Hemilabile Bidentate Research Articles

Three-Component Asymmetric Ni-Catalyzed 1,2-Dicarbofunctionalization of Unactivated Alkenes via Stereoselective Migratory Insertion.

An asymmetric 1,2-dicarbofunctionalization of unactivated alkenes with aryl iodides and aryl/alkenylboronic esters under nickel/bioxazoline catalysis is disclosed. A wide array of aryl and alkenyl nucleophiles are tolerated, furnishing the products in good yield and with high enantioselectivity. In addition to terminal alkenes, 1,2-disubstituted internal alkenes participate in the reaction, establishing two contiguous stereocenters with high diastereoselectivity and moderate enantioselectivity. A combination of experimental and computational techniques shed light on the mechanism of the catalytic transformation, pointing to a closed-shell pathway with an enantiodetermining migratory insertion step, where stereoinduction arises from synergistic interactions between the sterically bulky achiral sulfonamide directing group and the hemilabile bidentate ligand.

Palladium-Catalyzed Decarbonylative Alkylation of Acyl Fluorides.

Palladium-catalyzed decarbonylative alkylation reactions of acyl fluorides have been developed using alkylboranes having β-hydrogens. A wide range of functional groups were well tolerated, even at the high temperature required for decarbonylation. This protocol provides a diverse C(sp2)-C(sp3) bond formation via a highly efficient decarbonylative process. The hemilabile bidentate ligand DPPE plays a crucial role for retardation of the undesired β-hydride elimination.

Computationally Guided Catalyst Design in the Type I Dynamic Kinetic Asymmetric Pauson–Khand Reaction of Allenyl Acetates

The Rh(I)-catalyzed allenic Pauson-Khand reaction (APKR) is an efficient, redox-neutral method of synthesizing α-acyloxy cyclopentenones. An enantioselective APKR could provide access to chiral, nonracemic α-acyloxy and α-hydroxy cyclopentenones and their corresponding redox derivatives, such as thapsigargin, a cytotoxic natural product with potent antitumor activity. Rapid scrambling of axial chirality of allenyl acetates in the presence of Rh(I) catalysts enables the conversion of racemic allene to enantiopure cyclopentenone product in a dynamic kinetic asymmetric transformation (DyKAT). A combined experimental and computational approach was taken to develop an effective catalytic system to achieve the asymmetric transformation. The optimization of the denticity, and steric and electronic properties of the ancillary ligand (initially (S)-MonoPhos, 58:42 er), afforded a hemilabile bidentate (S)-MonoPhos-alkene-Rh(I) catalyst that provided α-acyloxy cyclopentenone product in up to 14:86 er. Enantioselectivity of the Rh(I)-(S)-MonoPhos-alkene catalyst was rationalized using ligand-substrate steric interactions and distortion energies in the computed transition states. This asymmetric APKR of allenyl acetates is a rare example of a Type I DyKAT reaction of an allene, the first example of DyKAT in a cyclocarbonylation reaction, and the first catalyst-controlled enantioselective APKR.

Kinetics, Mechanism and Theoretical Studies of Norbornene-Ethylene Alternating Copolymerization Catalyzed by Organopalladium(II) Complexes Bearing Hemilabile α-Amino-pyridine.

Cationic methylpalladium complexes bearing hemilabile bidentate α-amino–pyridines can serve as effective precursors for catalytic alternating copolymerization of norbornene (N) and ethylene (E), under mild conditions. The norbornyl palladium complexes in the formula of {[RHNCH2(o-C6H4N)]Pd(C7H10Me)(NCMe)}(BF4) (R = iPr (2a), tBu (2b), Ph (2c), 2,6-Me2C6H3 (2d), 2,6-iPr2C6H3 (2e)) were synthesized via single insertion of norbornene into the corresponding methylpalladium complexes 1a–1e, respectively. Both square planar methyl and norbornyl palladium complexes exhibit facile equilibria of geometrical isomerization, via sterically-controlled amino decoordination–recoordination of amino–pyridine. Kinetic studies of E-insertion, N-insertion of complexes 1 and 2, and the geometric isomerization reactions have been examined by means of VT-NMR, and found in excellent agreement with the results estimated by DFT calculations. The more facile N-insertion in the cis-isomers, and ready geometric isomerization, cooperatively lead to a new mechanism that accounts for the novel catalytic formation of alternating COC.

