Tuning the selectivity in iridium-catalyzed acceptorless dehydrogenative coupling of primary alcohols

An acceptorless dehydrogenative coupling of primary alcohols to carboxylic acids/carboxylates, esters, and Guerbet alcohols (via both homo- and cross-β-alkylation of the alcohols) in the presence of an N-heterocyclic carbene iridium(I) catalyst was developed under aerobic conditions. The product selectivity can be easily tuned among the products with a single catalyst through simple modification of the reaction conditions, such as the catalyst and base amounts, the choice of base, and the reaction temperature.

Acceptorless dehydrogenation of alcohols to carboxylic acids by palladium nanoparticles supported on NiO: delving into metal-support cooperation in catalysis

In this work, we have developed a simple NiO-supported Pd nanocatalyst (Pd@NiO) for oxidant-free dehydrogenative oxidation of primary alcohols to carboxylic acids along with hydrogen gas as a byproduct. The catalyst has been characterized by techniques like XRD, HRTEM, SEM-EDX, XPS and ICP-AES. The nanostructured Pd@NiO material showed excellent dehydrogenative oxidation activity and outperformed the activity of free NiO or Pd nanoparticles supported on silica/carbon as a catalyst, which could be attributed to synergistic effect of Pd and NiO. A diverse range of aromatic and aliphatic primary alcohols could be efficiently converted to their corresponding carboxylates in high yields with a catalyst loading as low as 0.08 mol%. Notably, highly challenging biomass derived heterocyclic alcohols such as furfuryl alcohol and piperonyl alcohol can also be efficiently converted to their corresponding acids. Moreover, our catalyst can convert benzyl alcohol to benzoic acid on a gram scale with 89% yield. Interestingly, the H₂ gas liberated in the reaction can also be used as a substrate for the hydrogenation of 3a to 4a in 65% yield. The nanostructured catalyst is highly reusable and no significant decrease in activity was observed after six reaction cycles. A kinetic study revealed that the reaction followed first-order kinetics with a rate constant of k = 1.47 × 10^-4 s^-1, under optimized conditions. The extent of reactivity of different functionalities towards dehydrogenation was also investigated using a Hammett plot showing good linearity.

Cyclometalated (NNC)Ru(II) complex catalyzed β-methylation of alcohols using methanol

Indolyl fragment containing phenanthroline based new ligands and their corresponding Ru(II) complexes were synthesized and fully characterized by various spectroscopic techniques. Catalytic activity of these newly synthesized cyclometalated (NNC)Ru(II) complexes...

Bulky PNP ligands blocking metal-ligand cooperation allow for isolation of Ru(0), and lead to catalytically active Ru complexes in acceptorless alcohol dehydrogenation

We synthesized two 4Me-PNP ligands which block metal-ligand cooperation (MLC) with the Ru center and compared their Ru complex chemistry to their two traditional analogues used in acceptorless alcohol dehydrogenation catalysis. The corresponding 4Me-PNP complexes, which do not undergo dearomatization upon addition of base, allowed us to obtain rare, albeit unstable, 16 electron mono CO Ru(0) complexes. Reactivity with CO and H 2 allows for stabilization and extensive characterization of bis CO Ru(0) 18 electron and Ru(II) cis and trans dihydride species that were also shown to be capable of C(sp2)-H activation. Reactivity and catalysis are contrasted to non-methylated Ru(II) species, showing that an MLC pathway is not necessary, with dramatic differences in outcomes during catalysis between i Pr and t Bu PNP complexes within each of the 4Me and non-methylated backbone PNP series being observed. Unusual intermediates are characterized in one of the new and one of the traditional complexes, and a common catalysis deactivation pathway was identified.

Recent Development in the Synthesis and Catalytic Application of Iridacycles

Cyclometallated complexes are well-known and have found many applications. This article provides a short review on the progress made in the synthesis and application to catalysis of cyclometallated half-sandwich Cp*Ir(III) complexes (Cp*: pentamethylcyclopentadienyl) since 2017. Covered in the review are iridacycles featuring conventional C,N chelates and less common metallocene and carbene-derived C,N and C,C ligands. This is followed by an overview of the studies of their applications in catalysis ranging from asymmetric hydrogenation, transfer hydrogenation, hydrosilylation to dehydrogenation.

Efficient methylation of anilines with methanol catalysed by cyclometalated ruthenium complexes

Cyclometalated ruthenium allow the effective and selective N-methylation of anilines under mild conditions.

