Diene Complexes Research Articles

Conformational Locking Induced Enantioselective Diarylcarbene Insertion into B-H and O-H Bonds Using a Cationic Rh(I)/Diene Catalyst.

Transition-metal-catalyzed enantioselective transformations of aryl/aryl carbene are inherently challenging due to the difficulty in distinguishing between two arene rings in the reaction process thus remain largely less explored. The few successful examples reported so far, without exception, have all been catalyzed by Rh(II)-complexes. Herein, we describe our successful development of a novel cationic Rh(I)/chiral diene catalytic system capable of efficient enantioselective B-H and O-H insertions with diaryl diazomethanes, allowing the access to a broad range of gem-diarylmethine boranes and gem-diarylmethine ethers in good yields with high enantioselectivities. Notably, previously unattainable asymmetric diarylcarbene insertion into the O-H bond was achieved for the first time. A remarkable feature of this newly designed Rh(I)/diene catalyst bearing two ortho-amidophenyl substitutents is that it can distinguish between two arene rings of the diaryl carbene through a stereochemically selective control of π-π stacking interactions. DFT calculations indicate that the rotation-restricted conformation of Rh(I)/diene complex played an important role in the highly enantioselective carbene transformations. This work provides an interesting and unprecedented stereocontrol mode in diaryl metal carbene transformations.

Asymmetric Cyclopropanation of Primary N-Vinylamides via Carbene Transfer Catalyzed by Cationic Rh(I)/Diene Complexes: An Unexpected Outer-Sphere Mechanism

Asymmetric Cyclopropanation of Primary <i>N</i>-Vinylamides via Carbene Transfer Catalyzed by Cationic Rh(I)/Diene Complexes: An Unexpected Outer-Sphere Mechanism

Enantioselective Si−H Insertion of Arylvinyldiazoesters Promoted by Rhodium(I)/Diene Complexes

AbstractAn enantioselective Si−H insertion between α‐arylvinyldiazoesters and silanes has been developed by employing rhodium(I)/chiral diene complex as the catalyst. This protocol offers a catalytic asymmetric approach to various chiral silanes containing α‐arylvinyl functionality at room temperature with good enantiocontrol (89–99% ee). Transformation of the resulting enantiomerically enriched α‐arylvinyl‐α‐silyl esters led to a variety of other valuable building blocks. The deuterium kinetic isotope effect experiments revealed a rhodium(I)‐mediated concerted Si−H insertion mechanism.

Synthesis of chiral boranes via asymmetric insertion of carbenes into B-H bonds catalyzed by the rhodium(I) diene complex.

Molecules with chiral boron atoms have been scarcely studied due to limited synthetic access. Herein, we report a new method for their synthesis via asymmetric insertion of arydiazoacetates into the B-H bonds of prochiral carbene-boranes NHC-BH2R. The reaction is catalyzed by the rhodium(I) complex with the chiral diene ligand tBu2-TFB, which can be conveniently prepared by diastereoselective coordination of the racemic diene with (S-Salox)Rh(CO)2. The target boranes were typically obtained in 75-90% yields with 90-95% ee.

A Complementary Experimental and Theoretical Approach for Probing the Surface Functionalization of ZnO with Molecular Catalyst Linkers

AbstractThe application of ZnO materials as solid‐state supports for molecular heterogeneous catalysis is contingent on the functionalization of the ZnO surface with stable self‐assembled monolayers (SAMs) of catalyst linker molecules. Herein, experimental and theoretical methods are used to study SAMs of azide‐terminated molecular catalyst linkers with two different anchor groups (silane and thiol) on poly and monocrystalline (0001, ) ZnO surfaces. Angle‐resolved and temperature‐dependent X‐ray photoelectron spectroscopy (XPS) is used to study SAM binding modes, thermal stabilities, and coverages. The binding strengths and atomistic ordering of the SAMs are determined via atom‐probe tomography (APT). Density functional theory (DFT) and ab initio molecular dynamics (AIMD) calculations provide insights on the influence of the ZnO surface polarity on the interaction affinity and conformational behavior of the SAMs. The investigations show that SAMs based on 3‐azidopropyltriethoxysilane possess a higher binding strength and thermal stability than the corresponding thiol. SAM surface coverage is strongly influenced by the surface polarity of ZnO, and the highest coverage is observed on the polycrystalline surface. To demonstrate the applicability of linker‐modified polycrystalline ZnO as a catalyst support, a chiral Rh diene complex is immobilized on the azide‐terminal of the SAM and its coverage is evaluated via XPS.

