Large Primary Kinetic Isotope Effect Research Articles

Visible-Light Acceleration of H2 Evolution from Aqueous Solutions of Inorganic Hydrides Catalyzed by Gold-Transition-Metal Nanoalloys.

Production of hydrogen (H2) upon hydrolysis of inorganic hydrides potentially is a key step in green energy production. We find that visible-light irradiation of aqueous solutions of ammonia-borane (AB) or NaBH4 containing "click"-dendrimer-stabilized alloyed nanocatalysts composed of nanogold and another late transition-metal nanoparticle (LTMNP) highly enhances catalytic activity for H2 generation while also inducing alloy to Au core@M shell nanocatalyst restructuration. In terms of visible-light-induced acceleration of H2 production from both AB and NaBH4, the Au1Ru1 alloy catalysts show the most significant light-boosting effect. Au-Rh and Au-PtNPs are also remarkable with total H2 release time from AB and NaBH4 down to 1.3 min at 25 °C (AuRh), 3 times less than in the dark, and Co is the best earth-abundant metal alloyed with nanogold. This boosting effect is explained by the transfer of plasmon-induced hot electron from the Au atoms to the LTMNP atoms facilitating water O-H oxidative addition on the LTMNP surface, as shown by the large primary kinetic isotope effect kH/kD upon using D2O obtained for both AB and NaBH4. The second simultaneous and progressive effect of visible-light irradiation during these reactions, alloy to Au core@M shell restructuration, enhances the catalytic activity in the recycling, because, in the resulting Au core@M shell, the surface metal (such as Ru) is much more active than the original Au-containing alloy surface in dark reactions. There is no light effect on the rate of hydrogen production for the recycled nanocatalyst because of the absence of Au on the NP surface, but it is still very efficient in hydrogen release during four cycles because of the initial light-induced restructuration, although it is slightly less efficient than the original nanoalloy in the presence of light. The dendritic triazole coordination on each LTMNP surface appears to play a key role in these remarkable light-induced processes.

(CDC)–Rhodium-Catalyzed Hydroallylation of Vinylarenes and 1,3-Dienes with AllylTrifluoroborates

Catalytic site-selective hydroallylation of vinyl arenes and 1,3-dienes is reported. Transformations are promoted by a readily accessible bidentate carbodicarbene–rhodium complex and involve commercially available allyltrifluoroborates and an alcohol. The reaction is applicable to vinyl arenes and aryl- or alkyl-substituted 1,3-dienes (30 examples). Allyl addition products are generated in 40–78% yield and in up to >98:2 site selectivity. Reaction outcomes are consistent with the intermediacy of a Rh(III)–hydride generated by protonation of Rh(I) by an acid. A number of key mechanistic details of the reaction are presented: (1) Deuterium scrambling into the product and starting alkene indicates reversible Rh(III)–H migratory insertion. (2) A large primary kinetic isotope effect is observed. (3) With substituted allyltrifluoroborates (e.g., crotyl-BF3K) mixtures of site isomers are generated as a result of transmetalation followed by Rh–(allyl) complex equilibration, consequently disproving outer-sphere addition of the allyl nucleophile to Rh(III)–(η3-allyl). (4) Stereochemical analysis of a cyclohexadiene allyl addition product supports a syn Rh(III)–hydride addition. (5) A Hammett plot shows a negative slope. Finally, utility is highlighted by a iodocyclization and cross metathesis.

Primary Deuterium Kinetic Isotope Effects: A Probe for the Origin of the Rate Acceleration for Hydride Transfer Catalyzed by Glycerol-3-Phosphate Dehydrogenase.

