Reactive Intermediate Chemistry

(Section 5) of TME. Finally, in a few cases, the value of D happens to be just three times that of E, in which case only three lines will be observed.

A sixor four-line ESR spectrum that can be fitted to a triplet spin Hamiltonian is strong evidence that the species in the sample embodies two unpaired electron spins. Support for the presence of a triplet spin system often can be found in the weak ms ¼ 2 line,75 which appears at one-half the field strength of the center of gravity of the ms ¼ 1 six-line pattern. This nominally ‘‘forbidden’’ ms ¼ 2 resonance results when the ESR spectrometer field and frequency produce a microwave quantum of energy just sufficient to jump the gap between the uppermost and lowermost triplet substates, that is, a transition over two quantum levels.

3.2. Biradicals or Radical Pairs?

However, what is not immediately obvious from this information alone is whether these spins are associated with a triplet pair of free radicals or with a true molecular triplet. For this purpose, the D value is indispensable, because from it, one can determine the average separation of the spins.

Gordy68 and Gordy and Morehouse76 derived an equation for calculating this average or effective distance based upon the assumption that the location of each spin dipole may be assigned approximately to a point.77 The result may be stated as in Eq. 1, in which 2D in gauss is the separation between the two outermost of the six (or four) lines, and R is the effective interelectronic distance in angstrom units:

2D ¼ 55; 600=R3 ð1Þ

One of the useful applications of the Gordy–Morehouse equation is in just this problem of distinguishing true molecular triplets from radical pairs. Thus, many non-Kekule´ triplets have 2D values between 450 and 600 G, corresponding to aver-

˚

age interelectronic distances of 5 and 4.5 A, respectively. Note that these distances would permit the two spins to reside within the boundaries of an ordinary molecule. Triplets with such separations may usually be assumed to be molecular triplets, rather than radical pairs. A radical pair with such a small separation of spins would have the two spin centers nearly in contact with each other. Distances

˚

of 4.5–5 A would be too close to accommodate even a single molecule of the solvent in such frequently used matrices as Freons or perfluorocyclohexane. Ready mutual quenching of the pair then would be expected, thus precluding the observation of the spectrum. Moreover, one would expect that the interradical distance should vary within a given matrix, since the radical pairs are not constrained by covalent bonds to a fixed separation. Further, the effective distance should vary from one solvent matrix material to another. In fact, just this behavior has been observed78 in matrix-immobilized preparations of radical pairs. These radical pairs show poorly resolved triplet spectra with broad lines, and the D values of such samples differ drastically with change of solvent. It is prudent, therefore, to make sure that the D value of a putative molecular triplet remains essentially constant in several different media.

3.3. Curie’s Law and Its Application to Assignments of the Ground-State Multiplicity

In the context of ESR spectroscopy, the Curie law may be stated in its simplest form as I ¼ C=T, where I is the intensity of an absorption line, T is absolute temperature, and C is a constant. A modified form of the law (Curie–Weiss law), I ¼ C=ðT yÞ, sometimes is needed when the plot of I versus 1/T has a non-zero intercept. In both cases, the plot should be linear if the paramagnetic species responsible for the signal is not engaged in an equilibrium with other species of different multiplicity. The most common candidate for such other species is a singlet, with spin of zero.

Traditionally, it has been implicitly assumed that equilibrium between a triplet state and a lower lying singlet state would be rapidly established. However, instances of slow interconversion are now known and will be discussed in Section 7.2.1. If spin equilibrium with a more stable singlet is rapid, one would expect downward curvature of the plot at lower temperature. Bear in mind that if the singlet lies too far below the triplet (more than 2 kcal/mol), the amount of triplet present may be too small to detect by ESR. A linear plot has been the major criterion for a triplet ground state until quite recently, but as will become apparent, that may change soon. An accidental near degeneracy of singlet and triplet (less than 10-20 cal/mol separation) also will give rise to a linear plot.

