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The Nonclassical Cation Controversy* Bull. Hist. Chem., VOLUME 25, Number 2 (2000 2 "WHAT'S IN A NAME?" FROM DESIGNATION TO DENUNCIATION - THE NONCLASSICAL CATION CONTROVERSY* Stphn . Wnnr, Wrtr lthn Inttt In l, Chrtphr . Wln, br f th outbursts by Brown's obduracy and duplicity (7)—all hInld rp t Unvrt Cll, ndn n ll, nt prtt ptr, n tht th ntlnl l (UC, pblhd ppr n th rrrnnt n hl rtltt f rvrd t t b ptt f "bnr rfr f th trpn drvtv, phn hdrhl l phl (8." In thr jd rv f th rd (. h rtn bln t th l f lld ntr pd Arntt nd rr vn rd th WnrMrn rrrnnt, hh r hr tn f hthr t ntttd n ntn f "pth trzd b hn n th rbnrbn bnd ltn ll n (." rthrr, th tp t rtnt r trnfrd nt prdt. h rr hv ltl ptntl fr nrtn ntntn. A rnnt hd ln pzzld nd fntd rn rntl 8 ppr n th rhtpl nnll ht, n th rprntd hlln t l n, nrbrnl tn (, ndd th n dtr nt l trtrl thr, hh rtd pn th ptlt f ndtn prfnd drnt n t rfr. ltl nvrn. r th UC rp, rrrnnt prt rbttl nd ntrrbttl pprd prvdd hhl vbl tt f thr ltrn thr r ltr (0. f rn htr. ntx p f Inld l I thr nthn t b nd b n xntn f Structure and Mechanism in Organic Chemistry r th ntrvr, thr thn ttlltn fr thn dvtd t WnrMrn rrrnnt (2. h " f th t prfl nd" (nd lrt ttl f Wln ppr nnnd t rt I f r n rn htr fhtn n thlv n fr n th "U f Itp n Chl tn." h nd ftn ndnfd fhn? h rpn n nnt tbr f WW II t hv prldd tnbl . Wrtn n 6 lltn f jr n th pprn f frthr ppr n th r. trbtn t th dpt, rtltt prd th nrd r th dln prn ht t f n nld f vln thr nd lvl hn tntn n t thrn pn r " f th t tht th ntrvr hd ffrdd (. r th h prfl nd nd prnlt n rn htr trl tndpnt, I blv tht n nl f th p (, 4." Whtvr th tt f th dptd n, th d n ntrbt t r ndrtndn f n nvtv th vd rdl ll. h prn nfnt : pl nr, . C. rn, t hv bn th th pt f n xprntl thn n th n trt f th r brbd nt. In n l td f rtn hn brtd xpl h d b J. D. Roberts of preparing to "trample some wonderful and complex little th hftn tt n jr bdpln flowers with his muddy boots (5, 6)." Lest one con- thn rn htr clude that Brown was more sinned against than sinning, th rl f Cld Wr fndn n th vltn f his antagonists claimed to have been provoked to these ptWrld Wr II htr 124 Bull. Hist. Chem., VOLUME 25, Number 2 (2000) the relationship of alternate theoretical formalisms ization (2). However, even this modified classical to differing representations of molecules and the scheme cannot accommodate the stereochemical results. conflicts that arise when new forms of represen- Wilson's ingenious but tentative solution to this tation are introduced dilemma was to propose that C-6 became only partly In this paper concentrate on the last of these issues; I bonded to C-2, while remaining partly bonded to C-1 as hope to treat others in subsequent publications. well (Fig. 2a). In this bridged cation the positive charge would then be divided between C-l and C-2. More- Wilson's results and his associated interpretation over, the electron pair originally binding C-6 to C-1 of them are as follows. Under the catalytic influence of would now be shared among or delocalized over three hydrogen chloride, camphene hydrochloride ionizes with centers—C-1, C-2 and C-6. Wilson proposed this delo- unexpected rapidity and also rearranges, producing only calized cation not as a fleeting transition state but rather one of two possible isomeric rearrangement products as a reaction intermediate, long-lived on the molecular (Fig. l). The speed with which the starting material lost time scale. The partial bonding between C-6 and C-l chloride ion suggested to Wilson that the organic cat- would preclude nucleophilic attack on C-1 from the endo ion, the direction, thus camphenyl ion, explaining the must be unusu- anomalous ally stable. It also formation of seemed highly only one prod- likely that the re- uct stereoiso- arrangement took mer. The pro- place following posed inter- ionization, after mediate which the rear- would thus be ranged cation re- neither a captured chloride camphenyl ion to give nor an isobornyl chlo- isobornyl ion ride. The entire but rather a process was re- mesomeric versible, with ion, i.e., a equilibrium fa- resonance hy- voring isobornyl brid of the chloride. Wilson two. Ever sought an ex- mindful of planatory mecha- what we r . Cll hn nism that was in might call the harmony with two signal findings: the ionization of cam- Lavoisier gambit—seize the nomenclature and hearts phene hydrochloride was much faster than anticipated and minds will follow—Ingold in 1951 named these (kinetic anomaly), and