There has been a recent drive to develop asymmetric catalytic methods to produce epoxides using environmentally benign oxidants, especially hydrogen peroxide. This critical review discusses the advances that have been made using both metal-based and organocatalytic homogeneous catalysts (142 references).

Amino acids have been known to catalyze organic reactions for many years, but their boronic acid counterparts are much less well-studied. Although there are a number of useful general approaches to the synthesis of protected aminoboronic acids, many practical challenges remain in the isolation and purification of free aminoboronic acids. Despite these issues, now several different chiral and achiral aminoboronic acids show promise as bifunctional organic catalysts. In this Account, we describe both advances in the synthesis of these aminoboronic acids and some of their underdeveloped potential in catalysis. The first aminoboronic acids that demonstrated catalytic properties, such as 8-quinoline boronic acid, enabled the hydrolysis and etherification of chlorohydrins. More recently, aminoboronic acids have effectively catalyzed direct amide formation. In addition, these catalysts can enable the kinetic resolution of racemic amines during the acylation process. Aminoboronic acids can also function as aldol catalysts, acting through in situ boronate enolate formation in water, and have facilitated tunable asymmetric aldol reactions, acting through the formation of an enamine. On the basis of these examples, we expect that these molecules can catalyze an even wider range of reactions. We anticipate many further discoveries in this area.

New insights into the mechanism for the transition metal-mediated oxidation of hydroxamic acids to give intermediate acyl nitroso species, with subsequent hetero-Diels-Alder trapping are presented. The activation of triphenylphosphine-ligated ruthenium-salen complexes is examined, and evidence is presented for the ruthenium-oxo species which are involved in the oxidative process of the hydroxamic acid. The observation of the lack of asymmetric induction involved in the intermolecular cycloaddition process involving the intermediate acyl nitrsoso species is explained, with the aid of comparing the ruthenium-salen-based systems with nitrosotoluene, and copper(I)/copper(II) BINAP-based catalysis of nitrosopyridine complexes. This study demonstrates the importance of secondary coordination to achieve asymmetric induction in nitroso-DielsAlder reactions.

New transition metal catalysts have been prepared and applied for in situ formation of acyl nitroso dienophiles.

No abstract