Oxido‐Hydroxido‐ and Oxido‐Aminato‐Osmium(V) Complexes with a Cyclohexanediamine‐Based Tetradentate Ligand as Active Oxidants for Dihydroxylation and Aminohydroxylation of Alkenes

Abstract

Six‐coordinate osmium(III) complexes coordinated by a cyclohexanediamine‐based tetradentate ligand [trans‐N,N′‐dimethyl‐N,N′‐bis(2‐pyridylmethyl)‐1,2‐cyclohexanediamine: BPMCN] are synthesized. The remaining two coordination sites are occupied by chloride (Cl–), triflate (OTf–), or hydroxide/water (OH–/H2O) to complete the octahedral osmium(III) centers; [OsIIIX2(BPMCN)]+ (X = Cl– or OTf–) and [OsIII(OH)(H2O)(BPMCN)]2+. The OsIII(OH)(H2O) complex catalyzes cis‐dihydroxylation and cis‐aminohydroxylation of styrene with hydrogen peroxide (H2O2) and chloramine‐T (TsNClNa) to yield the corresponding diol and aminoalcohol, respectively. The oxido‐hydroxido‐osmium(V) complex {[OsV(O)(OH)(BPMCN)]2+} is isolated as the active oxidant of the dihydroxylation. The oxidation of the OsIII(OH)(H2O) complex with chloramine‐T yields oxido‐aminato‐osmium(V) complex, [OsV(O)(NHTs)(BPMCN)]2+. Comparison of the crystal structure with those of the oxido‐hydroxido‐osmium(V) complex and related imido‐ and amino‐osmium complexes indicates that the –NHTs group works as a monoanionic aminate ligand and the osmium(V)–NHTs bond has a single bond character [Os–N; 2.058(4) Å]. The oxido‐aminato‐osmium(V) complex is revealed as the active oxidant for the aminohydroxylation by the direct reaction of the complex with styrene. The characterization of the complex allows us to propose the catalytic mechanism of the aminohydroxylation of alkenes.

Oxygen Atom Insertion into the Osmium–Carbon Bond via an Organometallic Oxido–Osmium(V) Intermediate

“…However, no experimental evidence for such a stepwise C–O bond formation mechanism has been reported, so far. On the other hand, we have successfully isolated reactive oxido–osmium­(V) complexes that can be adopted to cis -dihydroxylation and cis -aminohydroxylation of alkenes, oxidative cyclization of 1,5-dienes, and C–H bond activation of the aliphatic moiety of the supporting ligand. − Here, we prepare a cyclometalated osmium­(III) complex of phpy for examining its reactivity toward various oxidants to find that oxygen atom insertion into the Os–C bond takes place via a stepwise mechanism involving a high-valent osmium–oxido intermediate (Scheme b).…”