Recyclable catalysts for palladium-catalyzed aminations of aryl halides

Metal-catalyzed reactions for the C(sp2)–N bond formation: achievements of recent years

The review deals with the main catalytic methods for the C(sp2)–N bond formation, including Buchwald–Hartwig palladium-catalyzed amination of aryl and heteroaryl halides, renaissance of the Ullmann chemistry, i.e., the application of catalysis by copper complexes to form the carbon–nitrogen bond, and Chan–Lam reactions of (hetero)arylboronic acids with amines. Also, oxidative amination with C–H activation, which has been booming during the last decade, is addressed. Particular attention is paid to achievements in the application of heterogenized catalysts.

The bibliography includes 350 references.

Reactivity of phospha-Wittig reagents towards NHCs and NHOs

Phospha-Wittig reagents, RPPMe3 (R = Mes* 2,4,6-tBu3-C6H2; MesTer 2,6-(2,4,6-Me3C6H2)-C6H3; DipTer 2,6-(2,6-iPr2C6H3)-C6H3), can be considered as phosphine-stabilized phosphinidenes. In this study we show that PMe3 can be displaced by NHCs or NHOs. Interestingly, phosphinidene-like reactivity results in a subsequent C(sp2)-H activation of the exocyclic CH2 group in NHOs. This concept was further extended to allyl-apended NHOs, which resulted in phosphine-substituted allyl species.

An unexpected Staudinger reaction at an N-heterocyclic carbene-carbon center

The previously unreported carbene-phosphine adduct (IPr)PCl2N3 [IPr = (HCNDipp)2C:; Dipp = 2,6-iPr2C6H3] was synthesized and used as a synthon toward the elusive dichlorophosphazene monomer unit, [Cl2P=N]. (IPr)PCl2N3 was found to undergo halide and azide abstraction when combined with various electrophiles and its thermolysis yielded the unexpected Staudinger reaction product (IPr=N)PCl2.

Structurally versatile phosphine and amine donors constructed from N-heterocyclic olefin units

A general strategy for the synthesis of hindered N- and P-based donors is presented whereby the strongly electron releasing N-heterocyclic olefin (NHO) unit, IPr[double bond, length as m-dash]CH-, (IPr[double bond, length as m-dash]CH- = [(HCNDipp)2C[double bond, length as m-dash]CH](-); Dipp = 3,6-(i)Pr2C6H2) is linked to terminally bound phosphine and amine donors. Preliminary coordination chemistry is presented involving phosphine (IPr[double bond, length as m-dash]CH)PR2 (R = (i)Pr and Ph) and amine (IPr[double bond, length as m-dash]CH)NMe2 ligands and the Lewis acids BH3 and AuCl. Interestingly, (IPr[double bond, length as m-dash]CH)NMe2 binds AuCl through an exocyclic olefin unit, while the softer phosphorus centers in (IPr[double bond, length as m-dash]CH)PR2 coordinate to yield Au-P linkages; thus the reported NHO-based ligands exhibit tunable binding modes to metals.

From imine to amine: an unexpected left turn. Cis-β iron(ii) PNNP' precatalysts for the asymmetric transfer hydrogenation of acetophenone

A slight change in the iron catalyst structure (amine arm with PEt₂ to imine arm with PPh₂) results in a complete reversal of the enantioselectivity toward ketone reduction.

