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

Yafei Gao; Xuelian Li; Jeremiah E Stevens; Hao Tang; Jeremy M Smith

doi:10.1021/jacs.2c13350

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

J Am Chem Soc. 2023 Jun 7;145(22):11978-11987. doi: 10.1021/jacs.2c13350. Epub 2023 May 25.

Authors

Yafei Gao¹, Xuelian Li², Jeremiah E Stevens³, Hao Tang², Jeremy M Smith¹

Affiliations

¹ Department of Chemistry, Indiana University, Bloomington, Indiana 47405, United States.
² College of Chemistry and Materials Engineering, Wenzhou University, Wenzhou, Zhejiang 325035, P. R. China.
³ Department of Chemistry and Biochemistry, The Ohio State University, Columbus, Ohio 43210, United States.

PMID: 37227372
DOI: 10.1021/jacs.2c13350

Abstract

Transition metal catalyzed alkene double bond transposition usually involves metal hydride intermediates. Despite significant advances in the design of catalysts that dictate product selectivity, control over substrate selectivity is less advanced and transition metal catalysts that selectively transpose double bonds in substrates containing multiple 1-alkene functionalities are rare. Herein, we report that the three-coordinate high spin (S = 2) Fe(II) imido complex [Ph₂B(^tBuIm)₂Fe═NDipp][K(18-C-6)THF₂] (1-K(18-C-6)) catalyzes 1,3-proton transfer from 1-alkene substrates to afford 2-alkene transposition products. Mechanistic investigations involving kinetics, competition, and isotope labeling studies, supported by experimentally calibrated DFT computations, strongly support an unusual nonhydridic mechanism for alkene transposition that is enabled by the cooperative action of the iron center and basic imido ligand. As dictated by the pK_a of the allylic protons, this catalyst enables the regioselective transposition of C═C double bonds in substrates containing multiple 1-alkenes. The high spin (S = 2) state of the complex allows a wide scope of functional groups to be tolerated, including those that are typical catalyst poisons, such as amines, N-heterocycles, and phosphines. These results demonstrate a new strategy for metal-catalyzed alkene transposition with predictable substrate regioselectivity.