Bond Metathesis Reactions Research Articles

Intermolecular C-C/C-N σ-bond metathesis enabled by visible light.

Transition-metal-catalyzed double/triple bond metathesis reactions have been well-established due to the ability of transition-metal catalysts to readily interact with π bonds, facilitating the progression of the entire reaction. However, activating σ-bonds to induce σ-bond metathesis is more challenging due to the absence of π bonds and the high bond energy of σ bonds. In this study, we present a novel photo-induced approach that does not rely on transition metals or photosensitizers to drive C-C and C-N σ-bond metathesis reactions. This method enables the cross-coupling of tertiary amines with α-diketones via C-C and C-N single bonds cleavage and recombination. Notably, our protocol exhibits good compatibility with various functional groups in the absence of transition metals and external photosensitizers, resulting in the formation of aryl alkyl ketones and aromatic amides in good to high yields. To gain insights into the mechanism of this pathway, we conducted controlled experiments, intermediate trapping experiments, and DFT (Density Functional Theory) calculations. This comprehensive approach allowed us to elucidate the detailed mechanism underlying this transformative reaction.

Ring‐Expanded N‐Heterocyclic Copper(I) Boryl Complexes: The Structures of [(6‐Dipp)CuBcat], [(6‐Dipp)CuBneop], and [(6‐Dipp)CuBhex

Abstractσ‐Bond metathesis reactions between [(6‐Dipp)CuOtBu] (6‐Dipp=:C({Dipp}NCH2)2CH2, Dipp=2,6‐iPr2−C6H3) and three diboranes gave access to three new copper(I) boryl complexes [(6‐Dipp)CuBcat], [(6‐Dipp)CuBneop], and [(6‐Dipp)CuBhex] (cat=1,2‐O2C6H4; neop=(OCH2)2C(CH3)2; hex=OC(CH3)HCH2C(CH3)2O). Whilst [(6‐Dipp)CuBcat] and [(6‐Dipp)CuBneop] formed rapidly in toluene, access to [(6‐Dipp)CuBhex] required heating to 60 °C for days. The complexes were characterised by single‐crystal X‐ray crystallography which showed in all three cases that the systems were monomers and distorted‐linear at the copper atom. The stability of [(6‐Dipp)CuBneop] was found to be comparable to that of [(IPr*)Cu‐Bneop] (IPr*=1,3‐bis(2,6‐(diphenylmethyl)‐4‐methylphenyl)imidazol‐2‐ylidene); it persisted in solution for days with no sign of decomposition. [(6‐Dipp)CuBhex] is a rare crystallographically characterised example of a complex containing a boryl anion supported by the hexylene glycolato ligand.

C–N bond metathesis: mechanistic insight into palladium-catalyzed ring-closing using aminal species

A computational study was conducted to investigate the mechanism of a new type of C–N bond metathesis reaction, in which a reversible reductive elimination/oxidative addition mode is involved.

Taming Heavier Group 14 Imine Analogues: Accessing Tin Nitrogen [Sn=N] Double Bonds and their Cycloaddition/Metathesis Chemistry

AbstractA systematic study to access stable stannaimines is reported, by combining different heteroleptic stannylenes with a range of organic azides. The reactions of terphenyl‐/hypersilyl‐substituted stannylenes yield the putative tin nitrogen double bond, but is directly followed by 1,2‐silyl migration to give SnIIsystems featuring bulky silylamido ligands. By contrast, the transition from a two σ donor ligand set to a mixed σ‐donor/π‐donor scaffold allows access to three new stannaimines which can be handled at room temperature. The reactivity profile of these Sn=N bonded species is crucially dependent on the substituent at the nitrogen atom. As such, the Sn=NMes (Mes=2,4,6‐Me3C6H2) system is capable of activating a broad range of substrates under ambient conditions via 1,2‐addition reactions, [2+2] and [4+2] cycloaddition reactions. Most interestingly, very rare examples of main group multiple bond metathesis reactions are also found to be viable.

Taming Heavier Group 14 Imine Analogues: Accessing Tin Nitrogen [Sn=N] Double Bonds and their Cycloaddition/Metathesis Chemistry.

