Anti-Markovnikov Intermolecular Hydroamination of Alkenes and Alkynes: A Mechanistic View

Hydroamination, the addition of an N-H bond across a C-C multiple bond, is a reaction with a great synthetic potential. Important advances have been made in the last decades concerning catalysis of these reactions. However, controlling the regioselectivity in the amine addition toward the formation of anti-Markovnikov products (addition to the less substituted carbon) still remains a challenge, particularly in intermolecular hydroaminations of alkenes and alkynes. The goal of this review is to collect the systems in which intermolecular hydroamination of terminal alkynes and alkenes with anti-Markovnikov regioselectivity has been achieved. The focus will be placed on the mechanistic aspects of such reactions, to discern the step at which regioselectivity is decided and to unravel the factors that favor the anti-Markovnikov regioselectivity. In addition to the processes entailing direct addition of the amine to the C-C multiple bond, alternative pathways, involving several reactions to accomplish anti-Markovnikov regioselectivity (formal hydroamination processes), will also be discussed in this review. The catalysts gathered embrace most of the metal groups of the Periodic Table. Finally, a section discussing radical-mediated and metal-free approaches, as well as heterogeneous catalyzed processes, is also included.

Insights into the Chiral Phosphoric Acid-Catalyzed Dynamic Kinetic Asymmetric Hydroamination of Racemic Allenes: An Allyl Carbocation/Phosphate Pair Mechanism

Computational studies of chiral phosphoric acid (CPA)-catalyzed dynamic kinetic asymmetric hydroamination (DyKAH) of racemic allenes show that the reaction proceeds through a catalytic asymmetric model involving a highly reactive π-allylic carbocationic intermediate, generated from a racemic allene through an intermolecular proton transfer mediated by CPA, which also results in a high E/Z selectivity. Moreover, the distortion-interaction, atom in molecule, and electrostatic interaction analyses and space-filling models are employed on the basis of the DyKAH catalyzed by (S)-A5 (reaction 1) or (R)-A2 (reaction 2) to explain the high enantioselectivity and the controlling effects of SPINOL scaffolds on the signs of enantioselectivity. Our calculations indicate that the enantioselectivity of reactions 1 and 2 can be mainly ascribed to the favorable noncovalent interactions within the stronger chiral electrostatic environment created by the phosphoric acid in the preferential transition states. Finally, the effect of (S/R)-SPINOL-based CPAs on the signs of enantioselectivity can be explained by the different combination modes of substrates into the chiral binding pocket of the catalyst controlled by the chirality of SPINOL backbones. Overall, the new insights into the reaction rationalize the outcome and these key factors that affect the product enantioselectivity are important to guide the DyKAHs.