Hydroamination of Alkenes

Abstract

The addition of an amine NH-functionality to alkenes (including vinyl arenes, conjugated dienes, allenes or ring-strained alkenes), the so-called hydroamination, represents a simple and highly atom-economical approach for the synthesis of nitrogen-containing products. A large variety of catalyst systems are available, ranging from alkali, alkaline earth, rare earth, Group 4 and Group 5 metals, to late transition metal catalysts, and, less prominent, Bronsted and Lewis acid-based catalyst systems. The mode of operation of these catalyst systems can vary significantly and the different reaction mechanisms and the scope and limitations are discussed. While intramolecular hydroamination reactions can be readily achieved with a large number of catalyst systems, significantly fewer examples for the more challenging intermolecular hydroamination are known, especially for unactivated alkenes. The stereoselective hydroamination has also received significant attention due to the importance of chiral nitrogen-containing molecules in pharmaceutical industry. A variety of highly selective chiral catalyst systems have been developed for intramolecular hydroaminations, while examples of intermolecular asymmetric hydroaminations are scarce. Hydroamination in the context of this review article is defined as the addition of HNR2 across a non-activated, unsaturated carbon-carbon multiple bond. This review focuses on the hydroamination reaction of simple, non-activated alkenes. The addition of amines to slightly activated alkenes, such as vinyl arenes, 1,3-dienes, strained alkenes (norbornene derivatives, methylenecyclopropenes) and allenes is closely related and is covered as well. However, hydroamination reactions of alkynes and Aza-Michael reactions involving the addition of an N-H fragment across the conjugated or otherwise activated double bond of a Michael acceptor are not covered. The scope of amine types includes ammonia, primary and secondary aliphatic and aromatic amines, azoles, and hydrazines. N-Protected amines, such as ureas, carboxamides, and sulfonamides are covered as well, as they are important substrates for metal-free and late transition metal-based catalysts. The literature through January 2011 will be covered with two selected references from 2012 (comprising Table 3D). A supplemental reference list is provided for reports appearing February 2011 through April 2015. The chapter is organized by the nature of the carbon unsaturation to which the amine is added. Ranging from less reactive substrates such as ethylene and unactivated alkenes, to slightly activated substrates, such as vinyl arenes, and more activated substrates, including conjugated dienes, allenes and strained alkenes. Enantioselective hydroamination reactions, an area that has seen significant progress over the past decade, are discussed next. Finally, tandem hydroamination/carbocyclization reactions of aminodialkenes provide rapid access to complex alkaloidal skeletons. The tables at the end of the chapter are separated into five main sections: achiral intermolecular hydroamination, achiral intramolecular hydroamination, enantioselective intermolecular hydroamination, enantioselective intramolecular hydroamination, and tandem hydroamination/carbocyclization. Within the first four sections the tables are further divided into subsections based on the participating alkene, i.e. alkenes, vinyl arenes, dienes, allenes, and strained alkenes. Keywords: Hydroamination; heterofunctionalization; catalysis; asymmetric catalysis; nitrogen heterocycles; mechanism; alkenes; vinyl arenes, allenes, dienes, strained alkenes, amines; transition metal catalysis; alkali metals, alkaline earth metals, Bronsted acid catalysis; Lewis acid catalysis