A novel entry to xanthones by an intramolecular Diels-Alder reaction involving 2-(1,2-dichlorovinyloxy) aryl dienones

A wide array of synthetic methods are described in the literature for the preparation of xanthones—a prominent class of tricyclic molecules that occur widely in nature. Majority of these reported methods involve linking the two aromatic rings and forming the central pyrone ring using a variety of classical and non-classical cyclization strategies. In a conceptually different approach, we describe here a new xanthone synthesis wherein both the pyrone and the second aromatic rings were forged in a single step by an intramolecular cycloaddition reaction involving 2-(1,2-dichlorovinyloxy) aryldienones.     

A Facile Synthesis of Bridged‐tricyclic Skeletons via Intramolecular Diels‐Alder Reaction of Cyclic 1,3‐Dienes Containing an α,β‐Unsaturated Ester

Intramolecular Diels‐Alder reaction (IMDA) precursors are easily available starting from addition of ester functionalized zinc‐copper reagents to cyclohexadienyl‐ and cycloheptadienylirontricarbonyl cation salts. The resulting cyclic 1,3‐dienes containing an α,β‐unsaturated ester functionality underwent smoothly IMDA reaction to afford bridged tricyclic compounds. Bridged heterotricyclic skeletons were also available via IMDA reaction of cyclic 1,3‐dienes bearing an imine or aldehyde functionality.     

Role of Hydrocarbon Radicals CHx (x=1, 2, 3) in Graphene Growth: A Theoretical Perspective

Many outstanding properties of graphene are blocked by the existence of structural defects. Herein, we propose an important healing mechanism for the growth of graphene, which is produced via plasma‐enhanced chemical vapor decomposition (PECVD), that is, the healing of graphene with single vacancies by decomposed CH₄ (hydrocarbon radical CH_x, x=1, 2, 3). The healing processes undergo three evolutionary steps: 1) the chemisorption of the hydrocarbon radicals, 2) the incorporation of the C atom of the hydrocarbon radicals into the defective graphene, accompanied by the adsorption of the leaving H atom on the graphene surface, 3) the removal of the adsorbed H atom and H₂ molecule to generate the perfect graphene. The overall healing processes are barrierless, with a huge released heat of 530.79, 290.67, and 159.04 kcal mol^?1, respectively, indicative of the easy healing of graphene with single vacancies by hydrocarbon radicals. Therefore, the good performance of the PECVD method for the generation of graphene might be ascribed to the dual role of the CH_x (x=1, 2, 3) species, acting both as carbon source and as defect healer.     

Conjugation‐Promoted Reaction of Open‐Cage Fullerene: A Density Functional Theory Study

Density functional theory calculations are performed to study the addition mechanism of e‐rich moieties such as triethyl phosphite to a carbonyl group on the rim of a fullerene orifice. Three possible reaction channels have been investigated. The obtained results show that the reaction of a carbonyl group on a fullerene orifice with triethyl phosphite most likely proceeds along the classical Abramov reaction; however, the classical product is not stable and is converted into the experimental product. An attack on a fullerene carbonyl carbon will trigger a rearrangement of the phosphate group to the carbonyl oxygen as the conversion transition state is stabilized by fullerene conjugation. This work provides a new insight on the reactivity of open‐cage fullerenes, which may prove helpful in designing new switchable fullerene systems.     

Cyclodextrins in Polymer Modification: Diels–Alder Addition of Cyclopentadiene/Methylated‐β‐Cyclodextrin Complex on Unsaturated Polyester and Formation of a New Type of Polypseudorotaxane

Summary: Cyclopentadiene ( 1 ) was incorporated as a guest into the cavity of randomly methylated‐β‐cyclodextrin (me‐β‐CD) as a host, yielding the stable, water compatible cyclopentadiene/me‐β‐CD complex ( 1a ). We successfully attempted to use the synthesised complex in a Diels–Alder addition with a water‐soluble unsaturated polyester ( 2 ) derived from poly(ethylene glycol) and maleic anhydride. The reaction yielded a new type of polypseudorotaxane ( 3 ). Examination of the polypseudorotaxane ( 3 ) and a model inclusion complex of the starting unsaturated polyester with me‐β‐CD ( 2a ) showed that cyclodextrins are threaded onto the main chain in both cases. The cyclohexene moiety formed after the Diels–Alder addition does not act as a stopper, a dethreading process being evidenced and discussed.