A systematic study to access stable stannaimines is reported, by combining different heteroleptic stannylenes with a range of organic azides. The reactions of terphenyl-/hypersilyl-substituted stannylenes yield the putative tin nitrogen double bond, but is directly followed by 1,2-silyl migration to give SnII systems featuring bulky silylamido ligands. By contrast, the transition from a two σ donor ligand set to a mixed σ-donor/π-donor scaffold allows access to three new stannaimines which can be handled at room temperature. The reactivity profile of these Sn=N bonded species is crucially dependent on the substituent at the nitrogen atom. As such, the Sn=NMes (Mes=2,4,6-Me3 C6 H2 ) system is capable of activating a broad range of substrates under ambient conditions via 1,2-addition reactions, [2+2] and [4+2] cycloaddition reactions. Most interestingly, very rare examples of main group multiple bond metathesis reactions are also found to be viable.

Enantioselective ring-closing aminomethylamination of aminodienes enabled by modified Trost ligands

Optically active N -heterocycles are important building blocks in organic synthesis and medicinal chemistry. However, an enantioselective synthetic protocol with excellent compatibility across the normal-sized, medium-sized, and macro-sized rings remains largely rare. We herein report a Pd-catalyzed highly enantioselective ring-closing aminomethylamination of aminodienes with aminals via C–N bond metathesis, which provides rapid access to enantiomer-enriched 5- to 16-membered β-aminoallylated N -heterocycles (up to 98% ee) under mild conditions with the use of modified Trost ligands. In addition, the enantioselective synthesis of chiral tetrahydroisoquinolines (THIQs) has also been realized by this method. The unity of this method has been demonstrated by the formal synthesis of the natural product (+)-( 8S , 13R )-cyclocelabenzine. Mechanistically, the reaction goes through a challenging and less known stereoselective migratory insertion of alkene into an alkyl-Pd species. We also conducted a detailed analysis of the stereochemical outcome of the reaction, which provided an intriguing view of enantioselection. • Enantioselective migratory insertion of alkene to alkyl-palladium intermediates • Highly efficient synthesis of enantioenriched 5- to 16-membered N -heterocycles • Access to C-4 enantioenriched THIQs with excellent functional-group tolerance • First example of asymmetric C–N bond metathesis reaction Not only are N -heterocycles privileged structural motifs in natural alkaloids, but they are also widely utilized as key intermediates for the synthesis of pharmaceutical molecules and drug candidates. Efficient synthetic approaches toward optically active N -heterocycles, especially ones general for the construction of versatile ring systems, are of paramount importance. The enantioselective domino Heck cyclization/cross-coupling reactions are a powerful approach to functionalized chiral heterocyclic compounds. However, the use of sp 3 C-hybridized alkyl electrophiles in such sequential transformations remains largely elusive. Herein, with modified Trost-type ligands, we report an efficient Pd-catalyzed asymmetric ring-closing reaction that gives access to enantiomer-enriched 5- to 16-membered N -heterocycles. A Pd-catalyzed highly enantioselective ring-closing aminomethylamination of aminodienes with aminals via C–N bond metathesis has been established; it provides rapid and unique access to a wide range of optically active N -heterocycles bearing 5- to 16-membered rings in good to excellent enantioselectivities (up to 98% ee) under mild conditions with the use of modified Trost ligands. Mechanistically, the reaction goes through a challenging and less known stereoselective migratory insertion of alkene into an alkyl-Pd species. This success represents the first time that the Trost ligand has been used to induce asymmetry in Heck reactions.

Alternating Current Electrolysis Enabled Formal C-O/O-H Cross-Metathesis of 4-Alkoxy Anilines with Alcohols.

While multiple bond metathesis reactions, for example olefin metathesis, have seen considerable recent progress, direct metathesis of traditionally inert C-O single bonds is extremely rare and particularly challenging. Undoubtedly, metathesis reaction of C-O bonds is one of the most ideal routes for the value-added upgrading of molecules involving C-O bonds. Reported here is a new protocol to achieve the formal C-O/O-H cross-metathesis via alternating current electrolysis. Featuring mild reaction conditions, the protocol allows readily available 4-alkoxy anilines and alcohols to be converted into a wide range of valuable products in highly regioselective and chemoselective manner. Moreover, the present strategy can be used in the late-stage modification of pharmaceuticals as well as biologically active compounds, which demonstrated the potential application.

