Regio- and Stereoselective Hydroamination of Alkynes Using an Ammonia Surrogate: Synthesis of N-Silylenamines as Reactive Synthons

Group IV Complexes with Sterically Congested N-Aryl-adamantylcarbamidate Ligands

Metalation of N-(2,6-dibenzhydryl-4-tolyl)adamantane-1-carboxamide (1, Ar*N(H)-C(O)-Ad) with M(NMe2)4 (M = Ti, Zr, Hf) yields amidate complexes Ar*N=C(Ad)-O-Ti(NMe2)3 (2) as well as bis(amidate) compounds (Ar*N=C(Ad)-O)2M(NMe2)2 (M = Zr (3), Hf (4)). In 2, the amidate ligand acts as a monodentate base via the oxygen atom with the Ti center in a slightly distorted tetrahedral environment. The steric requirement of the amidate ligand stabilizes the small coordination number of four of the Ti atom. In congeners 3 and 4, two bidentate amidate ligands exist in the coordination spheres, leading to hexacoordinate group IV metal atoms. The small bite angles of the Zr- and Hf-bound amidate ligands lead to severe distortion of the octahedral environments of the Zr and Hf centers. Titanium compound 2 is an unsuitable choice to catalyze hydrofunctionalization of alkynes with amines and phosphane oxides and despite the significantly smaller pKa value of the carboxylic amide, formation of carboxamide 1 is the dominant reaction upon addition of amines or phosphane oxides to release intramolecular steric strain introduced by the very bulky adamantylamidato ligand.

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.

One-Pot Sequential Hydroamination Protocol for N-Heterocycle Synthesis: One Method To Access Five Different Classes of Tri-Substituted Pyridines

Tri-substituted pyridines are important scaffolds that can be found in a plethora of commercially available drugs. A one-pot general method for the selective synthesis of less explored/challenging patterns of tri-substituted pyridines is described. Hydroamination of alkynes with commercially available N-triphenylsilylamine generates N-silylenamines. These in situ generated N-silylenamines, upon reaction with α,β-unsaturated carbonyl compounds and subsequent oxidation, furnish 25 examples of selectively substituted 2,4,5-, 2,3,4-, 3,4,5-, 2,3,5-, and 2,3,6-trisubstituted pyridines in up to 78% yield. The reaction features high functional group compatibility providing an expeditious and general approach for the assembly of selectively substituted tri-substituted pyridine derivatives. The robustness and practicality of the reaction have been demonstrated in a gram-scale reaction.

A Boron-Nitrogen Double Transborylation Strategy for Borane-Catalyzed Hydroboration of Nitriles

Organoborane-catalyzed hydroboration of nitriles provides N,N-diborylamines, which act as efficient synthons for the synthesis of primary amines and secondary amides. Known nitrile hydroboration methods are dominated by metal catalysis. Simple and metal-free hydroboration of nitriles using diborane [H-B-9-BBN]₂ as a catalyst and pinacolborane as a turnover reagent is reported. The reaction of monomeric H-B-9-BBN with nitriles leads to the hydrido-bridged diborylimine intermediate; a subsequent sequential double hydroboration-transborylation pathway involving B-N/B-H σ bond metathesis is proposed.

N-Silylamines in catalysis: synthesis and reactivity

A summary of the catalytic synthesis and reactivity of N-silylated amines is presented. Dehydrocoupling of amines with silanes, hydrosilylation of imines and dealkenylative coupling of amines with vinylsilanes are three ways to achieve their catalytic syntheses. The resultant N-silylamines serve as substrates in a variety of reactions, including C-N and C-C bond forming reactions, and are preferred in transformations because of the facile Si-N hydrolytic cleavage to reveal free amine products upon reaction completion. This review highlights the distinct electronic properties of N-silyl amines, N-silyl imines and N-silyl enamines that result in complementary reactivity to that of parent non-silyl variants.

Hydrosilylation and Mukaiyama aldol-type reaction of quinolines and hydrosilylation of imines catalyzed by a mesoionic carbene-stabilized borenium ion

Aldimines and ketimines containing electron-donating and electron-withdrawing groups can be hydrosilylated with borenium catalysts at as low as 1 mol% catalyst loading at room temperature, providing the corresponding secondary amines in excellent yields. Reactions with 2-phenylquinoline gave the 1,4-hydrosilylquinoline product selectively which can be further functionalized in a one-pot synthesis to give unique γ-amino alcohol derivatives. Control experiments suggest that the borenium ion catalyzes both the hydrosilylation and subsequent addition to the aldehyde.

