Triplet carbenes diphenylcarbenes

It is evident from Table 11 that the rate of addition of the triplet bis(methoxy-carbonyl) carbene is somewhat slower than that of the singlet. Another important general rule may also be deduced from Table 11 the triplet carbene adds to dienes about 3—4 times faster than to olefins the reactivity ratio of 2,3-dimethyl-butadiene-1,3/pentene-l is 9.6 for triplet and 2.8 for the singlet. This ratio may be compared with that for diphenylcarbene (1,3-butadiene/hexene-l), which is > 100. 

However, be aware that the ratio of E/D is not always a reliable guide to the structure of triplet carbenes. For example, diphenylcarbene and fluorenylidene have very different geometries but almost exactly the same D and E values.If one really wants to learn about the geometry, one must label the carbene carbon with and measure the hyperfine coupling constants. 

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... 

Triplet diphenylcarbene has a Tl/2 of 2 ps in the presence of 1.0 M isoprene (Eisenthal et al., 1980). The lifetime of singlet phenylchlorocar-bene ranges from 5 to 500 ns in the presence of 1.0 M alkenes with various substituents (Turroetal., 1980). In 1.0M methanol singlet fluorenylidene and singlet diphenylcarbene have half-lives of 0.77 ns and 0.02 ns respectively (Zupancic and Schuster, 1980 Eisenthal et al., 1980). Triplet carbenes react very rapidly with 02. 

Diphenylcarbene (diphenylmethylene) can be generated from diphenyldiazomethane (373) by direct irradiation or by triplet sensitization.1156 The intermediate multiplicity then controls the subsequent reactions the singlet carbene inserts into the O—H bond of methanol, whereas the triplet carbene adds to an a I Irene (Scheme 6.173). It has been found that singlet and triplet diphenylcarbenes are in rapid equilibrium relative to the rates of reactions.1157,1158 Competitive quenching experiments (to obtain k1 and kTS) and laser flash spectroscopy (Section 3.7 to obtain k2 and kST) allowed the determination of the free energy difference between the singlet and triplet states of carbene ( 20kJ mol J). 

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. 

Using laser techniques, the rate of decomposition of the triplet carbene in thefluorenylidene system has been shown to be 3.6 x 10 s at room temperature This triplet state is very much more reactive than that of diphenylcarbene it has been suggested that the latter may be stabilized by the ability of the phenyl rings to rotate, so that the unpaired electrons that are in orthogonal orbitals may be more efficiently delocalized. 

Our work with diphenylcarbene had some technical issues due to overlapping absorptions but was nevertheless straightforward, as the matrix spectrum of the triplet carbene was available and the spectrum of benzhydryl radical was... 

We should also note in passing that triplet carbenes also seem to react with substituted styrenes with ambiphilic selectivity. Thus, both triplet 9-xanthylidene, 16, [53] and diphenylcarbene, 17, [54] add to X-C( H4CH=CH2 (X = p-MeO, p-M.e, H, p-C, p-Br, and w-Br), and afford broken Hammett patterns with k i minima at X = H (a = 0.0). The ambiphilicity of these triplet species is distinct from that of the singlet carbenes, MeOCCl or PhOCF, discussed above. The philicity of the latter can be understood by molecular orbital... 

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. 

Interest in carbenes in rigid media can be dated to the 1960s when Murray et al. reported diphenylcarbene as the first organic species with a triplet ground state. ... 

Rates and Activation Parameters. The first condensed-phase absolute rate measurement for a carbene-alkene addition was reported by Closs and Rabinow in 1976 flash lamp photolysis of diphenyldiazomethane generated (triplet) diphenylcarbene, which added to butadiene (in benzene) with k =... 

The reaction of triplet diphenylcarbene with pyridine has been well studied, and a mechanism proposed from kinetic data (90TL953). The carbenes generated from laser flash photolysis of alkylbromo- and alkylfluoro-diazirines were trapped by pyridine to form the pyridinium ylides... 

Comparison of the chemical reactions of various carbenes with those of methylene itself is also very interesting. Both diphenylcarbene52 and propargylene,52 which possess triplet configurations in their ground states, add nonstereospecifically to olefins. Moreover, diphenylcarbene reacts with oxygen54 to form an excited state of benzophenone.50... 

Carbenes react with la at comparable rates, and rate constants of 6.7 0.8 x 10 M s" and 6.7 x 10 M" s have been obtained for the reactions of diphenylcarbene and chlorophenylcarbene, respectively. Both series of experiments show a trend of more facile reaction of la with more electropositive centers. The triplet 1,4-diradical 22a (X = Ph max - nm) formed by the addition of diphenylcarbene to [l.l.ljpropellane,... 

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