Alternating Current Electrolysis Enabled Formal C−O/O−H Cross‐Metathesis of 4‐Alkoxy Anilines with Alcohols

AbstractWhile multiple bond metathesis reactions, for example olefin metathesis, have seen considerable recent progress, direct metathesis of traditionally inert C−O single bonds is extremely rare and particularly challenging. Undoubtedly, metathesis reaction of C−O bonds is one of the most ideal routes for the value‐added upgrading of molecules involving C−O bonds. Reported here is a new protocol to achieve the formal C−O/O−H cross‐metathesis via alternating current electrolysis. Featuring mild reaction conditions, the protocol allows readily available 4‐alkoxy anilines and alcohols to be converted into a wide range of valuable products in highly regioselective and chemoselective manner. Moreover, the present strategy can be used in the late‐stage modification of pharmaceuticals as well as biologically active compounds, which demonstrated the potential application.

Recent Advances in C—X Bond Metathesis Reactions

Recent Advances in C—X Bond Metathesis Reactions

Ionic-Liquid-Catalyzed Approaches under Metal-Free Conditions.

ConspectusMetal-free catalysis is a promising protocol to access chemicals without metal contamination. Ionic liquids (ILs) that are entirely composed of organic cations and inorganic/organic anions have emerged as promising alternatives to molecular solvents and metal catalysts due to their unique properties such as structural tunability, the coexistence of multiple interactions among ions (e.g., electrostatic interaction, hydrogen bonding, van de Waals forces, acid/base interactions, hydrophilic/hydrophobic interactions, etc.), unique affinity for a wide range of chemicals, good chemical and thermal stability, and quite low volatility. ILs have shown potential applications in various chemical processes.In this Account, we systematically described our most recent work on IL-catalyzed approaches under metal-free conditions. The first section presents the IL-catalyzed strategies toward the transformation of CO2 to value-added chemicals, focusing on the CO2-reactive IL-catalyzed CO2 transformation to various heterocycles and the IL-catalyzed reductive transformation of CO2 to chemicals. In these approaches, we designed task-specific ILs that are able to chemically capture and activate CO2 via forming anion-based carbonate/carbamate or cation-based carboxylate/carbamate intermediates, thus further accomplishing its transformation to a series of heterocycles including quinazoline-2,4(1H,3H)-diones, cyclic carbonates, 2-oxazolidinones, oxazolones, and benzimidazolones under metal-free conditions. For the IL-catalyzed approaches to reducing CO2 with hydrosilanes to chemicals, we employed ILs capable of activating the Si-H bond in hydrosilanes and the N-H bond in amine substrates via H-bonding, thus achieving the reductive transformation of CO2 to formamides, benzimidazoles, and benzothiazoles via cooperative catalysis. The second section describes our finding on the IL-catalyzed hydration of the C≡C bond in propargylic alcohols. Azolate anion-based ILs that can chemically capture CO2 via the formation of carbamates could serve as robust nucleophiles to attack the C≡C bond in propargylic alcohols and then efficiently catalyze the hydration of propargylic alcohols to produce α-hydroxy ketones with the assistance of atmospheric CO2 gas under metal-free conditions. The third section unveils the cooperative catalysis strategy of hydrogen bond donors and acceptors of ILs for chemical reactions. In the hydrogen-bonding catalysis protocols, cations of the ILs act as H-bond donors and anions, as acceptors, forming H-bonds with the reactant molecules, respectively, in opposite ways, which can cooperatively catalyze the ring-closing C-O/C-O bond metathesis reactions of aliphatic diethers to O-heterocycles, the dehydrative etherification of alcohols to ethers, and direct oxidative esterification of alcohols to esters. We believe that these IL-catalyzed metal-free processes and strategies display promising practical applications, and their commercialization would bring great benefits to the production of the as-afforded value-added chemicals.

Lewis base-free thiophosphonium ion: a cationic sulfur atom transfer reagent.

Phosphorus(v) sulfide and Lawesson's reagent are commonly used thionating reagents which are considered to operate after dissociation into highly reactive dithiophosphorane fragments. We report the synthesis and properties of a monomeric thiophosphonium ion [R2P[double bond, length as m-dash]S]+. The highly electrophilic species reacts with carbonyls in oxo-for-sulfido exchange reactions at room temperature and undergoes phosphorus-chalcogen bond metathesis reactions with phosphine chalcogenides.

Palladium-Catalyzed Ring-Closing Reaction via C-N Bond Metathesis for Rapid Construction of Saturated N-Heterocycles.

The ring-closing reactions based on chemical bond metathesis enable the efficient construction of a wide variety of cyclic systems which receive broad interest from medicinal and organic communities. However, the analogous reaction with C-N bond metathesis as a strategic fundamental step remains an unanswered challenge. Herein, we report the design of a new fundamental metallic C-N bond metathesis reaction that enables the palladium-catalyzed ring-closing reaction of aminodienes with aminals. The reactions proceed efficiently under mild conditions and exhibit broad substrate generality and functional group compatibility, leading to a wide variety of 5- to 16-membered N-heterocycles bearing diverse frameworks and functional groups.

