Aminolysis of Epoxides Using Iridium Trichloride as an Efficient Catalyst

Abstract

Iridium trichloride catalyzes the ring opening of epoxides by aryl, heterocyclic, or aliphatic amines under mild conditions. The reactions proceed at room temperature to afford the corresponding b-amino alcohols in excellent yields. In general, the aminolysis of cyclopentene oxide is faster than that of cyclohexene oxide in the presence of iridium trichloride as a catalyst.b-Amino alcohols are an important class of organic compounds that are widely used in natural products and medicinal chemistry and as chiral auxiliaries and ligands. 1,2 The most practical and commonly used method for synthesizing these compounds is the direct aminolysis of an epoxide with an excess of an amine at an elevated temperature. However, the need for high temperatures is disadvantageous for sensitive functional groups and can causes problems of regioselectivity. To eliminate such problems in the ring opening of epoxides by amine nucleophiles, several activators or promoters, such as Lewis acids or metal salts, have been introduced. 3-13 Nevertheless, in some cases yields are unsatisfactory, and several of these catalysts fail to bring about the cleavage of epoxides by deactivated or sterically hindered aromatic amines. Here, we report our results on a mild and efficient method for the synthesis of b-amino alcohols by nucleophilic ring opening of epoxides by aryl, heterocyclic, or aliphatic amines with iridium trichloride as a catalyst.It is generally believed that carbon-metal bonds in thirdrow transition-metal complexes are more stable than those of first-or second-row transition-metal complexes. Consequently, the third-row transition-metal complexes are expected to be very stable, limiting their use as catalysts in organic transformations. 14 Despite this, considerable progress has recently been made in the use of iridium complexes as catalysts in hydrogenation reactions and carbon-carbon and carbon-heteroatom bond-forming reactions. 14-17 The most commonly used catalysts for these transformations are dichloro(cyclooctadiene)iridium or dichloro(h 5 -pentamethylcyclopentadiene)iridium in the presence of a phosphine ligand, or iridium complexes that themselves contain phosphine ligands. Recently, iridium(III) chloride hydrate (IrCl 3 ·xH 2 O) was used to effect a Meinwald rearrangement for the conversion of styrene oxide and its derivatives into the corresponding aldehydes. 18 We therefore wondered whether iridium(III) chloride would catalyze the reaction of epoxides with aryl, heterocyclic, or aliphatic amines to give the corresponding amino alcohols.