Electron crystallography of chiral and non-chiral small molecules.

Electron crystallography has recently gained attentions in multiple fields of research, as it has been demonstrated to determine atomic structures for inorganic, organic, and macromolecular materials from nano-sized crystals that were not amenable to conventional X-ray crystallography. Here, we demonstrate continuous-rotation microcrystal electron diffraction (microED) in a 200 kV transmission electron microscope using a DE-64 camera-a low-noise direct electron detector that can accommodate a linear response up to ∼1200 electrons per pixel per second at 20 fps with 2x-hardware-binning, making it ideal for acquisition of high-quality diffraction patterns. We have used this method and camera to determine a 0.75 Å structure of an organic molecule, biotin, with an exceptional goodness-of-fit, as well as a 0.88 Å structure of a chiral molecule, L-serine.

Intermolecular sp3-C-H Amination for the Synthesis of Saturated Azacycles.

The preparation of substituted azetidines and larger ring, nitrogen-containing saturated heterocycles is enabled through efficient and selective intermolecular sp3-C-H amination of alkyl bromide derivatives. A range of substrates are demonstrated to undergo C-H amination and subsequent sulfamate alkylation in good to excellent yield. N-Phenoxysulfonyl-protected products can be unmasked under neutral or mild basic conditions to yield the corresponding cyclic secondary amines. The preparative convenience of this protocol is demonstrated through gram-scale and telescoped multistep procedures. Application of this technology is highlighted in a nine-step total synthesis of an unusual azetidine-containing natural product, penaresidin B.

Alkali Metal-Hydroxide-Catalyzed C(sp)-H Bond silylation.

Disclosed is a mild, scalable, and chemoselective catalytic cross-dehydrogenative C-H bond functionalization protocol for the construction of C(sp)-Si bonds in a single step. The scope of the alkyne and hydrosilane partners is substantial, providing an entry point into various organosilane building blocks and additionally enabling the discovery of a number of novel synthetic strategies. Remarkably, the optimal catalysts are NaOH and KOH.

Sodium Hydroxide Catalyzed Dehydrocoupling of Alcohols with Hydrosilanes.

An O-Si bond construction protocol employing abundantly available and inexpensive NaOH as the catalyst is described. The method enables the cross-dehydrogenative coupling of an alcohol and hydrosilane to directly generate the corresponding silyl ether under mild conditions and without the production of stoichiometric salt byproducts. The scope of both coupling partners is excellent, positioning the method for use in complex molecule and materials science applications. A novel Si-based cross-coupling reagent is also reported.

Catalytic C-H bond silylation of aromatic heterocycles.

This protocol describes a method for the direct silylation of the carbon-hydrogen (C-H) bond of aromatic heterocycles using inexpensive and abundant potassium tert-butoxide (KOt-Bu) as the catalyst. This catalytic cross-dehydrogenative coupling of simple hydrosilanes and various electron-rich aromatic heterocycles enables the synthesis of valuable silylated heteroarenes. The products thus obtained can be used as versatile intermediates, which facilitate the divergent synthesis of pharmaceutically relevant compound libraries from a single Si-containing building block. Moreover, a variety of complex Si-containing motifs, such as those produced by this protocol, are being actively investigated as next-generation therapeutic agents, because they can have improved pharmacokinetic properties compared with the original all-carbon drug molecules. Current competing methods for C-H bond silylation tend to be incompatible with functionalities, such as Lewis-basic heterocycles, that are often found in pharmaceutical substances; this leaves de novo synthesis as the principal strategy for preparation of the target sila-drug analog. Moreover, competing methods tend to be limited in the scope of hydrosilane that can be used, which restricts the breadth of silicon-containing small molecules that can be accessed. The approach outlined in this protocol enables the chemoselective and regioselective late-stage silylation of small heterocycles, including drugs and drug derivatives, with a broad array of hydrosilanes in the absence of precious metal catalysts, stoichiometric reagents, sacrificial hydrogen acceptors or high temperatures. Moreover, H2 is the only by-product generated. The procedure normally requires 48-75 h to be completed.

Silylation of C-H bonds in aromatic heterocycles by an Earth-abundant metal catalyst.

Heteroaromatic compounds containing carbon-silicon (C-Si) bonds are of great interest in the fields of organic electronics and photonics, drug discovery, nuclear medicine and complex molecule synthesis, because these compounds have very useful physicochemical properties. Many of the methods now used to construct heteroaromatic C-Si bonds involve stoichiometric reactions between heteroaryl organometallic species and silicon electrophiles or direct, transition-metal-catalysed intermolecular carbon-hydrogen (C-H) silylation using rhodium or iridium complexes in the presence of excess hydrogen acceptors. Both approaches are useful, but their limitations include functional group incompatibility, narrow scope of application, high cost and low availability of the catalysts, and unproven scalability. For this reason, a new and general catalytic approach to heteroaromatic C-Si bond construction that avoids such limitations is highly desirable. Here we report an example of cross-dehydrogenative heteroaromatic C-H functionalization catalysed by an Earth-abundant alkali metal species. We found that readily available and inexpensive potassium tert-butoxide catalyses the direct silylation of aromatic heterocycles with hydrosilanes, furnishing heteroarylsilanes in a single step. The silylation proceeds under mild conditions, in the absence of hydrogen acceptors, ligands or additives, and is scalable to greater than 100 grams under optionally solvent-free conditions. Substrate classes that are difficult to activate with precious metal catalysts are silylated in good yield and with excellent regioselectivity. The derived heteroarylsilane products readily engage in versatile transformations enabling new synthetic strategies for heteroaromatic elaboration, and are useful in their own right in pharmaceutical and materials science applications.