Highly Enantioselective Ruthenium/PNNP-Catalyzed Imine Aziridination: Evidence of Carbene Transfer from a Diazoester Complex

Enantiomerically Enriched Aziridine-2-carboxylates via Copper-Catalyzed Reductive Kinetic Resolution of 2H-Azirines

We present the first reductive kinetic resolution of racemic 2H-azirines to prepare optically enriched N-H aziridine-2-carboxylates, which are bench stable and readily diversifiable building blocks, concomitantly with the corresponding enantiomerically enriched 2H-azirines. The N-H aziridines were obtained with excellent diastereoselectivity (>20:1) and high enantioselectivity (up to 94%). A Hammett study revealed a linear free energy relationship between the ΔΔG⧧ of the diastereomeric transition states and the σp - values. density functional theory (DFT) calculations and non-covalent interaction analysis suggested that non-classical H-bonding interactions and edge-to-face aromatic interactions between the substrate and the ligand are responsible for the stereoselectivity and also for the substrate electronic effects observed in the Hammett study.

3-Arylaziridine-2-carboxylic Acid Derivatives and (3-Arylaziridin-2-yl)ketones: The Aziridination Approaches

Aziridination reactions represent a powerful tool in aziridine synthesis. Significant progress has been achieved in this field in the last decades, whereas highly functionalized aziridines including 3-arylated aziridine-2-carbonyl compounds play an important role in both medical and synthetic chemistry. For the reasons listed, in the current review we have focused on the ways to obtain 3-arylated aziridines and on the recent advances (mainly since the year 2000) in the methodology of the synthesis of these compounds via aziridination.

A One-Step Preparation of Tetradentate Ligands with Nitrogen and Phosphorus Donors by Reductive Amination and Representative Iron Complexes

The synthesis and use of the first examples of unsymmetrical, mixed phosphine donor tripodal NPP₂' ligands N(CH₂CH₂PR₂)₂(CH₂CH₂PPh₂) are presented. The ligands are synthesized via a convenient, one pot reductive amination using 2-(diphenylphosphino)ethylamine and various substituted phosphonium dimers in order to introduce mixed phosphine donors substituted with P/P', those being Ph/Cy (2), Ph/ⁱPr (3), Ph/ⁱBu (4), Ph/o-Tol (5), and Ph/p-Tol (6). Additionally, we have developed the first known synthesis of a symmetrical tripodal NP₃ ligand N(CH₂CH₂PiBu₂)₃ using bench safe ammonium acetate as the lone nitrogen source (7). This new protocol eliminates the use of extremely dangerous nitrogen mustard reagents typically required to synthesize NP₃ ligands. Some of these tetradentate ligands and also P₂NN' ligands N(CH₂-o-C₅H₄N)(CH₂CH₂PR₂)₂ (P₂NN'-Cy, R = Cy; P₂NN'-Ph, R = Ph) prepared by reductive amination using 2-picolylamine are used in the synthesis and reactions of iron complexes. FeCl₂(P₂NN'-Cy) (8) undergoes single halide abstraction with NaBPh₄ to give the trigonal bipyramidal complex [FeCl(P₂NN'-Cy)][BPh₄] (9). Upon exposure to CO(g), complex 9 readily coordinates CO giving [FeCl(P₂NN'-Cy)(CO)][BPh₄] (10), and further treatment with an excess of NaBH₄ results in formation of the hydride complex [Fe(H)(P₂NN'-Cy)(CO)][BPh₄] (11). Our previously reported complex FeCl₂(P₂NN'-Ph) undergoes double halide abstraction with NaBPh₄ in the presence of the coordinating solvent to give [Fe(NCMe)₂(P₂NN'-Ph)][BPh₄]₂ (12). Ligand 3 can be coordinated to FeCl₂, and upon sequential halide abstraction, treatment with NaBH₄, and exposure to an atmosphere of dinitrogen, the dinitrogen hydride complex [Fe(H)(NPP₂'-ⁱPr)(N₂)][BPh₄] (13) is isolated. Our symmetrical NP₃ ligand 7 can also be coordinated to FeCl₂ and, upon exposure to an atmosphere of CO(g), selectively forms [FeCl(NP₃)(CO)][BPh₄] (14) after salt metathesis with NaBPh₄. Complex 14 can be treated with an excess of NaBH₄ to give the hydride complex [Fe(H)(NP₃)(CO)][BPh₄] (15), which can further be deprotonated/reduced to the Fe(0) complex Fe(NP₃)(CO) (16) upon treatment with an excess of KH.

