Selective N-alkylation of amines using nitriles under hydrogenation conditions: facile synthesis of secondary and tertiary amines

Nitriles were found to be highly effective alkylating reagents for the selective N-alkylation of amines under catalytic hydrogenation conditions. For the aromatic primary amines, the corresponding secondary amines were selectively obtained under Pd/C-catalyzed hydrogenation conditions. Although the use of electron poor aromatic amines or bulky nitriles showed a lower reactivity toward the reductive alkylation, the addition of NH(4)OAc enhanced the reactivity to give secondary aromatic amines in good to excellent yields. Under the same reaction conditions, aromatic nitro compounds instead of the aromatic primary amines could be directly transformed into secondary amines via a domino reaction involving the one-pot hydrogenation of the nitro group and the reductive alkylation of the amines. While aliphatic amines were effectively converted to the corresponding tertiary amines under Pd/C-catalyzed conditions, Rh/C was a highly effective catalyst for the N-monoalkylation of aliphatic primary amines without over-alkylation to the tertiary amines. Furthermore, the combination of the Rh/C-catalyzed N-monoalkylation of the aliphatic primary amines and additional Pd/C-catalyzed alkylation of the resulting secondary aliphatic amines could selectively prepare aliphatic tertiary amines possessing three different alkyl groups. According to the mechanistic studies, it seems reasonable to conclude that nitriles were reduced to aldimines before the nucleophilic attack of the amine during the first step of the reaction.     

Photostabilizing activity of tertiary hindered amines

The influence of the N-alkyl group of tertiary hindered amines on the photostabilization of polymers was studied. The photostabilizing effects of the tertiary amine derivatives of 4-benzoyloxy-2,2,6,6-tetramethylpiperidine ( 1a ) in polypropylene were compared. All tertiary amine derivatives having α-H to hindered N showed higher effectiveness than 1a . Model liquid phase photoxidations were carried out by irradiating (UV-lamp) the solutions of tertiary hindered amines containing tert-butyl hydroperoxide as a photoinitiator. The tertiary hindered amines were oxidized more easily than corresponding parent hindered amine and converted to the parent amine, which was identified as its salt, resulting from the carboxylic acid produced from the N-alkyl group by oxidation. The thermal reaction of the tertiary hindered amines with tert-butyl hydroperoxide was also studied in the liquid phase. The tertiary hindered amines decomposed tert-butyl hydroperoxide more rapidly than the parent secondary hindered amine, and generated the parent amine. It was also found that the photostabilizing effects of tertiary hindered amines for polyolefins were higher than that of the parent secondary hindered amine.     

Willgerodt-Kindler reaction at room temperature: Synthesis of thioamides from aromatic aldehydes and cyclic secondary amines

A simple method for the synthesis of thioamide derivatives in DMSO at room temperature and at 120 °C has been developed. Total 27 compounds were prepared under both conditions via a one-pot, three component reaction between substituted aromatic aldehydes, elemental sulfur powder, and cyclic secondary amines. By optimizing the mole ratio of sulfur powder and amines, we have successfully carried out Willgerodt-Kindler reaction of aromatic aldehydes at room temperature. At 120 °C, it is catalyst free reaction with lower reaction time whereas at room temperature, due to the additional amine molecule, Willgerodt-Kindler reaction of aromatic aldehydes is successfully carried out at room temperature. On gram-scale, the reaction is successfully attempted under both conditions with good yields.     

Solid-phase synthesis of a library of hydroxyproline derivatives

The synthesis of a library of N-alkylated O-arylated hydroxyproline derivatives has been achieved on solid phase. The choice of O-protection and the optimization of the Mitsunobu reaction involving a secondary alcohol were key to the success of this synthesis. First, acylation of resin-bound amines with N-Fmoc-O-THP-hydroxyproline was accomplished readily. Subsequent deprotection of the Fmoc and reductive amination with different aldehydes resulted in the tertiary amine intermediate. The deprotection of the THP group by p-toluenesulfonic acid was followed by a Mitsunobu reaction with a series of phenols. Finally, the products were cleaved from the resin using trifluoroacetic acid to produce a 10 200 member library.     

