Trapping of 4-Halo-2-chlorobicyclo[2.1.1]hex-1-ene: DFT Calculations on This and Related Molecules
摘要:
The reaction of 1-chloro-3-trichloromethylbicyclo[1.1.1]pentane 5a with an excess of MeLi leads to 1,3-dichloro-3,4-dimethylbicyclo[2.1.1]hexane (7a) as the major product in 33% yield, as well as to the bicyclo[2,1.1]hexane derivatives 6a and 8a. C-13 labeling shows that 7a is formed through two routes, the minor one constituting a trapping reaction of the elusive bicyclo[2.1.1]hex-1-ene (3) by MeLi. This bridgehead olefin is also trapped in an ene reaction with alpha-methylstyrene leading to 18 in 8 % yield, DFT calculations at the B3LYP/6-311G(d,p)//B3LYP/6-31G(d) level of theory show that 3-chlorobicyclo[1.1.1]pent-1-yl-chlorocarbene (2a) in its singlet electronic state is local minimum on the corresponding energy hypersurface, It rearranges over a barrier of only 7.9 kcal mol(-1) to the strongly pyramidalized bridgehead olefin 3 a, which shows a high propensity for a second rearrangement (barrier 8.4 kcal mol(-1)) to give carbene 4a. Hydrogen migration of 4a to afford 1,3-dichlorobicyclo[2.1.1]hex-2-ene (20a) needs a somewhat higher barrier of 13.2 kcal mol(-1) and is not observed under the experimental conditions employed in this work.