PREPARATION AND STRUCTURE INVESTIGATIONS OF SIMPLE BICYCLE[1.1.0]BUTANONES

摘要

Four 1,3-dialkyl-substituted bicyclo[1.1.0]butanones have been prepared, All have been completely characterized by H-1 and C-13 NMR spectroscopy, but O-17 NMR proved to be particularly diagnostic. The di-t-Bu 4 and di-tert-amyl 5 members can be obtained in the condensed phase by working quickly and at subambient temperatures, but attempts to obtain X-ray quality crystals have not been successful. The synthetic procedure involves a very rapid and efficient 1,3-elimination of two bromine atoms from 1,3-dibromocyclobutan-2-ones using the salt PPN+Cr(CO)(4)NO-. This reaction is particularly well-suited for exploratory in situ low-temperature MMR experiments, Molecular orbital calculations show these ketones to have the typical bent bicyclobutane structure, but the central C1-C3 bond is very long and the carbonyl carbon is distinctly nonplanar. Both of these features can be attributed to a strong interaction of the C1-C3 ''bond'' with the carbonyl group, and several qualitative orbital interaction schemes are presented to show that bicyclobutanones have on one hand some oxyallyl character and, on the other, some similarities with the bonding in esters. Nucleophiles react with bicyclobutanones in a nonconcerted process to add across the C1-C3 bond. [References: 52]