Imine Substituents Research Articles

Synthesis and Coordination Chemistry of Iminophosphanes.

Iminophosphanes are a new group of 1,3-P,N-ligands, readily obtainable from secondary phosphanes and nitrilium ions, having a tunable N-donor site by means of varying the imine substituents. These ligands give, in high yields, monodentate gold complexes and bidentate rhodium and iridium complexes. Crystal structures are reported for both the ligands and the complexes.

Cobalt catalyzed z-selective hydroboration of terminal alkynes and elucidation of the origin of selectivity.

A bis(imino)pyridine cobalt-catalyzed hydroboration of terminal alkynes with HBPin (Pin = pinacolate) with high yield and (Z)-selectivity for synthetically valuable vinylboronate esters is described. Deuterium labeling studies, stoichiometric experiments, and isolation of catalytically relevant intermediates support a mechanism involving selective insertion of an alkynylboronate ester into a Co-H bond, a pathway distinct from known precious metal catalysts where metal vinylidene intermediates have been proposed to account for the observed (Z) selectivity. The identity of the imine substituents dictates the relative rates of activation of the cobalt precatalyst with HBPin or the terminal alkyne and, as a consequence, is responsible for the stereochemical outcome of the catalytic reaction.

Novel one-pot pseudo four component reaction: expeditious synthesis of functionalized imidazo[1,2-a]pyridines

A new and efficient one-pot synthesis of imidazo[1,2-a] pyridines is described. A C-3 imine substituent was installed on the imidazopyridine framework under mild conditions to form the corresponding products in good to excellent yields.

Cyclometalation and coupling of a rigid 4,5-bis(imino)acridanide pincer ligand on yttrium.

An extremely rigid NNN-donor proligand, 4,5-bis{(diphenylmethylene)amino}-2,7,9,9-tetramethylacridan, H[AIm2] was prepared in five steps starting from 5-methyl-2-aminobenzoic acid and 4-bromotoluene. Reaction of intensely orange H[AIm2] with LiCH2SiMe3 formed deep blue Li(x)[AIm2]x (x = 2 in the solid state), while reaction with [Y(CH2SiMe3)3(THF)2] (0.5 equiv.) afforded deep blue [Y(AIm2)(AIm)] (1; AIm = an AIm2 ligand cyclometalated at the ortho-position of one of the phenyl rings). Compound 1 slowly isomerizes to form green-brown 2, which contains a single trianionic, hexadentate ligand that features one amine, two imine, and three amido donors. The acridanide backbone and one imine group in each of the original AIm2 ligands is intact, but the two acridanide backbones are now linked by an isoindoline heterocycle. Yttrium in 2 is coordinated to six nitrogen donors and the ortho carbon of an isoindoline phenyl substituent. The intense colours of H[AIm2], Li(x)[AIm2]x and 1 were shown by TD-DFT calculations to arise from charge transfer transitions from the HOMO, which is localized on the acridanide ligand backbone, to the LUMO and LUMO+1, which are localized on the imine substituents. The conversion of 1 to 2 was studied by UV-Visible absorption spectroscopy and is first-order with a half-life of 7.8 hours at room temperature.

Asymmetric synthesis of α-branched amines via Rh(III)-catalyzed C-H bond functionalization.

The first asymmetric intermolecular addition of non-acidic C–H bonds to imines is reported. The use of the activating N-perfluorobutanesulfinyl imine substituent is essential for achieving sufficient reactivity and provides outstanding diastereoselectivity (>98:2 dr). Straightforward removal of the sulfinyl group with HCl yields the highly enantiomerically enriched amine hydrochlorides.

Rational Design of Rhodium Complexes Featuring κ4‐N,N,N,N‐ and κ5‐N,N,N,P,P‐Bis(imino)pyridine Ligands

AbstractThe addition of aminoalkyl‐substituted 2,6‐bis(imino)pyridine (or pyridine diimine, PDI) ligands to [(COD)RhCl]2 (COD = 1,5‐cyclooctadiene) resulted in the formation of rhodium monochloride complexes with the general formula (NPDI)RhCl (NPDI = iPr2NEtPDI or Me2NPrPDI). The investigation of (iPr2NEtPDI)RhCl and (Me2NPrPDI)RhCl by single‐crystal X‐ray diffraction verified the absence of amine arm coordination and a pseudo square‐planar geometry about rhodium. Replacement of the chloride ligand with an outer‐sphere anion was achieved by adding AgBF4 directly to (iPr2NEtPDI)RhCl to form [(iPr2NEtPDI)Rh][BF4]. Alternatively, this complex was prepared by chelate addition following the salt metathesis reaction between AgBF4 and [(COD)RhCl]2. Using the latter method, both [(NPDI)Rh][BF4] complexes were isolated and found to exhibit κ4‐N,N,N,N‐PDI coordination regardless of arm length or steric bulk. In contrast, the metallation of PPDI chelates featuring alkylphosphane imine substituents (PPDI = Ph2PEtPDI or Ph2PPrPDI) resulted in the formation of cationic complexes featuring κ5‐N,N,N,P,P‐PDI coordination in all instances, [(PPDI)Rh][X] (X = Cl, BF4). Adjusting the metallation stoichiometry allowed the preparation of [(Ph2PPrPDI)Rh][(COD)RhCl2], as validated by multinuclear NMR spectroscopy and single‐crystal X‐ray diffraction.

