Allyl radical chemistry

The discovery of 7r-allylic complexes is an exciting recent development in coordination chemistry. In these complexes the bonding of the allylic radical... 

The cation radical can undergo deprotonation to yield an allyl radical or nucleophilic attack by the solvent to produce a methoxyalkyl radical. Coupling of these radicals with the aromatic radical anion produces acyclic adducts. As an alternative, the anion radical can be protonated, ultimately giving reduction product. Thus, the degree of charge separation within the excited state complex dramatically influences the observable chemistry. 

As mentioned with benzyl groups, an allylic center is also quite susceptible to autoxidation chemistry (Fig. 109). The allylic hydrogen has a weak C-H bond dissociation energy due to the resonance stabilization energy of the resulting allylic radical (157). 

Is Delocalization a Driving Force in Chemistry Benzene, Allyl Radical, Cyclobutadiene and their Isoelectronic Species. 

S. S. Shaik, P. C. Hiberty, J.-M. Lefour, G. Ohanessian, J. Am. Chem. Soc. 109, 363 (1987). Is Delocalization a Driving Force in Chemistry Benzene, Allyl Radical, Cyclobutadiene and their Isoelectronic Species. 

Recently, iron catalysis gained general importance. Its catalytic chemistry has been summarized ([2] recent reviews [3, 4]). Iron(II) and iron(III) salts have a long history in radical chemistry. The former are moderately active in atom-transfer reactions as well as initiators for the Fenton reaction with hydrogen peroxide or hydroperoxides (reviews [5-12]). Important applications of this principle are the Kharasch-Sosnovsky reaction (the allylic oxidation of olefins) [13], which often... 

The development of ruthenium complexes for other applications in radical chemistry is still in its infancy, but seems well suited to future expansion, thanks to the versatility of ruthenium as a catalytically active center. Large avenues have not been explored yet and remain open to research. For instance, the development of methodologies for the asymmetric functionalization of C-H bonds remains a challenge. The Kharasch-Sosnovsky reaction [51,52],in which the allylic carbon of an alkene is acyloxylated, its asymmetric counterpart, and the asymmetric version of the Kharasch reaction itself are practically terra incognita to ruthenium chemistry, and await the discovery of improved catalysts. 

The preparation of a-selenoketones, esters, nitriles and related compounds can easily be performed via alkylation of the corresponding enolates or stabilized carbanions [21]. These compounds have found many synthetic applications in radical chemistry. In Eq. (9), a typical example involving a ketone is depicted [22]. The stability of a-selenoketones such as 41 is remarkable. Similar reactions with lactones have been performed. For instance, this approach has been applied to the stereoselective synthesis of oxygen-containing rings to either faces of a bicyclic structure [23]. The approach based on a-selenenylation/radical allyla-tion compares favorably with classical enolate allylation procedures, which usually leads to mixture of mono- and diallylated compounds. Furthermore, this strategy is excellent for the preparation of quaternary carbon centers [24] as shown by the conversion of 43 to 45, a key intermediate for the synthesis of fredericamycin A, [Eq. (10)] [25]. Similar reactions with sulfoxides [26] and phosphonates [27] have also been reported. 

The topic of 7r-allyl-transition metal chemistry is considerably broader than complexes involving the tt-CsHs ligand itself. A variety of complexes is now known in which the 7r-allyl radical is part of a carbocyclic ring system. Some examples of 7r-cyclopropenyl-, 7r-cyclobutenyl-, and tt-cyclopentenyl- transition metal complexes are presented below. 

Waglund, T, Cleasson, A, Stereoselective synthesis of the a-allyl C-glycoside of 3-deoxy-D-manno-2-octulosonic acid (KDO) by use of radical chemistry, Acta Chem. Scand., 46, 73 1992. 

Nechvatal, A. Allylic halogenation. Advances in Free-Radical Chemistry (London) 1972, 4,175-201. 

We shall use the allyl radical to illustrate how one can treat planar unsaturated organic molecules using multiconfigurational methods. Some properties of the groimd state will be studied and, in addition, the electronic spectrum. The system has acmally been used as a example in a course given at the Department of Theoretical Chemistry in Lund called Quantum Chemistry at Work and a number of students have performed the calculations. We shall use their results. Some of them were recently published [69]. 

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