In this lecture, an account is presented of new methodology which permits quantitative assessment of ring inversion and bond shifting barriers within 8annulene derivatives. With optically active cyclooctatetraenes in hand, it suffices to determine the overall racenization rate and to obtain independent experimental measurement of one additional rate constant for complete numerical definition of their dynamic parameters. The antiaronatic nature of planar delocalized cyclooctatetraenes is analyzed and compared in magnitude to the overall conformational energetics of these systems.The sequence of historical events which include (a) Willsttter's initial synthesis (1911)(1912)(1913) of cyclooctatetraene (COT),' (b) Hiickel's proposition (1931) that this 1#n system should be characterized by unfavorable sr-electron delocalization,2 and (c) Reppe's develomemt (19)48) of a process which lends itself to large scale commercial production of COT has caused this hydrocarbon to become a target of intense experimental and theoretical scrutiny.4 The considerable attention commanded by cyclooctatetraene is, in large part, the result of its position as the smallest nomplanar [)#nannulene.° As a direct consequence of its ground state tub conformation and alternating single and double bonds the COT ring system is capable of three fundamental structural changes. The first of these processes, termed ring inversion (RI), relates exclusively to conformationälmobility and is isodynamical.7 While flexing of the parent COT merely regenerates the starting material, similar conversion of 1 to 1' leads to production of the enantiomer, presumably via planar alternate transition state 2. Were 1 optically active, net racemization would therefore accompany this framework equilibration.Cyclooctatetraenes are also capable of bond shiftim (BS), a process which has usually been characterized by somewhat higher energy demands.7These equally isodynamical structural alterations, generally viewed as mediated by planar-delocalized transition states, e.g.3,7812 should also result in the racemization of chiral COT's. Of course, this event is dependent upon the bility of to produce bOth 1 and 1', an entirely plausible assumption (unsubstituted 3 has symmetry). Transition state 3 constitutes a particularly elusive 5pecie5'3 which should e endowed with "antiaromatic" n-electron delocalization. Remaining pivotal questions deal with whether planar 3 is truly involved and, if so, to what degree it is destabilized. One may therefore inquire of 3 the true nature of its geometry, the extent of eight-membered ring distortion, if any, which may occur with alteration in peripheral substitution, and, most importantly, the magnibude of its resonance principle equate the species with nonaromatic 2 from which it is experimentally distinguishable. Thus, to the extent that the hypothetical structures 2 and 3 are accurate, then the difference between the activation enthalpies for RI and BS in any COT will directly provide a value of the RE for the planar delocalized structu...
