Rate constants triplet carbenes

Triplet DPC is readily detectable at 315 nm and is quenched by TEMPOs. The bimolecular rate constant (feq) for carbene scavenging is determined by plotting the pseudo-first-order rate constant for carbene decay as a function of substrate concentration. The values of feq (2-3 x 10 s ) are practically the same for the reac-... 

The transient triplet carbene formed from irradiation of DABA in benzene can be observed to react with styrene. For this process laser spectroscopy reveals a bimolecular rate constant (ksty) equal to 1.2 x 107M-1s-1. The product of the reaction is the expected cyclopropane. This observation clearly supports the spectral assignment of the carbene made above. When deuteriated a-methylstyrene is substituted for styrene as a trap for the carbene, the cyclopropane that results is a 1 1 mixture of stereoisomers, (20) (Table 6). This finding indicates that BA is reacting in this sequence exclusively from its ground triplet state. 

The rate constants kTS and kST define an equilibrium constant (ATeq) connecting the singlet and triplet carbenes. An estimate of Ktq, and hence AGSX, for BA can be obtained from the experiments described above. The time resolved spectroscopic measurements indicate that BA reacts with isopropyl alcohol with a rate constant some five times slower than the diffusion limit (Table 7). This, in conjunction with the picosecond timescale measurements, gives a value for ksr. The absence of ether formation from the sensitized irradiation, when combined with the measured rate constant for reaction of 3BA with isopropyl alcohol, gives an upper limit for k-. These values give Keq and thus AGST 2 5.2 kcal mol-1 (Table 8). 

The species identified as XA reacts with styrene to give the expected cyclopropane. The rate constant for this reaction is ca 200 times less than the corresponding rate constant for 3BA (Table 6). Also, use of the deuterium-labeled a-methylstyrene reveals that the cyclopropanation occurs with essentially total retention of stereochemistry. Moreover, precisely the same result is obtained when this carbene is formed by triplet sensitization rather than by direct irradiation. These findings also point to a reaction originating from a singlet carbene. 

Laser flash photolysis of phenylchlorodiazirine was used to measure the absolute rate constants for intermolecular insertion of phenylchlorocarbene into CH bonds of a variety of co-reactants. Selective stabilization of the carbene ground state by r-complexation to benzene was proposed to explain the slower insertions observed in this solvent in comparison with those in pentane. Insertion into the secondary CH bond of cyclohexane showed a primary kinetic isotope effect k ikY) of 3.8. l-Hydroxymethyl-9-fluorenylidene (79), generated by photolysis of the corresponding diazo compound, gave aldehyde (80) in benzene or acetonitrile via intramolecular H-transfer. In methanol, the major product was the ether, formed by insertion of the carbene into the MeO-H bond, and the aldehyde (80) was formed in minor amounts through H-transfer from the triplet carbene to give a triplet diradical which can relax to the enol. 

Of course, the rate constants (ks and fex) are dependent on the substrates and, hence, the above criteria should be taken only as a general guide. For example, if one chooses a quencher that efficiently reacts with the triplet state, such as O2, carbenes with triplet ground states react efficiently with the quencher to give products such as the corresponding ketones, regardless of AEst values (see Section 6). 

When monitoring the transient due to triplet carbenes is difficult because of the inherent weak nature of the bands and/or severe overlapping with the absorption bands of the parent diazo compounds, it is more convenient to follow the dynamics of the triplet carbene by measuring the rate of the products formed by reaction of triplet carbenes with quenchers such as radicals (Section 5.3) and carbonyl oxides (Section 6.5). In this case, note that the observed rate constant (feobs) of a triplet carbene reaction is the sum of the decay rate constants of the triplet. These may include decay via an associated but invisible singlet with which the triplet is in rapid equilibrium. Thus in general. 

EXPERIMENTAL ESTIMATION OE S-T GAPS 397 TABLE 9.5. Rate Constants for Reactions of Triplet Carbenes with Methanol and AGsj... 

The observed rate constant (fcobs) of a triplet carbene reaction is the sum of all decay rate constants of the triplet. These may include decay via an associated but... 