Using Catalysts To Make Catalysts: Titanium-Catalyzed Hydroamination To Access P,N-Ligands for Assembling Catalysts in One Pot

Using a diamido-bis(amidate) titanium precatalyst, the hydroamination of alkynylphosphines afforded phosphinoenamine products. After reduction, 2-aminophosphines are prepared in excellent yield and on gram scale. A broad variety of alkynylphosphines and primary amines with different electronic and steric features are tolerated in this sequential transformation, enabling the rapid assembly of a collection of ligands. Additionally, intermediate phosphinoenamines can be used directly as proligands for coordination to transition metals using protonolysis or salt metathesis reactions. These transformations result in easy-to-use one pot protocols to prepare metal P,N-complexes for catalysis or small molecule activation.

Nonmetal-catalyzed hydroamination of ynamides with amines

A TfOH-catalyzed hydroamination of ynamides with primary and secondary amines under mild reaction conditions is described for the efficient synthesis of N-arylimines and ethene-1,1-diamines.

Vanadium Pyridonate Catalysts: Isolation of Intermediates in the Reductive Coupling of Alcohols

The reductive coupling of alcohols using vanadium pyridonate catalysts is reported. This attractive approach for C(sp³)-C(sp³) bond formation uses an oxophilic, earth-abundant metal for a catalytic deoxygenation reaction. Several pyridonate complexes of vanadium were synthesized, giving insight into the coordination chemistry of this understudied class of compounds. Isolated intermediates provide experimental mechanistic evidence that complements reported computational mechanistic proposals for the reductive coupling of alcohols. In contrast to previous mononuclear vanadium(V)/vanadium(III)/vanadium(IV) cycles, this pyridonate catalyst system is proposed to proceed by a vanadium(III)/vanadium(IV) cycle involving bimetallic intermediates.

Catalytic intramolecular hydroamination of aminoallenes using titanium and tantalum complexes of sterically encumbered chiral sulfonamides

Titanium and tantalum catalysts supported by readily prepared chiral sulfonamide ligands catalyze hydroamination of aminoallenes that lack N-protecting groups.

Titanium catalysis for the synthesis of fine chemicals – development and trends

Atlas as a Titan(ium) is holding the earth-abundant chemistry world. Titanium is the second most abundant transition metal, is a key player in important industrial processes (e.g. polyethylene) and shows much promise for diverse applications in the future.

Zirconium Hydroaminoalkylation. An Alternative Disconnection for the Catalytic Synthesis of α-Arylated Primary Amines

Primary amine products have been prepared using zirconium-catalyzed hydroaminoalkylation of alkenes with N-silylated benzylamine substrates. Catalysis using commercially available Zr(NMe₂)₄ affords an alternative disconnection to access α-arylated primary amines upon aqueous workup. Substrate-dependent regio- and diastereoselectivity of the reaction is observed. Bulky substituents on the terminal alkene exclusively generate the linear regioisomer. This atom-economic catalytic strategy for the synthesis of building blocks that can undergo further synthetic elaboration is highlighted in the preparation of trifluoroethylated α-arylated amines.

A silica-supported titanium catalyst for heterogeneous hydroamination and multicomponent coupling reactions

Highly dehydrated silica gel, SiO₂⁷⁰⁰, gave a material with a total surface hydroxyl density of 0.31 ± 0.05 mmol g^-1, 0.9 ± 0.1 Si-OH sites per nm². Treatment of this material with Ti(NMe₂)₄ gave Ti(NMe₂)₃/SiO₂⁷⁰⁰, which is 1.50% ± 0.07 Ti, where the titanium is bound to the surface, on average, through a single O-Si-Ti linkage. This material was tested for its properties as a catalyst for C-N bond forming reactions and was found to be a competent alkyne hydroamination and iminoamination catalyst. For iminoamination, which is the 3-component coupling of an alkyne, primary amine, and isonitrile, this heterogeneous catalyst was able to carry out some catalyses faster than previously reported homogeneous catalysts with lower catalyst loadings. The material is also a catalyst for the addition of aniline to dicyclohexylcarbodiimide to form a substituted guanidine. In addition, a known quinoline with biological activity was prepared using the heterogeneous catalyst in a one-pot procedure using half the catalyst loading of the previously reported synthesis.

