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The reaction mechanism and enantioselectivity of asymmetric Michael addition reaction between alkynone (R1) with α-angelica lactone (R2) catalyzed by chiral N, N'-dioxide-Sc(III) complex were investigated at the M06/6-31G(d,p) (acetonitrile, SMD) level. The α-angelica lactone substrate could isomerize to the active enolized form in the presence of Sc(OTf)3 reagent, assisted by the counter trifluoromethanesulfonate anion OTf-. The alkynone substrate and enolized angelica lactone (or its anion) coordinated to Sc(III) center of N,N'-dioxide-Sc(III) complex catalyst simultaneously, forming a high active hexacoordinate-Sc(III) complex. The catalytic reaction occurred via a two-step mechanism, in which C2 Cγ bond formation was predicted to be the chirality-controlling step as well as the rate-determining step (RDS), affording predominant S-enantiomer. The counterion OTf- facilitated C H construction as a proton-shuttle, producing mainly E-configuration product observed in experiment. The steric repulsion from the ortho-substituent of amide moiety as well as the chiral backbone of N, N'-dioxide-Sc(III) catalyst played the key role for chiral induction in the asymmetric reaction. The less destabilizing Pauli repulsion and more stabilizing attractive interaction, especially the orbital interaction, along the si-face attack pathway enhanced the enantiodifference of the two competing pathways for high enantioselectivity. The energy barriers for E/Z isomerization of S or R-enantiomer assisted by HOTf was as high as 34.6–35.0 kcal mol−1, indicating that the product with Z-conformation was difficult to be obtained. These results were in good agreement with experimental observations.

The reaction mechanism and enantioselectivity of the asymmetric [2 + 2] cycloaddition between an alkynone (R1) and a cyclic enol silyl ether (R2) were studied theoretically by the DFT method at the...

The mechanism and enantio- as well as diastereo-selectivity of asymmetric direct aldol reaction of isatin with α-angelica lactone catalyzed by chiral N, N'-dioxide-Sc(OTf)3 were studied at the B3LYP(SMD, THF)/6-31G(d,p) level. The calculations indicated that the reaction occurred along stepwise mechanism, that is, C C bond formation followed by H-transfer from α-angelica lactone to carbonyl group of isatin. The H-transfer step was predicted to be the rate-determining step(RDS), with the energy barrier of 23.3 ∼ 26.2 kcal mol−1 for four stereoisomeric products. The ortho-isopropyl groups of amide in ligand blocked the reacting site from re-face of isatin and induced the re-face of α-angelica lactone to approach to the si-face of isatin, leading to the predominant product with 3S, 2′S-configuration. In this asymmetric catalysis, the interaction energy term (ΔEint), especially stabilizing orbital energy (ΔEoi) and electrostatic energy (ΔVelstat), were the main contributors to the enantioselectivity of the reaction, while the activation strain energy (ΔEstrain) of α-angelica substrate enhanced the diastereodifferentiation of the two competing pathways, contributing to high diastereomeric ratio (d.r.). The turnover frequency (TOF) along the favorable si-face pathway was predicted to be 2.76 × 10−21 s−1, with the rate constant of k(T) = 7.5 × 10−2 exp(-108390/RT) dm3 mol−1 s−1 over 258.15 K ∼ 358.15 K temperature range. These results provided explanation for the good reactivity as well as excellent asymmetric induction of chiral N, N'-dioxide-Sc(OTf)3 in direct aldol reaction of isatin.

Solvent-thermal conversion of biomass was promising for obtaining value-added chemicals. However, little was known about the interactions between solvents and biomass in the process, which hindered the effective utilization of biomass. The effects of γ-valerolactone (GVL) and H2O on enhancing pubescens degradation via the cleavage of inter- and intramolecular linkages were studied. At 160 °C, H2O selectively promoted the cleavage of the intermolecular linkages by forming hydrogen bonds, making mainly contributions to hemicellulose dissolution, while GVL and H2O promoted lignin dissolution by forming hydrogen bonds with −OCH3 group of lignin. H2O promoted the cleavage of β-(1,4)-glycosidic bonds in hemicellulose derived oligomers to xylose, while the oxygen in the ring of GVL might interact with hydroxyl groups of xylose unit to enhance the dehydration of xylose to furfural, whereas GVL with H2O promoted the depolymerization of lignin to oligomers mainly including β-O-4′ and β–β′ linkages connecting to G a...

The mechanism and selectivity of the asymmetric Friedel–Crafts (F–C) alkylation reaction between indole and chalcone catalyzed by chiral N,N′-dioxide–Sc(III) complexes were investigated at the M06/6-311+G(d,p)//M06/[LANL2DZ,6-31G(d)](SMD,CH2Cl2) level. The reaction occurred via a three-step mechanism: (i) the C3–Cβ bond formation by interacting the most mucleophilic C3 center of indole with the most electrophilic Cβ center of chalcone; (ii) the abstraction of the proton at the C3 atom of indole by counterion OTf–; (iii) proton transfer from HOTf to the Cα atom of chalcone, generating the F–C alkylation product. The reaction preferred to occur along the favorable re-face attack pathway, producing the dominant R-product. The turnover frequency (TOF) of catalysis was predicted to be 1.59 × 10–7 s–1, with a rate constant of K(T) = 1.58 × 10–7 exp(−29057/RT) dm6·mol–2·s–1 over the temperature range of 248–368 K. Activation strain model (ASM) and energy decomposition analysis (EDA), as well as noncovalent inter...

