Wolff rearrangement carbene intermediate

There are several reactions that are conceptually related to carbene reactions but do not involve carbene, or even carbenoid, intermediates. Usually, these are reactions in which the generation of a carbene is circumvented by a concerted rearrangement process. Important examples of this type are the thermal and photochemical reactions of a-diazo ketones. When a-diazo ketones are decomposed thermally or photochemically, they usually rearrange to ketenes, in a reaction known as the Wolff rearrangement.232... 

Photoelimination of nitrogen from diazoketones is complicated by Wolff rearrangement of the intermediate carbene, as shown below for diazoaceto-phenone<35) ... 

When the Wolff rearrangement is carried out photochemically, the mechanism is basically the same,162 but another pathway can intervene. Some of the ketocarbene originally formed can undergo a carbene-carbene rearrangement, through an oxirene intermediate.168 This was shown by l4C labeling experiments, where diazo ketones labeled in the carbonyl group... 

Earlier studies [11,12] have shown that photo excitation of aqueous 2-chloro-phenol or 2-bromophenol leads to contraction of the aromatic cycle to give cyclopentadienic acids (an example of a Wolff rearrangement [13]), and to substitution of the halogen by OH (photohydrolysis), with moderate quantum yields (< = 0.01-0.04). A carbene (2-oxocyclohexa-3,5-dienylidene) was suggested as a possible intermediate in the ring contraction pathway [11]. 

In contrast to the carbene and carbenoid chemistry of simple diazoacetic esters, that of a-silyl-a-diazoacetic esters has not yet been developed systematically [1]. Irradiation of ethyl diazo(trimethylsilyl)acetate in an alcohol affords products derived from 0-H insertion of the carbene intermediate, Wolff rearrangement, and carbene- silene rearrangement [2]. In contrast, photolysis of ethyl diazo(pentamethyldisilanyl)acetate in an inert solvent yields exclusively a ketene derived from a carbene->silene->ketene rearrangement [3], Photochemically generated ethoxycarbonyltrimethyl-silylcarbene cyclopropanates alkenes and undergoes insertion into aliphatic C-H bonds [4]. Copper-catalyzed and photochemically induced cyclopropenation of an alkyne with methyl diazo(trimethylsilyl)acetate has also been reported [5]. 

Nitrogen extrusion from a-diazoketone and the 1,2-shift can occur either in a concerted manner or stepwise via a carbene intermediate known as the Wolff rearrangement (Scheme 2.58). a-Diazoketones undergo the Wolff rearrangement thermally in the range between room temperature and 750°C in gas-phase pyrolysis. Due to the formation of side products at elevated temperatures, the photochemical or silver-metal-catalyzed variants are often preferred that occur at lower reaction temperature. 

Because of their reactivity, it is frequently difficult to decide whether a free carbene has been generated or whether an electrophilic carbon undergoes a synchronous reaction. It is generally accepted that a free carbene is an intermediate in the Wolff rearrangement, in which a diazoketone rearranges to a highly reactive ketene. The versatile ketene intermediate then reacts to... 

In both the photolysis and pyrolysis of diazoketones carbene intermediates have been postulated by analogy with other diazo decompositions, but in the majority of cases the carbene cannot be trapped by olefins. Instead, ready migration of alkyl or aryl groups (bonded to the carbonyl group) to the divalent carbon atom takes place, known as the Wolff rearrangement... 

Irradiation of diazo quinoline 153 in alcohol provided the corresponding indole-3-carboxylic esters in 8-61% yield. Mechanistically, the transformation is a Wolff rearrangement and involves the formation of an intermediate carbene, ring contraction to ketene, and its solvolysis (76S754). 

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