Oxidation‐Triggered Ring‐Opening Metathesis Polymerization

Eight new N-Hoveyda-type complexes were synthesized in yields of 67-92 % through reaction of [RuCl2 (NHC)(Ind)(py)] (NHC=1,3-bis(2,4,6-trimethylphenylimidazolin)-2-ylidene (SIMes) or 1,3-bis(2,6-diisopropylphenylimidazolin)-2-ylidene (SIPr), Ind=3-phenylindenylid-1-ene, py=pyridine) with various 1- or 1,2-substituted ferrocene compounds with vinyl and amine or imine substituents. The redox potentials of the respective complexes were determined; in all complexes an iron-centered oxidation reaction occurs at potentials close to E=+0.5 V. The crystal structures of the reduced and of the respective oxidized Hoveyda-type complexes were determined and show that the oxidation of the ferrocene unit has little effect on the ruthenium environment. Two of the eight new complexes were found to be switchable catalysts, in that the reduced form is inactive in the ring-opening metathesis polymerization of cis-cyclooctene (COE), whereas the oxidized complexes produce polyCOE. The other complexes are not switchable catalysts and are either inactive or active in both reduced and oxidized states.

(3,5-Di-tert-butyl-2-ethoxybenzylidene)[2-(3,5-di-tert-butyl-1H-pyrazol-1-yl)ethyl]amine

The angles within the benzene ring in the title compound, C30H49N3O, ranging from 116.34 (16) to 124.18 (16)°, reflect the presence of electron-donating and electron-withdrawing substituents. The angles at the two electron-donating tert-butyl substituents are smaller than 120°, at the electron-withdrawing eth­oxy substituent larger than 120°, and at the imine substituent equal to 119.59 (16)°. The latter does not reflect the electron-donating nature of the imine group due to the presence of other substituents.

Rifamycin antibiotics—new compounds and synthetic methods. Part 3: Study of the reaction of 3-formylrifamycin SV with primary amines and ketones

In the third stage of our study concerning the search for new antibacterial rifamycin antibiotics, the reactions of 3-formylrifamycin SV (1) with a range of primary alkylamines and ketones of general structure R1–CH2–CO–R2 (R1H or alkyl and R2alkyl or aryl) has been investigated. A new synthetic method for the preparation of a new group of rifamycin derivatives with an α,β-unsaturated imine substituent at C-3 has been developed.These compounds showed a tendency to reversibly isomerise in organic solvents and, in the presence of water, to rapidly hydrolyse. The structures of four isolated microcrystalline compounds 2, 3, 4, 5 and a reaction's mechanism have been proposed on the basis of mass spectrometry results as well as (1D) and (2D) 1H and 13C NMR analysis. The new synthetic route reported herein is a promising pathway to new reactive rifamycins displaying broader capabilities than the plain 3-formylrifamycin SV.

Sulfur Ylide Promoted Synthesis of N‐Protected Aziridines: A Combined Experimental and Computational Approach

A range of N-protected aziridines [N-Tosyl (N-Ts), N-2-trimethylsilylethanesulfonamide (N-SES), N-tert-butoxycarbonylamido (N-Boc), and N-o-nitrobenzenesulfonamide (oNs)] were prepared in moderate to good yield and with high enantiomeric excess of both isomers starting from N-protected imines, using a sulfonium salt derived from Eliel's oxathiane. The diastereoselectivities of the reactions are influenced by the imine N-protecting group, the imine substituent, and the sulfide structure. An unusual cis selectivity was observed in the formation of N-tosyl-2-phenyl-3-tert-butylaziridine and N-o-trimethylsilylethanesulfonamide-2-phenyl-3-tert-butylaziridine, which was explained by using computational models. The analysis suggests that betaine formation in the case of N-tosyl-tert-butylaldimine aziridination using oxathiane benzyl sulfonium ylide 1' is reversible and that the selectivity is determined at the rotation step, which is unusual for semistabilized ylide aziridination. We have shown herein that the steric bulk of an imine substituent, in combination with a sterically demanding sulfonium ylide, can also affect the reversibility of the reaction. This is the first example of this sort involving aziridinations using semistabilized ylides.

