Pyrolysis mechanisms pyrrole

FIGURE 8.15 Pyrolysis mechanism of pyrrole proposed by Lifshitz et al. 

Zhai et al. [78] approved the opinion of Mackie et al. s, and they investigated the pyrolysis mechanisms of pyrrole by density functional theory calculations. The results show that the initial step of pyrrole pyrolysis, via a 1,2-hydrogen migration from nitrogen to an adjacent carbon, leads to 2H-pyrrole. The activation barrier was... 

The kinetics and mechanism of pyrrole pyrolysis were investigated by ab initio quantum-chemical calculations. It was revealed that pyrrole undergoes tautomerization to form 2H- and 37/-pyrroles prior to any thermal decomposition. It has been shown that the major product, HCN, arises from a hydrogen migration in pyrrole to form a cyclic carbene with the NH bond intact. Ring scission of the carbene leads to an allenic imine of HCN and propyne which is the lowest energy pathway. The 277-pyrrole... 

The reaction of ketocarbenoids with pyrroles leads to either substitution or cyclopropanation products, depending on the functionality on nitrogen. With N-acylated pyrrole (209) reaction of ethyl diazoacetate in the presence of copper(I) bromide generated the 2-azabicyclo[3.1.0]hex-3-ene system (210) and some of the diadduct (211 Scheme 44).162163 On attempted distillation of (210) in the presence of copper(I) bromide rearrangement to the 2-pyrrolylacetate (212) occurred, which was considered to proceed through the dipolar intermediate (213). In contrast, on flash vacuum pyrolysis (210) was transformed to the dihydropyridine (214). A plausible mechanism for the formation of (214) involved rearrangement of (210) to the acyclic imine (215), which then underwent a 6ir-electrocyclization. 

In general, pyrolysis of monocyclic 1,2,3-triazines 1 leads to acetylenes, nitriles and nitrogen.46, 295 297 300 Heating simple 1,2,3-triazines in a sealed tube led to the isolation of pyridines, pyrimidines, pyrazoles, pyrroles, pyridazine, and indeno[3,2-/>]pyridine, depending on the substituents bound to the 1,2,3-triazine ring and the reaction conditions.46 A mechanism formulated for the formation of these products involves an azete as an intermediate. 

In the pyrolysis of proteins, the dominant mechanism is the splitting-off of the amino acids rather than break-up of the backbone into smaller fragments which are characteristic of the original amino acids.Usually highly characteristic signals are found for the aromatic and sulphur containing amino acids, e.g., hydrogen sulphide from cystine and in combination with methionine pyrrole pyrrolidine and methylpyrrole for proline, phenyl, and cresol from tyrosine, toluene, styrene and phenylacetonitrile for phenylalanine indole, and methylindol from tryptophan. 

Pyrolysis and smoke studies of amino acids indicate that they are potential precursors of several nitrogen heterocyclic ring systems found in tobacco smoke. Proline has been shown to be efficiently converted to pyrrole upon pyrolysis (3219, 3724) and in addition has been proposed as a possible precursor of pyrocoll (2593, 4336). y-Amino acids and dicarboxy-lic amino acids are capable, under pyrolytic conditions, of forming 2-pyrrolidones (1967, 3079) and by a similar mechanism, A-amino acids can form 2-piperidones (1967, 3079). A side effect of the pyrolysis of amino acids is the formation of hydrogen cyanide. 

Since nicotine is the most abundant and best known tobacco alkaloid, its pyrolysis has been thoroughly studied [Woodward et al. (4275a), Jarboe and Rosene (1923a)]. More recent work [Kaburaki et al. (2006)] on the pyrolysis of nicotine and various alkyl-pyridines has resulted in a proposed mechanism for the thermal degradation of nicotine. .. Schmeltz [Schmeltz et al. (3499)] also studied nicotine and identified a number of previously unreported compounds in the nicotine pyrolysates. .. These included pyrrole, acenaphthene, indole, skatole, and anthracene and/or phenanthrene. However, the presence of dibenzacridines and dibenzcarbazole, previously reported in nicotine and pyridine pyrolysates, could not be confirmed [Van Duuren et al. (4027)]. 

---