A theoretical study on the protonation of cycloalkanes C_nH_2n (n = 3 to 6)

Abstract

Ab initio self‐consistent‐field molecular orbital calculations have been carried out for the CnH2n (n = 3 to 6) cycloalkanes and various conformers of their protonated forms. The calculated protonation energies for the sequence of conformers of the protonated forms follow the experimentally observed trend. Correlations between optimum CCC bond angles at the protonation site and the calculated protonation energies have been observed, and these correlations may be of some use in estimating protonation energy‐bond angle relations in other (strained) cyclic compounds when the central carbon atom of a CCC moiety is protonated.

Hydrocarbon Activation via Reversible 1,2-RH-Elimination from (tBu3SiNH)3ZrR: Synthetic, Structural, and Mechanistic Investigations

“…Consider the binding of an alkane or arene to an electropositive, d 0 zirconium center to be analogous to protonation of RH, whose enthalpy change is −PA, where PA is the proton affinity of the hydrocarbon. , Figure illustrates a plot of Δ G ⧧ for 1,2-RH-elimination versus the proton affinity of the corresponding RH. When available, appropriate experimental PA values were utilized, but several were obtained from semiempirical AM1 calculations.…”

“…A charge of +1 was used for the protonated alkanes, and GEO-OK conditions allowed the inclusion of five-coordinate carbon. Calculations were performed on both the protonated alkanes and the corresponding unprotonated alkanes (neopentane, isobutane, toluene, mesitylene; also, calculations were checked against literature values for methane, cyclopropane and cyclohexane). , The heats of formation were then used to determine the proton affinity by eq 43. In all cases protonation was site-specific; the carbon which would be bonded to the metal in the corresponding zirconium hydrocarbyl was chosen as the site of protonation (e.g., the benzylic carbon in toluene).…”

Reactions of cyclopentane on HY Zeolite

“…There is also considerable evidence for the existence of protonated cyclopropanes as reaction intermediates in solution (Collins, 1969;Lee, 1970). Protonated cyclopropane structures have also been the subject of a number of theoretical studies (Ghio and Tomasi, 1973;Lee et al, 1984). Theoretical studies on protonation of ring systems, including cyclopentane (Lee et al, 1984) concluded that protonation energies for cyclopentanes and cyclohexanes are lower than for cyclopropanes.…”

“…Protonated cyclopropane structures have also been the subject of a number of theoretical studies (Ghio and Tomasi, 1973;Lee et al, 1984). Theoretical studies on protonation of ring systems, including cyclopentane (Lee et al, 1984) concluded that protonation energies for cyclopentanes and cyclohexanes are lower than for cyclopropanes. Some recent experimental studies (Lee and KO, 1981; Lee and KO, 1980) on isotopic exchange in cyclobutane, cyclopentane and cyclohexane using ordinary acids such as H2S04, have indicated that initial protonation of the rings does indeed occur at ordinary temperatures.…”