Bioreduction of β-carboline imines to amines employing Saccharomyces bayanus

β-Carboline imine reductions mediated by Saccharomyces bayanus have been described achieving moderate to good enantiomeric excesses of the amine products. The enantiomeric excesses of the bioreduction showed a dependence on the imine substituents. Compounds presenting C 1–C 11 aliphatic substituent groups afforded amines with an ( S)-configuration, whereas C 15 and higher aliphatic- and aromatic-substituted β-carboline imines achieved inversion of the configuration in the final ( R)- 2 amine products. Based on this data, a model for the Saccharomyces reduction is proposed.

(2Z)-2-[(2,3-Dimethylphenyl)imino]-1,2-diphenylethanone

In the title compound, C22H19NO, the 2,3-dimethyl­anilinic group is planar with an r.m.s. deviation of 0.0226 Å. The phenyl rings with the carbonyl and imine substituents are also planar with r.m.s. deviations of 0.0019 and 0.0048 Å, respectively. These phenyl rings are oriented at dihedral angles of 74.70 (5) and 79.43 (5)°, respectively, with the 2,3-dimethyl­anilinic group, whereas the dihedral angle between them is 88.28 (4)°. Weak intra­molecular C—H⋯N hydrogen bonding occurs and completes an S(5) ring motif in the mol­ecule. In the crystal, weak π–π inter­actions are present between the carbonyl-containing phenyl rings at a centroid–centroid distance of 3.5958 (12) Å. C—H⋯π inter­actions between the 2,3-dimethyl­anilinic and the carbonyl-containing phenyl rings are also present, where the C—H group is from the former.

Cooperative effect of the camphor imine ligand in alkynols activation by trans-[MCl2(YNC10H14O)2] (M=Pd or Pt)

The cooperative effect of the camphor imine ligand (YNC10H14O) in the cyclization of 5-hexyn-1-ol and 4-pentyn-1-ol promoted by complexes trans-[MCl2(YNC10H14O)2] (M = Pd, Y = NH2, NHMe, NMe2, OH, Ph; Pt, Y = NH2, NHMe, NMe2) is established from a direct relation between the constants calculated for conversion of 5-hexyn-1-ol (A) into 2-methyl-2-pent-4-ynyloxy-tetrahydropyran (B) and 4-pentyn-1-ol (C) into 2-methyl-2-pent-4-ynyloxy-tetrahydrofuran (D) and the basic character of the camphor imine substituent (Y). In the catalytic process acid–base interactions between the alkynol and the coordinated camphor imine are supported by the structural characterization of [PdCl4][Me2NHNC10H14O]2.

Edging In with Iron

Chemistry Hydroboration is widely used in organic chemistry to append a borane and a hydrogen atom to respective carbons at either end of a double bond; the borane group can then be efficiently replaced to form a range of compounds such as alcohols. Wu et al. now show that an iron catalyst facilitates a variation on this reaction in 1,3-dienes: hydrocarbon derivatives with two double bonds separated by a single bond (C=C–C=C). Instead of adding boron and hydrogen to adjacent carbons, the reaction proceeds to add them at opposite edges of the four-carbon sequence, concomitantly creating a double bond between the central carbons. The catalyst forms in situ through reduction of an iminopyridine iron(II) complex by magnesium. In substrates substituted at one of the central carbons in the diene framework, the reaction is highly selective for the E product geometry about the newly formed double bond, complementing a previously developed palladium-catalyzed variant of this reaction class that favors the opposite stereochemistry. Varying the imine substituent on the ligand also allows the authors to modulate which end carbon receives the boron. J. Am. Chem. Soc. 131 , 10.1021/ja9048493 (2009).

Reduction Chemistry of Aryl- and Alkyl-Substituted Bis(imino)pyridine Iron Dihalide Compounds: Molecular and Electronic Structures of [(PDI)2Fe] Derivatives

Sodium amalgam reduction of the aryl-substituted bis(imino)pyridine iron dibromide complex, ((Et)PDI)FeBr2 ((Et)PDI = 2,6-(2,6-Et2-C6H3N=CMe)2C5H3N), under a dinitrogen atmosphere in pentane furnished the bis(chelate)iron compound, ((Et)PDI)2Fe. Characterization by X-ray crystallography established a distorted four-coordinate iron center with two kappa2-bis(imino)pyridine ligands. Reducing the steric demands of the imine substituent to either a less sterically encumbered aryl ring (e.g., C6H4-4-OMe) or an alkyl group (e.g., Cy, iPr, cis-myrtanyl) also yielded bis(chelate) compounds from sodium amalgam reduction of the corresponding dihalide. Characterization of the compounds with smaller imine substituents by X-ray diffraction established six-coordinate, pseudo-octahedral compounds. In one case, a neutral bis(chelate)iron compound was prepared by reduction of the corresponding iron dication, [(PDI)2Fe]2+, providing chemical confirmation of electrochemically generated species that were previously reported as too reducing to isolate. Magnetic measurements, metrical parameters from X-ray structures, Mössbauer spectroscopy, and open-shell, broken symmetry DFT calculations were used to establish the electronic structure of both types (four- and six-coordinate) of neutral bis(chelate) compounds. The experimentally observed S = 1 compounds are best described as having high-spin ferrous (S(Fe) = 2) centers antiferromagnetically coupled to two bis(imino)pyridine radical anions. Thus, the two-electron reduction of the diamagnetic, low-spin complex [(PDI)2Fe]2+ to [(PDI)2Fe] is ligand-based with a concomitant spin change at iron.

