Toward Green Acylation of (Hetero)arenes: Palladium-Catalyzed Carbonylation of Olefins to Ketones

Green Friedel–Crafts acylation reactions belong to the most desired transformations in organic chemistry. The resulting ketones constitute important intermediates, building blocks, and functional molecules in organic synthesis as well as for the chemical industry. Over the past 60 years, advances in this topic have focused on how to make this reaction more economically and environmentally friendly by using green acylating conditions, such as stoichiometric acylations and catalytic homogeneous and heterogeneous acylations. However, currently well-established methodologies for their synthesis either produce significant amounts of waste or proceed under harsh conditions, limiting applications. Here, we present a new protocol for the straightforward and selective introduction of acyl groups into (hetero)­arenes without directing groups by using available olefins with inexpensive CO. In the presence of commercial palladium catalysts, inter- and intramolecular carbonylative C–H functionalizations take place with good regio- and chemoselectivity. Compared to classical Friedel–Crafts chemistry, this novel methodology proceeds under mild reaction conditions. The general applicability of this methodology is demonstrated by the direct carbonylation of industrial feedstocks (ethylene and diisobutene) as well as of natural products (eugenol and safrole). Furthermore, synthetic applications to drug molecules are showcased

Methanol to Olefins over H‑MCM-22 Zeolite: Theoretical Study on the Catalytic Roles of Various Pores

H-MCM-22 zeolite bears three types of pores, supercages, sinusoidal channels, and pockets, and exhibits excellent catalytic performance in the process of methanol to olefins (MTO); however, the catalytic role that each type plays in MTO is still unclear. In this work, density functional theory considering dispersive interactions (DFT-D) was used to elucidate the contributions of various pores in H-MCM-22 to MTO. The results demonstrated that these three types of pores are different in their catalytic action on MTO, because of the large differences in pore size and shape that determine the space confinement and electrostatic stabilization effects. The formation of propene is predicted to take place in the supercages, where propene can be effectively produced through both polyMB and alkene cycles, with a relatively low free energy barrier as well as low enthalpy barrier and entropy loss for the rate-determining steps. In the sinusoidal channels, the free energy barrier of the methylation and cracking steps is elevated due to the space confinement and the reactivity of alkenes is also markedly depressed in the narrow channels, in comparison with those in the supercages; as a result, the contribution of the sinusoidal channels to the entire propene formation is minor. Meanwhile, the pockets are probably detrimental to MTO, as certain large intermediates such as 1,1,2,6-tetramethyl-4-isopropylbenzenium cations are easily formed in the pockets but are difficult to decompose due to the lack of an electrostatic stabilization effect from the zeolite framework, which elevates the total free energy barrier and may lead to a rapid deactivation of these active sites. In comparison with the difference in pore size and structure, the difference of various pores in the acid strength of the active sites exhibits an insignificant effect on their catalytic behaviors in MTO. The theoretical insights in this work are conducive to a subsequent investigation on the MTO mechanism and the development of better MTO catalysts and reaction processes

Polymethylbenzene or Alkene Cycle? Theoretical Study on Their Contribution to the Process of Methanol to Olefins over H‑ZSM‑5 Zeolite

Polymethylbenzene (polyMB) and alkene cycles are considered as two main routes forming light olefins in the process of methanol to olefins (MTO); however, the contribution that each cycle makes to MTO is still unclear. In this work, density functional theory considering dispersive interactions (DFT-D) was used to elucidate the catalytic roles that the polyMB and the alkene cycles may play in forming ethene and propene from methanol in MTO over H-ZSM-5. The results demonstrated that ethene and propene can be produced in nearly the same probability via the polyMB cycle, as they have a very close free energy height as well as a similar free energy barrier for the rate-determining steps. Via the alkene cycle, however, propene is the dominant product, because the methylation and cracking steps to get propene have a much lower free energy barrier in comparison with those to form ethene. As a result, ethene is predominantly formed via the polyMB cycle, whereas propene is produced via both the polyMB and the alkene cycles. The contribution of the alkene cycle is probably larger than that of the polyMB cycle, resulting in a high fraction of propene in the MTO products. Meanwhile, both cycles are interdependent in MTO, as the aromatic species generated by aromatization via the alkene cycle can also serve as new active centers for the polyMB cycle, and vice versa. Moreover, the catalytic activity of H-ZSM-5 zeolite is directly related to its acid strength; weaker acid sites are unfavorable for the polyMB cycle and then enhance relatively the contribution of the alkene cycle to forming light olefins. These results can well interpret the recent experimental observations, and the theoretical insights shown in this work may improve our understanding of the MTO mechanism, which are conducive to developing better MTO catalysts and reaction processes

Evolution of Aromatic Species in Supercages and Its Effect on the Conversion of Methanol to Olefins over H‑MCM-22 Zeolite: A Density Functional Theory Study