Large primary deuterium kinetic isotope effects (1° DKIEs) on enzyme-catalyzed hydride transfer may be observed when the transferred hydride tunnels through the energy barrier. The following 1° DKIEs on kcat/Km and relative reaction driving force are reported for wild-type and mutant glycerol-3-phosphate dehydrogenase (GPDH)-catalyzed reactions of NADL (L = H, D): wild-type GPDH, ΔΔG⧧ = 0 kcal/mol, 1° DKIE = 1.5; N270A, 5.6 kcal/mol, 3.1; R269A, 9.1 kcal/mol, 2.8; R269A + 1.0 M guanidine, 2.4 kcal/mol, 2.7; R269A/N270A, 11.5 kcal/mol, 2.4. Similar 1° DKIEs were observed on kcat. The narrow range of 1° DKIEs (2.4–3.1) observed for a 9.1 kcal/mol change in reaction driving force provides strong evidence that these are intrinsic 1° DKIEs on rate-determining hydride transfer. Evidence is presented that the intrinsic DKIE on wild-type GPDH-catalyzed reduction of DHAP lies in this range. A similar range of 1° DKIEs (2.4–2.9) on (kcat/KGA, M–1 s–1) was reported for dianion-activated hydride transfer from NADL to glycolaldehyde (GA) [Reyes, A. C.; Amyes, T. L.; Richard, J. P. J. Am. Chem. Soc.2016, 138, 14526–14529]. These 1° DKIEs are much smaller than those observed for enzyme-catalyzed hydrogen transfer that occurs mainly by quantum mechanical tunneling. These results support the conclusion that the rate acceleration for GPDH-catalyzed reactions is due to the stabilization of the transition state for hydride transfer by interactions with the protein catalyst. The small 1° DKIEs reported for mutant GPDH-catalyzed and for wild-type dianion-activated reactions are inconsistent with a model where the dianion binding energy is utilized in the stabilization of a tunneling ready state.

Primary deuterium kinetic isotope effects prolong drug release and polymer biodegradation in a drug delivery system

We have developed a chemically-controlled drug delivery system in which a drug is covalently attached via a carbamate to hydrogel microspheres using a β-eliminative linker; rate-determining proton removal from a CH bond adjacent to an electron withdrawing group results in a β-elimination to cleave the carbamate and release the drug. After subcutaneous injection of the hydrogel-drug conjugate, the drug is slowly released into the systemic circulation and acquires an elimination t1/2,β that matches the t1/2 of linker cleavage. A similar β-eliminative linker with a slower cleavage rate is installed into crosslinks of the polymer to trigger gel degradation after drug release. We have now prepared β-eliminative linkers that contain deuterium in place of the hydrogen whose removal initiates cleavage. In vitro model systems of drug release and degelation show large primary deuterium kinetic isotope effects of kH/kD ~ 2.5 to 3.5. Using a deuterated linker to attach the peptide octreotide to hydrogel-microspheres, the in vivo t1/2,β of the drug was increased from ~1.5 to 4.5 weeks in the rat. Similarly, the in vivo time to biodegradation of hydrogels with deuterium-containing crosslinks could be extended ~2.5-fold compared to hydrogen-containing counterparts. Thus, the use of primary deuterium kinetic isotope effects in a single platform technology can control rates of β-elimination reactions in drug release and polymer biodegradation rates.

Theoretical Investigation into Rate-Determining Factors in Electrophilic Aromatic Halogenation.

The halogenation of monosubstituted benzenes in aqueous solvent was studied using density functional theory at the PCM-M06-2 X/6-311G(d,p) level. The reaction with Cl2 begins with the formation of C atom coordinated π-complex and is followed by the formation of the σ-complex, which is rate-determining. The final part proceeds via the abstraction of the proton by a water molecule or a weak base. We evaluated the use of the σ-complex as a model for the rate-determining transition state (TS) and found that this model is more accurate the later the TS comes along the reaction coordinate. This explains the higher accuracy of the model for halogenations (late TS) compared to nitrations (early TS); that is, the more deactivated the substrate the later the TS. The halogenation with Br2 proceeds with a similar mechanism as the corresponding chlorination, but the bromination has a very late rate-determining TS that is similar to the σ-complex in energy. The iodination with ICl follows a different mechanism than chlorination and bromination. After the formation of the π-complex, the reaction proceeds in a concerted manner without a σ-complex. This reaction has a large primary hydrogen kinetic isotope effect in agreement with experimental observations.

Structure and Enzymatic Properties of an Unusual Cysteine Tryptophylquinone-Dependent Glycine Oxidase from Pseudoalteromonas luteoviolacea.

