C-13 NMR CHEMICAL-SHIFTS AND TRANSFER OF C-13 SPIN SATURATION AND THE STRUCTURES AND REARRANGEMENTS OF 9-BARBARALYL CATIONS, PROTONATED BARBARALONE, AND BARBARALONE

The usefulness of a recently developed technique for detection and quantitative study of exchange reactions,13C spin-transfer saturation, is demonstrated. It has been used to measure rates and elucidate reaction mechanisms for the degenerate rearrangements of 1,9-dimethyltricyclo[3.3.1,02,8]nona-3,6-dien-9-yl cation (1,9-dimethyl-9-barbaralyl cation) (2), 9-methyltricyclo[3.3.1,02,8]nona-3,6-dien-9-yl cation (9-methyl-9-barbaralyl cation) (3), protonated tricyclo[3.3.1.02,8]nona-3,6-dien-9-one (protonated barbaralone) (4), and tricyclo[3.3.1.02,8]nona-3,6-dien-9-one (barbaralone) (5). The ions were synthesized in Superacid using an ion generation apparatus for low-temperature synthesis (ca. 0 to ca. ~155°C). The13C chemical shifts of the ions show that they contain cyclopropylcarbinyl cation like structural elements. No significant charge delocalization into the double bonds was detected. Some of the13C spin relaxation times (T1) are also reported. Ion 3 is shown to undergo a 4 + 2 carbon scrambling through divinylcyclopropylcarbinyl cation-divinylcyclopropylcarbinyl cation rearrangements rather than through a regular Cope mechanism. This was inferred from the small rearrangement barrier, 7.63 + 0.21 (-0.18)kcal mol-1(-128.6°C) of 3 compared with those of 4(13.4+0.3(-0.2)kcal mol-1at -43.5°C) and 5 (10.2±0.1kcal mol-1at -93.2°C). The13C spin-transfer experiments with 2 show that this ion undergoes more extensive rearrangements than 3. The bridge constituted by C-1, -5, and -9 rotates in a stepwise fashion around the pseudoring made up by the six carbons C-2, -3, -4, -6, -7, and -8. The barrier for exchange of C-3.C-7 with C-2,C-8 and C-4,C-6 was 7.58 + 0.33 (-0.28) kcal mol-1at -128.0°C. Microscopic mechanisms for the “rotation” are discussed. The totally degenerate parent barbaralyl cation (1) is found to have a13C NMR spectrum at -136°C that shows essentially no signal at the S/N ratio of ca. 6 expected if there was no exchange. The NMR probe temperature was calibrated using a chemical reaction whose rate was determined in the probe as well as outside in a low-temperature thermostat (the ion generation apparatus). © 1979, American Chemical Society. All rights reserved.