The [4 + 2] Cycloaddition of 2‐Pyrone in Total Synthesis

Diels‐Alder reaction is one of the most important reactions in organic synthesis and has witnessed great achievements in total syntheses of complex natural products. In this review, we will focus on the [4 + 2] cycloaddition that utilizes 2‐pyrone as the diene component. Major advances of its applications in the total syntheses of natural products in the past fifty years will be discussed here.     

Molecular dynamics and metadynamics simulations of [2 + 2] photocycloaddition

Triplet state mechanism of [2 + 2] photocycloaddition forming a cyclobutane ring from two ethylenes is investigated in the context of photocatalysis. High‐level ab initio calculations are combined with ab initio adiabatic molecular dynamics and ab initio metadynamics for rare events modeling. In a photocatalytic scheme, a reactant reaches the triplet state either via intersystem crossing (ISC) or triplet sensitization. The model system adopts a biradical structure, which represents energy intersection with the ground state. The system either completes cyclization or undergoes fragmentation into two olefinic units. The potential and free energy surfaces of the cyclobutane/ethylenes system are mapped with multireference approaches describing possible reaction pathways. To obtain a full picture of a double bond photoreactivity, ab initio adiabatic dynamical calculations were used to estimate reaction yields and to model the effects of excess energy. The potential use of density functional theory based approaches for [2 + 2] photocycloaddition was investigated for future simulations and design of realistic photocatalytic systems. Dynamical aspects of [2 + 2] photocycloaddition via a triplet state manifold are investigated by combining ab initio multireference methods and ab initio molecular dynamics and metadynamics. The reaction pathways are studied for a model system of two ethylenes forming a cyclobutane ring to provide a basis for further studies on design of photocatalytic systems.     

Catalytic [2 + 2 + 2] cycloaddition polymerization of diyne–nitrile monomers in the presence of CoCl2‐6H2O/diphosphine/Zn

Cobalt‐catalyzed [2 + 2 + 2] cocycloaddition reaction of 1,6‐diynes and nitriles to generate substituted pyridines has been applied to the polymerization of diyne–nitrile monomers, the reaction of which proceeded smoothly in a step‐growth fashion to provide linear polymers comprising pyridine structures in the main chain. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 345–351     

Facile Synthesis of Spirooxindole‐Cyclohexenes via Phosphine‐Catalyzed [4 + 2] Annulation of α‐Substituted Allenoates

A phosphine‐catalyzed [4 + 2] annulation of α ‐substituted allenoate with exocyclic alkene moiety of oxindoles or indan‐1,3‐diones has been developed. Thus, under the catalysis of PPh₃ (20 mol%), a series of spirooxindole‐ or spiroindan‐1,3‐dione‐cyclohexenes have been obtained in moderate to excellent yields and regioselectivity from the annulations of α ‐methyl allenoates with 3‐methyleneoxindoles or 2‐methyleneindan‐1,3‐diones. This method offers an easy access to structurally novel spirocyclohexenes.     

Late‐Stage Formation of Carbon–Fluorine Bonds

In this account, we review work from our lab on the development of methods for carbon–fluorine bond formation, with an emphasis on late‐stage fluorination of functionalized small molecules and synthesis of ¹⁸F‐labeled molecules for potential use as tracers in positron emission tomography (PET). We attempt to highlight reactions that we feel are of particular practical relevance, as well as areas of research where there is still significant room for advancement.     

Theoretical studies of spin state‐specific [2 + 2] and [5 + 2] photocycloaddition reactions of n‐(1‐penten‐5‐yl)maleimide

N‐alkenyl maleimides are found to exhibit spin state‐specific chemoselectivities for [2 + 2] and [5 + 2] photocycloadditions; but, reaction mechanism is still unclear. In this work, we have used high‐level electronic structure methods (DFT, CASSCF, and CASPT2) to explore [2 + 2] and [5 + 2] photocycloaddition reaction paths of an N‐alkenyl maleimide in the S₁ and T₁ states as well as relevant photophysical processes. It is found that in the S₁ state [5 + 2] photocycloaddition reaction is barrierless and thus overwhelmingly dominant; [2 + 2] photocycloaddition reaction is unimportant because of its large barrier. On the contrary, in the T₁ state [2 + 2] photocycloaddition reaction is much more favorable than [5 + 2] photocyclo‐addition reaction. Mechanistically, both S₁ [5 + 2] and T₁ [2 + 2] photocycloaddition reactions occur in a stepwise, nonadiabatic means. In the S₁ [5 + 2] reaction, the secondary C atom of the ethenyl moiety first attacks the N atom of the maleimide moiety forming an S₁ intermediate, which then decays to the S₀ state as a result of an S₁ → S₀ internal conversion. In the T₁ [2 + 2] reaction, the terminal C atom of the ethenyl moiety first attacks the C atom of the maleimide moiety, followed by a T₁ → S₀ intersystem crossing process to the S₀ state. In the S₀ state, the second C C bond is formed. Our present computational results not only rationalize available experiments but also provide new mechanistic insights. © 2017 Wiley Periodicals, Inc.     