Unsaturated vicinal frustrated phosphane/borane Lewis pairs as ligands in gold(i) chemistry

The P/B FLP 6, formed by the 1,1-hydroboration of dimesitylphosphino(trimethylsilyl)acetylene with HB(C6F5)2 forms the Au(i)X complexes 9a (X: Cl) and 10a (X: NTf2), respectively. Both show marked AuB interactions. The P/B FLP isomer 8, featuring the bulky SiMe3 substituent at the carbon adjacent to boron forms gold complexes 9b and 10b, both of which show weaker AuB interactions. Complex 10a was employed in the catalytic hydroamination of a series of alkynes with p-toluidine. Complexes 9 and 10 were characterized by X-ray diffraction.

Gold(i) catalysed regio- and stereoselective intermolecular hydroamination of internal alkynes: towards functionalised azoles

Vinyl- and saturated azoles were synthesised through gold(i) catalysed regio- and stereoselective intermolecular hydroamination of internal alkynes and subsequent hydrogenation.

Catalytic intramolecular hydroamination of aminoallenes using titanium complexes of chiral, tridentate, dianionic imine-diol ligands

Alkylation of d- or l-phenylalanine or valine alkyl esters was carried out using methyl or phenyl Grignard reagents. Subsequent condensation with salicylaldehyde, 3,5-di-tert-butylsalicylaldehyde, or 5-fluorosalicylaldehyde formed tridentate, X2L type, Schiff base ligands. Chiral shift NMR confirmed retention of stereochemistry during synthesis. X-ray crystal structures of four of the ligands show either inter- or intramolecular hydrogen bonding interactions. The ligands coordinate to the titanium reagents Ti(NMe2)4 or TiCl(NMe2)3 by protonolysis and displacement of two equivalents of HNMe2. The crystal structure of one example of Ti(X2L)Cl(NMe2) was determined and the complex has a distorted square pyramidal geometry with an axial NMe2 ligand. The bis-dimethylamide complexes are active catalysts for the ring closing hydroamination of di- and trisubstituted aminoallenes. The reaction of hepta-4,5-dienylamine at 135 °C with 5 mol% catalyst gives a mixture of 6-ethyl-2,3,4,5-tetrahydropyridine (40-72%) and both Z- and E-2-propenyl-pyrrolidine (25-52%). The ring closing reaction of 6-methyl-hepta-4,5-dienylamine at 135 °C with 5 mol% catalyst gives exclusively 2-(2-methyl-propenyl)-pyrrolidine. The pyrrolidine products are obtained with enantiomeric excesses up to 17%.

Metal-catalyzed regiodivergent organic reactions

This review discusses metal-catalysed regiodivergent additions, allylic substitutions, CH-activation, cross-couplings and intra- or intermolecular cyclisations.

N-Silylenamines as Reactive Intermediates: Hydroamination for the Modular Synthesis of Selectively Substituted Pyridines

A modular and selective synthesis of mono-, di-, tri-, tetra-, and pentasubstituted pyridines is reported. Hydroamination of alkynes with N-silylamine using a bis(amidate)bis(amido)titanium(IV) precatalyst furnishes the regioselective formation of N-silylenamines. Addition of α,β-unsaturated carbonyls to the crude mixtures followed by oxidation affords 47 examples of pyridines in yields of up to 96%. This synthetic route allows for the synthesis of diverse pyridines containing variable substitution patterns, including pharmaceutically relevant 2,4,5-trisubstituted pyridines, using this one-pot protocol.

Catalytic Alkyne Arylation Using Traceless Directing Groups

By using Pd0 /Mandyphos, we achieved a three-component aminoarylation of alkynes to generate enamines, which are then hydrolyzed to either α-arylphenones or α,α-diarylketones. This Pd-catalyzed method overcomes established known pathways to enable the use of amines as traceless directing groups for C-C bond formation.