Bonding and Reactivity of a Diborabutadiene Fe Complex Synthesized by Boron–Boron Bond Activation

Though precious-metal multiboryl complexes feature prominently in catalytic C–H borylation schemes, the corresponding first-row transition multiboryl species have been comparatively underexplored. We set out to explore the iron coordination chemistry of phenylene-bridged diboryl chelates by targeting oxidative addition of strained cyclic diboron reagents. Under UV photolysis, Fe(CO)5 reacts with a four-membered 1,2-diboracycle via B–B bond cleavage to afford an Fe complex that is most consistent with a 1,4-dibora-1,3-butadiene Fe(0) tricarbonyl complex rather than the target diboryl Fe(II) tetracarbonyl complex. The unsaturated phenylene linker is essential for favoring this bonding arrangement. The Fe tricarbonyl product readily undergoes ligand substitution of CO by PMe3 and PMe2Ph to form diboryl iron dicarbonyl phosphine complexes. All three Fe complexes were characterized by IR and NMR spectroscopy, single-crystal X-ray diffraction, high-resolution mass spectrometry, and cyclic voltammetry, which allow for comparisons to be drawn between foundational Fe tricarbonyl diene complexes and their diboradiene analogues.

Rhodium(I)-Catalyzed Direct Enantioselective C-H Functionalization of Indoles.

A highly regiospecific vinylogous carbene insertion protocol for direct asymmetric C-H functionalization of indoles with arylvinyldiazoacetates has been developed. Under the catalysis of simple Rh(I)/chiral diene complexes, the reaction occurs solely at the vinylogous position of the vinylcarbenoid with exceptional E selectivity and enantiocontrol. It provides an efficient way to obtain an interesting class of chiral indole scaffolds bearing an α,β-unsaturated ester unit and a gem-diaryl carbon stereocenter in good yields (≤99%) with excellent enantioselectivities (≤96%) at room temperature.

Tethering chiral Rh diene complexes inside mesoporous solids: experimental and theoretical study of substituent, pore and linker effects on asymmetric catalysis

Improved enantioselectivity in the 1,2-addition was observed for chiral Rh norbornadiene catalysts immobilized on ordered mesoporous silica with small pores. Confinement effects were rationalized by experimental and computational studies.

Determination of accessibility and spatial distribution of chiral Rh diene complexes immobilized on SBA-15 via phosphine-based solid-state NMR probe molecules

We reveal a method to quantify accessibility and location of molecular catalysts, immobilized on SBA-15 and applied in 1,2-additions.

Phenyl Sulfones: A Route to a Diverse Family of Trisubstituted Cyclohexenes from Three Independent Nucleophilic Additions.