For various reasons discussed elsewhere,13 the Curie law plot is not informative at temperatures >77 K, so that measurements in the cryogenic region are imperative.

Finally, one uses low-temperature techniques for ESR measurements of nonKekule´ species not only because of the inherent high reactivity of the species, but also because unless the sample is immobilized or at least hampered in its motion by viscous solvent, rapid tumbling will cause the spectrum to suffer line shape distortions that impede the extraction of the zero-field splitting parameters.

4. SOME KEY EXAMPLES

4.1. Trimethylenemethane

4.1.1. ESR Spectrum. The TMM (4) derivatives had been proposed as reactive intermediates in the thermal rearrangements of methylenecyclopropanes,14,79,80 but the first unequivocal spectroscopic characterization of any TMM, in fact, of any non-Kekule´ compound, came from the ESR investigation by Dowd.25 He synthe-

seven-line hyperfine pattern with the binomial intensity relationship expected for six equivalent hydrogens. Later,81 the spectrum was shown to follow the Curie law. All these data were compatible with a threefold symmetric structure and a triplet ground state for TMM.

The hyperfine splitting is certainly a significant observation, since it implies the presence of six equivalent hydrogens, as would be required by a planar structure with a threefold rotation axis perpendicular to the central carbon. Although the synthesis of the carrier of the ESR spectrum from two different precursors (Scheme 5.1) and the ESR data leave little room for doubt that the TMM structure is correct, one could quibble that the hyperfine result establishes threefold symmetry only permissively rather than decisively. It is conceivable that the seven-line pattern could result from an accidental equivalence of electron-nuclear coupling constants or from a rapid conformational averaging process in a TMM of lower symmetry. This possibility seems unlikely, in view of the unanimous agreement of several high-level quantum mechanical structure computations,82 which point to a planar threefold symmetric configuration for triplet TMM.

Nevertheless, just that kind of accidental hyperfine equivalence may occur in the case of TME (5) (see below), which shows a nine-line pattern54 suggestive of eight equivalent hydrogens. Neither planar nor twisted TME can have eight truly equivalent hydrogens, so the hyperfine pattern is due to some cause other than molecular symmetry.

Note that ESR spectroscopy gives no direct information whatever on the singlet state. In principle, from a nonlinear Curie plot, one can deduce the presence of the singlet and estimate the energy with which it is separated from the triplet. However, the Curie plot is a very insensitive method for detecting the presence of a singlet of slightly higher energy. Even when the Curie plot is linear, the singlet may be present but indetectable if its energy lies more than 0.05 kcal/mol above the triplet.13 In some instances, which we discuss later, other spectroscopic tools may be applicable to this purpose.

4.1.2. The Chemistry of TMM: Ring Closure. A few instances have been reported of chemical reactions in which the parent TMM molecule is suspected,

with varying degrees of plausibility, to have been the reactive intermediate in chemical reactions. These examples have been extensively discussed in reviews.12,14,27,83 The

first example in which TMM is unequivocally involved is the reaction monitored by the disappearance of the triplet ESR signal in a matrix at low temperatures.84,85

However, there has been some uncertainty about the actual chemical process responsible for this observation. Dowd and Chow84,85 concluded that the actual che-

mical process is the ring closure to methylenecyclopropane (MCP). The primary evidence for this suggestion is the kinetic behavior of the reaction, which is first order. It is not easy to imagine another true unimolecular reaction of TMM. First-order kinetics also might be observed if the signal decay were simply the result of abstraction of a hydrogen atom from the matrix. This should have but apparently did not result in growth of the monoradical signal in the middle of

the ESR spectrum. Dowd and Chow pointed out that the conversion of triplet TMM to singlet methylenecyclopropane required at some point a change of multiplicity. They wrote the mechanism 3TMM ! 1TMM ! 1MCP for this process and concluded that the observed 7-kcal/mol barrier measured the energy gap between 3TMM and 1TMM.

