Reactive Intermediate Chemistry

for classical hydrogen atom transfer. Thus, QMT is proposed to be the mechanism for the matrix hydrogen atom transfer. The observation that the carbenes are completely stable in CCl4 matrices at 77–100 K suggests that the light hydrogen atom can tunnel, but the more massive Cl atom cannot.90

The members of the radical pairs generated by the hydrogen atom abstraction of the triplet carbenes usually diffuse apart in fluid solution. Product mixtures consisting of radical dimers, double hydrogen abstraction products and so on are formed (Scheme 9.6). In a rigid matrix, however, the members of the pairs are not able to diffuse apart (owing to the limited diffusibility within the matrix) and therefore recombine with high efficiency to give the C H ‘‘insertion’’ products upon thawing the matrix.

A hydrogen atom transfer reaction in which the reaction mechanism changes from a completely classical process in a soft warm glass to a completely quantum mechanical tunneling process in a cold hard glass has been more evidently demonstrated by using a technique with greater time resolution than a conventional EPR method: LFP. Thus, LFP of diazofluorene in a number of glasses generates singlet FL(23), which undergoes intersystem crossing to triplet 23.91

In contrast to the EPR studies, exponential decays of the triplet 23 are observed. The Arrhenius treatment of the data obtained in hydrogen donating glasses shows that there are two regions; a steep region at high temperatures and a flat, temperature-insensitive region with very low-activation parameters. Upon extrapolation of the high-temperature data to the low-temperature regime, one finds that the observed rate is hundreds of times faster than predicted. No such break in the Arrhenius plot is observed for the decays of 23 in perhalogenated solvents, which have no abstractable hydrogens.92 Thus, these broken Arrehnius plots observed in hydrogen donating solvents are best explicable in terms of a change in the reaction mechanism.

5.4.3. Why Can the Triplet Find a Tunneling Pathway? The exact reason why triplets can find a tunneling pathway, whereas singlet carbenes cannot is not clear. However, recent investigations on the low-temperature hydrogenation of carbenes provide some clues concerning this issue. It has been shown that a carbene with a triplet ground state, for example, phenyltrifluoromethylcarbene (65) undergoes hydrogenation when generated at 10 K in 2% H2/Ar matrix, followed by warming to 30 K in the dark to give the corresponding reduction product (66, Scheme 9.15). On the other hand, a carbene with singlet ground state, for example, phenyl(chloro)carbene (67) does not react at all upon annealing in an H2-doped Ar matrix even at 35 K (Scheme 9.15). Moreover, 65 is completely unreactive with D2 under comparable conditions, suggesting the presence of a large kinetic deuterium isotope effect. The results are interpreted as indicating that the triplet reacts through hydrogen-tunneling abstraction followed by recombination of the resultant radical pair to give the reduction product, whereas the singlet, requiring concerted addition, does not undergo tunneling reaction under cryogenic conditions.93

Energetics of carbenes and hydrogen reactions have been calculated at the B3LYP/6-31G** level of theory. They indicate that all the H2 additions are

418 TRIPLET CARBENES

Scheme 9.15

be very exothermic ( 80 90 kcal/mol), including those of chlorophenylcarbene (67). Thus overall exothermicity does not appear to be the reason for the lack of reactivity of the singlet carbene. Concerted addition of H2 to 67 is calculated to have a barrier of 12.4 kcal/mol, while hydrogen abstractions from H2 by triplet carbenes (65) are predicted to have significantly lower energy barriers (5.7 kcal/mol). Based on these calculations, it is suggested that the direct insertion of singlet 67 via quantum mechanical tunneling is less facile than stepwise reaction of the triplet carbenes 65 with H2, either because of a higher classical barrier or because of the lower probability of QMT involving two hydrogens simultaneously. However, much more reactive singlet carbenes, such as difluorovinylidene (F2C C:) appear to surmount the restrictions on concerted additions even at very low temperatures.94

