Molecular conformation of diazo-compounds. Part IV. Dipole moments and infrared spectra of some hindered α-diazo-ketones

Abstract

The conformations of the hindered diazo-ketones MeCOCMeN₂, Bu^tCOCBu^tN₂, PhCOCMeN₂, and o- and p-ClC₆H₄COCMeN₂ have been estimated in non-polar solvents from dipole moment and i.r. data. The t-butyl derivative has a single distorted configuration, with an angle of 140° between the planes containing the Bu^tCO and CBu^tN₂ systems. The results for the other compounds point to the occurrence of two pseudo-cis-trans-(distorted) conformers, with the possibility of partial diazo-group–carbonyl conjugation. In the ortho-chloro-derivative, where the phenyl ring is out of the COCRN₂(pseudo) molecular plane, both conformers should be almost planar. In the light of evidence on the thermal decomposition of these compounds, it is inferred that complete planarity of the cis–trans-conformers is not a prerequisite for the formation of ketens or unsaturated ketones; rather, their formation is related to the angle of rotation around the central C–C bond.