V. Fragmentation Reactions
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A. β-Fragmentation
Homolytic β-fragmentation of a radical is an elementary reaction that cleaves a bond to one of the atoms adjacent to a radical center. (Other names for β-fragmentation are β-cleavage and β-scission.) This type of reaction sometimes produces an unsaturated carbohydrate by expelling a noncarbohydrate radical (eq 27), and other times it gives a carbohydrate radical and an unsaturated noncarbohydrate (eq 28). Equation 29 illustrates the first of these possibilities with a reaction in which the radical 4 fragments to give C6H5SO2· and the unsaturated carbohydrate 5.21 Being able to form a stabilized radical such as C6H5SO2· is an essential factor in this reaction.
A β-fragmentation reaction producing a carbohydrate radical and an unsaturated noncarbohydrate is one driven by formation of a compound with a thermodynamically stabilized multiple bond (usually a carbon–oxygen double bond) and a radical that also is stabilized (usually by an oxygen atom attached to the radical center). The reaction shown in eq 30 fits this pattern because it produces formaldehyde and the oxygen stabilized radical 6.22 Forming an aromatic ring is another way for providing a substantial driving force for β‑fragmentation (eq 31).23 A further option for β-fragmentation is ring opening, a possibility that presents itself when a radical is centered on an atom attached to the ring (eq 32).24
B. Heterolytic β-Fragmentation
When a radical is centered on a carbon atom that has an effective nucleofuge attached to a neighboring carbon atom, the possibility exists for formation of a radical cation (eq 33). The bond from the neighboring carbon atom to the leaving group needs to be one that does not cleave homolytically with ease; otherwise, β-fragmentation producing ionic intermediates could be preempted by homolytic fragmentation. Heterolytic β-fragmentation occurs in the reaction shown in eq 34.25
C. α-Fragmentation
α-Fragmentation is an elementary reaction in which a bond attached to a radical center cleaves homolytically. This reaction is rare because it requires the energy-demanding step of bond breaking without the energetic compensation of bond formation. One situation in which α-fragmentation takes place is in the formation of the isonitrile and stabilized, sulfur-centered radical shown in eq 35.26 A second occurs in the fragmentation of the hypervalent radical shown in eq 36.19
D. Bond Homolysis
Bond homolysis either produces a pair of radicals (eq 37), or if the bond being broken is part of a ring system, a diradical. Thermal reaction cleaves the weakest bond in a molecule; thus, when the cobaloxime 7 is heated, the carbon–cobalt bond, one of the weakest covalent bonds known, breaks homolytically at temperatures well below those necessary for cleavage of other bonds in the molecule (eq 38).27 This bond homolysis involves electron transfer with cobalt acting as the electron acceptor.
Photochemical reaction offers a range of possibilities for bond homolysis (eq 39). Success depends both upon a compound being able to absorb the incident light and on this light supplying sufficient energy for bond breaking. Absorption of visible light provides the energy needed to cleave weaker covalent bonds, such as the iodine–oxygen bond in the reaction shown in eq 40.28 UV radiation is energetic enough to break stronger bonds, such as the carbon–carbon bonds in the reactions pictured in eq 41.29
Unlike thermal reaction, bond breaking during a photochemical process does not necessarily cleave the weakest bond in a molecule. Selectivity in bond breaking during photolysis results from a combination of factors that control the reactivity of electronically excited molecules. In the reaction shown in eq 41, for instance, excitation energy is quickly localized in the keto group in the substrate. This localization leads to one of the characteristic reactions of an excited aldehyde or ketone, namely, breaking the bond between the carbonyl carbon atom and one of its attached carbon atoms.30