Pyrolysis of tert-Butyl tert-Butanethiosulfinate, t-BuS(O)St-Bu: A Computational Perspective of the Decomposition Pathways

Abstract

A systematic theoretical study has been performed on the low pressure thermal decomposition pathways of t-BuS(O)St-Bu using the CCSD(T)/cc-pV(D+d)Z//B3LYP/6-311++G(2d,2p), CCSD(T)/cc-pV(D+d)Z//PBEPBE/6-311++G(2d,2p), and G3B3 level of theories. Rate constants for the unimolecular decomposition pathways are calculated using Rice−Ramsperger−Kassel−Marcus (RRKM) theory. On the basis of the experimental observation and theoretical predictions, the pyrolysis channels are considered as primary and secondary pyrolysis reactions. The primary decomposition via a five-membered transition state leads to the formation of tert-butanethiosulfoxylic acid (t-BuSSOH) and 2-methylpropene (C4H8) almost exclusively having low-pressure limit rate constant k(1)(0) = 4.67 × 10(−6)T(−4.67) exp(−11.64 kcal mol(−1)/RT) cm3 mol(−1) s(−1) (T = 500−800 K). The primary decomposition via a six-membered transition state is also identified, and that leads to the tert-butanethiosulfinic acid t-BuS(OH)S, which is the branched chain isomer of t-BuSSOH. The formation of t-BuSSOH is thermodynamically as well as kinetically favorable over t-BuS(OH)S formation, and therefore the second product could not be found experimentally. Furthermore, calculation on secondary pyrolysis pathways involving the decomposition of t-BuSSOH leads to the formation of 1-oxatrisulfane (trans-HSSOH and cis-HSSOH) and their branched isomer S(SH)OH. These three secondary product formation rates are competitive, but thermodynamics do not favor the formation of the branched isomer. Among the secondary pyrolysis products, trans-HSSOH is the most stable one, and its formation rate constant at low pressure is calculated to be k(3)(0) = 5.49 × 10(28)T(−10.70) exp(−36.22 kcal mol(−1)/RT) cm3 mol(−1) s(−1) (T = 800−1500 K). Finally, the secondary pyrolysis pathway from less stable product t-BuS(OH)S is also predicted, and that leads to trans-HSSOH and cis-HSSOH products with almost equal rates. A bond-order analysis using Wiberg bond indexes obtained by natural bond orbital (NBO) calculation predicts that the primary and secondary pyrolysis of t-BuS(O)St-Bu occur via E1-like mechanism.

New Reactions of N‐tert‐Butylimines; Formation of N‐Heterocycles by Methyl Radical Elimination on Flash Vacuum Thermolysis of N‐Benzylidene‐ and N‐(2‐Pyridylmethylidene)‐tert‐butylamines

“…An alternative pathway involves the formation of a tert ‐butyl radical as a result of homolysis of a tert ‐butylN bond, which is known to take place in tert ‐butyl azo‐compounds 5. The formation of isobutene from the tert ‐butyl radical is readily understood,4b but formation of NH‐imines would seem to require another process. In a previous study, McNab et al 6.…”

Thermolysis of 3,3,5,5‐Tetramethyl‐1,2,4‐trithiolane 1‐Oxide: First Matrix Isolation of the HOSS· Radical

“…The structure of 13 was elucidated by analysis of its rotational spectrum as well as that of its isotopologues 4b,4c. Recently, the mechanism of thermolysis of 12 , leading to 13 and S 2 O ( 14 ), was studied by means of computational methods 9. In accordance with earlier reported results,4a,4b the experiment conducted at 500 °C leads to elimination of isobutene ( 20 ) and the formation of 1‐oxatrisulfane derivative 19 (Scheme ).…”

“…The first step is an electrocyclic ring opening with a simultaneous H‐shift yielding 1‐oxatrisulfane derivative 22 . It corresponds to the mechanistic pathway postulated on the basis of computations carried out for 12 9. In both cases, the computed activation energies are below 30 kcal mol –1 .…”

“…It should be noted that the alternative pathway B corresponds to that computed for the fragmentation of 19 9. However, competitive homolytic fragmentation corresponding to pathway A (Scheme ) has not been considered in this report 9.…”

“…This important finding allows us to conclude that, in both cases, the reaction pathways are governed by analogous mechanisms in which sulfur radicals play a key role. The appearance and role of the sulfur radical 16 was not considered in either experimental or computational works focusing on the thermolyses of thiosulfinates 4,9…”