Ligand-Controlled Ruthenium-Catalyzed Borrowing-Hydrogen and Interrupted-Borrowing-Hydrogen Methodologies: Functionalization of Ketones Using Methanol as a C1 Source

Abstract

Herein we report simple, highly efficient, and phosphine-free N,C–Ru and N,N–Ru catalysts for ligand-controlled borrowing-hydrogen (BH) and interrupted-borrowing-hydrogen (I-BH) methods, respectively. This protocol has been employed on a variety of ketones using MeOH as a green, sustainable, and alternative C1 source to form a C–C bond through the BH and I-BH methods. Reasonably good substrate scope, functional group tolerance, and good-to-excellent yields at 70 °C are the added highlights of these methodologies. Controlled experiments reveal that an in situ formed formaldehyde is one of the crucial elements in this ligand-controlled selective protocol, which upon reaction with a ketone generates an enone as an intermediate. This enone in the presence of the N,C–Ru catalyst and N,N–Ru catalyst through the BH and I-BH pathways yields methylated ketones and 1,5-diketones, respectively.

Transition metal-catalysis in interrupted borrowing hydrogen strategy

“…Very recently, Biswal et al , reported phosphine-free ligand-controlled ruthenium-catalyzed IBH functionalization of ketones for the formation of C–C bond using methanol as a C1 Source. 47 In this report by changing the ligand system authors achieved both BH and IBH products as shown in Scheme 17. Optimization studies reveals that at low loading of base and Ru–N,C catalyst 35 gives the BH product (α-methyl propiophenone 37 ), wherein Ru–N,N catalyst 36 at high loading of base gives IBH product (1,5-diketones 38 ).…”

“…51 Later, the transition-metal-catalyzed coupling of indoles with alcohols is an alternate method developed by various research groups. 38–47 In 2007, Grigg and coworkers reported that the [{Cp*IrCl 2 } 2 ]-catalyzed ( i.e. Cat.…”

“…51 Later, the transition-metal-catalyzed coupling of indoles with alcohols is an alternate method developed by various research groups. [38][39][40][41][42][43][44][45][46][47] In 2007, Grigg and coworkers reported that the [{Cp*IrCl 2 } 2 ]-catalyzed (i.e. Cat.1) coupling of indoles with aliphatic alcohols (except methanol) to form C3alkylated indoles (only minor bisindolylmethanes were formed as by-products).…”

Synthesis, structural, and photophysical properties of pyrazolyl bis(pentafluorophenyl)boron complexes

“…7 In addition, substituted pyrazoles are also used as herbicides, pesticides, supramolecular entity units, UV stabilizers and ligands in coordination chemistry and catalysis, and also in photoinduced electron transfer systems. 8 However, their applications as fluorescent materials are not well studied. Hence, there is a growing interest in the preparation of structurally diverse pyrazoles and the investigation of their fluorescent behaviour.…”

Ruthenium(II)-Catalyzed Hydrogenation and Tandem (De)Hydrogenation via Metal–Ligand Cooperation: Base- and Solvent-Assisted Switchable Selectivity

“…Apart from these challenges, methanol has been utilized as a hydrogen source in the past few years for catalytic transfer hydrogenation and tandem (de)­hydrogenation reactions . Among a few selected examples, the utilization of the nickel­(I)–hydride complex by García, [Ir­(I)­Cl­(cod) 2 ] along with 1,2-bis­(diphenylphosphino)­ethane (dppe) by Obora, cyclometalated rhodium­(III) complex by Xiao, iridium­(I)- N -heterocyclic carbene (NHC) complex by Crabtree, cyclometalated ruthenium complexes by Beller, anionic iridium­(III) complex by Li, supported iridium catalyst by Loh, the combination of [RuCp*Cl 2 ] n and dpePhos by Seayad, bifunctional Ru­(II) complex by Kundu, PNHP–Pincer Ru catalysts by Kayaki, RuCl 3 by Natte, bifunctional ruthenium catalyst [( p -cymene)­Ru­(2,2′-bpyO)­(H 2 O)] by Li, ruthenium­(II) complexes with N -heterocyclic carbene–phosphine ligands by Ke, acridine-derived SNS–Ru pincer by Srimani, palladium phosphine and phosphine-free N,C–Ru and N,N–Ru catalysts by Chandresekhar and Venkatasubbaiah, and [Cp*Ir­(2,2′-bpyO)­(OH)]Na by Xing is noteworthy, in which methanol is utilized as both a hydrogen-transferring and methylating agent. Herein, we disclose the utilization of the interconvertible coordination mode (imino N → Ru and amido N–Ru) , of imidazole to Ru­(II)– para -cymene for solvent/base-assisted switchable transfer hydrogenation and tandem (de)­hydrogenation of unsaturated chalcones using methanol as a hydrogen-transferring and methylating agent.…”