The Stereochemical Course of the Generation and Interception of aSix‐Membered Cyclic Allene: 3δ2‐1H‐Naphthalene (2,3‐Didehydro‐1,2‐dihydronaphthalene)
The bromofluorocarbene adduct rac-1 of indene, possessing the fluorine atom at the endo position, is a useful substrate for the generation of the isonaphthalene 4 by the Doering–Moore–Skattebol reaction. By resolution of rac-1, an enantiomerically pure precursor of a six-membered cyclic allene was obtained for the first time. The treatment of (+)- or (–)-1, dissolved in 2,5-dimethyl-, 2-tert-butyl-5-methyl-, or 2,5-bis(tert-butyl)furan, with methyllithium gave rise to the [4+2] cycloadducts 6 of 4 to the furans. It was shown by means of HPLC on Chiralcel OD that the formation of 6 proceeded with about 40 % ee, and that this value was independent of the type and concentration of the furan, and the reaction temperature. The absolute configurations of the enantiomers 1, as well as those of the enantiomers 6, were determined by simulation of the CD spectra by quantum chemical methods and by the comparison of them with the experimental spectra. In the case of (+)-1, the reliability of this procedure was checked by X-ray crystal structure analysis. On the basis of these results, a model is proposed for the steric course of the reaction sequence, leading from a pure enantiomer 1 to the 70:30 mixtures of the product enantiomers 6. The use of indene as trapping reagent for 4 furnished the [2+2] cycloadduct 15. For the preparation of rac-15, the particularly simple one-pot procedure was employed, in which indene, tetrabromomethane, and methyllithium were combined and in which the dibromocarbene adduct 2 of indene serves as precursor of 4. Compound 2 could not be isolated, but was characterised by low-temperature 1H NMR spectra. The conversion of (+)- or (–)-1 into 4 in the presence of indene afforded 15 only with a very low enantioselectivity. The constitutions and the relative configurations of 15 as well as the compounds 8 and 16, which resulted from thermolysis of the cycloadducts 6b and 15, respectively, were elucidated by X-ray crystal diffraction. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)