Acyl iodides in organic synthesis: XI. Unusual N-C bond cleavage in tertiary amines

Acyl iodides reacted with excess primary and secondary amines in a way similar to acyl chlorides, yielding the corresponding carboxylic acid amide and initial amine hydroiodide. Reactions of tertiary amines with acyl iodides were accompanied by cleavage of the N-C bond with formation of the corresponding N,N-di(hydrocarbyl)carboxamide and alkyl iodide. In the presence of excess tertiary amine the latter was converted into quaternary tetra(hydrocarbyl)ammonium iodide.     

Pd/C-catalyzed Reductive Mono-N-alkylation of Nitrophenol Derivatives in One-pot Way

A range of different nitrophenol derivatives were converted in one‐pot to the corresponding secondary alkyl aminophenols in good to excellent yields by using ketones as alkyl source and hydrogen over 10 wt% Pd/C as reducing agent. In all examples, except for one, the secondary amine was the sole alkylation product isolated. When aldehydes were used as alkyl source, the corresponding tertiary amine as a sole alkylation product was isolated.     

Pt-Sn/γ-Al2O3-catalyzed highly efficient direct synthesis of secondary and tertiary amines and imines

Versatile syntheses of secondary and tertiary amines by highly efficient direct N‐alkylation of primary and secondary amines with alcohols or by deaminative self‐coupling of primary amines have been successfully realized by means of a heterogeneous bimetallic Pt–Sn/γ‐Al₂O₃ catalyst (0.5 wt % Pt, Pt/Sn molar ratio=1:3) through a borrowing‐hydrogen strategy. In the presence of oxygen, imines were also efficiently prepared from the tandem reactions of amines with alcohols or between two primary amines. The proposed mechanism reveals that an alcohol or amine substrate is initially dehydrogenated to an aldehyde/ketone or NH‐imine with concomitant formation of a [PtSn] hydride. Condensation of the aldehyde/ketone species or deamination of the NH‐imine intermediate with another molecule of amine forms an N‐substituted imine which is then reduced to a new amine product by the in‐situ generated [PtSn] hydride under a nitrogen atmosphere or remains unchanged as the final product under an oxygen atmosphere. The Pt–Sn/γ‐Al₂O₃ catalyst can be easily recycled without Pt metal leaching and has exhibited very high catalytic activity toward a wide range of amine and alcohol substrates, which suggests potential for application in the direct production of secondary and tertiary amines and N‐substituted imines.     

The ruthenium-catalyzed reduction and reductive N-alkylation of secondary amides with hydrosilanes: practical synthesis of secondary and tertiary amines by judicious choice of hydrosilanes

A triruthenium cluster, (mu3,eta2,eta3,eta5-acenaphthylene)Ru3(CO)7 (1) catalyzes the reaction of secondary amides with hydrosilanes, yielding a mixture of secondary amines, tertiary amines, and silyl enamines. Production of secondary amines with complete selectivity is achieved by the use of higher concentration of the catalyst (3 mol %) and the use of bifunctional hydrosilanes such as 1,1,3,3-tetramethyldisiloxane. Acidic workup of the reaction mixture affords the corresponding ammonium salts, which can be treated with a base, providing a facile method for isolation of secondary amines with high purity. In contrast, tertiary amines are formed with high selectivity by using lower concentration of the catalyst (1 mol %) and polymeric hydrosiloxanes (PMHS) as reducing agent. Reduction with PMHS encapsulates the ruthenium catalyst and organic byproducts to the insoluble silicone resin. The two reaction manifolds are applicable to various secondary amides and are practical in that the procedures provide the desired secondary or tertiary amine as a single product. The product contaminated with only minimal amounts of ruthenium and silicon residues. On the basis of the products and observed side products as well as NMR studies a mechanistic scenario for the reaction is also described.     

