Catalytic 1,3-Proton Transfer in Alkenes Enabled by Fe=NR Bond Cooperativity: A Strategy for pK _a-Dictated Regioselective Transposition of C=C Double Bonds

Transitionmetal catalyzed alkene double bond transpositionusuallyinvolves metal hydride intermediates. Despite significant advancesin the design of catalysts that dictate product selectivity, controlover substrate selectivity is less advanced and transition metal catalyststhat selectively transpose double bonds in substrates containing multiple1-alkene functionalities are rare. Herein, we report that the three-coordinatehigh spin (S = 2) Fe-(II) imido complex [Ph2B-( (t) BuIm)(2)Fe=NDipp]-[K-(18-C-6)-THF2] (1-K-(18-C-6)) catalyzes 1,3-proton transferfrom 1-alkene substrates to afford 2-alkene transposition products.Mechanistic investigations involving kinetics, competition, and isotopelabeling studies, supported by experimentally calibrated DFT computations,strongly support an unusual nonhydridic mechanism for alkene transpositionthat is enabled by the cooperative action of the iron center and basicimido ligand. As dictated by the pK (a) ofthe allylic protons, this catalyst enables the regioselective transpositionof C=C double bonds in substrates containing multiple 1-alkenes.The high spin (S = 2) state of the complex allowsa wide scope of functional groups to be tolerated, including thosethat are typical catalyst poisons, such as amines, N-heterocycles, and phosphines. These results demonstrate a new strategyfor metal-catalyzed alkene transposition with predictable substrateregioselectivity.