2-Methoxy-Δ3-1,3,4-oxadiazoline, a multipurpose precursor for the generation of a carbene, an ylide, or a diazo compound

Abstract

In the course of our investigation of the intermolecular reactions of foiled carbenes of the norborn-2-en-7-ylidene type, we have investigated the decomposition of methoxyoxadiazoline 1 in alcohols. Most of the reactions performed lead to products with an anti configuration confirming the participation of the double bond to the stabilization of the transition states. The thermal behavior of spirooxadiazoline 1 is quite different from the behavior of the parent 2-methoxy-2,5,5-trimethyl-Δ3-1,3,4-oxadiazoline. Photolysis of 1 leads to the carbene after prior formation of the diazo compound whereas thermolysis cleanly generates an extremely unstable carbonyl ylide 4 that immediately decomposes to the stabilizednucleophilic carbene 5 and methyl acetate without generation of 1-methoxyethylidene. Both conformers of 4 do not interconvert and, therefore, they have different life times. Nevertheless, we were able to trap syn ylide 4a with methanol. Calculations show that nonbonding interactions between an alkyl carbene and an ester are more significant than ylide formation. Synthetically, photolysis of oxadiazoline 1 in ethyl acetate has proven to give anti ethers in excellent yields. Moreover, we report the first reactions of a norbornenylidene derivative with O-H bonds in which no products resulting from a cationic rearrangement are formed. © Wiley-VCH Verlag GmbH & Co. KGaA, 2008.