Wolff rearrangements diazo compounds

In this section we first discuss photolytic reactions of arenediazonium salts and report on quinone diazides at the end of the section in the context of imaging technology. Diazoalkenes, non-quinonoid diazo ketones, and the Wolff rearrangement are treated in the book on aliphatic diazo compounds (Zollinger, 1995, Chap. 8). 

The overall reaction is reminiscent of the Wolff rearrangement of a-diazo carbonyl compounds which gives ketenes. Compounds 6 formally represent their phosphorus analogues and are sometimes also designated as phosphenes 11... 

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. 

Protected 6-amino-hexahydro-l,7-dioxopyrazolo[l,2-4]pyrazole-2-carboxylic acid 274 is available by a thermolytic decomposition of diazo compound 273 via the Wolff rearrangement. The starting compound is simply available by alkylation of racemic 272 with the corresponding bromoacetoacetate and subsequent diazo transfer reaction (Scheme 35) <1996TL4891>. 

Figure 5.29 pNPDP reacts with amine-containing compounds by its p-nitrophenyl ester group to form amide bonds. After photoactivation of the diazo derivative with UV light, a Wolff rearrangement occurs to a highly reactive ketene intermediate. This group can couple to nucleophiles such as amines. 

The use of copper as a catalyst in carbenoid transfer has its roots in the Amdt-Eistert reaction, Eq. 1 (3). Although the original 1935 paper describes the Wolff rearrangement of a-diazo ketones to homologous carboxylic acids using silver, the authors mention that copper may be substituted in this reaction. In 1952, Yates (4) demonstrated that copper bronze induces insertion of diazo compounds into the X-H bond of alcohols, amines, and phenols without rearrangement, Eq. 2. Yates proposal of a distinct metal carbenoid intermediate formed the basis of the currently accepted mechanistic construct for the cyclopropanation reaction using diazo compounds. 

In contrast to 2-alkylarylcarbenes, triplet carbonyl carbenes do not abstract H from 5- or e-CH bonds. Photolysis of diazo compounds (7) in methanol gave products due to Wolff rearrangement (8) and 0-H insertion (9). Sensitized photolysis led, in addition, to the H-abstraction product (10). Analysis of the results indicated that a large proportion of the insertion product (9) arises from the excited diazo compound and that spin inversion of the triplet carbene is faster than H-abstraction from the solvent. Intersystem crossing to the singlet state is a major reaction of all triplet carbonyl carbenes that are not rapidly scavenged intramolecularly. 

The Wolff rearrangement has been very successfully used in the synthesis of propellanes.40 The highly strained 3-diazo[3,2.2]propellan-2-one was photolyzed in dichloromethane at — 70 °C in the presence of dimethylamine to give A W-dimethyl tricyclo[2.2.2.0 u4]octane-2-carboxamide (5), a very highly strained interesting compound with inverted geometry at the bridgehead carbons,42 in 40% yield.41... 

Appropriately substituted diazo ketones have been converted into oxetanes in two instances by Wolff rearrangement processes. The structure of compound (52) was established by X-ray crystallography (69MI51300, 81CSC345). Reaction of 4,4-dibromo-2,2,5,5-tetramethyltetrahydro-3-furanone with aqueous base is a good method of preparation for 3-hydroxy-2,2,4,4-tetramethyloxetane-3-carboxylic acid (equation 90) (66JA1242). 

Diazocarbonyl compounds can also be prepared by C-acylation of diazoalkanes with polystyrene-bound acyl halides (Entry 6, Table 10.19). As an alternative to diazomethane, the more stable a-(trimethylsilyl)diazomethane may be used, which is sufficiently nucleophilic to react with acyl halides. On heating, the resulting a-(trimethyl-silyl)diazo ketones undergo Wolff rearrangement to yield ketenes, and have also been used as starting materials for the preparation of oxazoles [368]. 

Ketocarbenes (1) are usually generated from the corresponding diazo compounds (3).s Other sources which are occasionally used are a,a-dibromo compounds (4),9 sulfur ylides (5)10 and iodonium ylides (6 Scheme 2).11 The thermal or photochemical decomposition of diazo compounds in the presence of ir-systems is often complicated by indiscriminate side reactions, such as Wolff rearrangements,12 C—H insertions and hydride migrations. To avoid such problems, the use of metal-catalyzed decomposition of diazo compounds is generally preferred.1 2... 

An hDA reaction of the thermally generated (trialkylsilyl)vinylketene 888 with diethyl ketomalonate furnishes the 5,6-dihydropyran-2-one 889 in excellent yield. Protodesilylation of the cycloadduct 889 is achieved in quantitative yield upon its exposure to methanesulfonic acid (Scheme 244). A photochemical Wolff rearrangement of the silyl diazo compound 890 can also be used to generate an intermediate diene for reaction with diethyl ketomalonate to afford the 5,6-dihydropyran-2-ones 891 (Equation 358) <19990L641>. 

Unfortunately not only carbenes are formed when diazoacyl and related compounds are photolysed. A major product is that formed by rearrangement of the carbene (or perhaps the excited diazo compound) to a ketene the Wolff rearrangement (Fig. 3.9). 

Chavan and coworkers provide evidence that the Wolff rearrangement is facilitated by the formation of silver nanoclusters, which initiate electron transfer to the diazo compound providing 8. While the precise fate of this species remains to be firmly established, they suggest a multicycle process involving the intermediacy of a silver carbene 10 (Scheme 8.2).10 12 Decomposition of the silver carbene to the free carbene 14 precedes rearrangement to ketene 13, which is then trapped with water to provide the carboxylic acid 15 (Scheme 8.2). 

Fig. 14.29. Preparation of an a-diazoketone (compound E) from a ketone (A) and subsequent Wolff rearrangement of the a-diazoketone. Initially, A is transformed to give the enolate B of its a-formyl derivative. In a Regitz diazo group transfer reaction, this will then be converted into the a-diazoketone E. Ring contraction via Wolff rearrangement occurs and the 10-membered cyclic diazoketone C rearranges in aqueous media to give the nine-membered ring carboxylic acid E via the ketene D.

In the photochemistry of a-diazo carbonyl compounds, singlet carbenes undergo a Wolff rearrangement while triplet carbenes react with alkenes to afford cydopropanes. 

The loss of CO, S, SO, SO2, SO3, and N2 by thermolysis or photolysis has been used to make three- and four-membered rings for example, the cyclic sulfamidate 94 undergoes clean thermolysis at 70 C to form the vinyl aziridine 95 in excellent yield <2002T5979, CHEC-III(1.01.6.5)79> and Wolff rearrangement of diazo compounds 96 gives -lactams 97 (Scheme 51) <1973J(P1)2024>. 

Wolff rearrangement of /)-diazenyl-a -diazo ketone 83 in the presence of water, methanol (or even with primary and secondary amines) under varied reaction conditions comes with surprise Instead of the expected homologous acid 89, methyl ester 90 (or the respective amide), 2-phenylcinnolin-3(2E0-one (91) is the only product indicative of the anticipated in situ formation of ketene intermediate 88 (Scheme 22). Obviously, the apparent 67r-electrocyclization reaction prevails forming the isolated heterocyclic product 91, a so far unknown compound. 

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