Formation from homolytic bond cleavage

Processes accompanied by a decrease in volume, such as C—C bond formation, in which the distance between two carbon atoms decreases from the van der Waals distance of ca 3.6 A to the bonding distance of ca 1.5 A, are accelerated by raising the pressure and equilibria are shifted toward the side of products (AV < 0, AV < 0). The reverse reaction, a homolytic bond cleavage, leads to an increase in volume (AV / > 0, AV > 0). Pressure induces a deceleration of such a process and a shift in equilibrium toward the side of reactants. However, in an ionization, such as an ionic dissociation, the attractive interaction between the ions generated and the solvent molecules leads to a contraction... 

Figure 3.9b portrays homolytic bond formation by the recombination of radicals and is accompanied by charge transfer from A to B. The radicals must be singlet coupled. The interaction of triplet-coupled electron pairs is repulsive and does not lead to bond formation. The reverse process describes homolytic bond cleavage and results in singlet-coupled free radicals. 

Sensitization Via Electron Transfer. Pappas and Jilek analyzed the energetics of energy and electron transfer sensitization.(5j ) Their evidence indicated that inmost cases, sensitization could be explained by an electron transfer mechanism (see Figure 5). Sensitization is believed to involve the transfer of an electron from an excited photosensitizer (S ) to an onium molecule, which may involve the formation of an excited state complex. Product analysis and irreversible polarography previously determined from electrochemical studies, indicate that the reduced iodonium salt undergoes homolytic bond cleavage to form a phenyl radical and iodobenzene. ( ) The reduced sulfonium salt produces phenyl sulfide and a phenyl radical.(10)... 

Scheme 5.11 shows a general blueprint that serves as a useful framework for considering reaction mechanisms that involve biradical intermediates B. The scheme also applies to carbenes (Section 5.4.1) and nitrenes (Section 5.4.2). When formed by thermal or photochemical homolytic bond cleavage from the reactant molecule M, biradicals will initially have the multiplicity of their immediate precursors. Singlet biradicals ( B ) tend to be short-lived because spin-allowed bond formation leading back to the reactant M or to a... 

Type I Photoinitiators Unimolecular Photoinitiators. These substances undergo an homolytic bond cleavage upon absorption of light. The fragmentation that leads to the formation of radicals is, from the point of view of chemical kinetics, a unimolecular reaction. The number of initiating radicals formed upon absorption of one photon is termed the quantum yield of radical formation... 

The degradation can be photochemically induced (a) homolytic or (b) heterolytic cleavage at the weaker bonds. The photolysis of the type (a) may lead to elimination reactions and the type (b) may lead to free radical formation. The point of bond cleavage may not be the seat for light absorption. The energy can migrate from unit to unit until it finds itself at the seat of reaction. 

Examples of radical-mediated C-alkylations are listed in Table 5.4. In these examples, radicals are formed by halogen abstraction with tin radicals (Entries 1 and 2), by photolysis of Barton esters (Entry 3), and by the reduction of organomercury compounds (Entry 4). Carbohydrate-derived, polystyrene-bound a-haloesters undergo radical allylation with allyltributyltin with high diastereoselectivity (97% de [41]). Cleavage from supports by homolytic bond fission with simultaneous formation of C-H or C-C bonds is considered in Section 3.16. 

The products arising from the reaction of 431 with the alkyl-substituted silenes 149 and 150 suggest that the reaction occurs by a radical pathway, initiated by a homolytic Si—C bond cleavage of 431 and subsequent Si—Si bond formation giving the biradical 434. Intramolecular disproportionation of 434 gives 435, while 436 and 437 are the results of ring closure reactions without or with expulsion of tetramethylethene, respectively (equation 139)181. 

Photogeneration of MnRe(CO)io from irradiation of both Mn2(CO)io and Re2(CO)io also occurs, and such cross-coupling has become a definitive test of whether homolytic metal-metal bond cleavage occurs. Kinetics of recovery of ground state Mn2(CO)io absorption after flash irradiations are consistent with formation of Mn(CO)5 radicals from the excitation (13). 

When a molecule is irradiated, particularly with ultraviolet light, some bonds within the molecule can absorb this energy and undergo homolyt-ic cleavage to generate free radicals. These reactions include the formation of alkoxyl (RO ) radicals from alkyl hypoiodites (RO) or nitrites (RONO) or the generation of bromine atoms from bromine or A-bro-mosuccinimide. 

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