Cobalt(I) cobalamin and cobinamide are efficient catalysts of the hydrogenolysis of aliphatic chloro compounds. Taking chloroacetonitrile as example, the first requirement for high catalytic efficiency is fulfilled by the high reactivity of the Co(I) complex toward the substrate, leading to the alkylcobalt(III) complex. This is further reduced into the alkylcobalt(II) complex. However, the fact that these two reactions are fast is not enough to ensure an efficient catalysis: in DMF catalysis is very poor, while it is high in water. The experiments carried out in DMF with addition of an acid show that a crucial step in the catalytic process is the proton transfer decomposition of the alkylcobalt(II) complex, leading to the product, and closing the catalytic loop by regeneration of the cobalt(I) complex. Another important feature of these catalytic reactions is the role played by axial ligands present in the solution, particularly those that are produced by the catalytic reaction itself, namely, chloride ions and the counteranion of the added acid. The stronger these ligands, the more negative the potential required for the reduction of the alkylcobalt(III) complex. This amounts to a self-moderation effect: the more efficient catalysis, the slower its second step, i.e., the conversion of the alkylcobalt(III) complex into the alkylcobalt(II).
Neophyl radicals were generated by photoinduced electron transfer (PET) from a suitable donor to the neophyl iodide (1, 1-iodo-2-methyl-2-phenylpropane). The PET reaction of 1 with the enolate anion of cyclohexenone (2) afforded mainly the reduction products tert-butylbenzene (5) and the rearranged isobutylbenzene (6), arising from hydrogen abstraction of the neophyl radical (15) and the rearranged radical 16 intermediates, respectively. The photostimulated reaction of 1 with 2 in the presence of di-tert-butylnitroxide, as a radical trap, afforded adduct 10 in 57% yield. The photoinduced reaction of the enolate anion of acetophenone (3) with 1 gave the substitution products 11 (50%) and 12 (16%), which arise from the coupling of 3 with radicals 15 and 16, respectively. The rate constant obtained for the addition of anion 3 to radical 15 was 1.2 x 10(5) M(-)(1) s(-)(1), by the use of the rearrangement of this radical as a clock reaction. The anion of nitromethane (4) was almost unreactive at the initiation step, but in the presence of 2 under irradiation, it gave high yields (67%) of the substitution product 13 and only 2% of the rearranged product 14. When the ratio of 4 to 1 was diminished, it was possible to observe both substitution products 13 and 14 in 16% and 6.4% yields, respectively. These last results allowed us to estimate the coupling rate constant of neophyl radicals 15 with anion 4 to be at least of the order of 10(6) M(-)(1) s(-)(1). Although the overall quantum yield determined (lambda = 350 nm) for the studied reactions is below 1, the chain lengths (Phi(propagation)) for the reaction of 1 with anions 3 and 4 are 127 and 2, respectively.
[reaction: see text]. The photoinduced reactions of aryl halides with the thiourea anion afford arene thiolate ions in DMSO. These species without isolation, and by a subsequent aliphatic nucleophilic substitution, S(RN)1 reaction, oxidation, or protonation, yield aryl methyl sulfides, diaryl sulfides, diaryl disulfides, and aryl thiols with good yields (50-80%). This is a simple and convenient approach which involves the use of the commercially available and inexpensive thiourea in a "one-pot" two-step process for the synthesis of aromatic sulfur compounds.
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