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. Cyclophanes as molecular workbenches: Pseudo‐geminally substituted [2.2]paracyclophanes are “molecular workbenches” that may be used to study the question of how functional groups interact (see scheme). When a specific reaction begins at one of the functional groups the opposing one reacts like a neighboring group in an exactly defined geometrical position, which causes stereospecific product formation.