Pentamethylcyclopentadienyl osmium complexes that contain diazoalkane, dioxygen and allenylidene ligands: preparation and reactivity

Diazoalkane complexes [Os(η5-C5Me5)(N2CAr1Ar2)(PPh3){P(OR)3}]BPh4 (1, 2) [R = Me (1), Et (2); Ar1 = Ar2 = Ph (a); Ar1 = Ph, Ar2 = p-tolyl (b); Ar1Ar2 = C12H8 (fluorenyl) (c)] were prepared by reacting bromo-compounds OsBr(η5-C5Me5)(PPh3){P(OR)3} with an excess of diazoalkane in ethanol. The treatment of diazoalkane complexes 1 and 2 with acetylene under mild conditions (1 atm, RT) led to dipolar (3 + 2) cycloaddition affording 3H-pyrazole derivatives [Os(η5-C5Me5)(η1-[upper bond 1 start]N[double bond, length as m-dash]NC(C12H8)CH[double bond, length as m-dash]C[upper bond 1 end]H)(PPh3){P(OR)3}]BPh4 (6, 7) [R = Me (6), Et (7)] whereas reactions with terminal alkynes R1C[triple bond, length as m-dash]CH (R1 = Ph, p-tolyl, COOMe) gave vinylidene derivatives [Os(η5-C5Me5){[double bond, length as m-dash]C[double bond, length as m-dash]C(H)R1}(PPh3){P(OR)3}]BPh4 (8b-d, 9b-d) [R = Me (8), Et (9); R1 = Ph (b), p-tolyl (c), COOMe (d)]. Exposure to air of dichloromethane solutions of complexes 1 and 2 produced dioxygen derivatives [Os(η5-C5Me5)(η2-O2)(PPh3){P(OR)3}]BPh4 (10, 11) [R = Me (10), Et (11)]. Allenylidene [Os][double bond, length as m-dash]C[double bond, length as m-dash]C[double bond, length as m-dash]CR1R2 (12-14) [R1 = R2 = Ph (12, 13); R1 = Ph, R2 = Me (14)], vinylvinylidene [Os][double bond, length as m-dash]C[double bond, length as m-dash]C(H)C(Ph)[double bond, length as m-dash]CH2 (15) and 3-hydroxyvinylidene [Os][double bond, length as m-dash]C[double bond, length as m-dash]C(H)C(H)R2(OH) (16, 17) [R2 = Ph (16), H (17)] derivatives were also prepared. The vinylidene complex [Os(η5-C5Me5)([double bond, length as m-dash]C[double bond, length as m-dash]CH2)(PPh3){P(OMe)3}]BPh4 (8a) reacted with PPh3 to afford the alkenylphosphonium derivative [Os(η5-C5Me5){η1-C(H)[double bond, length as m-dash]C(H)PPh3}(PPh3){P(OMe)3}]BPh4 (18) whereas vinylidene complexes 8 and 9 reacted with water leading to the hydrolysis of the alkyne and the formation of carbonyl complexes [Os(η5-C5Me5)(CO)(PPh3){P(OR)3}]BPh4 (19, 20). The complexes were characterised by spectroscopic data (IR and NMR) and by X-ray crystal structure determination of [Os(η5-C5Me5){[double bond, length as m-dash]C[double bond, length as m-dash]C(H)p-tolyl}(PPh3){P(OEt)3}]BPh4 (9c), [Os(η5-C5Me5)(η2-O2)(PPh3){P(OMe)3}]BPh4 (10) and [Os(η5-C5Me5)(CO)(PPh3){P(OMe)3}]BPh4 (19).

