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

[Image: see text] Reactions of the hexahydride OsH(6)(P(i)Pr(3))(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 OsH(2)(η(2)-C(2)Me(2))(P(i)Pr(3))(2) (2). In toluene, at 80 °C, the coordinated hydrocarbon isomerizes into a η(4)-butenediyl form to afford OsH(2)(η(4)-CH(2)CHCHCH(2))(P(i)Pr(3))(2) (3). Isotopic labeling experiments indicate that the isomerization involves Me-to-C(Os) hydrogen 1,2-shifts, which take place through the metal. The reaction of 1 with 3-hexyne gives 1-hexene and OsH(2)(η(2)-C(2)Et(2))(P(i)Pr(3))(2) (4). Similarly to 2, complex 4 evolves to η(4)-butenediyl derivatives OsH(2)(η(4)-CH(2)CHCHCHEt)(P(i)Pr(3))(2) (5) and OsH(2)(η(4)-MeCHCHCHCHMe)(P(i)Pr(3))(2) (6). In the presence of pinBH, complex 2 generates 2-pinacolboryl-1-butene and OsH{κ(2)-H,H-(H(2)Bpin)}(η(2)-HBpin)(P(i)Pr(3))(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.