Activation Parameters for Ring Opening of (Bicyclo[n.1.1]alk-1-yl)methyl Radicals

Abstract

Ring opening of the (bicyclo[1.1.1]pent-1-yl)methyl (4), (bicyclo[2.1.1]hex-1-yl)methyl (5) and (bicyclo[3.1.1]hept-1-yl)methyl (6) radicals has been investigated by semiempirical molecular orbital theory MINDO/3 and AM1) and by experiment. Our results indicate that (4) ring-opens extremely rapidly, with an estimated energy barrier of less than 22 kJ mol-1. The higher homologues (5) and (6) rearrange more slowly with barriers of about 42.2 and 46.0 kJ mol-1 respectively. This trend is also observed in the MINDO/3 and AM1 results, with the latter giving the better agreement with experiment.

Bicyclo[1.1.1]pentanes, [n]Staffanes, [1.1.1]Propellanes, and Tricyclo[2.1.0.02,5]pentanes

“…This is in a reasonably good agreement with the value of 11.2 kcal/mol calculated at the UHF/6-31G* level of theory . For the 3-carboxymethyl-substituted radical 287 (Chart ) the experimental activation energy for the rearrangement is less than 5 kcal/mol . At a higher temperature (>210 K), the primary 3-methylenecyclobutylmethyl radical ( 295 ) product rearranges further into the 2-allylallyl radical ( 296 ). ,

123

…”

“…304 For the 3-carboxymethyl-substituted radical 287 (Chart 12) the experimental activation energy for the rearrangement is less than 5 kcal/mol. 306 At a higher temperature (>210 K), the primary 3-methylenecyclobutylmethyl radical (295) product rearranges further into the 2-allylallyl radical (296). 304,305 Photochemical decomposition of the Barton ester 67 in the presence of thiophenol as a hydrogen donor produced a mixture of 1-methylbicyclo[1.1.1]pentane (24) and the ring-opened 3-methylmethylenecyclobutane 297 in a ratio about 3:1 (Scheme 124).…”