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The properties and synthetic applications of carbonyl ylides constitute an active area of chemical research.1 Very recently, Griffin, Houk, and their co-workers2 published results of theoretical studies of structures and reactions of substituted carbonyl ylides One of the questions addressed in that paper concerns their fragmentation (eq 1), and the authors' conclusions about frag-X _ Y V©,/ *> + o +(i) mentation, on the basis of their studies and the literature, included the following:3 (a) Fragmentation of carbonyl ylide (X = Y = H) is endothermic by about 38 kcal/mol. (b) One amino substituent (X = NH2) decreases the thermodynamic barrier to fragmentation in either sense and path a, leading to aminocarbene, may actually be exothermic.4 (c) Thermal fragmentation of a carbonyl ylide from a coplanar ground state (0°,0°conformation5) is a disallowed process.Biosynthesis of Macrolide Antibiotics. 3.1

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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.

, 1646 (1984). 2-Aryl-2-methoxy-5.5-dimethyl-A" I ,3,4-oxadiazolines (4) and 5-aryl-2-methoxy-2,5-dimethyl-A3-1,3,4-oxadiazolines (5) were synthesized. Compounds 4 decompose in solution with first order kinetics. Rate constants are correlated with Hammett substituent constants (a-) with p(49.2"C) = 0.74 and 0.89 for CCI, and CD30D, respectively. The final products from 4 indicate that thermolysis involves the cleavage of both C-N bonds, to form Nz and, initially, a carbonyl ylide. Compounds 5, which were obtained as mixtures of cisltrans isomers containing several impurities, and which therefore gave poorer kinetic data, decomposed in CDCI, solution with ~( 4 5°C ) --1.1. Carbonyl ylide intermediates, similar to those from the closelyrelated compounds 4, were assumed on the basis of analogy and on the basis of partial identification of products. The effects of para substituents in the aryl groups of 4 and 5 show that the transition states have greater electron density at C-2 of 4 and at C-5 of 5 than do the starting materials. In spite of the increase in electron density at C-2 of 4, the transition state must be less polar, overall, than the ground state because rate constants for thermolysis of 4 in methanol are smaller than those for CCI, solvent. A plausible explanation for the substituent effects and the solvent effects is that the loss of N2 is concerted, with a transition state resembling more closely a spin paired 1,3-diradical than a I ,3-dipole. Alternative stepwise mechanisms, in which C2-N3 bond scission of 4 and C5-N4 bond scission of 5 are the rate-determining steps, leading to 1,5-diradical intermediates, can not be excluded on the basis of the evidence. intermtdiaires semblables a ceux observCs dans le cas des composts 4 qui leur sont trbs voisins. Les effets des substituants en position para des groupes aryles des composts 4 et 5 montrent que, par rapport aux produits de dtpart, les ttats de transition ont une plus grande densitt Clectronique en C-2 dans les composes 4 et en C-5 dans les composts 5. En dtpit d'une augmentation de la densit6 tlectronique en C-2, dans les composts 4, l'ttat de transition doit Ctre, dans I'ensemble, moins polaire que I'ttat fondamental puisque les constantes de vitesse de la thermolyse des composts 4 dans le mtthanol sont plus faibles que celles dans le CCI,. Une perte de N2 concertte, avec un Ctat de transition ressemblant de trbs prbs un diradical-1.3 de spin pairt plutbt qu'a un dipole, constitue une explication plausible des effets de substituant et de solvant. Les preuves obtenues n'excluent pas la possibilitt d'un micanisme par Ctapes dans lequel la scission de la liaison C2-N3 des composts 4 et de la liaison C5-N4 des composCs 5 constituerait l'ttape dtterminante et conduirait a la formation de diradicaux-1,5 intermtdiaires.[Traduit par le journal]

The principal product of gas phase thermal decomposition of 2-methoxy-2,5,5-trimethyl-Δ3-1,3,4-oxadiazoline (methoxyoxa-diazoline) was found to be 1-methoxy-1-[(1-methylethenyl)oxy]-ethane (vinyl ether). This arises from a selective 1,4-hydrogen shift in the carbonyl ylide intermediate. Fragmentation of the carbonyl ylide is the dominant process in the solution thermolysis of methoxyoxadiazoline. The phase and temperature dependence of this thermolysis is discussed in terms of the probable structure of the carbonyl ylide intermediate.

Thermolysis of a 2-methoxy-Δ3-1,3,4-oxadiazoline involves loss of N2 with formation of a carbonyl ylide. The fate of the carbonyl ylide depends on its environment and on the other substituents present. Thus, the ylides from 2-methoxy-2,5,5-trimethyl-Δ3-1,3,4-oxadiazoline (1) and from 2-methoxy-2-(4-methoxyphenyl)-5,5-dimethyl-Δ3-1,3,4-oxadiazoline (2) are trapped very efficiently by methanol. However, the ylide from 1 is trapped much less efficiently than that from 2 by dimethylacetylene dicarboxylate, cis-1,2-dichloroethylene, or norbornadiene. A major competitive process in the case of 1 is fragmentation of the ylide to carbonyl compounds and carbenes, the latter being trapped by alkenes to form cyclopropanes. An intramolecular 1,4-H transfer is also competitive under some conditions. The ylide from 2 does not appear to fragment, nor does it undergo the 1,4-H transfer, but it cyclizes efficiently to the oxirane in the absence of trapping agents.Preliminary estimates of rate constants for cyclization of the ylide from 2 to form the oxirane [Formula: see text] and for its additions to norbornadiene and to dimethylacetylene dicarboxylate (1 × 105 M−1 s−1 and 1 × 109 M−1 s−1, respectively) are reported. If it is assumed that the ylide from 1 would add to dimethylacetylene dicarboxylate with a similar rate constant, then the yield for that process can be used to place a lower limit of 1010 s−1 on the rate constant for fragmentation of that ylide at 31 °C.

. Can. J. Chem. 69, 1507Chem. 69, (1991.Carbonyl ylides 2, generated by thermolysis of alkoxyoxadiazolines 1 in chloroform, react with chloroform to form ketals (3) of l,l,l-trichloropropanone. The isotope effect at 80°C, determined by analysis of products from thermolysis of 1 in mixed solvent (CHC13/CDC13, both in large excess) was estimated to be kH/kD = 5. A mechanism involving concerted C-C1 and C-H bond formation between the ylide C atoms and the C1 and H atoms of chloroform is proposed. The ketals of l,l, 1-trichloropropanone are the first to be reported.