Biradicals, unreactivity

The oxygen molecule, a paramagnetic species with an unpaired electron on each atom, has already been referred to as biradical, albeit an unreactive one. The photochemical excitation of an anthracene to a biradical, or to something rather like one, has also been mentioned (p. 331) if this excitation is carried out in the absence of air or oxygen, instead of the trans-annular peroxide—(104)—a photo-dimer (130) is obtained ... 

In one case where unsymmetrical diketones were used [165], the stereochemical course of the reaction could be explained. The reduction of 1 -acyl-8-benzoylnaphthalenes afforded predominantly the corresponding cis and 1,2-acenaphthenediols in strongly acidic and alkaline media, respectively. The preferable formation of the cw-diol is rationalized as due to cyclization through intramolecular coupling of the biradical intermediate, while the trans-diol is formed by intramolecular nucleophilic addition of the benzylic carbanion intermediate to its unreacted acyl group. 

A study of the photochemical reactivity of salts of the amino ketone (44) with enantiomerically pure carboxylates has been reported. The irradiations involved the crystalline materials using A, > 290 nm and the reactions are fairly selective which is proposed to be the result of hindered motion within the crystalline environment. Some of the many results, using (S)-(—)-malic acid, R-(+)-malic acid and (2R,3R)-(+)-tartaric acid, are shown in Scheme 1. The principal reaction in all of the examples is a Norrish Type II hydrogen abstraction and the formation of a 1,4-biradical. This leads mainly to the cis-cyclobutanol (45) by bond formation or the keto alkene (46) by fission within the biradical. A very minor path for the malate example is cyclization to the trn 5-cyclobutanol (47). A detailed examination of the photochemical behaviour of a series of large ring diketones (48) has been carried out. Irradiation in both the solid phase and solution were compared. Norrish Type II reactivity dominates and affords two cyclobutanols (49), (50) and a ring-opened product (51) via the conventional 1,4-biradical. Only the diketone (48a) is unreactive... 

There are few experimental examples of longer distance intramolecular hydrogen abstraction. s-Hydrogen transfer forming 1,6-biradicals generally requires that y- and 8-hydrogen atoms be unavailable or unreactive [as, for example, in the p-(o-tolyl) propiophenone 291 (Scheme 6.127)].968... 

Both steric and electronic effects can influence the regioselectivity of ene reactions. For example, a strong preference for hydrogen abstraction from the more substituted side of the double bond of the perepoxide intermediates, generated from trisubstituted alkenes, is observed. This effect has been called the cis effect and explained on the basis of orbital interactions1423 and activation entropy differences.1457 Scheme 6.267 shows the product distribution after photooxygenation of three alkenes (561 563).1461 There is an apparent steric effect of the methyl versus 2-propyl substituents in the first two cases. The cyclopropyl moiety in the last example remains unreacted, which rules out the formation of a biradical intermediate (see also Special Topic 6.10). 

The naphthalene-1,3-dione derivatives (405) are photochemically transformed in benzene-t-butyl alcohol into the two products (406, 407).The reaction is proposed as a Norrish Type II process involving the biradical (408) as the key intermediate. The formation of both products can be accounted for from this intermediate either by bond formation or by the more unusual disproportionation to yield (407). A study of the dependence of product distribution with time has shown that while the olefinic product (407) can be produced from the biradical (408), there is also a secondary process in the direct reaction that cleaves the product (407) into the same biradical. The cleavage of this product is clearly dependent on the presence of the additional carbonyl function (perhaps via intramolecular energy transfer) since the acetate (409) is unreactive under the conditions of irradiation. 

Interestingly, direct irradiation of ester- or carbo g lic acid-bearing VCP afforded five-membered lactones in low yields with unreacted VCP fScheme 11.39. Eq. 2). Formation of the cyclic lactone was presumably due to resonance delocalization of the biradical intermediate into the carbonyl group, revealing an O-centered radical species 53, which recombines to afford dihydrofuran 54. This dihydrofuran undergoes addition of water, followed by elimination of alcohol, to yield the cyclic lactone product 56 fScheme 11.401. ... 

The formation of the quinoidal p-xylylene intermediate can be monitored by the appearance of a peak in the UV spectrum around 310 nm. This has been used to optimize reaction conditions for polymerizations involving unreactive sulfonium salts [48]. There has been some controversy over the precise nature of the polymer coupling reaction. The initial assumption was that the polymerization was a radical-promoted process [46]. The presence of radicals was very hard to prove, and the pendulum swung for a while toward an anionic mechanism [51]. However, careful work by Lahti and coworkers [53] showed that radical trapping reagents did indeed suppress the polymerization. As an example, the addition of TEMPO to the reaction mixture not only dramatically lowered the yields and molecular weights but also caused the disappearance of the spin label. The mechanism of radical initiation is unknown it may involve spontaneous coupling of two quinoidal p-xylylene intermediates to form a biradical. 

---