Synthesis of Five-Membered Osmacycles with Osmium–Vinyl Bonds from Hydrido Alkenylcarbyne Complexes

Treatment of the osmium hydrido butenylcarbyne
complex [OsH­{CC­(PPh3)CH­(Et)}­(PPh3)2Cl2]­BF4 (1) with
excess 2-chloro-4-cyanopyridine in the presence of H2O2 generates the fused osmacyclopentadiene 2. A
detailed mechanism of the conversion has been investigated with the
aid of in situ NMR experiments and the isolation of intermediates 3 and 4. In contrast, reaction of 1 with the propiolic acid ester HCCCOOMe produces the osmafuran 5. Analogous reactions of the osmium hydrido phenylethenylcarbyne
complex [OsH­{CC­(PPh3)CH­(Ph)}­(PPh3)2Cl2]­BF4 (6) with
nitriles/CO or HCCCOOMe were also studied, which result in
the formation of five-membered osmacycles [Os­{CHC­(PPh3)­CH­(Ph)­(OH)}­(CH3CN)­(PPh3)2CO]­(BF4)2 (7) and [Os­{CHC­(PPh3)­CH­(Ph)­(OH)}­(PhCN)­(PPh3)2CO]­(BF4)2 (8). In the presence of NEt3, 6 can convert
to the osmium hydrido phenylethenylcarbyne complex OsH­{CCCH­(Ph)}­(PPh3)2Cl2 (9) in wet acetonitrile,
presumably involving P–C bond cleavage. Similarly, 9 can react with HCCCOOMe with the aid of HBF4 to
give osmafuran 10.