View metadata, citation and similar papers at core.ac.uk brought to you by CORE Published in "Angewandte Chemie International Edition 45(40): 6616 - 6619, 20 provided by RERO DOC Digital Library which should be cited to refer to this work. Cyclobutadiene: The Antiaromatic Paradigm? Thomas Bally* Keywords: In memory of Satoru Masamune antiaromaticity · cyclobutadiene · thermochemistry (1928–2003) There is probably no other single widely accepted (and hardly disputed) London proposed in 1937 a very appeal- molecule that has fascinated experimen- that benzene, with its six cyclically ing model that accounts for these effects tal and theoretical chemists so consis- delocalized p electrons, represents the in terms of ring currents that are in- tently over the past 40 years as has paradigm of aromaticity. In view of the duced by the external magnetic field in cyclobutadiene (CBD). On average, famous Hckel 4n/4n + 2 electron rule, the system of cyclically delocalized 16 publications which deal, in one form one is tempted to rush to the conclusion p electrons.[5] or another, with the parent compound that CBD, with its four cyclically delo- There has been a vigorous debate in p C4H4 and 40 which deal with cyclo- calized electrons, is therefore the recent years about whether and to what butdienes in general have appeared in paradigm of antiaromaticity. extent these ring currents contribute to every year of this time span, and there However, this conclusion is not as the deshielding or shielding of protons are probably many more on derivatives straightforward as it might seem, be- attached to aromatic or antiaromatic and metal complexes. There is no sign cause in benzene the two Kekul reso- rings, respectively,[6–8] as manifested in that interest in CBD is about to wane, as nance structures contribute with equal 1H NMR spectra. At the focus of this the highest number of papers per year weights to the D6h equilibrium geome- debate stand the “nuclear independent appeared in 2000 and 2002, and in the try, whereas in CBD, with its rectangular chemical shifts” (NICS values)[9] that last five years two entire thematic issues structure having strongly localized sin- can be computed at any point in the of Chemical Reviews, both with many gle and double bonds, this is obviously space surrounding a molecule. It has references to CBD, were dedicated to not the case. Thus the notion of “bond become clear that the original propos- issues of aromaticity/antiaromaticity[1] delocalization” does not have the same al[9] to calculate NICS values at the and localization/delocalization,[2] that meaning in the two compounds, and one center of (anti)aromatic rings does not is, topics that are central to the debate of the issues of the continuing debate is provide a valid measure of currents on CBD. to what extent this affects the properties induced in the cycle of p electrons. Recently the first direct and reason- that are associated with (anti)aromatic- However, it seems that the sign and ably accurate determination of the en- ity. Another issue is of course how magnitude of the p component of the thalpy of formation of CBD has been exactly one should or should not assess magnetic-shielding (or NICS) tensor at achieved by the group of Kass.[3,4] Thus these properties. I will comment herein some distance (usually 1 ) above the http://doc.rero.ch they have provided a solid foundation on some aspects of this debate. ring provides a good indication of the for the discussion on the thermochem- Various criteria have been proposed direction and size of the ring current, ical consequences of antiaromaticity, over time to judge whether a molecule is even in systems that have only s elec- and I take this occasion to reflect on aromatic or not (it is perhaps worth trons.[10] recent developments in this field and to recalling that the term “aromatic” itself However, I will focus herein on the point out the pitfalls with which the implies that a pleasant odor must at one other class of “beacon” properties that resulting discussion is fraught. time have been considered as an impor- are characterictic of (anti)aromaticity, In spite of the great activity docu- tant criterion; thus, by the same token, namely thermochemical and/or kinetic mented in the opening paragraph, many antiaromatic compounds should smell ones (the two often being mixed up). myths surround this fascinating mole- unpleasantly), and it was generally as- Clearly, benzene enjoys a special ther- cule, most of which relate to the extent sumed that several of these criteria have modynamic stability which expresses of delocalization of its p electrons and/ to be met for a molecule to qualify for itself, for example, in the fact that its or the degree of antiaromaticity. It is this distinction. (mono)hydrogenation is endothermic Two types of characteristic traits of by 22 kJmolÀ1 (Scheme 1), in contrast aromaticity have emerged as “beacon” to that of