C-H borylation is a powerful and atom-efficient method for converting affordable and abundant chemicals into versatile organic reagents used in the production of fine chemicals and functional materials. Herein we report a facile C-H borylation of aromatic and olefinic C-H bonds with 2-aminophenylboranes. Computational and experimental studies reveal that the metal-free C-H insertion proceeds via a frustrated Lewis pair mechanism involving heterolytic splitting of the C-H bond by cooperative action of the amine and boryl groups. The adapted geometry of the reactive B and N centers results in an unprecedentently low kinetic barrier for both insertion into the sp(2)-C-H bond and intramolecular protonation of the sp(2)-C-B bond in 2-ammoniophenyl(aryl)- or -(alkenyl)borates. This common reactivity pattern serves as a platform for various catalytic reactions such as C-H borylation and hydrogenation of alkynes. In particular, we demonstrate that simple 2-aminopyridinium salts efficiently catalyze the C-H borylation of hetarenes with catecholborane. This reaction is presumably mediated by a borenium species isoelectronic to 2-aminophenylboranes.

Ansa-aminoborane 1 (ortho-TMP-C6H4-BH2; TMP = 2,2,6,6-tetramethylpiperid-1-yl), a frustrated Lewis pair with the smallest possible Lewis acidic boryl site (-BH2), is prepared. Although it is present in quenched forms in solution, and BH2 represents an acidic site with reduced hydride affinity, 1 reacts with H2 under mild conditions producing ansa-ammonium trihydroborate 2. The thermodynamic and kinetic features as well as the mechanism of this reaction are studied by variable-temperature NMR spectroscopy, spin-saturation transfer experiments, and DFT calculations, which provide comprehensive insight into the nature of 1.

ABSTRACT:We report the synthesis and reactivity of a chiral aminoborane displaying both rapid and reversible hydrogen activation. The catalyst shows exceptional reactivity in asymmetric hydrogenation of enamines and unhindered imines with stereoselectivity up to 99% ee. The reaction mechanism is analyzed via DFT calculations, which point to the importance of both repulsive steric and stabilizing intermolecular noncovalent forces in the stereodetermining hydride transfer step of the catalytic cycle.One of the most efficient and atom-economic ways of preparing chiral amines is through transition metal catalyzed asymmetric hydrogenation of prochiral imines and enamines. 1 The products, having chiral α-carbons, are important in synthetic chemistry due to their application as ligands, resolving agents, chiral auxiliaries and building blocks. Additionally, the chiral information and tendency to form hydrogen bonds, are essential features in molecular recognition, thus making them potential pharmaceuticals. 2 Drug substance preparation through transition metal catalysis requires tedious product purification due to strict demands on heavy metal residuals in the products. 3 Recently, main-group systems combining sterically hindered Lewis acids and bases have been reported to cleave molecular hydrogen heterolytically under mild reaction conditions. 4 The field commonly referred to as the chemistry of "Frustrated Lewis pairs" (FLP) has attracted an increased scientific and practical interest, mainly owing to their applicability as catalysts in homogeneous metal-free hydrogenations of imines, enamines, 5 N-heterocycles, 6 and first results have been disclosed for the hydrogenation of carbonyl compounds 7 as well. In this regard it is surprising that reports of the corresponding asymmetric reactions are still few. Intermolecular FLP systems involving chiral boranes have been successfully utilized for enantioselective imine hydrogenation. Use of inherently chiral terpene groups (like α-pinene and camphor) on boron has proven to be effective for hydrogenation of acetophenone N-arylimines and ee's up to 83% were reported (Figure 1, 1). 8 A recent development was achieved by introducing chirality through a binaphthyl backbone (2), enabling hydrogenation at room temperature and consequently increasing asymmetric induction. 9 In intramolecular FLP systems appropriate linking of the Lewis acid and base has proven to be crucial for dihydrogen activation as well as for the hydrogen transfer process. 10 In ansa-aminoboranes, the Lewis acid and base are in close vicinity, but bulky substituents hinder dative bond formation. Such structures generate high FLP reactivity and the substrate scope could be expanded to enamines, N-alkyl and alkyl imines. 5d,11 However, previous attempts to incorporate high reactivity of ansa-aminoboranes and asymmetric hydrogenation, namely the introduction of chiral amines via straightforward synthetic approaches into linked systems (Figure 1, 3), have resulted in only moderate enantioselectivities. 11...

A joint experimental-theoretical study of a bifunctional squaramide-amine-catalyzed Michael addition reaction between 1,3-dioxo nucleophiles and nitrostyrene has been undertaken to gain insight into the nature of bifunctional organocatalytic activation. For this highly stereoselective reaction, three previously proposed mechanistic scenarios for the critical CC bond-formation step were examined. Accordingly, the formation of the major stereoisomeric products is most plausible by one of the bifunctional pathways that involve electrophile activation by the protonated amine group of the catalyst. However, some of the minor product isomers are also accessible through alternative reaction routes. Structural analysis of transition states points to the structural invariance of certain fragments of the transition state, such as the protonated catalyst and the anionic fragment of approaching reactants. Our topological analysis provides deeper insight and a more general understanding of bifunctional noncovalent organocatalysis.