How Lewis Acids Catalyze Diels-Alder Reactions

The Lewis acid(LA)-catalyzed Diels-Alder reaction between isoprene and methyl acrylate was investigated quantum chemically using a combined density functional theory and coupled-cluster theory approach. Computed activation energies systematically decrease as the strength of the LA increases along the series I2 Collapse

Key Words

• Activation strain model
• Diels-Alder reactions
• Lewis acid catalysis
• Pauli repulsion
• density functional calculations

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MESH Headings Collapse

Grants

• NWO Nederlandse Organisatie voor Wetenschappelijk Onderzoek
• CTQ2016-78205-P and CTQ2016-81797-REDC MINECO
• Nederlandse Organisatie voor Wetenschappelijk Onderzoek

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Affiliation(s)

Pascal Vermeeren Department of Theoretical ChemistryAmsterdam Institute of Molecular and Life Sciences (AIMMS)Amsterdam Center for Multiscale Modeling (ACMM)Vrije Universiteit AmsterdamDe Boelelaan 10831081HVAmsterdamThe Netherlands
Trevor A. Hamlin Department of Theoretical ChemistryAmsterdam Institute of Molecular and Life Sciences (AIMMS)Amsterdam Center for Multiscale Modeling (ACMM)Vrije Universiteit AmsterdamDe Boelelaan 10831081HVAmsterdamThe Netherlands
Israel Fernández Departamento de Química Orgánica I and Centro de Innovación en Química Avanzada (ORFEO-CINQA)Facultad de Ciencias QuímicasUniversidad Complutense de Madrid28040MadridSpain
F. Matthias Bickelhaupt Department of Theoretical ChemistryAmsterdam Institute of Molecular and Life Sciences (AIMMS)Amsterdam Center for Multiscale Modeling (ACMM)Vrije Universiteit AmsterdamDe Boelelaan 10831081HVAmsterdamThe Netherlands Institute for Molecules and Materials (IMM)Radboud UniversityHeyendaalseweg 1356525AJNijmegenThe Netherlands

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How Alkali Cations Catalyze Aromatic Diels-Alder Reactions

We have quantum chemically studied alkali cation-catalyzed aromatic Diels-Alder reactions between benzene and acetylene forming barrelene using relativistic, dispersion-corrected density functional theory. The alkali cation-catalyzed aromatic Diels-Alder reactions are accelerated by up to 5 orders of magnitude relative to the uncatalyzed reaction and the reaction barrier increases along the series Li+ < Na+ < K+ < Rb+ < Cs+ < none. Our detailed activation strain and molecular-orbital bonding analyses reveal that the alkali cations lower the aromatic Diels-Alder reaction barrier by reducing the Pauli repulsion between the closed-shell filled orbitals of the dienophile and the aromatic diene. We argue that such Pauli mechanism behind Lewis-acid catalysis is a more general phenomenon. Also, our results may be of direct importance for a more complete understanding of the network of competing mechanisms towards the formation of polycyclic aromatic hydrocarbons (PAHs) in an astrochemical context.

One-Pot Synthesis of Renewable Phthalic Anhydride from 5-Hydroxymethfurfural by using MoO3 /Cu(NO3 )2 as Catalyst

Herein, a synthetic pathway to renewable phthalic anhydride (PA) from 5-hydroxymethfurfural (HMF) in one pot is reported. The commonly available catalysts MoO₃ and Cu(NO₃ )₂ play a crucial role in integrating the multiple steps of the reaction, namely decarbonylation of HMF to active furyl intermediate (AFI), oxidation of HMF to maleic anhydride (MA), Diels-Alder cycloaddition of AFI and MA, and subsequent dehydration, in one pot. Under mild reaction conditions, a 63.2 % yield of PA is obtained from HMF. Compared with the currently reported route to renewable PA based on the Diels-Alder cycloaddition of biomass-derived MA and furan, this convenient one-pot synthesis represents a great improvement in efficiency.

Dual Activation of Aromatic Diels-Alder Reactions

The unusually fast Diels-Alder reactions of [5]cyclophanes were analyzed by DFT at the BLYP-D3(BJ)/TZ2P level of theory. The computations were guided by an integrated activation-strain and Kohn-Sham molecular orbital analysis. It is revealed why both [5]metacyclophane and [5]paracyclophane exhibit a significant rate enhancement compared to their planar benzene analogue. The activation strain analyses revealed that the enhanced reactivity originates from 1) predistortion of the aromatic core resulting in a reduced activation strain of the aromatic diene, and/or 2) enhanced interaction with the dienophile through a distortion-controlled lowering of the HOMO-LUMO gap within the diene. Both of these physical mechanisms and thus the rate of Diels-Alder cycloaddition can be tuned through different modes of geometrical distortion (meta versus para bridging) and by heteroatom substitution in the aromatic ring. Judicious choice of the bridge and heteroatom in the aromatic core enables effective tuning of the aromatic Diels-Alder reactivity to achieve activation barriers as low as 2 kcal mol-1 , which is an impressive 35 kcal mol-1 lower than that of benzene.

The Diels-Alder reaction of 4,6-dinitrobenzofuroxan with 1-trimethylsilyloxybuta-1,3-diene: a case example of a stepwise cycloaddition

The reaction of 4,6-dinitrobenzofuroxan (DNBF) with 1-trimethylsilyloxybuta-1,3-diene (8) is shown to afford a mixture of [2+4] diastereomeric cycloadducts (10, 11) through stepwise addition-cyclization pathways. Zwitterionic intermediate sigma-adduct 9, which is involved in the processes, has been successfully characterized by (1)H and (13)C NMR spectroscopy and UV/visible spectrophotometry in acetonitrile. A kinetic study has been carried out in this solvent that revealed that the rate of formation of 9 nicely fits the three-parameter equation log k=s(E+N) developed by Mayr to describe the feasibility of nucleophile-electrophile combinations. This significantly adds to the NMR spectroscopic evidence that the overall cycloadditions take place through a stepwise mechanism. The reaction has also been studied in dichloromethane and toluene. In these less polar solvents, the stability of 9 is not sufficient to allow direct characterization by spectroscopic methods, but a kinetic investigation supports the view that stepwise processes are still operating. An informative comparison of our reaction with previous interactions firmly identified as prototype stepwise cycloadditions is made on the basis of the global electrophilicity index, omega, defined by Parr within the density functional theory, and highlighted by Domingo et al. as a powerful tool for understanding Diels-Alder reactions.