Transient biradicals

Photolysis of dibenzoylstilbene episulfoxide 20 represented the first method for preparation of monothiobenzil 22 [23,24]. A mechanism involving formation of the sultene 21 was postulated, the authors favoring a concerted rearrangement over a discrete biradical. While there is no evidence to rule out a concerted mechanism, this seems likely to be an example of carbonyl p-cleav-age, so a transient biradical is expected. The authors suggested that the sultene, posmlated but not actually detected in the reaction mixture, would decompose thermally in the same fashion as related dioxetanes. 

In the case of allene + NO reaction in solid Ar during the visible-light photolysis (585 nm), 2-allylnitrite radical was detected by Fourier transform infrared (FT-IR) spectroscopy, whereas excitation of dimethylacetylene - NO systems (610 nm) gives the acetylmethyliminoxy radical and dimethylketene [75]. As a result of oxygen atom transfer from NO to the central carbon atom, 2-allenylidoxy transient biradical is generated in allene ... 

We have also recently shown that Bergman (Scheme 8.7) and related reactions (not shown) [84, 85] of polyunsaturated hydrocarbons, with a 1,3,5-hexatriene skeleton form a branch inside a larger Cope reaction family characterized by a common 1,5-hexadiene structural unit. The examination of this whole family of reactions allowed us to derive a very simple rule for involvement of transient biradicals in Cope-like reactions of hydrocarbons A non-concerted reaction takes place when biradical intermediates are stabilized either by aUyl or aromatic resonance [84, 85]. 

Small, Jr., R. D. and Scaiano, J. C., Interaction of oxygen with transient biradicals photogenerated from y-methyl valerophenone, Chem. Phys. Lett., 48,354,1977. 

In the absence of 1,4-CHD, the biradical 55e undergoes an intramolecular 1,5-hydrogen shift to form 57, making it possible for an intramolecular radical-radical coupling to occur to produce 58 (Scheme 20.13). The fact that 58 was produced from 54e lends support to the formation of the a,3-didehydrotoluene biradical 55e as a transient reaction intermediate. It is also worth noting that the benzylic radical center in 55 is a stabilized triarylmethyl radical. 

The cyclic enediynyl sulfide 93 is also prone to undergo prototropic rearrangement (Scheme 20.21) [57]. When the l,8-diazabicydo[5.4.0]undec-7-ene (DBU)-induced isomerization was conducted in carbon tetrachloride, three cycloaromatized products, 96 to 98, were isolated, indicating the formation of the biradical 95a as a transient intermediate. In a polar solvent, such as methanol or ethanol, the formation of 99 can best be accounted for by regarding the biradical 95a as the zwitterion ion 95b. A related process involving the oxidation of 93 with selenium dioxide has also been reported [58],... 

TRANSIENT CHEMICAL SPECIES PERSISTENT RADICAL BIRADICAL Eree rotation,... 

K. It too gives the characteristic adducts obtained from the singlet, but the addition does not occur directly between the bicyclic hydrocarbon (28) and the alkene. Instead, the reaction occurs in two steps, first the reversible unimolecular ring opening of 28 to singlet biradical 14b, followed by a bimolecular capture of the latter (Scheme 5.5). Another hydrocarbon isomer 29 can be prepared as a transient intermediate. Its thermal conversion to the biradical 14b apparently occurs at even lower temperature. 

Figure 18.6. Calculated trace (solid line) resulting from the addition of transient absorbances due to biradical (dashed line) and free radical (dotted line)/ [Reproduced with permission from M. V. Encinas, R J. Wagner, and J. C. Scaiano, J. Am. Chem. Soc. 1980, 102, 1357. Copyright 1980 American Chemical Society.]...

Transient absorption spectra from valerophenone in Na-ZSM-5 and Cs-ZSM-5 could be assigned to triplet-triplet transitions no spectra which can be attributed to the hydroxy-1,4-biradicals were detected. Furthermore, the decay of the transient signals could not be fit to either a single or double exponential expression, and samples prepared under apparently identical conditions exhibited half-lives that varied by a factor of 2 [292], All of these spectroscopic observations suggest that the valerophenone molecules reside in a distribution of sites within a zeolite and migration among them under the experimental conditions is slow. Thus, the Norrish II photoproduct ratios must be interpreted in terms of not only the relative populations of alkanophenones at each site type, but also the quantum efficiencies of each and the conformational preferences of the intermediate BR in each environment. 

Conlin and coworkers have prepared (E)- and (Z)-l,l,2,3-tetramethylsilacyclobutanes 5 and have studied the mechanism of their thermal decomposition in order to gain insight into the stereochemistry of the thermal decomposition of silacyclobutanes20. The occurrence of transient 1,4-biradicals like 6 in [2 + 2] fragmentations is accompanied by a loss of the reactant stereochemistry. This can be rationalized by rotational processes in the diradical 6 (6a —> 6b) which compete effectively with the -scission steps yielding the silene 2 and E/Z 2-butene 7 (equation 3). 

