Regio- and stereoselective synthesis of boryl-substituted allylsilanes via transition metal-catalyzed silaboration

Regio- and stereo-controlled synthesis of boryl-substituted allylsilanes via transition metal-catalyzed additions of silylboranes to unsaturated organic compounds is described. Nickel-catalyzed reactions of (dimethylphenylsilyl)pinacolborane with 1,3-dienes, vinylcyclopropanes, and vinylcyclobutanes yielded 4-, 5-, and 6-boryl-substituted allylsilanes, respectively. Palladium-catalyzed addition of the silylborane to allenes took place at the more substituted C=C bond to yield 2-borylallylsilane selectively. The 2-borylallylsilanes served as useful allylation reagents in Lewis acid-mediated reactions with acetals and aldehydes. In addition to the simple allylation reactions, a cascade reaction to form the trans-9-boryl-1,2-benzooxadecalin skeleton and a cyclization reaction to form cyclic alkenylboranes were achieved by the use of 2-borylallylsilanes as key reagents. Reactions of methylenecyclopropanes were catalyzed by palladium and platinum catalysts. The reaction course, however, depended upon the substrate structure and the catalyst employed. For instance, cycloalkylidenecyclopropanes yielded 2-cycloalkylidene-3-boryl-1-silylpropanes selectively in the presence of a palladium catalyst, while 3-cycloalkylidene-3-boryl-1-silylpropanes were obtained selectively in the corresponding platinum-catalyzed reactions.     

Ni-catalyzed addition reaction of allylic selenides to alkynes

Nickel complex catalysts, Ni(cod)₂-2PPh₃ and Ni(cod)₂-dppb systems in particular, catalyze the addition reactions of phenyl allyl selenide to terminal alkynes to regioselectively afford 2-phenylseleno-1-allyl-1-alkenes in good to excellent yields. A mechanism that involves a η³-allyl-nickel complex is proposed on the basis of isolation, crystal structure determination and reactivity study of the complex.     

Ester-functionalized phthalonitriles and zinc phthalocyanines via palladium-catalyzed cyanation of 4,5-dichlorophthalates

Dicyanophthalates 3 were synthesized via Pd-catalyzed two-fold cyanation of the corresponding 4,5-dichlorophthalates with Zn(CN)₂. Appropriate modification of reaction conditions allowed one-pot synthesis of the corresponding zinc phthalocyanines 7 bearing eight peripheral alkyl ester groups. Powder X-ray diffraction study of a mesogenic zinc phthalocyanine bearing branched alkyl substituents revealed a rare case of a transition between two columnar rectangular liquid crystalline mesophases with different symmetry.     

Mechanistic study of palladium catalyzed S-S and Se-Se bonds addition to alkynes

The mechanistic study of palladium catalyzed S-S and Se-Se bonds addition to alkynes revealed the involvement of dinuclear transition metal complexes in the catalytic cycle. Coordination of alkyne to dinuclear transition metal complex was found to be the rate determining step of the reaction. An unusual phosphine ligand effect increasing the yield of addition reaction was found in the studied system. A new synthetic procedure was developed to perform the catalytic reaction using easily available Pd(II) complex. The scope of the reaction and the reactivity of S-S and Se-Se bonds toward alkynes were investigated. The X-ray structure of the product of S-S bond addition reaction showed favorable geometry for the possible application as a chelate ligand.     

Chelation-assisted carbon-hydrogen and carbon-carbon bond activation by transition metal catalysts

Herein we describe the chelation-assisted C-H and C-C bond activation of carbonyl compounds by Rh1 catalysts. Hydroacylation of olefins was accomplished by utilizing 2-amino-3-picoline as a chelation auxiliary. The same strategy was employed for the C-C bond activation of unstrained ketones. Allylamine 24 was devised as a synthon of formaldehyde. Hydroiminoacylation of alkynes with allylamine 24 was applied to the alkyne cleavage by the aid of cyclohexylamine.     

