CARBONYL YLIDES AND CARBENES FROM DELTA(3)-1,3,4-OXADIAZOLINES

Abstract

<p>A series of new, substituted (DELTA)('3)-1,3,4-oxadiazolines weresynthesized by oxidation of the corresponding hydrazones, eitherelectrochemically or with lead tetraacetate. 2-Methoxy-2,5,5-trimethyl-(DELTA)('3)-1,3,4-oxadiazoline 1 (R=R'=R''=CH(,3)), whenthermolyzed in CD(,3)OD, gave products of trapping of a R'' N=N OR R'' O R' A carbonyl ylide. In CCl(,4) or C(,6)H(,6), which are not efficient trapping solvents the ylide underwent fragmentation into carbenes and carbonyl compounds. As a check that the same intermediate was involved in both solvents 2-methoxy-2-(p-substituted)phenyl-5,5-dimethyl-(DELTA)('3)-1,3,4-oxadiazolines were thermolyzed in CCl(,4) and in CD(,3)OD. A Hammett correlation with (sigma)('-) was observed in both solvents, with similar slopes. Furthermore, changes in substituents at C(,5) also affected the rate of thermolysis of the oxadiazolines. These results led us to conclude that, oxadiazolines of type A thermolyze with concerted loss of nitrogen to give rise to carbonyl ylide intermediates. Attempts to trap the intermediates (ylide and carbenes) from the thermolysis of the oxadiazolines were successful. Some olefins and carbonyl compounds can trap both intermediates. The reaction of a carbene with a carbonyl compound to form a carbonyl ylide is the reverse of the ylide fragmentation mentioned above. Such reversibility was proved for one of the carbenes when it was found that thermolysis of 1 (R=R'=R''=CH(,3)) in acetone-d(,6) gave propene-d(,6) as one of the products. Thermolysis of 2-methoxy-2,5,5-trimethyl-(DELTA)('3)-1,3,4-oxadiazoline in CCl(,4) gave acetone and methoxymethyl carbene (43%) as well as methyl acetate and dimethyl carbene (54%). The 5,5-dicyclopropyl analogue thermolyzed in benzene to a carbonyl ylide which fragmented nearly cleanly to methyl acetate and dicyclopropyl carbene (> 90%). In CCl(,4) chlorinated products, derived from the reaction of dicyclopropylcarbene with CCl(,4), were obtained. Finally, thermolysis of 2-acetoxy-2,5,5-trimethyl-(DELTA)('3)-1,3,4-oxadiazoline in CCl(,4) afforded 1-acetoxyethyl-2-propenyl-ether(> 90%) derived from a 1,4-hydrogen shift in the ylide. The 5,5-dicyclopropyl analogue gave only 20% of the analogous 1,4-shift product and the major pathway involved a fragmentation of the ylide to give a rare acyloxycarbene, CH(,3)(')COAc.</p>