Acridine-Based SNS–Ruthenium Pincer Complex-Catalyzed Borrowing Hydrogen-Mediated C–C Alkylation Reaction: Application to the Guerbet Reaction

Abstract

Herein, we describe the application of SNS-based ruthenium pincer catalyst for Guerbet condensation reaction of primary alcohols to prepare β-alkylated dimer alcohols in good yields. Moreover, the scope of these complexes to convert ethanol to butanol was also investigated. Furthermore, the work was extended toward the C-alkylation of secondary alcohols with primary alcohols to give α-alkylated ketones. Several control experiments were also conducted that shows the involvement of “borrowing hydrogen” in the protocol.

Structurally Well-Defined PC(sp3)P Osmium Pincer Catalysts for Methylation of Alcohols, Indoles, Phenols, and Sulfonamides Using Methanol as a C1 Source

“…The most spectacular examples include unprotected indoles that can be methylated selectively into the branched homologues. On the other hand, when aliphatic primary and secondary alcohols such as octan-1-ol, dodecan-1-ol, and cyclohexanol were allowed to react with methanol, the desired products were obtained with moderate to good yields ( 7e – f ), apparently, due to unsynchronized starting alcohol/methanol dehydrogenation rates, which causes the accumulation of the dehydrogenated intermediates and facilitates competitive Guerbet homocoupling . Another limitation was the presence of unprotected amines that are readily methylated under the described reaction conditions, and, for example, attempted β-methylation of 2-(4-aminophenyl)­ethan-1-ol produced bis-methylated product in 85% yield (entry 7b ).…”

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.…”