Direct Oxidation of Primary Alcohols to Carboxylic Acids

Oxidation of primary alcohols to carboxylic acids is a fundamental transformation in organic chemistry, yet despite its simplicity, extensive use, and relationship to pH, it remains a subject of active research for synthetic organic chemists. Since 2013, a great number of new methods have emerged that utilize transition-metal compounds as catalysts for acceptorless dehydrogenation of alcohols to carboxylates. The interest in this reaction is explained by its atom economy, which is in accord with the principles of sustainability and green chemistry. Therefore, the methods for the direct synthesis of carboxylic acids from alcohols is ripe for a modern survey, which we provide in this review.

1 Introduction

2 Thermodynamics of Primary Alcohol Oxidation

3 Oxometalate Oxidation

4 Transfer Dehydrogenation

5 Acceptorless Dehydrogenation

6 Electrochemical Methods

7 Outlook

Cyclometalated Ruthenium Pincer Complexes as Catalysts for the α-Alkylation of Ketones with Alcohols

Ruthenium PNP pincer complexes bearing supplementary cyclometalated C,N‐bound ligands have been prepared and fully characterized for the first time. By replacing CO and H⁻ as ancillary ligands in such complexes, additional electronic and steric modifications of this topical class of catalysts are possible. The advantages of the new catalysts are demonstrated in the general α‐alkylation of ketones with alcohols following a hydrogen autotransfer protocol. Herein, various aliphatic and benzylic alcohols were applied as green alkylating agents for ketones bearing aromatic, heteroaromatic or aliphatic substituents as well as cyclic ones. Mechanistic investigations revealed that during catalysis, Ru carboxylate complexes are predominantly formed whereas neither the PNP nor the CN ligand are released from the catalyst in significant amounts.

A cyclometalated Ir(III)-NHC complex as a recyclable catalyst for acceptorless dehydrogenation of alcohols to carboxylic acids

In this work, we have synthesized two new [C, C] cyclometalated Ir(iii)-NHC complexes, [IrCp*(C∧C:NHC)Br](1a,b), [Cp* = pentamethylcyclopentadienyl; NHC = (2-flurobenzyl)-1-(4-methoxyphenyl)-1H-imidazoline-2-ylidene (a); (2-flurobenzyl)-1-(4-formylphenyl)-1H-imidazoline-2-ylidene (b)] via intramolecular C-H bond activation. The molecular structure of complex 1a was determined by X-ray single crystal analysis. The catalytic potentials of the complexes were explored for acceptorless dehydrogenation of alcohols to carboxylic acids with concomitant hydrogen gas evolution. Under similar experimental conditions, complex 1a was found to be slightly more efficient than complex 1b. Using 0.1 mol% of complex 1a, good-to-excellent yields of carboxylic acids/carboxylates have been obtained for a wide range of alcohols, both aliphatic and aromatic, including those involving heterocycles, in a short reaction time with a low loading of catalyst. Remarkably, our method can produce benzoic acid from benzyl alcohol on a gram scale with a catalyst-to-substrate ratio as low as 1 : 5000 and exhibit a TON of 4550. Furthermore, the catalyst could be recycled at least three times without losing its activity. A mechanism has been proposed based on controlled experiments and in situ NMR study.

Highly Efficient N-Heterocyclic Carbene/Ruthenium Catalytic Systems for the Acceptorless Dehydrogenation of Alcohols to Carboxylic Acids: Effects of Ancillary and Additional Ligands

The transition-metal-catalyzed alcohol dehydrogenation to carboxylic acids has been identified as an atom-economical and attractive process. Among various catalytic systems, Ru-based systems have been the most accessed and investigated ones. With our growing interest in the discovery of new Ru catalysts comprising N-heterocyclic carbene (NHC) ligands for the dehydrogenative reactions of alcohols, we designed and prepared five NHC/Ru complexes ([Ru]-1–[Ru]-5) bearing different ancillary NHC ligands. Moreover, the effects of ancillary and additional ligands on the alcohol dehydrogenation with KOH were thoroughly explored, followed by the screening of other parameters. Accordingly, a highly active catalytic system, which is composed of [Ru]-5 combined with an additional NHC precursor L5, was discovered, affording a variety of acid products in a highly efficient manner. Gratifyingly, an extremely low Ru loading (125 ppm) and the maximum TOF value until now (4800) were obtained.