For verification of this paper, please visit on http://www.socialresearchfoundation.com/innovation.php#8

Objectives: Flash vacuum pyrolysis (FVP) is a technique which allow preparation of many unusual products, which are difficult to synthesize by conventional methods.  A range of different experimental set-ups have been described, but the one used here is robust and easily constructed from standard laboratory equipment with the addition of a tube furnace and a simple vacuum line with pump. Despite the high temperatures involved (500-1000 °C). The high vacuum flow conditions mean that each molecule only spends a few milliseconds in the hot zone so the technique is actually quite gentle and sensitive products including chiral compounds can be obtained in good yield without significant decomposition or racemization. Synthesized products could be converted to products of further synthetic utility. Pyrolysis of o-phenylene sulfite analogs give rise to products resulting from loss of SO. The pyrolysis is consistent with the fragmentations in the mass spectrometer, which proceed with initial loss of SO from the molecular ions. Substituted o-phenylene sulfites fragment differently under electron-impact and pyrolytic conditions.

DeJongh D.C., Van Fossen R. Y., J. Org. Chem., 1972; 37: 1129-1135

Gas‐phase pyrolysis is a special synthetic method that is usually clean, convenient, and efficient, and that has advantages over other synthetic methods for accomplishing the same goals. Flow gas‐phase pyrolysis has long been used to provide a very powerful and useful alternative methodology in synthetic organic chemistry. Organic chemists have been performing flash vacuum pyrolysis reactions with the aim of synthesizing new compounds and/or to study reactive intermediates. An added advantage is that this technique is effectively free from solvation, hydrogen bonding, and protonation effects common in solution reactions. Gas‐phase pyrolysis reactions offer important routes for novel heterocyclization and selective synthesis. The flash vacuum pyrolysis (FVP) of o-phenylene sulfite related compounds is an important and high-yielding source of biologically active compunds. Early research on the pyrolysis of 1-acylbenzotriazoles only revealed formation of benzoxazoles, while the important formation of cyanocyclopentadienes was missed. Similarly, previous studies of the pyrolysis of 1-alkoxycarbonylbenzotriazoles revealed the formation of 2-alkoxybenzoxazoles, 1- and 2-alkylbenzotriazoles.

Synthesized products could be converted to products of further synthetic utility. Pyrolysis of o-phenylene sulfite analogs give rise to products resulting from loss of SO. The pyrolysis is consistent with the fragmentations in the mass spectrometer, which proceed with initial loss of SO from the molecular ions. Substituted o-phenylene sulfites fragment differently under electron-impact and pyrolytic conditions.

The experiment involves pyrolysis of the simple heterocyclic compound o-phenylene sulfite[10-11] (1) which is readily prepared by heating catechol (benzene-1,2-diol) with thiony chloride. When it is subjected to FVP at 750 °C and 10 -10-* Torr, it first loses sulfur monoxide to give o-benzoquinone (2) and this then loses carbon monoxide. The resulting cyclopentadienone (3) is a reactive 1,3-diene, which readily undergoes Diels-Alder cycloaddition with itself to form a [4 + 2] dimer. ° The SO fragment eliminated in the first step is an unusual and unstable oxide of sulfur, which disproportionates by a complex mechanism [12] to give ultimately SO, and elemental sulfur. The cyclopentadienone dimer (4) will be purified by preparative thin-layer chromatography. This technique is ideal for separation of complex mixtures on a scale of 0.1-1 g and involves running the mixture, applied as a linear band, up a large (20 × 20 cm) TLC plate of silica or alumina. After visualisation under UV light the component bands can be scraped off individually and the products extracted from the support material.

The mixture of pyrolysis products was dissolved in methylene chloride and treated as described below. Silica gel was used for TLC. The GC analyses used a thermal conductivity detector. Some of the ions could conceivably arise from pyrolysis in the ion source, followed by ionization. However, metastable ions suggest that this is not happening. Also, the mass spectra are the same from different instruments and at different ion source temperatures. Except where indicated, the diols used in the syntheses of the sulfites were obtained commercially from Aldrich or from Matheson Coleman and Bell. 

o-Phenylene sulfite and its ring-substituted analogs were prepared by a procedure based on one described by Green. The modified procedure is given below for preparation of o-phenylene sulfite. The data from the substituted sulfites are given in Table 1. Characteristic IR absorptions were observed at 1200-1220 cm-1.

o-Phenylene sulfite had bp 58° (1.5 mm); ir (neat) 1220, 1240, 1460 cm-1; uv (pentane) Xmax (absorbance) 273 (0.51), 269 (0.575), 265 nm (sh) (0.450); yield, 72%. o-Phenylene sulfite is a strong lachrymator which hydrolyzes to catechol readily.

Compound	Diol, g	Solvent	Yield %	B.P. 0C
2-methyl	10.0	C6H6	77	61
4-tert-Butyl (9)	10.0	C6H6	84	75
3,5-Di-tert-Butyl (10)	7.6	C6H6	78.5	110

Table-1

Mass Spectra:

o-Phenylene sulfite (1): m/e (rel. intensity) 156 (80), 108 (35), 92 (4), 80 (100), 64 (15.5), 63 (16), 52 (80), 51 (31), 50 (25), 48 (14), 39 (14.5), and 38 (11.5).

