A macrocyclic 2,19‐dioxo[3.3](3,3′) azobenzolophane by transition‐metal carbonyl complex‐mediated CO insertion and cyclization

Abstract

2,19-Dioxo(3.3](3,3')azobenzolophane (5) has been obtained in a total yield of 18% by way of CO insertion and cyclization of 1,2-bis[3-(bromomethyl)phenyl]-4,4-dimethyl-3,5-pyrazolidinedione (1) using C O~( C O )~ in acetonitrile followed by the removal of the N-protecting 4,4-dimethylmalonyl groups. 5 is obtained predominantly as the transltrans isomer 5a and can be isomerized by irradiation with h = 369 nm to a mixture of nearly equal amounts of the transltrans, cis/ trans, and cis/cis isomers 5a, 5b, and 5c. The thermal isomerization back to 5a is slow but fast upon irradiation with h. = 443 nm.Medium and macrocyclic cyclophanes are routinely prepared from open-chain precursors by cyclization involving nucleophilic substitution'). In particular the reaction between sulfides and benzyl halides can be used for an efficient cyclophane synthesis. We have used this method in combination with the rigid-group principle2) and with high dilution techniques3) for the synthesis of a series of diazacyclophanes4) containing an azobenzene moiety, but up to now all attempts to effect a sulfur extrusion from these thia-and dithiadiazacyclophanes failed ' ). The macrocyclic azobenzolophanes undergo an interesting cisltrans photoisomerism which affects the shape of the molecular cavity, and the synthesis of azobenzolophanes lacking the weak benzylic C -S bond would be of some advantage for photochemical experiments. However, the synthesis of sulfur-free diazacyclophanes by a cyclization and a C -C bond formation by nucleophilic substitution') has not been successful ' ). The reductive coupling of benzyl halides using various low-valent metals represents an elegant method of a C-C bond formation'). However, attempts to use these methods for the preparation of cyclophanes from 1,3-bis[4-(brommethyl)-phenyllpropane used as a model compound have failed although good yields have been obtained for the coupling of benzyl bromide under the same reaction conditions'). This agrees with the results recently published by Wey and Butenschon8).Transition-metal carbonyl complexes are known to catalyze the intermolecular formation of carbon -carbon bonds by CO insertion into carbon -halogen bondsg). By treatment of dihalides with these reagents an insertion of CO and the intramolecular formation of small cyclic ketones (n = 5 -7) occur'o). However, again no cyclic products are obtained in a reaction of 1,3-bis[4-(brommethyl)phenyl]propane with Ni(CO), or COZ(CO)~ under various reaction conditions". Instead, the corresponding carboxylic acids are formed by the known sequence of CO insertion and hydrolytic cleavage of the metal -carbon bonds6). Apparently, CO insertion and cyclization require a rather special stereochemistry of the organic dihalide. Therefore, we have tried a combination of the rigid-group principle and the CO insertion method to prepare sulfur-free diazacyclophanes by using 1,2-bis[4-(brommethyl)phenyl]-4,4-dimethyl-3,5-pyrazolidinedione and 1,2-bis[3-(brommethyl)phenyl]-4,4-dimethyl-3,5-pyrazolidinedione (l), respectively, w...

Correlation between the molecular structure and trans ↔ cis isomerization characteristics of azobenzenes

“…In particular, the steric factor in azobenzene can significantly vary the rate of thermal cis-to-trans isomerization. As an example the steric strain in azobenzenophanes, in which two azobenzenes are cyclically connected by relatively short atomic chains, influences the lifetime of the cis form, from seconds to years [24][25][26][27][28]. Tamaoki et al have reported that the lifetime of the cis-cis isomer in a xanthene-based cyclic azobenzene dimer was increased up to 6.4 years, and that an azobenzenophane, an azobenzene dimer connected at 3-and 3′-positions without spacers, formed thermodynamically stable cis form [27,28].…”