Utilization of a Trimethylsilyl Group as a Synthetic Equivalent of a Hydroxyl Group via Chemoselective C(sp³)–H Borylation at the Methyl Group on Silicon

A conversion of trimethylsilylalkanes into the corresponding alcohols is established based on an iridium-catalyzed, chemoselective C­(sp3)–H borylation of the methyl group on silicon. The (boryl­methyl)­silyl group formed by C­(sp3)–H borylation is treated with H2O2/​NaOH, and the resulting (hydroxy­methyl)­silyl group is converted into a hydroxyl group by Brook rearrangement, followed by oxidation of the resulting methoxy­silyl group under Tamao conditions. An alternative route proceeding through the formylsilyl group formed from a (hydroxy­methyl)­silyl group by Swern oxidation is also established. The method is applicable to substituted trimethyl­silyl­cyclo­alkanes and 1,1-dimethyl-1-sila­cyclo­pentane for conversion into the corresponding stereodefined cycloalkyl alcohols and 1,4-butanediol

Functionalization of Tetraorganosilanes and Permethyloligosilanes at a Methyl Group on Silicon via Iridium-Catalyzed C(sp³)–H Borylation

In the presence of an iridium 3,4,7,8-tetramethyl-1,10-phenanthroline catalyst, a methyl group on the silicon atom of alkyltrimethylsilanes undergoes selective C–H borylation with bis­(pinacolato)­diboron in cyclooctane at 135 °C to give alkyl­(borylmethyl)­dimethylsilanes. The C–H borylation of tetramethylsilane takes place efficiently at 100 °C. Permethyloligosilanes can also undergo C–H borylation without cleavage of the Si–Si bonds

Decatungstate-Catalyzed C(sp³)–H Alkylation of a Val Residue Proximal to the N‑Terminus Controlled by an Electrostatic Interaction

The decatungstate photocatalyst [W10O32]4– efficiently promoted the C­(sp3)–H alkylation of the trifluoroacetic acid salt of valine methyl ester (H-Val-OMe·TFA) with electron-deficient alkenes under UV irradiation. The electrostatic interaction between the cationic ammonium group (+NH3) of the main chain and anionic [W10O32]4– played an important role in this reaction. The influence of various protected amino acids in the C­(sp3)–H alkylation was investigated as the model reaction for the alkylation of Val-containing peptides. The introduction of an alkyne moiety into Val through this alkylation was successful, and successive copper-catalyzed azide–alkyne cycloaddition (CuAAC) was demonstrated. The C­(sp3)–H bond of a Val residue located at the second from the N-terminus was also successfully converted. C­(sp3)–H alkylation of oligopeptides containing two Val residues selectively proceeded proximally to the N-terminus

3‑Position-Selective C–H Trifluoromethylation of Pyridine Rings Based on Nucleophilic Activation

The first example of the 3-position-selective C(sp2)–H trifluoromethylation of pyridine rings was established. 3-Position-selective trifluoromethylation was achieved by the nucleophilic activation of pyridine and quinoline derivatives through hydrosilylation and successive electrophilic trifluoromethylation of the enamine intermediate. This reaction was applicable to perfluoroalkylation at the 3 position of the pyridine rings and late-stage trifluoromethylation of a bioactive molecule. Mechanistic studies indicated that the reaction proceeds via the formation of N-silyl enamine and trifluoromethylated enamine intermediates

Control of Site-Selectivity in Hydrogen Atom Transfer by Electrostatic Interaction: Proximal-Selective C(sp³)–H Alkylation of 2‑Methylanilinium Salts Using a Decatungstate Photocatalyst

Site-selective C­(sp3)–H alkylation of 2-methylanilinium salts via radical intermediates was developed. The anionic decatungstate photocatalyst ([W10O32]4–) interacts with the ammonium group of the substrate through electrostatic interaction and selectively abstracts a hydrogen atom from the proximal benzylic carbon atom under UV irradiation. A variety of 2-methylanilinium salts reacted with electron-deficient alkenes. The alkylated product was successfully converted into an aryl iodide via cleavage of the C–N bond and a tetrahydro­benzoazepinone derivative by intramolecular cyclization. Mechanistic studies clearly show the existence of the interactions between [W10O32]4– and the ammonium group

Catalytic Functionalization of Methyl Group on Silicon: Iridium-Catalyzed C(sp³)–H Borylation of Methylchlorosilanes

A methyl group of methylchlorosilanes undergoes C–H borylation in an iridium-catalyzed reaction with bis­(pinacolato)­diboron in cyclohexane at 80 °C, giving (borylmethyl)­chlorosilanes selectively

Functionalization of Tetraorganosilanes and Permethyloligosilanes at a Methyl Group on Silicon via Iridium-Catalyzed C(sp³)–H Borylation

In the presence of an iridium 3,4,7,8-tetramethyl-1,10-phenanthroline catalyst, a methyl group on the silicon atom of alkyltrimethylsilanes undergoes selective C–H borylation with bis­(pinacolato)­diboron in cyclooctane at 135 °C to give alkyl­(borylmethyl)­dimethylsilanes. The C–H borylation of tetramethylsilane takes place efficiently at 100 °C. Permethyloligosilanes can also undergo C–H borylation without cleavage of the Si–Si bonds

A (Borylmethyl)silane Bearing Three Hydrolyzable Groups on Silicon: Synthesis via Iridium-Catalyzed C(sp³)–H Borylation and Conversion to Functionalized Siloxanes

An iridium-catalyzed C­(sp³)–H borylation of X₃SiMe (X = hydrolyzable group) was established. A trialkoxy­(methyl)­silane bearing sterically demanding neopentyloxy groups (X = neopentyloxy) underwent C–H borylation at the methyl group on silicon, giving (borylmethyl)­tris­(neopentyloxy)­silane in 70% isolated yield. The choice of the hydrolyzable group X was the key to efficient and chemoselective C–H borylation; trialkoxy­(methyl)­silanes bearing sterically less demanding alkoxy groups (X = ethoxy, <i>n</i>-butyloxy, and isobutyloxy) suffered from C–H activation at the alkoxy groups, and trichloro­(methyl)­silane (X = Cl) failed to react. A trimethylsiloxy group could substitute the neopentyloxy groups of the borylated product by the reaction of trimethylsilanol in the presence of tetrabutylammonium fluoride