Au nanoparticle-catalyzed double hydrosilylation of nitriles by diethylsilane.

We present the first example of Au-catalyzed reduction of nitriles into primary amines. In contrast to monohydrosilanes which are completely unreactive, diethylsilane (a dihydrosilane) is capable of reducing aryl or alkyl nitriles into primary amines under catalysis by Au nanoparticles supported on TiO, a smooth double hydrosilylation pathway. The produced labile -disilylamines are readily deprotected by HCl in EtO to form the hydrochloric salts of the corresponding amines in very good to excellent yields.

Pd Nanoparticle-Catalyzed Stereospecific Mizoroki-Heck Arylation of -1,2-Disilylarylethylenes.

In the presence of catalytic amounts of Pd nanoparticles, generated from Pddba/Ag(I), -1,2-ditrimethylsilylarylethylenes undergo with aryl iodides a stereospecific Mizoroki-Heck arylation leading to -ditrimethylsilyldiarylethylenes. This chemoselectivity is in contrast to that of their trimethylgermyl analogues, which are arylated at the position of the C-Ge bonds. -1,2-Ditrimethylsilylarylethylenes are completely unreactive under the standard reaction conditions.

Substitution-Dependent Ring-Opening Hydrosilylation or Dehydrogenative Hydrosilylation of Cyclopropyl Aldehydes and Ketones Catalyzed by Au Nanoparticles.

The reaction between hydrosilanes and aryl-substituted cyclopropyl aldehydes or ketones catalyzed by Au nanoparticles supported on TiO provides two distinct ring-opening reaction motifs depending on the substituents. 2-Aryl-substituted cyclopropyl carbonyl compounds form linear enol ethers via formal silyl hydride addition on the carbon atom bearing the aryl group. Under the reaction conditions, the hydrosilylation adducts undergo hydrolytic silyl deprotection to form acyclic aldehydes or ketones.

Synthesis of γ-Pyrones and -Methyl-4-pyridones via the Au Nanoparticle-Catalyzed Cyclization of Skipped Diynones in the Presence of Water or Aqueous Methylamine.

Supported Au nanoparticles on TiO catalyze the hydration/6- cyclization of skipped diynones to γ-pyrones in aqueous dioxane, via triple bond activation. The isomeric 3(2)-furanones which could be formed through a competing and often prevailing 5- cyclization pathway using homogeneous ionic Au(I) catalysts were not seen. The reaction does not proceed via the initial 1,3-transposition of the skipped diynones to their corresponding conjugated 1,3-diynone isomers.

Synthesis of Vinylgermanes via the Au/TiO-Catalyzed -1,2-Digermylation of Alkynes and the Regioselective Hydrogermylation of Allenes.

In the presence of Au/TiO (1 mol %), terminal alkynes react quantitatively with stoichiometric amounts of the unactivated digermane MeGe-GeMe, forming exclusively -1,2-digermylated alkenes. We also establish the Au/TiO-catalyzed hydrogermylation of terminal allenes with EtGeH, which exhibits a highly regioselective mode of addition on the more substituted double bond forming vinylgermanes. Additionally, we provide preliminary results regarding the Pd nanoparticle-catalyzed C-C coupling of 1,2-digermyl alkenes with aryl iodides.

Reduction of the Diazo Functionality of α-Diazocarbonyl Compounds into a Methylene Group by NHBH or NaBH Catalyzed by Au Nanoparticles.

Supported Au nanoparticles on TiO (1 mol%) are capable of catalyzing the reduction of the carbene-like diazo functionality of α-diazocarbonyl compounds into a methylene group [C=(N) → CH] by NHBH or NaBH in methanol as solvent. The Au-catalyzed reduction that occurs within a few minutes at room temperature formally requires one hydride equivalent (B-H) and one proton that originates from the protic solvent. This pathway is in contrast to the Pt/CeO-catalyzed reaction of α-diazocarbonyl compounds with NHBH in methanol, which leads to the corresponding hydrazones instead.

Isomerization of Epoxides into Allylic Alcohols Catalyzed by Supported Au Nanoparticles on TiO: A Mechanistic Insight.

Based on primary and β-secondary isotope effects, it is established that the isomerization of epoxides into allylic alcohols catalyzed by supported Au nanoparticles proceeds via an unsymmetrical concerted transition state in which there is partial positive charge developing on the C-O carbon atom progressing C-H abstraction. A carbocationic intermediate can be formed only in cases of sufficient stabilization of the positive charge by appropriate substituents.

