Carbenes, nitrogen extrusion formation

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. 

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... 

Diazopyrazole (368) undergoes gas-phase thermal extrusion of two molecules of nitrogen and formation of the azirine 369 the same product is formed from the azide 370. This indicates the occurrence of the carbene-nitrene rearrangement 371 -> 372 (Scheme 72)432 Carbenes like 371 can be trapped in solution.432... 

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. 

The photochemical extrusion of nitrogen from silyl-substituted diazoacetates (hv > 300 nm) in the presence of various alkenes leads mainly to the formation of cyclopropanes (Table 4). Reactions of trimethylsilyl- and triisopropylsilyldiazoacetates with monosubstituted alkenes such as hex-1-ene or styrene (Table 4, entries 1-3) show interesting results. The formation of the thermodynamically less favored Z-isomer increases with growing steric demand of the silyl substituent. The cyclopropanation of ( )- and (Z)-but-2-ene (Table 4, entries 5 and 6) reveals that the addition reaction does not proceed completely stereospecifically. Small amounts of the wrong diastereomer can be detected, which is believed to arise from the triplet spin state of the carbene. Insertion into allylic C-H bonds occurs in the case of di- or trisubstituted alkenes (Table 4, entries 4-7). 

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. ... 

Flash-vacuum pyrolysis of a-benzotriazolyl-/ -oxO tributylphosphorus ylides results in extrusion of tributylphosphane to give 3-acetyl-1,2,4-benzotriazine (9) and 2-acetylbenzonitrile. The formation of these products is explained by the initial formation of acetyl(benzotri-azolyl)carbene which either forms 3-acetyl-l,2,4-benzotriazine (9) or 4-acetyl-l,2,3-benzotri-azine. Elimination of nitrogen from the 1,2,3-benzotriazine and 1,3-acetyl migration affords the isolated 2-acetylbenzonitrile.211... 

The formation of thioethers as by-products occurs because the triazolo-pyridines are in equilibrium with a diazo form, and the position of the equilibrium depends on the substitution pattern on triazolopyridine (050BC3905). The presence of an electron-withdrawing sulphoxide in position 6, as is the case in 98 or 99, shifts the equUibrium to the diazo form. The formation of thioethers 104 and 105 from the diazo form, can be explained on the assumption that a carbene intermediate formed by extrusion of nitrogen (07ARK297) is trapped by an oxygen transfer from the corresponding sulphoxide (67TL2363), which is reduced to a thioether (Scheme 21). 

---