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

Abstract 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(PCPCH2-iPr)(CH2CH2CH3)(CO)2]. This complex reacts with an excess of HBpin to form the Fe(II) pinacolborane complex [Fe(PCPCH2-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(PCPCH2-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.