CuH-Catalyzed Regioselective Intramolecular Hydroamination for the Synthesis of Alkyl-Substituted Chiral Aziridines

This report details a general and enantioselective means for the synthesis of alkyl-substituted aziridines. This protocol offers a direct route for the synthesis of alkyl-substituted chiral aziridines from achiral starting materials. Readily accessed allylic hydroxylamine esters undergo copper hydride-catalyzed intramolecular hydroamination with a high degree of regio- and enantiocontrol to afford the aziridine products in good to excellent yields in highly enantioenriched form. The utility of the products derived from this method is further demonstrated through derivatization of the chiral aziridine products to obtain a diverse array of functionalized enantioenriched amines.

Highly Selective Stable Isotope Labeling of Histidine Residues by Using a Novel Precursor in E. coli-Based Overexpression Systems

The importance of NMR spectroscopy in unraveling the structural and dynamic properties of proteins is ever-expanding owing to progress in experimental techniques, hardware development, and novel labeling approaches. Multiple sophisticated methods of aliphatic residue labeling can be found in the literature, whereas the selective incorporation of NMR active isotopes into other amino acids still holds the potential for improvement. In order to close this methodological gap, we present a novel metabolic precursor for cell-based protein overexpression to assemble ¹³ C/² H isotope patterns in the peptide backbone, as well as in side chain positions of a mechanistically distinguished histidine residue.

Brønsted Base-Mediated Aziridination of 2-Alkyl-Substituted-1,3-Dicarbonyl Compounds and 2-Acyl-Substituted-1,4-Dicarbonyl Compounds by Iminoiodanes

The synthesis of α,α-diacylaziridines and α,α,β-triacylaziridines from reaction of 2-alkyl-substituted-1,3-dicarbonyl compounds and 2-acyl-substituted-1,4-dicarbonyl compounds with arylsulfonyliminoiodinanes (ArSO2N=IPh) under Brønsted base-mediated atmospheric conditions is described. The reaction mechanism is thought to involve the formal oxidation of the substrate followed by aziridination of the ensuing α,β-unsaturated intermediate by the hypervalent iodine(iii) reagent.

Recent advances in chiral imino-containing ligands for metal-catalyzed asymmetric transformations

In this review, the recent applications of a variety of chiral imino-containing ligands classified by different types of metal-catalyzed asymmetric reactions are summarized. The progress made in this area would encourage us to design and synthesize more novel chiral imino-containing ligands, and explore their applications in metal-catalyzed asymmetric transformations.

Pentamethylcyclopentadienyl Half-Sandwich Diazoalkane Complexes of Ruthenium: Preparation and Reactivity

The diazoalkane complexes [Ru(η(5)-C5Me5)(N2CAr1Ar2){P(OR)3}L]BPh4 (1-4) [R = Me, L = P(OMe)3 (1); R = Et, L = P(OEt)3 (2); R = Me, L = PPh3 (3); R = Et, L = PPh3 (4); Ar1 = Ar2 = Ph (a); Ar1 = Ph, Ar2 = p-tolyl (b); Ar1Ar2 = C12H8 (c); Ar1 = Ph, Ar2 = PhC(O) (d)] and [Ru(η(5)-C5Me5){N2C(C12H8)}{PPh(OEt)2}(PPh3)]BPh4 (5c) were prepared by allowing chloro-compounds RuCl(η(5)-C5Me5)[P(OR)3]L to react with the diazoalkane Ar1Ar2CN2 in the presence of NaBPh4. Treatment of complexes 1-4 with H2O afforded 1,2-diazene derivatives [Ru(η(5)-C5Me5)(η(2)-NH═NH){P(OR)3}L]BPh4 (6-9) and ketone Ar1Ar2CO. A reaction path involving nucleophilic attack by H2O on the coordinated diazoalkane is proposed and supported by density functional theory calculations. The complexes were characterized spectroscopically (IR and (1)H, (31)P, (13)C, (15)N NMR) and by X-ray crystal structure determination of [Ru(η(5)-C5Me5)(N2CC12H8){P(OEt)3}2]BPh4 (2c) and [Ru(η(5)-C5Me5)(η(2)-NH═NH){P(OEt)3}2]BPh4 (7).

