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 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.

Ethynyl[2.2]paracyclophanes are shown to be useful substrates for the preparation of complex, highly unsaturated carbon frameworks. Thus both the pseudo-geminal- 2 and the pseudo-ortho-diethynylcyclophane 4 can be dimerized by Glaser coupling to the respective dimers 9/10 and 11/12. Whereas the former isomer pair could not be separated so far, the latter provided the pure diastereomers after extensive column chromatography/recrystallization. Isomer 11 is chiral and could be separated on a column impregnated with cellulose tris(3,5-dimethylphenyl)carbamate. The bridge-extended cyclophane precursor 18 furnished the ring-enlarged cyclophanes 19 and 20 on Glaser-Hay coupling. Cross-coupling of 4 and the planar building block 1,2-diethynylbenzene (1) yielded the chiral hetero dimer 22 as the main product. An attempt to prepare the biphenylenophane 27 from the triacetylene 24 by CpCo(CO)2-catalyzed cycloisomerization resulted in the formation of the cyclobutadiene Co-complex 26. Besides by their usual spectroscopic and analytical data, the new cyclophanes 11, 12, 19, 20, 22, and 26 were characterized by X-ray structural analysis.

Intramolecular reactions in pseudo-geminally substituted [2.2]paracyclophanes.

A selection of pseudo-geminally substituted [2.2]paracyclophanes, the alkynes 6, 7, 10, 11 a, and 11 b and the alkenes 8 and 9 were prepared for the study of intraannular reactions between functional groups in direct juxtaposition. Whereas 9 and 10 provide the corresponding cyclobutane and cyclobutene derivatives on irradiation (12 and 13, respectively), the bis-alkynes 7 and 11 b do not lead to a cyclobutadiene intermediate. In the latter case the "half-closed" butadiene derivative 17 was isolated. A Paterno-Büchi reaction took place on irradiation of 8 and 6, although the oxetene intermediate 21 produced in the second example did not survive the reaction conditions (ring-opening to 22). Bromine addition to 9, 10, and 7 occurred with high stereoselectivity (formation of the dibromides 27, 30, and 33, respectively), and is rationalized by postulating the formation of the cationic intermediates 26, 29, and 32, respectively. To study the interaction of a carbocation with a facing triple bond, the alcohol 34 was prepared from 6. On acid treatment ring closure to the triply-bridged phane 38 took place, accompanied by the hydration of the triple bond to the ketoalcohol 37. In an interesting intraannular [2+3]cycloaddition reaction the bis-acetylene 11 a, on treatment with n-butyl lithium, provided the cyclopentadiene derivative 42. That the two triple bonds of a pseudo-geminal diacetylene can engage in a cyclization reaction leading to the cyclopentadienone complex 44 was also shown by treating 11 b with iron pentacarbonyl.