Thermolysis of diazo compounds

The thermal decomposition of diphenyidiazomethane in aprotic (hydrocarbon) solvents gives benzophenone azine and tetraphenylethane as the principal identifiable products , while in hydroxylic solvents the appropriate derivative of benzhydrol accompanies the azine . Kinetic studies of the decomposition of diphenyidiazomethane in xylene and in 1-methylnaphthalene at about 100°C have shown that the reaction is first-order in diazo compound this has been interpreted as supporting the intermediate formation of di-phenylcarbene, viz- 

According to (3), the rate of disappearance of the diazo compound is given by 

1 ntegration and application of limiting conditions yields the expression 

Results for acetonitrile, benzene, and toluene solutions of diphenyldiazo-methane are given in Table 3. kjhi has been evaluated from the yield of ketazine, and k, has been derived from kobs hy means of eqn. (6). Values of A , do not differ significantly in the two aromatic solvents. The somewhat larger value of ki in acetonitrile indicates a small medium effect. Clearly, the nature of the solvent has a marked influence on the partitioning of the intermediate though not on its formation. 

In aqueous acetonitrile, the disappearance of diphenyidiazomethane followed first-order kinetics at all water concentrations . The observed velocity 

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The photolysis or thermolysis of diazo compounds results in the formation of carbenes. These reactions were identified as early as 1901 by Hantzsch and Lehmann D and Staudinger and Kupfer who decomposed diazomethane photochemically and thermally, respectively. The subsequent work of Hine 3) and Doering ) started the era of carbene chemistry. Excellent reviews of the chemistry of carbenes are available 5-27),... 

The reaction is assumed to involve initial formation of a carbene, by decomposition of the diazo compound with loss of nitrogen, followed by reaction of the electron-deficient carbene with the lone pair of electrons of the arsenic atom. Thermolysis of diazo compounds in copper-catalyzed reactions is known to provide singlet carbenes or carbenoid species (17). 

Enamine formation occurs by the thermolysis of diazo compounds (Scheme 150)67 109,278 284 288,304 332 453 454 via a carbene-like intermediate.284 332 When R1 = Ph, it enters into competition with hydrogen migration,284,332 and the electrophilic character of the carbene enhances the migration of the dimethylaminophenyl more than the phenyl.332 When triazoline synthesis is carried out at temperatures higher than that at which thermolysis of diazo compounds occurs, enamines are obtained exclusively, as in the addition of phenyl azide to cinnamic nitriles and ketones, with phenyl migration dominating in the nitrile.284 Enamine is also formed quantitatively in the reaction of ethyl diazoacetate with benzylideneaniline at 110°C.455... 

Thermolysis of 58a in butanol affords, together with 17% of 60a (R = C4H9) which evidences the intermediacy of the thiophosphene 59 a, a variety of partly atypical products which seriously impede the desired rearrangement38. Photolysis of 58b in methanol is also found to give only 18 % 1,2-P/C shift to form the heterocumulene 59b, from which the thiophosphinic rater 60b (R = CH3) results 39). As already mentioned in connection with the photolysis of diazo compounds of type 36 (see Sect. 2.2), Wolff rearrangement (9%) and O/H insertion (6%) once again compete with thiophosphinic ester formation. Moreover, solvolysis of the P(S)/C(N2) bond 391 prevents a greater contribution of carbene products to the overall yield. 

Certain transition metal complexes catalyze the decomposition of diazo compounds. The metal-bonded carbene intermediates behave differently from the free species generated via photolysis or thermolysis of the corresponding carbene precursor. The first catalytic asymmetric cyclopropanation reaction was reported in 1966 when Nozaki et al.93 showed that the cyclopropane compound trans- 182 was obtained as the major product from the cyclopropanation of styrene with diazoacetate with an ee value of 6% (Scheme 5-56). This reaction was effected by a copper(II) complex 181 that bears a salicyladimine ligand. 

