Water is truly ubiquitous. Approximately 80% of the earth’s surface is covered by water, although only 1% of this is drinkable water. Water is the solvent of life and makes up about 60% of the mass of the human body. The majority of synthetic organic chemistry carried out in research laboratories or industrial processes utilizes organic solvents, however. Organic solvents have a number of attractive features: they will dissolve a wide range of organic compounds, they come with a variety of properties, and they are often volatile and easily removed. Unfortunately, organic solvents are often toxic, flammable, and nonrenewable and have low heat capacities. In contrast, water is nontoxic and nonflammable, has a high heat capacity, and is relatively inexpensive. Of course water has some significant drawbacks as a solvent: it is a poor solvent for most organic molecules, and it is highly reactive with many classes of reagents. Because of these drawbacks, water is rarely used as a primary solvent in synthetic organic chemistry, although there is a growing body of work related to organic chemistry in water.1-3

Hydrophilic Ligands and Their Application in Aqueous-Phase Metal-Catalyzed Reactions

Solvents play a critical role in “greening” synthetic chemistry, and this is also true in catalytic organic synthesis. This review attempts to summarize the progress made in the past a few years on homogeneous and heterogeneous catalytic reactions in the non-conventional solvents, water, ionic liquids, supercritical carbon dioxide and fluorous carbons, with the focus on those catalyzed by transition metal complexes. The reactions covered include hydrogenation, hydroformylation, carbonylation, Heck reactions, Suzuki and Stille couplings, Sonogashira reactions, allylic substitution, olefin metathesis, olefin epoxidation and alcohol oxidation.

Toward green catalytic synthesis—Transition metal-catalyzed reactions in non-conventional media

Recent advances in green catalytic oxidations of alcohols in aqueous media

Oxidation from alcohols to carboxylic acids, a class of essential chemicals in daily life, academic laboratories, and industry, is a fundamental reaction, usually using at least a stoichiometric amount of an expensive and toxic oxidant. Here, an efficient and practical sustainable oxidation technology of alcohols to carboxylic acids using pure O2 or even O2 in air as the oxidant has been developed: utilizing a catalytic amount each of Fe(NO3)3·9H2O/TEMPO/MCl, a series of carboxylic acids were obtained from alcohols (also aldehydes) in high yields at room temperature. A 55 g-scale reaction was demonstrated using air. As a synthetic application, the first total synthesis of a naturally occurring allene, i.e., phlomic acid, was accomplished.

Iron Catalysis for Room-Temperature Aerobic Oxidation of Alcohols to Carboxylic Acids

Hydrochloric acid, a very inexpensive and readily available inorganic acid, has been found to cooperate exquisitely with NaNO2/TEMPO in catalyzing the molecular-oxygen-driven oxidation of a broad range of alcohol substrates to the corresponding aldehydes and ketones This transitionmetal-free catalytic oxidative conversion is novel and represents an interesting alternative route to the corresponding carbonyl compounds to the metal-catalyzed aerobic oxidation of alcohols The reaction is highly selective with respect to the desired product when carried out at room temperature in air at atmospheric pressure Notably, the use of very inexpensive NaNO2 and HCl in combination with TEMPO for this highly selective aerobic oxidation of alcohols in air at ambient temperature makes the reaction operationally and economically very attractive ne results of mechanistic studies, performed with the aid of electrospray ionization mass spectrometry (ESI-MS), are presented and discussed TEMPO, TEMPOH, and TEMPO+ were observed in the redox cycle by means of ESI-MS On the basis of these observations, a mechanism is proposed that may provide an insight into the newly developed aerobic alcohol oxidation

TEMPO/HCl/NaNO2 Catalyst: A Transition‐Metal‐Free Approach to Efficient Aerobic Oxidation of Alcohols to Aldehydes and Ketones Under Mild Conditions

An aqueous palladium catalyst that is stabilized by a water-soluble biquinoline-based ligand was employed for the aerobic oxidation of secondary and primary alcohols. Secondary alcohols afforded the corresponding ketones in high yield with selectivities greater than 90%. Aliphatic primary alcohols were fully oxidized to carboxylic acid products under the same reaction conditions, with various amounts of ester byproduct. Benzyl alcohol can be converted to a mixture of benzaldehyde and benzoic acid, where the relative amounts are dependent on the reaction conditions, while substituted benzylic alcohols gave primarily benzaldehyde derivatives. In contrast to related systems, the catalyst used in this study is tolerant of additional coordinating groups on the benzyl alcohol substrate. In all examples of alcohol oxidation, water is used as the only reaction solvent and air is used as the oxidant, thus these conversions could serve as environmentally benign “green” alternatives to traditional oxidation methods.

James P. Clarkson

Papers

Pd(II)-biquinoline catalyzed aerobic oxidation of alcohols in water