Synthesis, chemical and NMR spectral studies of 1,7,7-trimethyl-2,3-benzobicyclo[3.3.1]non-2-enes

Abstract

A detailed NMR study has been made into the preparation, structure and characterization of a series of 1,7,7-trimethyl-2,3-benzobicyclo[3.3.1]non-2-enes. This family of compounds was derived from the parent structure, A, with a number of endo and exo C-4 substituted and C-4 sp²-hybridized derivatives being prepared. [Image of 'structure A'] The general structural features of these systems make them very useful for the proton NMR investigations of "perpendicular" or "out-of-plane" diamagnetic ring-current shifts. The axial methyl group at C-7 is in the region of this diamagnetic shielding and, thereby, is shifted upfield relative to a normal methyl group. Substitution at C-4 was found to affect these chemical shifts, causing Me[subscript 7a] to move to even higher fields in both endo- and exo-substituted derivatives and these results are discussed. The carbon-13 NMR spectra of this series of compounds has been determined and the carbon assignments have been tabulated. As part of the structural study of these compounds, an x-ray crystal structure determination for exo-4-bromo-1,7,7-trimethyl-2,3-benzobicyclo[3.3.1]non-2-ene has been reported. These data, as well as vicinal coupling constants between H-5 and the protons at C-4 have been used to determine the stereochemistry of the C-4 substituted derivatives of structure A. A study has been made into the feasibility of employing C-4 nitrogen-substituted derivatives of structure A as a possible synthetic intermediate for the preparation of a new series of heteroaromatic bicyclononenes. Results have shown these intermediates to be unsuitable for the cyclization reaction to form isoquinolinoid systems, due to the extreme lability of the nitrogen-to-C-4 bond.