A practical ruthenium-catalyzed cleavage of the allyl protecting group in amides, lactams, imides, and congeners

A convenient methodology for the deprotection of N-allylic amide-like moieties was developed. The first examples accounting for the ruthenium-catalyzed deallylation of amides, lactams, imides, pyrazolidones, hydantoins, and oxazolidinones have been achieved by the sequential use of Grubbs carbene (isomerization step) and RuCl(3) (oxidation step). A variety of substrates, including enantiopure multifunctional beta- and gamma-lactams, can be employed.     

Ruthenium-catalyzed [1,n]-metallotropic shift (n = 3, 5) of alkynyl carbene complex intermediates

The ruthenium-catalyzed isomerization of diynes and triynes involving propargyl carboxylate moieties affords dienynes and dienediynes, respectively. The [1,n]-metallotropic shift (n = 3, 5) (carbene walk) of in situ generated alkynyl carbene complexes has been proposed for the catalytic isomerization reaction.     

Ruthenium-catalyzed olefin metathesis double-bond isomerization sequence

A novel ruthenium-catalyzed tandem ring-closing metathesis (RCM) double-bond isomerization reaction is described in this paper. The utility of this method for the efficient syntheses of five-, six-, and seven-membered cyclic enol ethers is demonstrated. It relies on the conversion of a metathesis-active ruthenium carbene species to an isomerization-active ruthenium-hydride species in situ. This conversion is achieved by using various additives. Scope and limitations of the different protocols are discussed, and some mechanistic considerations based on (31)P and (1)H NMR spectroscopic studies are presented.     

New selectivities from old catalysts. Occlusion of Grubbs’ catalysts in PDMS to change their reactions

This article describes new selectivities for Grubbs’ first and second generation catalysts when occluded in a hydrophobic matrix of polydimethylsiloxane (PDMS). Occlusion of catalysts in mm-sized slabs of PDMS is accomplished by swelling with methylene chloride then removing the solvent under vacuum. The catalysts are homogenously dissolved in PDMS yet remain catalytically active. Many substrates that react by olefin metathesis with Grubbs’ catalysts freely dissolved in methylene chloride also react by olefin isomerization with occluded catalysts. Eleven examples of substrates that exhibit dual reactivity by undergoing olefin isomerization with occluded catalysts and olefin metathesis with catalysts dissolved in methylene chloride are reported. Most of these substrates have olefins with allylic phosphine oxides, carbonyls, or ethers. Control experiments demonstrate that isomerization is occurring in the solvent by decomposition of the catalyst from a ruthenium carbene to a proposed ruthenium hydride. This work was extended by heating occluded Grubbs’ first generation catalyst to 100 °C in 90% MeOH in H₂O in the presence of various alkenes to transform the Grubbs’ catalyst into an isomerization catalyst for unfunctionalized olefins. This work demonstrates that occlusion of organometallic catalysts in PDMS has important implications for their reactions and can be used as a method to control which reactions they catalyze.     

Palladium-catalyzed reaction of allyl carbamates; allylation of carbonucleophiles,and protection-deprotection of amines

Allylation of carbonucleophiles with allylic carbamates under neutral conditions has been studied. The C-allylation of carbonucleophile is competitive with the N-allylation of amines, and the structure of amines is crucial for the selectivity. Bulky secondary amines gave the best results. Also a new method of protection-deprotection of amines as carbamates has been developed. Smooth deprotection is possible by the palladium-catalyzed reaction of allyl carbamates with formic acid. This method is particulary useful for primary amines, including optically active amino acids.     

Development of isomerization and cycloisomerization with use of a ruthenium hydride with N-heterocyclic carbene and its application to the synthesis of heterocycles

A pure ruthenium hydride complex with N-heterocyclic carbene (NHC) ligand was efficiently generated from the reaction of a second-generation Grubbs ruthenium catalyst with vinyloxytrimethylsilane and unambiguously characterized. This ruthenium hydride complex showed high catalytic activity for the selective isomerization of terminal olefin and for the cycloisomerization of 1,6-dienes. These reactions of N-allyl-o-vinylaniline lead to novel synthetic methods for heterocycles such as indoles and 3-methylene-2,3-dihydroindoles, which are useful synthons for bioactive natural products. These procedures address an important issue in diversity-oriented synthesis.     