Cycloisomerization of Conjugated Allenones into Furans under Mild Conditions Catalyzed by Ligandless Au Nanoparticles.

Au nanoparticles supported on TiO (1 mol %) catalyze the quantitative cycloisomerization of conjugated allenones into furans under very mild conditions. The reaction rate is accelerated by adding acetic acid (1 equiv), but the acid does not participate in the protodeauration step as in the corresponding Au(III)-catalyzed transformation. The process is purely heterogeneous, allowing thus the recycling and reuse of the catalyst effectively in several runs.

Supported Au Nanoparticles-Catalyzed Regioselective Dehydrogenative Disilylation of Allenes by Dihydrosilane.

Supported Au nanoparticles on TiO catalyze the unprecedented dehydrogenative disilylation of monosubstituted and 1,1-disubstituted allenes by Et SiH exclusively on the terminal double bond in a stereoselective manner. Treatment of the disilylation products with H O, in a one-pot operation also catalyzed by Au/TiO , leads to 3-alkylidene-1,2,5-oxadisilolanes, an unknown class of heterocyclic compounds, which are excellent scaffolds for the stereoselective synthesis of alkenes under Hiyama-type cross-coupling conditions.

Reaction of Aromatic Carbonyl Compounds with Silylborane Catalyzed by Au Nanoparticles: Silylative Pinacol-type Reductive Dimerization via a Radical Pathway.

Aromatic aldehydes and acetophenones undergo silylative pinacol-type reductive dimerization in their reaction with silylborane pinB-SiMePh (pin: pinacolato) catalyzed by supported Au nanoparticles on TiO. It is proposed that after initial activation of silylborane by Au nanoparticles and addition to the carbonyl functionality of an aromatic aldehyde or ketone, an aryl silyloxy radical is generated from the collapse of the intermediate adduct, which then dimerizes through a chain process. The silyloxy radical was almost quantitatively trapped in the presence of TEMPO.

Au Nanoparticle-Catalyzed Insertion of Carbenes from α-Diazocarbonyl Compounds into Hydrosilanes.

Supported Au nanoparticles on TiO catalyze the insertion of carbenes from α-diazocabonyl compounds into hydrosilanes. It is proposed that the transformation involves two modes of catalytic activation: formation of nucleophilic Au carbenes on the surface of nanoparticle via expulsion of N and activation of the Si-H bond of hydrosilane on Au nanoparticle, followed by coupling of the chemisorbed species. No external ligands or additives are required, while the process is purely heterogeneous, thus allowing the recycling and reuse of the catalyst.

Synthesis of Formate Esters and Formamides Using an Au/TiO₂-Catalyzed Aerobic Oxidative Coupling of Paraformaldehyde.

A simple method for the synthesis of formate esters and formamides is presented based on the Au/TiO₂-catalyzed aerobic oxidative coupling between alcohols or amines and formaldehyde. The suitable form of formaldehyde is paraformaldehyde, as cyclic trimeric 1,3,5-trioxane is inactive. The reaction proceeds via the formation of an intermediate hemiacetal or hemiaminal, respectively, followed by the Au nanoparticle-catalyzed aerobic oxidation of the intermediate.

Total Synthesis and Structural Revision of (+)-Yaoshanenolide B.

(+)-Yaoshanenolide B was synthesized employing as a key step an endo- and face-selective Diels-Alder reaction between natural R-(-)-α-phellandrene and the exocyclic double bond of a 5-methylene-2(5H)-furanone. The dienophile furanone was prepared by photooxygenation of a suitably substituted 2-thiophenylfuran followed by dehydration of the resulting γ-hydroxybutenolide. Through this synthesis, the initially proposed structure for (+)-yaoshanenolide B has been revised to the 1R,2S,4R,7R,1″S diastereomer.

Regiocontrolled Synthesis of γ-Hydroxybutenolides via Singlet Oxygen-Mediated Oxidation of 2-Thiophenyl Furans.

Photooxygenation of 2-thiophenyl-substituted furans in ethanol leads to the rapid, regiocontrolled, and quantitative synthesis of γ-hydroxybutenolides. The carbonyl group in butenolide holds the position of thiophenyl moiety in reacting furans. Decomposition of the initially formed [4 + 2] endoperoxide into products through a radical chain mechanism is proposed, as the fate of thiophenyl moiety is its transformation into ethyl phenylsulfenate (PhS-OEt) and diphenyldisulfide.

Ligandless Regioselective Hydrosilylation of Allenes Catalyzed by Gold Nanoparticles.

