Ketenes photochemical reactions

An a-diazo ketone 1 can decompose to give a ketocarbene, which further reacts by migration of a group R to yield a ketene 2. Reaction of ketene 2 with water results in formation of a carboxylic acid 3. The Woljf re arrangement is one step of the Arndt-Eistert reaction. Decomposition of diazo ketone 1 can be accomplished thermally, photochemically or catalytically as catalyst amorphous silver oxide is commonly used ... 

Merlic et al. were the first to predict that exposing a dienylcarbene complex 126 to photolysis would lead to an ort/zo-substituted phenolic product 129 [74a]. This photochemical benzannulation reaction, which provides products complementary to the classical para-substituted phenol as benzannulation product, can be applied to (alkoxy- and aminocarbene)pentacarbonyl complexes [74]. A mechanism proposed for this photochemical reaction is shown in Scheme 54. Photo activation promotes CO insertion resulting in the chromium ketene in-... 

Keywords Metal carbenes Photochemical reactions Metal-ketene complexes... 

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... 

Cyclohexadienones. The perturbation of the electronic absorption spectra by adsorption on silica gel can have a significant effect on subsequent photochemical reactions. For example, Hart has shown that 2,4-cyclo-hexadienones photochemically degrade cleanly in nonpolar solvents to a ketene (1) but in highly polar solvents or adsorbed on silica gel bicyclic ketone (2) is the predominant product.33 The absorption spectra indicate that in nonpolar solvents the lowest singlet state is the n,n state, from which the formation of the ketene proceeds. This n -n band is obscured by the band in polar media, inversion of the energy levels of the n,n, and the first... 

As mentioned in Section 7.2, when the electron transfer reaction between electron-rich alkenes and excited carbonyl compounds is energetically favorable, the RI pair becomes an important intermediate in photochemical [2 + 2] cycloaddition reactions (Scheme 7.5). The regioselectivity of these reactions may differ from that observed for the PB reaction involving 1,4-triplet biradical intermediates. Typical examples of PB reactions with very electron-rich alkenes, ketene silyl acetals (Eox = 0.9 V vs SCE), have been reported (Scheme 7.11) [27]. Thus, 2-alkoxyoxetanes were selectively formed as a result of the PB reaction with benzaldehyde or benzophenone derivatives, whereas a selective formation of 3-alkoxyoxetanes was observed in less electron-rich alkenes (see Scheme 7.9). When p-methoxybenzalde-hyde was used in the photochemical reaction, the regioselectivity was less than that observed in the case of benzaldehyde. This dramatic decrease in regioselectivity provided evidence that the selective formation of 2-alkoxyoxetanes occurred via RI pair intermediates. It should be noted that the stereoselectivity is also completely different from that associated with triplet 1,4-biradicals (vide infra). 

The first photoreactions were observed by Perst and Dimroth 215) when 2-acetoxy-2,4,6-triphenylquinol 1 in ethanol or benzene was left for some days in daylight. Only 2% of a new compound 3 was isolated. It was prepared, however, in 90% yield by irradiation of 1 with the short wave light of a mercury lamp. The photoisomer 3 proved to be the bicyclic exo acetate derivative. Heating it in benzene or toluene causes rearrangement back to the 2-acetoxy-quinol 1. Perst U8,216) later showed by extensive and careful experiments that a ketene 2 is the primary product of this photochemical reaction. It can be trapped with nucleophiles, such as dimethyl-... 

The photochemical reaction of ethoxycarbonyl azide with optically active ketene silyl acetals derived from 10-sulfamidoisoborneol were used to prepare a-amino esters 9 with satisfactory yields and diastereoselectivity65. The pure diastereomeric esters were obtained after chromatographic separation. 

SCHEME 12.1 Photochemical reactions of diazonaphtoquinone derivatives elimination of molecular nitrogen with formation of a carbene, followed by Wolff rearrangement to give a ketene that reacts with water traces to form a carboxylic acid. 

Complete cyclic conjugation is present also in derivatives of the as yet unknown 2-methylene-1,3-dioxin-4-one (5) and 2,5-dimethylene-l,3-dioxane-4,6-dione (6). These structures are cyclic acylals of ketene. Well-known and intensively studied are the 5-methylene Meldrum s acid derivatives (7) here the cyclic -conjugation is blocked at C-2. The reactivity of compounds of this type is discussed in Section 6.08.5.1. The cyclic a -unsaturated lactones (8) have also been thoroughly investigated, as their thermal and photochemical reactions are of great synthetic utility. These are dealt with in Section 6.08.5.2. 

Photfrfysis of a-Diazo Carbonyl Compounds - Some recent advances in the matrix photochemistry of diazoketones, including some heterocyclic species, have been reviewed. Flash photolysis of 10-diazo-9(10//)-phenanthrenone (35) in aqueous solution led to the detection of two transient species on the pathway to the final product, fluorene-9-carboxylic acid. These were identified, from solvent isotope effects and the nature of the observed acid-base catalysis, as fluorenylideneketene (36, X = CO) and the enol of fluorene-9-carboxylic acid (36, X = C(0H)2), formed by hydration of the ketene. In related studies, fluorenylideneketene was found to react with amines to give ylides as intermediates on the route to the amide final products. The product distribution from the photochemical reactions of 2-diazo-3-oxo-5,10,15,20-tetraphenylchlorins with alcohols strongly depends on the central metal ion of the irradiated diazoketones. ... 

In the heavily methyl substituted cyclohexadienones 723 which have been intensively studied, the only photochemical reaction is the formation of ketenes 724 88) 725 is formed in a thermal reaction from... 

A careful study of the formation of thiolesters R R CH CO SR , by the addition of a thiol to a keten, suggests that a radical path is followed. At least this accounts for the very irreprodudble rates. Photochemical reactions of 2-aminothiophenol (25) with carboxylic acids or cyclohexene provide a challenge from a mechanistic point of view the. S-methyl... 

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