Modular Approach to Heterogenous Catalysis. Manipulation of Cross-Coupling Catalyst Activity.

A new type of robust, heterogeneous, modular Pd catalyst with metal embedded in the gel matrix is presented. The regulatory element of its catalytic activity has been introduced via chemical changes in the gel. The concept is illustrated in a series of Suzuki-Miyaura cross-coupling reactions. The demonstrated catalyst activity variations depend on the structure of the gel.

On-demand degrafting and the study of molecular weight and grafting density of poly(methyl methacrylate) brushes on flat silica substrates.

We report on degrafting of surface-anchored poly(methyl methacrylate) (PMMA) brushes from flat silica-based substrates using tetrabutylammonium fluoride (TBAF) and determining their molecular weight distribution (MWD) using size exclusion chromatography (SEC). The grafted PMMA layer was synthesized using surface-initiated atom transfer radical polymerization (SI-ATRP) of MMA for polymerization times ranging from 6 to 24 h. X-ray photoelectron spectroscopy, ellipsometry, and time-of-flight secondary ion mass spectrometry were employed in tandem to characterize the degrafting process. The SEC eluograms were fit to various polymer distributions, namely Zimm-Schulz, ATRP in continuous stirred tank reactor, Wesslau, Schulz-Flory, and Smith et al. The ATRP model gives the best fit to the experimental data. The dry PMMA brush thickness and the number-average molecular weight (obtained from the MWD) suggest that the grafting density of the PMMA grafts is independent of polymerization time, indicating well-controlled/living growth of MMA. The observed polydispersity index (PDI) was higher than that generally observed in bulk grown polymers under similar conditions, indicating an effect due to chain confinement and crowding. We detect small but noticeable dependence of the polymer brush grafting density on the inhibitor/catalyst ratio. Higher inhibitor/catalyst ratio offers better control with lower early terminations, which results in a small increase in the apparent grafting density of the chains.

Direct Measurement of Molecular Weight and Grafting Density by Controlled and Quantitative Degrafting of Surface-Anchored Poly(methyl methacrylate).

We report on quantitative determination of the molecular weight distribution (MWD) and grafting density (σP) of polymer assemblies grown by controlled radical polymerization from flat substrates as a function of polymerization time and the ratio between the inhibitor and catalyst species. Specifically, we grow poly(methyl methacrylate) (PMMA) brushes on flat silica-based surfaces by surface-initiated atom transfer radical polymerization (SI-ATRP), cleave the PMMA grafts quantitatively using tetrabutyl ammonium fluoride (TBAF), and analyze their MWD by size exclusion chromatography equipped with a high-sensitivity differential refractive index detector. The polymer growth and degrafting processes are followed by ellipsometry, X-ray photoelectron spectroscopy, and time-of-flight secondary ion mass spectrometry. The σP is independent of polymerization time and increases with increasing SI-ATRP inhibitor/catalyst ratio. Specifically, σP increases from 0.48 ± 0.06 to 0.58 ± 0.06 chains/nm2 as the inhibitor/catalyst molar ratio increases from 0 to 0.015, respectively, providing evidence that high inhibitor/catalyst ratio offers better control of the SI-ATRP reaction, by lowering number of terminations, and leading to denser PMMA brush assemblies.

On the mechanism of the copper-mediated C-S bond formation in the intramolecular disproportionation of imine disulfides.

The mechanism of the copper-mediated disproportionation of aromatic imine disulfides to benzothiazoles in the gas phase is investigated by experimental and theoretical methods. Application of infrared multiphoton dissociation and hydrogen/deuterium exchange experiments combined with density functional theory (DFT) calculations of the relevant molecular structures and the associated infrared spectra allows the identification of the observed ionic intermediates. The theoretical investigation of the possible reaction pathways supported by collision-induced dissociation experiments provides a consistent mechanistic picture of the reaction catalyzed by a single copper(I) ion. Activation of the substrate proceeds via homolytic sulfur-sulfur bond cleavage, yielding metal complexes in the formal +3 oxidation state; carbon-sulfur coupling and hydrogen-atom transfer complete the transformation to the products. Exploratory studies demonstrate that in the gas phase, the disproportionation of the imine disulfide can also be mediated by other metal ions via different either homo- or heterolytic mechanisms without involving high-valent intermediates.

Metallothionein-inspired prototype of molecular pincer.

Study of Zn and Pb release from complexes with natural and synthetic amidothiol motifs inspired the design of a "molecular pincer" that scavenges quantitatively metals from liquid environment and releases them, on-demand, under very mild oxidative conditions.

Pd2+ and Cu2+ catalyzed oxidative cross-coupling of mercaptoacetylenes and arylboronic acids.

An oxidative thiolate scavenging concept in a metal catalyzed reaction is presented and demonstrated on the aerobic Pd and Cu catalyzed cross-coupling of mercaptoacetylenes with arylboronic acids. Synthetic value of the chemistry as the complementary tool to the Sonogashira protocol has been demonstrated on a series of functionalized mercaptoacetylene substrates.

Cu and Ag catalyzed oxidative arylthiation of terminal acetylenes.

A mild Cu or Ag catalyzed oxidative arylthiation of terminal acetylenes is introduced. The process, featuring metal catalyzed C-H bond activation as a key step, leads to the formation of highly substituted mercaptoacetylenes under unprecedented neutral conditions.

Ascorbate mediated copper catalyzed reductive cross-coupling of disulfides with aryl iodides.

