Reactive Intermediate Chemistry

light intensity and photolysis time. The results are interpreted as indicating that an excited triplet DPC reacts with the matrix by hydrogen atom abstraction to give ultimately the C H insertion product (see Section 5.4.1).133

In addition to the rather trivial differences mentioned above, laser irradiation can also lead to products as a result of reexcitaion of the carbenes. Thus, excitation of 30 in isooctane with a pulse of the 249-nm line from a KrF excimer laser results in the formation of 9,10-diphenylanthrancene (103), 9,10-diphenylphenanthrene (104), and fluorene, in addition to tetraphenylethylene (Scheme 9.31). Conventional lamp irradiation of 30 results in the formation of benzophenone azine as a major product. None of the products mentioned above are detected. Moreover, the yield of both 103 and fluorene increased markedly with increased laser power. While the details of the mechanism of this reaction are not certain yet, it is clear from the dependence on laser power that some of these products arise from carbene photochemistry.134

8.2. Spectroscopic Studies

More direct evidence for the intervention of excited states of triplet carbenes in reactions in solution is obtained by spectroscopic studies. Thus, picosecond lasers make it possible to study the quenching of carbene fluorescence by various substrates in solution at room temperature. Diphenylcarbene is generated upon laser photolysis of 30 and a second UV laser pulse is time delayed by 8 ns and is used to excite the carbene, thereby producing the excited triplet DPC (Scheme 9.32). The fluorescence of 3DPC* is then monitored with a streak camera. The fluorescence

Scheme 9.32

lifetime of 3DPC* in acetonitrile with no quencher is found to be 3.8 ns. The decay rate is markedly increased upon addition of methanol and isoprene.135 A plot of the decay constant against methanol concentration is used to obtain the bimolecular

values are to be compared to those known for the reaction of the lowest singlet and ground triplet states of DPC. The observed rapid diffusion-controlled reaction of methanol with DPC ( 2 1010 M 1 s 1) is thought to occur via the low-lying singlet state. The reaction of excited 3PDC with methanol is thus approximately two orders of magnitude slower than that of 1DPC. On the other hand, there is roughly a

The quenching rate constant by O2 is ð4:4 0:8Þ 1010 M 1 s 1. For comparison, the ground-state carbene reacts with a rate constant of 1:9 108 M 1 s 1. The increase in rate of over two orders of magnitude between ground and excited carbenes is much larger than the enhancement observed for free radicals. The high rate constant for the excited carbene is interesting because it suggests that the process cannot have any significant spin restrictions. Thus, 5/9 of the encounters will be quintet events, which must lead to the ground state of both oxygen and the carbene. One third of the encounters are triplet encounters, which requires one of the reactants to convert into its singlet state. Whether the reaction leads to the carbene singlet or singlet oxygen has not been determined yet.139

Carbene fluorescence in solution is usually red shifted by 25–30 nm with respect to the band position observed in matrix at 77 K. This shift is attributed to emission from nonequilibrated conformations at low temperature. In matrices, the carbene is produced in a locked conformation similar to that for the precursor diazo compound but, in solution, it approaches the thermodynamically favored configuration. This difference has been demonstrated by variable temperature EPR studies of sterically congested carbenes (see Section 3.1.1.3). So, in solution, the equilibrium conformation is reached rapidly and only fluorescence from the relaxed state is observed. In support of this suggestion, the shift for dimesitylcarbene is smaller than for other carbenes, indicating that shifts are smaller when the carbene structure is such that it restricts conformational change.

8.3. Geometry of Excited Triplet State Carbenes

Conventional EPR techniques have been successfully used to measure the D and E values of matrix-isolated carbenes in the ground triplet state because the steadystate concentration of triplet species is sufficiently high in the system. The technique cannot be used, however, for excited species having triplet lifetimes of the order of 10–100 ns, since their steady-state concentration is too low. The D parameters are estimated from the external magnetic field effect on the T–T fluorescence decay in a hydrocarbon matrix at low temperature. The method is based on the effect of the Zeeman mixing on the radiative and nonradiative decay rates of the T1–T0 transition in the presence of a weak field. The D values are estimated

by fitting the decay curve with that calculated for different D values. The D (T1) values estimated for nonplanar DPC (c1 symmetry) is 0.20 cm 1.140,141

