C₉H₈ Pyrolysis. o-Tolylacetylene, Indene, 1-Indenyl, and Biindenyls and the Mechanism of Indene Pyrolysis

Abstract

o-Tolylacetylene 5 is obtained by flash vacuum pyrolysis (FVP) of the isoxazolone 13a at 800 °C/10(-4) hPa. At 900-1000 °C the acetylene 5 isomerizes to indene 1, which reacts further by elimination of a hydrogen atom and dimerization of the 1-indenyl radical 9 to 1,1'-biindenyl 10. The latter undergoes partial isomerization to 3,3'-biindenyl 16, and further pyrolysis of the biindenyls yields higher polycyclic aromatic hydrocarbons (PAHs), particularly chrysene 2. C-H bond breakage in indene, which occurs with an activation energy of 80 ± 5 kcal/mol with formation of the 1-indenyl radical 9, has been the subject of much investigation in relation to hydrocarbon combustion, in particular the formation of chrysene and other PAHs from indene, which itself is formed in the combustion of toluene and other hydrocarbons. However, C-C bond breakage also needs to be considered. Calculations at the B3LYP/6-311+G(d,p) level indicate that key C-C bond breakages in indene have free energies of activation of ca. 80 kcal/mol. Positive entropies of activation make all these reactions more facile at high temperatures relevant to hydrocarbon combustion chemistry. C1-C2 bond breakage results in the formation of o-tolylvinylidene 6 and o-tolylacetylene 5. The reversible 1,2-shift interconverting 5 and 6 (the Roger Brown rearrangement) can lead to carbon scrambling in C3-labeled indene 1a, resulting in indene 1d carrying the label in positions 1, 2, and 3 and explaining the (14)C-labeling pattern observed by Badger et al. in the derived chrysene 2d. o-Tolylacetylene 5 and o-tolylvinylidene 6 should be considered as intermediates in models of the fuel-rich combustion of toluene, indene, and other hydrocarbons.

Ethynamine - Ketenimine - Acetonitrile - Rearrangements: A computational Study of Flash Vacuum Pyrolysis Processes

“…Acetylenes are known to undergo thermal interconversion with vinylidenes (the Roger Brown Rearrangement) 38 with an energy barrier of ~40 kcal/mol, 39,40,41 whereby the barrier for the Thus the overall barrier for the rearrangement of 3 to 4 is expected to be no more than about 113 kcal/mol relative to acetonitrile, or ~60 kcal/mol relative to ethynamine 3. This reaction therefore becomes the overall most preferred path for the isomerization 3à4, removing the need for TS1.…”

An experimental study of indene pyrolysis with synchrotron vacuum ultraviolet photoionization mass spectrometry

“…A few literature studies are related to the detailed chemical kinetics of indene. 6,9,[30][31][32][33][34] Badger and Kimber 9 performed gas chromatography (GC) measurements of indene pyrolysis, and observed a significant amount of benzofluorenes, benz[a]anthracene, benzophenanthrene and chrysene. Laskin and Lifshitz 6 measured thermal decomposition products of indene behind reflected shock waves using a GC.…”

“…Lifshitz et al 34 investigated the pyrolysis of 2-methyl-indene in a shock tube, and partially described the mechanism of indenyl radicals. Flash pyrolysis of indene and 2-ethynyl-toluene (C 2 HC 6 H 4 CH 3 ) was conducted by Ajaz et al, 30 and that of 3-methyl-4-(2 0 -methyl)benzylideneisoxazol-5(4H)-one by Wentrup et al 31 The quantum chemical calculations of Wentrup et al 31 explained the interconversion between indene and 2-ethynyl-toluene by Roger-Brown rearrangement. Measurements of Ajaz et al 30 and Wentrup et al 31 showed that the combination of indenyl or indene could produce large PAHs, such as bi-indenyl and chrysene.…”

Acetylenes from Aldehydes. Preparation of Ethynylphenols and Phenylacetylenes by Flash Vacuum Pyrolysis of Isoxazolones

“…As acetylenes have numerous important uses in organic synthesis and materials science, efficient syntheses of acetylenes are highly desirable. [1][2][3] Particularly noteworthy is the advent of 'click chemistry', which has made terminal acetylenes the molecules of choice for linking large molecular moieties. [3d-g] We have previously reported the synthesis of a variety of alkynes 3 in high yields using flash vacuum pyrolysis (FVP) of 4-methylideneisoxazol-5(4H)-ones 1, which themselves are readily synthesized from aldehydes and 3-methylisoxazol-5(4H)-ones.…”

“…[6] Although a 1,2-R shift is also possible, this has a higher activation barrier. [2,7] 3-Alkylidene-1-azirines may also be involved as transient intermediates. [6] In addition, ethynylamines RR 0 N-C C-H and RNH-C C-H and ketenimines R-N¼C¼CH 2 have also been generated by FVP of 4-aminomethyleneisoxazolones.…”