Catalyst-mediated decomposition of diazo compounds in the presence of C=S compounds has found application for the preparation of thiiranes in homogeneous systems (68,110,111). Recently, a convenient procedure for the preparation of geminal dichlorothiiranes from nonenolizable thioketones and chloroform under Makosza conditions was reported (112). Another approach to 2,2-dihalogenated thiiranes (e.g., 2,2-difluoro derivatives) involves the thermolysis of Seyferth reagents in the presence of thioketones (113,114a). Nucleophilic dimethoxycarbene was shown to add smoothly to adamantanethione to provide a unique approach to a thiiranone (5, 5 )-dimethylacetal (114b). 

The first carbene ever isolated was la, which was prepared using the most classical route to transient carbenes, namely, the decomposition of diazo compounds. The [bis(diisopropylamino)phosphino](trimethylsilyl)diazomethane precursor (la) was obtained by treatment of the lithium salt of trimethylsilyldiazomethane with 1 equiv of bis(diisopropylamino)chlorophosphine. Dinitrogen elimination occurs by photolysis (300 nm) or thermolysis (250 °C under vacuum) affording carbene la as a red oily material in 80% yield (Scheme 8.1). Carbene la is stable for weeks at room temperature and can even be purified by flash distillation under vacuum (10-2 Torr) at 75-80 °C. 

A potential problem in the use of diazo compounds as C atom precursors is the fact that intermediates in these reactions may act as C donors with the free atoms not involved. Indeed, the timing of the reactions in Eq. 6 is unknown and some of these intermediates may be bypassed in the thermolysis of 8. However, a comparison of the reactions of carbon from 8 with those of nucleogenic and arc generated carbon reveals quite similar products from many different substrates and provides circumstantial evidence for free C atoms in the decomposition of 8. 

Certain transition metal complexes catalyze the decomposition of diazo compounds, where the metal-bound carbene intermediates behave differently from the free species generated by their photolysis or thermolysis. 

Diazomethane and its simple analogs undergo cycloaddition to unsaturated compounds both directly and after conversion to carbenes. The direct cycloadditions are 1,3-dipolar for the most part and provide access to pyrazolines and pyrazoles. Intramolecular cyclizations were recognized as early as 1965 95 The two main methods used in generation of diazo compounds for subsequent intramolecular cycloaddition include thermolysis of tosylhydrazone salts and thermolysis of iminoaziridines. Decomposition of nitros-amines has also been employed. 

The loss of CO, S, SO, SO2, SO3, and N2 by thermolysis or photolysis has been used to make three- and four-membered rings for example, the cyclic sulfamidate 94 undergoes clean thermolysis at 70 C to form the vinyl aziridine 95 in excellent yield <2002T5979, CHEC-III(1.01.6.5)79> and Wolff rearrangement of diazo compounds 96 gives -lactams 97 (Scheme 51) <1973J(P1)2024>. 

Diazirines 179 and 181, the cyclic structural isomers of diazo compounds, are often employed as entry to carbenes. By thermolysis of diazirines methoxychloro- , cyanophenyl- and chlorophenyl carbenes are generated and are able to undergo addition across olefinic derivatives (equation 44). 

The reaction of dichlorocarbene with ketones and diamines results in near quantitative formation of a mixture piperazinones 584 and 585 (80JOC754). As shown in Section III,C,2, piperazine 78 [R = H, R + R = (CH2)s], the minor product of the Rh2(OAc)4-catalyzed decomposition of diazo ester 73, is the result of the dimerization of the intermediate ylide 76 (84JOC113). Tetrahydropyrazines were synthesized through ring expansion of imidazolidines. Thermolysis or photolysis of diazo compounds... 

The rate of carbene formation by pyrolysis of diazo compounds shows unexpected substituent effects. The unimolecular rate constant for thermolysis of diphenyldiazomethane is increased by any single para substituent, be it electron-donating or electron-withdrawing. If both rings are substituted, one by an electron-donating and the other by an electron-withdrawing substituent, then this disubstituted compound decomposes faster than a compound monosubstituted with either substituent. However, symmetrically disubstituted diphenyldiazomethanes decompose more slowly than do the compounds monosubstituted with the same substituents. 