Solvation largely accounts for the effect of N-alkylation on the properties of nickel(II/I) and chromium(III/II) cyclam complexes

The source of the effect of N-alkylation on the redox properties of Ni(II/I) and Cr(III/II) cyclam complexes has been investigated using DFT calculations. The structures of the anhydrous and hydrated complexes were optimized in the gas phase, and single point calculations were performed in a polarized continuum. The main results are the following: the decrease in outer sphere solvation upon N-alkylation is the major source of the relative stabilization of the lower oxidation state complexes by the tertiary amine ligands; tertiary amine nitrogen donors are stronger sigma-donors than the secondary amines, as predicted from the inductive effect of alkyls; steric strain elongates the metal-nitrogen bonds in the tertiary complexes and decreases the ligand strain energies; and the site of water binding to the complexes differs because of their different electronic structures (i.e., in the Ni complexes, the water molecules bind to the M[bond]N[bond]H sites, whereas in the Cr complexes they bind to the central metal cation). Outer sphere hydrogen bonding of water to the ligands in the coordination sphere lowers the ionization potentials by charge delocalization.     

Photocatalytic α‐Tertiary Amine Synthesis via C−H Alkylation of Unmasked Primary Amines

A practical, catalytic entry to α,α,α‐trisubstituted (α‐tertiary) primary amines by C?H functionalisation has long been recognised as a critical gap in the synthetic toolbox. We report a simple and scalable solution to this problem that does not require any in situ protection of the amino group and proceeds with 100 % atom‐economy. Our strategy, which uses an organic photocatalyst in combination with azide ion as a hydrogen atom transfer (HAT) catalyst, provides a direct synthesis of α‐tertiary amines, or their corresponding γ‐lactams. We anticipate that this methodology will inspire new retrosynthetic disconnections for substituted amine derivatives in organic synthesis, and particularly for challenging α‐tertiary primary amines.     

Simultaneous spectrophotometric analysis of aliphatic amines utilizing thermochromism of charge-transfer complexes with tetrabromophenolphthalein ethyl ester.

Aliphatic amines, such as n-hexylamine (primary), di-n-hexylamine (secondary) and tri-n-hexylamine (tertiary amine), react with tetrabromophenolphthalein ethyl ester molecules (TBPEH) to form reddish or red-violet charge-transfer complexes (CT complexes) in 1,2-dichloroethane (DCE). The absorption maxima of the CT complexes with all primary amines occur at around 560 nm, with secondary amines at 570 nm and, with tertiary amines at 580 nm. The CT complex formation constants with TBPEH in DCE increase in the order of the primary, secondary and tertiary amines, but their constants decrease quantitatively with an increase in temperature. This phenomenon (thermochromism) could be applied to the simultaneous spectrophotometric determination of primary amine and secondary amine, or secondary amine and tertiary amine in a mixed solution utilizing the difference of absorbance with temperature changes.     

Catalytic reductive alkylation of secondary amine with aldehyde and silane by an iridium compound

[reaction: see text] An efficient methodology for the reductive alkylation of secondary amine with aldehyde and Et(3)SiH using an iridium complex as a catalyst has been developed. For example, treatment of dibutylamine with butyraldehyde and Et(3)SiH (a 1:1:1 molar amount of amine, aldehyde, and silane) in 1,4-dioxane at 75 degrees C under the influence of a catalytic amount of [IrCl(cod)](2) gave tributylamine in quantitative yield. In this reaction, no reduction of aldehyde took place. It was found that IrCl(3), which is a starting material for preparation of iridium complexes such as [IrCl(cod)](2), acts as an efficient catalyst for the present reductive alkylation of amine. In addition, a cheaper, easy-to-handle, and environmentally friendly reducing reagent such as polymethylhydrosiloxane (PMHS) in place of Et(3)SiH was also useful. Thus, a variety of secondary amines could be alkylated by allowing them to react with aldehydes and PMHS in the presence of an iridium catalyst to afford the corresponding tertiary amines in good to excellent yields. From the deuterium label experiments, it was revealed that silane and water, generated during the formation of enamine by the reaction of amine and aldehyde, seem to behave as a hydrogen source. The catalytic cycle was discussed.     

