Steric requirements of sp 2-hybridised lone electron pairs. Part I. The conformations of 2-(pyridylmethylene)indan-1-ones

Some 2-(pyridylmethylene)indan-1-ones have been prepared and their conformations deduced from dipole moments. The 3-pyridyl derivative consists of approximately equal amounts of the cis- and the trans-form, but in the 2-isomer the form with the nitrogen cis to the CH2 group greatly predominates. The results indicate that the steric requirements of an sp2-hybridised lone pair on nitrogen are less than those of a hydrogen atom bonded to an sp2-hybridised carbon atom, when the reference probe is a CH2 group.