Room temperature solutions, triplet carbenes

Since most of the carbenes 1 have triplet ground states, ESR spectroscopy allows to see the unpaired electrons and determine the local symmetry at the carbene center and the amount of spin delocalization.13-18 Most of the ESR spectra of carbenes reported in the literature have been recorded in organic glasses or powder samples at temperatures between 4 and 77 K. Many carbenes are slightly colored and exhibit characteristic absorptions extending to the visible region of the spectrum. UV/vis spectroscopy not only provides information on the excited states of carbenes, which in many cases are the reactive species during precursor photolyses, but also links low temperature spectroscopy to LFP in solution at room temperature. 

Moritani 81,82) showed that the absorption spectra of dibenzo[a, djcyclohep-tatrienylidene and 10, 11-dihydro dibenzo[a, djcycloheptadienylidene can be recorded both at 77 °K and at room temperature. This proves that the same species is observed in the matrix and in solution, and is the triplet carbene. The lifetime of this carbene at room temperature was determined to be about 10 sec. 

C in THF, which cleanly led to the formation of (amino)(aryl)carbene Xllla (Scheme 8.8). ° A NMR signal at 8 = 314.2 ppm leaves no doubt of the formation of Xllla. Carbene Xllla is stable for days in solution at —50 °C but undergoes a C—H insertion reaction at room temperature within a few hours, giving rise to the 4,6-di-ferf-butyl-l,l-dimethyl-3-(methyl-rcrt-butylamino)mdane as the major product. It is interesting to note that this reaction, typical of transient singlet and triplet carbenes, has never been observed for diamino carbenes. This striking difference demonstrates the less perturbed character of carbene Xllla. 

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. 

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 DPC is then monitored with a streak camera. The fluorescence... 

Bis(2,4,6-tribromophenyl)carbene (106f) 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 Ik/el = 8.9 s ). The parameter fj/2, is estimated to be Is (Table 9.14). 

Tomioka H, Watanabe T, Hirai K et al. (1995) 2,2. 4,4, 6,6 -Hexabromodiphenylcarbene. The first stable triplet carbene in fluid solution at low temperature and in the crystal state at room temperature. J Am Chem Soc 1995 117 6376-6377... 

Itoh, T., Nakata, Y., Hirai, K., Tomioka, H., Triplet Diphenylcarbenes Protected by Trifluoromethyl and Bromine Groups. A Triplet Carbene Surviving a Day in Solution at Room Temperature, J. Am. Chem. Soc. 2006,128, 957 967. 

Whereas carbenes of type 17 are singlets the hexabromide 20 is a stable triplet both in solution and in the crystalline state as shown by ESR spectroscopy. Prepared by photolysis of a diazomethane precursor, crystalline 20 is stable for months at room temperature. [16]... 

In order to estimate the stability of triplet carbenes (19) under ambient conditions, laser flash photolysis ( LFP) [26] was carried out on the precursor diazomethanes (18) in solution at room temperature. The transient absorption bands formed upon the flash were recorded by a multi-channel detector. These bands were assigned to the triplet carbenes (19) by comparison with those obtained in matrix at low temperature. The kinetic information was then available by monitoring the decay of the transient absorption with oscillographic tracer. When triplet carbenes decayed unimolecularly, which is often so, lifetime (x) can be determined. However when the decay did not follow a single exponential, which is sometimes the case, x cannot be determined. In this case, a half-life (ti/2) is estimated from the decay curve as a rough measure of the stability. 

ESR parameters for triplet carbenes and nitrenes have been summarized, and it has been shown that phenylnitrene is produced predominantly (87-88%) in the singlet state by direct photolysis of phenyl azide in low-temperature matrices. The first spectroscopic observation of a singlet nitrene has been reported nanosecond-laser photolysis of 1-azidopyrene gives the So nitrene (A ax 450 nm) which has a lifetime of 22 nsec at room temperature (in benzene) and 34 nsec at 77 K in rigid solution. At room temperature it decays to the triplet ground state (Tj, A ax 415 nm) with a rate constant of about 4.4 x 10 sec T, is formed directly by biacetyl sensitized photolysis of the azide. The lifetime of the excited triplet (Tj) was about 7 nsec. Tj dimerizes to azopyrene. ... 

Hyperconjugation can stabilize triplet carbenes, but to a lesser extent than singlet carbenes, by donating C —C a or H electron density to a half-filled orbital of the carbene center. It is more difficult to stabilize triplet carbenes than singlet carbenes with bulky alkyl substituents, because the radical character of triplet carbenes makes them much more reactive with C-H bonds. Carbon-fluorine bonds are less reactive toward radicals, however, and the triplet carbene 37 was sufficiently stable to survive for nearly a day in solution at room temperature. ... 

A two-electron oxidation of allenes (1) (A = S, NMe) has been found to yield carbenes (2) (Scheme 1) dimerization and reaction with (2,2,6,6-tetraamethyl-piperidin-l-yl)oxyl (TEMPO) support a triplet ground state as predicted by computations. Triplet diphenylcarbenes bearing bulky substituents at the para positions have been generated and studied in rigid matrixes at low temperatures by electron spin resonance (ESR) and UV-Vis spectroscopy as well as by laser flash photolysis (LFP) in solution at room temperature. Their reactivity upon LFP was shown to be dominated by dimerization unless triplet carbene quenchers (such as oxygen and cyclohexadiene) were present. 

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