One pot synthesis of N-ethylaniline from nitrobenzene and ethanol

A novel method for the one pot synthesis of N-alkyl arylamines from nitro aromatic compounds and alcohols is proposed through the combination of the aqueous-phase reforming of alcohol for hydrogen production, the reduction of nitro aromatic compounds for the synthesis of aromatic amine and the N-alkylation of aromatic amine for the production of N-alkyl arylamine over an identical catalyst under the same conditions of temperature and pressure in a single reactor. In this process, hydrogen generated from the aqueous-phase reforming of alcohols was used in-situ for the hydrogenation of nitro aromatic compounds for aromatic amine synthesis, followed by N-alkylation of aromatic amine with alcohols to form the corresponding N-alkyl arylamines at a low partial pressure of hydrogen. For the system composed of nitrobenzene and ethanol, under the conditions of 413 K and PN2 = 1 MPa, the conversion degrees of nitrobenzene and aniline were 100%, the selectivity to N-ethylaniline and N, N-diethylaniline were 85.9% and 0%-4%, respectivity, after reaction for 8 h at the volumetric ratio of nitrobenzene:ethanol:water = 10:60:0. The selectivity for N, N-diethylaniline production is much lower than that through the traditional method. In this process, hydrogen and aromatic amines generated from the aqueous-phase reforming of alcohols and hydrogenation of nitro aromatic compounds, respectively, could be promptly removed from the surface of the catalyst due to the occurrence of in-situ hydrogenation and N-alkylation reactions. Thus, this may be a potential approach to increase the selectivity to N-alkyl arylamine.     

Development of a General Non‐Noble Metal Catalyst for the Benign Amination of Alcohols with Amines and Ammonia

The N‐alkylation of amines or ammonia with alcohols is a valuable route for the synthesis of N‐alkyl amines. However, as a potentially clean and economic choice for N‐alkyl amine synthesis, non‐noble metal catalysts with high activity and good selectivity are rarely reported. Normally, they are severely limited due to low activity and poor generality. Herein, a simple NiCuFeO_x catalyst was designed and prepared for the N‐alkylation of ammonia or amines with alcohol or primary amines. N‐alkyl amines with various structures were successfully synthesized in moderate to excellent yields in the absence of organic ligands and bases. Typically, primary amines could be efficiently transformed into secondary amines and N‐heterocyclic compounds, and secondary amines could be N‐alkylated to synthesize tertiary amines. Note that primary and secondary amines could be produced through a one‐pot reaction of ammonia and alcohols. In addition to excellent catalytic performance, the catalyst itself possesses outstanding superiority, that is, it is air and moisture stable. Moreover, the magnetic property of this catalyst makes it easily separable from the reaction mixture and it could be recovered and reused for several runs without obvious deactivation.     

Enthalpies of formation of primary, secondary, and tertiary amineborane adducts in tetrahydrofuran solution

The enthalpies of solution of primary, secondary, and tertiary amines in THF were determined from calorimetric experiments for five primary, five secondary, and three tertiary amines. The enthalpies of formation of amineborane adducts from borane and the corresponding amines in THF solution were also determined. The differences in adduct formation enthalpies from borane and the corresponding amines can be explained by taking into account steric effects and the chain length of the substituents on the amine. In general, as the alkyl chain length, branching, or the number of chains increases, the formation enthalpy of amineborane adducts is less exothermic. That is to say, the steric effect is more important in tertiary and secondary amines than in primary ones. The enthalpy of solution of linear primary amines in THF was more endothermic as the alkyl chain increased and a similar behavior was observed with linear secondary and tertiary ones. An analysis is made of the amine structural factors which affect the amineborane adduct formation.     

