1,4‐Dialkynylbutatrienes: Synthesis, Stability, and Perspectives in the Chemistry of carbo‐Benzenes

Abstract

The π-electron-rich C(8)-conjugated sequence of 1,4-dialkynylbutatrienes is identified as a fragile and fascinating motif occurring in carbo-benzene derivatives, and in Diederich's 1,4-bis(arylethynyl)- or 1,4-bis(triisopropylsilylethynyl)butatriene "capped" representatives, in particular, in tetraalkynylbutatriene. The family of symmetrical 1,4-dialkynylbutatrienes (E-C≡C)RC=C=C=CR(C≡C-E) is extended to functional caps (E=H, CH(3), C≡CPh, CPh=CHBr, or CPh=CBr(2)) with non-alkynyl substituents at the sp(2) vertices (R=Ph or CF(3)). The targets were selected for their potential in appealing retrosynthetic routes to carbo-benzenes, in which the aromatic C(18) macrocycle would be directly generated by sequential metathesis or reductive coupling processes. The functional 1,4-dialkynylbutrienes were synthesized by either classical methods used for the preparation of generic butatrienes (R'Li/CuX-mediated reductive coupling of gem-dihaloenynes or SnCl(2)/HCl-mediated reduction of 3,6-dioxy-octa-1,4,7-triyne precursors). Their spectroscopic and electrochemical properties are compared and analyzed on the basis of the relative extent of total conjugation.

First example of ring carbomer of 1,4-cyclohexadiene

“…The preparation of the carbocyclohexadiene 6 relies on a two-step strategy starting from the known [6] (which was isolated in a higher yield of 20 %). solution, exhibits one intense band at  max = 418 nm (Figure 1a).…”

“…First isolation in the tetraphenyl-dianisyl series of a carbo-mer of 1,3-cyclohexadiene 1, sub-reduction side-product of the [6]pericyclynediol 2 to the carbobenzene 3 [1].…”

“…The two trifluoromethyl groups of 2F, replacing two phenyl groups of 2, at two adjacent vertices allowed the selective fourelectron reduction to the carbo-cyclohexadiene target 1F, through the destabilization of the carbenium intermediate possibly generated by the acidic character of the reducing SnCl 2 /HCl system [6].…”

“…The first example of carbocyclohexadiene 1, reported in 2012 [1,2], resulted from an incomplete reduction of a [6]pericyclynic precursor 2 of the corresponding para-dianisyl-carbo-benzene original target 3 [3,4]. The conjugated ring carbo-mer of 1,3-cyclohexadiene 1 was the sole sub-reduced product separated from the reaction mixture by semi-preparative HPLC, without evidence of non-conjugated 1,4-isomers 4 or 5 (Scheme 1).…”

“…Since then, a series of bis-trifluoromethylated conjugated carbo-1,3-cyclohexadienes 1F was developed on the basis of a synthesis procedure involving bis-trifluoromethylated [6]pericyclynic precursors of type 2F (Scheme 2) [5].…”