A novel PNN ligand bearing an orthophenylene group and a primary amine was synthesized with the aid of a palladium-catalyzed amination and reacted with phosphonium dimers [–PR₂CH₂CH(OH)–]₂[Br]₂ R = Et, iPr, Cy, Ph, xylyl, and o-Tol, and [Fe(OH₂)₆]²⁺ to produce a new series of cis-β iron(ii) PNNP′ precatalysts cis-β-[Fe(Br)(CO)(PNNP′)]BPh₄ as a pair of diastereomers. The more stable orthophenylene amido group was chosen to imitate and replace the enamido moiety of a highly active iron precatalyst for the asymmetric transfer hydrogenation (ATH) of ketones in an attempt to prevent its deactivation caused by reduction of the enamido group. This objective was partially achieved using the complex with a PEt₂ group which catalyzed the transfer hydrogenation in isopropanol of 150 000 equivalents of acetophenone to racemic 1-phenylethanol. With a low acetophenone to catalyst ratio of 500 to 1, the catalytic activity was moderate and the enantiomeric excess (ee) of the product 1-phenylethanol ranged surprisingly from 94% (R) to 95% (S) depending on the nature of PR₂ and whether the precatalyst contained an imine or amine donor. The amine precatalyst with a PEt₂-group produced a more stable hydride species when activated, allowing the reaction mixture to be heated to 75 °C to obtain a TON of 8821 for acetophenone while retaining the high ee of 95% (S). The activation pathway in basic isopropanol (iPrOH) was studied for three precatalysts to elucidate that the cis-β precatalysts rearrange to form trans hydride complexes. The study suggests that the enantioselectivity of these complexes is determined by from which side of the PNNP′ plane the hydride transfer occurs.

Applications of Palladium-Catalyzed C-N Cross-Coupling Reactions

Pd-catalyzed cross-coupling reactions that form C–N bonds have become useful methods to synthesize anilines and aniline derivatives, an important class of compounds throughout chemical research. A key factor in the widespread adoption of these methods has been the continued development of reliable and versatile catalysts that function under operationally simple, user-friendly conditions. This review provides an overview of Pd-catalyzed N-arylation reactions found in both basic and applied chemical research from 2008 to the present. Selected examples of C–N cross-coupling reactions between nine classes of nitrogen-based coupling partners and (pseudo)aryl halides are described for the synthesis of heterocycles, medicinally relevant compounds, natural products, organic materials, and catalysts.

A triazine-phosphite polymeric ligand bearing cage-like P,N-ligation sites: an efficient ligand in the nickel-catalyzed amination of aryl chlorides and phenols

A reusable polymeric material containing P,N-ligation sites was prepared by a facile one-step route for application in transition-metal catalysis as a reusable P,N-ligand.

Assembly of Primary (Hetero)Arylamines via CuI/Oxalic Diamide-Catalyzed Coupling of Aryl Chlorides and Ammonia

A general and practical catalytic system for aryl amination of aryl chlorides with aqueous or gaseous ammonia has been developed, with CuI as the catalyst and bisaryl oxalic diamides as the ligands. The reaction proceeds at 105-120 °C to provide a diverse set of primary (hetero)aryl amines in high yields with various functional groups.

Rational ligand design for the arylation of hindered primary amines guided by reaction progress kinetic analysis

We report the Pd-catalyzed arylation of very hindered α,α,α-trisubstituted primary amines. Kinetics-based mechanistic analysis and rational design have led to the development of two biarylphosphine ligands that allow the transformation to proceed with excellent efficiency. The process was effective in coupling a wide range of functionalized aryl and heteroaryl halides under mild conditions.

Exploring the Catalytic Reactivity of Nickel Phosphine–Phosphite Complexes

In this study, we present an investigation into various nickel phosphite and phosphite–phosphine complexes for use in the Mizoroki–Heck and Suzuki–Miyaura cross-coupling reactions and the ammonia arylation reaction. In these coupling reactions, it was discovered that the Ni[P(OEt)3]4, (dppf)Ni[P(OPh)3]2, and (binap)Ni[P(OPh)3]2 catalysts were the most effective. In addition, an optimisation process for these catalytic systems as well as functional group compatibility are discussed.

Installation of protected ammonia equivalents onto aromatic & heteroaromatic rings in water enabled by micellar catalysis

A single set of conditions consisting of a palladium catalyst, a commercially available ligand, and a base, allow for several types of C-N bond constructions to be conducted in water with the aid of a commercially available "designer" surfactant (TPGS-750-M). Products containing a protected NH₂ group in the form of a carbamate, sulfonamide, or urea can be fashioned starting with aryl or heteroaryl bromides, iodides, and in some cases, chlorides, as substrates. Reaction temperatures are in the range of room temperature to, at most, 50 °C, and result in essentially full conversion and good isolated yields.