Sensitive fluorescent detection and Lewis basicity of aliphatic amines

In this contribution is reported the sensitive properties of the Zn(II) Schiff base complex, 1, in dichloromethane with respect a series of primary, secondary, and tertiary aliphatic amines through the study of fluorescence enhancement upon amine coordination to the Lewis acidic Zn(II) metal center with formation of 1:1 adducts. It is found that complex 1 exhibits selectivity and nanomolar sensitivity for primary and alicyclic amines. A distinct selectivity is also observed along the series of secondary or tertiary amines, paralleling the increasing steric hindrance at the nitrogen atom. The binding interaction can be related to the Lewis basicity of the coordinating amine; thus, complex 1 represents a suitable reference Lewis acid, and estimated binding constants within the investigated amine series can be related to their relative Lewis basicity. A relative order of the Lewis basicity can be established for acyclic amines, primary > secondary > tertiary, while an inverted order, tertiary > secondary ≈ primary (acyclic), is found in the case of alicyclic amines. The present approach represents a simple, suitable method to ranking the relative Lewis basicity of aliphatic amines in low-polarity, nonprotogenic solvents.     

Tandem carbocupration/oxygenation of terminal alkynes

[chemical reaction: see text]. A direct and general synthesis of alpha-branched aldehydes and their enol derivatives is described. Carbocupration of terminal alkynes and subsequent oxygenation with lithium tert-butyl peroxide generates a metallo-enolate. Trapping with various electrophiles provides alpha-branched aldehydes or stereo-defined trisubstituted enol esters or silyl ethers. The tandem carbocupration/oxygenation tolerates alkyl and silyl ethers, esters, and tertiary amines. The reaction is effective with organocopper complexes derived from primary, secondary, and tertiary Grignard reagents and from n-butyllithium.     

Orthogonal Catalysis for an Enantioselective Domino Inverse-Electron Demand Diels−Alder/Substitution Reaction

An enantioselective domino process for the synthesis of substituted 1,2-dihydronaphthalenes has been developed by the combination of chiral amines and a bidentate Lewis acid in an orthogonal catalysis. This new method is based on an inverse electron-demand Diels−Alder and a subsequent group exchange reaction. An enamine is generated in situ from an aldehyde and a chiral secondary amine catalyst that reacts with phthalazine, activated by the coordination to a bidentate Lewis acid catalyst. The absolute configuration of the product is controlled by chiral information provided by the amine. The formed ortho-quinodimethane intermediate is then transformed via a group exchange reaction with thiols. The new method shows a broad scope and tolerates a wide range of functional groups with enantiomeric ratios up to 91 : 9. All-in-all, this enantioselective synthesis tool provides an easy access to complex 1,2-dihydronaphthalenes starting from readily available phthalazine, aldehydes and thiols in a combinatorial way.     

General Reductive Amination of Aldehydes and Ketones with Amines and Nitroaromatics under H2 by Recyclable Iridium Catalysts

Heterogeneous iridium catalysts were prepared and applied for the reductive amination of aldehydes and ketones with nitroaromatics and amines using H₂ . The iridium catalysts were prepared by pyrolysis of ionic liquid 1‐methyl‐3‐cyanomethylimidazoulium chloride ([MCNI ]Cl) with iridium chloride (IrCl₃ ) in activated carbons. Iridium particles were well dispersed and stable in the N‐doped carbon materials from [MCNI ]Cl with activated carbon. The Ir@NC (600‐2h) catalyst was found to be highly active and selective for the reductive amination of aldehydes and ketones using H₂ and a variety of nitrobenzenes and amines were selectively converted into the corresponding secondary and tertiary amines. The Ir@NC (600‐2h) catalyst can be reusable several times without evident deactivation.     

Solution-phase library synthesis of furanoses

The solution-phase synthesis of amido-, urea-, and aminofuranoses was achieved. Alkylated furanose aldehydes were treated with primary amines in the presence of sodium triacetoxyborohydride to give secondary amines. Subsequent acylation with acid chlorides and isocyanates afforded amidofuranoses and ureafuranoses, respectively. Second, reductive amination of furanose aldehydes with secondary amines yielded tertiary amines. The resulting acetonides were treated with alcohols in the presence of acid to yield mixed acetals. In the library syntheses, functionalized scavenger resins were used in the purification of intermediates and products.