Manganese complexes reactivity with reductants

Reduction of the size of the substituents at the thiophosphinito ligand in 122 changes the reactivity of the manganese complex towards phosphaal-kynes markedly (Scheme 42). With R = Me, a third molecule of f-BuC=P... 

The last method, F, which uses NiCl2-2,2 -bipyridine complex, works well only for the most reactive aryl iodides [31]. It is mechanistically more complicated because here lead(II) bromide acts as the source of lead(0) which actually reduces Ni"(bpy)X2 back to the catalytically active Ni°(bpy). Since the 2,2 -bip)ridme 75 is somewhat more electron-rich ligand than simple 2,2 -bipyridine (5), the resulting nickel(II) complex employed in the related Chen s method is sUghtly more reactive with electron-poor aryl halides, e.g. bromides, but generally the method is appUcable only with aryl iodides [30]. For instance, both iodo- and bromobenzene gave biphenyl (8) in 98 and 88% yields, respectively, but 2-bromothiophene was coupled in only 22% yield [30], Scheme 8. Chromium(II) chloride in a catalytic amount was used as an intermediate reductant as it provides an effective redox system, Cr(II) / Cr(III) between Ni(0) / Ni(II) and metallic manganese as the ultimate reductant. 

One motivation for the characterization of the above compounds has been to more fully understand the involvement of such higher valent manganese porphyrin complexes in model systems which imitate the catalytic activity of monooxygenase cytochrome P-450 and related enzymes. The catalytic cycle of cytochrome P-450 appears to involve the binding and reduction of molecular oxygen at a haem centre followed by the ultimate formation of a reactive iron oxo complex which is responsible for oxidation of the substrate. For example, cytochrome P-450 is able to catalyse alkane hydroxylation with great selectivity. 

Finally, it can be stated that many reactivity patterns of free radical ions are equally found in oxidative and reductive transformations involving initial inner-sphere ET, such as in reactions with samarium iodide [389], low valent titanium [390] and titanocene complexes [391], manganese(III) [392], and CAN [393]. 

Since fef for complex formation is compounded of 2 and fe2 3, the net effect is not readily predictable. It should be noted that the observed value for fef has to be corrected for a reduction in the number of replaceable water molecules (by multiplying by a factor of 6/(6 — n) where n is the number of coordination positions occupied by the bound ligand). The net effect observed is a negative one, the reactivity order with NSA being Mg + > Mg(TP) > Mg(NTA) > Mg(ATP) whereas that for oxine [66] is more expectedly Mg > Mg(NTA) > Mg(ATP) > Mg(TP) ". Similar studies have been made with manganese(II), cobalt(II), zinc(II), and nickel(II) [70b]. 

---