Piano-Stool Iron Complexes as Precatalysts for gem-Specific Dimerization of Terminal Alkynes

Abstract

A series of piano-stool Fe−NHC complexes have been prepared and characterized. The NHC ligands used herein possess a benzyl and a mesityl wingtip groups and have different electronic structures within the NHC rings. The catalytic activities of these Fe complexes have been examined for the homodimerization of terminal alkynes.

Syntheses and Reactivity of Piano-Stool Iron Complexes of Picolyl-Functionalized N-Heterocyclic Carbene Ligands

“…In comparison to the metric parameters of HL in 1 , the C–N bonds in the C 5 N rings of the L – ligands in 2 are elongated and the C–C bonds of the C 5 N rings of the L – ligands in 2 display a more pronounced alternating long–short pattern. Such structural features are similar to those of the previosuly reported Mes L – ligand . The C–C bonds that are exocyclic with respect to the C 5 N rings in 2 , i.e., C5–C6 and C27–C28, are 1.360(4) are 1.353(5) Å in length, respectively, which are typical for C–C double bonds and much shorter than that in 1 (1.497(4) Å).…”

“…Moreover, various substituents on the wingtips of NHCs can be used to fine-tune the steric and electronic properties of the catalysts. Consequently, many transition-metal complexes featuring bidentate N-picolyl-NHC ligands have emerged recently, some of which have exhibited interesting stoichiometric and catalytic reactivity involving bifunctional behaviors. , …”

Reactivity of an Unprotected Mesoionic N-Heterocyclic Olefin

“…We have recently reported the catalytic activity of the iron complex of an MIC derived from a 1,2,3-triazolium salt bearing a benzyl group at the 4position, where the pro-carbenic position-5 of the triazole ring is unsubstituted. 11 Herein, we report the synthesis of an unprotected mNHO from the same 1,2,3-triazolium salt. The reactivity of this new mNHO toward various group 13 Lewis acids and CO 2 has been explored, where the isomerization of the Lewis acid adducts have been studied through a combination of experiments and computation.…”

Late 3d Metal-Catalyzed (Cross-) Dimerization of Terminal and Internal Alkynes

“…At first the precatalyst [Fe]-4 is activated by lithium alkynylide, generated by deprotonation of an alkyne via LiHMDS, to form complex F. Coordination of another alkyne leads to complex G, followed by migratory insertion to give complex H. σ-Bond metathesis releases the gem-enyne product 4 and the cyclometalated complex I. This complex is coordinated by another alkyne to form the η 2 -coordinated complex J, which isomerizes in the rate-determining step into the complex K. This complex undergoes another σ-bond metathesis into the active species F (Liang et al, 2019;Liang et al, 2020a). The high regioselectivity in this catalysis can be explained by complex G. If the alkyne would coordinate in the inverted manner, the head-to-head dimerization product would be released.…”

“…This complex is coordinated by another alkyne to form the η 2 -coordinated complex J , which isomerizes in the rate-determining step into the complex K . This complex undergoes another σ-bond metathesis into the active species F ( Liang et al, 2019 ; Liang et al, 2020a ). The high regioselectivity in this catalysis can be explained by complex G .…”