This proposal leads to a still unresolved puzzle: Theoretical calculations82 agree that the lowest singlet state of TMM, which has one of the methylene groups twisted out of the plane to a bisected conformation, lies some 0–7 kcal/mol below the planar singlet and 14–20 kcal/mol above the triplet. Thus, the computational results require that the activation energy for a ring closure to methylenecyclopropane by the Dowd–Chow mechanism be some 7–13 kcal/mol higher than that observed in the Dowd–Chow experiment. Clearly, if the calculations are properly reporting the situation, there is a substantial energy shortfall in the Dowd–Chow

the issues, the reader is referred to reviews of the problem.10,14,15,28 and to Section 4.1.4.

Additional spectroscopic information on the chemistry of the TMM triplet state now has been provided by the experiments of Maier et al.,86 who have prepared TMM in substantial quantities by the irradiation of methylenecyclopropane in a xenon matrix at 10 K in the presence of codeposited halogen atoms. This experiment has permitted them to record the infrared (IR) spectrum of TMM, all the observable bands of which are in agreement with those calculated by ab initio methods. They also were able to supply direct evidence for a photochemical ring-closure reaction, 3TMM ! methylenecyclopropane, by irradiation of the biradical at 254 nm. The disappearance of the TMM IR absorptions is accompanied by growth of the methylenecyclopropane bands. Of course, this observation cannot be taken as a demonstration that the reaction reported by Dowd and Chow, namely, thermal cyclization of 3TMM, actually occurs.

4.1.3. Bimolecular Trapping. Attempts to demonstrate bimolecular reactions of the parent unsubstituted TMM face a common difficulty: Even if adducts apparently formed by a transient-plus-reagent reaction are observed, how can one be sure that the bimolecular trapping step in the overall reaction actually involves the free transient in question and not the precursor or another transient? For example, the formation of a 35% yield87 of 3-vinylmethylenecyclopentane (21, Scheme 5.3) in the photolysis of 3-methylenecyclobutanone 12 in liquid butadiene (22) might plausibly be ascribed to trapping of a transient TMM, but other pathways can be written and have not been excluded. In one such pathway, the sequence of trapping and decarbonylation is simply reversed: The Norrish I intermediate 23 might be trapped and the resulting adduct acyl biradical 24 might lose carbon monoxide to give the same product 21.

hν

−CO

21

4.1.4. Determination of the Singlet–Triplet Gap in TMM by Electron Photodetachment Photoelectron Spectroscopy: Principles of the Method. The difficulty of determining the singlet–triplet gap in non-Kekule´ compounds by ESR methods led a reviewer in 198813 to remark that ‘‘[t]he need for a more reliable, more broadly applicable method for the determination of [the singlet–triplet gap] should now be obvious. Perhaps no other single development would do more to advance the study of non-Kekule´ molecules.’’ In the past few years, just such a method has emerged from advances in the techniques of gas phase chemistry and physics of ions. These innovations have led to the development of the method of electron photodetachment spectroscopy (PES), notably in the laboratories of Lineberger, Ellison, and their colleagues at the Joint Institute for Laboratory Astrophysics (JILA) at the University of Colorado. When combined with chemical procedures for the generation of gas-phase negative ions, the technique has been applied to the problem of non-Kekule´ molecules and other biradicals by the Colorado group and also by the late Robert Squires and Paul Wenthold and their colleagues at Purdue University. These efforts have now provided a major forward thrust in the direct measurement of the energy separations between spin multiplets.

