Non-aromatic endocyclic angle at the ring fusion with the aromatic ring in benzocyclopentene and -hexene, revisited

Abstract

X-ray geometries of monocycloalkenobenzenes with a fused-ring size of r = 5, 6 were obtained by the use of the Cambridge Structural Database. In contrast to Allen's study [Acta Cryst. (1981), B37, 900–906], the data were not averaged over exact or imposed m ad symmetry (m ad is the mirror plane going through the midpoint of the aromatic ring fusion bond and the aromatic bond parallel to it). The distribution of the non-aromatic endocyclic angle ∊ at the ring fusion clearly shows two and three, respectively, distinct curves for r = 5 and r = 6. The respective ranges are 104.3–112.6 and 104.4–123.2°. If one of the C atoms directly attached to the aromatic ring is sp 2 hybridized, the bond length e between the aromatic ring and this atom is shortened by some 0.044 (15) and 0.027 (12) Å for r = 5 and r = 6, respectively. However, for r = 5, e is only shortened by 0.030 (17) Å when both C atoms directly attached to the aromatic ring are sp 2 hybridized. For r = 5, the endocyclic angle ∊ is 1.2 (13)° smaller at the side involving the sp 2 hybridized C atom.