A novel process is described for the synthesis of di- and trisubstituted cyclohexenes from an arene. These compounds are prepared from three independent nucleophilic addition reactions to a phenyl sulfone (PhSO2R; R = Me, Ph, and NC4H8) dihapto-coordinated to the tungsten complex {WTp(NO)(PMe3)}(Tp = trispyrazolylborate). Such a coordination renders the dearomatized aryl ring susceptible to protonation at a carbon ortho to the sulfone group. The resulting arenium species readily reacts with the first nucleophile to form a dihapto-coordinated sulfonylated diene complex. This complex can again be protonated, and the subsequent nucleophilic addition forms a trisubstituted cyclohexene species bearing a sulfonyl group at an allylic position. Loss of the sulfinate anion forms a π-allyl species, to which a third nucleophile can be added. The trisubstituted cyclohexene can then be oxidatively decomplexed, either before or after substitution of the sulfonyl group. Nucleophiles employed include masked enolates, cyanide, amines, amides, and hydride, with all three additions occurring to the same face of the ring, anti to the metal. Of the 12 novel functionalized cyclohexenes prepared as examples of this methodology, nine compounds meet five independent criteria for evaluating drug likeliness. Structural assignments are supported with nine crystal structures, density functional theory studies, and full 2D NMR analysis.

α-Amino-iso-Butyric Acid Foldamers Terminated with Rhodium(I) N-Heterocyclic Carbene Catalysts.

To investigate how remotely induced changes in ligand folding might affect catalysis by organometallic complexes, dynamic α‐amino‐iso‐butyric acid (Aib) peptide foldamers bearing rhodium(I) N‐heterocyclic carbene (NHC) complexes have been synthesized and studied. X‐ray crystallography of a foldamer with an N‐terminal azide and a C‐terminal Rh(NHC)(Cl)(diene) complex showed a racemate with a chiral axis in the Rh(NHC) complex and a distorted 310 helical body. Replacing the azide with either one or two chiral L‐α‐methylvaline (L‐αMeVal) residues gave diastereoisomeric foldamers that each possessed point, helical and axial chirality. NMR spectroscopy revealed an unequal ratio of diastereoisomers for some foldamers, indicating that the chiral conformational preference of the N‐terminal residue(s) was relayed down the 1 nm helical body to the axially chiral Rh(NHC) complex. Although the remote chiral residue(s) did not affect the stereoselectivity of hydrosilylation reactions catalysed by these foldamers, these studies suggest a potential pathway towards remote conformational control of organometallic catalysts.

Contrasting Photochemical and Thermal Catalysis by Ruthenium Arsine Complexes Revealed by Parahydrogen Enhanced NMR Spectroscopy

AbstractThe thermal and photochemical reactivity of Ru(CO)3(dpae) (1) and Ru(CO)2(dpae)(PPh3) (2) towards H2 and diphenylacetylene is described. These reactions are monitored by NMR spectroscopy in conjunction with the parahydrogen induced polarisation (PHIP) effect, spatially resolved chemical shift imaging and the Only Parahydrogen Spectroscopy (OPSY) signal filtering method. The results are supported by DFT. The thermal and photochemical reactions of 1 with H2 proceed by CO loss and form Ru(H)2(CO)2(dpae) (3). 1 catalyses the formation of 1,2 diphenylethane, cis‐ and trans‐stilbene, and 1,2,3,4 tetraphenylbutadiene under 325 nm irradiation at 295 K in a reaction where Ru(CO)2(dpae)(η2‐CHPh=CPhCPh=CHPh) forms. When the same reaction is monitored under thermal conditions at 333 K the η2‐diene complex is no longer detected but hydride containing Ru(CHPhCH2Ph)(H)(CO)2(dpae) and Ru(CO)2(dpae)(trans‐stilbene) are seen. For 1, the photochemical promotion of hydrogenation through 325 nm irradiation results in an approximate 5.5‐fold increase in turnover at 333 K when compared to no irradiation. In contrast, 2 reacts thermally with H2 at 295 K through PPh3 and CO loss with both Ru(H)2(CO)(dpae)(solvent) and 3 being detected. Under irradiation, CO loss dominates and two isomers of Ru(H)2(CO)(dpae)(PPh3) form. While 2 forms the same 4 organic products at 295 K and a second isomer of Ru(CHPhCH2Ph)(H)(CO)2(dpae) alongside the diene complex no photochemical promotion of hydrogenation is observed.