Glycine oxidase from Pseudoalteromonas luteoviolacea (PlGoxA) is a cysteine tryptophylquinone (CTQ)-dependent enzyme. Sequence analysis and phylogenetic analysis place it in a newly designated subgroup (group IID) of a recently identified family of LodA-like proteins, which are predicted to possess CTQ. The crystal structure of PlGoxA reveals that it is a homotetramer. It possesses an N-terminal domain with no close structural homologues in the Protein Data Bank. The active site is quite small because of intersubunit interactions, which may account for the observed cooperativy toward glycine. Steady-state kinetic analysis yielded the following values: kcat = 6.0 ± 0.2 s-1, K0.5 = 187 ± 18 μM, and h = 1.77 ± 0.27. In contrast to other quinoprotein amine dehydrogenases and oxidases that exhibit anomalously large primary kinetic isotope effects on the rate of reduction of the quinone cofactor by the amine substrate, no significant primary kinetic isotope effect was observed for this reaction of PlGoxA. The absorbance spectrum of glycine-reduced PlGoxA exhibits features in the range of 400-650 nm that have not previously been seen in other quinoproteins. Thus, in addition to the unusual structural features of PlGoxA, the kinetic and chemical reaction mechanisms of the reductive half-reaction of PlGoxA appear to be distinct from those of other amine dehydrogenases and amine oxidases that use tryptophylquinone and tyrosylquinone cofactors.

Diastereoselective Lithiation of N-Benzyl Pyrroloimidazolones Derived from l-Proline Hydantoin

An N-benzyl pyrroloimidazolone derived from l-proline hydantoin undergoes asymmetric lithiation with n-BuLi/TMEDA in toluene to give products of electrophile quench (E+) that range from 87:13 to 91:9 diastereomeric ratio (dr). All products appear to have the same relative stereochemistry as determined by transmetalation of benzylic stannanes, which gave identical major diastereomers for several products as to what was observed by direct lithiation–substitution of the starting material. X-Ray crystallography of the major diastereomer of the benzophenone adduct established (R)-configuration at the benzylic center, i.e., anti stereochemistry with respect to the imidazolone. Lithiation of a selectively deuterated analogue of the starting material according to the optimized conditions, followed by benzophenone quench, gave diastereomeric products with far lower selectivity (53:47 dr) than lithiation of the non-deuterated analogue (91:9 dr) owing to a large primary kinetic isotope effect. These preliminary results imply that metalation of the N-benzyl pyrroloimidazolone may follow an asymmetric deprotonation pathway to give a benzylic carbanion that retains its configuration during electrophile quench.

Kinetic Analysis and Probing with Substrate Analogues of the Reaction Pathway of the Nitrile Reductase QueF from Escherichia coli

The enzyme QueF catalyzes a four-electron reduction of a nitrile group into an amine, the only reaction of this kind known in biology. In nature, QueF converts 7-cyano-7-deazaguanine (preQ0) into 7-aminomethyl-7-deazaguanine (preQ1) for the biosynthesis of the tRNA-inserted nucleoside queuosine. The proposed QueF mechanism involves a covalent thioimide adduct between preQ0 and a cysteine nucleophile in the enzyme, and this adduct is subsequently converted into preQ1 in two NADPH-dependent reduction steps. Here, we show that the Escherichia coli QueF binds preQ0 in a strongly exothermic process (ΔH = −80.3 kJ/mol; −TΔS = 37.9 kJ/mol, Kd = 39 nm) whereby the thioimide adduct is formed with half-of-the-sites reactivity in the homodimeric enzyme. Both steps of preQ0 reduction involve transfer of the 4-pro-R-hydrogen from NADPH. They proceed about 4–7-fold more slowly than trapping of the enzyme-bound preQ0 as covalent thioimide (1.63 s−1) and are thus mainly rate-limiting for the enzyme's kcat (=0.12 s−1). Kinetic studies combined with simulation reveal a large primary deuterium kinetic isotope effect of 3.3 on the covalent thioimide reduction and a smaller kinetic isotope effect of 1.8 on the imine reduction to preQ1. 7-Formyl-7-deazaguanine, a carbonyl analogue of the imine intermediate, was synthesized chemically and is shown to be recognized by QueF as weak ligand for binding (ΔH = −2.3 kJ/mol; −TΔS = −19.5 kJ/mol) but not as substrate for reduction or oxidation. A model of QueF substrate recognition and a catalytic pathway for the enzyme are proposed based on these data.

