Molecular rearrangements carbene reactions

In this review an attempt is made to discuss all the important interactions of highly reactive divalent carbon derivatives (carbenes, methylenes) and heterocyclic compounds and the accompanying molecular rearrangements. The most widely studied reactions have been those of dihalocarbenes, particularly dichlorocarbene, and the a-ketocarbenes obtained by photolytic or copper-catalyzed decomposition of diazo compounds such as diazoacetic ester or diazoacetone. The reactions of diazomethane with heterocyclic compounds have already been reviewed in this series. ... 

We add to the purely organic carbene rearrangements the reactions of carbenes with molecular oxygen (8-29), because they involve an oxygen rearrangement from the primary O-oxide (8.49) to the dioxirane (8.50), as shown by Dunkin and Shields (1986) for cyclopentadienylidene (8.48). 

Diazirine, fluoromethoxy-nitrogen extrusion, 7, 224 Diazirine, methylvinyl-rearrangement, 7, 221 Diazirines addition reactions to Grignard compounds, 7, 2 0 as carbene precursors, 7, 236 IR spectra, 7, 203 microwave spectrum, 7, 199 molecular spectra, 7, 202-204 nitrogen extrusion, 7, 223 NMR, 7, 202 photoconversion to diazoalkanes, 7, 234 photoisomerization, 7, 221 photolysis, 7, 225-227 quantum chemical investigations, 7, 197 reactions... 

Hydride and 1,2-alkyl shifts represent the most common rearrangement reactions of carbenes and carbenoids. They may be of minor importance compared to inter-molecular or other intramolecular processes, but may also become the preferred reaction modes. Some recent examples for the latter situation are collected in Table 23 (Entries 1-10, 15 1,2-hydride shifts Entries 11-15 1,2-alkyl shifts). Particularly noteworthy is the synthesis of thiepins and oxepins (Entry 11) utilizing such rearrangements, as well as the transformations a-diazo-p-hydroxyester - P-ketoester (Entries 6, 7) and a-diazo-p-hydroxyketone -> P-diketone (Entry 8) which all occur under very mild conditions and generally in high yield. 

Although some carbenes are reported not to add to cyclopropenes207, there are several examples of inter- and intra-molecular addition leading initially to the formation of bicyclobutanes. l,2-Diphenylcyclopropene-3-carboxylates are converted to a mixture of three stereoisomeric bicyclo[1.1.0]butanes by reaction with ethoxy-carbonylcarbene generated from the thermolysis of ethyl diazoacetate an additional product is the diene (278) which is apparently formed by rearrangement of an intermediate zwitter ion208). It should be noted, however, that cyclopropenes readily undergo addition to diazo-compounds, and that subsequent transformations may then lead to bicyclobutanes (see Section 8), and that a free carbene may therefore not be involved in the above process. 

This is called the Curtius rearrangement. There is no evidence for the existence of the free nitrene in this reaction, unlike in the Wolff rearrangement where there is some evidence for the existence of the free carbene, so probably the steps are concerted in this case. The driving force for this reaction is the production of molecular nitrogen. This provides so much energy, that it is possible to harness it to expand an aromatic ring. Indicate the pathway that is followed when an aryl azide is heated with phenylamine. 

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. 

As primary products of the reaction of silylenes with molecular oxygen the formation of either silanone (9-oxides 3 or the dioxasiliranes 4 can be expected. The homologous carbenes react exclusively to give carbonyl (9-oxides 5 which rearrange photochemically to dioxiranes 6 (Scheme 5.1). ... 

---