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

Abstract

The reaction of an aryne with an alkyne to generate ab enzocyclobutadiene (BCB) intermediate is rare.W er eport here examples of this reaction, revealed by Diels-Alder trapping of the BCB by either pendant or external electrondeficient alkynes.M echanistic delineation of the reaction course is supported by DFT calculations.Athree-component process joining the benzyne first with an electron-rich and then with an electron-poor alkyne was uncovered. Reactions in which the BCB functions in ararely observed role as a4p diene component in Diels-Alder reactions are reported. The results also shed new light on aspects of the hexadehydro-Diels-Alder reaction used to generate the benzynes.

CuI‐Mediated Bromoalkynylation and Hydroalkynylation Reactions of Unsymmetrical Benzynes: Complementary Modes of Addition

“…[9] Moreover,w eo bserved no alkynylation product formation in the absence of the copper catalyst. [10] We envision the reaction proceeding through amechanism such as that depicted in Figure 2b.T he initial thermal cycloisomerization is the rate-determining step in the overall cycle,b ecause the reaction rate is similar for triyne 10 regardless of the trapping agent used. [11] We propose that benzyne 11 initially undergoes nucleophilic addition with CuBr to produce the aryl copper(I) species 18,w hich then reacts with am olecule of the bromoalkyne to generate the Cu III adduct 19,from which reductive elimination would yield 12 a and close the cycle.…”

Mechanism and Regioselectivity of an Unsymmetrical Hexadehydro-Diels–Alder (HDDA) Reaction

“…The potential of forming a cyclobutadiene via bonds 1−1′ and 2−2′ is not likely due to high product instability (Figure S1), although Hoye et al postulated benzyne cyclobutadienes in a different HDDA study. 18 We were intrigued by the origins of this selectivity and report here a computational study on the HDDA reaction of 1.…”

Benzyne Cascade Reactions via Benzoxetenonium Ions and Their Rearrangements to o-Quinone Methides

“…4 Indeed, when R 1 is a hydrogen, the reaction proceeds through other pathways. 5 The highly substituted benzyne intermediate is accessed conveniently from a triyne precursor via a cycloisomerization event, namely, the hexadehydro-Diels-Alder (HDDA) reaction, 6 and therefore, the overall cascade is essentially a net thermal isomerization of the triyne.…”