Biradicals photochemically generated

The photochemically generated cyclopentane-1,3-diyl diradials (87) were part of a study of spin delocalization through the EPR Z)-paramctcr. These biradicals were a model system for cumyl and benzyl radicals and experimental data were combined with MO calculations to map the electronic effects on D by varying the aromatic substituent (Ar = heterocycle).218 This parameter was also measured for a related series of... 

In addition to 4 4- 2 cycloaddition, disilenes are capable of reacting with unsaturated compounds by the ene reaction. (Trimethylsilyl)trimethyldisilene (8), obtained by irradiation of 7, reacts with 2,3-dimethylbutadiene to yield the ene adduct 12 as well as the 4 + 2 cycloadduct 13 (Section II.A.3.a)52. The possibility has been considered52 that the ene reaction proceeds via a biradical resulting from a triplet disilene produced in the initial photochemical generation step (cf. equation 7). 

Stereoselective Addition of Photochemically Generated Radicals on Al-kenes. The biradical nature of the state of the reactive excited state... 

I, 4-biradicals involved in Norrish type 11 photoreaction of methanonaphthoquinones, Chem. Lett., 329, 1982 (c) Osuka, A., Shimizu, H., Chiba, M. H., Suzuki, H., and Maruyama, K., Type 11 photoreaction of la,7a-dihydro-l//-cyclopropa[h]naphthalene-2,7-diones photochemical generation of type 11 biradicals from cyclobutanols,/. Chem. Soc., Perkin Trans. 1, 2037, 1983. 

Finally, thermally induced isomerizations which generate carbon-centered biradical organic molecules have been shown to serve as alternative for conventional chemical and photochemical methods [71]. A straightforward procedure to accomplish such biradicals was described by Myers using a thermal conversion of yne-allenes [72]. According to this scheme, Wang and coworkers [73] heated 3-178 in 1,4-cyclohexadiene to 75 °C and obtained 3-181 in 22% yield via the biradicals 3-179 and 3-180 (Scheme 3.48). 

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... 

Irradiation of ketone (182) with a mercury lamp generates biradicals which are formed via n-TT electron transition. The reactive oxygen center of the biradical abstracts a hydrogen atom via 1,6-H shift, and the formed carbon-centered biradical couples intramolecularly to produce 5-membered compound (183) (eq. 3.70) Eq. 3.71 shows the same type of photochemical cyclization through the generation of a biradical, 1,9-H shift, and finally intramolecular coupling of the formed carbon-centered biradical [192-199]. 

Benzophenone (Amax = 340 nm, log e = 2.5, n-ir electronic transition) can be used as a photochemical reagent and eq. 4.25 shows a radical Michael-addition reaction with benzophenone. The formed benzophenone biradical (triplet state, Tx) abstracts an electron-rich a-hydrogen atom from methyl 3-hydroxypropanoate (62) to generate an electron-rich a-hydroxy carbon-centered radical [III], then its radical adds to the electron-deficient (3-carbon of a, (3-unsaturated cyclic ketone (63) through the radical Michael addition. The electrophilic oxygen-centered radical in the benzophenone biradical abstracts an electron-rich hydrogen atom from methyl 3-hydroxypropanoate (62) [70]. So, an a-hydroxy carbon-centered radical [III] is formed, and an electron-deficient a-methoxycarbonyl carbon-centered radical [III7] is not formed. 

In addition to serving as substrates with which to further study the effects of various substituents on the photochemical transformations of cyclobutanones, 3-methylenecyclobutanones are capable of generating the theoretically interesting trimethylenemethane biradical species (32) by way of the photodecarbonylation reaction. Indeed, Dowd and Sachdev (33) observed a triplet ESR spectrum upon photolysis of 3-methylenecyclobutanone [69]... 

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