MACRO RINGS .36. RING EXPANSION RACEMIZATION AND ISOMER INTERCONVERSIONS IN [2.2]PARACYCLOPHANE SYSTEM THROUGH A DIRADICAL INTERMEDIATE

When heated to approximately 200°, [2.2]paracyclophane and its derivatives cleave at the benzyl-to-benzyl bond to give the p,pʹ-dimethylenebibenzyl diradical (or derivatives of this diradical), whose fate depends on the medium. For example, optically pure (–)-4-carbomethoxy[2.2]paracyclophane underwent racemization at 200° at a rate little dependent on medium polarity. Without solvent, pseudo-gem-bromoacetyl[2.2]paracyclo-phane3 (pseudo-gem-XIII) and pseudo-m-bromoacetyl[2.2]paracyclophane (pseudo-meta-XIII) when heated at 200° provide the same equilibrium mixture of pseudo-gem and pseudo-meta isomers in which the latter dominates by a factor of 5.8. At the same temperature, pseudo-p- and pseudo-o-bromoacetyl[2.2]paracyclophane (XIII isomers) equilibrate to give a mixture in which the latter dominates by a factor of 1.2. Similar equilibrations have been applied to a large number of disubstituted [2.2]paracyclophanes. When heated at 250° in p-diisopropylbenzene, [2.2]paracyclophane (I) gave a 21% yield of p,pʹ-dimethylbibenzyl. When heated in either dimethyl maleate or dimethyl fumarate at 200°, [2.2]paracyclophane gave diester products in which the diradical intermediate had added across the double bond. The resulting cis- and trans-2,3-dicarbomethoxy[2.4]paracyclophanes (cis- and trans-XVII) were formed in almost equal amounts (30% each) from either starting ester. The two esters were identified by their interconversion, degradation to the known [2.4]paracyclophane (XXII), and by partial asymmetric hydrolysis of the derived trans anhydride (racemate) with brucine and water to give optically active trans acid (XX) and trans anhydride (XXIII). Of the two diesters, trans-XVII was demonstrated to dominate in an equilibrated (base) mixture by a factor of greater than 200. Racemization or isomerization in the [2.2]paracyclophane system was found to occur about three powers of ten faster than hydrogen abstraction from p-diisopropylbenzene. The ring expansion process by addition to olefin was one to two powers of ten faster than the same hydrogen atom abstraction process. © 1969, American Chemical Society. All rights reserved.