Wolff rearrangement thermolysis

Scheme 10.14 gives some other examples of Wolff rearrangement reactions. Entries 1 and 2 are reactions carried out under the classical silver ion catalysis conditions. Entry 3 is an example of a thermolysis. Entries 4 to 7 are ring contractions done under photolytic conditions. Entry 8, done using a silver catalyst, was a step in the synthesis of macbecin, an antitumor antibiotic. Entry 9, a step in the synthesis of a drug candidate, illustrates direct formation of an amide by trapping the ketene intermediate with an amine. 

Thermolysis of 58a in butanol affords, together with 17% of 60a (R = C4H9) which evidences the intermediacy of the thiophosphene 59 a, a variety of partly atypical products which seriously impede the desired rearrangement38. Photolysis of 58b in methanol is also found to give only 18 % 1,2-P/C shift to form the heterocumulene 59b, from which the thiophosphinic rater 60b (R = CH3) results 39). As already mentioned in connection with the photolysis of diazo compounds of type 36 (see Sect. 2.2), Wolff rearrangement (9%) and O/H insertion (6%) once again compete with thiophosphinic ester formation. Moreover, solvolysis of the P(S)/C(N2) bond 391 prevents a greater contribution of carbene products to the overall yield. 

A tandem Wolff rearrangement reaction followed by cyclization leads to the formation of thieno[3,2-f]pyrans in moderate yields as shown in Scheme 26 <1998T6457>. A key step of the reaction involves thermolysis of diazoketophosphonate 97. 

The loss of CO, S, SO, S02 and N2 by thermolysis or photolysis has been used to make three- and four-membered rings for example, thiiranes (67) are obtained from (66) (CHEC 5.06.4.4). A2-1,2,3-Triazolines give aziridines and Wolff rearrangement of (68) gives (69). 

Photolysis or thermolysis of 1 in the presence of A -benzylideneaniline (R = H) and A -(diphenyl-methylene)aniline (R = Ph) afforded 5,6-diphenyl- and 5,6,6-triphenyl-l,2-bis(diphenylme-thylene)-5-azaspiro[2.3]hexan-4-one (5), respectively. These reactions can be interpreted in terms of the cycloaddition of the dimethyleneketene (resulting from the Wolff rearrangement) with azobenzene and imines." " ... 

Thermolysis of N,N-dialkyldiazoacetamides in matrices results in formation of ketenes by Wolff rearrangement, as well as competitive formation of C—H insertion products, with ketenes favored for cyclic precursors. Matrix photolysis in argon of 65 forms an unobserved carbene, which leads mainly to the P-lactam 66 and small amounts of the ketene 67 as identified by IR spectroscopy (Eqn (4.34)). Photolysis of 65 in a CO matrix gives capture of the carbene by CO forming ketene rotational isomers syn-68 and anti-68, similarly identified by IR spectroscopy (Eqn (4.35)), while N,N-dimethyl and N,N-diethyl diazoamides form insertion products. ... 

Flash vacuum thermolysis with Ar-matrix isolation or matrix photolysis of 2-phenylbenzo-l,3-dioxan-4-one as well as other precursors gave the ketene 6-carbonylcyclohexa-2,4-dienone 112, as detected by the IR absorption at 2134 cm Flash vacuum thermolysis above 600 °C or matrix photolysis resulted in formation of the fulvenone 114, proposed to arise by decarbonylation of 112 and Wolff rearrangement. The use of C-labeled precursor gave 114 with scrambling revealing the intermediacy of bisketene intermediates 113 in the reaction (Scheme 4.21). 

Other classes of heterocycles undergo thermolytic fragmentation to give imido-nitrenes, as typified by the thermolysis of 1,5-diphenyltetrazole 41, the intermediates 42 can either cyclize on to an aromatic ring to form benzimidazoles 43 or undergo a Wolff-type rearrangement to carbodiimides 44 (Scheme 7). Thermal decomposition of 2-acyltetrazoles... 

A plethora of products is obtained when either elemol (273), in the presence of p-nitrobenzoic acid (or benzoic acid), or elemyl-p-nitrobenzoate is pyrolysed at about 200 °C. These include the elemenes (274)—(276) together with the selinenes (277)—(281). The sequence of formation and the factors affecting the thermolyses are discussed in depth, together with the data for the thermolysis of dihydro-geijerene (282). This latter compound has been synthesized from germacrone (227) by a Cope rearrangement to j3-elemenone (283) followed by a retro-aldoliza-tion to (284) and subsequent Wolff-Kishner reduction. 

Imidoylnitrenes are aza analogs of both vinylnitrenes and acylnitrenes. Not unexpectedly, therefore, the two major reactions are cyclization onto aromatic rings, forming benzimidazoles, and a Wolff-type rearrangement to carbodiimides. 1,5-Diphenyltetrazole (118) forms a mixture of 2-phenylbenzimidazole (119) (23%) and diphenylcarbodiimide (120) (76%) on thermolysis above 200°C.173,174 However, only the benzimidazole was formed by photolysis.175... 

Iminocarbenes can be generated by thermolysis or photolysis of triazoles. Like the oxocarbenes they ondergo a Wolff-tixe rearrangement 29,30) and also carbene-carbene interconversion via a sjunmetrical intermediate, most likely the... 

The reaction is reversible, for the gas-phase thermolysis of benzocyclopropene under mild conditions pelds the Wolff-type rearrangement product of 22, namely fulvenallene (24) 44). Several other precursors of carbene 22 are known 44-48) (Scheme 3, Table 4). 

Another instance of the sulfoxide as an interested spectator in a photochemical reaction is the generation of a-sulfinyl carbenes [115]. Photolysis (or thermolysis) of the sulfmyl substituted pyrazoline 227 was shown by trapping experiments to generate the carbene 228, which undergoes a Wolff-type rearrangement to generate the sulfine. After loss of sulfur, the ketone 230 is isolated in near quantitative yield. 

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