Direct detection of 3DPC* is made by time-resolved EPR spectroscopy. In this method, 3DPC is first generated by photolysis of 30 in a hydrocarbon matrix at 16 K and is excited by a 465-nm laser, which corresponds to a T–T absorption of the T0 state of DPC. The transient triplet spectrum of the species having a decay rate of 160 ns at 16 K is assigned to the EPR spectrum of 3DPC*. The ZFS parameters

Examination of the data indicates that for all the carbenes studied, the D (T1) values are significantly smaller than the D (T0) values. Since D is proportional to

438 TRIPLET CARBENES

1/r3, r being the average separation between the two unpaired electrons, the observed decrease of D indicates that the average separation of r is significantly larger in the T1 state than in the T0 state. The electronic structure of the excited T1 state involves the promotion of the unpaired electrons from the p orbital to a p* orbital. Calculations performed on DPC (14a) and 2-naphthylphenylcarbene (b-51) indicate that this p* orbital has a minor contribution from the carbenic 2pp atomic orbital. Thus p > p:* excitation results in a delocalization of the unpaired onto the rings and to a decrease of the corresponding spin density on the carbenic center, while the spin density originating from the s unpaired electrons is retained. As a consequence, the average separation between the two unpaired electrons is increased in the T1 state relative to T0 ground state, leading to a D (T1) values significantly smaller than the D(T0).

8.4. Reactivity Differences between Triplet and Excited Triplet Carbenes

It is very interesting to examine the origin of the big difference in the reactivity between the ground and excited triplet carbene. A simple model for 3DPC, 1DPC, and 3DPC* is given by the orbital filling scheme involving the highest occupied nonbonding molecular orbitals (s, p) and the antibonding (p*) orbital of the carbene. The ZFS parameters support this idea. It can be seen by this representation that 1DPC and 3DPC* will be similar to each other insofar as they both possess an empty low-lying orbital. The ground 3DPC, on the other hand, has no empty lowlying orbitals available to it (Scheme 9.33). This simple scheme offer an explanation for the fact that the 3PDC* would undergo certain reactions that 3PDC would not, and 3PDC* would appear to resemble 1PDC in certain of its reactions.136

p↑

Scheme 9.33

It was shown that the patterns of the relative rates of reaction of 1DPC and 3DPC* with alcohols are essentially identical and followed the relative acidity of the alcohols (MeOH > i-PrOH > t-BuOH) and showed a kinetic deuterium isotope effect on reaction with the OH bond (Table 9.13). These results indicate that 3DPC* attacks the O H bond rather than the C H bond of the alcohol. If the C H bonds of the alcohol were attacked by 3DPC*, as in the ground triplet state reaction, then one would expect 2-propanol would react faster than methanol. Lack of any discernable quenching of 3DPC* by diethyl ether and THF indicates that 3DPC*

does not appear to react with alcohol by initial attack on the heteroatom to generate an ylide type species, and that simple C H abstraction a to the oxygen is not important. It is proposed therefore that 3DPC* reacts with alcohols in a manner analogous to the 1DPC reaction with alcohol. It has been demonstrated that 1DPC reacts with methanol by accepting a proton on the unpaired electrons in the s orbital to generate a carbocation intermediate. This mechanism cannot be opperating in the reaction of 3DPC*. Perhaps the greater energy of 58 kcal/mol available for 3DPC* account for the reaction.136

On the other hand, the formation of a dimeric product in the laser photolysis of 30 in the absence of quenchers can be qualitatively interpreted by taking into account the spin distribution in 3DPC*. Thus, the reaction of 3DPC* with 3DPC according to Scheme 9.34 will lead to the observed products. Such reactions would be favored in the presence of an intense excimer laser, which creates a high concentration of 3DPC*.140

Scheme 9.34

9. PERSISTENT TRIPLET CARBENES

Now that we know the nature of triplet carbenes143 in great detail, it will be very challenging to attempt to stabilize this highly reactive species so that they can be isolated under normal conditions. This goal is especially intriguing because triplet carbenes are importent from a practical viewpoint; Because of its unusual high spin state, triplet carbenes are an attractive spin source for organomagnetic materials.