H-MCM-22 zeolite is a potential catalyst for the conversion of methanol to olefins (MTO). Previous studies indicated that three types of pores in H-MCM-22, viz., the supercages, sinusoidal channels, and pockets, are different in their catalytic action; however, the evolution of aromatic species in the supercages and its effect on MTO are still highly controversial. In this work, density functional theory considering dispersive interactions (DFT-D) was used to investigate the evolution of aromatic species including their formation, reactivity, and deactivation behavior in the supercages; the active role of the supercages in catalyzing MTO was elucidated. The results demonstrated that benzene can be generated in the supercages through aromatization of light olefins; after that, polymethylbenzenes (polyMBs) are formed through methylations, in competition with the construction of naphthalenic species. Both polyMBs (e.g., hexamethylbenzene) and polymethylnaphthalenes (polyMNs, e.g. dimethylnaphthalene) exhibit high reactivity as the hydrocarbon pool species in forming light olefins. Owing to the appropriate electrostatic stabilization and space confinement effects, naphthalenic species in the supercages are inclined to serve as the active intermediates to produce light olefins rather than act as the coke precursors in the initial period of MTO; as a result, the supercages contribute actively to the initial activity of H-MCM-22 in MTO, though they may be prone to deactivation in the later reaction stage in comparison with the sinusoidal channels. The insights shown in this work help to clarify the evolution of aromatic species and the active role of the supercages in MTO over H-MCM-22, which is of benefit to the development of better MTO catalysts and reaction processes

Stability and Reactivity of Intermediates of Methanol Related Reactions and C–C Bond Formation over H‑ZSM‑5 Acidic Catalyst: A Computational Analysis

On the basis of density functional theory including dispersion correction [ωB97XD/6-311+G­(2df,2p)//B3LYP/6-311G­(d,p)], the thermodynamics and kinetics of the reactions of CH₃OH and CH₃OCH₃ over H-ZSM-5 have been systematically computed. For the reaction of the methylated surface (CH₃OZ) with CH₃OH, CH₃OCH₃ formation is kinetically controlled and the competitive formation of CH₂O + CH₄ is thermodynamically controlled, in agreement with the observed desorption temperatures of CH₃OH, CH₃OCH₃, and CH₂O under experimental conditions. For the reaction between ZOCH₃ and CH₃OCH₃, the formation of the framework stabilized (CH₃)₃O⁺ is kinetically controlled, consistent with the NMR observation at low temperature, and the competitive formation of surface CH₃OCH₂OZ + CH₄ is thermodynamically controlled. On the basis of the thermodynamically more favored CH₂O and CH₃OCH₂OZ, there are two parallel routes for the first C–C bond formation, from the coupling of CH₃OCH₂OZ with CH₃OH and CH₃OCH₃ as well as from the coupling of CH₂O with CH₃OH and CH₃OCH₃. The most important species is the methylated surface (CH₃OZ), which can react with CH₃OH and CH₃OCH₃ to form the corresponding physisorbed CH₂O and chemisorbed CH₃OCH₂OZ, and they can further couple with additional CH₃OH and CH₃OCH₃ to result in first C–C formation, verifying the proposed formaldehyde (CH₂O) and methoxymethyl (CH₃OCH₂OZ) mechanisms

Product Distribution Control for Glucosamine Condensation: Nuclear Magnetic Resonance (NMR) Investigation Substantiated by Density Functional Calculations

Selective conversion of glucosamine (GlcNH₂) to deoxyfructosazine (DOF) and fructosazine (FZ) with additives was investigated. Significantly enhanced yield of DOF can be improved to 40.2% with B­(OH)₃ as the additive. Chemical shift titration (via one-dimensional nuclear magnetic resonance (1D ¹H and ¹³C NMR)) and two-dimensional nuclear magnetic resonance (2D NMR) including ¹H–¹³C HSQC and ¹H–¹H COSY are used to investigate intermolecular interactions between B­(OH)₃ and GlcNH₂. Diffusion-ordered NMR spectroscopy (DOSY) was further employed to identify intermediate species. Mechanistic investigation by NMR combined with electron spray ionization–mass spectroscopy (ESI-MS) discloses that a mixed 1:1 boron complex was identified as the major species, shedding light on the promotional effects of B­(OH)₃, which is substantiated by density functional theory (DFT). Boron coordination effects make ring-opening and subsequent dehydration reaction thermodynamically and kinetically more favorable. Dehydration of dihydrofructosazine is a key step in controlling overall process (49.7 kcal/mol). Interestingly, chelating effect results in substantial reduction of this free-energy barrier (31.5 kcal/mol). Notably, FZ was gradually becoming the main product (yield up to 25.3%), with H₂O₂ as the oxidant