Robert A. Schambach Whinier College Whinier, California 90608
Teaching Aromaticity, Conjugation, and Enolization
An understanding of the delocalization of electrons in organic compounds iscentral to a knowledge of organic chemistry. In teaching undergraduates ahout aromaticity, conjuzation. and enohation.-this author has found it useful to present examples of compounds in whfch delocalizationeffects are sustained in the presence of potentially interfering, saturated carbon atoms. These examples of homoaromaticity, homoconiueation, and homoenolization serve a three-fold purpose.-~&t, due to their unique nature the examples stimulate the student's imagination and help reinforce the basic concepts heing discussed. Second, since the examples are not presented in most organic textbooks, the student is helped to realize that organic chemistry is much more than 1200 pages of text, and that it is a living, growing body of knowledge. Third, the student is helped to understand that there are few hard and fast rules in organic chemistry, and that as a learner he is heing presented with generalizations to which there are many exceptions. Hornoaromatlclty Although it is difficult to find a definition of aromaticity with which all organic chemists will agree, the importance of aromaticity and Huckel's rule is widely recognized (I). An interesting example that may he introduced when discussing these two concepts is the bishomoaromatic carhanion (I) derived from bicyclo 13.2.lIocta-2,6-diene (11). Brown and Occolowitz (21 reoorted that (11) . is more reactive than the analogous hicycloalkene (111) towards allylic proton-deuterium exchange hy a factor of in t-BuOKDMSO a t 50°C. ~~
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They argue that the increase in acidity is due to the aclditional double bond in (111, which allows for the formation of an anion, (I), stabilized by both of the carbon-carbon double bonds. This cyclic system contains six electrons (four from the 2 n double bonds and two from the anion). and orovides for electron delocalization and accompanying resonance stabilization. iuxtanosition (The term "homo" is usuallv used to indicate the . . of one saturated linkage, most often a single carbon, with respect to a reference system. Anion (I) is termed bishomoaromatic due to the presence of two saturated carbons, C1 and Cs, in the molecule.) Winstein and coworkers (3) prepared solutions conmining the sodium and ootnssium salts of (1)and srudied theanionic magnetic resonance (pmr) techniques. species using The spectra of (I), as compared with (111, indicate a delocalized bishomoaromatic structhe containing an appreciable ring current. Briefly, the signal for protons II6.7 is shifted downfield indicating the effect of a deshielding ring current on the "isolated" hydrogens. In addition, the bridgehead protons, Hl,5,undergo very little change in chemical shift. This is also in accord with a bishomoaromatic system in which the r electron density is situated on C2,3,4.6,7. As a teaching device, this example reinforces the Huckel rule by showing an aromatic system containing six electrons. It also shows that there is an interaction between the two
"isolated" ?r systems, the allylic system on C2,3,4, and the ethylenic system on C6.7, and clearly demonstrates that the two saturated carbons, Cia, do not completely insulate the two r systems from one another. Most current texts discuss the aromatic properties of the and the corresponding tropylium cyclopentadienyl anion (N) cation (V). When discussing these systems, i t is convenient to mention the stable homotropylium cation (VI) easily formed via the addition of antimony pentachloride to equimolar amounts of HCI and cyclooctatetraene in nitromethane (4,5).
As structure (VI) implies, there is asextet of electrons distributed over seven carbon atoms. The pmr spectrum indicates an appreciable ring current, and thk species is formally. a homoaromatic cation. The pmr spectrum shows peaks a t T = 1.4,3.4,4.8, and 10.6 having areas of 5:2:1:1. The assignment is C2-6. C1.7, C 8 ~ s 9CBH~. The large difference in the chemical shifts of Ha and Hb on C g supports the existence of a ring current produced in the homoaromatic cation, (VI). H,, residing over the homoaromatic ring, is shielded, while Hb, heing closer to the plane of the rine. is deshielded (6). A t h r d example
