Mechanism of hydroboration in ether solvents. A model ab initio study

Clark, Timothy; Wilhelm, Dieter; Schleyer, Paul von Ragué

doi:10.1039/c39830000606

Cited by 20 publications

(11 citation statements)

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“…As can be seen, formation of the precomplex Int0 a / Int0 b is obviously endothermic in view of the Gibbs free energies (7.2/5.0 kcal/mol, respectively) but exothermic with respect to the enthalpy energies (−4.4/–7.4 kcal/mol, respectively). This is basically consistent with previous reports − since it was actually the relative enthalpy energies that were adopted to determine whether the 3-center precomplex forms or not. Actually, combination of the two reactants is an entropic penalty process, which will certainly result in higher Gibbs barrier than enthalpy barrier, let alone that the entropic penalty based on the ideal gas-phase model is often overestimated because the suppressing effects of the solvent and pressure on the translational and rotational degrees of freedom of the reactants cannot be properly accounted for by the gas-phase model. ,, …”

Section: Results and Discussionsupporting

confidence: 91%

“…Despite intense experimental and theoretical studies regarding the reaction mechanism of direct − or transition-metal-catalyzed ,,− hydroborations, the detailed mechanistic steps or transition states are still of considerable dispute, although there has been some common consensus. For example, the uncatalyzed reaction should proceed through a 4-center transition state, with or without , a loose 3-center π complex formed in the early stage.…”

Section: Introductionmentioning

confidence: 99%

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Unravelling the Origins of Hydroboration Chemoselectivity Inversion Using an N,O-Chelated Ir(I) Complex: A Computational Study

Bai

Zhang

et al. 2019

J. Org. Chem.

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It has been widely reported that the transition-metal catalysts can invert the chemoselectivity of hydroboration of multifunctional substrates, but the fundamental reasons for this inversion have rarely been studied. In this work, mechanistic details and chemoselectivity of the hydroboration of various E–C (E = CH, CH2, O, N, etc.) multiple bonds has been explored through density functional theory calculations, including reactions using free HBCy2 or captured HBCy2 by an N,O-chelated Ir(I) complex which includes a phosphoamidate group. The computational results demonstrate the formation of a 3-center π complex before the 4-center transition state when using free HBCy2. When the Ir(I) complex is added, the HBCy2 would be captured to form a metallaheterocyclic Shimoi-type intermediate in which a δ-[M]···H–B agostic interaction appears. Afterward, the E–C functional group intersects this hemilabile interaction rather than the PO···B coordination bond, and the following hydride transfer is found to be rate-determining. The frontier molecular orbital analysis discloses that the olefins that have typical delocalized π electrons tend to give more effective overlap with the LUMO of free HBCy2, but the aromatic substituents could significantly reduce its reactivity due to the enhanced electron delocalization. The polarized carbonyl group facilitates σ coordination with the metal center, but the vinyl group favors interaction through π coordination. The superiority of σ coordination results in the priority of hydroboration of aldehyde over the olefins when using the captured HBCy2. Thus, it is the conversion of the more favored bonding groups by the transition metal that should be fundamentally responsible for the chemoselectivity inversion.

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Section: Results and Discussionsupporting

confidence: 91%

Section: Introductionmentioning

confidence: 99%

Unravelling the Origins of Hydroboration Chemoselectivity Inversion Using an N,O-Chelated Ir(I) Complex: A Computational Study

Bai

Zhang

et al. 2019

J. Org. Chem.

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“…Similar role of solventa s an ucleophile has been reported recently. [4e] Moreover, BH(NHMe) 2 ,a na nalogue of intermediate 4,w as recently detected by 11 BNMR during at rapping experiment for dehydropolymerization of MeH 2 NBH 3 by Rh catalyst [4h] (Figure S4 in the Supporting Information). Later,acyclohexene (C)d islodges the Scheme1.Conventional mechanism of NH 2 =BH 2 trapping by cyclohexene.…”

