Photochemical reactions Wolff rearrangement

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... 

Danheiser et al. developed a new aromatic annotation methodology for the total s)mthesis of hyellazole (245) by irradiation of the heteroaryl a-diazo ketone 675 in the presence of 1-methoxypropyne (590). This reaction proceeds via the photochemical Wolff rearrangement of the heteroaryl a-diazo ketone 675 to generate a vinylketene, followed by a cascade of three pericyclic reactions. 

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... 

The key step of the Amdt-Eistert Homologation is the Wolff-Rearrangement of the diazoketones to ketenes, which can be accomplished thermally (over the range between r.t. and 750°C, photochemically or by silver(I) catalysis. The reaction is conducted in the presence of nucleophiles such as water (to yield carboxylic acids), alcohols (to give alcohols) or amines (to give amides), to capture the ketene intermediate and avoid the competing formation of diketenes. 

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>. 

Fig. 11.24. Mechanisms of the photochemically initiated and Ag(I)-catalyzed Wolff rearrangements with formation of the ketocarbene E and/or the ketocarbenoid F by dediazotation of the diazoketene D in the presence of catalytic amounts ofAg(I). E and F are converted into G via a [1 2]-shift of the alkyl group R1. N2 and an excited carbene C are formed in the photochemically initiated reaction. The excited carbene usually relaxes to the normal ketocarbene E, and this carbene E continues to react to give G. The ketocarbene C may on occasion isomerize to B via an oxacyclopropene A. The [l,2-]-shift of B also leads to the ketene G.

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. 

The intramolecular insertion into the N—H bond of j8-lactams was used successfully in the synthesis of bicyclic ring systems. Photochemical, in contrast to Rh(II)-catalyzed, decomposition of diazo ester 62 was found to occur far less selectively. In the photolytic reaction, the imide 63 is the major product. It presumably arises by a photolytic Wolff rearrangement to a ketene intermediate, which is trapped intramolecularly. With Rh2(AcO)4 catalyst the Wolff rearrangement is suppressed and 62 undergoes ring closure to 64 nearly quantitatively (80TL31). 

R.L. Danheiser and co-workers generated a key vinylketene intermediate via tandem Wolff rearrangement-ketene-alkyne cycloaddition to utilize it in a photochemical aromatic annulation reaction Danheiser benzannulation) for the total synthesis of the phenalenone diterpene salvilenone. ... 

For photochemically generated (2-biphenylcarbonyl)phenylcarbene, several competing intramolecular reactions are observed, namely cyclopropanation of an aromatic nucleus (followed by norcaradiene to cycloheptatriene tautomerization), Wolff rearrangement, C-H insertion, and a carbene-to-carbene rearrangement (see Houben-Weyl, Vol. E19b, pl282). 

Explain the following concepts and keywords photochromism photoswitching photoinitiator photoaffinity labelling photo-Wolff rearrangement photoactivatable compound Barton reaction photochemical trigger photolabile linker photoamination redox photosensitization. 

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