Preparation of diazoalkane complexes of iron(ii)

The preparation of diazoalkane complexes of iron(ii) and their reactivity toward acrylonitrile are described.

Diazoalkane complexes of ruthenium with tris(pyrazolyl)borate and bis(pyrazolyl)acetate ligands

Diazoalkane complexes [Ru(Tp)(N2CAr1Ar2)(PPh3)L]BPh4 ( and ) [Tp = tris(pyrazolyl)borate; L = P(OMe)3, P(OEt)3; Ar1 = Ar2 = Ph; Ar1 = Ph, Ar2 = p-tolyl; Ar1Ar2 = C12H8] were prepared by allowing chloro-compounds RuCl(Tp)(PPh3)L to react with diazoalkane in the presence of NaBPh4. Acrylonitrile CH2[double bond, length as m-dash]C(H)CN reacts with diazoalkane complexes to give 3H-pyrazole derivatives [Ru(Tp){N[double bond, length as m-dash]NC(Ar1Ar2)CH(CN)CH2}(PPh3){P(OMe)3}]BPh4 and [Ru(Tp){N[double bond, length as m-dash]NC(Ar1Ar2)CH2C(H)CN}(PPh3){P(OMe)3}]BPh4 (). Diazoalkane complexes [Ru(bpza)(N2CAr1Ar2)(PPh3)2]BPh4 () [bpza = bis(pyrazolyl)acetate] were also prepared. All complexes were characterised by IR and NMR spectroscopy and X-ray crystal structure determination of [Ru(Tp){N2C(Ph)(p-tolyl)}(PPh3){P(OMe)3}]BPh4 (). The differences exhibited by [Ru(Tp){N2C(Ph)(p-tolyl)}(PPh3){P(OMe)3}](+) and [Ru(Cp){N2C(Ph)(p-tolyl)}(PPh3){P(OMe)3}](+), as regards coordination of the diazoalkane ligand and reactivity towards alkenes, were explained on the basis of a comparative DFT study.

Preparation and reactivity of diazoalkane complexes of ruthenium stabilised by an indenyl ligand

