Degenerate rearrangement of long-lived 9-cyclopropyl-9,10-dimethyl-9,10-dihydrophenanthren-9-ylium: Migration of cyclopropyl group

Abstract

Dynamic NMR study showed that long-lived 9-cyclopropyl-9,10-dimethyl-9,10-dihydrophenanthren-9-yl cation generated from 9-cyclopropyl-9,10-dimethyl-9,10-dihydrophenanthren-9-ol in HSO 3 F-SO 2 ClF-CD 2 Cl 2 at -100 to -75°C undergoes fast degenerate 1,2-shift of the cyclopropyl group. The Arrhenius parameters for this process were determined: E a = 33.7 kJ/mol, log A = 13.0. R = cyclopropyl (a), Et (b), Me (c), Ph (d), CH 2 =CH (e).Rearrangements of carbocations via 1,2-shift of a migrating group may be regarded as specific intramolecular electrophilic substitution reactions [1]. Dynamic NMR studies on long-lived carbocations gave experimental data on the mechanism and velocities of such reactions [2]. We previously showed [3] that the energy barriers to degenerate carbocationic rearrangements involving 1,2-shift of hydrocarbon migrating groups in long-lived carbocations may be estimated by quantum-chemical calculations in terms of the density functional theory (DFT). The series of the examined rearrangements included such migrating groups as alkyl, aryl, and vinyl, and the rearrangement was considered to be intramolecular electrophilic substitution reaction at a tetragonal or trigonal carbon atom. Analogous rearrangements of carbocations in which the carbocationic center is capable of interacting with cyclopropyl group attracted considerable interest. Although the corresponding carbon atom in the cyclopropane ring is formally tetragonal, it is well known that cyclopropane resembles alkenes in chemical properties.1,2-Shifts are observed in solvolysis and cationoid rearrangements involving short-lived cationic intermediates [4-8]; however, we have found no published data on a analogous 1,2-shifts in long-lived carbocations. Therefore, the goal of the present study was to generate a long-lived carbocation having a cyclopropyl group at the α-carbon atom with respect to the cationic center and examine its degenerate rearrangement via 1,2-shift of the cyclopropyl group; it was also interesting to estimate the migrating ability of cyclopropyl group.We anticipated that long-lived carbocation with a cyclopropyl group at the α-carbon atom with respect to the cationic center could be generated in superacidic medium at low temperature, i.e., under the conditions hampering acid-catalyzed opening of cyclopropane ring. As shown previously, quite appropriate models for estimating migrating ability of different groups R are long-lived 9-R-9,10-dimethyl-9,10-dihydrophenanthrenyl cations I [2]. Such cations characteristically undergo degenerate rearrangements via 1,2-shift of the group R (Scheme 1).We planned to generate cation Ia from the corresponding alcohol II as shown in Scheme 2. The results of quantum-chemical calculations on the relative stabilities of cations III and Ia gave us grounds to believe that the fraction of cation Ia in equilibrium mixture