"Inverse-electron-demand" ligand substitution in palladium(0)-olefin complexes

Synthesis and Reactivity of Ruthenium(BINAP)(PPh3)

Ru(BINAP)(PPh3)HCl cleanly reacts with LiCH2TMS to give Ru(BINAP)(PPh3) (1) that has been fully characterized, including by X-ray diffraction (BINAP and TMS stand for (2,2'-bis(diphenylphosphino)-1,1'-binaphthyl and trimethylsilyl respectively). In sharp contrast with other carbonyl-free phosphine complexes of Ru(0), 1 demonstrates a strikingly high thermal stability and no propensity for intramolecular C-H activation (cyclometalation). Yet 1 coordinates acetonitrile and readily exchanges its PPh3 ligand with alkenes and dienes, thus behaving like a "masked" 16-e Ru(0) species. Electron-poor alkenes coordinate more readily than electron-rich ones, which testifies for the nucleophilic character of the Ru(0)-BINAP fragment. While being thermally stable, 1 is highly reactive and is capable of activating C-H and N-H bonds, and even of cleaving an inert N-Et bond. The combination of high reactivity and stability originates from the P,arene-chelation by the BINAP ligand, i.e., the coordinated π-arene stabilizes Ru(0) to prevent cyclometalation, yet it can slide upon substrate coordination, thereby enabling a variety of inert bond activation reactions to occur under mild conditions.

Catalytic enantioselective C-H functionalization of alcohols by redox-triggered carbonyl addition: borrowing hydrogen, returning carbon

The use of alcohols and unsaturated reactants for the redox-triggered generation of nucleophile-electrophile pairs represents a broad, new approach to carbonyl addition chemistry. Discrete redox manipulations that are often required for the generation of carbonyl electrophiles and premetalated carbon-centered nucleophiles are thus avoided. Based on this concept, a broad, new family of enantioselective C-C coupling reactions that are catalyzed by iridium or ruthenium complexes have been developed, which are summarized in this Minireview.

Pd(Quinox)-Catalyzed Allylic Relay Suzuki Reactions of Secondary Homostyrenyl Tosylates via Alkene-Assisted Oxidative Addition

Pd-catalyzed allylic relay Suzuki cross-coupling reactions of secondary alkyl tosylates, featuring a sterically-hindered oxidative addition and precise control of β-hydride elimination, are reported. The identification of a linear free energy relationship between the relative rates of substrate consumption and the electronic nature of the substrate alkene suggests that the oxidative addition requires direct alkene involvement. A study of the effect of chain length on the reaction outcome supports a chelation-controlled oxidative addition.

Scope and mechanism of the Pt-catalyzed enantioselective diboration of monosubstituted alkenes

The Pt-catalyzed enantioselective diboration of terminal alkenes can be accomplished in an enantioselective fashion in the presence of chiral phosphonite ligands. Optimal procedures and the substrate scope of this transformation are fully investigated. Reaction progress kinetic analysis and kinetic isotope effects suggest that the stereodefining step in the catalytic cycle is olefin migratory insertion into a Pt-B bond. Density functional theory analysis, combined with other experimental data, suggests that the insertion reaction positions platinum at the internal carbon of the substrate. A stereochemical model for this reaction is advanced that is in line both with these features and with the crystal structure of a Pt-ligand complex.

Enantioselective conversion of primary alcohols to α-exo-methylene γ-butyrolactones via iridium-catalyzed C-C bond-forming transfer hydrogenation: 2-(alkoxycarbonyl)allylation

Upon exposure of acrylic ester 1 to alcohols 2a-i in the presence of a cyclometalated iridium catalyst modified by (-)-TMBTP, catalytic C-C coupling occurs, providing enantiomerically enriched 5-substituted α-exo-methylene γ-butyrolactones 3a-i. Bromination of the methylene butyrolactone products followed by zinc-mediated reductive aldehyde addition provides the disubstituted α-exo-methylene γ-butyrolactones 6a and 6b with good to excellent levels of diastereoselectivity.

Catalytic enantioselective Grignard Nozaki-Hiyama methallylation from the alcohol oxidation level: chloride compensates for π-complex instability

Methallyl chloride serves as an efficient allyl donor in highly enantioselective Grignard Nozaki-Hiyama methallylations from the alcohol or aldehyde oxidation level via iridium catalyzed transfer hydrogenation. Under identical conditions, methallyl acetate does not react efficiently. Double methallylation of 1,3-propanediol provides the C(2)-symmetric adduct as a single enantiomer, as determined by HPLC analysis.