Reactive species can be stabilized either thermodynamically or kinetically. Thermodynamic stabilization is usually done electronically by perturbation of the conjugated p system. As we have seen, electronic effects usually play an important role in stabilizing the singlet state. The stabilized singlet state becomes less reactive. They can be even isolated under ambient conditions in some cases.144 Kinetic stabilization, on the other hand, is usually achieved by retarding the decay processes of the species in question. Sterically bulky substituents are introduced around the reactive center in order to prevent it from reacting with external reagents. Triplet states are also stabilized by electronic effects and their reactions through the upper lying singlet state may be suppressed. In this light, kinetic stabilization using steric protecting groups should be more effective for generating persistent triplet carbenes. Moreover, the introduction of sterically bulky groups

440 TRIPLET CARBENES

around the carbenic center must expand the carbene bond angle, which results in the thermodynamic stabilization of the triplet state relative to the singlet state.

This method was shown to be very useful by Zimmerman and Paskovich in 1964 in their attempts to prepare a hindered divalent species completely unreactive toward external reagents.145 They generated dimesitylcarbene (19c ¼ 106b) and bis(2,4,6-trichlorophenyl)carbene (106d).145 Although those carbenes were not stable enough to be isolated, they exhibited unusual chemical properties. Thus, in solution at room temperature, these carbenes did not react with the parent diazo compound to give azine but dimerized instead to give tetrakis(aryl)ethylene (107) in 70–80% yield. Dimesitylcarbene decayed by intramolecular attack at a o-methyl group to form benzocyclobutene (110), a reaction that is not observed for 2-methyl- diphenylcarbenes under similar conditions (Scheme 9.35).

X = Me

2

+

Scheme 9.35

The formation of olefinic dimerization products as the main product is rare in the decomposition of diazo compounds, whereas formation of ketazine is virtually omnipresent. The authors explained these data by assuming that the hindered diarylcarbenes do not have accessible singlet counterparts, because the singlet would require a smaller carbene angle and incur severe aryl–aryl repulsion. As a result of severe steric hindrance and consequent resistance to external attack by solvent, the

hindered triplet diarylcarbene concentration builds up to the point at which dimerization occurs.

These considerations clearly suggest that kinetic stabilization is a far better way to stabilize the triplet states of carbenes than thermodynamic stabilization. It is also important to note that thermodynamic stabilization usually results in the perturbation of electronic integrity of the reactive center, as has been seen in the case of phosphinocarbene (111)144a and imidazol-2-ylidene (112).144b On the other hand, kinetic stabilization affects the original electronic nature only slightly. According to the definition, a carbene is a molecule in which the two unpaired electrons can be localized on one carbon center. Therefore, if the unpaired electrons are excessively delocalized, one may wonder whether one should still call the species a carbene.146 In this regard, a carbene stabilized by steric protection can be regarded as a ‘‘real’’ stable carbene.

Triplet diphenylcarbene is chosen as a prototypical triplet system as it has been the most extensively studied of diarylcarbenes and abundant basic data are available. Alkyl, halo and fluoroalkyl groups have been used as kinetic protectors for 3DPCs (Scheme 9.36).

9.1. Triplet Diphenylcarbenes Protected by Alkyl Groups

The tert-butyl group is known as one of the most effective kinetic protectors and many reactive molecules have been stabilized and even isolated by using this group. For example, the divalent center of silylenes and germylenes, the heavy atom analogues of carbenes, have been shown to be blocked by tert-butyl groups.

Photolysis of [2,4,6-tris(tert-butylphenyl]phenyldiazomethane (105a) results in the formation of 3-phenylindane (113) almost exclusively. This product is most probably produced from the photolytically generated carbene (106a) through insertion into the C H bonds of the o-tert-butyl groups (Scheme 9.36). The LFP of 105a in degassed benzene in the presence of benzophenone as a triplet energy sensitizer produced transient absorption bands (340 nm) ascribable to the triplet carbene (3106a), which followed a first-order decay(ki ¼ 7:97 103 s 1 at 20 C). This observation means that 3106a decays mainly by abstracting hydrogen intramolecularly from the methyl group of an o-tert-butyl group to form the indane (113). The lifetime was estimated to be 125 ms, which is only 60 times longer than that observed for ‘‘parent’’ DPC (t ¼ 2 ms) under the identical conditions.147

The methyl group is the smallest alkyl group and is generally not considered as an effective kinetic protector. However, Zimmerman and Paskovich145 showed that decay pathways of dimesitylcarbene are suppressed both interand intramolecularly by nearby methyl groups (Scheme 9.35).

