Laser flash photolysis singlet carbenes

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). 

Carbon tunneling in a second singlet chlorocarbene has also been proposed. It has proved impossible to observe noradamantylcarbene 73 spectroscopically, either by solution laser flash photolysis or with matrix isolation at low temperatures. It has been suggested that the carbene rearranges too rapidly, possibly via carbon tunneling, to adamantene (74). 

A number of minima corresponding to oxonium ylides and H-bonded structures were found on the potential-energy surface for reaction of singlet carbenes with water and alcohols." Laser flash photolysis revealed that the rates of reaction between cyclopentadienylidene or fluorenylidene and alcohols increased with alcohol acidity and had linear Bronsted plots with slopes of 0.061 and 0.082, respectively.100 These results point to protonation with a very early transition state or to concerted OH insertion. For tetrachlorocyclopentadienylidene, the results showed that ylide formation (100) is predominant. 

Similarly, pyridine traps both carbenes 55 and 56 which are effectively generated under laser flash photolysis from precursors 53 and 54, respectively. Carbene 56 was found to have greater bimolecular reactivity than analogue 55. Since singlet carbene 55 is nonplanar, the filled hybrid orbital of the carbene can now interact with the 7t -system of the carbonyl. This additional stability can be attributed to the lone pairs of the carbonyl coordinating with the empty p orbital of the carbene (Scheme 9) <2001JA6061, 2002TL7>. 

Condensed-phase carbenes will often react with the surrounding solvent medium, but they can also combine with other solute molecules. Indeed, these intermolecular reactions commonly occur at rates that are limited only by diffusion. Consequently, such carbenes are just short-lived intermediates. Nevertheless, arylhalocarbenes tend to have much longer lifetimes than typical alkylhalocarbenes, e.g., three orders of magnitude longer, because the latter also undergo rapid 1,2-H shift to form al-kenes.168,169 Indeed, the lifetime (t) of singlet carbene 64 (2max = 310 nm)170 in 2,2,4-trimethylpentane (isooctane) is reported to be ca. 3.6 ps, as determined via laser flash photolysis (LFP) of diazirine 63.171... 

Scaiano et al. (1989) characterized the parent benzoquinone oxide, various derivatives thereof being obtained from the corresponding cyclic diazo ketones as well as from 9-diazofluorene, and from diazodiphenylmethane by laser flash photolysis at room temperature. The parent benzoquinone oxide has an absorption maximum at 410 nm. The rate constants of 2-diazo-l,2-benzoquinone and of the carbene of this diazo compound with singlet oxygen were found to be 1.0 x 10 m s and... 

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. 

In general, non aromatic ground state singlet carbenes do not have useful chromophores for laser flash photolysis studies. 

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. 

Photodissociation of a diazo compound produced a carbene for the initial intermediate species. The lifetime of the formyl carbene produced by the laser irradiation of formyldiazomethane was investigated [111] (Scheme 3). Two different rise parts [fast and slow (900 ps)] were observed in the TG signal and the slower component is assigned to the lifetime of the singlet formylcarbene. In the presence of methanol, the singlet carbene reacts with methanol to produce the inserted product. The rate constant of the insertion reaction was determined as k = 4.3 x 109 M 1 s. The observed rate constant was consistent with that obtained from a flash-photolysis experiment. 

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