Calix[4]arene Schiff bases—potential ligands for fluorescent studies

A series of novel double-armed calix[4]arene derivatives incorporating imine substituents were synthesised from the Schiff-base reaction of 25,27-bis[2-[(1-formyl-2-phenyl)oxy]ethyl]- p- tert-butylcalix[4]arene ( 1) with the appropriate amine or hydrazone. All compounds were characterised by various spectroscopic and analytical techniques, and in three cases, by X-ray crystallographic studies. In the case of compound 2, inclusion compounds were synthesised with both m-xylene and dimethylformamide and their X-ray structures revealed these inclusion sites—between the pendant arms and in the upper cavity, respectively. In all cases, the pendant arms are bent away from each rather than adopting a face-to-face conformation.

Insight into the Mechanism of Platinum-Catalyzed Cross-Coupling of Polyfluoroaryl Imines

We recently reported the first examples of Pt-catalyzed methylation of polyfluorinated arenes as a means to generate partially fluorinated aromatic building blocks. The reaction is selective for cleavage of C−F bonds ortho to an imine substituent and allows for preferential cleavage of C−F bonds in the presence of considerably weaker bonds (e.g., C−Br). We report herein the proposed mechanism for this reaction. Initial C−F activation generates a Pt(IV)−F intermediate, which then reacts with (CH3)2Zn via transmetalation. Subsequent reductive elimination from the Pt(IV) species generates a new sp2−sp3 C−C bond. In addition, we have identified a species present during the catalytic reaction and propose this to be the catalyst resting state.

Do Reaction Conditions Affect the Stereoselectivity in the Staudinger Reaction?

The stereochemistry is one of the critical issues in the Staudinger reaction. We have proposed the origin of the stereoselectivity recently. The effects of solvents, additives, and pathways of ketene generation on the stereoselectivity were investigated by using a clean Staudinger reaction, which is a sensitive reaction system to the stereoselectivity. The results indicate that the additives, usually existed and generated in the Staudinger reaction, and the pathways of the ketene generation do not generally affect the stereoselectivity. The solvent affects the stereoselectivity. The polar solvent is favorable to the formation of trans-beta-lactams. The addition orders of the reagents affect the stereoselectivity in the Staudinger reaction between acyl chlorides and imines. The addition of a tertiary amine into a solution of the acyl chloride and the imine generally decreases the stereoselectivity, which is affected by the interval between additions of the acyl chloride and the tertiary amine, and the imine substituents. Our current results provide further understanding on the stereochemistry of the Staudinger reaction between acyl chlorides and imines and on the factors affecting the stereochemistry and also provide a method to prepare beta-lactams with the desired relative configuration via rationally tuning the stereoselectivity-controlling factors in the Staudinger reaction.

Asymmetric aziridine synthesis by aza-Darzens reaction of N-diphenylphosphinylimines with chiral enolates. Part 2: Inversion of diastereoselectivity

The aza-Darzens (‘ADZ’) reactions of N-diphenylphosphinyl (‘ N-Dpp’) imines with chiral enolates derived from N-bromoacetyl 2 S-2,10-camphorsultam proceed in generally good yield to give N-diphenylphosphinyl aziridinoyl sultams. However, the stereoselectivity of the reaction is dependent upon the structure of the imine substituent: when the chiral enolate was reacted with arylimines substituted in the ortho-position, mixtures of cis- and trans-2′ R,3′ R-aziridines were obtained, often with a complete selectivity in favour of the trans-isomer.

A Selective and Rapid Access to Six‐ or Seven‐Membered Ring Iminosugars via 6,8‐Diazabicyclo[3.2.1]oct‐6‐ene Intermediates

AbstractA selective and rapid access to six‐ or seven‐membered ring iminosugars is reported. The key step of the strategy involves the highly regio‐ and stereoselective reduction of 6,8‐diazabicyclo[3.2.1]oct‐6‐ene intermediates, depending on the nature of imine substituents. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)