Preparation, Structural Properties and Thermal Isomerization of Hex‐3‐ene‐1,5‐diyne Bridged [2.2]Paracyclophanes

Abstract

The [2.2]paracyclophane moiety is used as a spacer to connect the ends of a hex-3-ene-1,5-diyne unit, a π-system that on thermolysis usually cycloaromatizes to a benzene ring (Bergman cyclization). For the preparation of the pseudo-geminally-bridged system 9, the diacetylene 3 was chain-extended to the diol 16, which after conversion to the pseudo-geminal dibromide 17 was ring-closed by treatment with LiHMDS/HMPA to the [2.2]paracyclophane enediyne 9. Whereas the McMurry coupling of the pseudo-ortho bisaldehyde 24 resulted in the formation of the hexadienyne-bridged cyclophane 27, the pseudo-ortho-bridged hydrocarbon 11 was obtained by preparing first the diol 28 from 24, converting the latter into the dioxolane 29, which in the last step furnished the olefin 11 by treatment with Tf2 O/EtN(iPr)2 . The authentic Bergman product 10 of the pseudo-gem-bridged hexenediyne 9 was synthesized by a conventional sequence starting from the ethynyl formyl substrate 18. Since the pseudo-ortho-enediyne-bridged hydrocarbon 11 is thermally labile, its benzannelated derivate 34 was prepared. No classical Bergman cyclization reactions could be observed for any of the [2.2]paracyclophane-bridged hexenediynes prepared here. In the pseudo-gem-series the fulvenes 14 and 15 were the only products that could be identified under thermal conditions (McMurry coupling); the benzannelated substrate 34 gave the benzofulvene-bridged cyclophane 36 on photolysis. Bergman cyclizations yielding fulvene derivatives are extremely rare. The mechanism of the cyclization of 9 and 34 is discussed, using compliance constants. The structure assignments of the hydrocarbons synthesized in this study are based on spectroscopic studies as well as X-ray structural analyses for 9, 10, 11, 27, and 34.

Building complex carbon skeletons with ethynyl[2.2]paracyclophanes

“…The “crossed” derivative 22 , with its relatively short extra acetylenic bridges and with the clamping effect of the ring C5–C10, is reminiscent of the more strained derivatives presented in the previous paper [1]. The cyclophane rings remain parallel, with a moderate twist angle of 7.8°; they subtend interplanar angles of ca.…”

“…We have discussed these in detail in our previous paper [1], and summarize them here as follows: the single bonds in the bridges, C1–C2 and C9–C10, are elongated and the sp 3 angles at these atoms widened; the sp 2 angles at the bridgehead atoms C3, C6, C11 and C14 are narrowed; the rings display a flattened boat conformation in which the bridgehead atoms lie ca. 0.12–0.16 Å out of the plane of the other four atoms; these planes are approximately parallel to each other, as are the vectors between the bridgehead atoms, and the non-bonded contacts between bridgehead atoms are necessarily short (2.7–2.8 Å) [4].…”

“…3.07 Å (this is also the case for the other “double paracyclophane” structures presented here (see below) and will not be mentioned again explicitly). The slight bowing of the extra bridges, with angles at the sp carbon of 170–173°, can be recognized in the Figure; we regard this angle as a “soft” parameter [1]. The angle between the two halves of the molecule, expressed as the interplanar angle between the plane of C4, C5, C7, C8 and its symmetry-equivalent, is 77°, and this is clearly imposed by the “crossed” bridge geometry.…”

Cyclophanes, Part 73: diastereoselective dimerization of an α,β-unsaturated ketone. Structure of all-trans-1,3-dibenzoyl-2,4-di([2.2]paracyclophan-4-yl)cyclobutane

“…The cyclophane units display the usual features reflecting the strain of the system (see e.g., [4]); thus the single bonds in the bridges are elongated (av. 1.580 Å) except for the possibly disordered bridge (see below), the sp 3 bridge angles are widened (av.…”

Synthesis, structure, and first reactions of a new class of thiacyclophanes

“…We have recently described the first [2.2]paracyclophane derivatives possessing such lengthened unsaturated bridges: 2 The pseudo-geminally substituted dienyne 3 and the enediyne 4, as well as its pseudo-ortho isomer 5 (Scheme 2), were prepared and some of their chemical properties studied. One of the most often employed strategies to prepare bridged aromatic systems involves the preparation of sulfides, which D r a f t are subsequently subjected to ring contraction.…”