Chlorine atom abstraction photochemical initiation

Photolysis of Mn2(CO)8(phen) in the presence of CCU, however, results in the formation of a mixture of MnCl(CO)5 and MnCl(CO)3phen. The reaction, which involves the initial photohomoloysis of the Mn-Mn bond followed by chlorine atom abstraction from CCU by the resulting Mn radicals, is discussed in more detail in a later section of this chapter. In other cases, however, photolysis leads to the dissociation of a carbonyl ligand rather than to homolysis of the intermetallic bond. The quantum yields of these particular reactions are dependent on both the metal and the ligands in the complex. The photodissociation of a carbonyl ligand has also been proposed as the initial step in the photochemical disproportionation reactions of these complexes/ In this case two electron donor ligands such as THF or a tertiary phosphine induce the photodisproportionation of the complex (CO)sM-M(C0)3 (diimine) (M = Mn, Re) into a cationic complex M(CO)3(diimine)L and an anionic complex M(CO)s (Refs. 99 and 100) ... 

The process is believed to initiate with formation of chlorine atom (either thermally or photochemically), which then abstracts a hydrogen from methane. 

Photochemical reactions of t-BuOCl generally involve hydrogen atom abstraction by t-BuO, a radical which is considerably more selective than Cl . In the reaction of t-BuOCl with 2,3-dimethylbutane at 64 °C, there is a report of a selectivity identical to that of Cl upon photolysis but a higher selectivity with initiation by Bz202. Photolysis of t-BuOCl may lead to chlorine atoms, hydrogen chloride and molecular chlorine whereas BZ2O2 leads to the t-BuO radical chain. Reaction 29 will shift t-BuOCl... 

The first step is the thermal or photochemical breaking of the chlorine-chlorine bond. This initiation reaction is followed by the first propagation step (Rg. 11.40), the abstraction of a hydrogen atom from methane by a chlorine atom to produce a methyl radical and hydrogen chloride. In the second propagation step, the methyl radical abstracts a chlorine atom from a chlorine molecule to give methyl chloride and another chlorine atom that can carry the chain reaction forward. There are many possible termination reactions Figure 11.40, which shows the overall mechanism, includes only one. 

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