Understanding the mechanism and reactivity of Pd-catalyzed C-P bond metathesis of aryl phosphines: a computational study.

Transition metal-catalyzed single bond metathesis has recently emerged as a useful strategy for functional group transfer. In this work, we explored the mechanism and reactivity profile of Pd/PhI-cocatalyzed C-P bond metathesis between aryl phosphines using density functional theory (DFT) calculations. The overall single bond metathesis involves two Pd(ii)-catalyzed C-P reductive eliminations and two Pd(0)-catalyzed C-P oxidative additions, which allows the reversible C-P bond cleavage and formation of the phosphonium cation. Distortion/interaction analysis indicates that the facile C-P bond cleavage and formation of the phosphonium cation are due to the involvement of coordinating aryl phosphine in the process. In addition, the substituent effects on the reaction kinetics and thermodynamics of metathesis were computed, which provides helpful mechanistic information for the design of related single bond metathesis reactions.

Crystalline Divinyldiarsenes and Cleavage of the As=As Bond.

The first divinyldiarsenes [{(NHC)C(Ph)}As]2 (NHC=IPr 3 a, SIPr 3 b; IPr=C{(NAr)CH}2 ; SIPr=C{(NAr)CH2 }2 ; Ar=2,6-iPr2 C6 H3 ) are reported. Compounds 3 a and 3 b were prepared by the reduction of corresponding chlorides {(NHC)C(Ph)}AsCl2 (NHC=IPr 2 a, SIPr 2 b) with Mg. Calculations revealed a small HOMO-LUMO energy gap of 3.86 (3 a) and 4.24 eV (3 b). Treatment of 3 a with (Me2 S)AuCl led to the cleavage of the As=As bond to restore 2 a, which is expected to proceed via the diarsane [{(IPr)C(Ph)}AsCl]2 (4). Remarkably, 4 as well as 2 a can be selectively accessed on treatment of 3 a with an appropriate amount of C2 Cl6 . Moreover, 3 a readily reacts with PhEEPh (E=Se or Te) at room temperature to give {(IPr)C(Ph)}As(EPh)2 (E=Se 5 a; Te 5 b), revealing the cleavage of As=As and E-E bonds and the formation of As-E bonds. Such highly selective stepwise oxidation (3 a→4→2 a) and bond metathesis (3 a→5 a,b) reactions are unprecedented in main-group chemistry.

Controlled scrambling reactions to polyphosphanes via bond metathesis reactions.

Triphosphanes R'2PP(R)PR'2 (9a,c: R = Py; 9b R = BTz), 1,3-diphenyl-2-pyridyl-triphospholane 9d and pentaphospholanes (RP)5 (13: R = Py; 18: R = BTz) are obtained in high yield of up to 98% from the reaction of dipyrazolylphosphanes RPpyr2 (5: R = Py; 6: R = BTz; pyr = 1,3-dimethylpyrazolyl) and the respective secondary phosphane (R'2PH, R' = Cy (9a,b), t Bu (9c); PhPH(CH2)2PHPh (9d)). The formation of derivatives 9a-d proceeds via a condensation reaction while the formation of 13 and 18 can only be explained by a selective scrambling reaction. We realized that the reaction outcome is strongly solvent dependent as outlined by the controlled scrambling reaction pathway towards pentaphospholane 13. In our further investigations to apply these compounds as ligands we first confined ourselves to the coordination chemistry of triphosphane 9a with respect to coinage metal salts and discussed the observation of different syn- and anti-isomeric metal complexes based on NMR and X-ray analyses as well as quantum chemical calculations. Methylation reactions of 9a with MeOTf yield triphosphan-1-ium Cy2MePP(Py)PCy2 + (10 +) and triphosphane-1,3-diium Cy2MePP(Py)PMeCy2 2+ (11 2+) cations as triflate salts. Salt 11[OTf]2 reacts with pentaphospholane 13 in an unprecedented chain growth reaction to give the tetraphosphane-1,4-diium triflate salt Cy2MePP(Py)P(Py)PMeCy2 2+ (19[OTf]2) via a P-P/P-P bond metathesis reaction. The latter salt is unstable in solution and rearranges via a rare [1,2]-migration of the Cy2MeP-group followed by the elimination of the triphosph-2-en-1-ium cation [Cy2MePPPMeCy2]+ (20 +) to yield a novel 1,4,2-diazaphospholium salt (21[OTf]).

