The conversion of glycerol selectively to lactic acid has been accomplished in high yields (ca. 90 %) by a NNN pincer-Ru catalyst. DFT explains the role of Ru-P bond and...
We report the use of ruthenium–NNN‐pincer complexes of the type (R2NNN)RuCl2(PPh3) (R = tBu, iPr, Cy and Ph) for the catalytic N‐alkylation of primary amines under solvent‐free conditions. For the first time, the base that is required to promote these reactions is generated in situ from the alcohol by the use of sodium. The resulting sodium alkoxide regenerates the alcohol substrate while acting as the water scavenger thus mitigating the need of an additional base. Among the catalysts screened, (tBu2NNN)RuCl2(PPh3) (0.02 mol‐%) gives very high turnovers and good yields at 140 °C. The (tBu2NNN)RuCl2(PPh3) catalyzed N‐alkylation tolerates a variety of amine and alcohol substrates. While excellent turnover (29000) was obtained for the (tBu2NNN)RuCl2(PPh3) (0.002 mol‐%) catalyzed alkylation of aniline with cyclohexyl methanol, the turnovers obtained in the corresponding catalytic methylation of p‐anisidine was also very high (12000). The (tBu2NNN)RuCl2(PPh3) catalyzed reactions have also been accomplished under open‐vessel conditions resulting in a net dehydrogenative coupling reaction. This protocol has been used to transform benzene‐1,2‐diamines to benzimidazoles with high productivity (12000 turnovers). DFT studies indicate that while β‐hydride elimination is rate‐determining (RDTS: 24.31 kcal/mol) for the alcohol dehydrogenation segment which is endothermic, insertion of the imine is rate‐determining (RDTS: 11.26 kcal/mol) for its hydrogenation that is exothermic.
The
synthesis and characterization of a pincer-nickel complex of
the type (
iPr2NNN)NiCl2(CH3CN) is reported here. We have demonstrated the utility of
this pincer-nickel complex (0.02 and 0.002 mol %) for the catalytic
N-alkylation of amines using various alcohols. Under solvent-free
conditions, while the highest yield (ca. 90%) was obtained for the
alkylation of 2-aminopyridine with naphthyl-1-methanol, excellent
turnovers (34000 TONs) were observed for the alkylation of 2-aminopyridine
with 4-methoxybenzyl alcohol. To demonstrate the synthetic utility
of these systems, high-yield reactions (up to 98%) have been probed
for representative substrates with a higher loading of the pincer-nickel
catalyst (4 mol %). DFT studies indicate that while β-hydride
elimination is the RDS for alcohol dehydrogenation, the N-alkylated
product can be formed either via hydrogenation with a rate-determining
σ-bond metathesis or by alcoholysis that has imine insertion
as the RDS. All of the corresponding resting states have been observed
by HRMS (ESI) analysis. The labeling experiments are also complementary
to DFT studies and show evidence for the involvement of the benzylic
C–H bond in the RDS with a k
CHH/k
CHD value of about 2.5. This method
has been applied to accomplish efficient (2000 TONs) dehydrogenative
coupling leading to various benzimidazoles.
A series of NNN pincer‐ruthenium complexes (R2NNN)RuCl2(PPh3) (R=Cyclohexyl (Cy), t‐butyl (tBu), i‐propyl (iPr) and phenyl (Ph)) have been synthesized and characterized. These pincer‐ruthenium complexes have been used to catalyse the Kharasch addition or atom transfer radical addition (ATRA) of carbon tetrachloride to styrene. Among the pincer‐ruthenium catalysts screened for the Kharasch addition, the catalytic activity followed the order (Cy2NNN)RuCl2(PPh3)>(iPr2NNN)RuCl2(PPh3)≫(Ph2NNN)RuCl2(PPh3). The oxidation of Ru(II) is easier with (Cy2NNN)RuCl2(PPh3) and (iPr2NNN)RuCl2(PPh3) in comparison with
RuCl2(PPh3) as indicated by cyclic voltammetry studies. The catalyst precursor (R2NNN)RuCl2(PPh3) itself is the resting state of the reaction. The rate determining step involves the generation of the five‐coordinate 16‐electron ruthenium(II) species (R2NNN)RuCl2. Owing to weaker binding of triphenyl phosphine to ruthenium, the generation of catalytically active 16‐electron species (Cy2NNN)RuCl2 and (iPr2NNN)RuCl2 are more favourable. The complex (Cy2NNN)RuCl2(PPh3) demonstrates very high productivity (5670 turnovers after 48 h at 140 °C) in the absence of any co‐catalyst radical initiator. To the best of our knowledge, our turnovers (ca. 5670) are much higher than that reported hitherto. Quantum mechanical calculations demonstrate that the path involving the activation of carbon tetrachloride by (Cy2NNN)RuCl2 is more favoured than the path where carbon tetrachloride is activated by (Cy2NNN)RuCl2(η2‐styrene). Density functional theory (DFT) and kinetic studies are in accord with the widely accepted mechanism involving the single electron transfer (SET) from ruthenium(II) to chloride radical with concomitant generation of a benzyl radical which is trapped by the resulting ruthenium(III) species.magnified image
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.