Diradicals carbon-centered

The fragmentation/cyclization ratio is determined by the relative orientation of the respective molecular orbitals, and thus by the conformation of diradical species 2. The quantum yield with respect to formation of the above products is generally low the photochemically initiated 1,5-hydrogen shift from the y-carbon to the carbonyl oxygen is a reversible process, and may as well proceed back to the starting material. This has been shown to be the case with optically active ketones 7, containing a chiral y-carbon center an optically active ketone 7 racemizes upon irradiation to a mixture of 7 and 9 ... 

Substantial advances have been made in understanding, preparing, and detecting the carbon-centered delocalized or localized diradicals. But the silicon-centered diradicals are much less explored. Search for other stable localized singlet diradicals remains a goal of experimental and theoretical scientists. 

The short lifetimes of carbon-centered monoradicals are generally reduced in the case of diradicals due to their propensity to form covalent bonds. It has been suggested that stable diradicals may be observable from highly strained bicyclic molecules where the TS for inversion is a diradical. Unfortunately, only persistent diradicals have been obtained in this way. Akin to this approach, in a recent attempt to generate the oxyallyl diradical, Sorensen and co-workers synthesized two substituted bicyclobutanones hoping to stretch and homolytically break the central bond using bulky substituents, which would also stabilize the diradical. Though the bicyclobutanones did not yield the desired oxyallyl derivative, the X-ray structures showed... 

A one-electron reduction of the bond between an aliphatic carbon and a halogen leads to a halogen anion and a carbon-free radical. A good example is the reduction of carbon tetrachloride as discussed earlier in this chapter. The first product in the reduction is the trichloromethyl-free radical. Carbon-centered radicals are not very reactive with biological molecules, but they react very rapidly with molecular oxygen (a diradical) to form a peroxy-free radical (Fig. 5.15), which is quite toxic (10). 

Such a reaction may occur on treatment of the benzo-fused bicyclic sulfonium salt 37 with various bases under aerobic conditions. The incipient allylic diradical 39 is trapped as 1,3-diradical by molecular oxygen to give a mixture of up to 35% diastereomeric Spiro-1,2-dioxolanes 40. The spirocyclopentene 41 is obtained in a competing path in which allylic diradical 39 is cyclized as a 1,5-carbon-centered diradicaE . 

A number of diradicals (also called biradicals) are known,and the thermodynamic stability of diradicals has been examined. Orbital phase theory has been applied to the development of a theoretical model of localized 1,3-diradicals, and used to predict the substitution effects on the spin preference and S-T gaps, and to design stable localized carbon-centered 1,3-diradicals. When the unpaired electrons of a diradical are widely separated, for example, as in CH2CH2CH2CH2, ... 

The group ofWalborsky probably has described one of the first true anionic/radi-cal domino process in their synthesis of the spirocyclopropyl ether 2-733 starting from the tertiary allylic bromide 2-730 (Scheme 2.161) [369]. The first step is a Michael addition with methoxide which led to the malonate anion 2-731. It follows a displacement of the tertiary bromide and a subsequent ring closure which is thought to involve a SET from the anionic center to the carbon-bromine anti bonding orbital to produce the diradical 2-732 and a bromide anion. An obvious alternative Sn2 halide displacement was excluded due to steric reasons and the ease with which the reaction proceeded. 

Homolytic cleavage of b—d bond gives a diradrical intermediate. If the centers bearing the unpaired elections in a diradical are well separated (for example by a long carbon chain) there will be little interaction between them and each will act as an independent monoradical function. Usually the centers are close enough for molecular interaction and it eliminates their reaction as separate chemical entities. 

The Diels-Alder reaction is the best known and most widely used pericyclic reaction. Two limiting mechanisms are possible (see Fig. 10.11) and have been vigorously debated. In the first, the addition takes place in concerted fashion with two equivalent new bonds forming in the transition state (bottom center, Fig. 10.11), while for the second reaction path the addition occurs stepwise (top row, Fig. 10.11). The stepwise path involves the formation of a single bond between the diene (butadiene in our example) and the dienophile (ethylene) and (most likely) a diradical intermediate, although zwitterion structures have also been proposed. In the last step, ring closure results with the formation of a second new carbon carbon bond. Either step may be rate determining. 

It has been argued128-124 that singlet methylene adds directly to the carbon double bond to form a cyclopropane ring by a three-center mechanism involving essentially simultaneous formation of two carbon-carbon bonds. We believe, however, that the detailed mechanism must include formation of a short lived diradical as the initial step for both singlet and triplet methylene. The diradical may undergo ring closure or struc-... 

A related example is given in equation 1751. Here, both roles of the n-7t excited state are involved. The antibonding electron density opens that a,/J bond which will afford a dicyano-stabilized odd-electron center. Its oxygen p v orbital attacks the ipso carbon of the phenyl group on the cyclopropane ring to give the spiro-diradical species shown. This then opens to photoproduct. 

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