C-C versus C-H bond activation of alkynes by early second-row transition metal atoms

The reactions of Y (a2D), Zr (a3F), Nb (a6D), Mo (a7S), and electronically excited-state Mo* (a5S) with propyne (methylacetylene) and 2-butyne (1,2-dimethylacetylene) were investigated using crossed molecular beams. For all of the metals studied, reactions with propyne led to H2 elimination, forming MC3H2. For Y + propyne, C-C bond cleavage forming YCCH + CH3 also was observed, with an energetic threshold in good agreement with an earlier determination of D0(Y-CCH). For Y + 2-butyne, three reactive channels were observed: YC4H4 + H2, YC3H3 + CH3, and YC3H2 + CH4. The C-C bond cleavage products accounted for 21 and 27% of the total products at Ecoll = 69 and 116 kJ/mol, respectively. For Zr and Nb reactions with 2-butyne, competition between H2 and CH4 elimination was observed, with C-C bond cleavage accounting for 12 and 4% of the total product signal at Ecoll = 71 kJ/mol, respectively. For reactions of Mo and Mo* with 2-butyne, only H2 elimination was observed. The similarity between reactions involving two isomeric species, propyne and allene, suggests that H atom migration is facile in these systems.     

Synthesis of cyanofuroxans from 4-nitrofuroxans via CC bond forming reactions

A substitution reaction of 4-nitrofuroxans to prepare 4-cyanofuroxans is described. This substitution reaction was complicated by the reverse reaction and a judicious choice of cyanide source was important to enable this direct synthesis process. The optimized reaction conditions showed an excellent applicability for the synthesis of a range of 4-cyanofuroxans. 3-Cyanofuroxans, known to be thiol-mediated nitric oxide donors, could also be obtained by the thermal or photochemical isomerization of 4-cyanofuroxans. The developed cyanation of furoxans is a rare example of CC bond-forming reaction on a furoxan ring.     

Sulfate additives generate robust and highly active palladium catalysts for the cyanation of aryl chlorides

The use of sulfate additives such as H₂SO₄ greatly increases the reactivity of palladium catalysts for the cyanation of aryl and heteroaryl chlorides and renders them more robust toward adventitious air. Using this method, a wide variety of aromatic and heteroaromatic nitriles were prepared in high yield.     

Synthesis of indoles via palladium-catalyzed C-H activation of N-aryl amides followed by coupling with alkynes

A convenient and efficient method for the construction of indole skeleton was developed via Pd-catalyzed C-H activation of N-aryl amides and subsequent coupling with alkynes. Both stoichiometric and catalytic versions have been successfully achieved.     

Polymerization of acenaphthylene by transition metal catalysts

The polymerization of acenaphthylene (ACN) was examined in the presence of the group V and VI transition metal salts such as WCl₆, MoCl₅, TaCl₅, and NbCl₅, as catalysts under various reaction conditions. These transition metal salts were found to be effective catalysts for the polymerization of ACN. The polymerization of ACN by WCl₆ in chlorobenzene proceeded at a high initial rate when the monomer to catalyst mole ratio was 200. In addition, it was observed that aromatic solvents generally were found to be superior to aliphatic solvents for both conversion and molecular weight. The structure of the resulting polymers was characterized by means of NMR, IR, UV, and x-ray diffraction. Emission properties were also investigated. Fluorescence emission spectra of the polymers obtained by WCl₆ as a catalyst varied strongly depending on the polymerization solvent. Thus, it appears that the polyacenaphthylene produced by WCl₆ was a different configuration dependent on the polymerization solvents used.     

Facile synthesis of arylboronic esters by palladacycle-catalyzed bromination of 2-arylbenzoxazoles and subsequent borylation of the brominated products

An efficient and facile synthesis of arylboronic esters bearing the benzoxazole moiety has been described using a new family of palladacycle: cyclopalladated ferrocenylimines as the catalysts. This reaction includes two concessive steps: bromination of 2-arylbenzoxazoles and subsequent borylation. The bromination of para-C-H bond was an electrophilic substitution process by using NBS as the brominating reagent, and the brominated products were determined by HMBC (¹H-detected heteronuclear multiple bond correlation) spectra. Particularly, the borylation step could be carried out successively only after removal of the solvent to afford the arylboronic esters in moderate to good yields.     

Selective,catalytic carbon-carbon bond activation and functionalization promoted by late transition metal catalysts

The selective catalytic activation and functionalization of carbon-carbon bonds in a series of substituted cyclopropane substrates has been developed using commercially available transition metal catalysts. Catalytic hydrogenation and olefination procedures, tolerant of a range of functional groups, have been discovered. Introduction of a chelate-assisting substituent such as [PPh2] is effective in altering the kinetic selectivity and lowering the activation barrier for the catalytic processes.     

