Biradical intermediates singlet

Sensitized irradiation of (398) below — 30 °C gave only (399) photo-isomerization occurs at higher temperatures, and the formal ( 2 + 2 -l- 2) process probably involves biradical intermediates. Singlet rearrangements appear to be involved in the photochemical conversion of (400 both syn and anfi-isomers) in methanol into naphthalene, methyl 1-naphthylacetate, and 7-carbomethoxy-2,3-benzonorcaradiene (the major product). The anti-chloroketone (401), but not its sy -isomer, likewise underwent photorearrangement to the norcarene derivative, thus illustrating the stereospecificity. 

When pyrrole is irradiated, only decomposition products were obtained. Theoretical data can fit this statement (Fig. 6). In fact, the direct irradiation populates the excited singlet state, which can be converted into the Dewar pyrrole or into the corresponding triplet state. Clearly, the intersystem crossing to the triplet state allows the system to reach the lowest energy state. The excited triplet state can give the biradical intermediate, and this intermediate can give either the decomposition... 

Also in this case calculation results fit the experimental data (Fig. 7) [99H(50)1115]. In fact, the singlet excited state can evolve, giving the Dewar thiophene (and then isomeric thiophenes) or the corresponding excited triplet state. This triplet state cannot be converted into the biradical intermediate because this intermediate shows a higher energy than the triplet state, thus preventing the formation of the cyclopropenyl derivatives. 

In the photochemical isomerization of isoxazoles, we have evidence for the presence of the azirine as the intermediate of this reaction. The azirine is stable and it is the actual first photoproduct of the reaction, as in the reaction of r-butylfuran derivatives. The fact that it is able to interconvert both photochemically and thermally into the oxazole could be an accident. In the case of 3,5-diphenylisoxazole, the cleavage of the O—N bond should be nearly concerted with N—C4 bond formation (8IBCJ1293) nevertheless, the formation of the biradical intermediate cannot be excluded. The results of calculations are in agreement with the formation of the azirine [9911(50)1115]. The excited singlet state can convert into a Dewar isomer or into the triplet state. The conversion into the triplet state is favored, allowing the formation of the biradical intermediate. The same results [99H(50)1115] were obtained using as substrate 3-phenyl-5-methylisoxazole (68ACR353) and... 

Computational results are reported for the isomerization of 1,4,5-trimethyl-imidazole (99MI233). They show that the isomerization occurs through the Dewar isomer arising from the excited singlet state. The formation of the triplet state is energetically favored however, the biradical intermediate cannot be produced because it has higher energy than the excited triplet state. 

Calculations account for the experimental results (Fig. 20) (99MI233). The first excited singlet state (which accounts for the absorption at 269 nm calculated value 267 nm) was converted into the corresponding triplet state (69 kcal moF experimental 68 kcal moF ). The triplet state gives the cleavage of the S—N bond with the formation of the biradical intermediate. This intermediate leads to the product. 

Steady-state kinetics. The cycloaddition reaction between the singlet ground state of 2-isopropylidene cyclopentane-1,3-diyl ( = S ) with acrylonitrile (A) is believed to occur by way of a biradical intermediate (BR),17... 

A simple example serves to illnstrate the similarities between a reaction mechanism with a conventional intermediate and a reaction mechanism with a conical intersection. Consider Scheme 9.2 for the photochemical di-tt-methane rearrangement. Chemical intnition snggests two possible key intermediate structures, II and III. Computations conhrm that, for the singlet photochemical di-Jt-methane rearrangement, structure III is a conical intersection that divides the excited-state branch of the reaction coordinate from the ground state branch. In contrast, structure II is a conventional biradical intermediate for the triplet reaction. 

In view of the results obtained for attack of acetone singlets on 1-methoxy-butene to yield singlet biradicals (partial loss of stereochemistry due to bond rotation in the biradical), acetone attack on dicyanoethylene to yield oxetane stereospecifically via a similar biradical intermediate is difficult to envision. Thus a new mechanism must be developed to account for these results. 