Understanding the Mechanisms of Unusually Fast HH, CH, and CC Bond Reductive Eliminations from Gold(III) Complexes

Carbon-carbon bond reductive elimination from gold(III) complexes are known to be very slow and require high temperatures. Recently, Toste and co-workers have demonstrated extremely rapid CC reductive elimination from cis-[AuPPh3 (4-F-C6 H4 )2 Cl] even at low temperatures. We have performed DFT calculations to understand the mechanistic pathway for these novel reductive elimination reactions. Direct dynamics calculations inclusive of quantum mechanical tunneling showed significant contribution of heavy-atom tunneling (>25 %) at the experimental reaction temperatures. In the absence of any competing side reactions, such as phosphine exchange/dissociation, the complex cis-[Au(PPh3 )2 (4-F-C6 H4 )2 ](+) was shown to undergo ultrafast reductive elimination. Calculations also revealed very facile, concerted mechanisms for HH, CH, and CC bond reductive elimination from a range of neutral and cationic gold(III) centers, except for the coupling of sp(3) carbon atoms. Metal-carbon bond strengths in the transition states that originate from attractive orbital interactions control the feasibility of a concerted reductive elimination mechanism. Calculations for the formation of methane from complex cis-[AuPPh3 (H)CH3 ](+) predict that at -52 °C, about 82 % of the reaction occurs by hydrogen-atom tunneling. Tunneling leads to subtle effects on the reaction rates, such as large primary kinetic isotope effects (KIE) and a strong violation of the rule of the geometric mean of the primary and secondary KIEs.

Carboxylate-assisted C(sp³)-H activation in olefin metathesis-relevant ruthenium complexes.

The mechanism of C–H activation at metathesis-relevant ruthenium(II) benzylidene complexes was studied both experimentally and computationally. Synthesis of a ruthenium dicarboxylate at a low temperature allowed for direct observation of the C–H activation step, independent of the initial anionic ligand-exchange reactions. A first-order reaction supports an intramolecular concerted metalation–deprotonation mechanism with ΔG⧧298K = 22.2 ± 0.1 kcal·mol–1 for the parent N-adamantyl-N′-mesityl complex. An experimentally determined ΔS⧧ = −5.2 ± 2.6 eu supports a highly ordered transition state for carboxylate-assisted C(sp3)–H activation. Experimental results, including measurement of a large primary kinetic isotope effect (kH/kD = 8.1 ± 1.7), agree closely with a computed six-membered carboxylate-assisted C–H activation mechanism where the deprotonating carboxylate adopts a pseudo-apical geometry, displacing the aryl ether chelate. The rate of cyclometalation was found to be influenced by both the electronics of the assisting carboxylate and the ruthenium ligand environment.

Kinetic and Mechanistic Studies of Carbon-to-Metal Hydrogen Atom Transfer Involving Os-Centered Radicals: Evidence for Tunneling

We have investigated the kinetics of novel carbon-to-metal hydrogen atom transfer reactions, in which homolytic cleavage of a C-H bond is accomplished by a single metal-centered radical. Time-resolved IR spectroscopic measurements revealed efficient hydrogen atom transfer from xanthene, 9,10-dihydroanthracene, and 1,4-cyclohexadiene to Cp(CO)2Os(•) and (η(5)-(i)Pr4C5H)(CO)2Os(•) radicals, formed by photoinduced homolysis of the corresponding osmium dimers. The rate constants for hydrogen abstraction from these hydrocarbons are in the range 1.5 × 10(5) M(-1) s(-1) to 1.7 × 10(7) M(-1) s(-1) at 25 °C. For the first time, kinetic isotope effects for carbon-to-metal hydrogen atom transfer were determined. Large primary deuterium kinetic isotope effects of 13.4 ± 1.0 and 16.8 ± 1.4 were observed for the hydrogen abstraction from xanthene to form Cp(CO)2OsH and (η(5)-(i)Pr4C5H)(CO)2OsH, respectively, at 25 °C. Temperature-dependent measurements of the kinetic isotope effects over a 60 °C temperature range were carried out to obtain the difference in activation energies (E(D) - E(H)) and the pre-exponential factor ratio (A(H)/A(D)). For hydrogen atom transfer from xanthene to (η(5)-(i)Pr4C5H)(CO)2Os(•), the (E(D) - E(H)) = 3.3 ± 0.2 kcal mol(-1) and A(H)/A(D) = 0.06 ± 0.02 values suggest a quantum mechanical tunneling mechanism.

