The sequence of rearrangements of carbenes 6 has been studied. Precursors for 6 were 1-halo-3-(n-halomethyl)bicyclo[1.1.1]pentane derivatives 11, which were obtained by addition of polyhalomethanes to
the central bond of [1.1.1]propellane derivative 13 via radical chain processes. Reaction of compounds 11
with MeLi or sodium bis(trimethylsilyl)amide generates carbenes 6, which are stabilized by a carbene (6)−bridgehead olefin−carbene (9) rearrangement, followed by an intramolecular CH insertion reaction of carbenes
9 to afford tetracyclononane derivatives 15 in good yields. The carbenes 6 show only slight preferences in the
rearrangement to alkenes 7 and 8. Both could be trapped with α-methylstyrene as ene adducts 16 and 17. The
strongly twisted bridgehead olefin 7 has only the option to rearrange to carbene 9, whereas 8 could give 9 or
10, but prefers to give 9, in accordance with results of DFT calculations. The tetracyclononanes 15c and 15d
contain halide atoms in the 1,3-positions; on treatment with t-BuLi both compounds form a new CC bond by
reductive dehalogenation giving rise to the formation of a [2.1.1]propellane derivative 23 as short-lived
intermediate.
The reaction of 1-chloro-3-trichloromethylbicyclo[1.1.1]pentane 5 a with an excess of MeLi leads to 1,3-dichloro-3,4-dimethylbicyclo[2.1.1]hexane (7 a) as the major product in 33 % yield, as well as to the bicyclo[2.1.1]hexane derivatives 6 a and 8 a.13 C labeling shows that 7 a 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 amethylstyrene 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 (2 a) in its singlet electronic state is local minimum on the corresponding energy hypersurface.It rearranges over a barrier of only 7.9 kcal mol À1 to the strongly pyramidalized bridgehead olefin 3 a, which shows a high propensity for a second rearrangement (barrier 8.4 kcal mol
À1) to give carbene 4 a. Hydrogen migration of 4 a to afford 1,3-dichlorobicyclo[2.1.1]hex-2-ene (20 a) needs a somewhat higher barrier of 13.2 kcal mol À1 and is not observed under the experimental conditions employed in this work.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.