Frontispiece: Interplay of Polarity and Confinement in Asymmetric Catalysis with Chiral Rh Diene Complexes in Microemulsions

The influence of anionic and cationic co-surfactants in nonionic microemulsions on the performance of chiral Rh-norbornadiene complexes with different counterions in the 1,2-addition of phenylboronic acids to N-tosylimines is described. Catalytic studies revealed the unique role of the counterion and the type of co-surfactant. NMR experiments and the calculated intrinsic binding orbitals suggest that the anion exchange with co-surfactant accelerates the formation of the catalytically active Rh-OH complex, thus increasing the yield and affecting the enantioselectivity. For more information, see the Full Paper by M. Kirchhof, K. Abitaev and co-workers on page 16853 ff.

Interplay of Polarity and Confinement in Asymmetric Catalysis with Chiral Rh Diene Complexes in Microemulsions.

Microemulsions provide a unique opportunity to tailor the polarity and liquid confinement in asymmetric catalysis via nanoscale polar and nonpolar domains separated by a surfactant film. For chiral diene Rh complexes, the influence of counterion and surfactant film on the catalytic activity and enantioselectivity remained elusive. To explore this issue chiral norbornadiene Rh(X) complexes (X=OTf, OTs, OAc, PO2F2) were synthesized and characterized by X‐ray crystallography and theoretical calculations. These complexes were used in Rh‐catalyzed 1,2‐additions of phenylboroxine to N‐tosylimine in microemulsions stabilized either exclusively by n‐octyl‐β‐D‐glucopyranoside (C8G1) or a C8G1‐film doped with anionic or cationic surfactants (AOT, SDS and DTAB). The Rh(OAc) complex showed the largest dependence on the composition of the microemulsion, yielding up to 59 % (90 %ee) for the surfactant film doped with 5 wt% of AOT as compared to 52 % (58 %ee) for neat C8G1 at constant surfactant concentration. Larger domains, determined by SAXS analysis, enabled further increase in yield and selectivity while the reaction rate almost remained constant according to kinetic studies.

Silica–Polystyrene Hybrid Core/Shell Microparticles of Rhodium–Chiral Diene Complexes as Catalysts for Asymmetric 1,4-Addition Reactions

Heterogeneous catalysts for fine chemical synthesis play a crucial role in establishing an efficient chemical process. Immobilization of homogeneous catalysts is an important method for the preparation of heterogeneous catalysts; however, it is often accompanied by a loss of catalytic activity due to limited mass transport in the support matrix. Here, we have designed core/shell type polystyrene shell supports on silica with low degrees of cross-linking to improve accessibility to reaction sites. It was found that rhodium–chiral diene complexes on core/shell particles showed higher catalytic activity in comparison those on conventional supports and even better catalytic activity than the corresponding homogeneous catalyst in asymmetric 1,4-addition reactions. Our results may provide a general strategy to obtain many other heterogeneous catalysts with high activities.

Preparation of solutions of copper and nickel en and dien complexes- an illustration of the Jahn-Teller effect

With the advent of methods that effectively solve the problems of modeling metal ions in the gas, water, and solid phases, computational tools are becoming increasingly important in chemical research. here we consider both quantum and classical strategies for modeling systems containing metal ions that have been developed over the past few decades. This experiment was performed to study solutions of copper (II) and Nickel (II) complexes containing the following ligands, such as ammonia, diene, ene, and water in molar ratios, and to characterize the visible spectrum of the prepared solutions. The research was divided into three parts. In the first part, Nickel complexes were prepared and their formulas were determined using the job method. The second part of the experiment was performed to obtain EN-complexes of copper (II), and in the last part diene complexes were obtained. Dien and en were used as ligands.

Regiospecific and Enantioselective Arylvinylcarbene Insertion of a C-H Bond of Aniline Derivatives Enabled by a Rh(I)-Diene Catalyst.

