Bridgehead Carbocations via Carbene Fragmentation: Erasing a 10¹⁰ Kinetic Preference

Abstract

1-Norbornyloxychlorocarbene (1-NorOCCl), 1-bicyclo[2.2.2]octyloxychlorocarbene (1-BcoOCCl), and 1-adamantyloxychlorocarbene (1-AdOCCl) were generated in dichloroethane (DCE) by photolysis of the appropriate diazirines. The exclusive product in each case was the bridgehead alkyl chloride formed by fragmentation of the carbene to [R(+) OC Cl(-)] ion pairs, loss of CO, and cation-anion collapse. In mixtures of methanol and DCE, each carbene gave three products: RCl, ROH, and ROMe. RCl and ROMe resulted from competition between ion pair collapse and methanol capture of the cation. ROH resulted from methanol capture of the carbene (before fragmentation), followed by eventual methanolysis and hydrolysis of ROCH(Cl)OMe. The ratios of carbene capture to carbene fragmentation fell in the order 1-NorOCCl > BcoOCCl > 1-AdOCCl; 1-Nor(+) was the least stable cation and the slowest to form by fragmentation, so that this carbene was the most readily captured. This trend was accentuated in methanol-pentane mixtures, where ionic fragmentation was further slowed in the less polar solvent. Laser flash photolysis with either UV or time-resolved infrared (TRIR) monitoring permitted the determination of the absolute rate constants for fragmentations of the carbenes in DCE at 25 degrees C. The rate constants (s(-1)) were: 1-NorOCCl (3.3 x 10(4)), 1-BcoOCCl (1.5 x 10(5)), and 1-AdOCCl (5.9 x 10(5)). The rate constants decreased in the order of increasing strain in the resulting bridgehead carbocation, but the range of rate constants was compressed to a factor of only approximately 18. This constrasts with the factor of 10(10) by which the acetolysis of 1-AdOTs at 70 degrees C exceeded that of 1-NorOTs. The fragmentation of 1-NorOCCl to the ion pair was 3 x 10(15) times faster than the acetolysis of 1-NorOTs. The activation energies were measured as 9.0 kcal/mol (log A = 11.2 s(-1)) for the fragmentation of 1-NorOCCl and 4.4 kcal/mol (log A = 8.44 s(-1)) for that of 1-BcoOCCl both in DCE. B3LYP/6-31G computed activation energies in simulated DCE were 14.6 and 2.7 kcal/mol, respectively.