Reactive Intermediate Chemistry

data, showing that dimerization to form an ethylene is the main decay pathway for

3106d under these conditions. From the decay curve, the dimerization rate constant (kd) was determined to be 1:2 107 M 1 s 1. 154 The half-life was determined to

be 18 ms, which is four orders of magnitude larger than that observed for parent DPC but is about one-tenth that of 3106d (Scheme 9.35, Table 9.14).

Perchlorinated DPC 3106e154 also gave a carbene dimer when generated in degassed benzene and decayed in second-order kinetics. The dimerization rate constant (kd) was found to be 2:5 106 M 1 s 1, which is approximately one order of magnitude smaller than that determined for 3106e. This observation suggests that the dimerization rate is sharply decreased as four additional chlorine groups are introduced at the meta positions, obviously because of the buttressing effect. The half-life is estimated to be 28 ms.

On the other hand, kCHD increases only slightly as more chlorine atoms are introduced. This result is somewhat surprising in the light of the marked decrease in the dimerization rate caused by the four meta buttressing chloro substituents. It can be understood to indicate that electrophilicity of the carbene center is increased as more chlorine groups are introduced on the phenyl rings (Table 9.14). Thus, the attack of a bulky substrate (e.g., chlorinated DPC) on the carbenic center must be severely restricted, while the rate of abstraction of a very small hydrogen atom from a very efficient H donor is still controlled by the electrophilicity of the carbenic center.

Perchlorophenyl groups, which have an enormous effect on the stability of arylmethyl radicals, have only a slight effect on the stability of DPCs. This fact suggests how unstable triplet DPCs are and how much more difficult they are to stabilize than arylmethyl radicals.

Bromine groups appear to be more promising as a protecting group toward a triplet carbene center because their van der Waals radius is similar to that of methyl

overhang the reactive site more effectively. In addition, the value of x1, which gives a measure of the strength of the spin-orbit interaction, is increased rather dramatically in going from chlorine (586 cm 1) to bromine (2460 cm 1).156 Therefore intersystem crossing from the nascent singlet carbene to the triplet should be accelerated by introducing a bromine atom.

Bis(2,4,6-tribromophenyl)carbene (106f)157 was easily generated by photolysis of the precursor diazomethane (105f) and was characterized by EPR spectroscopy (Scheme 9.14). The triplet carbene generated in a degassed solution at room temperature decayed very slowly, persisting for at least 30 s. The decay was found to be second order (2k=el ¼ 8:9 s 1). The parameter t1=2, is estimated to be 1 s (Table 9.14).

The reaction patterns observed for 3106f were analogous to those observed with chlorinated and methylated DPCs. However, the structure and yield of the dimer were notably different from those observed for other persistent DPCs (e.g., 106b and 106c,d), which produced tetraarylethylenes in fairly good yield ( 80%). Generation of 3106f in a degassed benzene solution resulted in the formation

446 TRIPLET CARBENES

of a tarry matter, from which a relatively small amount (20%) of dimeric product was isolated. The dimer was identified as a phenanthrene derivative (115) rather than a simple carbenic dimer, tetraarylethylenes (107f). A possibility is that triplet carbenes undergo dimerization to form tetraarylethylenes (107f), which subsequently cyclize to lead to phenanthrene. The fact that the carbene dimer is formed in a complicated form in surprisingly small amount suggests that the carbene center is effectively blocked by four o-Br groups.

The observations clearly indicated that bromine groups shield the carbene center better than methyl groups, as expected. One may wonder whether through-space interaction between the triplet carbene center and the o-bromine groups may play a role in stabilizing the triplet. The EPR data clearly indicate, however, that there is no such interaction at least in a matrix at low temperature.

Anomalous effects of para-substituents on the stabilities of triplet brominated DPC were observed in this case (Scheme 9.36, the Table 9.14).157b,c The LFP stu-

dies show that bis(2,6-dibromophenyl)carbene 3106g, which has tert-butyl groups at para positions, decayed very slowly: much slower than 3106f. It took >200 s before all the signals due to 3106g disappeared completely. The decay was found to be the second order (2k=el ¼ 0:36 s 1), and t1=2 was estimated to be 16 s, suggesting that 3106g is some 20 times longer lived than 3106f. In marked contrast, bis(2,6-dibromophenyl)carbene 3106h, which has methyl groups at the para positions, decayed rapidly within 1 s. The decay was found to be first order in this case, and the lifetime is estimated to be 0.21 s (t1=2 ¼ 0:22 s).

