No abstract

We report the syntheses and catalytic activity of a series of piano-stool iron complexes with the general formula [FeClCp*(NHC)] (where NHC = N-heterocyclic carbene) toward the gem-specific dimerization of terminal alkynes. In comparison to our first-generation catalyst, the newly synthesized catalyst is more active and features the same geminal specificity. Both the experimental and computational data are presented herein.

We report a gem-specific homo- and cross-dimerization of terminal alkynes catalyzed by a well-defined iron(II) complex containing Cp* and picolyl N-heterocyclic carbene (NHC) ligands, and featuring a piano-stool structure. This catalytic system requires no additives and is compatible with a broad range of substrates, including those with polar functional groups such as NH and OH.

This review article summarizes recent development of homogeneous iron N-heterocyclic carbene catalysts.

1,3-Enynes are core structures of various natural products or pharmaceuticals and are broadly used synthons in organic synthesis. Metal-catalyzed alkyne dimerization is a desirable preparation method, due to its perfect atom economy and the readily available alkyne substrates. Controlling the regio- and stereoselectivity remains a challenge, due to the competing formation of the head-to-tail (gem) and head-to-head (E/Z) isomers. Although catalytic systems based on noble metals have been extensively studied, there has been a growing interest to replace these noble metals with environmentally benign and inexpensive alternatives. In this Perspective, we highlight recent advances in catalytic alkyne dimerization, without the use of noble metals.

We report a novel mesoionic N-heterocyclic olefin (mNHO) derived from a 1,2,3-triazolium salt. With the carbenic position unprotected, the mNHO can tautomerize into the mesoionic carbene (MIC) form at room temperature. The reactivity of this new mNHO with various group 13 Lewis acids and CO2 have been studied experimentally and computationally.

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.

Complexes [MClCp*(HL)] (1[Fe]/1[Ru]) (where HL = 1-mesityl-3-(pyridin-2-ylmethyl)imidazol-2-ylidene) were synthesized from the reaction of in situ generated HL ligand and [FeClCp*(TMEDA)] (where TMEDA is N,N,N',N'-tetramethylethylenediamine) or [RuClCp*], respectively. The deprotonation of 1[Fe]/1[Ru] with 1 equiv of LiHMDS led to cyclometalation through the o-Me of the mesityl group, forming [MCp*(L-κC,κC',κN)] 2[Fe]/2[Ru]. The coordinatively unsaturated compounds [MCp*(HL)]BPh 3[Fe]/3[Ru] were prepared from 1[Fe]/1[Ru] and halide scavenger NaBPh. Complex 3[Ru] showed agostic interactions between the o-Me group of the mesityl moiety and the metal center in solution and the solid state. When the vacant coordination site of 3[Fe]/3[Ru] is occupied by CO, the resulting [MCp*(CO)(HL)]BPh 4[Fe]/4[Ru] can be deprotonated with 1 equiv of KHMDS at the pyridylic position to afford complexes [MCp*L'(CO)] 5[Fe]/5[Ru], where the L' ligand chelates to the metal center through the nitrogen donor atom of the dearomatized pyridine ring and the carbene carbon. Complex 2[Fe] reacted rapidly with CO to afford the simple ligand substitution product [FeCp*(L-κC,κC')(CO)] 6[Fe], where the L acts as a bidentate chelating ligand through the carbene carbon and benzylic carbon. Under the same condition, the reaction of 2[Ru] with CO forms [RuCp*L″(CO)] 7[Ru], where the L″ ligand (an isomer of L and L') chelates to the metal center through the carbene carbon and a pyridyl carbon. Complexes 3[Fe]/3[Ru] reversibly bind dinitrogen to form [MCp*(HL)(N)]BPh 8[Fe]/8[Ru]. 3[Ru] reversibly binds dihydrogen to give [MCp*(H)(HL)]BPh 9[Ru], while no reaction was observed between 3[Fe] and H. The reaction of 3[Ru] with dioxygen led to the isolation of a stable side-on O complex [RuCp*(HL)(O)]BPh 10[Ru], while the reaction of 3[Fe] with dioxygen led to an intractable mixture of products.