Sequential catalytic isomerization and allylic substitution. Conversion of racemic branched allylic carbonates to enantioenriched allylic substitution products

A catalytic protocol for the conversion of readily accessible racemic, branched aromatic allylic esters to branched allylic amines, ethers, and alkyls has been developed. Palladium-catalyzed isomerization of branched allylic esters to terminal allylic esters, followed by sequential iridium-catalyzed allylic substitution, gave the branched allylic products in good yield with high regioisomeric and enantiomeric selectivity. Both electron-rich and electron-poor branched allylic esters gave products in >90% ee. High enantiomeric excesses were also observed for the products from the reactions of 2-thienyl acetates and dienyl carbonates.     

New insights into the mechanism of palladium-catalyzed allylic amination

A comparative investigation into palladium-catalyzed allylic amination of unsubstituted aziridines and secondary amines has been carried out. The use of NH aziridines as nucleophiles favors formation of valuable branched products in the case of aliphatic allyl acetates. The regioselectivity of this reaction is opposite to that observed when other amines are used as nucleophiles. Our study provides evidence for the palladium-catalyzed isomerization of the branched (kinetic) product formed with common secondary amines into the thermodynamic (linear) product. In contrast, the branched allyl products obtained from unsubstituted aziridines do not undergo the isomerization process. Crossover experiments indicate that the isomerization of branched allylamines is bimolecular and is catalyzed by Pd(0). The reaction has significant solvent effect, giving the highest branched-to-linear ratios in THF. This finding can be explained by invoking the intermediacy of sigma-complexes, which is consistent with NMR data. The apparent stability of branched allyl aziridines towards palladium-catalyzed isomerization is attributed to a combination of factors that stem from a higher degree of s-character of the aziridine nitrogen compared to other amines. The reaction allows for regio- and enantioselective incorporation of aziridine rings into appropriately functionalized building blocks. The resulting methodology addresses an important issue of forming quaternary carbon centers next to nitrogen. The new insights into the mechanism of palladium-catalyzed allylic amination obtained in this study should facilitate synthesis of complex heterocycles, design of new ligands to control branched-to-linear ratio, as well as absolute stereochemistry of allylamines.     

Amide Synthesis from Alcohols and Amines Catalyzed by Ruthenium N‐Heterocyclic Carbene Complexes

The direct synthesis of amides from alcohols and amines is described with the simultaneous liberation of dihydrogen. The reaction does not require any stoichiometric additives or hydrogen acceptors and is catalyzed by ruthenium N‐heterocyclic carbene complexes. Three different catalyst systems are presented that all employ 1,3‐diisopropylimidazol‐2‐ylidene (IiPr) as the carbene ligand. In addition, potassium tert‐butoxide and a tricycloalkylphosphine are required for the amidation to proceed. In the first system, the active catalyst is generated in situ from [RuCl₂(cod)] (cod=1,5‐cyclooctadiene), 1,3‐diisopropylimidazolium chloride, tricyclopentylphosphonium tetrafluoroborate, and base. The second system uses the complex [RuCl₂(IiPr)(p‐cymene)] together with tricyclohexylphosphine and base, whereas the third system employs the Hoveyda–Grubbs 1st‐generation metathesis catalyst together with 1,3‐diisopropylimidazolium chloride and base. A range of different primary alcohols and amines have been coupled in the presence of the three catalyst systems to afford the corresponding amides in moderate to excellent yields. The best results are obtained with sterically unhindered alcohols and amines. The three catalyst systems do not show any significant differences in reactivity, which indicates that the same catalytically active species is operating. The reaction is believed to proceed by initial dehydrogenation of the primary alcohol to the aldehyde that stays coordinated to ruthenium and is not released into the reaction mixture. Addition of the amine forms the hemiaminal that undergoes dehydrogenation to the amide. A catalytic cycle is proposed with the {(IiPr)Ru^II} species as the catalytically active components.     