The mechanism and selectivity of the asymmetric cycloaddition reactions of 3-vinylindole towards methyleneindolinone (Diels–Alder reaction) or ketimine (aza-Diels–Alder reaction) were investigated theoretically by DFT and ONIOM methods. The non-catalytic DA reaction occurred in a two-stage one-step mechanism with the energy barriers of 17.0–29.6 kcal mol−1. Reactivity index analysis indicated that the cycloaddition tended to occur by the most nucleophilic terminal C4 center in 3-vinylindole and the most electrophilic Cβ center in methyleneindolinone with lower activation energies, which accounted for the regioseletivity observed in experiment. For non-catalytic reactions, the slightly exo-preference stemmed from the stabilizing π-π stacking of two indole rings in substrates. In the presence of chiral N,N'-dioxide-Ni(II) complex, the bulky ortho-iPr group in aniline of ligand provided sufficient steric shielding around dienophile from the si-face attack in Diels–Alder reaction or re-face attack in aza-Diels–Alder reaction by diene, resulting in predominant products with excellent enantioselectivity (the favorable re-face attack in Diels–Alder reaction or si-face attack in aza-Diels–Alder reaction). A “pocket-like” chiral cavity constructed by chiral catalyst enhanced exo-diastereofacial selectivity significantly by maximizing the overlap of the two indole rings of substrates to avoid unfavorable steric repulsion between indole ring of 3-vinylindole and chiral ligand. Compared with catalytic aza-Diels–Alder reaction of 3-vinylindole and ketimine, the improved catalytic activity of chiral N,N'-dioxide-Ni(II) catalyst in Diels–Alder reaction of 3-vinylindole with methyleneindolinone contributed to the high turnover frequency (TOF = 1.05 × 106 s−1). These results were in good agreement with experimental observations.

A two-step method was adopted to produce furfural from the selective dissolution and conversion of hemicellulose in pubescens. First, in GVL(γ-valerolactone)-H2O co-solvent at 160 °C, H2O promoted the cleavage of chemical bonds linking hemicellulose, lignin, and cellulose, and GVL further helped the co-dissolution of hemicellulose (93.6 wt %) and lignin derivatives (80.2 wt %), leaving a high purity cellulose (83.3 wt %). Heating to 200 °C, the liquid system obtained with NaCl and THF added, achieved the maximum yield of 76.9 mol % with 82.2% selectivity to furfural based on the moles of converted hemicellulose using a 5 wt % pubescens to solvent ratio. It was demonstrated that NaCl with GVL promoted the depolymerization of oligomers to small molecular products (Mw < 150 Da), while the co-contribution of NaCl and co-solvent improved the selectivity to furfural. Cl– could interact strongly with C-OH-2,3,4 of the xylose unit, and the dehydration of xylose to form furfural might first occur on C-OH-4 of xylo...

The structure and electronic properties of metallic complexes formed by coordinating chiral N,N′-dioxide ligands with different amino acid skeletons or straight-chain alkyl spacers (linkage) to Mg2+ and Ca2+ are studied at the B3LYP-D3(BJ)/6-311G**(SMD, CH2Cl2)//ONIOM (B3LYP/6-31G*: UFF) (SMD, CH2Cl2) level. The N-oxide unit in the ligand exhibits a stronger O-donor ability than the carbonyl of the amide in the formation of the chiral N,N′-dioxide–Mg(II) catalyst. Coordination of the chiral N,N′-dioxide ligand to the metal center forms a pocket-like chiral environment (“chiral pocket”), which can be characterized by four structural descriptors, which are the bite angle αO1–M–O2, the average M–O distance, the torsion angle θ2 (C1–O1⋯O2–C2) and the dihedral angle (DC1–N3–C7–C8 or DC2–N4–C9–C10). The Lewis acidity of the metal ion decreases when the number of –CH2 spacers between two N-oxide units increases from 1 to 5, which is attributed to increasing Pauli repulsion as well as deformation of fragments. The stereoselectivity of asymmetric carbonyl–ene reaction is dependent on the blocking effect of ortho-iPr of aniline on the reaction site of isatin, which could be adjusted by changing the linkage and chiral backbone as well as metal ion. An unfavorable steric arrangement in the re-face attack pathway translated into a more destabilizing activation strain of the ene substrate, enhancing the enantiodifferentiation of two competing pathways for the desired (R)-product. The counterion might change the catalytic species as well as the associated chiral pocket by taking part in coordination towards the metal center, consequently affecting the reaction mechanism and stereoselectivity.

Herein, metal–ligand bonding features in a chelation N,N′-dioxide–Sc(III) complex have been addressed using the DFT method at the M06/6-31+G** Level. The donor–acceptor interaction between the carbonyl substrate and Sc(III)-based catalyst is analyzed in detail by the activation strain model (ASM), energy decomposition analysis (EDA), and natural orbital for chemical valence (NOCV) calculations. The orbital interaction is the major contributor to N,N′-dioxide–[Sc(OTf)]2+ bonding, whereas the electrostatic interaction plays a more important role than orbital interaction in the activation of a carbonyl compound in hexacoordinate N,N′-dioxide–Sc(III) complexes. The substituents in the amide group of the N,N′-dioxide ligand (L) affect the electrostatic energy as well as the orbital energy between the CH2O and Sc(III)-based catalyst by adjusting the Lewis acidity of the metal centre. The complex with ortho-diisopropylphenyl groups in the ligand exhibits a higher reactivity towards CH2O. Compared to OiPr, the counter ion OTf in the Sc(III)-complex enhances the Lewis acidity of the metal centre and facilitates the activation of CH2O by promoting electron density flow from CH2O to the metal fragment. The high catalytic performance of the N,N′-dioxide–Sc(III) complex towards PhCHO and chalcone is attributed to their good nucleophilicity that results in a more stabilizing electrostatic and orbital interaction between the N,N′-dioxide–[Sc(OTf)]2+ complex and carbonyl substrate.