In a simplified description, the experiment consists of three stages: (1) a radical anion formally related to the non-Kekule´ neutral compound is generated by some suitable process; (2) the ions are mass selected and subjected to irradiation by a 351-nm laser to photodetach electrons; and (3) these electrons are allowed to pass through an energy analyzer that measures their kinetic energy. The last step generates a photoelectron spectrum, from which important information can be deduced about the vibrational states of the radical anion and about the binding energy with which the photoelectrons (PE) were held in the anion before they were detached. The amount of this energy will depend on the state of the neutral

biradical produced in the photodetachment. If two spin states of the neutral differ in energy, as they will barring an accidental degeneracy, two separate band systems will appear in the PE spectrum. Conservation of energy requires that the sum of the energy of the reactants (the laser photon and the radical anion) must equal the sum of the energy of the products (the photoelectron produced and the particular state of the neutral produced, see Eqs. 2 and 3).

Thus, the process that leads to the lower energy neutral must give a photoelectron of higher energy, whereas the process that leads to the higher energy neutral must give a photoelectron of lower energy. The separation between the band origins of the PE band corresponding to the two product neutral states directly gives the energy difference between them. Some authors report this directly as measured in kinetic energies of the electrons, and others report it as the ‘‘electron binding energy,’’ which is simply the difference between the photon energy (3.51 eV) and the energy of the photoelectron. Clearly, if the PE data are reported as electron binding energies, the more stable neutral state will be represented by the lower photoelectron binding energy.

4.1.5. Application to TMM. A convincing example of the power of this method is the determination of the singlet–triplet separation in TMM. The TMM negative ion (27) is prepared by the gas-phase reaction shown in Scheme 5.4.88 The reaction of 1,1-di[(trimethylsilyl)methyl]ethene (25) with fluoride ion in the gas phase removes one trimethylsilyl group and gives rise to the anion 26, which in a subsequent step, is treated with molecular fluorine in the flowing afterglow source of a negative ion photoelectron spectrometer.

4, TMM

Scheme 5.4

This step generates the TMM radical anion 27, which is selected for electron photodetachment of the mass 54 (TMM) anions from the beam.82 Two PE band

systems are observed in the PE spectrum in which the origin bands are separated by 0.699 eV (16.1 kcal/mol). Since computational results indicate the conformation of the radical anion to be planar, and since the photoelectron detachment is a Franck– Condon transition, the authors ascribe the observed separation to the energy difference between the planar forms of the singlet and triplet states of TMM. The energy separation between planar and the more stable bisected TMM neutral is calculated to be 0–3 kcal/mol, which suggests that the separation of the triplet from the lowest energy singlet is 13–16 kcal/mol.

One can hardly argue with the conclusion that experiment now matches the theoretically calculated energy gap for the planar forms, 17 kcal/mol. The picture is complicated somewhat by the fact that the gap between the triplet and the bisected singlet is not given directly by the experiment, and logically one could quibble that the extraction of this number has itself required a calculation. However, sustaining that argument requires that the calculations be entirely accurate for the planar forms but suddenly lurch into a large error for the gap between the triplet and the

bisected singlet forms. Instead, it seems justified to move ahead with the authors’ conclusion that this gap is 13–16 kcal/mol rather than the 7 kcal/mol proposed84,85

earlier.

Note that the PE experiment does not identify by simple inspection which of the spin states is more stable, the singlet or the triplet. However, the assignment of the lower energy state to the triplet seems entirely reasonable, in view of Dowd’s demonstration that the triplet is the ground state in matrices. Further confirmation of this may come from a complete vibrational analysis of the richly detailed PE spectrum.82

Further applications of these methods are discussed later in Section 5.1.

4.2. Ring-Constrained Derivatives of TMM

The study of these substances eventually was to evolve into a broad development of the chemistry of TMM derivatives. The original motivation was to expand earlier findings89 on the stereospecific conrotatory thermal (300 C) ring opening of the 2,3-dimethylbicyclo[2.1.0]pentanes to 2,5-heptadienes, plausibly via 3,4- dimethylcyclopentane-2,5-diyls. The planned follow-up experiment was to observe whether the insertion of a methylene group would generate a triplet TMM species (see Section 4.2.1), and thereby cause the ring opening to become nonstereospecific. This experiment was never completed, but observations in the early stages of it led to the study of several of such non-Kekule´ compounds.

