Theoretical Calculations on the Wittig Reaction Revisited

A series of Wittig reactions was calculated at the HF/3-21G* and B3LYP/6-31G* levels to understand the origin of the different product selectivities for different classes of ylides. Both alkylidenetriphenylphosphorane (nonstabilized ylide) and benzylidenetriphenylphosphorane (semistabilized ylide) yielded two types of transition states (TS) with a nearly planar and a puckered structure. The planar TS gave trans oxaphosphetane (OP), whereas the puckered TS led to cis OP. In contrast to previous semiempirical calculations, the present calculations showed that while a planar trans TS is more stable than a puckered cis TS for the semistabilized ylide, a puckered cis TS is more stable for the reaction of the nonstabilized ylide with benzaldehyde. These calculated selectivities agree with experiment. The carbonyl carbon kinetic isotope effects (KIEs) were computed at HF/3-21G* for the reactions of benzaldehyde with butylidenetriphenylphosphorane and with benzylidenetriphenylphosphorane. The reaction of the semistabilized ylide gave 13C KIE of 1.051 at 0 °C, which is in qualitative agreement with the experimental KIE. In contrast, 13C KIE for the reaction of the nonstabilized ylide with benzaldehyde was calculated to be 1.039, disagreeing with the experimental isotope effect of unity. This implies that although the product selectivity is reproduced by a pair of the planar trans TS and the puckered cis TS, the latter may not be the true rate-determining TS for the cis-OP formation process for the nonstabilized ylide reaction

Structural Determination on Yb@C₇₈ Reveals an Unexpected Relationship of Yb@C_2<i>n</i> (2<i>n</i> = 74–80)

With combined quantum chemical and statistical thermodynamic methods, we performed a systemic investigation on the endohedral metallofullerene, Yb@C78, in order to determine its cage structure and the metal position. Our results revealed that Yb@C78 possesses an IPR-satisfying cage structure, C2v(24107)-C78, which is obviously different from the cage structures of previously found C78-based metallofullerenes. Interestingly, the internal metal is found to be located upon a pyracylene motif apart from the 2-fold axis of the C2v(24017)-C78 cage, displaying another new example of metallofullerenes with the internal metal locating asymmetrically. The reason why ytterbium exhibits such an anomalous location stems from a comprehensive effect of the three main following factors: coordination number of ytterbium, surface curvature release, and electrostatic interaction. More importantly, an unexpected relationship among the cage structures of Yb@C2n (2n = 74–80) through C2 insertion and Stone–Wales transformations is found, offering opportunities to further investigate the formation mechanism of endohedral fullerenes

Cycloaddition of Benzyne to Armchair Single-Walled Carbon Nanotubes: [2 + 2] or [4 + 2]?

The reaction mechanism and regioselectivity of cycloaddition reactions of benzyne to armchair single-walled carbon nanotubes were investigated with quantum chemical methods. The [2 + 2] cycloaddition reaction follows the diradical mechanism, whereas the [4 + 2] cycloaddition reaction adopts the concerted mechanism. More importantly, the [2 + 2] product is always more stable thermodynamically than the [4 + 2] ones, regardless of the diameter, while the [4 + 2] cycloaddition becomes kinetically more favored as the diameter goes up

Hydrazine and Thermal Reduction of Graphene Oxide: Reaction Mechanisms, Product Structures, and Reaction Design

The density functional theory method (M05-2X/6-31G(d)) was used to investigate reaction mechanisms for deoxygenation of graphene oxides (GOs) with hydrazine or heat treatment. Three mechanisms were identified as reducing epoxide groups of GO with hydrazine as a reducing agent. No reaction path was found for the hydrazine-mediated reductions of the hydroxyl, carbonyl, and carboxyl groups of GO. We instead discovered the mechanisms for dehydroxylation, decarbonylation, and decarboxylation using heat treatment. The hydrazine de-epoxidation and thermal dehydroxylation of GO have opposite dependencies on the reaction temperature. In both reduction types, the oxygen functionalities attached to the interior of an aromatic domain in GO are removed more easily, both kinetically and thermodynamically, than those attached at the edges of an aromatic domain. The hydrazine-mediated reductions of epoxide groups at the edges are suspended by forming hydrazino alcohols. We provide atomic-level elucidation for the deoxygenation of GO, characterize the product structures, and suggest how to optimize the reaction conditions further

The First Stable Heteracyclopropabenzene: Synthesis and Crystal Structure of a Silacyclopropabenzene

The First Stable Heteracyclopropabenzene:  Synthesis and Crystal Structure of a Silacyclopropabenzen

1,6,7-Trigermabicyclo[4.1.0]hept-3-en-7-yl: The Isolable Bicyclic Germyl Radical

The one-electron oxidation reaction of potassium 3,4-dimethyl-1,6,7-tris(tri-tert-butylsilyl)-1,6,7-trigermabicyclo[4.1.0]hept-3-en-7-ide (2-·K+) with tris(pentafluorophenyl)borane in THF results in the formation of stable 3,4-dimethyl-1,6,7-tris(tri-tert-butylsilyl)-1,6,7-trigermabicyclo[4.1.0]hept-3-en-7-yl (3•), representing the first bicyclic germyl radical with the bicyclo[4.1.0]hept-3-ene skeleton. The germyl radical 3• was characterized by X-ray crystallographic analysis as well as ESR spectroscopy, showing that it has a near-planar Ge-radical center

Dichlorocarbene Addition to C₆₀ from the Trichloromethyl Anion: Carbene Mechanism or Bingel Mechanism?

