Reactions of an Osmium–Hexahydride Complex with 2‑Butyne and 3‑Hexyne and Their Performance in the Migratory Hydroboration of Aliphatic Internal Alkynes

Reactions of the hexahydride OsH6(PiPr3)2 (1) with 2-butyne and 3-hexyne
and the behavior of the resulting species toward pinacolborane (pinBH)
have been investigated in the search for new hydroboration processes.
Complex 1 reacts with 2-butyne to give 1-butene and the
osmacyclopropene OsH2(η2-C2Me2)­(PiPr3)2 (2). In toluene, at 80 °C, the coordinated hydrocarbon isomerizes
into a η4-butenediyl form to afford OsH2(η4-CH2CHCHCH2)­(PiPr3)2 (3). Isotopic labeling experiments
indicate that the isomerization involves Me-to-COs hydrogen
1,2-shifts, which take place through the metal. The reaction of 1 with 3-hexyne gives 1-hexene and OsH2(η2-C2Et2)­(PiPr3)2 (4). Similarly to 2, complex 4 evolves to η4-butenediyl derivatives OsH2(η4-CH2CHCHCHEt)­(PiPr3)2 (5) and OsH2(η4-MeCHCHCHCHMe)­(PiPr3)2 (6). In the presence of pinBH, complex 2 generates
2-pinacolboryl-1-butene and OsH­{κ2-H,H-(H2Bpin)}­(η2-HBpin)­(PiPr3)2 (7). According to
the formation of the borylated olefin, complex 2 is a
catalyst precursor for the migratory hydroboration of 2-butyne and
3-hexyne to 2-pinacolboryl-1-butene and 4-pinacolboryl-1-hexene. During
the hydroboration, complex 7 is the main osmium species.
The hexahydride 1 also acts as a catalyst precursor,
but it requires an induction period that causes the loss of 2 equiv
of alkyne per equiv of osmium.