What is the origin for this anomalous effects of para substituents? The inspection of ZFS parameters clearly indicates that there are no significant changes in E=D values as the para substituents are varied (Table 9.14). These observations indicate that the para substituents do not have much effect on the geometries of brominated DPCs. The reactivities of brominated DPCs toward typical triplet quenchers, that is, oxygen and 1,4-cyclohexadiene, are also not significantly affected by the para substituents (Table 9.14).

These observations clearly support the naı¨ve idea that the para substituents exert almost no effect at remote carbenic center of brominated DPCs, at least in terms of steric congestion. Furthermore, electronic effects do not seem important enough to change the reactivity of brominated DPCs so significantly. For example there is rather small distribution in the Hammett and spin-delocalizing substituent constants.

Benzene is recognized as a very unreactive solvent, especially for triplet carbenes. Therefore, the most reactive reactants under these conditions must be the triplet carbenes themselves. However, the products obtained from the photolysis in benzene consist of a highly complex mixture containing small amounts of carbene dimers. It is then possible that the simple dimerization of brominated DPCs must suffer from severe steric repulsion and, therefore, the carbene is forced to react at other positions. The most probable reactive sites are the aromatic rings, where spin can be delocalized.

Trityl radicals are known to undergo either methyl para or para–para coupling, depending on the substituent patterns.158 Thus, it is likely that the brominated DPCs also undergo similar coupling. The coupling reactions of trityl radicals are not suppressed by ‘‘reactive’’ substituents. For example, (p-bromophenyl)diphenylmethyl undergoes methyl-para coupling.159 On the other hand, tri(p-tolyl)methyl undergoes rapid disproportionation to yield tri(p-tolyl)methane and a quinoid compound that rapidly polymerizes.160 However, coupling at the para positions is retarded by the presence of a tert-butyl group at this position.161

9.3. Triplet Diphenylcarbenes Protected by Trifluoromethyl Groups

The trifluoromethyl group has been regarded as an ideal kinetic protector of carbenes because it is much bigger than methyl and bromine groups, and, more importantly, C F bonds are known to be almost the only type of bond unreactive toward carbenes.162 For example, trifluoromethylated DPCs generated in the gas phase at high temperature (350 C/10 4 Torr) gave trifluoromethylated fluorene derivatives as a result of carbene– carbene rearrangement. The CF3 group remained intact even at this high temperature.163

However it is almost impossibly difficult to prepare a precursor diphenyldiazomethane bearing four ortho CF3 groups. The precursor we were able to prepare is diphenyldiazomethane (105i), which has two ortho bromine groups in addition to two CF3 groups (Scheme 9.36).

Triplet carbene (106i)164 was easily characterized by EPR spectra recorded after irradiating the diazomethane precursor in MTHF glass at 77 K. The triplet signals were found to be persistent even at a significantly higher temperature. For example, a significant decay of the triplet signals began only at 210 K in MTHF. In more viscous solvents, 3106i was able to survive at much higher temperature. In triacetin, for example, no appreciable changes were observed even at 230 K. A measurable decay of the signals was observed only at 273 K (0 C), at which the ‘‘first-order’’ half-life (t1=2) was 10 min (Table 9.14).

To estimate the lifetime of triplet carbene 106i under our standard condition, that is, in degassed benzene at 20 C, LFP of 105i was carried out. However, the lifetime of 3106i was too long to be monitored by the LFP system, and a conventional UV–vis spectroscopic method was more conveniently employed in this case. The transient signals due to 3106i did not disappear completely even after 1 h under these conditions. The decay was analyzed as a sum of two exponential decays

448 TRIPLET CARBENES

(k1 ¼ 1:9 10 3 s 1; t1 ¼ 9:0 min and k2 ¼ 1:0 10 5 s 1; t2 ¼ 1602 min).165 Since the half-life estimated by dynamic ESR studies is in the order of 10 min, we assume that the first-decaying component is assigned to the triplet carbene, while the slower decaying one is assigned to a secondary product formed during the decay pathway of the triplet carbene. Thus, a triplet carbene having a lifetime over several minutes was realized for the first time.164

The rate constants of 3106i with typical triplet quenchers, that is, oxygen and 1,4-cyclohexadiene, were estimated and compared with those observed for the longest-lived triplet carbene known thus far. It is clear that reactivities decrease by 2 orders of magnitude as two o-bromine groups in 3106g are replaced with two CF3 groups.