Asymmetric insertion of an arylvinylcarbenoid into the C-H bond for direct enantioselective C(sp2)-H functionalization of aniline derivatives catalyzed by a rhodium(I)-diene complex was developed for the first time. The reaction occurred exclusively at the uncommon vinyl terminus site with excellent E selectivity and enantioselectivities, providing various chiral γ,γ-gem-diarylsubstituted α,β-unsaturated esters with broad functional group compatibility under simple and mild conditions. It provides a rare example of the asymmetric C-H insertion of arenes with selective vinylogous reactivity. Synthesis applications of this protocol were featured by several versatile product transformations. Systematic DFT calculations were also performed to elucidate the reaction mechanism and origin of the uncommon enantio- and regioselectivity of the Rh(I)-catalyzed C(sp2)-H functionalization reaction. The measured and computed inverse deuterium kinetic isotope effect supports the C-C bond-formation step as the rate-determining step. Attractive interactions between the chiral ligand and substrates were also proposed to control the enantioselectivity.

Iron-Catalyzed Vinylsilane Dimerization and Cross-Cycloadditions with 1,3-Dienes: Probing the Origins of Chemo- and Regioselectivity.

The selective, intermolecular, homodimerization and cross-cycloaddition of vinylsilanes with unbiased 1,3-dienes, catalyzed by a pyridine-2,6-diimine (PDI) iron complex is described. In the absence of a diene coupling partner, vinylsilane hydroalkenylation products were obtained chemoselectively with unusual head-to-head regioselectivity (up to >98% purity, 98:2 E/Z). In the presence of a 4- or 2-substituted diene coupling partner, under otherwise identical reaction conditions, formation of value-added [2+2]- and [4+2]-cycloadducts, respectively, was observed. The chemoselectivity profile was distinct from that observed for analogous α-olefin dimerization and cross-reactions with 1,3-dienes. Mechanistic studies conducted with well-defined, single-component precatalysts (MePDI)Fe(L2) (where MePDI = 2,6-(2,6-Me2-C6H3N═CMe)2C5H3N; L2 = butadiene or 2(N2)) provided insights into the kinetic and thermodynamic factors contributing to the substrate-controlled regioselectivity for both the homodimerization and cross cycloadditions. Diamagnetic iron diene and paramagnetic iron olefin complexes were identified as catalyst resting states, were characterized by in situ NMR and Mössbauer spectroscopic studies, and were corroborated with DFT calculations. Stoichiometric reactions and computational models provided evidence for a common mechanistic regime where competing steric and orbital-symmetry requirements dictate the regioselectivity of oxidative cyclization. Although distinct chemoselectivity profiles were observed in cross-cycloadditions with the vinylsilane congeners of α-olefins, these products arose from metallacycles with the same connectivity. The silyl substituents ultimately governed the relative rates of β-H elimination and C-C reductive elimination to dictate final product formation.

Pt(ii)-Chiral diene-catalyzed enantioselective formal [4 + 2] cycloaddition initiated by C–C bond cleavage and elucidation of a Pt(ii)/(iv) cycle by DFT calculations

A Pt(ii)-chiral diene complex demonstrates a high catalytic activity in the enantioselective formal [4 + 2] cycloaddition along with C–C bond cleavage of biphenylene at room temp. DFT calculations elucidated that the present catalysis involves a rare Pt(ii)/Pt(iv) cycle.

Azulene Functionalization by Iron-Mediated Addition to a Cyclohexadiene Scaffold.

The functionalization of azulenes via reaction with cationic η5-iron carbonyl diene complexes under mild reaction conditions is demonstrated. A range of azulenes, including derivatives of naturally occurring guaiazulene, were investigated in reactions with three electrophilic iron complexes of varying electronic properties, affording the desired coupling products in 43–98% yield. The products were examined with UV–vis/fluorescence spectroscopy and showed interesting halochromic properties. Decomplexation and further derivatization of the products provide access to several different classes of 1-substituted azulenes, including a conjugated ketone and a fused tetracycle.