N-Heterocyclic Carbene Boranes: Dual Reagents for the Synthesis of Gold Nanoparticles.

N-Heterocyclic carbenes (NHCs) have drawn considerable interest in the field of nanomaterials chemistry as highly stabilizing ligands enabling the formation of strong and covalent carbon-metal bonds. Applied to gold nanoparticles synthesis, the most common strategy consists of the reduction of a preformed NHC-AuI complex with a large excess of a reducing agent that makes the particle size difficult to control. In this paper, we report the straightforward synthesis of NHC-coated gold nanoparticles (NHC-AuNPs) by treating a commercially available gold(I) precursor with an easy-to-synthesize NHC-BH3 reagent. The latter acts as both the reducing agent and the source of surface ligands operating under mild conditions. Mechanistic studies including NMR spectroscopy and mass spectrometry demonstrate that the reduction of gold(I) generates NHC-BH2 Cl as a by-product. This strategy gives efficient control over the nucleation and growth of gold particles by varying the NHC-borane/gold(I) ratio, allowing unparalleled particle size variation over the range of 4.9±0.9 to 10.0±2.7 nm. Our strategy also allows an unprecedented precise and controlled seeded growth of gold nanoparticles. In addition, the as-prepared NHC-AuNPs exhibit narrow size distributions without the need for extensive purification or size-selectivity techniques, and are stable over months.

Organopotassium-Catalyzed Silylation of Benzylic C(sp3 )-H Bonds.

Benzylsilanes have found increasing applications in organic synthesis as bench-stable synthetic intermediates, yet are mostly produced by stoichiometric procedures. Catalytic alternatives based on the atom-economical silylation of benzylic C(sp3 )-H bonds remain scarcely available as specialized directing groups and catalytic systems are needed to outcompete the kinetically-favored silylation of C(sp2 )-H bonds. Herein, we describe the first general and catalytic-in-metal undirected silylation of benzylic C(sp3 )-H bonds under ambient, transition metal-free conditions using stable tert-butyl-substituted silyldiazenes (tBu-N=N-SiR3 ) as silicon source. The high activity and selectivity of the catalytic system, exemplified by the preparation of various mono- or gem-bis benzyl(di)silanes, originates from the facile generation of organopotassium reagents, including tert-butylpotassium.

Catalytic Disproportionation of Formic Acid to Methanol by using Recyclable Silylformates.

A novel strategy to prepare methanol from formic acid without an external reductant is presented. The overall process described herein consists of the disproportionation of silyl formates to methoxysilanes, catalyzed by ruthenium complexes, and the production of methanol by simple hydrolysis. Aqueous solutions of MeOH (>1 mL, >70 % yield) were prepared in this manner. The sustainability of the reaction has been established by recycling of the silicon-containing by-products with inexpensive, readily available, and environmentally benign reagents.

Autocatalytic Carbonyl Arylation through In Situ Release of Aryl Nucleophiles from N-Aryl-N'-Silyldiazenes.

A method for the catalytic generation of functionalized aryl alkali metals is reported. These highly reactive intermediates are liberated from silyl-protected aryl-substituted diazenes by the action of Lewis basic alkali metal silanolates, resulting in desilylation and loss of N2 . Catalytic quantities of these Lewis bases initiate the transfer of the aryl nucleophile from the diazene to carbonyl and carboxyl compounds with superb functional-group tolerance. The aryl alkali metal can be decorated with electrophilic substituents such as methoxycarbonyl or cyano as well as halogen groups. The synthesis of a previously unknown cyclophane-like [4]arene macrocycle from a 1,3-bisdiazene combined with a 1,4-dialdehyde underlines the potential of the approach.

Silyl Formates as Surrogates of Hydrosilanes and Their Application in the Transfer Hydrosilylation of Aldehydes.

