The carbon dioxide (CO2) capture and utilization has attracted a great attention in organic synthesis. Herein, an unpresented transient stabilization effect (TSE) of CO2 is disclosed and well applied ...to the electrochemical hydrogenation of azo compounds to hydrazine derivatives. Mechanistic experiments and computational studies imply that CO2 can capture azo radical anion intermediates to protect the hydrogenation from potential degradation reactions, and is finally released through decarboxylation. The promotion effect of CO2 was further demonstrated to work in the preliminary study of electrochemical reductive coupling of α‐ketoesters to vicinal diol derivatives. For the electrochemical reductive reactions mentioned above, CO2 is indispensable. The presented results shed light on a different usage of CO2 and could inspire novel experimental design by using CO2 as a transient protecting group.
A new transient stabilization effect (TSE) of CO2 is identified and applied to the electrochemical hydrogenation of azo compounds and the reductive coupling of α‐ketoesters. The TSE influence of CO2 means that overactive intermediates can be captured by CO2 to prevent their decomposition or side reaction to complete the synthesis that is difficult to achieve by direct means. The CO2 can be released later through a decarboxylation process.
Ni‐catalyzed cross‐coupling of unactivated secondary alkyl halides with alkylboranes provides an efficient way to construct alkyl–alkyl bonds. The mechanism of this reaction with the Ni/L1 ...(L1=trans‐N,N′‐dimethyl‐1,2‐cyclohexanediamine) system was examined for the first time by using theoretical calculations. The feasible mechanism was found to involve a NiI–NiIII catalytic cycle with three main steps: transmetalation of NiI(L1)X (X=Cl, Br) with 9‐borabicyclo3.3.1nonane (9‐BBN)R1 to produce NiI(L1)(R1), oxidative addition of R2X with NiI(L1)(R1) to produce NiIII(L1)(R1)(R2)X through a radical pathway, and CC reductive elimination to generate the product and NiI(L1)X. The transmetalation step is rate‐determining for both primary and secondary alkyl bromides. KOiBu decreases the activation barrier of the transmetalation step by forming a potassium alkyl boronate salt with alkyl borane. Tertiary alkyl halides are not reactive because the activation barrier of reductive elimination is too high (+34.7 kcal mol−1). On the other hand, the cross‐coupling of alkyl chlorides can be catalyzed by Ni/L2 (L2=trans‐N,N′‐dimethyl‐1,2‐diphenylethane‐1,2‐diamine) because the activation barrier of transmetalation with L2 is lower than that with L1. Importantly, the Ni0–NiII catalytic cycle is not favored in the present systems because reductive elimination from both singlet and triplet NiII(L1)(R1)(R2) is very difficult.
In the nick(el) of time! The feasible mechanism of Ni‐catalyzed alkyl–alkyl Suzuki cross‐coupling was found to be a NiI–NiIII catalytic cycle consisting of three steps: transmetalation, oxidative addition, and reductive elimination. Tertiary alkyl halides are not reactive because of the high barrier of reductive elimination. Ni/L2 is a much better catalyst for unactivated alkyl chlorides than Ni/L1 due to the lower barrier of transmetalation for the Ni/L2 system (see scheme).
The ligand-dependent selectivities in Ullmann-type reactions of amino alcohols with iodobenzene by β-diketone- and 1,10-phenanthroline-ligated CuI complexes were recently explained by the ...single-electron transfer and iodine atom transfer mechanisms ( Jones G. O. ; Liu P. ; Houk K. N. ; Buchwald S. L. J. Am. Chem. Soc. 2010, 132, 6205. ). The present study shows that an alternative, oxidative addition/reductive elimination mechanism may also explain the selectivities. Calculations indicate that a CuI complex with a negatively charged β-diketone ligand is electronically neutral, so that oxidative addition of ArI to a β-diketone-ligated CuI prefers to occur (and occur readily) in the absence of the amino alcohol. Thus, coordination of the amino alcohol in its neutral form can only occur at the CuIII stage where N-coordination is favored over O-coordination. The coordination step is the rate-limiting step and the outcome is that N-arylation is favored with the β-diketone ligand. On the other hand, a CuI complex with a neutral 1,10-phenanthroline ligand is positively charged, so that oxidative addition of ArI to a 1,10-phenanthroline-ligated CuI has to get assistance from a deprotonated amino alcohol substrate. This causes oxidative addition to become the rate-limiting step in the 1,10-phenanthroline-mediated reaction. The immediate product of the oxidative addition step is found to undergo facile reductive elimination to provide the arylation product. Because O-coordination of a deprotonated amino alcohol is favored over N-coordination in the oxidative addition transition state, O-arylation is favored with the 1,10-phenanthroline ligand.
