Neil A. Trout

Phenylgermane as a suitable precursor for laser flash photolysis measurements of germylene kinetics

Abstract R. Becerra, S.E. Boganov, M.P. Egorov, O.M. Nefedov and R. Walsh [Chem. Phys. Lett. 260 (1996) 433] reported the first direct kinetic measurements of germylene, GeH2, using two precursors, 3,4-dimethylgermacyclopentene and phenylgermane. They report that phenylgermane produces anomalously low reaction rate constants. We have re-examined this issue and measure rate constants using phenylgermane that agree with the values reported by Becerra et al. using the precursor 3,4-dimethylgermacyclopentene. The first direct rate constant measurement for GeH2+phenylgermane is reported. All rate constants are found to be independent of the total pressure (measured to 50 Torr). Photolysis of phenylgermane at 248 and 193 nm yields the same values for the rate constants within the experimental uncertainty, suggesting that relaxation of vibrationally excited germylene is not altering the measured reaction rate constants.

Transmission of polar substituent effects in the adamantane ring system as monitored by19F NMR

An extensive series of (E)/(Z)‐5‐substituted(X)adamant‐2‐yl fluorides (2 and 3, respectively) and (E)/(Z)‐4‐substituted(X)adamant‐1‐yl fluorides (4 and 5, respectively) were synthesized and characterized and their 19F chemical shifts measured in several solvents. Correlation of the 19F substituent chemical shifts (SCS) against polar field parameters (σF) together with comparisons against the 19F SCS of 4‐substituted(X)bicyclo[2.2.2]oct‐1‐yl fluorides (1) provide unequivocal evidence for the importance of electrostatic field and ‘through‐three‐bond’ electron delocalization (double hyperconjugation) effects as long‐range modes of transmission of polar effects in these saturated systems. The former effect is shown to be a function not only of spatial factors (angles and distance) but also the ‘stiffness’ of the C—F σ bond. The latter electronic mechanism is clearly the dominant factor regulating the 19F SCS of 2 and 4 but is switched ‘off’ in 3 and 5 as a result of specific stereoelectronic requirements. ©1998 John Wiley & Sons, Ltd.

Gas-phase facial diastereoselectivity of equatorial and axial 4-chloro-adamant-2-yl cations

The acid-catalyzed addition of CH3(18)OH to 2-methylene-adamantanes bearing a chlorine atom in the 4-equatorial (1e) or 4-axial (1a) position has been investigated in the gas phase, at 760 Torr, in the 40-120 degrees C temperature range. Two different experimental approaches were employed: (1) by adding neutral CH3(18)OH to the 2-methyl-4-Cl-adamant-2-yl cation, generated by protonation of the corresponding 2-methylene-4-Cl-adamantane (the extracomplex reaction) and (2) by reaction of 2-methylene-4-Cl-adamantane with CH3(18)OH2+, generated by methylation of H2(18)O (the intracomplex reaction). The crucial role of the nature of the noncovalent intermediates involved along the reaction coordinates emerges from the difference between the results obtained in the extracomplex and intracomplex reactions for both substrates investigated. The kinetic and stereochemical results indicate that the 4-Cl substituent plays a different role depending on its equatorial or axial orientation. Examination of the experimental results in the light of MP2/6-31G* theoretical calculations provides important information about the intrinsic factors governing the facial diastereoselectivity of trigonal carbocations. The effects due to differential face solvation phenomena emerge from the comparison of the present gas-phase results with those obtained from strictly related studies in solution.

The nature of electronic interactions governing the control of Π-facial selectivity in the capture of 5-substituted(X)-2-adamantyl radicals : electrostatic versus hyperconjugative effects

Abstract An electrostatic rather than a hyperconjugative effect appears to be the dominant factor governing the control of Π-facial selectivity in the capture of 5-substituted(X)-2-adamantyl radicals.