Rearrangements involving diazo compounds

The Bamford-Stevens decomposition of tosylhydrazones by base has been applied to steroids, although not extensively. It has been demonstrated that the reaction proceeds via a diazo compound which undergoes rapid decomposition. The course of this decomposition depends upon the conditions in proton-donating solvents the reaction has the characteristics of a process involving carbonium ions, and olefins are formed, often accompanied by Wagner-Meerwein-type rearrangement. In aprotic solvents the diazo compound appears to give carbene intermediates which form olefins and insertion products ... 

Photolysis ofbenzylchlorodiazirine (3) in the presence of tetramethylethylene (TME) is known to produce ( )- and (Z)-/l-chlorostyrene (4) and the cyclopropane (5). Plots of [5]/[4] vs [TME] are curved, consistent with the existence of two pathways for the formation of the alkenes (4). Benzylchlorocarbene (BnClC ) was generated by laser flash photolysis of the phenanthrene (6) in the presence of TME. In this case, plots of [5]/[4] vs [TME] are linear, mling out the possibility that the second pathway to the alkenes (4) involves reaction of a carbene-alkene complex. Time-resolved IR spectroscopy revealed that diazirine (3) rearranges to the corresponding diazo compound, but this process is too inefficient to account for the curvatures. It is proposed that the second pathway to alkene formation involves the excited state of the diazirine. 

The diazotization reaction generates the same type of /i-hydroxy carbocation that is involved in the pinacol rearrangement. (See Section 5.6 in Part A for a discussion of the formation of carbocations from diazo compounds.)... 

These reactions involve addition of the diazoester to an adduct of the carbonyl compound and the Lewis acid. Elimination of nitrogen then triggers migration. Sections B and C of Scheme 10.2 give some additional examples of pinacol rearrangements involving diazo and diazonium intermediates. 

A. Fiirstner, Olefin Metathesis and Beyond, Angew. Chem. Int. Ed. Engl. 2000, 39, 3012. D. M. Hodgson, F. Y. T. M. Pierand, and P. A. Stupple, Catal3dic Enantioselective Rearrangements and Cycloadditions Involving Ylides from Diazo Compounds, Chem. Soc. Rev. 2001, 30, 50. 

The Cu(I)-catalyzed decomposition of (alkynyloxysilyl)diazoacetates 119 furnishes the silaheterocycles 120 and/or 121 (equation 30) in modest yield63. In these cases, the photochemical extrusion of nitrogen from 119 does not lead to defined products and the thermal reaction is dominated by the 1,3-dipolar cycloaddition ability of these diazo compounds. In mechanistic terms, carbene 122 or more likely a derived copper carbene complex, is transformed into cyclopropene 123 by an intramolecular [1 + 2] cycloaddition to the triple bond. The strained cyclopropene rearranges to a vinylcarbene either with an exo-cyclic (124) or an endocyclic (125) carbene center, and typical carbene reactions then lead to the observed products. Analogous carbene-to-carbene rearrangements are involved in carbenoid transformations of other alkynylcarbenes64. 

Chavan and coworkers provide evidence that the Wolff rearrangement is facilitated by the formation of silver nanoclusters, which initiate electron transfer to the diazo compound providing 8. While the precise fate of this species remains to be firmly established, they suggest a multicycle process involving the intermediacy of a silver carbene 10 (Scheme 8.2).10 12 Decomposition of the silver carbene to the free carbene 14 precedes rearrangement to ketene 13, which is then trapped with water to provide the carboxylic acid 15 (Scheme 8.2). 

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. 

All-isomer is therefore regarded as the product of decomposition of the i2 diazo derivative (ii) via a carbene intermediate (12) (see p, 341). In hydroxylic solvents the 12-diazo compound (ii) suffers protonation, probably at C(i2) (c/. p. 340), and the resulting diazonium ion (13) rearranges via the C(i2> carbonium ion with elimination of N2. A possible alternative reaction sequence involving iV-protonation of the diazo compound is also illustrated (14). Present evidence does not permit... 

The metal-catalyzed decomposition of diazo compounds 603 provides access to a variety of cephem derivatives (Scheme 30). Copper-mediated cyclization of diazo ketone 603 (R = Et) gives 3-oxocepham through sulfo-nium ylide formation followed by j8-elimination (80H1999). The formation of cephem derivatives 604 and 605 presumably involves the intermediacy of ylide 606 (R = Ac, ArCH2) and its subsequent rearrangement. A more sophisticated mechanism is proposed for the formation of 607 and 608 (83H205). 

The diene arises through the diazo compound 4 via a retro-[3 -f 2] reaction and diazo compounds of this type have been detected or isolated in several photolyses of 2,3-di-azabicyclo[3.1.0]hex-2-enes. The workup procedure for this particular reaction involves selective removal of the diene by ozonolysis, a chemical method of separating the products which may have applications in related systems since chromatography often proves difficult. In some cases, the intermediate diazabicyclohexenes also prove very sensitive and difficult to handle, rearranging to pyridazines under the influence of acid, base " or light and sometimes, as in the synthesis of methyl 4,4-dimethyl-l,3-diphenylbicyclo[1.1.0]butane-2-carboxyl-... 

The cycloaddition of a diazo compound to an alkyne followed by extrusion of nitrogen under various conditions from an isolated pyrazole represents one of the standard routes to cyclopropenes (Section 3.2.1.1.). In certain cases the reaction is carried out under conditions which might involve direct addition of a carbene derived from the diazo species, e.g. formation of 7, 8, and9. For8(R = cyclopropyl), the product rearranged under the reaction conditions to 1-cyclopropyl-1-trimethylsilylallene. ... 

Catalytic enantioselective rearrangements and cycloadditions involving ylides from diazo compounds 01CSR50. 

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