Pd-catalyzed carbonylative conjugate addition of dialkylzinc reagents to unsaturated carbonyls

The Pd-catalyzed addition of organozinc reagents to unsaturated carbonyls in the presence of carbon monoxide provides 1,4-diketones in good yield. The reaction was studied with a number of substituted cyclic and acyclic ketones as well as alpha,beta-unsaturated aldehydes.

W18O49 Nanowires as Ultraviolet Photodetector

Photodetectors in a configuration of field effect transistor were fabricated based on individual W18O49 nanowires. Evaluation of electrical transport behavior indicates that the W18O49 nanowires are n-type semiconductors. The photodetectors show high sensitivity, stability and reversibility to ultraviolet (UV) light. A high photoconductive gain of 104 was obtained, and the photoconductivity is up to 60 nS upon exposure to 312 nm UV light with an intensity of 1.6 mW/cm2. Absorption of oxygen on the surface of W18O49 nanowires has a significant influence on the dark conductivity, and the ambient gas can remarkably change the conductivity of W18O49 nanowire. The results imply that W18O49 nanowires will be promising candidates for fabricating UV photodetectors.

Palladium-catalyzed aerobic alcohol oxidation supported by a new type of α-diimine ligands

A series of new ionic sulfonated α-diimine ligands were prepared and employed as supporting ligands for the palladium-catalyzed aerobic alcohol oxidation. The electronic properties, rigidity, and steric hindrance of ligands have remarkable influences upon the catalytic activity of the palladium complexes. The best catalytic result could be achieved using ligand 9 with an electron-withdrawing sulfonyl substituent together with bulky groups (iso-propyl in 2,6-positions). This diimine/palladium-catalyzed system is highly efficient for the oxidation of various benzylic alcohols, and showed moderate yields for the secondary aliphatic alcohols and cyclic alcohols.Key words: α-diimine, palladium, aerobic oxidation, alcohol.

Unusual reactivity of a sterically hindered diphosphazane ligand, EtN{P(OR)(2)}(2), (R = C(6)H(3)(Pr(I))(2)-2,6) towards (eta(3)-allyl)palladium precursors

The reactivity of (eta(3)-allyl)palladium chloro dimers [(1-R-eta(3)-C(3)H(4))PdCl](2) (R = H or Me) towards a sterically hindered diphosphazane ligand [EtN{P(OR)(2)}(2)] (R = C(6)H(3)(Pr(i))(2)-2,6), has been investigated under different reaction conditions. When the reaction is carried out using NH(4)PF(6) as the halide scavenger, the cationic complex [(1-R-eta(3)-C(3)H(4))Pd{EtN(P(OR)(2))(2)}]PF(6) (R = H or Me) is formed as the sole product. In the absence of NH(4)PF(6), the initially formed cationic complex, [(eta(3)-C(3)H(5))Pd{EtN(P(OR)(2))(2)}]Cl, is transformed into a mixture of chloro bridged complexes over a period of 4 days. The dinuclear complexes, [(eta(3)-C(3)H(5))Pd(2)(mu-Cl)(2){P(O)(OR)(2)}{P(OR)(2)(NHEt)}] and [Pd(mu-Cl){P(O)(OR)(2)}{P(OR)(2)(NHEt)}](2) are formed by P-N bond hydrolysis, whereas the octa-palladium complex [(eta(3)-C(3)H(5))(2-Cl-eta(3)-C(3)H(4))Pd(4)(mu-Cl)(4)(mu-EtN{P(OR)(2)}(2))](2), is formed as a result of nucleophilic substitution by a chloride ligand at the central carbon of an allyl fragment. The reaction of [EtN{P(OR)(2)}(2)] with [(eta(3)-C(3)H(5))PdCl](2) in the presence of K(2)CO(3) yields a stable dinuclear (eta(3)-allyl)palladium(I) diphosphazane complex, [(eta(3)-C(3)H(5))[mu-EtN{P(OR)(2)}(2)Pd(2)Cl] which contains a coordinatively unsaturated T-shaped palladium center. This complex exhibits high catalytic activity and high TON's in the catalytic hydrophenylation of norbornene.

Organic azides: an exploding diversity of a unique class of compounds

Since the discovery of organic azides by Peter Griess more than 140 years ago, numerous syntheses of these energy-rich molecules have been developed. In more recent times in particular, completely new perspectives have been developed for their use in peptide chemistry, combinatorial chemistry, and heterocyclic synthesis. Organic azides have assumed an important position at the interface between chemistry, biology, medicine, and materials science. In this Review, the fundamental characteristics of azide chemistry and current developments are presented. The focus will be placed on cycloadditions (Huisgen reaction), aza ylide chemistry, and the synthesis of heterocycles. Further reactions such as the aza-Wittig reaction, the Sundberg rearrangement, the Staudinger ligation, the Boyer and Boyer-Aubé rearrangements, the Curtius rearrangement, the Schmidt rearrangement, and the Hemetsberger rearrangement bear witness to the versatility of modern azide chemistry.

