ISSN:
0894-3230
Keywords:
Organic Chemistry
;
Physical Chemistry
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
,
Physics
Notes:
The 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.
Additional Material:
1 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/poc.610030602
