Recent advances in the application of nanoparticles in the selective reduction of carbon–carbon double bonds

Herein, we present recent advances in the application of metal nanoparticles in the selective hydrogenation of C–C double bonds. The review focuses on reduction methods of alkenes, arenes, and aromatic heterocycles, which were classified according to transition metals used as catalysts. The majority of described systems concern direct hydrogenation, which is of particular importance to industrial processes. Nonetheless, interesting transfer hydrogenation protocols were also developed, which may be incredibly convenient for laboratory purposes. Some of the methods are distinguished with excellent chemoselectivity making them the perfect tool for the synthesis of compounds containing reducible functional groups. Apart from noble metals, the application of earth-abundant ones as catalysts was a subject of studies, and the related methods were highlighted.     

Insertion of arynes into the carbon–oxygen double bond of amides and its application into the sequential reactions

The reaction of arynes, generated from ortho-(trimethylsilyl)aryl triflates, with the CO bond of formamides gave salicylaldehyde derivatives via the formation of formal [2+2] adducts. The sequential transformation of arynes into ortho-disubstituted arenes, o-aminoalkylphenols or o-hydroxyalkylphenols, was achieved by one-pot procedure using dialkylzincs.     

Novel transformation of molybdenum-coordinated 2-pyrazinecarbonitrile ligand: Nucleophilic addition of cyanide to carbon–nitrogen triple bond

The control of the presence of OH⁻/CN⁻ nucleophiles in an aqueous-ethanolic solution of [Mo(CN)₄O(pcn)]²⁻ anions (pcn = 2-pyrazinecarbonitrile) allows for selective transformation of the organonitrile ligand. The nucleophilic addition of CN⁻ to the triple bond of the pendant nitrile group in pcn results in reduction of the nitrile group to an imine which is subsequently deprotonated and coordinated to the molybdenum(IV) atom in a bidentate mode to afford the complex anion [Mo(CN)₃O(pnccn)]²⁻ (Hpnccn = pyrazinecyanoimine), as revealed by its X-ray crystal structure. Such a synthetic and crystallographic demonstration of the reduction of an organonitrile with the capture of a metal-bound imine intermediate is uncommon. Elemental analysis verified the formulation [PPh₄]₂[Mo(CN)₃O(pnccn)]·2.5H₂O (1) and the physicochemical behaviour of [Mo(CN)₃O(pnccn)]²⁻ was investigated using spectroscopy (IR, UV–Vis and NMR) and cyclic voltammetry. The compound is the first known example with coordinated primary imine group among cyanocomplexes of Mo. The structural and spectroscopic properties of 1 are compared with those of the compounds resulting from the competitive metal-assisted ligand hydrolysis. The equilibrium constant for the protonation of the cyanoimine ligand, pK = 5.54 ± 0.03, has been determined by the spectrophotometric titrations at room temperature.     

Metal free amide synthesis via carbon–carbon bond cleavage

A metal-free oxidative coupling of methyl ketones and primary amines to amides has been developed. The reaction tolerates a variety of functional groups, and is operationally simple. The reaction is proposed to go through a radical pathway to form the triiodomethyl ketone intermediate and the amide is formed by the nucleophilic attack of amine on triiodomethyl ketone carbonyl.     

The chemistry of the carbon–transition metal double and triple bond: annual survey covering the year 1999

This survey is intended to be a comprehensive summary of articles that report on the synthesis, reactivity, or properties of compounds featuring a multiple bond between carbon and a transition metal. Reactions which employ metal carbene complexes as transient intermediates generated through well-established routes [Russ. Chem. Bull. 48 (1999) 16] are not covered unless there is some effort to characterize the carbene complex intermediate. Although a determined effort has been made to include patents, in general only patents which are listed in or at the end of Organometallics section of Chemical Abstracts (Section 29) are included; patents which appeared in Chemical Abstracts in the year 1999 have been included. Only compounds which feature a multiple bond between one carbon atom and one transition metal are discussed in this survey, thus bridging carbene and carbyne complexes are not covered unless there is a multiple bond to at least one transition metal. The complexes of stable carbenes with transition metals have not been included; since the π-donation component of these complexes is minimal, there is no formal carbonmetal multiple bond [J. Chem. Soc., Chem. Commun. (1997) 1963; Polyhedron 16 (1997) 3879]. This survey has been divided into two sections, metal carbene (or alkylidene) complexes and metal carbyne (or alkylidyne) complexes; the carbene complex section represents the vast majority of this article. The metal carbene section has been organized according to metal, starting from the left side of the periodic table. The ionic model [R.H. Crabtree, The Organometallic Chemistry of the Transition Metals, second ed., Wiley-Interscience, New York, 1994, pp. 25–31] has been employed for the discussion of oxidation states and ligand electron count throughout this survey. A special section focusing on alkene metathesis has been included prior to the discussion of carbene complexes of individual metals. The metal carbyne section has been organized according to reaction type.     

