(Acyloxy)carbenes from thermolysis of oxadiazolines in solution. 1-Acetoxyethylidene and 1-acetoxypropylidene

Abstract

Thermolysis of 2-acetoxy-5,5-dicyclopropyl-2-methyl-A1 23-l,3,4-oxadiazoline in CC14 at 79.5 °C afforded biacetyl, acetyl chloride, and dicyclopropyl ketone, among other products. It is proposed that the oxadiazoline loses nitrogen, in the first step, to form a carbonyl ylide. The latter then fragments, primarily to dicyclopropyl ketone and 1acetoxyethylidene [ (acetoxymethy 1) carbene]. That carbene is partitioned between 1,2 acyl transfer to form biacetyl and abstraction of Cl from CC14 to form 1-acetoxy-1-chloroethyl radical. The latter undergoes ß scission to form acetyl chloride and acetyl radical, which in turn abstracts from CC14 to form more acetyl chloride. Similarly, 2acetoxy-5,5-dicyclopropyl-2-ethyl-A3-l,3,4-oxadiazoline decomposed in CC14 to form 2,3-pentanedione, acetyl chloride, propionyl chloride, dicyclopropyl ketone, and other products. Again, the -diketone is accounted for in terms of an (acyloxy)carbene precursor (1-acetoxy-l-propylidene) and the formation of two acyl chlorides supports the hypothesis that such a carbene can abstract Cl from CC14, with subsequent ß scission of the resulting radical. 1 -Acetoxyethylidene was trapped with neat 2-acetoxypropane and with neat 2-methoxypropane to form cyclopropanes.

Reactions of acetoxy(methylthio)carbene and S-methyl thiopyruvate

“…Formation of 6 probably occurs by concerted acetyl transfer from oxygen to the carbene carbon, as was found for other acyloxy carbenes (2)(3)(4)(5)(6)(7)(8). Formation of compounds 6 and 8 is most easily formulated in terms of zwitterionic intermediates (9) formed by reaction of 6 with 5 (Scheme 4).…”

“…Thermolysis of 1 is relatively complex, affording the carbene as well as 2-diazopropane as shown in the abbreviated Scheme 1. The carbene did not fragment to radicals, but it rearranged to methyl pyruvate (3) by acetyl migration from oxygen to the carbene carbon, a known process for other acetoxycarbenes (2)(3)(4)(5)(6)(7).…”

Reactions of 4-tert-butyl-5-methylene-3,3-dimethyl-Δ1-1,2,4-triazoline with dimethyl acetylenedicarboxylate

“…Ring opening of 13 to 14 is presumably suppressed because loss of N2 from 13, to generate 15, is faster. Azomethine ylide 15 isomerizes to 16 by H-atom transfers analogous to those that convert some carbonyl ylides to en01 ethers (30). In CH2C12 those transfers may be intramolecular but in CH30H there is a stepwise process, probably involving protonation at the cyclobutyl carbon followed by deprotonation from the isopropylidene function.…”

Substituent effects on rates of formation of methoxy-substituted carbonyl ylides from 2-aryl-2-methoxy- and 5-aryl-2-methoxy-Δ3-1,3,4-oxadiazolines

“…Although that mechanism could account for some additional features, such as very low yields of cycloadduct from thermolysis of compounds like 3b in the presence of neat acrylonitrile, it has been pointed out (10) that the rate factor (k30/k36) is smaller than might be expected for a stepwise mechanism. Moreover, it has been found subsequently that both trialkylacetoxy- (24) and trialkylalkoxy-substituted carbonyl ylides (25,26) can fragment to carbenes and carbonyl compounds more rapidly (27) than they can close to oxiranes. An intramolecular 1,4-hydrogen shift is also fast relative to oxirane formation, in some systems.…”

“…In view of the considerable potential of carbonyl ylides, not only as intermediates for cycloadditions but also as sources of carbenes (24)(25)(26)(27), we began studies aimed at elucidating the mechanisms of ylide formation from oxadiazolines and of intramolecular and intermolecular ylide reaction^.^ We report here some substituent effects and solvent effects on rate constants for unimolecular thermolysis of oxadiazolines 4 and 5. 5.22.…”