Hydroboration of Alkynes Catalyzed by Fe(II) PCP Pincer Complexes

A stereo- and regioselective Fe-(II)-catalyzed hydroboration of both terminal and internal alkynes with pinacolborane (HBpin) is described. The hydroboration reaction is in all cases highly syn-selective. The reaction requires no additives and proceeds with a catalyst loading of 1-2 mol % at 45-55 °C. An active pre-catalyst is the bench-stable alkyl Fe-(II) complex [Fe-(PCP^CH2-iPr)-(CH₂CH₂CH₃)-(CO)₂]. This complex reacts with an excess of HBpin to form the Fe-(II) pinacolborane complex [Fe-(PCP^CH2-iPr)-(κ²-H₂BPin)-(CO)], which is an equally active pre-catalyst but more difficult to handle, as it decomposes in the absence of substrates. Both pre-catalysts afford the highly reactive hydride catalyst [Fe-(PCP^CH2-iPr)-(H)-(CO)] under catalytic conditions. A broad variety of aromatic and aliphatic alkynes were efficiently and selectively borylated. Mechanistic insights are provided based on experimental data and DFT calculations.