Iron-Catalyzed Trimerization of Terminal Alkynes Enabled by Pyrimidinediimine Ligands: A Regioselective Method for the Synthesis of 1,3,5-Substituted Arenes

Abstract

The development of pyrimidine-based analogues of the well-known pyridinediimine (PDI) iron complexes enables access to a functional-group-tolerant methodology for the catalytic trimerization of terminal aliphatic alkynes. Remarkably, in contrast to established alkyne trimerization protocols, the 1,3,5-substituted arenes are the main reaction products. Preliminary mechanistic investigations suggest that the enhanced π-acidity of the pyrimidine ring, combined with the hemilability of the imine groups coordinated to the iron center, facilitates this transformation. The entry point in the catalytic cycle is an isolable iron dinitrogen complex. The catalytic reaction proceeds via a 1,3substituted metallacycle, which explains the observed 1,3,5-regioselectivity. Such a metallacycle could be isolated and represents a rare 1,3-substituted ferracycle obtained through alkyne cycloaddition.

Regioselectively switchable alkyne cyclotrimerization catalyzed by a Ni(ii)/bidentate P-ligand/Zn system with ZnI2 as an additive

“…To the best of our knowledge, such a 1,3,5-regioisomer synthesis from alkynes, with the advantages of simple catalytic systems, mild conditions, cheap reagents and excellent outcomes, has not yet been disclosed. [11][12][13][14][15][16] Substrate scope. I.…”

“…In contrast, very limited catalytic systems were found to selectively prepare the 1,3,5-substitution pattern of poly-substituted benzenes, which feature geometric symmetry and electronic uniformity and could have intriguing applications in materials science and biochemistry. 10 The synthesis of 1,3,5-substituted benzenes usually involves expensive reagents, elaborately designed ligands or structurally complicated complexes, as shown in the cases of mononuclear titanium, 11 iron 12 and indium 13 complexes, dinuclear chromium 14 and niobium 15 catalysts and polymer-supported cobalt 16 catalysts (Scheme 1A(b)). A very recent publication 17 reported the Ni( ii )–B(OH) 2 co-catalyzed cycloaddition reaction of terminal alkynes for the highly regioselective synthesis of 1,3,5-substitution products at a relatively high temperature (Scheme 1A(c)).…”

Polymer-Supported-Cobalt-Catalyzed Regioselective Cyclotrimerization of Aryl Alkynes

“…Organic light-emitting diodes, − dendrimers, , and fullerene fragments − can also be synthesized from 1,3,5-triarylbenzenes. The common synthesis routes for 1,3,5-triarylbenzenes are multistep Suzuki–Miyaura-type reactions, − acid-catalyzed triple condensation reactions of aryl methyl ketones, − and transition-metal-catalyzed cyclotrimerization reactions of aryl alkynes. − Although several transition metals, including cobalt, − nickel, − , rhodium, − and ruthenium, , are known to catalyze cyclotrimerization reactions, the selective formation of 1,3,5-triarylbenzene has rarely been achieved. − The transition-metal-catalyzed cyclotrimerization of alkynes usually generates a mixture of 1,2,4-triarylbenzene as a major product and 1,3,5-triarylbenzene as a minor product (Scheme a). − …”

Hydrogen-Bonding Controlled Nickel-Catalyzed Regioselective Cyclotrimerization of Terminal Alkynes

“…In recent years, the predominant formation of the 1,3,5isomer was observed in some reactions with elaborately designed catalysts such as the dinuclear Rh complex, 9 the dinuclear Nb catalyst, 10 the dinuclear Cr complex, 11 the halfsandwiched cobalt complex, 12 cycloheptatrienyliron, 13 and iron−pyrimidinediimine complexes. 14 practicability of these methods. In 2001, Jiang and coworkers reported a CuCl 2 -facilitated palladium-catalyzed regioselective cyclotrimerization of alkynes leading to 1,3,5-trisubstituted benzenes.…”

“…In recent years, the predominant formation of the 1,3,5-isomer was observed in some reactions with elaborately designed catalysts such as the dinuclear Rh complex, the dinuclear Nb catalyst, the dinuclear Cr complex, the half-sandwiched cobalt complex, cycloheptatrienyliron, and iron–pyrimidinediimine complexes . However, very limited examples have shown excellent selectivities and high yields with practical value.…”