1‐Phenyl‐1,2‐cyclohexadiene: Generation, Interception by Activated Olefins, Dimerisation and Trimerisation

Abstract

Four possible precursors of 1-phenyl-1,2-cyclohexadiene (2) were examined, namely, 6,6-dibromo-1-phenylbicyclo[3.1.0]hexane, (1alpha,5alpha,6alpha)-6-bromo-6-fluoro-1-phenylbicyclo[3.1.0]hexane, 1-bromo-2-phenylcyclohexene and 1-bromo-6-phenylcyclohexene. All four compounds could be converted into 2, as demonstrated by the products of the interception of 2 with activated olefins. Styrene, 1,1-diphenylethene, indene, furan and 2,5-dimethylfuran were employed as such. Whereas the first three gave [2+2] cycloadducts of 2, the last two provided one [4+2] cycloadduct each. To create the [2+2] cycloadducts, the pi bond of 2 that is more remote from the phenyl group reacted, whereas the pi bond of 2 conjugated with the phenyl group exclusively produced the [4+2] cycloadducts. The generation of 2 in the absence of a trapping reagent brought about relatively good yields of a dimer or a trimer of 2 depending on the mode of the liberation of 2. Being derivatives of triphenylene, the dimer as well as the trimer have unusual structures, thereby indicating that a phenyl group is participating in the formation of these compounds. The most surprising structure of the trimer was elucidated by X-ray crystal diffraction. As to the mechanisms, diradical intermediates are proposed both for the cycloadditions and for the dimerisation. The initial steps of the latter seem to proceed also in the trimerisation.

1‐Phenyl‐1,2‐cyclohexadiene: Astoundingly High Enantioselectivities on Generation in a Doering–Moore–Skattebøl Reaction and Interception by Activated Olefins

“…[5] The pure enantiomers and the non-racemic mixtures were identified as to their constitution and, where applicable, their diastereomeric nature by NMR spectroscopy.…”

“…Previously, we had obtained the racemic [4+2] cycloadduct rac-6 of rac-7 with this furan on treatment of 19 with potassium tert-butoxide in 9 % yield. [5] The chiral and enantiomerically pure base now brought about a 26 % yield of 6 and ent-6, the ratio of which was determined from the specific rotation ( www.chemeurj.org in the presence of 2,5-dimethylfuran with formation of pure 6, we believe that the equilibration of (M)-and (P)-7 did not proceed here, either. Given this prerequisite, the asymmetric induction of the enantiomerically pure potassium menthoxide is low, that is, the base recognised only a very small difference between the enantiotopic protons of the 6-methylene group of 19.…”

“…The results are comprehensively reported in the preceding paper. [5] Results and Discussion Generation of the cyclic allenes (M)-and (P)-7 from the pure enantiomers 5 and ent-5 and trapping of the intermediates 7: As shown in the preceding paper, [5] the bromofluoro-A C H T U N G T R E N N U N G carbene adduct rac-5 of 1-phenylcyclopentene is a useful precursor of rac-7 with regard to the Doering-Moore-Skattebøl reaction. For the enantioselective generation of (M)-and (P)-7, rac-5 had to be separated into the pure enantiomers 5 and ent-5.…”

“…This conclusion can be drawn even from the finding that 9+ent-9 and 10+ent-10 are formed in a ratio of close to 1:1, although the equilibrium ratio is 1:10 or even smaller. [5] As illustrated by Scheme 5, the four transition states 18 ad are conceivable for the attack of the central allene carbon atom of (M)-7 at the methylene group of styrene. Because of the mutual proximity of the two phenyl groups, 18 b should be the least favourable transition state.…”

“…The formulas represent absolute configurations. As with 5 and ent-5, those of the products (6,(8)(9)(10) were established by resolution of the racemic products [5] by HPLC using Chiralcel columns and comparison of the experimental with the calculated CD spectra (see the end of the Experimental Section). Figures 2, 3, 4 and 5 display these comparisons.…”