Stereochemical analysis of exo-methylenebenzocycloalkanes: evidence from carbon-13 nuclear magnetic resonance chemical shifts, 13C–13C nuclear spin couplings, and force field calculations

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Abstract
A series of exo-methylenebenzocycloalkanes with ring sizes 5–8 have been prepared, enriched with 13C and also deuterated at the exocyclic position. 13C chemical shifts are found to be influenced by the degree of planarity between the aromatic ring and the exocyclic methylene function. 13C–13C couplings through one, two, and three bonds have been measured. The 1JCC values correlate very well with %S character calculations. Values of 3JCC are found to be influenced by both dihedral angle and valence angle. The relative propensity for these compounds to isomerize to endocyclic material is discussed. Some earlier spectral assignments for the indene derivative are shown to be in error.

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