posted on 1996-09-06, 00:00authored byGiovanni Cerioni, Rudolf Janoschek, Zvi Rappoport, Thomas T. Tidwell
The properties of substituted cyclobutene-1,2-diones
1 are examined by the use of 17O
NMR
spectroscopy and theoretical calculations and compared to those of
cyclopropenones 2 and other
models. Cyclobutene-1,2-diones have less negative charge per
oxygen compared to cyclopropenones,
and electron donation by substituents enhances the negative charge on
oxygen. Calculated 17O
chemical shifts reproduce the measured trends. The dianions of
squaric and deltic acids are highly
stabilized by negative charge delocalization to the
oxygens.