Laser flash photolysis triplet carbenes

Platz, M.S. Maloney, V.M. Laser Flash Photolysis Studies of Triplet Carbenes. In Kinetics and Spectroscopy of Carbenes and Biradicals, M.S. Platz, Ed. Plenum New York, 1990 ... 

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. 

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... 

The potential of laser flash photolysis in the study of carbene reactions with heteroatoms has come to be recognized in recent years. A number of kinetic studies using this technique have been carried out with carbene precursors in nitrile solvents.122-127 An absorption band at 470 nm was observed in the laser flash photolysis of diazofluorene (246) in inert solvents. This band was assigned to triplet fluorenylidene (247). In acetonitrile, however, a second band was also detected at 400 nm and whose buildup is concurrent with the decay at 470 nm.122 Laser flash experiments in other nitrile solvents (i.e., benzonitrile and pivalonitrile) also produced a transient absorption band which is very similar to that observed in acetonitrile. The band at 400 nm was assigned to an intermediate nitrile ylide (248). This absorption could be quenched on addition of an electron-deficient olefin providing good support for its... 

Reviews on laser flash photolysis studies of arylhalocarbenes and triplet carbenes and on the solution photochemistry of carbenes and biradioals have been published. 

Simon and Peter (1984), and Griller et al. (1984) have reviewed laser flash photolysis of carbene formations. Absolute rates of singlet-triplet interconversions /tst and have been summarized for carbenes by Eisenthal et al. (1985) and by Schuster (1986). 

We add to these investigations a recent study of Tomioka s group (Hirai et al., 1994), in which reaction of O2 was used for trapping a triplet carbene and where it was possible to observe the triplet carbene at ambient temperature by laser flash photolysis. (2,4,6-Tri( err-butyl)phenyl)phenyldiazomethane (8.51) afforded photo-lytically 4,6-di( er butyl)-l,l-dimethyl-3-phenylindan (8.53) almost quantitatively. It is an insertion of the intermediate carbene into a CH bond of the tert-hMiy group at C(2). No solvent adducts were found in benzene, cyclohexane or methanol. As... 

Laser flash photolysis has, as usual, illuminated the problem. Jones and Rettig photodecomposed 9-diazofluorene (188) in hexafluorobenzene and cw-4-methyl-2-pentene mixtures, and showed that the degree of stereoselectivity in the cyclopropane products depended on the concentration of the alkene. Laser flash photolysis showed that the first detectable intermediate in the photolysis reaction is the triplet carbene, and suggests that the product studies are consistent with initial formation of a singlet fluorenylidene which has an extremely short life (less than 5 ns) before forming the triplet. The singlet can be trapped only by high alkene concentrations while the more stable triplet is easily trapped. 

At that time, it was not possible to measure any of the rate constants of Scheme 1 directly but in some cases it was possible to measure ratios of rate constants or to determine if spin equilibration was much faster or slower than intermolecular reactions. Organic chemists could then only dream of determining the absolute rate constants of Scheme 1. This would become possible around 1980 with the invention of laser flash photolysis with nanosecond (ns) time resolution. But successful application of this tool would require knowledge of the electronic spectra of singlet and triplet carbenes. Low temperature spectroscopy was enormously helpful in this regard. 

The situation with 1-naphthylcarbene is by no means unique. The invisibility of singlet and triplet non-aromatic carbenes makes studying their kinetics even more challenging. In fact, in our opinion the most interesting carbenes lack useful chromophores for laser flash photolysis studies. 

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. 

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. 

---