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 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.