Stable bis(diisopropylamino)cyclopropenylidene (BAC) as ligand for transition metal complexes

Several nickel(0), palladium(II), and rhodium(I) complexes have been prepared using for the first time the stable bis(diisopropylamino)cyclopropenylidene (BAC). Based on single crystal X-ray diffraction studies and spectroscopic data, the structural and electronic properties of these complexes are discussed. Moreover, their similarities and differences with the analogous NHC complexes are emphasized.     

Synthesis of aryl phosphines via phosphination with triphenylphosphine by supported palladium catalysts

The palladium catalyzed phosphination of functionalized aryl bromides, triflates, and chlorides with triphenylphosphine to yield aryldiphenylphosphines was catalyzed by thermally stable palladium catalysts supported on charcoal and aluminia. The addition of NaI promoted both the rates and yields in the phosphination with Pd/C.     

Hydrogenation of imino bonds with half-sandwich metal catalysts

Imines can be reduced to afford synthetically important amines via a number of means, of which half-sandwich metal complex-effected reduction has gained particular prominence in the past one decade or so. This Feature Article aims to summarise the progress made with such metal catalysts, placing emphasis on our own work. The article covers transfer hydrogenation and hydrogenation, and finishes with a brief account of catalyst immobilisation and mechanistic understanding.     

Encapsulation of transition metal catalysts by ligand-template directed assembly

Encapsulated transition metal catalysts are presented that are formed by templated self-assembly processes of simple building blocks such as porphyrins and pyridylphosphine and phosphite ligands, using selective metal-ligand interactions. These ligand assemblies coordinate to transition metals, leading to a new class of transition metal catalysts. The assembled catalyst systems were characterized using NMR and UV-vis spectroscopy and were identified under catalytic conditions using high-pressure infrared spectroscopy. Tris-3-pyridylphosphine binds three mesophenyl zinc(II) porphyrin units and consequently forms an assembly with the phosphorus donor atom completely encapsulated. The encapsulated phosphines lead exclusively to monoligated transition metal complexes, and in the rhodium-catalyzed hydroformylation of 1-octene the encapsulation of the catalysts resulted in a 10-fold increase in activity. In addition, the branched aldehyde was formed preferentially (l/b = 0.6), a selectivity that is highly unusual for this substrate, which is attributed to the encapsulation of the transition metal catalysts. An encapsulated rhodium catalyst based on ruthenium(II) porphyrins and tris-meta-pyridyl phosphine resulted in an even larger selectivity for the branched product (l/b = 0.4). These encapsulated catalysts can be prepared easily, and various template ligands and porphyrins, such as tris-3-pyridyl phosphite and ruthenium(II) porphyrins, have been explored, leading to catalysts with different properties.     

Stereospecific palladium-catalyzed borylation reaction of 1-alkenyl halides with diispropylaminoborane

The palladium-catalyzed borylation reaction of 1-alkenyl halides with diisopropylaminoborane leads efficiently and stereospecifically to the corresponding trans-1-alkenyl(diisopropylamino)boranes in good yields.     

Selective cis-addition of C-H bonds of pyrroles and thiophenes to alkynes catalyzed by a dinuclear palladium complex

Pyrroles and thiophenes reacted with alkynes in the presence of dinuclear palladium complexes with high stereoselectivity (cis-addition) in almost all cases. While regioselectivity in the reaction with pyrroles depended on substituents on the nitrogen atom and alkynes, all reactions of thiophenes afforded 2-alkenylthiophenes.     

Dinitrogen activation by low-coordinate transition metal complexes

In this article, several avenues in the ongoing computational study of first row transition metal β-diketiminate dinitrogen complexes are discussed. Analysis of monometallic N–N bond length changes reveals that upon complexation of free N₂, side-bound N₂ is 0.018–0.054?Å longer than analogous end-bound N₂. Although the same isomeric preferences across the 3-D series were calculated for bimetallic β-diketiminate N₂ complexes, the N–N bond lengths and hence N₂ activation was found to be greater compared to the monometallic species. This present research demonstrates that a useful starting point for activated dinitrogen complexes is the choice of bimetallic supporting ligands, which unlike monometallic ligands allow both metals to activate N₂ in a concerted fashion.