Rearrangements in the excited states

III. Rearrangements in the Excited State vs. Carbene-Alkene Complexes. .. 57... 

III. REARRANGEMENTS IN THE EXCITED STATE VS. CARBENE-ALKENE COMPLEXES... 

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). 

There is no clear-cut distinction between the photochromic anils and the molecules discussed in Section III since fluorescence can be observed In most compounds. The main difference seems to be the stabilization of an intermediate (produced by proton transfer) by geometrical rearrangement in the excited state, which is possible in the photochromic anils but not in the "fluorescent" compounds. 

With the intent to selectively derivatize CyD oligosaccharides,80 Abelt enlisted nitrogenous carbene precursors as labile guests.81 The involvement of supramolecular carbenes could be inferred based on intramolecular products stemming from 1,2-H shifts. Of course, 3//-diazirines that possess a-C-H bonds, like 3-methyl-3-phenyl-3//-diazirine (6) (Scheme 2), are susceptible to rearrangements in the excited state (RIES) that mimic the results of carbene 1,2-H shifts,82 e.g., 6 —>10 (Scheme 2). 

Recently, diazirine rearrangement in the excited state (RIES) that mimics the result of cyclopropylcarbene fragmentation has been postulated. For examples, see (a) Ref. 128d,e (b) Thamattoor, D.M., Jones Jr., M., Pan, W. and Shevlin, P.B. (1996). Tetrahedron Lett. 37, 8333-8336... 

Rearrangement in the excited state (RIES) of diazirine 45 might be the actual route to enyne 47 because MeOH, which is relatively reactive and in excess, is expected to trap carbene 46 completely... 

This concerns essentially the photoactivated diazirine 56 which may react diieclly by a rearrangement in the excited state (RIES) or ring-open to the labile diazo compound 61. Under slightly acidic conditions, protonation of the carbene to carbenium ion 63 and the intermediacy of diazonium ion 62 should also be considered. 

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. 

Wakahara, T., Niino, Y, Kato, T, Maeda, Y, Akasaka, T., Liu, M.T.H., Kobayashi, K., and Nagase, S., A Nonspectroscopic Method to Determine the Photolytic Decomposition Pathways of 3-Chloro-3alkyldiazirine Carbene, Diazo and Rearrangement in the Excited State, /. Am. Chem. Soc., 124, 9465, 2002. 

Clearly, rearrangements do occur in the excited states of diazirine and diazo carbene precursors. Kinetic studies of carbenic rearrangements need to consider the possible intervention of RIES when absolute rate constants are partitioned between competitive rearrangement pathways on the basis of product distributions.28... 

Formation of specific complexes in the excited states ( exciplexes )f 35 52 85) Exciplexes are complexes not present in the ground state that form due to the extensive redistribution of electron density that occurs upon excitation. Among exciplexes, there may be some whose formation does not require substantial nuclear rearrangements and thus occurs rather rapidly even at 77 K. The formation of exciplexes is accompanied by a spectral shift to longer wavelengths. It is postulated that the fluorescence from tryptophan in proteins in a variety of cases is fluorescence from tryptophan exciplexes)35 85) In studies of the effects of environmental dynamics on the spectra, the exciplexes may be considered as individual fluorophores. 

The different electron distribution in the excited state also may lead to other types of reactions. As an example, alkenes and polyenes display a low intermole-cular reactivity, but undergo extremely fast rearrangements, since the tt bonding character dramatically diminishes in the excited state. Thus, free rotation becomes feasible and, where appropriate, electrocyclic and sigmatropic processes take place (Figure 3.3). 

Di-TT-methane rearrangements are typical examples of reactions that occur in the excited state exclusively. These rearrangements have never been observed in the ground-state chemistry of 1,4-unsaturated compounds. 

We believe that the rearrangements of the di-ir-methane type observed in the DCA-sensitized irradiations of 1-aza- and 2-aza-1,4-dienes are important because the di-ir-methane process has been considered until now a paradigm of reactions that take place in the excited-state manifold only. Our results show that rearrangements of this type can also occur in the ground states of radical-cation intermediates. This opens the possibility of promoting di-ir-methane-type rearrangements by alternative thermal means. 

Steinfatt proposed an alternative mechanism for the formation of excited aminophth-alate, based on the concept of dioxirane-carbene mediated chemiexcitation, which is also attributed to other chemiluminescent systems ° °. After the attack of hydrogen peroxide on the diazaquinone 27 carbonyl carbon, a perhydrolysis step is postulated to result in the intramolecular dioxirane-carbene system (32) in the excited state ° ° . This species presumably rearranges to 3-aminophthalate dianion while still in the singlet-excited state (Scheme 23). Although this is a very interesting mechanistic proposal, it is based on experimental evidence obtained with indirect phthaloyl peroxide chemiluminescence and no further evidence corroborates this proposal. 

For more than 30 years di-7i-methane rearrangements have stood as the paradigm reactions that occur in the excited state manifold exclusively. However, recent studies have shown that rearrangements of the di-7r-methane type can also occur in ground state of radical-cation and radical-anion... 

This reasoning was used by the author in 1961 to rationalize the ubiquitous photochemical cyclization of butadienes to cyclobutenes here it was noted that the excited state has a high 1,4-bond order. The same reasoning was applied 6,12) to understanding the key step of cyclohexadienone rearrangements (vide infra). Still another example is the decreased central bond order in the excited state of stilbene which, as Daudel has noted 13), is in accord with photochemical cis-trans interconversion. 

The primary step of the rearrangement involves vinyl-vinyl (or more generally) 7t-7T bridging in the excited state. It is readily seen that such bridging of two vinyl (or 7t) moieties in proximity to one another corresponds to a locus with a high bond order. This is depicted in Equation 4. 

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