Section: Computational Detailsmentioning

confidence: 70%

“…Interestingly,d ecades back, the mechanistic intricacies of hydroboration of alkenes by BH 3 ÀTHF have generated much debate. Although some experimental and theoretical studies have implicated the role of BH 3 ÀTHF in formation of the initial p complex between ethylene, [11] others have suggested that free BH 3 is produced, which leads to the p complex formation. [12] Our DFT studies with cyclohexene( C )h ydroboration with BH 3 ÀTHF are consistentw ith the former mechanistic scenario.…”

Section: Computational Detailsmentioning

confidence: 99%

Theoretical Investigation on the Chemistry of Entrapment of the Elusive Aminoborane (H₂NBH₂) Molecule

Malakar

Bhunya

Paul

2015

Chemistry A European J

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Aminoborane (H2 N=BH2 ) is an elusive entity and is thought to be produced during dehydropolymerization of ammonia borane, a molecule of prime interest in the field of chemical hydrogen storage. The entrapment of H2 N=BH2 through hydroboration of exogenous cyclohexene has emerged as a routine technique to infer if free H2 N=BH2 is produced or not during metal-catalyzed ammonia borane dehydrogenation reactions. But to date, the underlying mechanism of this trapping reaction remains unexplored. Herein, by using DFT calculations, we have investigated the mechanism of trapping of H2 N=BH2 by cyclohexene. Contrary to conventional wisdom, our study revealed that the trapping of H2 N=BH2 does not occur through direct hydroboration of H2 N=BH2 on the double bond of cyclohexene. We found that autocatalysis by H2 N=BH2 is crucial for the entrapment of another H2 N=BH2 molecule by cyclohexene. Additionally, nucleophilic assistance from the solvent is also implicated for the entrapment reaction carried out in nucleophilic solvents. In THF, the rate-determining barrier for formation of the trapping product was predicted to be 16.7 kcal mol(-1) at M06 L(CPCM) level of theory.

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“…However, conclusive mechanistic studies that support this generally accepted mechanism have not been conducted. To support and shed some light in terms of the observed reactivities and mechanistic behaviour, computational modelling of hydroboration reactions have been shown to be useful [16]. Yet, simple kinetic information in the literature dates back to the 1980s.…”

Section: Introductionmentioning

confidence: 98%

Kinetics and mechanism of hydroboration reactions of HBBr2·SMe2 and HBCl2·SMe2 – Application of 11B NMR spectroscopy

Jaganyi

Xaba

Mzinyati

et al. 2007

Journal of Organometallic Chemistry

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Mechanism of hydroboration in ether solvents. A model ab initio study

Cited by 20 publications

References 0 publications

Unravelling the Origins of Hydroboration Chemoselectivity Inversion Using an N,O-Chelated Ir(I) Complex: A Computational Study

Unravelling the Origins of Hydroboration Chemoselectivity Inversion Using an N,O-Chelated Ir(I) Complex: A Computational Study

Theoretical Investigation on the Chemistry of Entrapment of the Elusive Aminoborane (H₂NBH₂) Molecule

Kinetics and mechanism of hydroboration reactions of HBBr2·SMe2 and HBCl2·SMe2 – Application of 11B NMR spectroscopy

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Mechanism of hydroboration in ether solvents. A model ab initio study

Cited by 20 publications

References 0 publications

Unravelling the Origins of Hydroboration Chemoselectivity Inversion Using an N,O-Chelated Ir(I) Complex: A Computational Study

Unravelling the Origins of Hydroboration Chemoselectivity Inversion Using an N,O-Chelated Ir(I) Complex: A Computational Study

Theoretical Investigation on the Chemistry of Entrapment of the Elusive Aminoborane (H2NBH2) Molecule

Kinetics and mechanism of hydroboration reactions of HBBr2·SMe2 and HBCl2·SMe2 – Application of 11B NMR spectroscopy

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Product

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Theoretical Investigation on the Chemistry of Entrapment of the Elusive Aminoborane (H₂NBH₂) Molecule