ru species during catalysis remains the hydride complex, and H(2) pressure has no effect on either the ee or the turnover frequency. Step (1), H(-) transfer, can be carried out stoichiometrically if the H(2) is replaced by a coordinating solvent. A methyl substituent on the Cp ring decreases the H(-) transfer rate and the turnover frequency slightly. Electron-donating substituents on the phosphine increase the H(-) transfer rate and increase the turnover frequency up to a point: eventually the hydride ligand (i.e., the one in CpRu(dmpe)H) becomes sufficiently basic to deprotonate the iminium cation to the corresponding enamine, and this pre-equilibrium competes with H(-) transfer. Ionic hydrogenation of enamines is possible when a Ru(H(2)) cation (i.e., [CpRu(dppm)(eta(2)-H(2))](+)) is used as the catalyst and the enamine is more basic than the product amine. Ionic hydrogenation of an alpha,beta-unsaturated iminium cation saturates both the C=C and the C=N bonds. A C=N bond is more reactive toward ionic hydrogenation than a C=C one, but in some cases (i.e., CH=CH(2)) the latter may compete with H(2) for a coordination site and decrease the turnover frequency.

The opening of epoxides typically requires electrophilic activation, and subsequent nucleophilic (SN2) attack on the less substituted carbon leads to alcohols with Markovnikov regioselectivity. We describe a cooperative catalysis approach to anti-Markovnikov alcohols by combining titanocene-catalyzed epoxide opening with chromium-catalyzed hydrogen activation and radical reduction. The titanocene enforces the anti-Markovnikov regioselectivity by forming the more highly substituted radical. The chromium catalyst sequentially transfers a hydrogen atom, proton, and electron from molecular hydrogen, avoiding a hydride transfer to the undesired site and resulting in 100% atom economy. Each step of the interconnected catalytic cycles was confirmed separately.

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