The polypseudorotaxane synthesized here.     

Diels–Alder and Retro‐Diels–Alder Cycloadditions of (1,2,3,4,5‐Pentamethyl)cyclopentadiene to La@C2v‐C82: Regioselectivity and Product Stability

One of the most important reactions in fullerene chemistry is the Diels–Alder (DA) reaction. In two previous experimental studies, the DA cycloaddition reactions of cyclopentadiene (Cp) and 1,2,3,4,5‐pentamethylcyclopentadiene (Cp*) with La@C_2v‐C₈₂ were investigated. The attack of Cp was proposed to occur on bond 19 , whereas that of Cp* was confirmed by X‐ray analysis to be over bond o . Moreover, the stabilities of the Cp and Cp* adducts were found to be significantly different, that is, the decomposition of La@C_2v‐C₈₂Cp was one order of magnitude faster than that of La@C_2v‐C₈₂Cp*. Herein, we computationally analyze these DA cycloadditions with two main goals: First, to compute the thermodynamics and kinetics of the cycloadditions of Cp and Cp* to different bonds of La@C_2v‐C₈₂ to assess and compare the regioselectivity of these two reactions. Second, to understand the origin of the different thermal stabilities of the La@C₈₂Cp and La@C₈₂Cp* adducts. Our results show that the regioselectivity of the two DA cycloadditions is the same, with preferred attack on bond o . This result corrects the previous assumption of the regioselectivity of the Cp attack that was made based only on the shape of the La@C₈₂ singly occupied molecular orbital. In addition, we show that the higher stability of the La@C₈₂Cp* adduct is not due to the electronic effects of the methyl groups on the Cp ring, as previously suggested, but to higher long‐range dispersion interactions in the Cp* case, which enhance the stabilization of the reactant complex, transition state, and products with respect to the separated reactants. This stabilization for the La@C₈₂Cp* case decreases the Gibbs reaction energy, thus allowing competition between the direct and retro reactions and making dissociation more difficult.     

Ab Initio Study on the Mechanism of Cycloaddition Reaction between Silylene Carbene （H_2Si=C：） and Acetone

The mechanism of cycloaddition reaction between singlet silylene carbene and acetone has been investigated with CCSD(T)//MP2/6-31G method. From the potential energy profile, it can be predicted that the reaction has two competitive dominant reaction pathways. One consists of two steps: (1) the two reactants (R1, R2) firstly form a four-membered ring intermediate (INT4) through a barrier-free exothermic reaction of 585.9 kJ/mol; (2) Then intermediate (INT4) isomerizes to CH3-transfer product (P4.1) via a transition state (TS4.1) with energy barrier of 5.3 kJ/mol. The other is as follows: on the basis of intermediate (INT4) created between R1 and R2, intermediate (INT4) further reacts with acetone (R2) to form the intermediate (INT5) through a barrier-free exothermic reaction of 166.3 kJ/mol; Then, intermediate (INT5) isomerizes to a silicic bis-heterocyclic product (P5) via a transition state (TS5), for which the barrier is 54.9 kJ/mol. The presented rule of this reaction: the [2+2] cycloaddition effect between the π orbital of silylene carbene and the π orbital of π-bonded compounds leads to the formation of a four-membered ring intermediate (INT4); The unsaturated property of C atom from carbene in the four-membered ring intermediate (INT4) results in the generation of CH3-transfer product (P4.1) and silicic bis-heterocyclic compound (P5).     

