Ernest W. Della

Bicyclo[1.1.1]pent-1-yl: A tertiary alkyl radical with enhanced reactivity☆

Abstract Absolute rate constants for reactions of bicyclo[1.1.1]pent-1-yl radicals with α-methylstyrene (1.4 × 10 7 M −1 s −1 ) and 1,4-cyclohexadiene (4.6 × 10 5 M −1 s −1 ) at 25° C were measured by laser flash photolysis. This bridgehead radical is more reactive than tert -butyl which we attribute to its high s -character and the absence of steric shielding of the radical center.

Measurement of the 13C hyperfine tensors for cubyl and related bridgehead radicals

13 C hyperfine features have been detected by electron paramagnetic resonance (EPR) spectroscopy from a range of bridgehead radicals following γ-irradiation of the corresponding bromides at 77 K. For cubyl and its 4-methoxycarbonyl derivative, both parallel and perpendicular components of the 13C tensor were resolved, but only the parallel components could be detected for the 4-bromo derivative. The full 13C tensors were also obtained for bicyclo[2.2.1]heptyl and tricyclo[3.1.1.03,6]heptyl radicals, but for bicyclo[2.2.1]hexyl, bicyclo[2.2.2]octyl and bicyclo[1.1.1]pentyl radicals only the parallel components were apparent, the perpendicular regions being obscured by other signals.A linear correlation is found between A∥ and the one-bond C–H (1JC–H) coupling in the nuclear magnetic resonance (NMR) spectra of the parent bridgehead alkanes and is used in the estimation of isotropic coupling constants for the bridgehead radicals, even in those cases where A⊥ is unknown.

Cubyl and 4-fluorocubyl radicals

The ESR spectra of cubyl and 4-fluorocubyl radicals were observed in solution; they indicated that hyperconjugation was slight, but that significant spin density reaches C(3) and C(4).

Bridgehead carbocations: formation of a 1-bicyclo[2.1.1]hexyl cation as the primary intermediate in the solvolysis of 3-methoxycarbonylbicyclo[2.1.1]hexyl triflate

The 3-methoxycarbonylbicyclo[2.1.1]hexyl cation is the primary intermediate in the solvolysis of 3-methoxycarbonylbicyclo[2.1.1]hexyl triflate.

Hyperconjugation in strained bridgehead cyclobutyl cations: an ab initio study of bicyclo[1.1.1]pent-1-yl cubyl and norcubyl cations

Ab initio molecular orbital calculations (MP2/6-31G**) provide strong evidence that the bicyclo[1.1.1]pent-1-yl, cubyl and tricyclo[3.1.1.03,6]hept-6-yl (6-norcubyl) cations are principally stabilized by hyperconjugative interaction of the strained α–β and β–γ carbon–carbon bonds with the (vacant) cationic p-orbital; the degree of interaction is strongly dependent on the geometry of the charged four-membered ring in each case.

Enhanced solvolytic reactivity of 1-bromobicyclo[3.1.1]heptane: intermediacy of a stabilised bridgehead carbenium ion

1-Bromobicyclo[3.1.1]heptane displays greater solvolytic reactivity than t-butyl bromide and produces a bridgehead carbenium ion which appears to be stabilised by transannular orbital interactions.