Reactive Intermediate Chemistry

The parity methods predict a high-spin ground state, but as Borden and Davidson59 were the first to point out, PMP and its derivatives have disjoint connectivity 57 and, hence, should have a small multiplet gap with a likely singlet ground state.

So far, there have been no experimental tests of the subtle distinction between the parity and disjoint mnemonics on the parent PMP, which remains an unknown substance. However, a number of PMP derivatives have been synthesized, starting

with the dicarbene 58, a disjoint, equal-parity structure that was studied by Itoh as early as 1978148,149 and shown by ESR spectroscopy to consist of both triplet and

quintet states. These observations have been expanded in a series of variations,

including some examples of disjoint-unequal parity cases, in the laboratories of Iwamura and co-workers150–152 and of Lahti and co-workers.153 These efforts

now have produced important tests of the qualitative rules (see structures 58–62 and Scheme 5.12).

Note that compounds (58–61b) are all equal-parity molecules and all disjoint, but compounds 62a–c are of unequal parity but also disjoint by the Borden–Davidson criterion.

In each of the cases, ESR spectroscopy, or magnetic susceptibility measurements, or both, give evidence that more than one spin state is present and that one of them is a singlet. Although the energy gaps between the multiplets are very small (Scheme 5.12), the singlet is the ground state throughout the series. A

m, p' disjoint

S = (8 - 8)/2 = 0

m, m' non-disjoint

S = (9 - 7)/2 = 1

Scheme 5.12

number of factors might have been expected to perturb the test. For example, it seems likely that the aryl rings will be twisted out of coplanarity with the rest of the p system, which potentially could diminish electronic interactions determinative of the ground state. It is remarkable that even when the gap is as small as a few calories, the preference for the singlet persists.

10. CONCLUSION AND OUTLOOK

In principle, the domain of non-Kekule´ molecules should extend over territory as broad as that of ordinary Kekule´ molecules. Aside from the potentially huge applications in synthesis and in materials science, there are many intriguing theoretical and mechanistic questions to be explored.

One example that comes readily to mind is the mechanism of generation of highspin non-Kekule´ intermediates from spin-zero precursors, a transformation that requires a change in electronic multiplicity along the reaction path. Some of the questions that can be asked about such reactions include whether the high-spin intermediate is formed in a statistical mixture of spin substates or instead in predominantly one substate. Very little is known experimentally about such processes,154 but the answers could provide much insight into the dynamics of reactions in which surface crossings must play a part.

A related area is the chemistry and physics of electronically excited non-Kekule´ molecules. Many of the non-Kekule´ molecules have absorption in the near-UV or even in the visible. The description of the excited states of these species offers a

broad field of exploration for both experimentalists and theoreticians. That this will be a challenging problem already is suggested by the experience in the case of the excited states of TMB (see Section 6).

It may be fairly said that the field of non-Kekule´ compounds has expanded from a few curious and esoteric molecules to an active domain of inquiry and a potential source of materials for practical application. Theory and experiment have interacted fruitfully in these developments, but some serious limitations remain. For example, for many years, experimentalists have operated under the imperative of structural simplification. The ideal is to construct molecules that embody the features of theoretical interest but are as free as possible of complicating impedimenta such as extra substituents or other structural elements left over from the synthesis itself and not readily removed.

There are several motivations for this minimalist program. One of them is a selfimposed if rather vaguely articulated wish to exclude perturbing features and to focus only on those aspects of chemical behavior seen as generalizable. This technique contains the implicit assumption that the ‘‘pure’’ or ‘‘unperturbed’’ test molecule really does meet those requirements, an assumption often not easily demonstrated.

Also, there is an esthetic element mixed into this motivation, which probably is derived from the tradition of stripped-down test molecules in physical organic chemistry, in which studies of the parent member of a series tend to be valued more highly than those of its derivatives. One thinks, for example, of the emphasis placed on studies of the unsubstituted molecules methylene, norbornyl cation, cyclobutadiene, tetrahedrane, benzyne, and so on. The higher valuation also is associated in some cases with the formidable difficulties experienced by experimentalists in the synthesis and observation of these species.

On the other hand, it is also true that in some cases, whole fields of investigation that were out of reach with the parent molecules opened up to scrutiny with derivatives. Examples given in this chapter include most of the cycloaddition chemistry of TMM and TME (Sections 4 and 5).

Nevertheless, the importance of the parent molecule remains undeniable, and the reason for it forms the second motivation for the minimalist approach: It is the traditional way to bring theory and experiment together for mutual comparison. For decades, chemists have been aware of the limitations on theoretical techniques imposed by computational capacity. Everyone dreamed of the day when the properties of any reasonably sized molecule could be calculated to any desired degree of accuracy. In fact, some computational enthusiasts announced about 25 years ago— prematurely, in my opinion—that the day already had arrived. Most chemists were skeptical then; they continued under the assumption that the most practical comparisons of theory and experiment would involve experiments on the simplest possible test molecules. Thus, those comparisons accepted the limitations that theory still operated under.