H–D kinetic isotope effects of alcohol electrooxidation on Au, Pd and Pt electrodes in alkaline solutions

The identification of rate-determining step (RDS) is vital for understanding reaction mechanisms and designing advanced catalysts. Here, we systemically studied the H–D kinetic isotope effects (KIEs) on electrooxidation of ethanol, isopropanol, and ethylene glycol (EG) on Au, Pd and Pt electrodes in alkaline solution by combining cyclic voltammetry and in situ FTIR spectroscopy. Large primary KIEs are observed on Au electrode: 8.4 for ethanol, 6.2 for EG and 7.9 for isopropanol when α-H of alcohols are substituted by deuterium, but much lower KIEs (<3.2) were observed on Pt, and the values on Pd are between Au and Pt. According to the criteria (KIEs=6–10) for CH (D) breaking/forming as RDS, we conclude that the cleavage of the α-H from CH bond is the RDS for alcohol electrooxidation on Au, but not on Pt. On Pd, the RDS depends on the structure of alcohols. The difference can be rationally correlated with the binding affinity of hydrogen on precious metal surfaces.

The substrate oxidation mechanism of pyranose 2‐oxidase and other related enzymes in the glucose–methanol–choline superfamily

Enzymes in the glucose-methanol-choline (GMC) oxidoreductase superfamily catalyze the oxidation of an alcohol moiety to the corresponding aldehyde. In this review, the current understanding of the sugar oxidation mechanism in the reaction of pyranose 2-oxidase (P2O) is highlighted and compared with that of other enzymes in the GMC family for which structural and mechanistic information is available, including glucose oxidase, choline oxidase, cholesterol oxidase, cellobiose dehydrogenase, aryl-alcohol oxidase, and pyridoxine 4-oxidase. Other enzymes in the family that have been newly discovered or for which less information is available are also discussed. A large primary kinetic isotope effect was observed for the flavin reduction when 2-d-D-glucose was used as a substrate, but no solvent kinetic isotope effect was detected for the flavin reduction step. The reaction of P2O is consistent with a hydride transfer mechanism in which there is stepwise formation of d-glucose alkoxide prior to the hydride transfer. Site-directed mutagenesis of P2O and pH-dependence studies indicated that His548 is a catalytic base that facilitates the deprotonation of C2-OH in D-glucose. This finding agrees with the current mechanistic model for aryl-alcohol oxidase, glucose oxidase, cellobiose dehydrogenase, methanol oxidase, and pyridoxine 4-oxidase, but is different from that of cholesterol oxidase and choline oxidase. Although all of the GMC enzymes share similar structural folding and use the hydride transfer mechanism for flavin reduction, they appear to have subtle differences in the fine-tuned details of how they catalyze substrate oxidation.

Mechanistic Investigations of a Stable, Highly Active, Extremely Sterically Shielded Molecular Gold Catalyst

AbstractAn N‐heterocyclic carbene gold complex IPr**AuNTf2 has been synthesized, spectroscopically investigated, structurally characterized, and used as a highly active and stable catalyst in the Hashmi phenol synthesis (IPr**=1,3‐di‐p‐tolylimidazol‐2‐ylidene with four di‐tert‐butylbenzhydryl ortho substituents, Tf=trifluoromethansulfonyl). A side reaction comprises an irreversible arene oxide ring opening with subsequent 1,2 methyl shift. The advantage of the steric demand of the ancillary ligand was explained by higher equilibrium concentrations of the cationic gold species, the circumvention of inactive dinuclear intermediates, and the inhibition of catalyst decomposition pathways. The methanol addition–hydration of alkynes featured a turnover‐limiting proton transfer step of an alkenylgold catalyst resting state, indicated by a large primary kinetic isotope effect and an alkyne competition experiment.