Prevention of undesirable isomerization during olefin metathesis

1,4-Benzoquinones have been found to prevent olefin isomerization of a number of allylic ethers and long-chain aliphatic alkenes during ruthenium-catalyzed olefin metathesis reactions. Electron-deficient benzoquinones are the most effective additives for the prevention of olefin migration. This mild, inexpensive, and effective method to block olefin isomerization increases the synthetic utility of olefin metathesis via improvement of overall product yield and purity.     

Palladium‐Catalyzed [3+3] Annulation of Vinyl Chromium(0) Carbene Complexes through Carbene Migratory Insertion/Tsuji–Trost Reaction

Vinyl chromium(0) Fischer carbene complexes were employed as the source of π‐allylic palladium species for catalytic [3+3] annulation under palladium catalysis. Mechanistically, this transformation is proposed to involve carbene migratory insertion and intramolecular Tsuji–Trost reaction as the key steps. Substituted six‐membered heterocyclic flavonones and quinolines are obtained, depending on the nucleophilic functional group on the coupling partners.     

Direct catalytic azidation of allylic alcohols

A direct catalytic azidation of primary, secondary, and tertiary allylic alcohols has been developed. This new azidation reaction affords the corresponding allylic azides in high to excellent yields and regioselectivities. The reaction provides straightforward access to allylic azides that are valuable intermediates in organic synthesis, including the preparation of primary amines or 1,2,3-triazole derivatives.     

Synthesis and ring-opening metathesis polymerization of eight-membered unsaturated lactams and related monomers

Novel eight-membered ring unsaturated lactams were synthesized and tested as monomers for the ruthenium-catalyzed ring-opening metathesis polymerization (ROMP). The reaction of a N-protected cyclic alkeneamine was also investigated. The Grubbs’ benzylidene complexes RuCl₂(=CHPh)(PCy₃)₂ or RuCl₂(=CHPh)(PCy₃)(IMesH₂) and selected ruthenium–arene species bearing either phosphine or stable Arduengo-type N-heterocyclic carbene ligands served as catalyst precursors. In most cases, isomerization of the starting materials took place and only 1-benzyl-1-aza-2-ketocyclooct-5-ene afforded a polymeric product. This polyamide was characterized by numerous analytical techniques.     

Synthetic Aspects of Metal-Catalyzed Oxidations of Amines and Related Reactions

The metabolism of amines is governed by a variety of enzymes such as amine oxidase, flavoenzyme, and cytochrome P-450. A wide variety of compounds are produced such as ammonia and alkaloids in selective and clean oxidation reactions that proceed under mild reaction conditions. Simulation of the functions of these enzymes with simple transition metal complex catalysts may lead to the discovery of biomimetic, catalytic oxidations of amines and related compounds. Indeed, metal complex catalyzed oxidations have been found to proceed with high efficiency. The first section of this review discusses the dehydrogenative oxidations of amines with transition metal catalysts by transition metal catalysts that simulate amine oxidase. The second section highlights the catalytic oxidation of secondary amines to nitrones by simulation of flavoenzymes. The third section describes the simulation of the function of cytochrome P-450 with lowvalent ruthenium complexes and peroxides. Biomimetic ruthenium-catalyzed oxidations of tertiary amines, secondary amines, and other substrates such as amides, β-lactams, nitriles, alcohols, alkenes, ketones, and even nonactivated hydrocarbons can be performed selectively under mild conditions. These three general approaches provide highly useful strategies for synthesis of fine chemicals and biologically active compounds such as alkaloids, amino acids, and β-lactams.     

Ruthenium-catalyzed linear selective allylic aminations of monosubstituted allyl acetates

The ruthenium-catalyzed highly linear selective allylic amination of monosubstituted allylic acetates with secondary amines was developed. The regioselectivity was controlled by the Ru₃(CO)₁₂/2-DPPBA catalyst, and a linear-type aminated product was obtained as a single regioisomer.     