The monitoring techniques are used to obtain the absolute rate constants of DPC with other substrates (Q) known to react with triplet carbenes. The experiments are carried out at several reagent concentrations and the experimental pseudo-first-order rate constant, k bs, is plotted against the substrate concentration. It can be shown that feobs is expressed by Eq. 22... 

TABLE 9.9a. Rate Constants for the Reaction of Triplet Carbenes with Hydrogen Donor Snbstrates... 

It is not easy to explain why the triplet reactions that are energetically much less favored than those of the singlets become dominant at low temperature. Based on Ea and log A measured for triplet carbene abstraction (see Section 5.3), one can estimate the rate constant at 77 K to be <10 M s, suggesting that triplet carbene reactions in matrices at 77 K should not occur. Obviously, reactions of carbenes within matrices are controlled by factors that are not operating in solution phase, as one might expect from dramatic changes in reaction conditions. 

Absolute rate constants for the representative triplet carbene-Oa reaction in solution at room temperature are listed in Table 9.12. The second-order rate constants are close to 10 M s, near the diffusion-controlled rate. Thus, the reaction with O2 is taken as evidence for the presence of the triplet state of the carbene. 

It is interesting to note here that the value of the rate constant for DPC (14a), 1-NC (a-12), methoxycarbonylphenylcarbene (53), and FL (23) with O2 are 5 X 10 , (3.5 0.7) X 10 , 8.6 x 10 and (1.4 0.2) x 10 AT s-, respectively. The difference in the rate constants is not as large as that observed for the hydrogen atom abstraction rate constants for those carbenes (Table 9.9) and do not reflect the difference in the magnitude of AGst- The reason is probably because the rate constant of triplet carbenes is very fast and because the singlet states do not interact with triplet oxygen because of the spin restriction. 

The bimolecular rate constant for the reaction of DPC with butadiene is determined to be 6.5 X 10 M s . Isoprene can be employed as a selective trap for triplet carbenes. Styrene is also shown to be an efficient trap for triplet carbene. (E)-p-Deutero-a-methylstyrene (89) is a very convenient reagent to diagnose the mult-plicity of the reacting carbene because it reacts with both singlet and triplet carbenes with different stereochemical outcomes. The stereochemistry of the adduct cyclopropane (90) can be easily judged by NMR (Scheme 9.28). For example, BA (22) reacts with styrene with total loss of stereochemistry, while in the reaction with dimethoxy FL (23a), the expected cyclopropane is obtained with complete retention of stereochemistry. The rate constants for the additions are (1.2 0.2) x 10 and... 

Fluorescence quenching studies can establish the rate constant at which a certain substrate interacts with the excited carbene, but they cannot provide any independent mechanistic information. Absorption studies are somewhat more informative in that the primary product of reaction can sometimes be detected directly. In the reaction of di-p-tolylcarbene with CCI4, the radical, (MeC6H4)2CCl, obviously formed as a result of abstraction of Cl atom from the substrate, is detected. Its formation can be monitored to give a rate constant of 1.1 x 10 M s for the excited state, which should be compared with a rate of 2 x 10 M s for ground-state triplet DPC. ... 

The rather short lifetime (a few nanoseconds) of the triplet excited carbene makes extensive studies of intermolecular processes difficult. However, the excited-state lifetime (60 ns) of triplet dimesitylcarbene (19c) is exceptionally large, probably because of decreased efficiency of both intermolecular and intramolecular deactivation pathways. Intermolecular rate constants for the reaction with CCI4, O2 and 1,4-cyclohexadiene have been determined. 

The rate constants of 1061 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 orders of magnitude as two o-bromine groups in 106g are replaced with two CF3 groups. 

Assuming that singlet nitrene reacts with alkanes at near diffusion controlled rates allowed deduction of a rate constant of singlet-to-triplet nitrene intersystem crossing (ISC) of 2-8 X 10 s . This ISC rate is slower than in carbenes, but significantly faster than with arylnitrenes, which are discussed in a subsequent section. 

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