In alkaline media, Fremy’s salt oxidizes alkanols by a hydrogen atom transfer mechanism

In aqueous alkali, Fremy’s salt (potassium nitrosodisulfonate dimer), homolyses nearly exclusively to the monomer radical anion, nitrosodisulfonate (NDS). In this media, NDS almost quantitatively oxidizes benzyl alcohol (PhCH 2OH) to benzaldehyde (PhCHO), itself being reduced to hydroxylamine disulfonate (HNDS). The reaction is very nearly first-order in [NDS], [alkanol] and in [OH −]. However, with progressive addition of HNDS, decay kinetics of NDS gradually deviates from first-order. Ultimately, with sufficient excess of HNDS, the reaction becomes second-order in [NDS]. The consumption ratio, (ΔPhCH 2OH]/Δ[NDS]), is ∼2. PhCD 2OH manifests a large primary kinetic isotope effect ( k H/ k D = 11.6). Substituted benzyl alcohols (RBzCH 2OH) with R-groups withdrawing electron density from the O–H bond accelerated the reaction; those with R-groups donating electron density to the O–H bond retarded the reaction. The conversion of 2-propanol to 2-propanone is much slower compared to that of benzyl alcohol to benzaldehyde. An alpha-H atom transfer mechanism seems logical.

Quantitative Production of Compound I from a Cytochrome P450 Enzyme at Low Temperatures. Kinetics, Activation Parameters, and Kinetic Isotope Effects for Oxidation of Benzyl Alcohol

Cytochrome P450 enzymes are commonly thought to oxidize substrates via an iron(IV)-oxo porphyrin radical cation transient termed Compound I, but kinetic studies of P450 Compounds I are essentially nonexistent. We report production of Compound I from cytochrome P450 119 (CYP119) in high conversion from the corresponding Compound II species at low temperatures in buffer mixtures containing 50% glycerol by photolysis with 365 nm light from a pulsed lamp. Compound I was studied as a reagent in oxidations of benzyl alcohol and its benzylic mono- and dideuterio isotopomers. Pseudo-first-order rate constants obtained at -50 degrees C with concentrations of substrates between 1.0 and 6.0 mM displayed saturation kinetics that gave binding constants for the substrate in the Compound I species (K(bind)) and first-order rate constants for the oxidation reactions (k(ox)). Representative results are K(bind) = 214 M(-1) and k(ox) = 0.48 s(-1) for oxidation of benzyl alcohol. For the dideuterated substrate C(6)H(5)CD(2)OH, kinetics were studied between -50 and -25 degrees C, and a van't Hoff plot for complexation and an Arrhenius plot for the oxidation reaction were constructed. The H/D kinetic isotope effects (KIEs) at -50 degrees C were resolved into a large primary KIE (P = 11.9) and a small, inverse secondary KIE (S = 0.96). Comparison of values extrapolated to 22 degrees C of both the rate constant for oxidation of C(6)H(5)CD(2)OH and the KIE for the nondeuterated and dideuterated substrates to values obtained previously in laser flash photolysis experiments suggested that tunneling could be a significant component of the total rate constant at -50 degrees C.

Heterocycle Formation via Palladium-Catalyzed Intramolecular Oxidative C−H Bond Functionalization: An Efficient Strategy for the Synthesis of 2-Aminobenzothiazoles

N-arylthioureas are converted to 2-aminobenzothiazoles via intramolecular C-S bond formation/C-H functionalization utilizing an unusual cocatalytic Pd(PPh(3))(4)/MnO(2) system under an oxygen atmosphere at 80 degrees C. This method eliminates the need for an ortho-halo substituted precursor, instead achieving direct functionalization of the ortho-aryl C-H bond. Mechanistic observations, including a large intramolecular primary kinetic isotope effect of 5.9, reveal a reaction pathway inconsistent with an electrophilic palladation mechanism.