Highly efficient redox isomerization of allylic alcohols at ambient temperature catalyzed by novel ruthenium-cyclopentadienyl complexes--new insight into the mechanism

A range of ruthenium cyclopentadienyl (Cp) complexes have been prepared and used for isomerization of allylic alcohols to the corresponding saturated carbonyl compounds. Complexes bearing CO ligands show higher activity than those with PPh3 ligands. The isomerization rate is highly affected by the substituents on the Cp ring. Tetra(phenyl)methyl-substituted catalysts rapidly isomerize allylic alcohols under very mild reaction conditions (ambient temperature) with short reaction times. Substituted allylic alcohols have been isomerized by employing Ru-Cp complexes. A study of the isomerization catalyzed by [Ru(Ph5Cp)(CO)2H] (14) indicates that the isomerization catalyzed by ruthenium hydrides partly follows a different mechanism than that of ruthenium halides activated by KOtBu. Furthermore, the lack of ketone exchange when the isomerization was performed in the presence of an unsaturated ketone (1 equiv), different from that obtained by dehydrogenation of the starting allylic alcohol, supports a mechanism in which the isomerization takes place within the coordination sphere of the ruthenium catalyst.     

Effective synthesis of bicyclodienes via palladium-catalyzed asymmetric allylic alkylation and ruthenium-catalyzed cycloisomerization

[n.3.0]Bicycles (n = 3–6) can be synthesized using palladium-catalyzed asymmetric allylic alkylation followed by ruthenium-catalyzed cycloisomerization. New types of triarylphosphino-1,2-diaminooxazoline ligands show the same high levels of enantioselectivity observed with Trost ligand when employed in Pd-catalyzed allylic alkylation reactions. The enyne products of these allylic alkylation reactions were further elaborated using a Ru-catalyzed redox isomerization process, for which a mechanism is proposed.     

Thiyl radical-mediated cleavage of allylic C-N bonds: scope, limitations, and theoretical support to the mechanism

Thiols mediate the radical isomerization of allylic amines into enamines. The reaction results in the cleavage of the allylic C-N bond, after treatment with aqueous HCl. The mechanism involves the abstraction of an allylic hydrogen alpha to nitrogen by thiyl radical, followed by a return hydrogen transfer from the thiol to the carbon gamma to nitrogen in the intermediate allylic radical. The scope and limitations of the reaction with respect to the nature of the thiol, to the structure of the allylic chain, and to the nature of the substituents at nitrogen were investigated. The experimental results were interpreted on the ground of DFT calculations of the C-Halpha BDE in the starting allylic amines, and of the C-Hgamma BDE in the resulting enamines. The efficiency of the initial hydrogen transfer is the first requirement for the reaction to proceed. A balance must be found between the S-H BDE and the two above-mentioned C-H BDEs. The incidence of stereoelectronic factors was analyzed through NBO calculations performed on the optimized geometries of the starting allylic amines. Additional calculations of the transition structures and subsequent tracing of the reaction profiles were performed for the abstraction of Halpha from both the allyl and the prenyl derivatives by p-TolS(*). The latter allowed us to estimate the rate constant for the abstraction of hydrogen by thiyl radical from an N-prenylamine and an N-allylamine.     

Towards "designer" surfaces: functionalisation of self-assembled monolayer (SAM) on colloidal gold by alkene metathesis

A variety of groups like a Fischer carbene complex, an N-hydroxysuccinimide or a ferrocene derivative have been grafted by ruthenium-catalyzed cross-metathesis reaction with terminal alkene groups on monolayer-protected gold clusters as a mild and convenient strategy to anchor functional molecules.     

A general,highly enantioselective method for the synthesis of D and L alpha-amino acids and allylic amines

Catalytic and enantioselective synthesis of amino acids is a subject of intense interest in the field of asymmetric catalysis. Traditionally, researchers have concentrated their efforts largely on the design and discovery of enantiopure catalysts for the Strecker reaction, alkylation of tert-butyl gylcinate-benzophenone, electrophilic amination of carbonyl compounds, and hydrogenation of N-acyl-aminoacrylic acid; however, the scope of these reactions is limited. In this paper, we report on a different approach to amino acids based on an expeditious route to enantiopure allylic amines. A highly enantioselective and catalytic vinylation of aldehydes leads to allylic alcohols that are then transformed to the allylic amines via Overman's [3,3]-sigmatropic rearrangement of imidates. Oxidative cleavage of the allylic amines furnishes the amino acids in good yields and excellent ee's. The scope and utility of this method are demonstrated by the synthesis of challenging allylic amines and their subsequent transformation to valuable nonproteinogenic amino acids, including both D and L configured (1-adamantyl)glycine.