9.4. Triplet Polynuclear Aromatic Carbenes

Introduction of aryl groups on methylene results in a significant decrease in D values. For example, D decreases from 0.69 to 0.515 on going from methylene to phenylcarbene.13 The D values decrease further as the aromatic ring is changed from phenyl to 1-naphthyl to 9-anthryl (Table 9.3). Similar trends are seen in diarylcarbene systems. The D values of DPC, di-1-naphthylcarbene (15) and di-9- anthrylcarbene (16a) are 0.4055, 0.3157, and 0.113 cm 1,13 respectively. These data clearly suggest that the unpaired electrons are more extensively delocalized as the number of the aromatic rings is increased, and hence the triplet states are thermodynamically stabilized.

Among the many triplet diarylcarbenes known, triplet di-9-anthrylcarbene (16a)166 shows the smallest D (0.113 cm 1) and E (0.0011 cm 1) values ever

reported. This observation means that the carbene has an almost linear and perpendicular geometry with extensive delocalization of the unpaired electrons into the anthryl portions of the molecule.167 The extensive delocalization is expected to stabilize this carbene thermodynamically, while the perpendicular geometry of the anthryl group stabilizes the carbene center kinetically, through shielding with the four peri-hydrogens. That is, the electronic factors and the molecular structure of the molecule seem ideal for the formation of a stable triplet carbene.

In spite of those highly favorable structural factors, 316a is very ephemeral.168 Its lifetime in degassed benzene is 0.5 ms,169 which is shorter even than that of parent triplet DPC. Product analysis studies have shown that 316a forms a trimer of dianthrylcarbene (116) as the main product (50–60%) (Scheme 9.37).170 The trimer is the one formed as a result of a threefold coupling at position 10 of the carbene. This observation suggests that delocalization of the unpaired electrons in 316a leads to their leaking out from the carbene center to position 10, where sufficient spin density builds up for the trimerization to take place. At the same time, the lack of formation of olefin-type dimers through coupling of two units of 316a at their carbene centers indicates that the carbene center itself is indeed well shielded and stabilized.

The above observations indicate that the stability of 316a would increase if the trimerization reaction were somehow suppressed. Thus, an attempt was made to generate a dianthrylcarbene having a substituent at position 10.

Scheme 9.37

The triplet carbene (16b) generated by photolysis of bis[9-(10-phenyl)anthryl]- diazomethane in MTHF glass at 77 K gave EPR signals very similar to those observed for dianthrylcarbene (16a) (Scheme 9.37). The ZFS parameters (D ¼ 0:105 cm 1, E ¼ 4:4 10 4 cm 1) derived from the signals are essentially the same as those obtained for 316a. These observations indicate that the phenyl groups are not in the same plane as the anthryl rings owing to the repulsion between orthoand peri-hydrogens. The EPR signals of 16a and 16b differ only in their thermal stability. When the MTHF glass containing 16a was warmed gradually, the signals due to 16a started to disappear at 90 K, whereas no significant decay of the signals of 316b was observed up to 240 K. The signal of 16b started to decay only at270 K ( 0 C), but was still visible when heating to 300 K ( 27 C).

To probe the stability of 316b under normal conditions, the precursor diazomethane was photolyzed in degassed benzene at room temperature. The bands due to 316b decayed cleanly, showing isosbestic points, persisting for more than 3h before disappearing completely. The decay curve was analyzed in terms of second-order kinetics (2k=el ¼ 5:2 10 4 s 1). The half-life for 316b was estimated to be 19 min. This carbene is the longest-lived triplet thus far generated in our research group.29

9.5. Toward Persistent High-Spin Polycarbenes

Triplet carbenes are regarded as one of the most effective sources of high spin. The magnitude of the exchange coupling between the neighboring centers is large, and

450 TRIPLET CARBENES

the photolytic production of polycarbenes is possible even in solid solution at cryo-

genic temperature under helium. Therefore, in situ magnetic measurements are easy if poly(diazo) precursors are available.171,172 Actually, Iwamura and co-workers

prepared a ‘‘starburst’’ type nonadiazo compound, 1,3,5-tris[[30,50-bis[phenyl(1,1- diazanediyl)-methyl]phenyl] (1,1-diazanediyl)benzene (117), and have demonstrated that the nine diazo groups are photolyzed therefrom at low temperature to give a pentadecet ground state (118) (Scheme 9.38).173 However, those systems

X

X

X

X X

Scheme 9.38

lack the stability necessary for characterization under ambient conditions and have an inherent drawback for further extension to usable magnetic materials. Moreover, the synthesis of the precursor diazo compounds becomes more difficult and laborious as the number of carbene units increases.

