Excitation carbenes

The second intermediate s identity has been debated since the mid-1980s. In 1984, Liu and Tomioka suggested that it was a carbene-alkenc complex (CAC).17 Similar complexes had been previously postulated to rationalize the negative activation energies observed in certain carbene-alkene addition reactions.11,30 A second intermediate is not limited to the CAC, however. In fact any other intermediate, in addition to the carbene, will satisfy the kinetic observations i.e., that a correlation of addn/rearr vs. [alkene] is curved, whereas the double reciprocal plot is linear.31 Proposed second intermediates include the CAC,17 an excited carbene,31 a diazo compound,23 or an excited diazirine.22,26 We will consider the last three proposals collectively below as rearrangements in the excited state (RIES). 

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. 

One approach, which has proved revealing in the case of 9-carbena-fluorene and which may be generally applicable, is to examine the influence of additives which can deactivate electronically excited carbenes without undergoing reaction themselves (e.g. hexafluoro-benzene) or which react preferentially with carbenes of a particular spin state (e.g. oxygen and butadiene, which are both thought to react preferentially with triplet carbenes). The technique is analogous to well-... 

Fig. 11.24. Mechanisms of the photochemically initiated and Ag(I)-catalyzed Wolff rearrangements with formation of the ketocarbene E and/or the ketocarbenoid F by dediazotation of the diazoketene D in the presence of catalytic amounts ofAg(I). E and F are converted into G via a [1 2]-shift of the alkyl group R1. N2 and an excited carbene C are formed in the photochemically initiated reaction. The excited carbene usually relaxes to the normal ketocarbene E, and this carbene E continues to react to give G. The ketocarbene C may on occasion isomerize to B via an oxacyclopropene A. The [l,2-]-shift of B also leads to the ketene G.

It seems likely that the first step is the formation of a vibrationally excited carbene,... 

The photolysis of methylisopropyldiazirine yields 2-methylbut-2-ene, 3-methylbut-l-ene, and cis- and irans-l,2-dimethylcyclopropane. The pyrolysis yields the same hydrocarbons, though once again in somewhat different relative yields. Again these differences can be rationalized in terms of a vibrationally excited carbene formed by the photochemical reaction. The results are shown in Table III. 

Some arylcarbenes, prepared by photodecomposition of the corresponding diazo compounds, undergo intersystem crossing so effectively that the alkenes present are mainly attacked by triplet-excited carbenes. This type of reactivity has, in particular, been associated with nitro-substituted arylcarbenes. The reaction between photochemically generated 4-nitrophenyl-carbene and (Z)- and ( )-but-2-ene, which has been studied in particular detail, has shown that nonstereospecific carbene addition may become just as important as the stereospecific reaction. ... 

The view that electronic states of different multiplicity need not be considered cannot easily be ruled out, since both deactivation of vibrationally excited carbenes and intersystem crossing between singlet and triplet states are brought about by collision with other molecules. The difficulty is not restricted to reactions in the gas phase in solution, collisional deactivation and collision-induced intersystem crossing can still be expected to compete with collisions leading to chemical reaction. However, the parallelism between the variation in stereospecificity in the gas-phase addition of methylene to the 2-butenes with the pressure of inert gas (Frey, 1959, I960 Anet et al., 1960 Bader and Generosa,... 

In light of the above results, the involvement of another intermediate is required but the question of its identity remains. A number of intermediates have been proposed, including a carbene-olefin complex (COC), an excited carbene, a diazo compound, an excited-state diazirine, and a biradical. These proposed intermediates are potentially involved in two types of mechanisms the COC mechanism and the rearrangement in the excited-state (RIES) mechanism. 

The remaining proposed intermediates involve rearrangement fi om the excited diazirine. This type of explanation was first put forward by Frey and then more recently by Platz and co-workers. - - - In this mechanism, an intermediate is formed from the excited diazirine that leads directly to the rearrangement products (Scheme 5). This second intermediate is not defined in the kinetic scheme, but it has been proposed to be an excited carbene, a biradical, a diazo compound, or the excited diazirine itself AH of these proposed intermediates are thought to allow for concomitant extrusion of nitrogen and rearrangement... 

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