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

Abstract

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.

Homogeneous catalysis with polyhydride complexes

“…In agreement with this, the hydroboration rate decreases by increasing the borane concentration; the catalysis is inhibited for borane : alkyne molar ratios >10. 181…”

Craig‐Hückel Hybrid Aromatic Metalla‐dehydro[11]annulenes Constructed by a Formal [10+1] Cycloaddition Reaction

“…In addition to the alkynyl group, there are two trans P(OMe) 3 groups, one Cl atom and two carbon atoms (C1 and C10) connected to the osmium center, forming an octahedral structure. To investigate the nature of interactions between the triple bond and the osmium atom, the known complex I [14] reported by Esteruelas et al. (Figure 2), in which the alkyne and the osmium center form an osma‐cyclopropene, was used for comparison.…”

How to Break the Law: trans‐Hydroboration and gem‐Hydroboration of Alkynes

“…The ruthenacyclopropene G generated via the dominant oxidative hydrogen migration process features the boryl group trans to the H‐atom and is hence perfectly set up for release of the trans ‐addition product by reductive boryl migration with formation of adduct H in the first place. Since the empty p‐orbital at boron assists in lowering the barrier of this step, the oxidative hydrogen migration is rate‐determining [56,60] …”