only isobornyl chloride was species nrtt ions (12). formed, although its isomer, bornyl chloride, was the more stable of the two (stereochemical anomaly). Fig- The novelty of Wilson's explanation lay in the idea ure 1 shows a "classical" mechanism for the rearrange- that the s electrons of the C-1/C-6 bond could be delo- ment. However, this mechanism explains neither the calized over more than two centers. The division of kinetic nor the stereochemical anomaly. The first bonding electrons into two types, s and p, had been anomaly can be accounted for by assuming that chlo- worked out several years earlier by the theoretician Erich ride ion loss and carbon skeleton rearrangement are con- Hückel (13). By treating the s electrons like the local- certed; that is, the C-6/C-2 bond is formed simulta- ized electron pairs of classical Lewis theory, while al- neously with cleavage of the C-2/Cl bond, thus bypass- lowing the p electrons to be delocalized over more than ing free camphenyl ion altogether. Such an assisted ion- two nuclei, the Heckel theory nicely rationalized the well ization would indeed result in an accelerated rate of ion- established reactivity differences between single and Bull. Hist. Chem., VOLUME 25, Number 2 (2000) 125 multiple bonds. The theoretically sophisticated knew Winstein and the Wilson paper arrived at Harvard at that the s/p division was but one possible representation about the same time in the fall of 1939. Winstein was of multiple bonds, and that a strict boundary between taking up a National Research Council fellowship in localized and delocalized electrons was illusory. How- Bartlett's laboratory, fresh from doctoral and ever, since the thesis of a qualitative difference between postdoctoral work with Howard Lucas at Caltech, where s and p electrons helped make sense of a raft of chemi- he studied metal ion-alkene complexes and neighbor- cal and spectroscopic data, it came to be accepted as an ing group participation, both subjects with close affini- accurate description of the actual state of affairs. ties to the Wagner-Meerwein rearrangement (19, 20). Winstein apparently first read Wilson's piece on Sep- Incorporating the s/p dichotomy within the already tember 22, 1939. In the course of the following three very successful conventions for drawing molecular struc- weeks he wrote out no fewer than 33 pages of notes on tures was not easy, and representing electron delocal- this paper and the antecedent literature, including deri- ization was particularly tricky. In the case of benzene it vations of the kinetic equations and verification of the required at least two Lewis structures (resonance struc- calculations. He even went so far as to check the tures), both of which were fictional. The properties of Eastman catalog for the prices of camphene, borneol, the real benzene molecule are such that a single Lewis and other compounds necessary to continue the project structure is inadequate to express them, and resonance (21). It seems fair to conclude that Winstein was not theory is one way of dealing with that inadequacy with- only deeply impressed by the Wilson paper but was out abandoning classical molecular representations (14, making definite plans to pursue his own investigations 15). in the area. Wilson was proposing analogously that there was He did not act on this plan for almost a decade. only one organic ion involved in the camphene hydro- However, starting in 1949 at UCLA, Winstein began chloride rearrangement, that the ion was easily formed publishing solvolytic studies of a simplified version of because it was resonance stabilized, and that this stabi- Wilson's molecule that retained its most important struc- lization required the delocalization of a pair of s elec- tural feature, the strained bicyclic ring system (Fig. 2b) trons (Fig. 2a). Wilson's proposal occupied a mere line (22). He mustered an assortment of kinetic, stereochemi- in his paper and was only put forward as a possibility, cal, and theoretical tools to establish the reality of cat- but its initial reception was apparently cool (16, 17). In ionic intermediates with delocalized s electrons. In con- addition to struggling with the correct structure of the current investigations at MIT and then at Caltech, Rob- ionic intermediate, Wilson also had to decide how to erts used 4C labeling to uncover the full complexity of represent it.
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