Diazocarbonyl compounds rearrangement

Although 2-acyl-2//-azirines are known to give oxazoles upon irradiation, the reaction is wavelength dependent, and isoxazoles are formed at some wavelengths, as they are in the thermal rearrangement of 2-acyl-2//-azirines.<74TL29,75JA4682> Since the thermal reaction of diazocarbonyl compounds with nitriles leads to oxazole formation, it would seem that mechanistic path C is unlikely in these reactions. 

Several comparative studies are available in the literature, which clearly shows that, according to the latest results, tetrafluoro-pheny/ azides (AZ), trifluoro-methyl-phenyl diazirines (DZR), and particularly benzophenone (BP) are the best choice (Scheme 3.). Diazocarbonyl compounds, which played a historically role in the evolution of PAL, can also be considered together with aryl-diazonium salts [6] (not shown). For aryl azides the more common unsubstituted photophore is presented. It should be noted that tetrafluoro-phenyl azides have an increased reactivity towards CH-groups and they do not rearrange. Other substituted phenyl azides can readily alter the excitation wavelength allowing the application of milder or differential photoactivation. 

Table 4.16. Generation and rearrangement of ammonium ylides from diazocarbonyl compounds and tertiary amines.

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. 

Photochemical or thermal extrusion of molecular nitrogen from ot-diazocarbonyl compounds generates a-carbonylcarbenes. These transient species possess a resonance contribution from a 1,3-dipolar (303, Scheme 8.74) or 1,3-diradical form, depending on their spin state. The three-atom moiety has been trapped in a [3 + 2] cycloaddition fashion, but this reaction is rare because of the predominance of a fast rearrangement of the ketocarbene into a ketene intermediate. There are a steadily increasing number of transition metal catalyzed reactions of diazocarbonyl compounds with carbon-carbon and carbon-heteroatom double bonds, that, instead of affording three-membered rings, furnish hve-membered heterocycles which... 

Recent study on sulfonium ylide [2,3]-sigmatropic rearrangement has been focused on the development of new catalytic systems, including new catalysts and alternative carbene precursor other than commonly used a-diazocarbonyl compounds. Besides the most commonly used Cu(i) and Rh(ii) catalysts, Fe com-... 

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]. 

Only insertion into the S-C(2) bond of 4-amino-2,5-dihydrothiophene-3-carbonitriles is observed on reaction with a-diazocarbonyl compounds in the presence of rhodium(ll) acetate. The ring expansion is regioselective and leads to the 4-cyano-3,6-dihydro-277-thiopyrans there is no evidence for the 5-cyano isomer (Scheme 152) <2000JPR494, 2001M721>. The reaction follows a different sequence when applied to 2-amino-4,5-dihydrothiophene-3-carboni-triles. The initial product is a 1,4-oxathiocine 447 which rearranges thermally to a 3,4-dihydro-2//-thiopyran (Scheme 153) <1996LA725>. 

The bicyclic cyclopropyl ketone shown in equation 174—easily accessible from the corresponding diazocarbonyl compound—could be transformed into two bicyclic divinylcyclopropanes with different functionality and positioning of the cyclopentene double bond. Rearrangement of these compounds leads to the bicyclo[12.1]octane series Very recent syntheses of the terpenes sinularene and quadrone include a pivotal Cope process of divinylcyclopropanes generated and rearranged in an analogous fashion (equation 175) . 

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. 

Cyclopropanation products were not isolated from furan and the following diazocarbonyl compounds diethyl 2-diazopent-3-enedioate, ° dimethyl diazomalonate (see also Table 13, entry 9), 6-diazopenicillinates (though not with benzofuran). The carbenoid reaction [copper(II) sulfate, 80 °C] of l-diazopropan-2-one with benzofuran yields 1-acetyl-1 a,6b-dihydro-l//-cyclopropa[ )]benzofuran. ° However, when furan is cyclopropanated with a-di-azo ketones, 6-(l-oxoalkyl)-2-oxabicyclo[3.1.0]hex-3-enes are either not obtained or rearrange slowly even at 20°C to the isomeric 1,4-diacylbuta-l,3-dienes. 

---