Generation of Carbenes by Thermal Cycloelimination

Abstract

The growth of interest in carbenes has led to a need for methods that allow their generation in unimolecular decomposition processes, possible also in the gas phase, without troublesome by-products. Besides the decomposition of diazo compounds, thermal cycloeliminations from carbocycles and heterocycles with odd numbers of ring atoms have been found to be suitable for this purpose. The problems, structural requirements, and scope of such cycloelimination reactions are discussed in the present progress report. The advantages of carbene generation by thermal cycloelimination are demonstrated with reference to dialkoxycarbenes, which are very difficult to obtain in any other way.

Structure of a spiro-fused 2,2-dioxa-Δ3-1,3,4-oxadiazoline

“…[1] were longer than the equatorial C-aryl bonds, 8 the uncertainties mean that the difference is not significant. Reactions of 6 -Thermolysis of 6 in benzene-d 6 at 100 o C was followed by 1 H NMR spectroscopy, with toluene as internal standard for integration.…”

“…The system investigated (6) could have either a preference for the azo-axial conformation (6a) or the alkoxy-axial conformation (6b), Scheme 3. According to many studies of the anomeric effect 7 , alkoxy substitutents have a preference for the axial position when the competition is with an alkyl group or with H. Although the source of the anomeric effect has been the subject of debate, it is clear that the more electron-withdrawing substituent, in systems that are sterically unbiased (eq [1]) adopts the axial position. 8 Based on that precedent, one would predict that the azo group (sigma p-phenylazo= 0.33) 9 would outrank an alkoxy group in terms of axial preference, in a system such as 6, eq [2].…”

Carbene reactions. Part 16. Thermolyses of norbornadiene-7-spiro-2′-(1′,3′-dithiolane)S-oxides

“…The formation of indan (31%) and cisstilbene was accompanied by small amounts of indene and benzaldehyde. We assumed that instead of cycloelimination, rearrangement of the norbornadiene (9) to the benzyl derivative (14) could have happened, since it seemed conceivable that pyrolysis of the sulphone (14) could proceed to indan and stilbene. However, thermolysis of an authentic sample of bis-sulphone (14) led to benzaldehyde (51%) and phenylcyclopropane (19%) instead.…”

Studies on dimethylsilylene

“…-Norbornadienes have a rich chemistry. Thermally they eliminate the elements of carbene and leave a residue of benzene at temperatures in the range of 100-200® C. The temperature is determined by the stability of the fragments produced (82) . îïhile this reaction is a symmetry-allowed cheleotropic fragmentation, it is generally believed that this reaction proceeds via a diradical intermediate.…”

“…îïhile this reaction is a symmetry-allowed cheleotropic fragmentation, it is generally believed that this reaction proceeds via a diradical intermediate. (82) .…”