The first example of Au-catalyzed hydrosilylation of allenes is presented using recyclable gold nanoparticles as catalyst, without the requirement of any external ligands or additives. The hydrosilane addition takes place on the more substituted double bond of terminal allenes in a highly regioselective manner. The observed regioselectivity/reactivity modes are attributed to steric and electronic factors.

cis-Semihydrogenation of alkynes with amine borane complexes catalyzed by gold nanoparticles under mild conditions.

Supported gold nanoparticles catalyze the semihydrogenation of alkynes to alkenes with ammonia borane or amine borane complexes in excellent yields and under mild conditions. Internal alkynes provide cis-alkenes, making this protocol an attractive alternative of the classical Lindlar's hydrogenation.

Furans and singlet oxygen--why there is more to come from this powerful partnership.

The purpose of this article is to give a taste of just how powerful the union between furans and photochemically-generated singlet oxygen is proving to be as a synthetic tool and to suggest that this chemistry is only now really coming of age. In attempting to achieve this goal, its progress from mechanistic curiosity to rapidly maturing applied science will be followed. It will be shown how the field has reached a point where the diversity of product structures attainable is expanding all the time at a tremendous pace and how this expansion allows for a wide variety of important developments from the discovery of new materials and methods for DNA-crosslinking, to the delineation of more sustainable synthetic technologies.

Nanogold-catalyzed cis-silaboration of alkynes with abnormal regioselectivity.

The first example of gold-catalyzed silaboration of alkynes with PhMe2SiBpin is documented in the presence of supported gold nanoparticles. In the case of terminal alkynes, the reaction proceeds at ambient conditions in very good yields and the regioselectivity is opposite to that observed in the presence of Pd or Pt catalysts. The abnormal regioselectivity is attributed to steric factors imposed by the Au nanoparticle during the 1,2-addition of silylborane to the alkyne.

Supported gold nanoparticle-catalyzed cis-selective disilylation of terminal alkynes by σ disilanes.

Supported gold nanoparticles on metal oxides (1 mol %) catalyze for the first time the cis-selective disilylation of terminal alkynes by 1,2-disilanes in isolated yields up to 94%. It is likely that the reaction proceeds through oxidative insertion of the σ Si-Si bond of disilanes on gold followed by 1,2-addition to the alkyne.

Aryl-substituted cyclopropyl acetylenes as sensitive mechanistic probes in the gold-catalyzed hydration of alkynes. Comparison to the Ag(I)-, Hg(II)-, and Fe(III)-catalyzed processes.

The gold-catalyzed hydration of 2-phenyl- or 2,2-diphenylcyclopropyl acetylene, sensitive probes to trace the formation of vinyl carbocations, provides exclusively the corresponding cyclopropyl methyl ketones. On the other hand, in the Ag(I)- or Fe(III)-catalyzed hydration, a profound vinyl carbocationic character appears in the initially formed metal-alkyne complexes, as judged by the partial (Ag(+)) or exclusive (Fe(3+)) formation of allene-type rearrangement products. These findings provide clear evidence for subtle electronic differences in metal-alkyne complexes, including Au(I or III), Ag(I), Fe(III), and Hg(II).

Gold nanoparticles-catalyzed activation of 1,2-disilanes: hydrolysis, silyl protection of alcohols and reduction of tert-benzylic alcohols.

Gold nanoparticles supported on TiO(2) catalyze under mild conditions the activation of a series of 1,2-disilanes towards hydrolysis and alcoholysis, with concomitant evolution of H(2) gas. For the case of tert-benzyl alcohols, the main or only pathway is reduction to the corresponding alkanes.

Cyclization of 1,6-enynes catalyzed by gold nanoparticles supported on TiO2: significant changes in selectivity and mechanism, as compared to homogeneous Au-catalysis.

Gold nanoparticles supported on TiO(2) (1.2 mol %) catalyze, for the first time under heterogeneous conditions, the cycloisomerization of a series of 1,6-enynes in high yields. In several cases, the product selectivity differs significantly as compared to homogeneous Au(I)-catalysis.

Functionalized 3(2H)-furanones via photooxygenation of (β-keto)-2-substituted furans: application to the biomimetic synthesis of merrekentrone C.

Photooxygenation of (β-keto)-2-substituted furans leads, in a one pot operation, to functionalized 3(2H)-furanones with good to excellent yields. This methodology was applied as a key-step to the concise and biomimetic synthesis of the sesquiterpene merrekentrone C. The precursor to merrekentrone C, keto difuran, was synthesized using a cross coupling of α-iodo-3-acetylfuran with an alkenyl furan under Fenton-type conditions.