Density functional theory calculations have been performed to explore the Ru‐catalyzed decarbonylative annulation reaction of 3‐hydroxy‐2‐phenyl‐chromone with an alkyne to synthesize ...spiroindenebenzofuranones. A Ru(II)−Ru(0)−Ru(II) rather than a Ru(II)−Ru(IV)−Ru(II) transformation was found involved in the decarbonylation process, which is responsible for the sequence of alkyne insertion/decarbonylation. Oxidative addition of C(carbonyl)−C(carbonyl) bond to Ru(0) and the Ru(II)−C(sp2) bond formation were confirmed to be favorable for the decarbonylation, meanwhile oxidative addition of C(carbonyl)−C(carbonyl) bond to Ru(0) is likely to be the rate‐determining step for the entire catalytic cycle. It is predicted that the regeneration of the catalyst was achieved by the oxidation of air oxygen in the absence of other oxidants. The current theoretical study provides new insights into the decarbonylative annulation.
DFT calculations have been used to explore the Ru‐catalyzed decarbonylative annulation of 3‐hydroxy‐2‐phenyl‐chromone with alkynes to synthesize spiro‐indenebenzofuranones. A Ru(II)−Ru(0)−Ru(II) transformation was found to be involved in the decarbonylation process. The oxidative addition of C(carbonyl)−C(carbonyl) bond to Ru(0) is likely the rate‐determining step. The regeneration of the catalyst is likely achieved by oxidation with air oxygen.
The first copper‐catalyzed regiodivergent cyanoboration of internal allenes with B2pin2 (bis(pinacolato)diboron) and NCTS (N‐cyano‐N‐phenyl‐p‐toluenesulfonamide) derivatives is reported. The β,γ‐ and ...α,β‐cyanoborylated products were synthesized with high regio‐ and stereo‐selectivity. Computational studies revealed that nucleophilic addition of allylcopper or related intermediates on cyanation reagent is the regio‐ and stereo‐determining step, while transmetalation with B2pin2 is the rate‐determining step. The nucleophilic addition step proceeds via inner‐sphere mechanism in the CuI/P(o‐tol)3 and CuI/Xantphos (P(o‐tol)3=tris(o‐methylphenyl)phosphine, Xantphos=4,5‐bis(diphenylphosphino)‐9,9‐dimethylxanthene) catalytic systems and via outer‐sphere mechanism in the CuII/Xantphos catalytic system, respectively.
The first copper‐catalyzed regiodivergent cyanoboration of internal allenes with B2pin2 and NCTS derivatives has been developed. MeOH was identified as the necessary additive for the regeneration of LCu‐Bpin species. Computational studies revealed that the copper‐mediated allylic cyanation is the regio‐ and stereo‐determining step, while the MeOH‐assisted transmetalation with B2pin2 is the rate‐determining step.
An unprecedented type of reaction for Cu-catalyzed trifluoromethylation of terminal alkenes is reported. This reaction represents a rare instance of catalytic trifluoromethylation through C(sp3)–H ...activation. It also provides a mechanistically unique example of Cu-catalyzed allylic C–H activation/functionalization. Both experimental and theoretical analyses indicate that the trifluoromethylation may occur via a Heck-like four-membered-ring transition state.