Special reactions of α,β-unsaturated ketones: Part II. Structure and stereochemistry of perhydro-5a,11a-epoxydibenz[b,f]oxepin-4a-ol,a bridged Diels–Alder dimer of 2-methylenecyclohexanone

The paper deals with the structure elucidation of (4aRS,5aRS,9aRS,11aRS)-perhydro-5a,11a-epoxydibenz[b,f]oxepin-4a-ol ( 6 ), which is unexpectedly formed from 2-dimethylaminomethylcyclohexanone hydrochloride ( 1 ·HCl) in the presence of phenylguanidine in boiling ethanol. In detail, two molecules of 2-methylenecyclohexanone (generated from 1 by elimination of dimethylamine) afforded via Diels–Alder reaction, subsequent addition of water and ring closure a product C₁₄H₂₂O₃, which finally proved to be the title compound. The structure of the tetracyclic hemiacetal 6 was deduced from one- and two-dimensional NMR experiments (1D TOCSY, DEPT-135, ¹H,¹H-COSY, gs-HSQC, gs-HMBC, gs-HSQC-TOCSY). In particular, the one-dimensional selective TOCSY and the gradient-selected HSQC-TOCSY, performed as a pseudo-3D experiment without the evolution period in the TOCSY part, proved to be very helpful for the unambiguous assignment of signals in overcrowded regions. NOE data confirmed the stereochemistry of epoxydibenzoxepinol 6 . Copyright © 2001 John Wiley & Sons, Ltd.     

{Sn10[Si(SiMe3)3]4}2−: A Highly Reactive Metalloid Tin Cluster with an Open Ligand Shell

The reaction of a Sn^ICl solution with LiSi(SiMe₃)₃ gave the anionic metalloid tin cluster {Sn₁₀[Si(SiMe₃)₃]₄}^2? ( 7 ) in good yield. The arrangement of the ten tin atoms in the cluster core can be described as a distorted centaur polyhedron. Quantum chemical calculations suggest that there are 26 bonding electrons in the cluster core, which may be described as an arachno cluster in agreement with Wade’s rules. NMR and mass spectrometric investigations showed that 7 is highly reactive, which may be due to the open ligand shell. The easily available tin atoms in 7 thereby open the door to further subsequent reactions, in which 7 may act as a building block to larger cluster aggregates.     

过渡金属氧化物(M2O5)+m=1,2(M=V, Nb, Ta)与C2H4气相反应机理的密度泛函研究

采用密度泛函理论研究了过渡金属钒族氧化物阳离子团簇(M2O5)+m=1,2(M=V, Nb, Ta)与C2H4气相反应机理. 反应为(M2O5)m++C2H4→(M2O5)m-1M2O4++C2H4O, 反应物先化合生成C—O键相连的化合物, 经过过渡态后M—O键断裂, 从而发生氧原子转移到碳氢化合物上的反应. 对于V2O5+与C2H4的反应, 存在经顺式和反式两种过渡态结构路径, 从能量上看, 经反式过渡态结构的路径更有利. 计算结果表明, 发生反应时C2H4与钒氧化物阳离子反应大量放热, 而与铌、钽氧化物阳离子反应却放热较少甚至不放热, 这与实验结果一致. 钒、铌、钽氧化物阳离子团簇发生氧转移反应活性不同的原因是金属-氧键的强弱不同所致.     

Exploring the Potential Energy Surface of E2P4 Clusters (E=Group 13 Element): The Quest for Inverse Carbon‐Free Sandwiches

Inverse carbon‐free sandwich structures with formula E₂P₄ (E=Al, Ga, In, Tl) have been proposed as a promising new target in main‐group chemistry. Our computational exploration of their corresponding potential‐energy surfaces at the S12h/TZ2P level shows that indeed stable carbon‐free inverse‐sandwiches can be obtained if one chooses an appropriate Group 13 element for E. The boron analogue B₂P₄ does not form the D_4h‐symmetric inverse‐sandwich structure, but instead prefers a D_2d structure of two perpendicular BP₂ units with the formation of a double B?B bond. For the other elements of Group 13, Al–Tl, the most favorable isomer is the D_4h inverse‐sandwich structure. The preference for the D_2d isomer for B₂P₄ and D_4h for their heavier analogues has been rationalized in terms of an isomerization‐energy decomposition analysis, and further corroborated by determination of aromaticity of these species.     

