Highly Conjugated π‐Systems Arising from Cannibalistic Hexadehydro‐Diels–Alder Couplings: Cleavage of C−C Single and Triple Bonds

Abstract

We have investigated the cannibalistic self-trapping reaction of an ortho-benzyne derivativeg eneratedf rom 1,11-bis(p-tolyl)undeca-1,3,8,10-tetrayne in an HDDA reaction. Without adding anys pecific trapping agent, the highly reactiveb enzyne is trapped by anotherb isdiyne molecule in at least three different modes.W eh ave isolated and characterized the resulting products and performed high-level calculations concerning the reactionm echanism. During the cannibalistic self-trapping process, either aC Ct riple bond or an sp-sp 3 CÀCs ingle bond is cleaved. Up to seven rings and nine CÀCb onds are formed startingf rom two 1,11bis(p-tolyl)undeca-1,3,8,10-tetrayne molecules. Our experimentsa nd calculations provide considerable insight into the variety of reactionp athways which the ortho-benzyned erivative,g enerated from ab isdiyne, can take when reacting with another bisdiynem olecule.

Direct observation ofo-benzyne formation in photochemical hexadehydro-Diels–Alder (hν-HDDA) reactions

“…S6 †). The "open" conformation was shown to be the structure of groundstate Me-BD-Me in the solid state by single-crystal X-ray diffraction, 94 and our DFT calculations on Me-BD-Me also indicate a preference for the open conformation with DG(Open) À DG(Closed) ¼ À3.3 kJ mol À1 in favor of the open conformer. However, it is logical to assume that the "closed" conformation leads to an increased HDDA reaction probability between the two -C^C-C^Cunits in the excited state because the two units have to approach each other for the reaction to occur.…”

“…1, path 3 gray) were also isolated and identied. 94 Although the isolated products arose from a thermal reaction, it has been reported that the HDDA products arising from both the thermally and photochemically generated o-benzyne intermediate are the same. 83 Thus, it was assumed that the electronic state of the o-benzyne intermediate is the same in both cases.…”

“…products were separated using ash chromatography. The structural characterization conrmed a product with a dimeric structure, i.e., a naphthalene derivative, 21,94 denoted Selftrapping-1 with its structure illustrated in Fig. 1 (path 2, green).…”

“…This correspondence indicates that HDDA-generated o-benzyne is probably self-trapped by an extra aryl butadiyne moiety of Me-BD-Me (for a plausible reaction mechanism see ref. 94). With the most plausible assignment of band 3 thus in place, band 2 at $370 nm can be concluded to be the o-benzyne species.…”

“…Me-BD-Me was synthesized according to the literature. 94 The general information on chemical synthesis and isolation is summarized in ESI Note 1. † In all transient absorption spectroscopic experiments, samples were dissolved in spectroscopic-grade solvents (Sigma-Aldrich) as received.…”