Silyl formates are investigated for the first time as surrogates of hydrosilanes. In the presence of a well-defined ruthenium catalyst, these reagents are shown to promote the chemoselective reduction of a variety of aldehydes by transfer hydrosilylation. Mechanistic investigations have shown that genuine hydrosilane species are avoided during catalysis. The mechanism involves a sequence of decarboxylation/insertion/transmetallation steps.

Fast and highly chemoselective alkynylation of thiols with hypervalent iodine reagents enabled through a low energy barrier concerted mechanism.

Among all functional groups, alkynes occupy a privileged position in synthetic and medicinal chemistry, chemical biology, and materials science. Thioalkynes, in particular, are highly useful, as they combine the enhanced reactivity of the triple bond with a sulfur atom frequently encountered in bioactive compounds and materials. Nevertheless, general methods to access these compounds are lacking. In this article, we describe the mechanism and full scope of the alkynylation of thiols using ethynyl benziodoxolone (EBX) hypervalent iodine reagents. Computations led to the discovery of a new, three-atom concerted transition state with a very low energy barrier, which rationalizes the high reaction rate. On the basis of this result, the scope of the reaction was extended to the synthesis of aryl- and alkyl-substituted alkynes containing a broad range of functional groups. New sulfur nucleophiles such as thioglycosides, thioacids, and sodium hydrogen sulfide were also alkynylated successfully to lead to the most general and practical method yet reported for the synthesis of thioalkynes.

Formoxyboranes as hydroborane surrogates for the catalytic reduction of carbonyls through transfer hydroboration.

A new class of Lewis base stabilized formoxyboranes demonstrates the feasibility of catalytic transfer hydroboration. In the presence of a ruthenium catalyst, they have shown broad applicability for reducing carbonyl compounds. Various borylated alcohols are obtained in high selectivity and yields up to 99%, tolerating several functional groups. Computational studies enabled to propose a mechanism for this transformation, revealing the role of the ruthenium catalyst and the absence of hydroborane intermediates.

Silyl formates as hydrosilane surrogates for the transfer hydrosilylation of ketones.

A transfer hydrosilylation of ketones employing silyl formates as hydrosilane surrogates under mild conditions is presented. A total of 24 examples of ketones have been successfully converted to their corresponding silyl ethers with 61-99% yields in the presence of a PNHP-based ruthenium catalyst and silyl formate reagent. The crucial role of the ligand for the transformation is demonstrated.

Silylation of O-H bonds by catalytic dehydrogenative and decarboxylative coupling of alcohols with silyl formates.

The silylation of O-H bonds is a useful methodology in organic synthesis and materials science. While this transformation is commonly achieved by reacting alcohols with reactive chlorosilanes or hydrosilanes, we show herein for the first time that silylformates HCO2SiR3 are efficient silylating agents for alcohols, in the presence of a ruthenium molecular catalyst.

Metal-free disproportionation of formic acid mediated by organoboranes.

In the presence of dialkylboranes, formic acid can be converted to formaldehyde and methanol derivatives without the need for an external reductant. This reactivity, in which formates serve as the sole carbon and hydride sources, represents the first example of the disproportionation of formate anions under metal-free conditions. Capitalizing on both experimental and computational (DFT) mechanistic considerations, the role of transient borohydride is highlighted in the reduction of formates and this reactivity was further exemplified in the methylation of TMP (2,2,6,6-tetramethylpiperidine) and in the transfer hydroboration reactions for the reduction of aldehydes.

Metal-free dehydrogenation of formic acid to H2 and CO2 using boron-based catalysts.

Formic acid is at the crossroads of novel sustainable energy strategies because it is an efficient H2 carrier. Yet, to date, its decomposition to H2 relies on metal-based catalysts. Herein, we describe the first metal-free catalysts able to promote the dehydrogenation of formic acid. Using dialkylborane derivatives, HCOOH is decomposed to H2 and CO2 in the presence of a base with high selectivity. Experimental and computational results point to the involvement of bis(formyloxy)borates as key intermediates in the C-H bond activation of a formate ligand.