4.2.1. 2-Methylenecyclopentane-1,3-diyls 14: Synthesis and ESR Properties. The first member of the 2-methylenecyclopentane-1,3-diyl series to be synthesized was the 2-isopropylidene derivative 14b,26(Scheme 5.2) followed by

a number of others (14a, 14c–f ), including the unsubstituted methylene compound 14a.27,90 Each of these substances can be generated from the corresponding diazene

(13), either thermally or photochemically. This and many other related studies have been reviewed repeatedly,14,15,27 and further studies on the ESR spectra of related

derivatives continue.91 The most important conclusions may be briefly summarized (Scheme 5.5):

CH3

H3C

N

N

13b

Scheme 5.5

1.Both singlet and triplet forms of the 2-alkylidenecyclopentane-1,3-diyls 14 undergo cycloadditions in high yield with alkenes 32 by characteristic stereochemical and regiochemical pathways. The singlet gives predominantly fused adducts 30 formed by concerted syn addition, whereas the triplet gives stereorandom and regiorandom (bridged 31 and fused 30) adducts by a stepwise addition (Scheme 5.5). The regiochemistry in the singlet additions is consistently rationalized by a frontier orbital model.

2.The relative rates of reaction of the singlet TMM derivative 114b with a series of alkenes (32) parallel those of a conjugated diene with the same alkenes in Diels–Alder reactions. These relative rates also are well correlated by the frontier orbital model for a concerted reaction. The absolute rates of the biradical cycloadditions are many orders of magnitude greater than those of

the model dienes. The relative rates of the alkenes in the cycloadditions of the triplet biradical 314b, on the other hand, follow the reactivity order of their

addition reactions with monoradicals.

3.The bicyclic hydrocarbon full-valence isomer (28) of the 2-isopropylidene- cyclopentane-1,3-diyl (15b) can be prepared and kept at temperatures below

However, it soon became clear that matters might not be so simple. There is a fundamental difference in the connectivity of the p centers in TME and that in

¨ 56,94

TMM. Huckel recognized a similar difference in the case of the two

Schlenk–Brauns hydrocarbons 1 and 2, and the theory was put on a modern basis and generalized by Borden and Davidson.59

Simply stated, the rule is that if the molecule can be mentally constructed (see Scheme 5.7) by joining two radical units at positions that have NBMO coefficients of (nominally) zero, as in the hypothetical formation of TME from two allyl radicals 35, then the exchange interaction approaches zero.

H

H

Scheme 5.7

Since it is the exchange interaction that determines to a large extent the energy separation of the multiplet states to first order, the physical basis for the triplet preference thus vanishes. In that case, other electron correlation effects tend to stabilize the singlet selectively.

The NBMOs of the singlet state of such a biradical are called disjoint in recognition of the fact that they can be considered to be confined to different sets of atoms. The net result is that in such molecules, the singlet–triplet gap will be small, and the singlet may become the ground state. Borden and Davidson59 pointed out that TME (5) is the parent molecule of this non-Kekule´ class, which we now call disjoint.

A second argument leading to the same conclusion comes from valence bond theory and can be expressed by a simple scheme involving the parity (starredness or unstarredness) of an alternant hydrocarbon or heteroatom substituted derivative thereof. 60,61 The conclusions are supported by other approaches involving parity

arguments.95 Moreover, they are extended to non-alternant systems in papers by other authors.96–100 The generalized version of the rule may be stated in the follow-

ing form: If the portion of the conjugated p system intervening between the unpaired spin centers has an even number of electrons, the molecule will be low spin, and if the number is odd, it will be high spin. The number of such electrons is calculated by assigning one electron to each intervening p center and performing the count on the resonance structure that minimizes the number of intervening p centers. Thus, the number is 2 in TME and 1 in TMM, corresponding to low and high spins, respectively.