SYNTHESIS AND DYNAMIC BEHAVIOR OF (1,5)CYCLOOCTATETRAENOPHANES - EFFECT OF DISTAL ATOM BRIDGING ON RACEMIZATION RATES AND ELECTROCHEMICAL REDUCIBILITY

The [5]-, [6]-, [8]-, and [10](1,5)cyclooctatetraenophanes as well as non-C2-symmetric methyl homologues of these 1,5-bridged [SJannulenes have been prepared conveniently via a route that holds promise of considerable generality. The scheme consists of applying the Cook-Weiss procedure to a cyclic -diketone, transforming the carbonyl groups of the product into a pair of olefinic sites (with or without attachment of the methyl substituent), preparation of the bracketed semibullvalene via nickel carbonyl promoted cyclization of the doubly allylic dibromide, and finally flash vacuum pyrolysis of the semibullvalene. Cyclic voltammetric studies performed on the title compounds have quantified not only the standard electron-donating capacity of the pendant alkyl groups, but also the consequences of nonbonded steric interactions associated with maintaining a bridged structure as the cyclooctatetraene ring flattens. Microscale preparation of the dianion of 5d permitted measurement of its1H NMR spectrum. Whereas 5b–d reacted with (–)-ertifo-bornyltriazolinedione, the methyl congeners 15 and 21 did not. Chromatographic separation of the urazole diastereomers derived from 5c and 5d proved possible. Whereas hydrolysis-oxidation of individual adducts provided access to optically active 5c, 5d was invariably already racemized upon isolation. Thus, the latter experiences very rapid conversion to its enantiomer. The activation parameters for racemization within 5c were determined, and it is argued that these reflect exclusively the energetic costs of bond shifting within its [8]annulene core. © 1990, American Chemical Society. All rights reserved.