Iminocarbenes

Fischer carbene complexes are valuable C3 building blocks for the formal [3C+2S] carbo- and heterocyclisation reactions [55]. Thus, not only the traditional a,/J-unsaturated but also aryl and iminocarbene complexes have been used to get a great variety of compounds derived from the [3C+2S] reaction with different C2 counterparts. 

Iminocarbene complexes of chromium and tungsten are useful isolable synthetic equivalents to nitrile ylides having the advantage that the range of 1,3-dipo-larophiles is not limited to electron-acceptor substrates and can be extended to electronically neutral as well as to electron-rich systems [56] (Scheme 18). 

The regioselectivity observed in these reactions can be correlated with the resonance structure shown in Fig. 2. The reaction with electron-rich or electron-poor alkynes leads to intermediates which are the expected on the basis of polarity matching. In Fig. 2 is represented the reaction with an ynone leading to a metalacycle intermediate (formal [4C+2S] cycloadduct) which produces the final products after a reductive elimination and subsequent isomerisation. Also, these reactions can proceed under photochemical conditions. Thus, Campos, Rodriguez et al. reported the cycloaddition reactions of iminocarbene complexes and alkynes [57,58], alkenes [57] and heteroatom-containing double bonds to give 2Ff-pyrrole, 1-pyrroline and triazoline derivatives, respectively [59]. 

Fig. 2 Reaction of an iminocarbene complex of chromium with an ynone...

The significant behaviour of the compound 12 on electron impact ionization, i.e. the formation of a dominating ionic species, m/z 92, 14, has been rationalized as shown in scheme 1885. It should be noted that the intermediacy of the ionized iminocarbene was proposed along with the possibility that an oxygen transfer takes place directly from the nitro group to the annelated benzene ring. Similarly, ionized 2- and 3-azafulven-6-ones were demonstrated to be formed from the appropriate heterocycles85. 

Thermal or photochemical extrusion of nitrogen from 1-arylbenzotriazoles (114) leads to the formation of carbazoles (Scheme 14). The mechanism is believed to involve cyclization of a diradical (115) or an iminocarbene (116) to the 4a/f-carbazole (117) followed by an aromatizing hydrogen... 

For the isomeric 2//-isoxazolin-5-ones 347, elimination of a nitrile is impossible and only CO2 is lost upon pyrolysis to give the iminocarbenes 348, which cyclize as shown to provide a useful synthesis of fused imidazoles 349. A wide range of examples has been examined as illustrated by the products 350-353, which are generally formed in almost quantitative yield using FVP at 500-600 °C165,166. 

In contrast to the isoxazolones, the isomeric 4//-oxazolin-5-ones which lack the weak N—O bond usually lose only CO upon pyrolysis. Thus, FVP of a variety of compounds 354 at 600°C gives the acylketenimines 355167. The 2//-oxazolin-5-ones do lose CO2 to give iminocarbenes, which may undergo rearrangement, as in the formation of 357 from... 

The first two derivatives in the series, monuron and metobromuron, are related to the 4-haloanilines. Their primary photochemistry has been studied by Boulkamh and Richard by means of nanosecond absorption spectroscopy [80]. The transients detected from both compounds in aqueous solution could be assigned to the N-substituted 4-iminocarbene, imino-p-benzoquinone-O-oxide and anilino radical from a complete analogy of their spectral and reactive behavior with that of the species obtained from 4-chloroaniline [55,57]. The quantum yields of carbene formation were determined to be = 0.051 for monuron and

halogen-substituted phenylurea derivatives underwent the same heterolytic dehalogenation process as the 4-haloanilines, which could be understood with reference to the protonability of the amine nitrogen, as in the case of 4-chloro-N,N-dimethylanilinc [55]. 

Ar-Thioacylisoxazol-5(2/7)-ones 174 lose carbon dioxide under photochemical conditions and undergo intramolecular cyclization of the iminocarbene 175 to afford thiazoles 176 in good yields as shown in Scheme 68 <1997J(P1)2673>. 

Vapor phase pyrolysis of two unsymmetrically substituted 1,2,3-triazole isomers, like (75) and (76), involves the antiaromatic l//-azirine derivative (77) which rearranges to two isomeric 2//-azirines (78) and (79) (71CC1519, 73JCS(Pi)555> and other products. Iminocar-benes (80) are most likely involved as intermediates. Furthermore, nitrogen has been extruded from several IH- 1,2,3-triazoles by flash vacuum pyrolysis. 1-Alkyl-1,2,3-triazoles (81) give nitriles (82) and (83) (via Wolff rearrangement) and (hydroxy)isoquinolines (84) and (85) (by [1,4-H] transfer in the iminocarbene) (75JCS(P1)1). 

The major thermal reaction of 2/f-azirines involves C(2)—N bond cleavage as shown in Equation (6) to form vinyl nitrenes (47). Cleavage of the C—C bond to produce iminocarbenes (48), diradical... 

A cyclic iminocarbene was transferred from 2-diazo-5-isopropyl-3,6-dimethoxy-2,5-dihy-dropyrazine to an alkene under unusually mild conditions. This diazo compound decomposes under the conditions of its synthesis from 2,5-dihydropyrazine 8, and in the presence of cy-cloalkenes, 2,5-dihydrospiro[pyrazine]bicyclo[n.l.O]alkanes 9 (n = 3-5) were obtained (an experimental procedure is given in Houben-Weyl Vol. E19b, pi 194). 

On photolysis of azido(o-cycloalkylidene)cinnamates 3, the iminocarbene intermediate was efficiently intercepted by intramolecular cyclopropanation only if n = 1. For n = 2-4, the corresponding cyclopropanes were obtained as minor products only. 

Migration of groups other than hydrogen has been observed in photochemically generated nitrile ylides (iminocarbenes). 43... 

The iminocarbenes 33 (Scheme 7) may also be regarded as nitrile ylides (35). Ab initio calculations45 indicate that nitrilium betaines are best described as hybrids of the bent dipolar and carbenic structures (e.g., 33 and 35). Consequently, the generation of the nitrile ylide 35 in the gas phase should give the same product as obtained from the azirine 31. This was confirmed by flash vacuum pyrolysis of the appropriate oxazol-5(2//)-one (36), which gave a quantitative yield of the azadiene 34.46... 

The dimethylamino-2//-azirine 39 undergoes exclusive C—C bond breaking by flash vacuum pyrolysis (350 C, 0.1 torr).50 In this case, the intermediate iminocarbene (or nitrile ylide) 40 is stabilized by the amino group. Only the carbene product 42 and not the nitrene product 41 was isolated (Eq. 13). 

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