Stereoselective cyanation of 4-formyl and 4-imino-β-lactams: application to the synthesis of polyfunctionalized γ-lactams

The stereoselective reaction of 4-oxoazetidine-2-carbaldehydes and their corresponding imines with cyanide-based reagents give β-lactam α-aminonitriles, which are chameleonic building blocks for the controlled synthesis of a variety of new compounds including functionalized γ-lactams, succinimide derivatives, and diamino-lactams derivatives in optically pure form.     

Stereoselective synthesis of 3-butylphtalide via CBS catalytic reduction

Optically active 3-butylphtalide of high enantiomeric excesses (up to 93% e.e.) was synthesized from the reduction of o-pentanoylbenzoic ester with borane using B-methyloxyoxazaborolidine (1b) as the chiral catalyst.     

Stereoselective reduction of N-phthaloyl α-amino ketones: an expeditious synthesis of statine

A highly diastereoselective synthesis of a protected statine derivative via syn-selective LiAlH(OBu-t)₃ reduction of a leucine derived N-phthaloyl α-amino ketone is described.     

Stereoselective allylation of α-hydroxy aldimines and its application to the formal synthesis of (−)-β-conhydrine

Stereoselective allylation of N-p-methoxyphenyl (PMP)-substituted α-hydroxy aldimines is described. Several Lewis acids (BF₃·OEt₂, SnCl₄, TiCl₄, ZnCl₂, and MgBr₂·OEt₂) were employed to mediate the allylation reactions. The addition of the allyl group generates a new stereocenter and affords the syn vicinal amino alcohol. Formal synthesis of (?)-β-conhydrine (1) was accomplished via syn-selective allyl addition to N-PMP-substituted α-hydroxy aldimine.     

Electrogeneration of the molybdenum-molybdenum double bond from thiolate-bridged dinuclear precursors: carbon monoxide catalysed isomerisation about the MoMo bond

The irreversible electroreduction of any of the three distinct dimolybdenum thiolate complexes [{MoCp(CO)}₂(μ-SMe)₃]⁺ and [{MoCp(CO)X}₂(μ-SMe)₂] (Cp = η⁵-C₅H₅; X = Cl or Br) gives a common intermediate which we identify as cis-[{MoCp(CO)}₂(μ-SMe)₂] (I). This intermediate slowly isomerises to the isolable product trans-[{MoCp(CO)}₂(μ-SMe)₂]; the isomerisation is catalysed by carbon monoxide, a process which we suggest takes place through CO binding across and elimination from the molybdenum-molybdenum double bond of (I).     

Investigation of lipase-catalyzed Michael-type carbon–carbon bond formations

Conjugate additions of carbon nucleophiles to appropriate acceptor molecules were investigated with respect to the synthetic potential and stereochemistry of the products. Reactions of short-chain α,β-unsaturated ketones and mono-substituted nitroalkenes with CH-acidic carboxylic ester derivatives were catalyzed by various immobilized lipases. Using methyl nitroacetate complete conversion with methyl vinyl ketone and trans-β-nitrostyrene was obtained. The reactions proceeded without enantioselectivity. Evidence for enzyme catalysis is provided by the observation that after denaturation of Candida antarctica lipase B or inhibition no reaction took place. Docking studies with the chiral addition product methyl 2-methyl-2-nitro-5-oxohexanoate did not reveal any specific binding mode for this reaction product, which would have been the requirement for stereoselective additions. These results support the experimental findings that the conjugate addition takes place without enantiopreference.     

