Lewis acid complexes of 1,2-diketones and their derivatives. The synthesis of seven-membered rings

The structures and charge distributions of Lewis and complexes of butane-2,3-dione, 1, cyclohexane-1,2-dione, 2, and 2-methoxycyclohex-2-en-1-one, 5, have been investigated by 1 H nmr spectroscopy. A 1:1 complex was formed between 1 and SnCl 4 which appeared to have a bridged structure. In the case of 2 and 5 both oxygens were again involved in complex formation with TiCl 4 or SnCl 4 . In each case the large downfield shift of the vinyl proton …