Consequences of Surface Oxophilicity of Ni, Ni-Co, and Co Clusters on Methane Activation.

This study describes a new C-H bond activation pathway during CH4-CO2 reactions on oxophilic Ni-Co and Co clusters, unlike those established previously on Ni clusters. The initial C-H bond activation remains as the sole kinetically relevant step on Ni-Co, Ni, and Co clusters, but their specific reaction paths vary. On Ni clusters, C-H bond activation occurs via an oxidative addition step that involves a three-center (H3C···*···H)⧧ transition state, during which a Ni-atom inserts into the C-H bond and donates its electron density into the C-H bond's antibonding orbital. Ni-Co clusters are more oxophilic than Ni; thus, their surfaces are covered with oxygen adatoms. An oxygen adatom and a vicinal Co-atom form a metal-oxygen site-pair that cleaves the C-H bond via a σ bond metathesis reaction, during which the Co inserts into the C-H bond while the oxygen abstracts the leaving H-atom in a concerted, four-center (H3C···*···H···O*)⧧ transition state. Similarly, Co clusters also catalyze the σ bond metathesis step, but much less effectively because of their higher oxophilicities, much stronger binding to oxygen, and less effective hydrogen abstraction than Ni-Co clusters. On Ni-Co and Co clusters, the pseudo-first-order rate coefficients are single-valued functions of the CO2-to-CO ratio (or H2O-to-H2 ratio), because this ratio prescribes the oxygen chemical potentials and the relative abundances of metal-oxygen site-pairs through the water-gas shift equilibrium. The direct involvement of reactive oxygen in the kinetically relevant step leads to more effective CH4 turnovers and complete elimination of coke deposition on Ni-Co bimetallic clusters.

Cp2TiCH2CH2(SiEt2CHMe2)]+, an Alkyl-titanocene(IV) Complex Containing an Unconventional Ti←C(β)-Si Mode of Bonding

The compound [Cp2Ti(Me)(CD2Cl2)][B(C6F5)4] reacts with triethylvinylsilane (TEVS) to give the γ-agostic σ-alkyl complex [Cp2TiCH2CHMe(SiEt3)]+ (IV), the product of a single 1,2-insertion of TEVS into the Ti-Me bond. On heating, complex IV undergoes a CH bond activation process to give the isomeric species [Cp2TiCH2CH2(SiEt2CHMe2)]+ (V) in which a Ti–C(β) interaction (Ti←C(β)-Si) is facilitated by an unusual γ-silyl effect. The structures of IV and V and the nature of the isomerization process were explored by DFT methods which suggest that isomerization proceeds via an intramolecular 1,5-σ bond metathesis reaction which involves ε-agostic interactions.

C-N and N-H Bond Metathesis Reactions Mediated by Carbon Dioxide.

Herein, we report CO2 -mediated metathesis reactions between amines and DMF to synthesize formamides. More than 20 amines, including primary, secondary, aromatic, and heterocyclic amines, diamines, and amino acids, are converted to the corresponding formamides with good-to-excellent conversions and selectivities under mild conditions. This strategy employs CO2 as a mediator to activate the amine under metal-free conditions. The experimental data and in situ NMR and attenuated total reflectance IR spectroscopy measurements support the formation of the N-carbamic acid as an intermediate through the weak acid-base interaction between CO2 and the amine. The metathesis reaction is driven by the formation of a stable carbamate, and a reaction mechanism is proposed.

ChemInform Abstract: Ring Expansion Reactions of Ethyl Cyclopropylideneacetate and Benzosilacyclobutenes: Formal σ Bond Cross Metathesis.

AbstractThe title reaction affords E/Z‐mixtures of benzosilacycloheptanes.

Ring expansion reactions of ethyl cyclopropylideneacetate and benzosilacyclobutenes: formal σ bond cross metathesis

The ring expansion reaction of ethyl cyclopropylideneacetate ( 1) with benzosilacyclobutenes proceeded smoothly in the presence of Ni(cod) 2-TOPP (tribiphenyl-2-yl phosphite) to give benzosilacycloheptenes in good yields. This formal σ bond metathesis reaction proceeded in a selective manner. The Ni-catalyzed reaction of 1 with biphenylenes was also examined.