Stereoselective routes to aryl substituted γ-butyrolactones and their application towards the synthesis of highly oxidised furanocembranoids

Titanium chelate addition of aryl nucleophiles to cyclopropyl aldehyde 6 followed by a tin-catalyzed one-pot retro-aldol, acetalisation and lactonisation sequence afforded cis and trans γ-aryllactone acetals. A γ-furyllactone derived by this approach was further transformed in two steps to model compounds for the oxidised northeastern sectors of selected Pseudopterogorgia diterpenoids.     

A new proposed mechanism for the cathodic reduction of a carbon–chlorine bond in 2-acetylphenyltrichloroacetate

The cathodic reduction of 2-acetylphenyltrichloroacetate at a controlled potential of −0.5 V (vs SCE) yields 3,3-dichloro-4-hydroxy-4-methyl-3,4-dihydrocoumarin. The charge consumption is 1e⁻/substrate molecule, which is in agreement with a non-conventional R–X→R⁻+X mechanism.     

Stereoselective synthesis of conjugated all-trans-tetraenes. Application to the synthesis of β-parinaric acid methyl ester

in this paper is reported the stereoselective synthesis of all-trans-tetraenes by reductive elimination of 1,8-dibenzoate-2,4,6-trienes with sodium amalgam. The method was applied to the syntheses of 4E, 6E, 8E, 10E-heptatetraene and β—parinaric acid methyl ester.     

Aza-annulation of β-enaminolactones: application to the synthesis of enantiopure difunctionalized bicyclic lactams

Diastereoselective aza-annulation of seven-membered β-enaminolactones 2 gives access to bicyclic heterocyles 5. Fragmentation of molecule 5a with lithium methoxide affords cis or trans bicyclic lactams 8 with excellent stereoselectivities.     

Stereoselective alkylation–reduction of β-keto nitriles by the fungus Curvularia lunata

The alkylation–reduction (AR) reaction of β-keto nitriles by growing cells of Curvularia lunata CECT 2130 has been explored. The reaction conditions for the ethylation of benzoylacetonitrile have been optimized in terms of both yield and stereoselectivity and the mechanism of this biotransformation was studied. The scope of this reaction has been extended to other alkylations (R=Et, Pr, Bu, iso-Bu) and to a series of aromatic and heteroaromatic substrates, yielding the corresponding optically active α-alkyl β-hydroxy nitriles. The yield (directly related to competing carbonyl reduction reaction) depends on the substrate bulk, whereas the enantiomeric and diastereomeric excesses (both up to 98%) seem to be dependent on several factors.     

Catalytically active CNT–polymer-membrane assemblies: From synthesis to application

A new membrane electrode assembly set up for catalytic processes containing carbon nanotubes has been developed. The process includes the nanotube synthesis, sputter deposition of platinum as catalyst and the membrane casting. Aligned nanotube carpets were grown from toluene/ferrocene solutions and sputtered with platinum. Subsequently the assembly was investigated using cyclic voltammetry to confirm a sufficient catalyst activity. A procedure was developed to embed the carbon nanotubes doped with catalyst into SPEEK membranes, while preserving the aligned structure and keeping some surface area of the catalyst-doped nanotubes free of membrane material to allow for easy access to reactants. So far the best results were obtained using an aligned but somewhat loose nanotube structure and a deposition of 0.034 mg/cm² Pt, forming a combination of small catalyst clusters and a thin film. The assemblies are optimized in respect to application in fuel cells and functional membranes.     

Stereoselective additions to the exocyclic CC bond of some α-alkylidene-(+)-camphor derivatives

Stereoselective additions to the exocyclic CC double bond of some (1R,3E,4S)-3-alkylidene-1,7,7-trimethylbicyclo[2.2.1]heptan-2-ones and (1R,4E,5S)-4-alkylidene-1,8,8-trimethyl-2-oxabicyclo[3.2.1]octan-3-ones were studied. All additions took place predominantly from the less hindered endo-face of the methylidene compounds to give the corresponding exo-adducts as the major isomers. Thus, catalytic hydrogenations afforded the α-alkylated (1R,3R,4R)-1,7,7-trimethylbicyclo[2.2.1]heptan-2-ones and (1R,4R,5R)-1,8,8-trimethyl-2-oxabicyclo[3.2.1]octan-3-ones in 28–100% de. Similarly, 1,3-dipolar cycloadditions of 2,4,6-trisubstituted benzonitrile oxides gave the corresponding spiro cycloadducts in 66–100% de. The structures were determined by 2D NMR techniques, NOESY spectroscopy and X–ray diffraction.