Rhodium(I)‐Catalyzed [4 + 2] Cycloaddition Reactions of 2‐Alkylenecyclo‐butanols with Alkynes and (E)‐2‐Nitroethenylbenzene through C(sp2)—C(sp3) Bond Cleavage

An intermolecular [4 + 2] cycloaddition was realized through C—C bond cleavage in the presence of Rh(I) catalyst. The selective ring opening of 2‐alkylenecyclobutanols enables the generation of active alkenylrhodium species, which underwent smooth cross addition over alkynes and (E)‐2‐nitroethenylbenzene, leading to highly substituted all‐carbon six‐membered rings in a single step and in a complete atom economy.     

Synthesis of Novel Tricyclic 1,5‐Benzothiazepine Derivatives Bearing Quinoline Moiety via [2 + 2] Cycloaddition Reaction

A series of new tricyclic 1,5‐benzothiazepine derivatives were synthesized by the reaction of 1,5‐benzothiazepine containing 2‐phenoxy‐quinoline with chloracetyl chloride and phenoxyacetyl chloride. The structures of the target compounds were confirmed by IR, ¹H NMR MS, and elemental analysis.     

Geometric and electronic structures of silicon fluorides (N = 4 – 6) and potential energy surfaces for dissociation reactions → SiF4 + F– and → + F–

The geometric and electronic structures of a series of silicon fluorides (n = 4 ? 6) were computationally studied with the aid of density functional theory (DFT) method with B3LYP and M06‐2X functionals and coupled cluster (CCSD and CCSD(T)) methods with 6‐311++G(d,p) basis set. The nature of the Si‐F bonds in these compounds was analyzed in the framework of the natural bond orbital theory and natural resonance theory. Energy characteristics (heats of reactions and energy barriers) of the dissociation reactions → SiF₄ + F^– and → + F^– were calculated using the DFT and CCSD methods. The potential energy surface of elimination of a fluoride anion from has a specific topology with valley‐ridge inflection points corresponding to bifurcations of the minimal energy reaction path. © 2016 Wiley Periodicals, Inc.     

Enzyme‐Inspired Room‐Temperature Lithium–Oxygen Chemistry via Reversible Cleavage and Formation of Dioxygen Bonds

Li‐O₂ batteries are promising energy storage systems due to their ultra‐high theoretical capacity. However, most Li‐O₂ batteries are based on the reduction/oxidation of Li₂O₂ and involve highly reactive superoxide and peroxide species that would cause serious degradation of cathodes, especially carbon‐based materials. It is important to explore lithium‐oxygen reactions and find new Li‐O₂ chemistry which can restrict or even avoid the negative influence of superoxide/peroxide species. Here, inspired by enzyme‐catalyzed oxygen reduction/oxidation reactions, we introduce a copper(I) complex 3 N‐Cu^I (3 N=1,4,7‐trimethyl‐1,4,7‐triazacyclononane) to Li‐O₂ batteries and successfully modulate the reaction pathway to a moderate one on reversible cleavage/formation of O?O bonds. This work demonstrates that the reaction pathways of Li‐O₂ batteries could be modulated by introducing an appropriate soluble catalyst, which is another powerful choice to construct better Li‐O₂ batteries.     

Synthesis and Crystal Structure of Novel β‐Lactam Derivatives Bearing Quinoline Moiety via [2 + 2] Cycloaddition

A series of new β‐lactam derivatives 4a , 4b , 4c , 4d , 4e , 4f , 4g , 4h , 4i , 4j , 4k , 4l were synthesized from [2 + 2] cycloaddition reaction of imines containing quinoline moiety 3a , 3b , 3c , 3d , 3e , 3f with ketene generated in situ from chloroacetyl chloride in dry Et₃N. The structures of the prepared compounds were characterized by IR, MS, ¹H NMR, and elemental analysis. In addition, the structure of ( 4i ) was determined by single crystal X‐ray crystallography.     

β‐Substituted Terthiophene [2]Rotaxanes

Towards polythiophene polyrotaxanes : The β‐substituted terthiophene [2]rotaxanes have been synthesized (see figure). Basic optical and electrochemical properties of the synthesized [2]rotaxanes are also reported.

     

Two‐Dimensional Networks of [AuCl4]– and [AuBr4]– Anions

Several salts of protonated amines and aza‐aromatics with [AuCl₄]^– and [AuBr₄]^– anions contain two‐dimensional (“square”) anionic networks that display short halogen ··· halogen contacts. The Au₄ quadrilaterals formed by neighboring anions of the networks are to a good approximation squares, with sides of around 7.5 Å for tetrachloridoaurates and 8 Å for tetrabromidoaurates.