Smith and his students have extended their productive studies to include bis[l,2,3-triazole] examples (Eqs. 14,15). The first reaction appears to be an exception and related systems provide much lower yields. Hie diazo compound (15.1>6) can be reduced in high yield using hydrazine and palladium. The members of Smith s group have also made a detailed study of the thermolysis of diazo-1,2,3-triazoles in various aromatic solvents and have found useful amounts of bis-products (Eq. 16). ... 

Formation of cluster opened methano- and imino[60]fullerenes (fulleroids and azafulleroids) thermal [3+ 2]-cycloadditions of diazo compounds or azides lead to the formation of fulleropyrazolines or fullerotriazolines. The thermolysis of such adducts after extrusion of N2 affords as kinetic products the corresponding [5,6] -bridged methano and iminofullerenes with an intact 60 7T-electron system and an open transannular bond (Scheme 6) [113-128]. The corresponding [6,6] -bridged structures with 58 rr-electrons and a closed transannular bond are formed only in traces. 

Thermal or photochemical decomposition of diazo compounds and diazirines gives carbenes. The formation of very stable gaseous nitrogen compensates for the formation of the unstable carbene. Photolysis or thermolysis of a ketene-Hke diazo compound eliminates a stable molecule carbon monoxide (CO) to yield a carbene. The reactions are not widely used since ketenes are not readily available precursors, and tend to polymerize under the reaction conditions (Scheme 5.5). 

Photolysis and Thermolysis. The absorption maxima of diazo compounds BTf and BP in methanol were at 396 nm and 392 nm as shown in Table 1. 

Thermal conversion of diazirines to linear diazo compounds was postulated occasionally and proved by indirect methods. The existence of a diazo compound isomeric to diazirine (197) was proved spectroscopically on short thermolysis in DMSO (76JA6416). An intermediate diazoalkane was trapped by reaction with acetic acid, yielding the ester (198) (77JCS(P2)1214). 

In the case of sodium 2-(diarylmethylene)cyclopentanone tosylhydrazones 3, however, thermolysis gives the 3//-1,2-benzodiazepines 6 in good yield selected examples are shown. It is suggested that steric constraints in the diazo compounds 4 favor the [1,7] ring closure. The reaction proceeds by way of the intermediates 5, which rearrange to the products by a [1,5] shift of hydrogen.115... 

Cyclizations of doubly unsaturated diazo compounds containing a thiophene ring within rather than at the end of the diene system to yield thicnodiazepines have also been reported. Thus, thermolysis of the sodium salt 7 gives the l//-thieno[3,2-r/]-2,3-diazepine 9. The intermediate 8 rearranges to the more stable product 9 by a symmetry allowed [1,5] shift of hydrogen.14,1... 

More recently, Williams has described the one pot synthesis of 2-substituted oxazoles 11 by the thermolysis of triazole amides 9 the reaction does not proceed photo-chemically.<92TL1033> Although the reaction does not involve addition to a nitrile, it is an interesting application of a diazo compound since the proposed zwitterionic intermediate 10 is a resonance form of a diazo imine, so formally the reaction may be thought of as a thermal decomposition of a diazo imine (Scheme 6). 

The triplet state is usually the ground state for non-conjugated structures, but either species can be involved in reactions. The most common method for generating nitrene intermediates, analogous to formation of carbenes from diazo compounds, is by thermolysis or photolysis of azides.246... 

Carbenes are commonly generated by irradiation or pyrolysis of an appropriate diazo-compound (2). Apart from differences readily traced to the change in temperature, the chemical properties of the carbenes formed from photolysis and from thermolysis are usually quite similar. These observations... 

Direct irradiation or thermolysis of a diazo-compound (2) is believed to generate the carbene initially in its singlet spin state. Triplet sensitization (5) is presumed to give the triplet carbene directly without first forming its singlet state via the triplet diazo-compound. In some cases, careful comparison of the results of direct irradiation experiments with those from triplet sensitization can provide useful information to identify the spin state initiating a reaction. 

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