7,14-Dichloro[1.1]metacyclophane:  A Highly Strained Aromatic Intermediate

Organic molecules with abnormal geometric parameters

“…265 A decrease in the length of the bridge in cyclophane molecules results in a dramatic increase in their reactivity. The unsubstituted cyclophanes with the smallest n (with a saturated bridge) known to date, [4]paracyclophane (146) 272,273 and [1.1]paracyclophane (147), 274 can be stabilised only in inert matrices at low temperatures, while [4]metacyclophane (148) 275 and an extremely unstable [1.1]metacyclophane derivative (compound 149) 276 were detected as short-lived intermediates. A highly unstable and, apparently, the most strained among the known [n]paracyclophanes, bicyclo[4.2.2]decapentaene, has been synthesised.…”

Highly Strained Cyclophanes

“…Treatment of 26 with t-BuOK in DMSO, in an attempt to synthesize dichloro[1.1]metacyclophane 27, led to the formation of 28 and 29 in 9 and 10% yield, respectively (Scheme 3.13) [10]. The formation of 28 has been rationalized in terms of the attack on 27 by an adventitious hydroxide ion at one of the bridgehead positions followed by ring opening.…”

Unusual reactivity of highly strained cyclophanes

“…Thus, [6]paracyclophane gave a 1:3 mixture of its meta and ortho isomers (Scheme 10), presumably because at the intermediate stage of 25, deprotonation to the relatively unstrained [6]metacyclophane competes with a second Wagner-Meerwein rearrangement to 26 which on deprotonation yields benzocyclooctene (27). 19 Surprisingly different is the course of reaction when [4]paracyclophane, obtained by irradiation of its Dewar isomer, is intercepted by acid (Scheme 11).…”

“…Probably 63 was indeed formed, but under the reaction conditions it did not survive because it was further transformed by the strong base to give 66 and 67 in about 10% yield. 27 The formation of 66 can easily be explained as shown in Scheme 17: attack of base, in casu inevitable traces of OH À , at one of the bridgehead positions (cf. the transformation 6 → 35 in Scheme 12), followed by ring opening and formation of a benzylic carbanion eventually gives 66; on the basis of this mechanistic reasoning, the structure of 66 forms good evidence for the intermediacy of 63.…”