In order to overcome these difficulties for realization of usable macroscopic spins, we have to find a way to stabilize triplet carbene centers and then to connect them in ferromagnetic fashion. As we have fairly stable triplet carbenes, the next step should be to explore a way to connect them while retaining a robust p-spin polarization. The topology-mediated molecular design of neutral organic highspin systems with p conjugation has been well established in the field of purely organic magnetics.172 In order to use polybrominated triplet DPCs as persistent spin units, and to construct as high spins as possible, we designed 1,3,5-tris- (ethynyl)phenyl as a two-dimensional linker.

The reaction scheme to realize the coupling necessary to generate the persistent carbene precursor 122 is illustrated in Scheme 9.39.174 Diazo groups are generally labile, and, hence, these groups are usually introduced at the last step of the

Br

X

Br

Br Br

(a) Me3SiC_CH/(Ph3P)2PdCl2/Cul; (b) KOH/MeOH; (c) 1,3,5-I 3C6H3/(Ph3P)2PdCl2/Cul;

(d) hν (λ>300 nm).

Scheme 9.39

synthesis when preparing not only mono(diazo) but also poly(diazo)compounds.174 Application of this strategy to these highly sterically congested diazo compounds, prepared mostly by base treatment of N-nitrosocarbamates, is almost impossibly difficult. Fortunately, however, the polybromodiphenyldiazomethanes 119 and 120 were found to be stable enough to survive in the presence of Pd(0) and Cul under Sonogashira coupling reaction conditions.175 The four bromine groups at the ortho positions effectively protect the carbene center from the external reagents and are also able to protect the diazo carbon effectively.

Moreover, each of the four ortho carbons is also protected from the approach of external reagents by these bromine groups. This stabilization enabled us to introduce connecting groups at the para position of the monomer diazo compound (e.g., 119) to give the functionalized monomer 120, and then to connect it to an appropriate linker satisfying the topological requirement for intramolecular

452 TRIPLET CARBENES

ferromagnetic spin alignment. In practice, we were able to introduce an ethynyl functional group very selectively at the para position in a reasonable yield. Deprotection of the trimethylsilyl group proceeded almost quantitatively to give 120. Subsequent coupling with 1,3,5-triiodobenzene took place smoothly to produce tris(diazo) compound 121, in which three carbene precursor units were properly introduced so as to generate tris(carbene) (122), a molecule connected in a ferromagnetic fashion.173

Photolysis of 121 in MTHF glass at 5 K gave a fine-structure EPR spectrum that was different from that observed on the photolysis of the corresponding monodiazomethane. The salient features of organic high-spin species apparently failed to appear in the observed spectrum. Canonical peaks dominate in the g 2 range, and the key peaks appearing in the wings away from the g 2 region are only vague. It is obvious that conventional continuous wave EPR (CW EPR) spectroscopy fails to give evidence of the spin identification of the tris(carbene).

In order to identify the spin multiplicity of the tris(carbene), field-swept twodimensional electron spin transient nutation (2D-ESTN) spectroscopy was used.176 This technique is based on pulsed fourier transform (FT) EPR spectroscopic methods and is capable of elaborating straightforward information on electronic and environmental structures of high-spin species even in amorphous materials, information that conventional CW EPR cannot provide. The nutation spectra unequivocally demonstrated that the observed fine structure spectrum is due to a septet spin state.

A nutation peak arising from any doublet species of byproducts was not detected, implicating a remarkable chemical stability of the tris(carbene) 122. Furthermore, the marked thermal stability of 122 manifested itself during the annealing process. A significant change in the EPR signal shape was observed when the matrix temperature was raised >140 K and maintained for 15 min. New fine-structure signals were formed irreversibly with temperature. They were also shown to originate in a septet ground state by 2D-ESTN spectroscopy. This result is in sharp contrast with that observed for septet-state benzene-1,3,5- tris(phenylmethylene), which has been shown to be persistent only up to 85 K177 in rigid glasses.

These studies show that the diazo compounds prepared to generate persistent triplet carbenes are also very persistent and therefore survive during the further chemical procedures to functionalize these precursor molecules. The building blocks thus prepared are very useful as they can be introduced very easily on the properly designed p-toplogical linkers, ad libitum. Moreover, the undesirable bimolecular side reactions that generate chemical defects among the topologically controlled high-spin (S ¼ 1) centers for extended spin alignment are rigorously avoided. These findings open a new door to the design and preparation of new persistent high-spin molecules for more practical organic ferromagnetics.