On the other hand, it has been shown that even at very low temperature highly electrophilic triplet carbenes undergo thermal H2 additions in H2–Ar matrix as well as C H insertion into CH4. For example, triplet tetrafluoro-4-oxocyclohexa-2,5- dienylidene (69a) generated in an H2-doped Ar matrix reacts with H2 even at 35 K to give tetrafluoro-4-oxocyclohexa-2,5-dienone (70a). It also reacts with CH4 at 40 K to give a formal C H insertion product (71a). Interestingly, no deuterium isotope effects are observed in this case; the same reaction takes place at qualitatively the same rate in D2 and CD4. The reactivity decreases on going from tetrachloro (69b) to tetrahydro derivatives (69c). The tetrahydro derivative does not react at all under the same conditions (Scheme 9.16). DFT calculations (B3LYP/6-31G(d)) predict that the reaction of carbene (69) with H2 to give cyclohexadienenone (70) is highly exothermic (91.4 and 73.8 kcal/mol for the fluoro and chloro derivatives, respectively) but do not reveal a transition state for the reaction. These calculations suggest a thermal reaction with an extremely small or absent barrier. This unusual reaction is explained in terms of the philicity of triplet carbene.95 It has been proposed that the electron affinities (EA) can be used as a measure of the carbene philicity. The EA values were calculated [B3LYP/6- 311 þþ G(d,p)] for fluoro (69a), chloro (69b), and hydro (69c) derivatives are 3.32, 3.08, and 2.05 eV, respectively.96 The EAs for phenyl chlorocarbene (singlet, 167) and diphenylcarbenes (314) are 1.52 and 1.48 eV, respectively. These are well below the electrophilicity of the hydro derivative (69c).

420 TRIPLET CARBENES

1150 min. These observations implicate a QMT mechanism for alkene formation.97

A similar QMT mechanism is proposed in a thermal 1,4-H shift of mesitylcarbene at 11 K.98

In contrast, 1-phenylethylidene (74), which can undergo 1,2-hydrogen migration to form styrene (75), was found to be thermally stable in argon or xenon matrix at 10 K. The carbene decay to give styrene only when warming to 65 K in xenon matrix (Scheme 9.18). From the disappearance rate constant for 74, a energy barrier

Scheme 9.18

of 4.7 kcal/mol (at 65 K) was estimated. Thus, the thermal rearrangement of triplet

1-phenylethylidene to styrene likely occurs upon thermal population of the S1 state of 74 at 65 K.97

A 1,2-hydrogen shift in benzylchlorocarbene (76), which has a singlet ground state, forms b-chlorostyrene (77). This reaction has been found to involve a QMT. Thus, the carbene was found to decay thermally at temperature as low as 10 K. In contrast, deuterated carbene (76-d2) was stable at higher temperatures (up to 42 K) (Scheme 9.18). Arrhenius plots of the decay kinetics were obtained by following the reaction between 10 and 34.5 K in an argon matrix. Again the small temperature dependence of the rate at lower temperature and the curved Arrhenius plot at higher temperature zone were noted. From extrapolation of the data at room temperature, a rate of 10 2 s 1 at 80 K is expected. This value indicates that tunneling is important at low temperature.98

The difference in thermal stability of singlet benzylchlorocarbene (76) compared with that of triplet methylphenylcarbene (74) was noted, and was interpreted as being caused by the different spin states. In solution, a minimum barrier of 4.3 kcal/mol was assessed for the triplet carbene, which is in the range of the value which had been measured for 76 in solution. Tunneling at low temperatures was excluded for the triplet species since it is stable up to 30 K in argon. Intersystem crossing is not necessary in the reaction of the singlet species. Therefore, spin control may play an important role in the triplet molecule, resulting in a minimum activation barrier in the order of magnitude of the singlet–triplet gap.99

Singlet carbenes are also known to undergo intramolecular C H insertion when structurally favored. For example, tert-butylchlorocarbene (78) gives rise to

