Kinetics of cyclopropyl radical reactions. 3. Study of some 1-substituted cyclopropyl radicals by EPR spectroscopy. The inversion barrier for 1-methylcyclopropyl

Silicon Analogues of Cyclopropyl Radical Derived from a Highly Stable Cyclic Disilene Compound Featuring a Si-Br Bond

A halogen-substituted cyclic disilene compound, bromocyclotrisilene, Si3Br(Eind)3 (3a), bearing fused-ring bulky Eind (a: R1 = R2 = Et) groups, has been synthesized as an extraordinarily air-stable compound by the reduction of 1,2-dibromodisilene, (Eind)BrSi═SiBr(Eind) (2a), or tribromosilane, (Eind)SiBr3 (1a), with the Mg or Li metal. The X-ray diffraction analysis of 3a showed that the disilene moiety has an almost planar, but slightly trans-bent structure. Even though 3a is quite air-stable both in solutions and in the solid state, its Si-Br bond is reactive under reducing conditions. The further treatment of 3a with the Li metal leads to the formation of room-temperature thermally stable silicon homologues of the cyclopropyl radical, i.e., the cyclotrisilanyl radicals (6a) [6a(syn) and 6a(anti)], via intramolecular C-H bond activation in a transient silicon homologue of the cyclopropenyl radical, i.e., the cyclotrisilenyl radical, [Si3(Eind)3]• (5a). The formation mechanism of 6a from 5a is discussed based on the theoretical calculations. The unique structural and electronic properties of these Si3 three-membered ring species incorporating the Eind groups have been experimentally and theoretically investigated.

Ring-Opening Dynamics of the Cyclopropyl Radical and Cation: the Transition State Nature of the Cyclopropyl Cation

We provide compelling experimental and theoretical evidence for the transition state nature of the cyclopropyl cation. Synchrotron photoionization spectroscopy employing coincidence techniques together with a novel simulation based on high-accuracy ab initio calculations reveal that the cation is unstable via its allowed disrotatory ring-opening path. The ring strains of the cation and the radical are similar, but both ring opening paths for the radical are forbidden when the full electronic symmetries are considered. These findings are discussed in light of the early predictions by Longuet-Higgins alongside Woodward and Hoffman; we also propose a simple phase space explanation for the appearance of the cyclopropyl photoionization spectrum. The results of this work allow the refinement of the cyclopropane C-H bond dissociation energy, in addition to the cyclopropyl radical and cation cyclization energies, via the Active Thermochemical Tables approach.

High-Temperature Isomerization of Benzenoid Polycyclic Aromatic Hydrocarbons. Analysis through the Bent Bond and Antiperiplanar Hypothesis Orbital Model

L. T. Scott has discovered the 1,2-swapping of carbon and hydrogen atoms which is known to take place on benzenoid aromatics (up to ∼1000 °C range). For example, ¹³C-1-naphthalene is specifically converted to ¹³C-2-naphthalene, and there is evidence that this occurs through the formation of benzofulvene and a naphthalene-carbene intermediate. Application of the bent bond/antiperiplanar hypothesis leads to the postulate that higher in energy pyramidal singlet diradical intermediates can be used to propose a mechanism that rationalizes various atom rearrangements on benzenoid aromatics and related isomeric compounds.

Generation and Structure of Unique Boriranyl Radicals

Three-member ring boracyclopropanes (boriranes) with N-heterocyclic carbene substituents were prepared by a recently discovered route. H atoms were selectively abstracted from the boron atoms by t-butoxyl radicals and this enabled boriranyl radicals to be detected and characterized by EPR spectroscopy for the first time. Their EPR parameters indicated they had planar π-character. From competition experiments, the rate constant for H atom abstraction was determined and found to be about 2 orders of magnitude less than for NHC-boranes. The B-H BDE of an NHC-borirane was estimated to be about 95 kcal mol^-1.

Slit Discharge IR Spectroscopy of a Jet-Cooled Cyclopropyl Radical: Structure and Intramolecular Tunneling Dynamics

High-resolution infrared spectra of a jet-cooled cyclopropyl radical are reported for the first time, specifically sampling the in-phase antisymmetric CH2 stretch (nu7) vibration. In addition to yielding the first precise gas-phase structural information, the spectra reveal quantum level doubling into lower (+) and upper (-) states due to tunneling of the lone alpha-CH with respect to the CCC plane. The bands clearly reveal intensity alternation due to H atom nuclear spin statistics (6:10 and 10:6 for even:odd Ka+Kc in lower (+) and upper (-) tunneling levels, respectively) consistent with C2v symmetry of the cyclopropyl-tunneling transition state. The two ground-state-tunneling levels fit extremely well to a rigid asymmetric rotor Hamiltonian, but there is clear evidence for both local and global state mixing in the vibrationally excited nu7 tunneling levels. In particular, the upper (-) tunneling component of the nu7 state is split by anharmonic coupling with a nearly isoenergetic dark state, which thereby acquires oscillator strength via intensity sharing with this bright state. From thermal Boltzmann analysis of fractional populations, tunneling splittings for a cyclopropyl radical are estimated to be 3.2 +/- 0.3 cm(-1) and 4.9 +/- 0.3 cm(-1) in the ground and nu7-excited states, respectively. This analysis indicates ground-state stereoracemization of the alpha-CH radical center to be a very fast process [k approximately 2.0(4) x 10(11) s(-1)], with the increase in the tunneling rate upon CH2 in-phase asymmetric stretch excitation consistent with ab initio predictions of equilibrium vs transition-state zero-point energies. Modeling of the ground-state-tunneling splittings with high level ab initio 1D potentials indicates an improved V0 = 1115 +/- 35 cm(-1) barrier height for alpha-CH inversion through the cyclopropyl CCC plane.