Synthesis, characterization and a reactivity study of some allyl palladium complexes bearing bidentate hemi-labile carbene or mixed carbene/PPh3 ligands

With the aim at synthesizing novel allyl complexes that can potentially act as catalysts in the Tsuji–Trost catalyzed reaction, we have synthesized and characterized some allyl and 2-Meallyl palladium derivatives with one hemilabile bidentate or two strong mono-coordinating spectator ligands. The hemilabile ligands are constituted by one nitrogen heterocyclic carbene (NHC) fragment acting as the pivot bearing a labile wing with a pyridine nitrogen or sulfur atom as the second stabilizing atom. One of two monodentate ligands is in all cases PPh3 whereas the other is a mono- or partially coordinated hemilabile carbene. The complexes were characterized by standard spectroscopic methods and elemental analysis and in two cases by SC-XRD technique.The reactivity of two selected complexes toward the Tsuji–Trost reaction was tested by stoichiometric allyl amination carried out with piperidine and the results of such a mechanistic investigation integrated by a computational study are also reported in this paper.

Synthesis of Hemilabile Cyclic (Alkyl)(amino)carbenes (CAACs) and Applications in Organometallic Chemistry.

A versatile methodology, involving readily available starting materials, allows for the synthesis of stable hemilabile bidentate cyclic (alkyl)(amino)carbenes (CAACs) featuring alkene, ether, amine, imine, and phosphine functionalities. The stability of the free carbenes has been exploited for the synthesis of copper(I) and gold(I) complexes. It is shown that the pendant imine moiety stabilizes the gold(III) oxidation state and enables the C-C bond oxidative addition of biphenylene to the corresponding cationic gold(I) complex. The latter and the corresponding copper(I) complex show high catalytic activity for the hydroarylation of α-methylstyrene with N,N-dimethylaniline, and the copper(I) complex promotes the anti-Markovnikov hydrohydrazination of phenyl acetylene with high selectivity.

Coordination chemistry of highly hemilabile bidentate sulfoxide N-heterocyclic carbenes with palladium(II).

Imidazolium salts, [RS(O)-CH2 (C3 H3 N2 )Mes]Cl (R=Me (L1a), Ph (L1b)); Mes=mesityl), make convenient carbene precursors. Palladation of L1a affords the monodentate dinuclear complex, [(PdCl2 {MeS(O)CH2 (C3 H2 N2 )Mes})2 ] (2a), which is converted into trans-[PdCl2 (NHC)2] (trans-4a; N-heterocyclic carbene) with two rotamers in anti and syn configurations. Complex trans-4a can isomerize into cis-4a(anti) at reflux in acetonitrile. Abstraction of chlorides from 4a or 4b leads to the formation of a new dication: trans-[Pd{RS(O)CH2(C3H2N2)Mes}2](PF6)2 (R=Me (5a), Ph (5b)). The X-ray structure of 5a provides evidence that the two bidentate SO-NHC ligands at palladium(II) are in square-planar geometry. Two sulfoxides are sulfur- and oxygen-bound, and constitute five- and six-membered chelate rings with the metal center, respectively. In acetonitrile, complexes 5a or 5b spontaneously transform into cis-[Pd(NHC)2(NCMe)2](PF6)2. Similar studies of thioether-NHCs have also been examined for comparison. The results indicate that sulfoxides are more labile than thioethers.

Experimental and Theoretical Studies on the Platinum-Mediated Selective C(sp)–Si Bond Cleavage of Alkynylsilanes

A series of cis-alkynyl(silyl)platinum(II) complexes was prepared via the chemoselective C(sp)–Si bond cleavage of alkynylsilanes by a platinum(0) complex ligated with the P–N hemilabile bidentate ligand. The coordination of the triple bond to the platinum center triggers selective C(sp)–Si bond cleavage. Hammett plots of the 31P{1H} NMR spectroscopic properties (δ and J values) reflect an electronic effect on platinum(II) complexes; trans substituents of arylethynyl groups are influenced, but cis-positioned silyl groups are not affected, as evidenced by 29Si{1H} NMR. In comparison, Hammett plots show that C(sp)–Si bond cleavage rates are accelerated by electron-rich alkynylsilanes, which is opposite to the ordinary oxidative addition of aryl halides to transition metals often observed in catalytic cross-coupling reactions. A DFT calculation reveals that intermediates and transition states are stabilized by electron-rich alkynylsilanes and that the five-membered hemilabile P–N ligand is essential, in which a reactive electron-deficient 14-electron platinum(0) species is produced via the dissociation of nitrogen, giving rise to a monodentate phosphine coordination. Electron-rich alkynylsilanes allow decreased π back-donation from the platinum center to the ligand, accelerating the dissociation of the more labile nitrogen. Steric congestion between diisopropylphosphino and silyl groups thermodynamically disfavors C(sp)–Si bond cleavage.

N-Benzyl-N-phosphino-tert-butylsulfinamide and Its Coordination Modes with Ir(I), Cu(I), Pd(II), and Pt(II): P,S or P,O?