2-Methyl-o-phenylene sulfite (7): m/e (rel. intensity) 170 (58), 122 (6), 106 (4), 105 (12), 94 (100), 78( 30), 77 (14), 66 (96), 65 (22), 63 (12.5) , 52 (16), 51 (24), 50 (11), 48 (12), 40 (24), 39 (54), and 38 (14).

4-tert-Butyl-o-phenylene sulfite (9): m/e (rel. intensity) 212 (25.5), 197 (100), 169 (2), 149 (2), 133 (6.5), 121 (4), 105 (11.5) , 93 (5), 91 (7), 79 (8), 78 (3), 77 (5), 65 (4), 53 (4), 52 (5), 51 (8), 41 (8), and 39 (8.5).

3,5-Di-tert-butyl-o-phenylene sulfite (10): m/e (rel. intensity) 268 (26), 253 (100), 189 (6.5), 149 (8), 119 (9), 105 (9), 91 (8), 75 (28), and 41 (17).

Discussion:

Pyrolysis of o-phenylene sulfite (1), 2-methyl-o-phenylene sulfite (7), 4-tert-butyl-o-phenylene sulfite (9) and 3,5-di-tert-butyl-o-phenylene sulfite (10) give rise to products resulting from loss of SO. For these compounds, the pyrolysis is consistent with the fragmentations in the mass spectrometer, which proceed with initial loss of SO from the molecular ions.

4-tert-butyl- (9) and 3,5-di-tert-butyl- (10) substituted o-phenylene sulfites fragment differently under electron-impact and pyrolytic conditions. Electron impact leads to loss of a methyl radical from a teri-butyl group, presumably forming a very stable tertiary benzylic cation or a ring-expanded analog. The molecules SO and S0₂ are lost at nearly the same extent from these M —CH₃· ions. Upon pyrolysis of 9 and 10, products resulting from SO loss are observed.

We have generally observed that there is less similarity between the two processes as the molecules become increasingly aliphatic. Striking similarity has also been observed in the case of biphenylylene cyclic sulfites. Electron-impact fragmentation of biphenylylene-2,2' sulfite leads to competitive losses of SO and SO₂. The importance of the loss of SO increases as the electron voltage is lowered, until the two losses are nearly equal in importance.

In spite of that, major products from the pyrolysis of benzophenone-2,2' sulfite are due to the loss of S0₂, whereas SO and S0₂ are lost competitively from the molecular ions with almost the same importance at 70 eV. It seems that the mass spectra of aromatic cyclic sulfites can be used to predict the most probable products of pyrolysis. As aliphatic substituents are added which can lead to very stable cations, the mass spectra are dominated by cleavage to stable ions, whereas the pyrolysis results resemble those of the unsubstituted analogs.

In the present review we see that, how o-phenylene sulfite and their substituted analogs undergoes pyrolysis, and from dimer products. These dimer products can form different heterocyclic products at various temperatures. Heterocyclic aromatic compounds are known to show a large range of ecotoxic effects, e.g. acute toxicity, developmental and reproductive toxicity, cytotoxicity, photoinduced toxicity, mutagenicity, and carcinogenicity. Synthetic heterocycles have widespread therapeutic uses such as antibacterial, antifungal, antimycobacterial, trypanocidal, anti-HIV activity, antileishmanial agents, of biochemical systems. The constantly accelerating rate genotoxic, antitubercular, antimalarial, herbicidal, analgesic, anti-inflammatory, muscle relaxants, anti-convulsant, anticancer etc. 

1. Brown R. E. C., Pyrolytic Methods in Organic Chemistry, Academic Press, New York, 1980.
2. Wentup C., Adv. Heterocycl. Chem., 1981; 28: 231-361.
3. Wiersum U. E., Recl. Trav, Chim. Pays-Bas, 1982; 101: 317-332.
4. Karpf M., Angew. Chem., Int. Ed. Engl., 1986; 25:414-430.
5. Aitken R. A., Thomas A. W., , John Wiley & Sons, Inc., New York, 1997; supplement A3:473-536.
6. McNab H., Aldrichim. Acta, 2004; 37: 19-26.
7. Wentrup C., Aust. J. Chem., 2014; 67: 1150-1165.
8. Dejongh D.C., Van Fossen R. Y., Bourgeois C. F., Tetrahedron Lett., 1967; 8: 271-276.
9. DeJongh D.C., Van Fossen R. Y., J. Org. Chem., 1972; 37: 1129-1135.
10. Aitken, R. A., Aitken K. M., Carruthers P. G., Jean M.-A., Slawin, A. M. Z. Chem. Commun. 2013; 49: 11367–11369.
11. Harras, M., Milius, W., Aitken, R. A., Schobert R., J. Org. Chem. 2017; 82: 579–597.
12. Hegelund, F., Wugt Larsen R., Aitken R. A., Aitken K. M., Palmer M. H., J. Mol. Spectrosc. 2007; 246: 198–212.