A DFT Study on the Conversion of Aryl Iodides to Alkyl Iodides: Reductive Elimination of R−I from Alkylpalladium Iodide Complexes with Accessible β‐Hydrogens

DFT calculations have been performed on the palladium‐catalyzed carboiodination reaction. The reaction involves oxidative addition, alkyne insertion, C?N bond cleavage, and reductive elimination. For the alkylpalladium iodide intermediate, LiOtBu stabilizes the intermediate in non‐polar solvents, thus promoting reductive elimination and preventing β‐hydride elimination. The C?N bond cleavage process was explored and the computations show that PPh₃ is not bound to the Pd center during this step. Experimentally, it was demonstrated that LiOtBu is not necessary for the oxidative addition, alkyne insertion, or C?N bond cleavage steps, lending support to the conclusions from the DFT calculations. The turnover‐limiting steps were found to be C?N bond cleavage and reductive elimination, whereas oxidative addition, alkyne insertion, and formation of the indole ring provide the driving force for the reaction.     

茂金属配合物[(eat5-C5H4R)Mo(CO)3 ]2(R: SiMe3 ,Si2Me5)的电子结构研究

运用G98W,采用Lanl2dz基组,对茂金属配合物[(eat5-C5H4R)Mo(CO)3]2 (R: SiMe3, Si2Me5)进行从头算研究,探讨配合物结构单元的稳定性、分子轨道能量、原子净电荷布居规律,以及一些前沿分子轨道的组成特征等,结果表明,标题配合物结构在能量上是稳定的,作为结构单元而存在.     

Preparation of 2-Alkyl- and 2-Acylpropenals from 5-(Trifluoromethanesulfonyloxy)-4H-1,3-dioxin: A Versatile Acrolein α-Cation Synthon

5-(Trifluoromethanesulfonyloxy)-4H-1,3-dioxin (3) participates in a variety of nucleophilic substitution reactions with cuprate reagents or in palladium catalyzed cross-coupling reactions to provide 5-substituted-4H-1,3-dioxins 5. Upon thermolysis, these compounds undergo facile retrocycloaddition reactions to generate the corresponding 2-substituted acroleins which, if necessary, can be trapped in situ with dienes or heterodienophiles. In particular, the heretofore unknown 2-acylacroleins can be generated using this methodology and trapped with enol ethers to afford 5-acyl-3,4-dihydro-2H-pyrans (6g,h), a substructural unit common to many natural products.     

Configurations of the [Pd(NO2)4]2- Complex Anion: Crystal Structure of Ag4A2[M(NO2)4]3 (M = Pd, Pt; A = K, Rb)

An integrated Xray diffraction study was performed on polycrystals and single crystals of three new isostructural phases with general formula Ag₄A₂[M(NO₂)₄]₃ (M = Pd, Pt; A = K, Rb). Data on the crystal structure solution (CAD4 diffractometer, MoK radiation, graphite monochromator = 2–30° are presented. In one crystallographically independent [M(NO₂)₄]^2- complex anion, the planar square coordination of the central atom is completed to 4 + 2 by two oxygen atoms at a distance of 3.02–3.12 in the other anion, it is completed to 4 + 1 + 1 by an oxygen atom at a distance of 3.12–3.30 and an Ag⁺ cation at a distance of 3.04–3.11 . Part of the Ag⁺ cations form Ag - Ag dimers with a distance of 3.03–3.07. Crystalchemical analysis of known structures containing [Pd(NO₂)₄]^2- complex anions was performed. It has been established that in none of the cases do any of the possible limiting configurations occur.     

A new method for the generation of stable phenoxyl radicals by the reaction of [(Me3Si)2N]2E (E = Ge,Sn) with sterically hindered phenols

An unusual reaction of diaminogermylene and diaminostannylene with sterically hindered phenols which leads to the formation of stable phenoxyl radicals in high concentrations was found. The reaction mechanism was proposed. Amides [(Me₃Si)₂N]₂E (E = Ge, Sn) were investigated electrochemically.