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

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 Tf₂O/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 .