Here we studied the coordination mode of optically pure N-benzyl-N-phosphino-tert-butylsulfinamide (1a) toward Ir(I), Cu(I), Pd(II), and Pt(II). Ligand 1a can work as a hemilabile bidentate P,O or P,S or monodentate P ligand depending on the nature of the metal and its electronic and steric environment. X-ray analysis confirmed that soft metals, such as Ir or Pt, favor the P,S coordination mode, while harder ones, such as Cu(I), favor the formation of P,O five-membered-ring chelates. Finally, analysis of the resonances of the methylene and t-Bu groups in coordinated 1a allowed us to predict a P,S coordination mode for the corresponding (1a)PdCl2 complex 7.

Alternating ethylene-norbornene copolymerization catalyzed by cationic organopalladium complexes bearing hemilabile bidentate ligands of α-amino-pyridines

Cationic methylpalladium complexes with hemilabile bidentate ligands of alpha-amino-pyridines, in the form of {[R(1)HNCR(2)H(o-C(6)H(5)N)]Pd(Me)(NCMe)}(BF(4)) (R(1) = (i)Pr, (t)Bu, Ar R(2) = H, Me) have been found to be effective precursors for catalytic copolymerization of ethylene and norbornene under mild conditions. The copolymer products exhibit predominant alternating microstructures which are evidenced by NMR and mass spectrometry as well as a kinetic analysis according to the Finman-Ross relationship.

New methods in asymmetric catalysis based on new hemi-labile bidentate ligands

Abstract Chiral bidentate hemi-labile bis(phosphine) monoxide ligands were shown to be quite effective in various copper-catalyzed transformations. Among them, the nucleophilic addition to imines, the conjugate addition to α,β-unsaturated nitro derivatives, and the conjugate reduction of α,β-unsaturated sulfones generally gave good to excellent yields and high enantiomeric excesses.

Crystal structure of trans-dichloro-1,3-propanediamine-bis[(2-methoxyethyl) diphenylphosphine]ruthenium (II),RuCl2(C3H10N2)(C15H17OP)2

Article Crystal structure of trans-dichloro-1,3-propanediamine-bis[(2-methoxyethyl) diphenylphosphine]ruthenium (II),RuCl2(C3H10N2)(C15H17OP)2 was published on April 1, 2006 in the journal Zeitschrift für Kristallographie - New Crystal Structures (volume 221, issue 1-4).

A Family of Simple Amide‐Derived Air‐Stable P,O‐Ligands for Suzuki Cross‐Coupling of Unactivated Aryl Chlorides.

AbstractFor Abstract see ChemInform Abstract in Full Text.

A family of simple amide-derived air-stable P,O-ligands for Suzuki cross-coupling of unactivated aryl chlorides

A family of air-stable amide-derived phosphine (Aphos) ligands engineered on simple N, N-dialkyl aryl amide scaffolds has been designed and prepared by one-pot synthesis from the amides in high yields. The Aphos ligands have been used, in analogous to their atropisomeric variations, as hemilabile bidentate P,O-ligands in various Pd-catalyzed C–N and C–C bond forming reactions. We present here our results on the highly efficient Suzuki cross-coupling reactions of unactivated and/or sterically hindered aryl chlorides with arylboronic acids and a relationship of Aphos structures with catalytic efficacy.

Binuclear Ruthenium Macrocycles Formed via the Weak-Link Approach

The "weak-link approach" for the synthesis of metallomacrocycles has been used to synthesize a series of novel Ru(II) macrocycles in high yield. RuCl2(PPh3)3 has been reacted with two different phosphino-alkyl-ether hemilabile ligands, 1,4-(PPh2(CH2)2O)2C6H4 and 1,4-(PPh2(CH2)2OCH2)2C6H4. The hemilabile bidentate ligand coordinates to Ru(II) centers through both the P and O atoms to form bimetallic "condensed intermediates". The weak Ru-O bonds have been selectively cleaved with CO, 1,2-diaminopropane, and pyridine to yield large open macrocycles. This is the first example of the weak-link approach employed to synthesize macrocycles with Ru, and metal centers in general that have more than four coordination sites.

An N-heterocyclic carbene as a bidentate hemilabile ligand: a synchrotron X-ray diffraction and density functional theory studyElectronic supplementary information (ESI) available: experimental details and characterization data; table of results for hydrogenation of 3-pentanone; Gaussian 98 summary for the W and Mo models; ORTEP plot of 1W and crystal data. See http://www.rsc.org/suppdata/cc/b3/b303762b/

The N-heterocyclic carbene ligand IMes was shown by synchrotron crystallography and DFT computations to adopt a hemilabile bidentate coordination mode in CpM(CO)2(IMes)+B(C6F5)4− (M = Mo, W), with a CC bond of one mesityl weakly coordinated to the metal.