The LFP of the precursor diazomethane (105b) in degassed benzene produced a transient band (330 nm) easily ascribable to the triplet carbene (3106b). This band disappeared more slowly than those of tert-butylated 106a. The decay kinetics of the transients indicate that the absorption at 330-nm decays within 1 s to generate a new species with a maximum 370 nm, which is too long lived to be monitored by the LFP system. The decay of the initial bands is kinetically correlated with the growth of the secondary species. Product analysis of the spent solution showed the presence of 107b and 110 (Scheme 9.35). The initially formed transient with a maximum at 330 nm is assigned to the carbene and the second to o-quinodimethane (114). The latter species is probably formed as a result of the 1,4-H shift from the o-methyl to the carbene center of 3106b and undergoes cyclization leading to benzocyclobutene (110, Scheme 9.35). The decay rate (kd) of the carbene was determined to be 1:1 10 s 1, while the growth rate (ki) was 1.5 s 1. From the decay curve, a half-life (t1=2) of 3106b was estimated to be160 ms, while the lifetime (t) based on ki was 666 ms.148,149 This lifetime is roughly more than three orders of magnitude greater than that of DPCs protected by tert-butyl groups.

Those studies suggest that acyclic alkyl groups encounter a limitation when used as protecting groups for triplet carbenes because, as they are brought closer to the carbene center in order to shield it from the external regents, they are easily trapped by the reactive center that they are expected to protect. In this light, it is crucial to develop a protecting group that is sterically congesting but unreactive toward triplet carbenes.

Bicycloalkyl groups are regarded as an attractive protector because bridgehead C H bonds are relatively unsusceptible to hydrogen abstraction,150 and yet the bridging chains will act as protectors. Thus, bis[octahydro-1,4,:5,8-di(ethano)- anthryl]carbene (106c)151 (Scheme 9.36) showed a significantly smaller E=D value (0.0265) than that observed for ‘‘parent’’ DPC (E=D ¼ 0:0464) or 106b, an openchain ‘‘counterpart’’ of 106c (Table 9.14). This observation suggests that the central angle of 106c is significantly expanded because of the steric influence of the bicyclohexyl groups. The LFP studies have shown that triplet 106c exhibited a transient band at 320–330 nm and a weak broad band at 480 nm. The transient absorption bands decayed in second-order kinetics (2k=el ¼ 8:4 s 1), but no new absorption was formed (Table 9.14). The approximate half-life of 339 is estimated to be 1.5 s, which is to be compared with that of 3106b (t1=2 ¼ 0:16 s). Thus, the formation of all-hydrocarbon triplet carbenes having a half-life over a second under normal conditions was realized for the first time.

Bicyclohexyl groups act as an ideal kinetic protector of triplet carbene not only by quenching the intramolecular hydrogen-donating process but also by inhibiting dimerization of the carbene center.

444 TRIPLET CARBENES

TABLE 9.14. Zero-Field Splitting Parametersa and Kinetic Parametersb for Persistent Triplet Diarylcarbenes (106)

aIn MTHF at 77 K.

bIn benzene at room temperature.

cIndicated by half-life (t1=2) or lifetime (t) depending on the mode of decay.

Note here that the reactivity (kO2 or kCHD) of 106c toward typical triplet quenchers (O2 or CHD) is somewhat higher than that of 106b (Table 9.14). Comparison of the optimized geometries of 106c and 106b obtained by the AM1/UHF method reveals that there is slightly more space around the carbene center in 106c than there is in 106b. This observation suggests that 106c is subject to attack of a small particle such as hydrogen more easily than 106b.

9.2. Triplet Diphenylcarbenes Protected by Halogens

Alkyl groups are attractive kinetic protectors for triplet carbenes. However, they are potentially reactive toward triplet carbenes and, hence, will not be able to shield the reactive center completely. In this respect, we need to explore protecting groups that are almost completely unreactive toward triplet carbenes. Halogens are generally reactive toward the singlet state of carbene but are not reactive with the triplet.

The enormous effect of perchlorophenyl groups on the stability of arylmethyl radicals has been well documented by a series of reports by Ballester.152 Thus, perchlorotriphenylmethyl has been shown to have a half-life on the order of 100 years in solution at room temperature in contact with air. It has therefore been termed an inert free radical. Even perchlorodiphenyl(chloro)methyl has been shown to be stable.153 The perchlorophenyl group may be expected to exert a similar stabilizing effect on triplet DPCs.

The stability of bis(2,4,6-trichlorophenyl)carbene145,154 generated by Zimmerman in 1964, was measured by using LFP. The transient absorption due to 3106d, however, was found to disappear disappointingly quickly (within 100 ms). The decay was found to be second order, in accordance with the product analysis