Effects of transmetalation on the mechanisms of copper-catalyzed phenolic oxidative coupling reactions

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The mechanism of initiation of phenolic oxidative coupling by the tetranuclear copper( I) complex [pyCuCl] 4 is affected by transmetalation. [pyCuCl] 4 is stoichiometrically transmetalated by equimolar (M(NS) 2 reagents (M is Co, Ni, Cu or Zn) at room temperature in nitrobenzene under N 2 to give equimolar solutions of air-sensitive complexes py 3Cu 3M(NS)Cl 4 ( IIa-d, respectively) and py and 1 mol of insoluble Cu(NS)(s). The corresponding reaction with equimolar Co(NS) 3 gives equimolar solutions of py 3Cu 3Co(NS) 2Cl 4 ( III) and py and 1 mol of Cu(NS)(s). Here, py is pyridine and NS is monoanionic S-methyl isopropylidenehydrazinecarbodithioate. Second- order reduction of O 2 by [pyCuCl] 4 gives the tetranuclear oxocopper(II) complex (μ- 4-O)py 4Cu 4Cl 4O ( Ib). Reduction of O 2 by IIc in the presence of equimolar py gives Ib and the disulfide co-product N 2S 2. The corresponding reductions of O 2 by IIa, IIb, IId and III give products py 3CuMCl 4O 2 ( IVa, IVb, IVd and IVa, respectively) and N 2S 2. Insertion of O 2 through the Cl 4 cores of the reductants is the rate-determining step in each system. However, different product core structures are indicated by different spectral sensitivity to oxidation co-product N 2S 2 and by different rate laws for their oxidation of 2,6-dimethylphenol (DMPOH) to the corresponding diphenoquinone. These core structures apparently contain either a weakly basic CuOM unit (in IVa and IVb) or a strongly basic Cu v O unit (in Ib and IVd). As a result, the rates of oxidation of DMPOH by Ib and IVd are independent of [DMPOH]. Opening of the CuOM unit to give a weakly basic site M v O is proposed to explain induction periods in the IVa, IVb/DMPOH systems, which then proceed at rates which are proportional to [DMPOH]. The origin of different py 3, 4Cu 3MCl 4O 2 product core structures is discussed.