10. CONCLUSION AND OUTLOOK

Much of the interest in carbene chemistry derives from the complexity induced by the presence of two electronic states, singlet and triplet. In this respect, it is crucial

to know the energy gap between the two states with great accuracy in order to understand and control apparently complex reactions of carbenes.

We have seen that high-level computational studies are able to predict EST of even fairly large systems in the gas phase quite precisely. They are also able to analyze the structural factors, influencing the relative stability (conjugation with p system, hyperconjugation, carbene bond angle, and so on). On the other hand, in the light of the experimental suggestions that the singlet state is stabilized relative to the triplet state in polar solvents, it is highly desirable to estimate EST in solution with an equal accuracy.

Recent development of the time-resolved spectroscopic techniques has enabled us to study the dynamic properties of these highly reactive species in a quantitative manner, and provides an opportunity to estimate EST experimentally. Although we are still not able to obtain all four key rate constants (kST, kTS, kS, and kT) of a carbene to determine its GST, a rough experimental evaluation of EST has been successfully made for several arylcarbenes with rather small S–T gaps based on the assumption of preequilibrium mechanism. The data, along with the theoretical estimates, have helped us understand the nature of carbene and its chemistry in a quantitative way. However, the critical assumption can be criticized, and a new mechanism, surface crossing, is proposed. A response to this criticism can be offered if the kinetic data can be measured under the conditions under which the key reaction is carried out. At present, only in one case has the direct observation of both states been made simultaneously by using TRIR techniques. It will not be long before those key rate constants become available for more carbenes with the aid of spectroscopic methods with faster time resolution.

The other issue involved in triplet carbene chemistry is the involvement of a higher lying singlet state especially when the final products from the both states are the same. This situation is seen especially in the reaction with C H bonds. Although CIDNP provides unequivocal evidence for the spin state of precursor carbenes, the interpretation of reactivity obtained by time-resolved spectroscopy is always hampered by the intervention of a higher lying singlet state, especially when the S–T gap is small. Moreover, philicity of not only singlet but also triplet state must also to be taken into account.

The role of spin in the reaction is an especially interesting and important issue in carbene chemistry. Apparent violation of the spin selection rule, such as the reaction of ground state singlet carbene with triplet molecular oxygen in matrices as well as in solution, and the reaction of triplet ground-state carbenes with (singlet) CO, CO2, and N2 in matrices, are challenging issues for the near future.

Finally, a persistent triplet carbene under normal conditions is still an on-going target, although a fairly stable one with a half-life over several minutes is available. Although the recent synthesis of stable singlet counterparts at room temperature greatly encourages the attempt, the inherent nature of triplet state, such as high reactivity toward oxygen makes the attempt far more difficult. However, it is obviously not beyond our wildest dream. The realization of a stable triplet carbene and its use as a spin source to construct organic magnetic materials will be another example to demonstrate that the research in physical organic chemistry eventually results in production of a idea for the practical world.

454 TRIPLET CARBENES

SUGGESTED READING

W. T. Borden, Ed.; Diradicals, John Wiley & Sons, Inc., New York, 1982.

C. Wentrup, Reactive Intermediates, John Wiley & Sons, Inc., New York, 1984.

G.B. Schuster, ‘‘Structure and Reactivity of Carbenes Having Aryl Substituents,’’ Adv. Phys. Org. Chem. 1986, 22, 311.

M. S. Platz and V. M. Maloney, ‘‘Laser Flash Photolysis Studies of Triplet Carbenes,’’ in

Kinetics and Spectroscopy of Carbenes and Biradicals, M. S. Platz, Ed., Plenum, New York, 1990, pp. 239–352.

W.Kirmse, ‘‘Carbenes and O-H Bond,’’ in Advances in Carbene Chemistry, Vol. 1, U. Brinker, Ed., JAI Press, Greenwich, CT, 1994, pp. 1–57.

W.Kirmse, ‘‘Carbene Protonation,’’ in Advances in Carbene Chemistry, Vol. 3, U. Brinker, Ed., JAI Press, Greenwich, CT, 2001, pp. 1–51.

M. S. Platz, ‘‘Atom-Transfer Reactions of Aromatic Carbenes,’’ Acc. Chem. Res. 1988, 21, 236.

H.Iwamura, ‘‘High-Spin Organic Molecules and Spin Alignment in Organic Molecular Assemblies,’’ Adv. Phys. Org. Chem. 1990, 26, 179.

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