hexatriene, which is exother- [*] Prof. T. Bally features. The first relates to magnetic mic by about 114 kJmolÀ1 (the exact Chemistry Department properties, in particular the anisotropy number depends on the choice of con- University of Fribourg of the diamagnetic susceptibility and its formations). As there is no evident 1700 Fribourg (Switzerland) E-mail: [email protected] exaltation, which distinguishes aromatic reason why one or the other of the Homepage: compounds from polyenes. Following conjugated dienes that result from these http://www-chem.unifr.ch/tb/bally/ suggestions by Pauling and Londsdale, hydrogenations should suffer from any 1 mation of polyenes can be composed CBD, one must make some assumption from experimentally based bond[15] or about its strain energy. The most rea- group increments.[17] Such increments sonable such assumption would seem to allow one to calculate the energy of a be that the strain energy increases (purely hypothetical) cyclic polyene that linearly in the series cyclobutane/cyclo- benefits from all the conjugation energy butene/CBD. In this case the antiaro- but not the special contribution resulting matic destabilization energy is given by from the cyclic arrangements of double the enthalpy change for the isodesmic bonds that allows for equivalent reso- Reaction (1).[3] nance structures in the case of benzene. The extra energy gained or lost in Scheme 1. Heats of hydrogenation of various À this way (that is, the Dewar resonance compounds in kJmol 1.[11] energy) is then equated with the differ- ence between the experimental enthalpy special destabilization, and because of atomization or formation of the However, the above assumption both compounds may be regarded as compound under scrutiny and its hypo- merits some consideration. The fact that essentially strain-free, the 136 kJmolÀ1 thetical polyene reference. If one uses the strain energy increases by a mere difference must a result of some special the latest version of Benson-type group 4 kJmolÀ1 on going from cyclobutane to stability of benzene compared to hexa- increments,[18] this energy difference cyclopropane indicates that the energet- triene. By the same token, the high amounts to 87 kJmolÀ1 in benzene. I ic cost of angular deformation (Baeyer exothermicity of the hydrogenation of believe that this procedure and the strain) cannot account for this property CBD is an indication of a similar resulting Dewar resonance energy rep- alone. Two different explanations have thermochemical destabilization of this resent the best and most transparent been advanced to account for this dis- compound (relative to butadiene, for way to assess the aromatic resonance crepancy. The first argues that the example). stabilization in benzoid hydrocarbons. increase in angular strain between the As long as the argument remains on The results are indeed in good accord two compounds is largely balanced by such a semiquantitative level, there is no with determinations of the same quan- an increase in the strength of the CÀH problem. However, if one wants to tity—usually by much more complicated bonds, owing to rehybridization of the quantitate the “(anti)aromatic (de)sta- procedures—relative to comparable ring carbon atoms, and that the strain bilization energy” more precisely, one (experimental or theoretical) standards. energy of cyclopropane is therefore invariably hits upon the question of In the case of benzene (or benzenoid anomalously low, whereas that of cyclo- what exactly one should take as a hydrocarbons) the assessment of the butane is rather normal.[19] The second reference, which could be either a real Dewar resonance energy is greatly fa- explanation refers to the proposal, orig- molecule or a theoretical construct.[12] cilitated by the fact that this is the only inally advanced by Dewar,[20] that cyclic There is an enormous body of literature nonadditive effect that needs to be delocalization of 4n + 2or4n s electrons dealing with this very controversial considered, because such compounds in CÀC bonds may also lead to aromatic question, literature which has been are assumed to be free of strain (all stabilization or destabilization, respec- http://doc.rero.ch reviewed elsewhere.[13] However, an im- bond angles have the ideal value of 1208 tively.[10] Accordingly, cyclopropane is portant point is that one should carefully for sp2-hybridized carbon atoms, and the stabilized by this so-called s aromaticity distinguish between the total energy separations between nonbonding hydro- (which manifests itself, for example, in that is gained by putting double bonds gen atoms are larger than the sum of negative NICS values above the ring into (linear or cyclic) conjugation and their van der Waals radii). plane), whereas cyclobutane is destabi- the energy gained (or lost) exclusively by However, this is clearly not the case lized by s antiaromaticity (positive virtue of the conjugation being cyclic.