Efficient Organoruthenium Catalysts for α‐Alkylation of Ketones and Amide with Alcohols: Synthesis of Quinolines via Hydrogen Borrowing Strategy and their Mechanistic Studies

Abstract

A new family of phosphine free organometallic ruthenium(II) catalysts (Ru1–Ru4) supported by bidentate NN Schiff base ligands (L1–L4 where L1=N,N‐dimethyl‐4‐((2‐phenyl‐2‐(pyridin‐2‐ylmethyl)hydrazineylidene)methyl) aniline, L2=N,N‐diethyl‐4‐((2‐phenyl‐2‐(pyridin‐2‐ylmethyl)hydrazineylidene)methyl)aniline, L3=N,N‐dimethyl‐4‐((2‐phenyl‐2‐(pyridin‐2‐yl)hydrazineylidene)methyl)‐ aniline and L4=N,N‐diethyl‐4‐((2‐phenyl‐2‐(pyridin‐2‐yl)hydrazineylidene)methyl) aniline) was prepared and characterized. These half‐sandwich complexes acted as catalysts for C−C bond formation and exhibited excellent performance in the dehydrogenative coupling of ketones and amides. In the synthesis of C–C bonds, alcohols were utilized as the alkylating agent. A broad range of substrates, including sterically hindered ketones and alcohols, were well tolerated under the optimized conditions (TON up to 47000 and TOF up to 11750 h−1). This ruthenium (II) catalysts were also active towards the dehydrogenative cyclization of o‐amino benzyl alcohol for the formation of quinolines derivatives. Various polysubstituted quinolines were synthesized in moderate to excellent yields (TON up to 71000 and TOF up to 11830 h−1). Control experiments were carried out and the ruthenium hydride intermediate was characterized to support the reaction mechanism and a probable reaction pathway of dehydrogenative coupling for the C−C bond formation has been proposed.

SmI2‐mediated C‐alkylation of Ketones with Alcohols under Microwave Conditions: A Novel Route to Alkylated Ketones

“…Moreover, Figure 4 illustrates a plausible dehydrogenative annulation mechanism in light of available literature and control experiments. Under standard condition, oxidation of 2‐aminobenzyl alcohol to 2‐aminobenzaldehyde takes place [17] . Followed by, aldol condensation with acetophenone to give intermediate I .…”

“…Under standard condition, oxidation of 2aminobenzyl alcohol to 2-aminobenzaldehyde takes place. [17] Followed by, aldol condensation with acetophenone to give intermediate I. As the reaction proceed equilibrium would shift towards ketone intermediate II which on dehydrogenative cyclisation affords product 4 a.…”

Atom‐Economic Alk(en)ylations of Esters, Amides, and Methyl Heteroarenes Utilizing Alcohols Following Dehydrogenative Strategies

“…In a recent report, Ghosh and co‐workers reported the bidentate Ru−NN complexes as effective catalysts for the α‐alkylation of ketones and amides [14] . They have employed four related complexes with different chelation size and groups on aryl ring of the ligands.…”

“…In a recent report, Ghosh and co-workers reported the bidentate RuÀ NN complexes as effective catalysts for the αalkylation of ketones and amides. [14] They have employed four related complexes with different chelation size and groups on aryl ring of the ligands. The complexes with six membered chelate show excellent activity at very low concentration of 0.01 mol%, however, the ligand aryl ring substituents do not show any considerable difference in their activities (Scheme 10).…”

“…Subsequently, Balaraman et al demonstrated the activity of analogues MnÀ PNP pincer complexes for the same reaction. [21] After a careful study on the impact of substituents at the ligand back bone, they have identified the complex bearing cyclohexyl substituted phosphine group (14) as the best catalyst which displayed excellent activity at 0.5 mol% loading in presence of KO t Bu (1.2 eq.) at 80-110 °C (Scheme 17).…”