Ketenes cleavage

While this process is not directly observable, Hess Law says that the endothermicity of this reaction is the sum of the next three reactions, equations 15-17. The sum is 566 kJmoC, some 215 kJmol more endothermic than the aforementioned ketene cleavage. We can conclude that lithiation and dianion formation significantly weaken the CC bond in the dilithium enolate as the lithium enolate has a considerably stronger CC bond than ketene. 

In the second case, fission C-l/C-2 gives an a-substituted pyrroleninium ion (LXXVIIa) which loses ketene (cleavage 3/4) with formation of the a-methylene-iV-methylpyrroleninium radical ion (m/e 97). This, in turn, eliminates a proton and appears in the spectrum with m/e 96 (LXXVIII). The latter is probably a 2-methylene-iV-methylpyrrolinium ion since deuteration in either position [2D in C-6 and C-7 (LXXIc) or 4D in C-2 and C-4 (LXXIb)] is marked by a shift by 2 mass units. 

Irradiation of 3,5-disubstituted isoxazoles in alcoholic solvents gave reaction products such as acetals incorporating the reaction solvent. The use of triethylamine in acetonitrile media produced ketene-aminals by reductive ring cleavage. The reductive ring cleavage product was also obtained by irradiation of the isoxazole in alcohol in the presence of copper(II) salts (Scheme 3) (76JCS(P1)783). 

Unsubstituted 3-alkyl- or 3-aryl-isoxazoles undergo ring cleavage reactions under more vigorous conditions. In these substrates the deprotonation of the H-5 proton is concurrent with fission of the N—O and C(3)—-C(4) bonds, giving a nitrile and an ethynolate anion. The latter is usually hydrolyzed on work-up to a carboxylic acid, but can be trapped at low temperature. As shown by Scheme 33, such reactions could provide useful syntheses of ketenes and /3-lactones (79LA219). 

An interesting use of the diketone type of ketene dimer (34) is reduction and cleavage to B,y-unsaturated aldehyde (35),... 

Cyclohexanones undergo type I cleavage to produce a mixture of ketenes and aldehydes by hydrogen transfer,<1-3)... 

Type I Cleavage Reactions Proceeding through Ketene Intermediates... 

The following reactions are believed to result from addition to ketene intermediates produced from type I cleavage reactions ... 

This result eliminates an a-cleavage pathway to form ketene since deuterium in this case should have appeared a to the carbonyl group of the ester. This result is, however, compatible with the charge-transfer mechanism. 

Two of the three general types of secondary reactions resulting from photochemical a-cleavage of carbonyls, namely molecular rearrangement and hydrogen transfer to yield aldehydes or ketenes, have been discussed. The third type of reaction observed, decarbonylation, will be discussed in this section. The discussion will begin with the decarbonylation of small ring carbonyls. By way of example of this type of reaction, diphenylcyclopropenone decarbonylates upon photolysis to yield diphenylacetylene(57) ... 

Other non-traditional preparations of 1,2,3-triazoles have been reported. The rearrangement in dioxane/water of (Z)-arylhydrazones of 5-amino-3-benzoyl-l,2,4-oxadiazole into (2-aryl-5-phenyl-27/-l,2,3-triazol-4-yl)ureas was investigated mechanistically in terms of substituents on different pathways <06JOC5616>. A general and efficient method for the preparation of 2,4-diary 1-1,2,3-triazoles 140 from a-hydroxyacetophenones 139 and arylhydrazines is reported <06SC2461>. 5-Alkylamino-] //-], 2,3-triazoles were obtained by base-mediated cleavage of cycloadducts of azides to cyclic ketene acetals <06S1943>. Oxidation of N-... 

In a noteworthy series of studies, Herndon has shown that cyclopropylcarbenes can be used as four-carbon components in molybdenum- and tungsten-mediated [4 + 2 + l]-reactions with alkynes and carbon monoxide (CO). These reactions give cycloheptadienones in moderate yields and with moderate selectivity (Equations (26)—(28)). The mechanism of this reaction is proposed to proceed through a series of steps involving metathesis, GO insertion, ketene formation, cyclopropane cleavage, and finally reductive elimination (Scheme 43).133... 

---