In 1978, we observed that flash photolysis of butyrophenone produced acetophenone enol as a transient intermediate, which allowed us to determine the acidity constant KE of the enol from the pH-rate profile (section pH Rate Profiles ) of its decay in aqueous base.4 That work was a sideline of studies aimed at the characterization of biradical intermediates in Norrish Type II reactions and we had no intentions to pursue it any further. Enter Jerry Kresge, who had previously determined the ketonization kinetics of several enols using fast thermal methods for their generation. He immediately realized the potential of the photochemical approach to study keto enol equilibria and quickly convinced us that this technique should be further exploited. We were more than happy to follow suit and to cooperate with this distinguished, inspiring, and enthusing chemist and his cherished wife Yvonne Chiang, who sadly passed away in 2008. Over the years, this collaboration developed into an intimate friendship of our families. This chapter is an account of what has been achieved. Several reviews in this area appeared in the years up to 1998.5 10... 

Stufkens and Hard observed light induced formation of zwitterions and biradicals from the diimine clusters [Os3(CO)io(iPr-AcPy)] and [Os3(CO)io (dmb)] using picosecond UV/vis and nanosecond IR spectroscopies. Picosecond transient spectra indicated generation of the ligand anion radical and evolution over approximately 50 ps to a much longer lived transient that was attributed to an opened cluster with an associated ligand on the diimine metal. Figure 15 shows the proposed evolution of species in the photoreaction [93,94]. 

The development of the two-color and laser jet approaches has also allowed the study of the photochemical behavior of excited states of reaction intermediates, i.e., transient species that are chemically distinct from the original ground or excited state, such as neutral and ion radicals, biradicals, carbenes, and ylides. In fact, the study of excited reaction intermediates has been more comprehensive than the study of upper states. Originally, the short-lived nature of the ground-state transient itself led to the incorrect assumption that the excited transient would be too short-lived to participate in any chemical or photophysical processes other than deactivation to the ground state. However, this is now known not to be the case and some surprising differences between the ground- and excited-state behavior of reaction intermediates have been observed. 

Similar transient behavior—formation of the charge transfer complex— was observed following irradiation of the radical derived from l-bromo-(2-bromomethyl)naphthalene, i.e., the l-bromo-2-naphthylmethyl radical (158) [99]. However, product evidence was ambiguous with respect to the production of the 1,3-biradical intermediate (Scheme 10). 

Notably, benzophenone is not consumed since it is regenerated in the process, o-phthalaldehyde is another interesting promoter for cationic polymerization based on pyridinium salt chemistry. Scanio and co-workers reported that UV irradiation of o-phthalaldehyde leads to the formation of 1,4-biradical transients via intramolecular hydrogen abstraction [55] ... 

Here, pathway 1 (reaction 1) is the coordinated addition of ozone (1) to ethylene (2), which proceeds through the formation of a weakly-boimd complex that transforms into primary ethylene ozonide (PO) or 1.2.3-trioxolene upon passing through the symmetrical transient state (TSl). Pathway 2 (reaction 2, the DeMore mechanism [15]) involves the collision during spontaneous orientation of the reagents (3) and the rotational transition to the biradical transient state (TS2) (4) followed by the formation of the same PO. Proceeding from the above-said, we supplement this pathway with the reaction of detachment of molecular oxygen and the formation of intermediate biradical (5) the latter may either decompose with the formation of formaldehyde (6) and carbene (7) or transform into acetaldehyde (8) or epoxide (9). Finally, pathway 3 involves the transition of ozone into the triplet state (10). This pathway is similar to reaction 2. Here, the same biradical (5) is formed it transforms into the... 

The transient absorption spectrum obtained from the pulse radiolysis of CMS solutions in benzene as shown in Fig. 2 (b) is very similar to the absorption spectrum obtained in a solid resist film as shown in Fig. 2 (a). However, the absorption in solution around 500 nm is mainly due to the complex of benzene with chlorine atoms whereas in the solid film, it is due to the complex of the phenyl ring of CMS with Cl. The absorption due to Pj is observed in both solid CMS film and CMS solutions in benzene. The absorption around 320 nm may be due to the biradical of benzene. The very short lived species with absorption around 500 nm is mainly due to the benzene excimer with a small contribution from the CMS excimer. Benzene solutions of CMS have proven to be a very good model system for reactions occurring in solid films of CMS. 

The decay of the carotenoid radical cation absorption of C +-P-C6o occurs on the micro second time scale in the frozen glass. It is accompanied by the rise of C-P-Ceo generated by charge recombination of the C -P-Ceo biradical, which is formed with a quantum yield of 0.07. The major component of the decay of the - C-P-Ceo transient has a time constant of 10 ps, which is a typical lifetime for a carotenoid triplet state. The absorption of C -P-Ceo " at 77 K does not decay exponentially, but an average decay rate of 7.5 x 10 s may be calculated from the data [155]. Time-resolved experiments have allowed detection of the EPR resonances of the C +-P-C6o biradical and C-P-Ceo- The spin-polarization of the carotenoid triplet spectrum verifies formation of this state by the radical pair... 

Transient triplet biradicals have been trapped by reaction with oxygen the procedure is claimed to provide an effective method for estimating the lifetimes of these species (Adam et al. ). Streith et al. have described the first example of a pyridino-cyclopropene (26). 

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