Efficient depolymerization methods are critical to the sustainable production of fuels and chemicals from biomass. Ligand-controlled selective C(sp3)–O and Ar–C(sp3) cleavages of β-O-4 lignin model ...compounds were realized with vanadium catalysts under redox-neutral conditions or air atmosphere. To clarify the mechanism and the origin of selectivity, a joint theoretical and experimental study was performed herein. First, with the aid of density functional theory (DFT) calculations, an updated mechanism involving VV, VIV, and VIII complexes was discovered for the C(sp3)–O cleavage process catalyzed by the Schiff base vanadium complexes with an overall free energy barrier of 34.9 kcal/mol. Meanwhile, a detailed catalytic cycle involving novel stepwise O–O/Ar–C(sp3) cleavage was clarified for the Ar–C(sp3) cleavage process catalyzed by the bis(8-oxyquinolate) coordinated vanadium complexes, having an overall free energy barrier of 28.8 kcal/mol. Further analysis based on the energetic span model revealed that the switchable selectivity results from the different T1 (ground triplet state)–HOMO separation/charge dispersion effects of ligands and the different formal oxidation states of the TOF-determining transition state (TDTS) in the C(sp3)–O and Ar–C(sp3) cleavage processes. Finally, control experiments of base and oxygen pressure were conducted to validate the conclusions from DFT studies regarding the role of bases and the TDTS step in the Ar–C(sp3) cleavage process.
Abstract
Highly effective and selective noble metal-free catalysts attract significant attention. Here, a single-atom iron catalyst is fabricated by saturated adsorption of trace iron onto zeolitic ...imidazolate framework-8 (ZIF-8) followed by pyrolysis. Its performance toward catalytic transfer hydrogenation of furfural is comparable to state-of-the-art catalysts and up to four orders higher than other Fe catalysts. Isotopic labeling experiments demonstrate an intermolecular hydride transfer mechanism. First principles simulations, spectroscopic calculations and experiments, and kinetic correlations reveal that the synthesis creates pyrrolic Fe(II)-plN
3
as the active center whose flexibility manifested by being pulled out of the plane, enabled by defects, is crucial for collocating the reagents and allowing the chemistry to proceed. The catalyst catalyzes chemoselectively several substrates and possesses a unique trait whereby the chemistry is hindered for more acidic substrates than the hydrogen donors. This work paves the way toward noble-metal free single-atom catalysts for important chemical reactions.
Cu‐catalyzed alkylboration of alkenes with bis(pinacolato)diboron ((Bpin)2) and alkyl halides provides a ligand‐controlled regioselectivity‐switchable method for the construction of complex ...boron‐containing compounds. Here, we employed DFT methods to elucidate the mechanistic details of this reaction and the origin of the different regioselectivity induced by Xantphos and Cy‐Xantphos. The calculation results reveal that the catalytic cycle mainly proceeds through the migratory insertion of alkenes on Cu‐Bpin complex, the oxidative addition of alkyl halides, and the reductive elimination of a C−C bond. Meanwhile, the rate‐ determining step is the oxidative addition of alkyl halides and the regioselectivity‐determining step is the migratory insertion of alkenes. The bulky cyclohexyl group of Cy‐Xantphos facilitates the approach of the substituents of alkenes to Bpin in the migratory insertion step and thus leads to the Markovnikov products. The less bulky phenyl group on Xantphos prefers keeping the substituents of alkenes away from the Bpin moiety in the migratory insertion step and thus results in anti‐Markovnikov products.
Flip the switch! Cu‐catalyzed alkylboration of alkenes with bis(pinacolato)diboron ((Bpin)2) and alkyl halides provides a ligand‐controlled regioselectivity‐switchable method for the construction of complex boron‐containing compounds (see scheme). DFT methods were employed to elucidate the mechanistic details of this reaction and the origin of the different regioselectivity induced by Xantphos and Cy‐Xantphos.
Transition-metal-catalyzed C–H activation represents one of most attractive research fields in modern organic chemistry while theoretical studies have become a popular and effective tool for ...elucidating mechanism nowadays. The recent achievements in the cross field of the two orientations are reviewed in this article. The first part introduced the advances in theoretical study on transition-metal-catalyzed C–H activation. The elegant work reported mainly in and after 2013, classified according to the mechanisms of C–H activation, were covered. The second part provided an analysis on the distribution of quantum-chemical methods, solvation models and basis sets in the collected theoretical studies.