Scheme 9.19

1,1-dimethyl-2-chlorocyclopropane (79) as a result of insertion of carbene into the C H bond of methyl group (Scheme 9.19). When 78 was generated in a nitrogen matrix at low temperature, it was detected by IR and UV–vis spectroscopy. However, the carbene decay on standing at 11 K in the dark produces the cyclopropane. The insertion rate was insensitive to temperature; warming the matrix from 11 to 30 K caused no abrupt increase in the disappearance of the carbene. The deuterated carbene (78-d9) was found to be thermally stable as the carbene survived up to 45 K. Thus, there is a minimal temperature dependence of the reaction at low temperatures and an unusual isotopic sensitivity of the kinetics all of this evidence suggests that the 1,3-CH insertion reaction of the carbene 78 involves QMT. The formation of C C bond during the reaction also suggests that heavy atom tunneling might be involved.100

5.4.5. Tunneling Reactions at Elevated Temperatures. Thus far, we have seen QMT reactions of carbenes at very low temperature. Many examples of QMT in reactions at elevated temperatures have also been reported. In fact, hightemperature QMT was demonstrated long before the low-temperature examples were discovered.

At cryogenic temperatures, the tunneling reaction proceeds mainly through the zero-point level and is sometimes referred to as deep tunneling. Low-temperature QMT is easily recognizable because deep tunneling is many orders of magnitude faster than the classical process, which is negligible at very low temperature. At elevated temperatures tunneling can occur through vibrational levels close to the top of the barrier. Consequently, the rates of a classical process and of a QMT process at ambient temperature may be comparable. A contribution of QMT to the overall rate of reaction at elevated temperatures can be revealed by measurement of kinetic isotope effects as a function of temperature. Differential activation energies greater than the difference in carbon–hydrogen and carbon–deuterium bond zero point energies [Ea(D)-Ea(H) >1.2 kcal/mol], and unusual ratio of preexponential factors (AH=AD < 0:7) signal a contribution of QMT to the overall rate of a reaction at elevated temperatures.101

In order to determine whether QMT may contribute to the overall reaction of diarylcarbenes with hydrogen atom donors in solution at ambient temperature, kinetic isotope effects for the benzylic hydrogen atom abstractions of the triplet states of several diarylcarbenes with toluene–toluene-d8 in fluid solution were determined over the temperature ranges of 75 to 135 C. The results are very much dependent on the structure of the carbene (Table 9.11).102 The differential

422 TRIPLET CARBENES

TABLE 9.11. Differential Activation Parameters of Triplet Carbene H(D) Atom Transfer

kinetic isotope effects observed for 1-naphthylphenylcarbene (a-51), 2- chlorodiphenylcarbene (14u), and 2-trifluoromethyldiphenylcarbene (14v) were much larger than predicted by complete loss of all zero-point energy in the transition state. This observation indicates that there is a contribution of QMT to the H(D) atom transfer process. For diphenylcarbene (14a), dibenzocycloheptadienylidene (18), and 4-biphenylphenylcarbene (14l), the differential kinetic isotope effects were barely consistent with a completely classical atom-transfer reaction. For fluorenylidene (23) and anthronylidene (52), the data were completely consistent with a purely classical atom-transfer process. The lifetime of the carbenes undergoing QMT are probably longer than those of the other carbenes because of steric hindrance about the carbene carbon. The substituent may also widen the bond angle at the carbene carbon. These effects will decrease the amount of s character in the singly occupied orbitals of the carbene and lower the reactivity still further. Thus, the trends are interpreted as indicating that the relatively slow rate of the classical atom-transfer reaction in the hindered carbenes allows a QMT pathway to contribute more prominently to the overall hydrogen atom-transfer rate.102

A contribution of QMT in solution at room temperature is also observed in 1,2- hydrogen rearrangement of a singlet carbene.103,104

6. REACTIONS WITH OXYGEN

Molecular oxygen is an important participant in reactions of triplet carbene because of its triplet ground state and its ubiquity as an impurity in reaction