A study of a novel degenerate carbocation rearrangement of the 4,9‐dimethyl‐9‐barbaralyl cation by dynamic 13C and 1H NMR spectroscopy in superacid

Abstract

AbstractThe 4,9‐dimethyltricyclo [3.3.1.02,8]nona‐3,6‐dien‐9‐yl (4,9‐dimethyl‐9‐barbaralyl) cation (3) was generated from 4,9‐dimethyl‐9‐barbaralol (5) at −135°C in two different superacid mixtures [FSO3HSO2CIFSO2F2(1:6:1) and FSO3HSO2CIFSO2F2CHCl2 (2:10:10:1 by volume)]. Its 1H and 13CNMR spectra show a strong temperature dependence in the range −150 to −125°C. The changes in band shapes with temperature show that the following exchanges take place: 4‐methyl with 9‐methyl, C‐4 with C‐9, C‐1 with C‐3 and C‐2 with C‐8. C‐5, C‐6 and C‐7 are found not to exchange rapidly either with each other or with the other carbons in 3. The mechanism of this novel rearrangement is suggested to involve the bicyclic 2,7‐dimethylbicyclo [3.2.2] nona‐3,6,8‐trienyl cation and the secondary barbaralyl cation 4,6‐dimethyltricyclo [3.3.1.02,8] nona‐3,6‐dien‐9‐yl as intermediates rather than 7,8‐dimethyl bicyclo [3.2.2] nona‐3,6,8‐trienyl cation, which does not have a methyl group on the allyl cation moiety. Comparisons with rearrangement mechanisms for other barbaralyl cations were also made. The rate constant for the degenerate rearrangement of 3 is 160 s−1 at −140°C, which corresponds to ΔG≠s = 26 kJ mol−1 (6·3 kcal mol−1). At −125°C ion 3 rearranges non‐degenerately to the 1,4‐bishomotropylium cation 1,8‐dimethylbicyclo [4·3·0] nona, 2,4,7‐trienyl (4) with k = 3 × 10−4 s−1. A mechanism for this rearrangement and the synthesis and purification of the ion precursor 4 are also reported.