Today, however, one senses that there has been an exponential growth in computational power.64 This change in circumstances raises the question of whether we should reexamine our attitudes. Is it possible that the time is at hand when

theoreticians and experimentalists can share the burden of mutual adjustment more equally? Can experimentalists now—or at least soon—hope that the most powerful computational techniques will be applied to the many non-Kekule´ test molecules and other species yet to be imagined that fall short of the classical minimalist ideal? As an experimentalist, I certainly hope so, and I venture the prediction that the coming postclassical era of physical organic research will be as rich in discoveries as the present classical one.

ACKNOWLEDGMENTS

It is a pleasure to acknowledge helpful correspondence and discussions with C. J. Cramer, G. B. Ellison, P. M. Lahti, M.S. Platz, R. R. Squires, and P. G. Wenthold.

SUGGESTED READING

Physical basis of Hund’s rule: (a) J. A. Berson, in The Chemistry of the Quinonoid Compounds, Vol. II, S. Patai and Z. Rappoport, Eds., John Wiley & Sons, Inc., New York, 1988.

(b) W. Kutzelnigg, ‘‘Friedrich Hund and Chemistry,’’ Angew. Chem. Intl. Ed. Engl. 1996, 35, 573.

ESR spectroscopy: (a) J. E. Wertz and J. R. Bolton, in Electron Spin Resonance: Elementary Theory and Practical Applications; McGraw Hill, New York, 1972. (b) J. A. Berson, in The Chemistry of the Quinonoid Compounds, Vol. II, S. Patai and Z. Rappoport, Eds., John Wiley & Sons, Inc., New York, 1988. (c) W. Gordy, in Theory and Applications of Electron Spin Resonance, Vol. 15, A. Weissberger series Ed., W. West, Ed., John Wiley & Sons, Inc., New York, 1980, p. 589. (d) E. Wasserman, W. A. Yager, and L. C. Snyder, ‘‘Electron spin resonance (E.S.R.) of the triplet states of randomly oriented molecules,’’ J. Chem. Phys. 1964, 41, 1763.

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Added Notes

1.A.-T. Wu, et al.155 have very recently observed triplet spectra from the irradiation of N-tosylpyrroles lacking the diazene group in various glasses at 4 K and have attributed these, without further structural evidence, to radical pairs resulting from the photocleavage of the N Ts bond. The D values are essentially independent of the matrix or the temperature up to 77 K. In the

case of the spectra from N-tosylpyrrole itself and the diazene 45f, the D values correspond, when delocalization is taken into account, to actual

˚

separation between the partners of about 3.1–3.5 A in the spin-dipolar

approximation (with neglect of spin-orbit perturbations) (Section 3.2). The authors propose that the triplet spectra previously attributed105,130 to N-tosyl-

3,4-dimethylenepyrrole (46f ) may be better interpreted as due to a radical pair resulting from N Ts cleavage but leaving the diazene group of 45f intact.

There are two groups of questions associated with this hypothesis. The first group concerns all of the spectra observed by Wu et al.155 If the species

responsible for the ESR signals observed are radical pairs, they behave quite differently from other radical pairs in frozen media (Section 3.2), which give unresolved ESR spectra and different spectral widths as a function of the

The separation of 3.1–3.5 A is close to the sum (3.3 A) of the van der Waals

radii of S and N. One interpretative difficulty now becomes apparent: What keeps the caged radicals, which on this basis are essentially in physical contact, from recombining by formation of a covalent bond? Such close contacts of partners have been reported in cages in the crystalline state but apparently not in glassy media.

The second group concerns the properties of the species from the diazene 45f, which in the interpretation of Wu et al. is thought to be a radical pair resulting from N Tos cleavage only. However, the ESR spectrum obtained from photolysis of the model compound N-tosylpyrrole itself at 77 K is very weak or not observed. Under the same conditions, the diazene 45f gives a signal at least 100 times as intense.130 It is difficult to imagine why the quantum yield for radical-pair formation by N Ts bond homolysis apparently should be increased to this extent by the presence of the remote nonreacting diazene function. Also, it is not obvious how the radical-pair hypothesis would account for the observation130 that a species embodying the non-Kekule´ biradical 46f unit (shown not to be the blue singlet biradical) can be trapped chemically by thawing triplet-rich glasses from irradiation of

diazene 45f. Instead, the latter two observations are readily accommodated in the original interpretation by Bush et al.105,130 that the ESR spectrum arises

from a molecular triplet, 46f, derived by photodeazetation of 45f.

2.Useful applications of the Evans proton NMR shift method156,157 have permitted the determination of effective magnetic moments meff (see Section 8) and spin multiplicities for several high-spin radical–cation species.158,159 This work makes available a simple procedure for such

assignments when the spin carriers are stable in solution.

3.The closest approaches to triangulene 6 have been made by introduction of oxygen atoms as in the trianion biradical 68, which has a triplet ground state. Several related derivatives have also been prepared.160

O−

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