Addition and Cyclization Reactions in the Thermal Conversion of Hydrocarbons with an Enyne Structure, 5. High‐Temperature Ring Closures of 1,3‐Hexadien‐5‐ynes to Naphthalenes – Competing Reactions via Isoaromatics, Alkenylidene Carbenes, and Vinyl‐type Radicals

Abstract

The 4-substituted 1-phenyl-1-butene-3-ynes la-c and the 2-ethynylstyrenes 7a-c were subjected to high-temperature pyrolysis. The cycloisomerization products isolated suggest that these are formed by three competing processes: by (i) an electrocyclic or a molecule-induced, (ii) an alkenylidene carbene controlled, and (iii) a radical-controlled ring-closure process. To estimate the relative importance of these three reactions here mentioned, the substrates have been isomerized in oxygen-free nitrogen and in nitrogen proportionally substituted by toluene at 700 and 650 "C, respectively. The relative contributions of these isomerizations depend not only on the conversion temperature but also on the substituent R in 1 or 7.Highly unsaturated acetylenes -among them 1,3-hexadiene-5-ynes -are known to be formed as reactive intermediates in fuel-rich hydrocarbon flames.They have been considered as an important source of the formation of aromatic hydrocarbons in the hot waste gases from combustion, but there is little experimental evidence about the details of the actually occurring processes. Under conditions similar to those in the down-stream section of combustion, concerning a temperature range from 1000 down to < 500 "C, it is quite likely that unimolecular reactions, like electro- In order to learn more about these competing reactions, we decided to study the cycloisomerization of the hexadien-ynes l a -l c (Scheme 1) and 7a-7c (Scheme 2) to the corresponding naphthalenes at 700 and 650 "C, respectively, and additionally the temperature dependence of the naphthalene formation from l a at temperatures between 625 and 750 "C in greater detail. From our previous studies we knew that the unimolecular electrocyclic (molecule induced) ring closures and those involving alkenylidene carbenes are independent of the nature of the diluent gas whereas the corresponding radical reactions can be significantly influenced by chemically modified diluents.In this paper, we describe a procedure which allows one to calculate the proportions in which electrocyclizations/ molecul induced ring closures as well as reaction cascades via alkenylidene carbenes and vinyl-type radicals take part in the cyclization of substituted 1,3-hexadien-5-ynes. ResultsThermal Conversion of 1-Phenyl-1-buten-3-ynes la-l c Trimethylsilyl-substituted cisltvuns-1 -phenyl-l-buten-3-yne (lc) was prepared by Pd-catalyzed coupling of pbromostyrene and trimethylsilylacetylene as described in It served as a starting compound for pyrolysis runs and also for the synthesis of l a . For the latter purpose, l c was almost quantitatively desilylated by treatment with an alcoholic sodium hydroxide solution, [141[151 and the resulting I -phenyl-l-buten-3-yne was converted into l a by deuterium exchange with a 5% solution of NaOD in D20.

Cycloaromatization of Open and Masked 1,3-Hexadien-5-ynes − Mechanistic and Synthetic Aspects

“…[14] Finally, the type C mechanism requires the generation of vinyl-type radicals by addition of chain-carrier radicals (preferably H atoms) to the CϪC triple bond [8,9,15] (see also ref. [16] ).…”

“…Section 5 and ref. [8] ), resulting firstly in a significant increase in importance of the type B mechanism with increasing temperature. Since, simultaneously, the type C mechanism gradually decreases in importance with increasing temperature, because of the displacement of the equilibrium according to RϪCϵCϪR ϩ H • Ǟ ǟ RϪCHϭ C • ϪR, to the disadvantage of the vinyl-type radicals, most authors tend to interpret high-temperature cycloaromatization processes in terms of the type B process (cf.…”

“…ref. [8] ] or under reduced pressure [flash vacuum pyrolysis (FVP) [22,25] ], has proven to be the most effective method for optimization of hexadienyne cycloaromatizations.…”

“…Thus, when the pyrolysis was carried out at 650°C, the partially ''masked'' 1-phenyl-1-buten-3-yne (46) gave naphthalene (47) in a 90% yield, but 1-methylene-1H-indene (48) (9%) and trace amounts of azulene (49) were also formed (Scheme 12). [8] Painstaking supplementary experiments [7] revealed in addition that 48 was exclusively formed by radical pathways, but 49 by the vinylidenecarbene route only (cf. Scheme 15 in Section 6).…”

“…Scheme 7), but they also provide insights into the details of the mechanisms operating in such reactions, [7Ϫ10] and especially into the temperature dependence of the mechanisms involved. [8] We have been actively engaged in this research since 1989 and this Microreview summarizes selected syntheses of aromatic compounds by thermal conversion of various 1,3-hexadien-5-ynes under different thermal conditions. The results obtained allow a clear differentiation to be made between the three types of mechanism.…”