Wolff rearrangements diazocarbonyl compounds

A density functional study has been made of the competition between Wolff rearrangement and [1,2]-H shift in /S-oxy-a-diazocarbonyl compounds. Silver-catalysed decomposition of a-diazoketones (88 n = 0), derived from A-tosyl a-amino acids in methanol, gave rise to mixtures of products of Wolff rearrangement (89) and direct insertion of the carbene into the NH bond (90). The -amino acid derived species (88 n = 1) gave rise to products of Wolff rearrangement. 

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]. 

The ester group in II is suggestive—although it is not a proof—of the intermediacy of a ketene, and ketene production in diazocarbonyl chemistry usually implies a Wolff rearrangement. The construction of a three-carbon chain on the other side of the ketone is a confirmation of this prediction. In turn, the Wolff rearrangement requires an a-keto carbene precursor that is the fate of diazo compounds exposed to ultraviolet light (wavelength lower than 3200 A). All this is translated into the mechanism depicted in Scheme 43.1. 

The singlet form of carbenes substituted with a carbonyl group is much lower in energy than the triplet, so the photolysis of a-diazocarbonyl compounds gives singlet carbenes. The photo-Wolff rearrangement (Chapter 2) of a-diazoketones occurs upon photolysis of these compounds. 

Carbenes from a-Diazocarbonyl Compounds The Wolff Rearrangement and the Arndt-Eistert Reaction... 

The Wolff rearrangement of a-diazocarbonyl compounds (8.58, R = H, alkyl, aryl, OR) has great synthetic importance because in most cases the ketenes formed react smoothly with water, alcohols, and amines (Scheme 8-34). An early application that still has considerable importance is the homologization of carboxylic acids (Arndt-Eistert reaction Arndt and Eistert, 1935). As shown in Scheme 8-34, the reaction starts from the chloride of the acid RCOOH, which leads to an a-diazo ketone with diazomethane (R = H), followed by the Wolff rearrangement and the hydrolysis of the ketene intermediate to give the homologous carboxylic acid (8.59, R =H). In alcohols and amines esters (8.60) and amides (8.61, R = H), respectively. 

Photolysis of a-diazo-jS-diketones showed initial formation of an a-oxo-ketene, which decarboxylated to an oxocarbene, then underwent a conventional Wolff rearrangement. a-Diazocarbonyl compounds in which the carbonyl group is part of an ester function can undergo the Wolff rearrangement, but conventional carbene reactions can compete more readily. 

The isomerization of the a -oxo-carbenes (197) and (199) via the oxiren (198) has been investigated, using labelled diazocarbonyl compounds as carbene sources (Scheme 10). ° The distribution of the labelled carbon in the products showed that the Wolff rearrangement proceeds in both cases, but mainly from (199). 

Scheme 3.6 Electronic structure of a-diazocarbonyl compounds and mechanism of the Wolff rearrangement.

The oldest method for the formation of a ketene, used by Staudinger in his studies on diphenylketene, is the reduction of an a-haloacyl halide with activated zinc. Most often, the ketene components used in the Staudinger reaction are usually produced by either of two ways the elimination of an acyl chloride (or less frequently another activated carboxyl derivative) in the presence of a base, or the Wolff rearrangement of a-diazocarbonyl compounds.The ketene is usually generated in situ in the presence of the imine however, if the ketene is stable enough, it may be prepared separately and then introduced into reaction with the imine. Other methods to produce ketenes have been used less often in the Staudinger reaction due to incompatibility with the imine component or p-lactam product or due to the harsh conditions required, such as the high temperatures employed in the pyrolysis of acid anhydrides or ketone acylals. 

The Wolff rearrangement of a-diazocarbonyl compounds offers perhaps the cleanest means of generating a ketene in situ without the... 

Brown s book on organic synthesis via boranes, ° and reviews on sulphene chemistry, the use of activated metals in organic and organometallic chemistry, the Wolff rearrangement of a-diazocarbonyl compounds, and ring contractions and expansions of vicinally disubstituted cyclobutanes and cyclopropylmethyl compoundsare also of appreciable relevance to bridged carbocyclic chemistry. Other pertinent, but more specialized, reviews are recorded in the various sections of this chapter. 

Several studies have indicated that the conformation of the diazo precursor plays a critical role in the pathway of the Wolff rearrangement. Kaplan and co-workers were the first to demonstrate that diazocarbonyl compounds exist as an equihbrium mixture of sy and anti forms and that concerted rearrangement... 

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