C=N cleavage reactions

Whether these radicals serve as electron donors (to yield iminium cations) or as hydrogen atom donors (to yield vinylamines) will depend on the nature of the reaction partner, particularly on its reduction potential. In any case, the quantum yield of C N cleavage reactions are low, since they require several consecutive processes, i.e. successive transfer of an electron, a proton, and a second electron, or a mechanistic equivalent thereof. 

These C N cleavage reactions suggest that it may be possible to develop HDN catalysts that will eliminate nitrogen from unsaturated organonitrogen compounds without first hydrogenating the hydrocarbon framework. 

In alcoholic solution, in addition to the carbene-forming C-N cleavage reaction a concurrent heterolytic dissociation has recently been proposed which gives rise to a ketene-diazonium ion pair, viz. 

Salamo is one of the most useful ligands for the preparation of multi-metal complexes because of its excellent affinity for various metal ions and stability of the ligands toward a C=N cleavage reaction. In addition, salamo metal complexes are usually much more soluble in common organic solvents than the corresponding salen complexes. 

In contrast to the well-defined photochemical behavior of 1-azirines the thermal reactions of these compounds have been studied less thoroughly (68TL3499). The products formed on photolysis of azirines can best be rationalized in terms of an equilibration of the heterocyclic ring with a transient vinylnitrene. Thus, products formed from the thermolysis of azirines are generally consistent with C—N cleavage. For example, the vinylnitrene generated from the thermolysis of azirine (149) can be trapped with phosphines (72CCS6S). 

In the meantime temperature-dependent stopped-flow measurements were conducted on the latter complex in order to determine the activation parameters of the N-N cleavage reaction (24). Plots of the absorption intensity at 418 nm vs. time at T — —35 to +15°C indicate biphasic kinetics with two rate constants 0bs(p and obs(2)> in analogy to our measurements of the tungsten complex. This time, however, both rates depended upon the acid concentration. Interestingly much smaller rate constants 0bs(i) and 0bs(2)> were found for all acid concentrations than given by Henderson et al. for his (single) rate constant kobs (up to 1 order of magnitude). Furthermore plots of 0bs(i) and kohs(2) vs. the acid concentration showed no saturation behavior but linear dependencies with slopes k and k and intercepts k und k, respectively (s — acid dependent and i — acid independent), Eq. (2) ... 

Predictably, 1,2,4-triazole is alkylated preferentially at the 1-position [36, 38,39]. Specific alkylation at the 4-position can be achieved by the initial reaction with dibromomethane to form the bis-triazol-l-ylmethane (see below), followed by quat-emization of the triazole system at the 4-position and subsequent C-N cleavage of the 1,1 -methylenebistriazolium salts [40]. 1,2,3-Benztriazole yields a mixture of the isomeric 1- and 2-alkylated derivatives [41]. The 1-isomer predominates, but the ratio depends on whether the reactions are conducted in the presence, or absence, of a nonpolar organic solvent (Table 5.33). Higher ratios of the 1-isomer are obtained under solidrliquid two-phase conditions. Thus, alkylation of 1,2,3-benztriazole with benzyl chloride produces an overall yield of 95% with the l- 2-isomer ratio of ca. 5.7 1 similar reactions with diphenylmethyl and triphenylmethyl chlorides gives overall yields of 95% (9 1 ratio) and 70% (100% 1-isomer), respectively [38], 6-Substituted purines are alkylated at the N9-atom and reaction with 1-bromo-3-chloropropane yields exclusively the 9-chloropropyl derivative (cf. reaction wi phenols) [42]. 

Under favourable circumstances, the initially formed /V-ylid reacts further through C-N cleavage. Thus, in the presence of a strong nucleophile, such as a phenoxide anion, the quaternary dichloromethylammonium cation forms an ion-pair with the phenoxide anion (Scheme 7.27), which decomposes to yield the alkyl aryl ether and the /V-formyl derivative of the secondary amine [22, 23]. Although no sound rationale is available, the reaction appears to be favoured by the presence of bulky groups at the 4-position of the aryl ring. In the absence of the bulky substituents, the Reimer-Tiemann reaction products are formed, either through the breakdown of the ion-pair, or by the more direct attack of dichlorocarbene upon the phenoxide anion [22,23],... 

An electrochemical reaction on the azepine carbamates 19 resulted in a ring contraction to give the N-substituted anilines 20 in moderate yield (Equation 1). Mechanistically, initial oxidation of the carbamate to give the radical cation 21, followed by electrocyclic rearrangement to 22, C-N cleavage, and then reduction was proposed <1998H(48)1151>. 

The involvement of an aldehyde-enolate as a ligand on IBX has also been postulated in a first oxidative C C bond cleavage reaction using IBX. N-Protected amino alcohols 14 have been oxidized with IBX in DMSO to the corresponding imides 15 [17]. 

Methyl radicals were indeed detected at short delay times in flash photolysis , and ESR studies have demonstrated the presence of CHj and NO2 radicals . In addition, large yields of CH are formed in the photolysis, presumably by abstraction reactions of CHj . Recently, experiments by kinetic flash photolysis have shown that the primary step leads to C-N cleavage and that the formation of methyl nitrite is explained by step (b). In contrast to previous results the... 

Early kinetic experiments on the thermal decomposition of nitro compounds established that for the simplest derivative, nitromethane, the process was first order, but that the reaction was chemically complex owing to further reactions between the products and nitromethane. Cottrell et re-examined the nitromethane pyrolysis and reported values of = 53.2 kcal.mole" and log A = 13 for the Arrhenius parameters of the homogeneous decomposition a radical mechanism was proposed, initiated by C-N cleavage... 

Within experimental error, the activation energy is the same as that measured for the condensed phase pyrolysis of hexanitroethane" , 38.9 kcal.mole, also suggesting a simple rate determining rupture of a C-N bond in the latter reaction. Slightly smaller activation energies and pre-exponential factors have been report-ed for C(N02)4 and C2(N02)e decompositions (Table 30). Trinitroalkanes" also follow first-order kinetics and it appears that C-N cleavage is rate-determining. Table 30 lists the Arrhenius parameters of thermolysis. 

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