Peter P. Gaspar

The quest for triplet ground state silylenes

Abstract The scarcity of triplet silylenes compared with triplet carbenes can be understood in terms of the sizes of the valence orbitals. The larger size of the silicon orbitals leads to a decrease in the repulsion of the nonbonding electrons in the singlet state and hence their energy-lowering separation in the triplet state is less capable of compensating an attendant promotion energy. Calculations suggested that the effect of bulky substituents must be supplemented by a reduction of their electronegativity in order to reduce the promotion energy to the point that a triplet ground state can be achieved at an attainable bond angle. The culmination of this approach has been the generation of a silylene (t-Bu)3SiSiSi(iPr)3 that appears to react from its triplet ground state.

Sonochemical synthesis of 1,1-dimethyl-1-germacyclopent-3-enes and the extrusion of dimethylgermylene upon their pyrolysis

Abstract The ultrasound-promoted reaction of dichlorodimethylgermane with lithium in the presence of substituted 1,3-dienes results in the formation of the corresponding germacyclopent-3-enes in good yields. The gas-phase flow pyrolysis of 1,1,3-trimethyl- and 1,1,3,4-tetramethyl-1-germacyclopent-3-enes leads to the extrusion of dimethylgermylene that has been trapped by addition to 1,3-dienes.

The phenylcarbene rearrangement revisited

Abstract The evolution of mechanistic ideas about the phenylcarbene rearrangement has been reviewed, and three closely linked problems have been identified toward whose solution this research has been aimed: 1. Why do the ratios of the stable end products from the rearrangements of o-, m- and p-tolylmethylene differ when all three reactions have been throught to pass through a common intermediate? 2. Why does the rearrangement of 2-methylcycloheptatrienylidene lead to exclusive formation of styrene? 3. What is the mechanism of styrene formation from o-tolylmethylene? New mechanisms have been proposed in which m- and p-tolylmethylene can rearrange to styrene without necessarily being converted to o-tolylmethylene. The formation of a small amount of 2,6-dimethylstyrene from the rearrangement of 3,4,5-trimethylphenylmethylene is viewed as evidence for such a mechanism, and a set of interconverting norcaradienylidenes are believed to be the crucial intermediates. Other alternatives are considered and rejected on the basis of the rearrangement products of 3,5-dimethyl- and 3,4,5-trimethylphenylmethylene.

The methanium ion, CH5+. Evidence for the structure of a nonclassical ion from reaction studies by ion cyclotron resonance spectroscopy

Studies of proton and deuteron transfer from methanium ions to various substrates and collision‐induced dissociation of isotopically substituted methanium ions suggest that under the low‐energy conditions of the ICR experiment, the addition of a hydrogen ion to methane produces a methanium ion whose hydrogens are structurally and chemically distinguishable. The reactions of methanium ions formed in the low energy electron impact ionization of CH4–CD4 mixtures are compatible with a methanium ion of Cs symmetry in which intramolecular rearrangement does not occur. Reaction studies on the methanium ions formed by the exoergic proton transfer from D3+ to methane indicate that rapid intramolecular rearrangements occur in these ions.

Gas phase reactions of SiF2 with F2 and Cl2

Abstract Reactions of SiF 2 radicals have been studied in a fast-flow system. Rate constants at 295 K of (4.7±0.3)×10 −13 cm 3 molecule −1 s −1 for the reaction of SiF 2 + F 2 , and (5.1±0.6)×10 −13 cm 3 molecule −1 s −1 for SiF 2 + Cl 2 were obtained. No reaction was observed with O 2 and H 2 . SiF 2 was detected by laser-induced fluorescence, and lifetime observations and an excitation spectrum are reported.

Laser photolysis of 2-phenylheptamethyltrisilane

Two transient absorptions have been detected in the 266 nm laser photolysis of 2-phenylheptamethyltrisilane in cyclohexane solution at room temperature, and their time evolution was recorded in the presence and absence of air and added trapping agents. The shorter-lived 440 nm transient (t1/2 < 20μs) is tentatively assigned to the silylene :SiMePh and the more persistent 380 nm transient to the disilene MePhSiSiMePh. The reactivity of this silylene is much lower than had been expected.

Addition of fluorenylidene to 1,2-dichloroethylenes. Effects of additives and the question of spin-state equilibrium

Photolyse du diazo-9 fluorene dans les cis- et trans-dichloro-1,2 ethylene et le styrene. Stereospecificite de la cyclopropanation. Mecanismes. Etude des produits formes

Silylene additions to cyclopentadiene and 1,3-cyclohexadiene. Evidence for the addition mechanism

Abstract Additions of SiH2, SiCl2 and Si(CH3)2 to cyclopentadiene and of Si(CH3)2 to 1,3-cyclohexadiene were carried out by copyrolysis of an appropriate disilane and the diene. All observed products are believed due to 1,2-addition of silylenes forming bicyclic vinylsilacyclopropane derivatives which undergo non-concerted rearrangements.

2,6-Dimethoxyphenylphosphirane Oxide and Sulfide and their Thermolysis to Phosphinidene Chalcogenides—Kinetic and Mechanistic Studies

Abstract 2,6-Dimethoxyphenylphosphirane is readily converted to its oxide and sulfide whose pyrolysis and (perhaps) photolysis lead to the generation of phosphinidene chalcogenides Ar–P Z (Z=O,S). Trapping experiments were carried out under conditions similar to the kinetic studies of the phosphirane chalcogenide pyrolyses that confirmed the formation of free Ar–P Z. The trapping experiments were in accord with carbene-like character for Ar–P Z, and the activation parameters suggest a non-least motion pathway for the addition of Ar–P Z to olefins. This represents quantitative evidence for the validity of the predictions of frontier-orbital theory for species that undergo reactions with small (or no) enthalpic barriers.

The disappearance of aryl carbenes in a reactive matrix

Abstract The disappearance of diphenylmethylene, fluorenylidene and phenylmethylene in isobutylene and fluorolube was monitored by esr spectroscopy at temperatures between 77°K and 171°K.