The concept of using ascorbic acid as a mediator/ reducing agent in a Cu(I) catalyzed process is introduced and further demonstrated on a cross-coupling reaction of aryl iodides with disulfides.

Mechanistic insights into a copper-disulfide interaction in oxidation of imines by disulfides.

The concept of using disulfides as an oxidant for Cu(i) is introduced as part of a Cu-catalyzed process leading to the formation of benzothiazole from an iminodisulfide under an inert atmosphere.

Copper/ascorbic acid dyad as a catalytic system for selective aerobic oxidation of amines.

A new, environmentally benign system based on an ascorbic acid (vitamin C)/copper dyad is presented, which facilitates the oxidation of amines to carbonyl compounds. The oxidation utilizes aerobic oxygen as the ultimate electron scavenger, which accepts electrons in a cascade-like fashion via a Cu(+)/Cu(2+) and ascorbic/dehydroascorbic acid couple. Dehydroascorbic acid, the oxidation agent, finally oxidizes amine substrates in a selective fashion after forming a Schiff base intermediate. The selectivity as well as generality of the method is demonstrated on various substrates.

Thioimides: new reagents for effective synthesis of thiolesters from carboxylic acids.

Thioimides and carboxylic acids are used as the precursors for the convenient synthesis of thiolesters in the phosphine mediated process. Cyclic and acyclic thioimides show equal efficiency, furnishing the desired thiolesters in good to excellent yields. The general, highly efficient transformation tolerates various functional groups and the resulting thiolesters are obtained in high purity after a simple separation. The reaction scope has been demonstrated on the preparation of several highly functionalized target molecules.

A new paradigm for carbon-carbon bond formation: aerobic, copper-templated cross-coupling.

Thiol esters and boronic acids react to produce ketones under aerobic conditions in the presence of catalytic quantities of a CuI or CuII salt. The reaction occurs at reasonable rates between room temperature and 50 degrees C at neutral pH using thiol esters derived from bulky 2 degrees amides of thiosalicylamides such as those based on N-tert-butyl-2-mercaptobenzamide. In this mechanistically unprecedented reaction system, the carbon-carbon bond formation occurs through templating of the thiol ester and the boronic acid at copper; the system is rendered catalytic in copper under the aerobic conditions.

Cu(I)-mediated reductive amination of boronic acids with nitroso aromatics.

[reaction: see text] A mild method for the reductive amination of aryl boronic acids with nitroso aromatic compounds is reported. This C-N bond formation is mediated by a stoichiometric amount of CuCl as both a catalyst and a reducing agent. Alternatively, 10% Cu(I)-3-methylsalicylate (CuMeSal) catalyzes the same reaction in the presence of either ascorbic acid or hydroquinone as the terminal reducing agent. Diarylamines bearing a variety of functional groups can be obtained in good yields.

Switchable catalysis: modular synthesis of functionalized Pyrimidinones via selective sulfide and halide cross-coupling chemistry.

[reaction: see text] A general demonstration of orthogonal selectivity of the Liebeskind-Srogl cross-coupling protocol compared to the Suzuki-Miyaura and Stille variants is reported.

Ketone synthesis under neutral conditions. Cu(I) diphenylphosphinate-mediated, palladium-catalyzed coupling of thiol esters and organostannanes.

[reaction: see text] A versatile approach to ketone synthesis is described. The reaction relies on the palladium-catalyzed, copper diphenylphosphinate-mediated coupling of thiol esters with organostannanes under neutral reaction conditions. This reaction complements the previously described coupling of thiol esters with boronic acids that used dual thiophilic-borophilic activation methodology.

A mild, nonbasic synthesis of thioethers. The copper-catalyzed coupling of boronic acids with N-thio(alkyl, aryl, heteroaryl)imides.

[reaction: see text] A new synthesis of thioethers is described. The reaction of boronic acids with aryl, heteroaryl, and alkyl N-thioimides in the presence of catalytic quantities of a Cu(I) carboxylate affords good to excellent yields of thioethers. This reaction takes place in the absence of a base under mild conditions (THF, 45-50 degrees C, 2.5-12 h) and represents an interesting complement to known methods for thioether synthesis.

Heteroaromatic thioether-boronic acid cross-coupling under neutral reaction conditions.

[reaction: see text] Pi-deficient heteroaromatic thioethers undergo efficient palladium-catalyzed cross-coupling with boronic acids mediated by copper(I) thiophene-2-carboxylate.

New Lewis-Acidic Molybdenum(II) and Tungsten(II) Catalysts for Intramolecular Carbonyl Ene and Prins Reactions. Reversal of the Stereoselectivity of Cyclization of Citronellal.

New Mo(II) complexes BnEt(3)N(+)[Mo(CO)(4)ClBr(2)](-) (A) and Mo(CO)(5)(OTf)(2) (B) and their W(II) congeners D and E have been developed as catalysts for the title reactions. Unlike other Lewis acids, the latter catalysts exhibit cis-stereoselectivity in the cyclization of citronellal (1 --> 3 with A and 1 --> 5 with B). Isotopic labeling allowed formulation of the reaction mechanism, according to which these complexes act as bulky Lewis acids, eta(1)-coordinated to the carbonyl oxygen. The stereochemistry appears to be controlled by the protruding ligand L(p), which dictates the boatlike transition state III. The kinetically formed cis-alkenol 3 can be equilibrated by [Mo(CO)(4)Br(2)](2) (C) or ZnCl(2) to its trans-epimer 2 via a retro-ene reaction.