Pincer Iron Hydride Complexes for Alkene Isomerization: Catalytic Approach to Trisubstituted (<i>Z</i>)‑Alkenyl Boronates

An iron dichloride complex [Fe]­Cl2 supported by a pincer phosphine–pyridine–imidazoline (PNNimid) ligand {[Fe]­Cl2 = (PNNimid)­FeCl2}, upon activation with NaHBEt3, catalyzes the isomerization of 1,1-disubstituted alkenyl boronates to synthetically valuable but previously difficult-to-access trisubstituted (Z)-alkenyl boronates with excellent regio- and stereoselectivity. The loading of the catalyst activator relative to iron was found to affect the selectivity and catalytic efficiency. In situ solvent-assisted electrospray ionization mass (SAESI-MS) studies revealed the generation of two catalytically competent species depending on the Fe/NaHBEt3 ratios: the reaction of [Fe]­Cl2 with 1.5 equiv NaHBEt3 predominantly formed a monohydride chloride [Fe]­HCl, while treatment with 3 equiv NaHBEt3 furnished a dihydride [Fe]­H2. In addition, the iron alkyl intermediates resulting from the insertion of the alkenyl boronate into the Fe–H bonds of [Fe]­HCl and [Fe]­H2 were successfully captured by SAESI-MS. The iron hydride catalysts are sensitive to the steric properties of the alkene substrates: the monohydride [Fe]­HCl is efficient for the synthesis of less hindered alkyl-bearing trisubstituted (Z)-alkenyl boronates, whereas the dihydride [Fe]­H2 is a favorable catalyst for generation of sterically more demanding aryl-substituted products. The synthetic utility of these trisubstituted (Z)-alkenyl boronate products was demonstrated by stereoselective synthesis of multisubstituted conjugated dienes and cyercene A.