Diazoalkane complexes [Ru(η(5)-C9H7)(N2CAr1Ar2)(PPh3)L]BPh4 (1-3) [L = PPh3, P(OMe)3, P(OEt)3; Ar1 = Ar2 = Ph; Ar1 = Ph, Ar2 = p-tolyl; Ar1Ar2 = C12H8 fluorenyl] were prepared by allowing chloro-complexes [RuCl(η(5)-C9H7)(PPh3)L] to react with an excess of diazoalkane in ethanol. Complexes 1-3 reacted with ethylene CH2=CH2 (1 atm) and maleic anhydride [ma, CH=CHCO(O)CO] to afford η(2)-alkene complexes [Ru(η(5)-C9H7)(η(2)-CH2=CH2)(PPh3)L]BPh4 (4, 5) and [Ru(η(5)-C9H7){η(2)-CH=CHCO(O)CO}(PPh3)L]BPh4 (7). Further, complexes 1-3 underwent cycloaddition with acrylonitrile CH2=C(H)CN, giving 1H-pyrazoline derivatives [Ru(η(5)-C9H7){η(1)-N=C(CN)CH2C(Ar1Ar2)NH}(PPh3)L]BPh4 (6). Treatment of diazoalkane complexes 1-3 with acetylene CH[triple bond, length as m-dash]CH under mild conditions (1 atm, room temperature) led to dipolar cycloaddition, affording 3H-pyrazole complexes [Ru(η(5)-C9H7)-{η(1)-N=NC(Ar1Ar2)CH=CH}(PPh3)L]BPh4 (8), whereas reaction with terminal alkynes RC≡CH (R = Ph, p-tolyl, Bu(t)) gave vinylidene derivatives [Ru(η(5)-C9H7){=C=C(H)R}(PPh3)L]BPh4 (9). The latter reacted with nucleophiles such as amines and alcohols to give amino- and alkoxy-carbene derivatives [Ru(η(5)-C9H7){=C(NHPr(n))(CH2Ph)}(PPh3)L]BPh4 (11) and [Ru(η(5)-C9H7){=C(CH3)(OEt)}(PPh3)L]BPh4 (10), respectively. In addition, complexes 9 reacted with phenylhydrazine to afford nitrile derivatives [Ru(η(5)-C9H7)(N[triple bond, length as m-dash]CCH2R)(PPh3)L]BPh4 (12) and phenylamine, whereas the reaction with water led to hydrolysis of the alkyne and formation of carbonyl complexes [Ru(η(5)-C9H7)(CO)(PPh3)L]BPh4 (13). Lastly, treatment of vinylidene complexes with the phosphines PPh3 and P(OMe)3 afforded alkenylphosphonium derivatives [Ru(η(5)-C9H7){C(H)=C(R)PPh3}(PPh3)L]BPh4 (14) and [Ru(η(5)-C9H7){C(R)=C(H)P(OMe)3}(PPh3)L]BPh4 (15), respectively. Compound [Ru(η(5)-C9H7){C(H)=C(H)PPh3}(PPh3)L]BPh4 (16) was also prepared. The complexes were characterised by spectroscopy (IR and NMR) and X-ray crystal structure determinations of [Ru(η(5)-C9H7){N2C(C12H8)}(PPh3){P(OEt)3}]BPh4 (3c), [Ru(η(5)-C9H7){=C=C(H)Ph}(PPh3){P(OEt)3}]BPh4 (9d) and [Ru(η(5)-C9H7){C(H)=C(Ph)PPh3}(PPh3){P(OEt)3}]BPh4 (14d).

Hydrolysis of coordinated diazoalkanes to yield side-on 1,2-diazene derivatives

Diazoalkane complexes [Ru(η(5)-C5Me5)(N2CAr1Ar2)(PPh3){P(OR)3}]BPh4 [R = Me (1), Et (2); Ar1 = Ar2 = Ph (a); Ar1 = Ph, Ar2 = p-tolyl (b); Ar1Ar2 = C12H8 (c)] were prepared by allowing chloro complexes RuCl(η(5)-C5Me5)(PPh3)[P(OR)3] to react with diazoalkane Ar1Ar2CN2 in ethanol. The treatment of compounds 1 and 2 with H2O afforded 1,2-diazene derivatives [Ru(η(5)-C5Me5)(η(2)-NH═NH)(PPh3){P(OR)3}]BPh4 (3 and 4) and ketone Ar1Ar2CO. A reaction path involving nucleophilic attack by H2O on the coordinated diazoalkane is proposed. The complexes were characterized spectroscopically (IR and NMR) and by X-ray crystal structure determination of [Ru(η(5)-C5Me5)(η(2)-NH═NH)(PPh3){P(OMe)3}]BPh4 (3).

Selective transfer hydrogenation and hydrogenation of ketones using a defined monofunctional (P^N(Bn)^N(Bn)^P)-Ru(II) complex

A defined (P^N^N^P)-Ru complex possessing tertiary amines within the ligand backbone proved to be highly active both in transfer hydrogenations and hydrogenations of a variety of ketones. As compared to the existing catalytic systems, no bifunctional activation of H2 or of the substrate by the metal center and a secondary amine within the ligand backbone is required to obtain high activities at catalyst loadings of down to 10 ppm.