Accelerated Ligand Metalation in a β-Diketiminato Scandium Dimethyl Complex Activated with Bis(pentafluorophenyl)borane

Equimolar reactions of LScMe2 (L = (Ar)NC(tBu)CHC(tBu)N(Ar); Ar = 2,6-iPr2-C6H3) and HB(C6F5)2 proceed through an isolable ion pair 2 to a metalated scandium borate 3 with loss of methane. Multinuclear NMR experiments confirm the proposed structure, which was also verified by synthesis via alternative routes and derivatization. Deuterium labeling studies offer insight into the mechanism of methane loss, which occurs through C−H activation of an abstracted methide rather than the direct intramolecular metalation commonly observed for β-diketiminato scandium alkyl complexes. A large primary kinetic isotope effect of kH/kD = 8.7(6) was observed for the decomposition of 2, which corroborates the proposed mechanism while also implicating a highly reactive four-membered scandocycle intermediate, LSc((H)CH2B(C6F5)2). Reaction of 2 equiv of HB(C6F5)2 with LScR2 yields μ2-hydridoborate complexes [LScCH3][(μ-H)2B(C6F5)2] (5), while 4 equiv of borane react to afford the bis-μ2-hydridoborate complex [LSc][(μ-H)2B(C6F...

Synthesis and mechanistic investigations of the decomposition of β-hydrogen containing W(VI) dialkyl complexes: β-H elimination vs. β-H abstraction

The dialkyl complexes containing β-hydrogen atoms:W(NPh)[ o-(Me 3SiN) 2C 6H 4]R 2 (R = Et, n-Pr, i-Bu, CH 2CH 2Ph) have been prepared.These compounds are indefinitely stable in solution at room temperature in the absence of air and moisture.The diethyl complex reacts with PMe 3 at room temperature ( t 1/2 = 12 min) to give ethane and the W(IV) ethylene complex, W(NPh)[ o-(Me 3SiN) 2C 6H 4](η 2-CH 2CH 2)(PMe 3) 2.A mechanistic study of this reaction has revealed that the reaction is first order in W and PMe 3, with Δ H ‡ = 5.1(0.6) kcal mol −1 and Δ S ‡ = −54.9(2.0) cal mol −1 K −1.A large primary kinetic isotope effect ( k H/ k D = 6.1(0.5) at 20 °C) was observed for the transformation.These observations are consistent with the reaction proceeding via direct H transfer from the β-carbon of one ethyl group to the α-carbon of the other ethyl group.The (bis)phenethyl complex reacts with PMe 3 at elevated temperature to give ethylbenzene and the W(IV) styrene complex, W(NPh)[ o-(Me 3SiN) 2C 6H 4](η 2-CH 2CHPh)(PMe 3) 2.Significantly different activation parameters (Δ H ‡ = 13.7(0.8) kcal mol −1 and Δ S ‡ = −36.7(2.3) cal mol −1 K −1) and a phosphine concentration dependent kinetic isotope effect ( k H/ k D = 6.4(5), [PMe 3] = 0 M; k H/ k D = 4.3(3), [PMe 3] = 0.4 M; k H/ k D = 2.4(4), [PMe 3] = 0.8 M; extrapolated to room temperature) were observed, suggesting two competing pathways (β-hydride elimination and β-H abstraction) are accountable for the transformation.

1,10-Phenanthroline catalysis of the chromium(VI) oxidation of alcohols: a kinetic and mechanistic study

Several primary, secondary and tertiary alcohols are oxidised in acidic solutions using CrVIas an oxygen donor and 1,10-phenanthroline (Phen) as catalyst. A large primary kinetic isotope effect, kie(kH/kD∼ 10) suggests a linear transition state in the noncatalytic system, whereas a moderate kinetic isotope effect observedin the catalytic pathway can be associated with a cyclic transition state. The kD2O/kH2Oratio does not seem to indicate any significant solvent isotope effect. CrIII inhibition, reflected in the firstorder plots, can be explained by exchange reactions. PhCH2OH reacts ca. 7 times faster than MeOH. Moderately negative ρ* values indicate ahydride abstraction mechanism. Imperfect correlation is observed in the LFER between catalytic and noncatalytic reaction systems.Enhanced reactivity has been observed in the presence of 2,2′-bipyridyl, whereas the catalytic activity is insignificant with picolinicacid, imidazole and oxalic acid. Consideration of the kinetic behaviour of catalytic and noncatalytic reaction pathways provides valuable mechanistic insight into the systems studied.