In Chapter 3 we discussed two photochemical reactions characteristic of simple carbonyl compounds, namely type II cleavage and photoreduction. We saw that photoreduction appears to arise only from carbonyl triplet states, whereas type II cleavage often arises from both the excited singlet and triplet states. Each process was found to occur from discrete biradical intermediates. In this chapter we will discuss two other reactions observed in the photochemistry of carbonyls, type I cleavage and oxetane formation. 

Qualitatively, the interaction diagram would closely resemble that in Fig. 3, since electron-donating substituents in both addends would raise the molecular levels of both the carbonyl compound and the olefin. Only the energy gap, E(n)-> F(n), would increase, the net result being that the calculated ratio of concerted to biradical reaction, Eqs. 40 and 41, should be even closer to unity than in the formaldehyde-ethylene case. Detailed calculations 38> support this conclusion, so PMO theory predicts that the overall stereochemical results are due to a combination of concerted (singlet) and biradical (triplet) mechanisms. This explanation agrees with the experimental facts, and it bypasses the necessity to postulate differential rates of rotation and closure for different kinds of biradical intermediates. 

Casal et al., 1984). On the other hand, reaction of a singlet carbene with oxygen should be quite slow. Not only do spin restrictions demand formation of a high-energy biradical intermediate, but oxygen does not normally react rapidly with electrophiles. 

A study on mechanistic aspects of di-ir-methane rearrangements has been published recently [72]. The kinetic modeling of temperature-dependent datasets from photoreactions of 1,3-diphenylpropene and several of its 3-substituted derivatives 127a-127d (structures 127 and 128) show that the singlet excited state decays via two inactivated processes, fluorescence and intersystem crossing, and two activated processes, trans-cis isomerization and phenyl-vinyl bridging. The latter activated process yields a biradical intermediate that partitions between forma-... 

Interest in the aza-Bergman reaction and the influence of protonation on 2,5-pyridyne and its derivatives was (once more) directly stimulated by the search for less toxic antimmor drugs. One approach to increase the selectivity is to decrease the reactivity of the biradical intermediate in hydrogen-abstraction reactions.In this regard, Chen has established a simple model that correlates the reactivity of singlet biradicals with their singlet-triplet gap This model is based on the assumption that the reactivity of the triplet biradical is... 

Although valence isomerization reactions of aromatic compounds have found little by the way of practical application, they are a fascinating area for mechanistic and theoretical study. The details are not completely dear, but it seems that, for benzene itself, benzvalene arises from the lowest excited singlet state, perhaps by way of a biradical intermediate (3.32) that could also be a precursor to fulvene bicyclohexadiene is probably produced from the second excited singlet state. For some other aromatic compounds the electronic nature of 5, and S2 may be reversed, or at least the states are much closer in energy, so that the preference for benzvalene or bicyclohexadiene formation under conditions of long-wavelength irradiation can be rationalized. 

The rules of orbital symmetry conservation apply only to concerted reactions in photochemical processes these are usually those of singlet excited states, since the triplet states often lead to long-lived biradical intermediates. 

Until recently, biradical intermediates were not implicated in singlet-state Norrish Type II reactions. Concerted reaction was a possibility Recovered starting material from photolysis of optically active ketones is not racemized when the photolyses are carried out in the presence of large amounts of 1,3-dienes,... 

Zwitterions [32] and biradicals [33] have been also proposed as intermediates. These mechanistic possibilities were insufficient for rationalizing the lack of correlation between the reaction rates and solvent polarity [14,31], An early theoretical study by Goddard [33] favored a biradical intermediate. However, isotope effect studies [34], the lack of Markovnikov-type directing effects [14,31], and the fact that radical scavengers have no effect on the reaction eliminated a biradical mechanism for the singlet-oxygen-alkene ene reaction. 

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