Nitrogen extrusion carbenes

Photolysis of dlazirines to nitrogen and carbenes is a general reaction and plays a greater role in carbene chemistry than photolysis of linear diazo compounds. Whereas the latter are often obtained only under the conditions of their thermal decomposition from suitable precursors, diazirines are obtainable in a pure state in most cases. Photolysis has the further advantage to permit nitrogen extrusion at atmospheric pressure, even with low-boiling materials. 

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... 

Photolysis of dialkyl- and alkyl-diazirines has been broadly investigated since the products formed gave evidence of shortlived intermediates possessing excess vibrational energy. As shown in Scheme 1 diazirines (3) were photoactivated to (235) by irradiation with a medium pressure mercury lamp with quantum yields smaller than one this means reversibly. Nitrogen extrusion yielded carbenes (236). These rearranged to the primary products, alkenes and... 

Diazo compounds have been extensively used in the preparation of three-membered carbocycles either as carbene sources or as precursors for 1-pyrazolines or 3//-pyrazoles. Nitrogen extrusion from pyrazolines is particularly valuable for the synthesis of alkylcy-clopropanes, since the direct carbene route is impractical, as a matter of fact, owing to rapid intramolecular processes in alkylcarbenes. The cycloaddition of diazo compounds to unsaturated bonds to give 1-pyrazolines and 3/f-pyrazoles usually proceed in a concerted manner, and hence is stereospecific. In the subsequent nitrogen extrusion from the adducts,... 

As stated above, an alkoxycarbonyl(disilanyl)carbene rearranges to an acylsilene, but not to a silylketene (Scheme 5). For diazoacetate 62, the result of a matrix-isolation experiment (Table 3) can also be reproduced when the nitrogen extrusion is induced by vacuum pyrolysis at 360 °C, by thermolysis of the neat compound at 180 °C or by photolysis in... 

Nitrogen extrusion and the 1,2-shift can occur either in a concerted manner or stepwise via a carbene intermediate ... 

Nitrogen extrusion by diiron nonacarbonyl affords a mixture of two isometric carbene iron complexes (111) rather than (110). The fact that a crossover product is formed is taken as evidence for the intermediacy of selenirene and carbenes (112a,b). 

Nitrogen extrusion from a-diazoketone and the 1,2-shift can occur either in a concerted manner or stepwise via a carbene intermediate known as the Wolff rearrangement (Scheme 2.58). a-Diazoketones undergo the Wolff rearrangement thermally in the range between room temperature and 750°C in gas-phase pyrolysis. Due to the formation of side products at elevated temperatures, the photochemical or silver-metal-catalyzed variants are often preferred that occur at lower reaction temperature. 

As with other diazo compounds, cyclopropanation with 1 or 2 can proceed either via a photo-chemically or thermally generated carbene, in a transition-metal-catalyzed reaction, or by [3 -I- 2] cycloaddition of the diazo function to the C-C double bond followed by nitrogen extrusion from the thermally unstable cycloadduct. 

Iron carbonyl complex formation of 1,2,3-selenadiazoles has been studied by Rees and co-workers. Nitrogen extrusion by di-iron nonacarbonyl gave a mixture of two isomeric carbene iron complexes 163 and 164. The... 

The mechanism of transition metal-catalyzed nitrogen extrusion from diazo compounds is not known in detail, but there is general agreement that metal-carbene complexes are formed in this process (Scheme 3) [6,7,11,28,29,30,31, 32]. Attempts to detect these elusive species in catalytic reactions have not been... 

Kobayashi et al. elaborated an unusual pathway for the synthesis of 2,3-bis(trifluoromethyl)indoles [32], Photolysis of the l-phenyl-4,5-bis(tri-fluoromethyl)-lH-l,2,3-triazole in hexane proceeded very slowly to afford the indole 68 in 44 % yield. It was proposed, that after homolytic nitrogen extrusion, the carbene 67 was formed. Intramolecular cyclization led to the indole 68. 

Dibromocarbene generated under phase-transfer conditions inserts into the a-C-H bond of ( -l,3-hexadiene)Fe(CO)3 to give (74) a similar reaction occurs with (chd)Fe(CO)3. Diazomethane reacts with triene (75) to give (76) on oxidation with Ce +, nitrogen extrusion to yield the cyclopropane derivative occurs, in addition to decomplexation. Reaction of (75) with the sulphur ylide (CH3)2 S=CHC02Me proceeds similarly to give (77).Methods for the addition of carbene and dichlorocarbene to the unco-ordinated double bonds of (cot)Fe(CO)3 and (chpt)Fe(CO)3 have been described. 

This chapter focuses on the generation of carbenes by extrusion of nitrogen (Tables 91.1 through 91.8). Several other methods are possible, such as the formation from tosylhydrazone salts, oxiranes, aziridines, dioxolanes, and pyrazolenines from cyclopropanes by photocydoehmination, from transition-metal carbene complexes or by a-elimination, from ot,a-dihalogen compounds, or from base treatment of N-nitrosocarbamate. ... 

As part of a mechanistic and synthetic study of nucleophihc carbenes the spirocyclic 4(5/l)-oxazolone 18 has been obtained from benzoyl isocyanate (Scheme 6.1) Thermal extrusion of nitrogen from the 1,3,4-oxadiazoline 14 produced the carbonyl ylide 15 that fragmented via loss of acetone to the aminooxycarbene 16. Spectroscopic data [gas chromatography-mass spectrometry (GC-MS), infrared (IR), proton and C-13 nuclear magnetic resonance ( H and NMR)] of the crude thermolysate was consistent with 18. The formation of 18 was rationalized to result from nucleophihc addition of 16 to benzoyl isocyanate followed by cyclization of the dipolar intermediate 17. Thermolysis of 19 and 21 under similar reaction conditions afforded 20 and 22 respectively, also identified spectroscopically as the major products in the thermolysate. 

Aziridines can be prepared directly from double-bond compounds by photolysis or thermolysis of a mixture of the substrate and an azide.812 The reaction has been carried out with R = aryl, cyano, EtOOC, and RS02, as well as other groups. The reaction can take place by at least two pathways. In one, the azide is converted to a nitrene, which adds to the double bond in a manner analogous to that of carbene addition (5-50). In the other pathway a 1,3 dipolar addition (5-46) takes place to give a triazoline (which can be isolated), followed by extrusion of nitrogen (7-46). Evidence for the nitrene pathway is most compelling for... 

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. 

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