Biradicals implications

Calculations allow one to justify the observed behavior (Fig. 19) (99MI233). In the case of 3- and 5-phenylisothiazole, the reaction should implicate a Dewar isomer, because the excited triplet isothiazole derivative cannot be converted into the corresponding biradical. 

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,... 

The highly functionalized chroman 436 is formed with complete asymmetric induction upon irradiation of the allene 437. Labeling experiments implicate an intermediate biradical 438 that undergoes a stereospecific 1,5-hydrogen shift to furnish the alkynic moiety seen in the product (Scheme 99) <1997TL8789>. 

As far as CIDNP is concerned, the most important difference between radical pairs and biradicals is that the interdiffusion of the paramagnetic centres is restricted in the latter case. This has two implications. 

The photolytic cleavage of alkyl aryl sulfoxides has been shown to occur via initial C—S bond homolysis, in accordance with the common mechanistic assumption. Secondary and tertiary alkyl groups show high chemoselectivity for alkyl C—S cleavage. Uniquely, alkene products have been isolated, formed by disproportionation of the initial alkyl radical, with the formation of benzaldehyde and racemization of primary alkyl compounds. An investigation into the photochemical conversion of N-propylsulfobenzoic imides into amides in various solvents revealed a solvent dependence of the observed mechanism. In ethanol, sulfur dioxide extension forms a biradical which abstracts a hydrogen atom from the solvent, whereas in aromatic solvents biradical formation by a single electron transfer is implicated. The photolysis and thermolysis of l,9-bis(alkylthio)dibenzothiophenes and /7-aminophenyl disulfide have been studied. 

Yields are occasionally good (up to 90%), but more often considerable starting material remains even after very long irradiation times and byproducts resulting from allylic 1,3-rearrangement and cisjtrans isomerization are formed. Stereochemical information of starting materials is lost in the product. Triplet biradicals have been implicated as intermediates. ... 

In a similar reaction, 3-alkynylallyl chlorides 12 cyclized to (isomeric) mixtures of 1 -chloro-2-alkynylcyclopropanes 13 on direct irradiation at 254,300 and 313 nm in hydrocarbon solvents. This reaction does not occur in the absence of the 3-alkynyl group, or in the presence of acetone. Singlet biradicals have been implicated as intermediates. ... 

The trans stereoselectivity may easily be rationalized as a result of sterics, although direct irradiation of the 1-phenylcyclohexene gives a similar ratio of trans cis stereoselectivity. The singlet reaction presumably does not involve a biradical intermediate of significant lifetime. Thus, the trans selectivity may be enhanced by sterics in the case of the triplet reaction. The preference for trans isomer may be inherent to the approach of the alkenes or, perhaps, it is a function of selective reversibility of biradical intermediates (see Scheme 1). Caldwell has published a formula for predicting the potential for photocycloaddition of alkenes and arenes in the singlet excited state [42], His analysis implicates an exciplex. 

MINDO/3 calculations predicted that the valence isomer of butalene, singlet 1,4-benzyne (XXI), should be more stable than butalene itself [211], although MNDO calculations predict the opposite [212], Both indicate appreciable delocalisation of electrons in the periphery. Irrespective of whether (XX) or (XXI) represents the energetically preferred structure, valence tautomorism between them may take place readily and there is chemical evidence which implicates the possible presence of a biradical structure in reactions which may generate butalene [213]. 

Carbene-like nitrile ylides have been further implicated in the photorearrangements of substituted 2/ir-azirines (78) and (79) which proceed by a 1,1-cycloaddition reaction (Scheme 7). The process is believed to involve a biradical or zwitterionic intermediate and, in all cases examined, the product is the bicyclic system (80), although at... 

The spirocyclic hexadienone (590) has been implicated in the photochemical rearrangement of 4-phenylchroman-3-one to 2-phenylchroman-3-one in benzene. The mechanism proposed involves cleavage and recoupling of the resultant biradical to give (590) which gives the product by a 1,3-sigmatropic shift. 

The photo-addition of methyl aqgtopyruvate (359) to cyclopentadiene (solvent) results in the formation of two [4 + 2] and two [2 + 2] adducts. The [4 -i- 2] adducts are the result of stereospecific, but not stereoselective, reaction and the [2 + 2] adducts lack the usual cyclobutane ring, presumably because of its cleavage by a photo-enolization mechanism. Direct or sensitized irradiation of (360) affords the crossed [2 + 2] adducts (362) and (363) as well as the normal [2 + 2] product (361) in 85 10 15 ratio. The formation of two crossed products implicates a biradical mech-amsm. ... 

The persisting mechanistic query on this nitrogen extrusion concerns stepwise versus concerted breakage of the two C-N bonds in DBH (1) the stepwise CN-bond rupture necessarily implicates a nitrogen-containing intermediate, namely the diazenyl biradical DZ, whereas the concerted process passes through the nitrogen-free DR species (Scheme 5). 

Computational and experimental evidence exist in favor of the stepwise nitrogen elimination in the photolysis of azoalkanes, which implicate the intervention of a diazenyl radical species. Thus, a recent theoretical study on the parent DBH discloses the singlet diazenyl biradical DZ as the lowest-energy transient on n,7i excitation. Similarly, computational results on the photolysis of azomethane suggest a stepwise mechanism for the denitrogenation. Indeed, simple symmetry considerations in terms of the Dauben-Salem-Turro theory on photochemical transformations predict a stepwise process. " ... 

---