Pyrolytic dimerization

Figure 1. Formation of PCDDs by pyrolytic dimerization of chlorophenate...

Tetrathianes. (1) Oxidative dimerization of a,a-disuhstituted alkanedithioic acid dianions (Scheme 38) or 1,1-dithiols (Equation 17) - very limited examples and a-monosuhstituted alkanedithioic acids decompose (2) reductive or pyrolytic dimerization of gi OT-disulfenyl dichlorides (Equation 18) - only malonate-derived examples (3) reaction of a-chloro sulfenyl chlorides with sodium trithiocarhonate (Equation 19) - only malonate-derived examples (4) sodium thiophenoxide-catalyzed reaction of thioketones with elemental sulfur (5) reaction of benzo-furan-3(2//)-one with S2CI2" (6) UV irradiation of a CS2 solution of a diazirine (7) reaction of a 2,2,4-trisubstituted-1,3-dithietane with Oxone (Scheme 54)". Method (4) is the most convenient and general of these. 

Thioketenes can be prepared in several ways, from carboxylic acid chlorides by thionation with phosphorus pentasulfide [1314-80-3], P2S5, from ketene dithioacetals by p-elimination, from 1,2,3-thiadiazoles with flash pyrolysis, and from alkynyl sulfides (thioacetylenes). The dimerization of thioketenes to 2,4-bis(alkylidene)-l,3-dithietane compounds occurs quickly. They can be cleaved back pyrolytically (63). For a review see Reference 18. 

The characteristic pyrolytic process for acid chlorides is loss of HC1 and this occurs for the bicyclic example 235 on FVP at 800 °C to give cyclohexa-1,2-diene 238 by way of the ketene 236 and carbene 237 as shown113. The product can be directly observed by low-temperature IR and forms a [2 + 2] dimer on warming up. FVP of >-toluoyl chloride 239 at 630 °C also results in loss of HC1 to provide a dependable large-scale synthesis of benzocyclobutenone 240 in excellent yield114. Pyrolysis of trichloroacetyl chloride, CI3C—COC1, over a bed of zinc at 420 °C results in dechlorination... 

The dimers D, 4 readily undergo electrochemical, irreversible, oxidation under anaerobic conditions on the pyrolytic graphite electrode, with oxidation peak potentials, determined by linear sweep voltammetry on 1 mM solutions, as shown in Table V). Note the much lower values of E for dimers Dj 3, relative to D4, testifying to the greater susceptibility of the former to oxidation. 

The reduction pathway for 2-thiopurine at the DME, and the pyrolytic graphite electrode (PGE), embraces three steps 166). The first is a one-electron reduction to a free radical which, below pH 5, undergoes dimerization and/or a second one-electron reduction to the 1,6-dihydro derivative. Above pH 5 the second wave is not observed and the free radical dimerizes to 6,6 -6ri-(l,6-dihydro-2-thiopurine) which, in turn, was postulated to undergo further reduction to 1,6-dihydropurine, accounting for the third wave l66). 

Acyclic alkenic sulfur compounds have also been obtained via pyrolytic rDA reactions. A general route to thioacrylamides utilizing rDA reactions under FVP conditions has been developed. The rDA reaction of the appropriate adducts yields thioacrylamide, V-methylthioacrylamide and NA -dimethyl-thioacrylamide in approximately quantitative yield (equation 13). The high reactivity of these a,3-unsaturated thioamides had previously limited their availability. a,3-Unsaturated acyclic chio-ketones such as (17) were prepared via a rDA reaction using a FVP technique. These highly reactive species dimerize to 4//-l,3-dithiins (18) on warming to-60 °C (equation 14). ... 

The sugar components ofDNA- and RNA-based nucleic acids participate in the formation of nucleoside subunits and under pyrolytic conditions will lead to a specific 2-methylfuran (la) residue. RNA-based nucleic acids will also exhibit fragment ions due to 2-methyl-4-hydroxyfuran (lb) (Fig. 3). This behavior is typical of ribose derivatives. However, the attachment of the fragments la or lb to other ions formed in the fragmentation processes is unusual. The la unit, for example, undergoes dimerization, and ions con-... 

By comparing the results for chlorinated polypropylene with those for polypropylene, it can be concluded that the two materials undergo very different pyrolytic reactions. Typical for polypropylene is the formation of fragments of the polymeric backbone with formation of monomer, dimer, etc., or with cleavage of the backbone in random places and formation of compounds with 3n, 3n-1, and 3n+1 carbon atoms (see Section 6.1). Pyrolysis of the chlorinated compound leads to a significant amount of HCI and also char. Very few chlorinated compounds are identified in the pyrolysate, since the elimination of HCI leaves very few chlorine atoms bound to carbons. Some aromatic hydrocarbons are formed by a mechanism similar to that of poly(vinyl chloride) pyrolysis. The elimination of HCI leads to the formation of double bonds, and the breaking of the carbon backbone leads to cyclization and formation of aromatic compounds. The reactions involved in this process are shown below for the case of formation of 1,3-dimethylbenzene ... 

The pyrolysis of 1-halo-l-methylsilacyclobutanes yielded primarily the head-to-tail dimers of the expected 1-halosilenes as well as significant amounts of their hydrogen halide adducts. In several of these reactions polymers were obtained as well159,186. The pyrolytic behavior of halogenated silacyclobutanes appears to be quite complicated, and although it is certainly possible that the initial products are ethylene and 1,1-dihalosilene almost exclusively, this has not been established beyond reasonable doubt. 

Thermal silylcarbene-to-silene rearrangements have been known for a long time1. The pyrolytic product from trimethylsilyldiazomethane, 1,1,2-trimethylsilene, was trapped in an argon matrix230, and the pyrolysis of bis(trimethylsilyl)diazomethane126 was reported to produce fair amounts of 2,4-bis(trimethylsilyl)hexamethyl-l,3-disilacyclobutane, the expected dimerization product of 2-(trimethylsilyl)-1,1,2-trimethylsilene. A second product was the disilane expected from an ene addition of one... 

A remarkable result of the above investigation75 is the finding that the direct dimerization of 1-methylsilene under the conditions of the pyrolytic experiments represents a nearly negligible path toward the formation of the observed 1,3-dimethyl-1,3-... 

---