The reactions of C60 and trichloromethyl anion (CCl3−) via both the Bingel mechanism and the carbene mechanism were comparably studied by means of density functional theory (DFT) computations. The Bingel mechanism is highly competitive as compared with the carbene mechanism that leads to the formation of C60(CCl2). Unlike the carbene mechanism with a weak regioselectivity and solvent sensitivity, the Bingel mechanism yields the [6,6]-C60(CCl2) isomer as the exclusive product and favors highly polar solvents. The results receive strong experimental support and simultaneously rationalize these experimental findings

Is the Isolated Pentagon Rule Always Satisfied for Metallic Carbide Endohedral Fullerenes?

Quantum-chemical calculations reveal that metallic carbide endohedral fullerene Y2C2@C84 possesses a novel fullerene cage, C1(51383)-C84, with one pair of pentagon adjacencies. One of the encapsulated yttrium atoms is located on the adjacent pentagons, while the other stays on a hexagonal ring in the fullerene cage. As one of numerous metallic carbide endohedral fullerenes, Y2C2@C1(51383)-C84 is the first example that violates the well-known isolated pentagon rule (IPR). More interestingly, compared with the fact that Sc2C2@C84 has a conventional IPR-satisfying cage, D2d(51591)-C84, Y2C2@C84 utilizes the novel fullerene cage C1(51383)-C84 with one pair of pentagon adjacencies

Dispersion Force Effects on the Dissociation of “Jack-in-the-Box” Diphosphanes and Diarsanes

The dissociation of the sterically encumbered diphosphanes and diarsanes [:E­{CH­(SiMe₃)₂}₂]₂ (E = P or As) and [:E­{N­(SiMe₃)₂}₂]₂ (E = P or As) into :Ė{CH­(SiMe₃)₂}₂ or :Ė{N­(SiMe₃)₂}₂ radical monomers was studied computationally using hybrid density functional theory (DFT) at the B3PW91 with the 6–311+G­(d) basis set for P and As, and the 6–31G­(d,p) basis set for other atoms. The structures were reoptimized with the dispersion corrected B3PW91–D3 method to estimate dispersion force effects. The calculations reproduced the experimental structural data for the tetraalkyls with good accuracy. Without the dispersion correction, negative dissociation energies of −10.3 and −6.5 kcal mol^–1 were calculated for the phosphorus and arsenic tetraalkyls, indicating that the radical monomers are more stable. In contrast, the incorporation of dispersion force effects afforded high, positive dissociation energies of +37.6 and +37.1 kcal mol^–1 that favored dimeric structures. The dissociation energies (without dispersion) calculated for the tetraamido-substituted dimer are also negative, but changed to positive values of +29.3 and +32.5 kcal mol^–1 upon optimization with the D3 dispersion term. In contrast to earlier calculations, which indicated that the release of accumulated strain energy within the tetraalkyl dimers was the driving force for dissociation to monomers (i.e., the “Jack-in-the-Box” molecular model), the current calculations show that dispersion force attractive interactions exceed those of ligand relaxation and stabilize the dimeric structures. Single-point MP2 (second-order Møller–Plesset perturbation theory) calculations including dispersion effects afforded dissociation energies of 30.4 and 30.8 kcal mol^–1 for the tetraalkyl species, suggesting that the addition of the D3 dispersion term to the B3PW91 functional may overestimate such forces by 7–8 kcal mol^–1. It is concluded that the balance of dispersion forces and entropic effects are the major determinants of the dissociation equilibria

Mechanism and Dynamic Correlation Effects in Cycloaddition Reactions of Singlet Difluorocarbene to Alkenes and Disilene

Mechanisms of the cycloaddition reactions of singlet difluorocarbene (CF2) to alkenes and disilene were studied using CASSCF, MR-MP2, CR-CC(2,3), and UB3LYP methods in combination with basis sets up to 6-311++G(3d,p). The CASSCF(4,4) energies suggest that the cycloadditions all follow the stepwise mechanism. However, energies calculated using the MR-MP2(4,4) and CR-CC(2,3) methods in combination with the 6-311G(d) or larger basis sets consistently show that the reactions follow a concerted mechanism. The stepwise mechanisms predicted at the CASSCF level are “artificial” because of their neglect of dynamic electron correlation effects. The importance of dynamic electron correlation in determining the mechanistic nature of the reactions is explained through knowledge of the reacting system’s geometries and charges along the reaction path