Mild conditions for deuteration of primary and secondary arylamines for the synthesis of deuterated optoelectronic organic molecules

Deuterated arylamines demonstrate great potential for use in optoelectronic devices, but their widespread utility requires a method for large-scale synthesis. The incorporation of these deuterated materials into optoelectronic devices also provides the opportunity for studies of the functioning device using neutron reflectometry based on the difference in the scattering length density between protonated and deuterated compounds. Here we report mild deuteration conditions utilising standard laboratory glassware for the deuteration of: diphenylamine, N-phenylnaphthylamine, N-phenyl-o-phenylenediamine and 1-naphthylamine (via H/D exchange in D₂O at 80 °C, catalysed by Pt/C and Pd/C). These conditions were not successful in the deuteration of triphenylamine or N,N-dimethylaniline, suggesting that these mild conditions are not suitable for the deuteration of tertiary arylamines, but are likely to be applicable for the deuteration of other primary and secondary arylamines. The deuterated arylamines can then be used for synthesis of larger organic molecules or polymers with optoelectronic applications.

Transitioning organic synthesis from organic solvents to water. What's your E Factor?

Traditional organic chemistry, and organic synthesis in particular, relies heavily on organic solvents, as most reactions involve organic substrates and catalysts that tend to be water-insoluble. Unfortunately, organic solvents make up most of the organic waste created by the chemical enterprise, whether from academic, industrial, or governmental labs. One alternative to organic solvents follows the lead of Nature: water. To circumvent the solubility issues, newly engineered "designer" surfactants offer an opportunity to efficiently enable many of the commonly used transition metal-catalyzed and related reactions in organic synthesis to be run in water, and usually at ambient temperatures. This review focuses on recent progress in this area, where such amphiphiles spontaneously self-aggregate in water. The resulting micellar arrays serve as nanoreactors, obviating organic solvents as the reaction medium, while maximizing environmental benefits.

Mild and highly selective palladium-catalyzed monoarylation of ammonia enabled by the use of bulky biarylphosphine ligands and palladacycle precatalysts

A method for the Pd-catalyzed arylation of ammonia with a wide range of aryl and heteroaryl halides, including challenging five-membered heterocyclic substrates, is described. Excellent selectivity for monoarylation of ammonia to primary arylamines was achieved under mild conditions or at rt by the use of bulky biarylphosphine ligands (L6, L7, and L4) as well as their corresponding aminobiphenyl palladacycle precatalysts (3a, 3b, and 3c). As this process requires neither the use of a glovebox nor high pressures of ammonia, it should be widely applicable.

An examination of the palladium/Mor-DalPhos catalyst system in the context of selective ammonia monoarylation at room temperature

An examination of the [{Pd(cinnamyl)Cl}(2)]/Mor-DalPhos (Mor-DalPhos = di(1-adamantyl)-2-morpholinophenylphosphine) catalyst system in Buchwald-Hartwig aminations employing ammonia was conducted to better understand the catalyst formation process and to guide the development of precatalysts for otherwise challenging room-temperature ammonia monoarylations. The combination of [{Pd(cinnamyl)Cl}(2)] and Mor-DalPhos afforded [(κ(2)-P,N-Mor-DalPhos)Pd(η(1)-cinnamyl)Cl] (2), which, in the presence of a base and chlorobenzene, generated [(κ(2)-P,N-Mor-DalPhos)Pd(Ph)Cl] (1 a). Halide abstraction from 1 a afforded [(κ(3)-P,N,O-Mor-DalPhos)Pd(Ph)]OTf (5), bringing to light a potential stabilizing interaction that is offered by Mor-DalPhos. An examination of [(κ(2)-P,N-Mor-DalPhos)Pd(aryl)Cl] (1 b-f) and related precatalysts for the coupling of ammonia and chlorobenzene at room temperature established the suitability of 1 a in such challenging applications. The scope of reactivity for the use of 1 a (5 mol %) encompassed a range of (hetero)aryl (pseudo)halides (X = Cl, Br, I, OTs) with diverse substituents (alkyl, aryl, ether, thioether, ketone, amine, fluoro, trifluoromethyl, and nitrile), including chemoselective arylations.