Crossing, intersystem biradical

SPIN-ORBIT COUPLING AND INTERSYSTEM CROSSING IN BIRADICALS... 

A possible explanation for the absence of 16 could be found by considering that intermediates 14 and 15 should be triplet biradicals. Intersystem crossing in 14 would afford a singlet biradical that cyclizes to the cyclopropyl imine 11a. However, intersystem crossing within biradical 15 would yield the zwitte-rionic intermediate 17, which is not stabilized by phenyl substitution, as in the case of 6 (Scheme 2). Literature precedents show that nonstabilized 1,3-dipolar intermediates similar to 17 do not cychze to three-membered heterocycles but undergo different reactions yielding a complex mixture of products. This interpretation is speculative at this point and it will be necessary to carry out additional studies to establish the accuracy of this proposal. 

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... 

The results described above represent the first example of the FR mechanism (Scheme 1). Semiempirical calculations on this molecule showed that the intersystem crossing to the excited triplet state is favored The reaction cannot be sensitized by xanthone because the triplet state of 3,4-diphenyl-1,2,5-oxadiazole is lower than that of xanthone. The cleavage of the triplet state to the biradical is favored, considering the relative energy of this intermediate (Fig. 23) (OOOUPl). 

The first step of the reaction involves the (n, it ) excited state of the carbonyl compound reacting with the ground-state alkene. For aromatic ketones, rapid intersystem crossing from the excited singlet state to the excited triplet state occurs, forming initially a 1,4-biradical and then the oxetane ... 

The formation of the A-vinylaziridine 70 in the photoreaction of 68 deserves additional comment. Depending on the multiplicity, the intermediate 72 formed by path b could be a triplet 1,3-biradical. However, if intersystem crossing occurs along the reaction coordinate, the singlet biradical must be considered as a dipolar azomethine ylide. According to literature precedents, both intermediates, the 1,3-biradical and the ylide, will cyclize to form the observed aziridine. This is the first case in a DPM process where a zwitterion can be postulated as a possible intermediate. 

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-... 

The photoreactivity of o-methyl acetophenone 11 has been studied exten-sively it is somewhat different from 1 because the singlet excited ketone (Sik) in 11 intersystem crosses to its triplet state in less than quantitative yields, as observed for 1 (Scheme 8). Thus, Sik in 11 decays by both intramolecular H-atom abstraction to form exclusively photoenol Z-13 and intersystem crossing to Tik of 11. Haag et al. estimated that Tik of 11 has a lifetime of 10 ns in benzene and decays by intramolecular H-atom abstraction to form biradical 12. The maximum... 

Laser flash photolysis of 46 showed results similar to those obtained for 45. The lifetimes and yields of Z and E photoenols from 46 are comparable to those obtained for 56. Similarly, laser flash photolysis of 47 reveals that the major reactivity pattern of 47 is intramolecular H-atom abstraction to form Z-58 and E-58 even though no products were observed that can be attributed to the formation of photoenol 58. Laser flash photolysis of 47 in methanol showed formation of biradical 57 ( max 330 nm, r = 22ns), which was efficiently quenched with oxygen (Scheme 32). Biradical 57 intersystem crosses to form Z-58 and E-58, which have maximum absorption at 400 nm. Enols Z-58 to E-58 were formed in the approximate ratio of 1 4. Enol Z-58 had a lifetime of 6.5)0,s in methanol, but its lifetime in dichloro-methane was only 110 ns. The measured lifetime of E-58 in methanol was 162)0,s, while it was 44 ms in 2-propanol. Thus, E-58 is considerably shorter-lived than E-56. Furthermore, E-58 is also shorter-lived than the analogous E-59 (Scheme 33), which cannot decay by intramolecular lactonization and has a lifetime of 3.6 ms in methanol. Thus, we proposed that E-58 undergoes solvent-assisted reketonization that is facilitated by the intramolecular H-atom bonding, as shown in Scheme 34. 

Benzaldehyde reacts in its triplet state. This way, a triplet biradical is formed as an intermediate. For the formation of the products, intersystem crossing into... 

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