New Thermal Routes to ortho-Benzyne

Abstract

There is experimental evidence that intermediate ortho-benzynes can be made by intramolecular [2 + 4] cycloaddition of a 1,3-diyne with an alkyne. Computations by several groups support a concerted mechanism for the cycloaddition of butadiyne with acetylene. High temperature benzyne cycloreversion has also been demonstrated experimentally; this may in fact be a common reaction in hydrocarbon fuel combustion. Following leads from earlier pyrolysis experiments, herein we predict that cycloaddition of benzyne with butadiyne can proceed by a stepwise mechanism to 2,3-naphthyne. However, a slightly lower energy path leads to a benzocyclobutadiene. ortho-Benzyne can be generated by solution-phase and solid-phase reaction in a microwave reactor. We have developed the method of microwave flash pyrolysis (MFP) for high temperature solid-phase microwave reactions. MFP reaction of phthalic anhydride, a classic benzyne precursor, results in a typical suite of products expected from a relatively high concentration of benzyne.

The Benzyne Story

“…[5b] In fact, the current values for these strain energies are 54 (cyclopropene) and 50 kcal mol −1 (benzyne) (1 cal = 4.184 J). [6,7] The enthalpy of formation of benzyne is ∼107 kcal mol −1 . [8] Huisgen and Rist (University of Munich) provided further evidence for the benzyne structure (3-methoxybenzyne 4) by demonstrating that identical mixtures of carboxylic acids were obtained on treatment of ortho-and meta-fluoroanisole with phenyllithium followed by carboxylation with CO 2 (Scheme 4).…”

“…[45] The formation of benzyne by microwave flash pyrolysis of phthalic anhydride is reviewed herein. [7] The mechanism of the deceptively simple formation of benzyne in the thermal reactions (Eqn 3) is still under debate. [19,43c,46] …”

Flash‐Vakuumpyrolyse – Techniken und Reaktionen

“…[251] Graphit kann als thermischer Sensibilisator eingesetzt werden. [251] Graphit kann als thermischer Sensibilisator eingesetzt werden.…”

“…Wenn die Aktivierungsenergien nicht zu hoch und die Produkte nicht zu instabil sind, ist es manchmal mçglich, die gleichen Produkte wie bei konventioneller FVP zu erhalten, indem die Reaktion in Fest-oder Lçsungsphasen unter Mikrowellenbestrahlung durchgeführt wird. [251] Graphit kann als thermischer Sensibilisator eingesetzt werden. [252] Beispielsweise ergab die Mikrowellenbestrahlung von Phthalsäureanhydrid auf Graphit bei 150 We ine Mischung von Benzol, Biphenyl, Biphenylen und Naphthalin ähnlich wie sie bei FVP erhalten wird, woraus sich auf das Durchlaufen von Didehydrobenzol als Zwischenstufe schließen lässt.…”

Benzocyclobutadienes: An Unusual Mode of Access Reveals Unusual Modes of Reactivity

“…58.5 kcal•mol −1 for 9 to 10 , Figure S1 in the SI) and to proceed with a low activation energy. [5] Therefore, the paucity of reports of alkyne + benzyne [2+2] reactions may reflect inefficiency in the subsequent manifold of reactions into which an initially formed BCB intermediate enters rather than inherent difficulty in its generation. In the initial discovery, Stiles et al (Figure 1a) rationalized the formation of dibenzocyclooctatetraenes 3 by dimerization of BCB intermediates 2 .…”