Oxidative Electrochemical Switching in Dithienylcyclopentenes, Part 2: Effect of Substitution and Asymmetry on the Efficiency and Direction of Molecular Switching and Redox Stability

The electrochemical and spectroelectrochemical properties of a series of C5-substituted dithienylhexahydro- and dithienylhexafluorocyclopentenes are reported. The effect of substitution at C5 of the thienyl moiety on the redox properties is quite dramatic, in contrast to the effect on their photochemical properties. The efficiency of electrochemical switching is dependent both on the central cyclopentene unit and on the nature of the substituents, whereby electron-donating moieties favour oxidative electrochemical ring-closure and vice versa. Asymmetrically substituted dithienylcyclopentenes were investigated to explore the ring-closure process in more detail. The results indicate that electrochemically induced ring-closure occurs via the monocation of the open form. In the presence of electroactive groups at C5 of the thienyl ring (e.g., methoxyphenyl) initial oxidation of these groups is followed by intermolecular electron transfer, which drives ring-closure of the open forms.

Modulation of the Electronic Structure of Semiconducting Nanotubes Resulting from Different Metal Contacts

Examined in this paper is the role of the metal electrode influencing the structure and electronic properties of semiconducting carbon nanotubes near the interface at low bias. Specifically, we present quantum-chemical calculations of finite sections of a (8,0) semiconducting single wall nanotube contacted with gold and palladium clusters. The calculations at the density functional level of theory, which included full geometry optimizations, indicate the formation of bonds between the metal atoms of the electrode and the carbon atoms of the nanotube. The local work function of the metal electrode can be expected to exhibit significant variations as a result of this bond formation. Compared to the gold-contacted nanotubes, the palladium-contacted nanotubes have a small but interesting increase in both length and diameter. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of the gold-contacted nanotube are shown localized at the edges. In contrast, the HOMO and LUMO of the palladium-contacted nanotube are extended over the entire nanotube and the metal cluster contacted to it, providing thereby a better conduction path in the contact region of the electrode and the nanotube. The involvement of the highly directional d orbitals in the interactions involving the palladium cluster leads to an enhanced pi electron density in the nanotube. This enhanced pi electron density is synonymous with an improved electron transmission.

Chemo- and Regioselective Intermolecular Cyclotrimerization of Terminal Alkynes Catalyzed by Cationic Rhodium(I)/Modified BINAP Complexes: Application to One-Step Synthesis of Paracyclophanes

A highly regioselective intermolecular cyclotrimerization of terminal alkynes has been developed based on the use of the cationic rhodium(I)/DTBM-Segphos complex. This method can be applied to a variety of terminal alkynes to provide 1,2,4-trisubstituted benzenes in high yield and with high regioselectivity. A chemo- and regioselective intermolecular crossed-cyclotrimerization of dialkyl acetylenedicarboxylates with a variety of terminal alkynes has also been developed based on the use of the cationic rhodium(I)/H8-BINAP complex, furnishing 3,6-disubstituted phthalates in high yields. It constitutes a highly efficient new method for intermolecular crossed-cyclotrimerization of two different monoynes in terms of catalytic activity, chemo- and regioselectivity, scope of substrates, and ease of operation. The versatility of this new crossed-alkyne cyclotrimerization procedure is demonstrated through its application to one-step synthesis of a [6]metacyclophane and [7]-[12]paracyclophanes from the corresponding terminal alpha,omega-diynes. Mechanistic studies have revealed that the chemo- and regioselectivity of this crossed-alkyne cyclotrimerization are determined by the preferential formation of a specific rhodium metallacycle derived from a terminal alkyne and a dialkyl acetylenedicarboxylate.

Catalysts for Cross-Coupling Reactions with Non-activated Alkyl Halides

Despite the problems inherent to metal-catalyzed cross-coupling reactions with alkyl halides, these reactions have become increasingly important during the last few years. Detailed mechanistic investigations have led to a variety of novel procedures for the selective cross-coupling of non-activated alkyl halides bearing beta hydrogen atoms with a variety of organometallic nucleophiles under mild reaction conditions. This Minireview highlights selected examples of metal-catalyzed coupling methods and is intended to encourage chemists to exploit the potential of these approaches in organic synthesis.