Silylboronic esters bearing a N,N-dialkylaminomethyl group on the silicon atoms were synthesized and used in palladium-catalyzed reactions. Five silylboronic esters were synthesized through a reaction of the [(N,N-dialkylaminomethyl)diorganosilyl]lithiums with i-PrOB(pin). An addition reaction of the silylboronic ester to ethynylbenzene (silaboration) took place efficiently by a phosphine- and isocyanide-free palladium catalyst to give (E)-1-silyl-2-boryl-1-phenylethene in a regio- and stereoselective manner. On the other hand, the silylboronic esters underwent β-elimination reactions in the presence of a palladium catalyst and styrene to afford 1,3-disilacyclobutanes and aminoboranes in good yields. This may be due to the generation of silene intermediates during the reaction. It should be noted that use of styrene as a ligand was essential for the generation of the silene species.

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.

AbstractHighly enantioselective cycloisomerization of N-methylanilines, bearing o-alkenyl groups, into indolines is established. An iridium catalyst bearing a bidentate chiral diphosphine effectively promotes the intramolecular addition of the C(sp3)−H bond across a carbon–carbon double bond in a highly enantioselective fashion. The reaction gives indolines bearing a quaternary stereogenic carbon center at the 3-position. The reaction mechanism involves rate-determining oxidative addition of the N-methyl C−H bond, followed by intramolecular carboiridation and subsequent reductive elimination.

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, n-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.

A 4,4′-bipyridine-based catalyst system for diboration of pyrazine derivatives was established. The catalyst cycle consists of the following two steps: (1) reductive addition of the boron–boron bond of bis(pinacolato)diboron to 4,4′-bipyridine to form N,N′-diboryl-4,4′-bipyridinylidene and (2) oxidative boryl transfer from the intermediate to pyrazine to give N,N′-diboryl-1,4-dihydropyrazine with regeneration of 4,4′-bipyridine.

1-Alkyl-2-methylenecyclopropanes react with silylboronic esters under mild conditions in the presence of a phosphine-free platinum catalyst, giving 3-substituted 2-boryl-4-silyl-1-butenes through selective cleavage of the less hindered proximal C–C bond of the cyclopropane ring. The steric bulk of the silyl group of the silylboronic esters was critical for efficient formation of the silaboration products, and i-PrPh2Si–B(pin) was developed as a silylboronic ester of choice.Keywords: C−C bond cleavage; homogeneous catalysis; methylenecyclopropane; organoboron compound; organosilicon compound; platinum catalyst; selectivity; silaboration

The C(sp3)–H bonds located on the methyl groups of an isopropyl group participate in iridium-catalysed C–H borylation with bis(pinacolato)diboron via a significant rate acceleration caused by a catalytic amount of t-BuOK.

Reaction of N-alkylisoindoles with (bromoethynyl)triisopropylsilane afforded 1,3-bis(triisopropylsilylethynyl) isoindoles in high yields. The formal C–H alkynylation proceeds under transition-metal-free conditions through [4 + 2] cycloaddition of the pyrrole ring of isoindole with bromoalkyne followed by ring-opening of the product.

Cyclic alkenylborates have been synthesized regioselectively via a palladium-catalyzed regioselective silaboration of terminal alkynes with ClMe2Si–B(pin) followed by basic hydrolysis. The cyclic borates undergo cross coupling with 4-iodoanisole in the absence of an external base.

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.

Pyridine undergoes addition of pinacolborane at 50 °C in the presence of a rhodium catalyst, giving N-boryl-1,2-dihydropyridine in a high yield. The selective 1,2-hydroboration also takes place in the reactions of substituted pyridines. In the reaction of 3-substituted pyridines, 3-substituted N-boryl-1,2-dihydropyridines are formed regioselectively.

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.

Pyrazines and substituted pyrazines undergo addition of boron reagents having B–B, Si–B, and H–B bonds under transition-metal-free conditions, leading to high-yield synthesis of N-borylated 1,4-dihydropyrazines and 1,2,3,4-tetrahydropyrazine.

The addition of silylboronic esters to pyridine takes place in toluene at 50 °C in the presence of a palladium catalyst to give N-boryl-4-silyl-1,4-dihydropyridines in high yield. The regioselective 1,4-silaboration also proceeds in the reaction of 2-picoline and 3-substituted pyridines, whereas 4-substituted pyridines undergo 1,2-silaboration to give N-boryl-2-silyl-1,2-dihydropyridines regioselectively.

The stereochemical course of the stereospecific Suzuki–Miyaura coupling of enantioenriched α-(acetylamino)benzylboronic esters with aryl bromides can be switched by the choice of acidic additives in the presence of a Pd/XPhos catalyst system. Highly enantiospecific, invertive C–C bond formation takes place with the use of phenol as an additive. In contrast, high enantiospecificity for retention of configuration is attained in the presence of Zr(Oi-Pr)4·i-PrOH as an additive.

The palladium-catalyzed silylene transfer to 2-alkenylindoles from a silylboronic ester bearing a diethylamino group on the silicon atom takes place efficiently, resulting in the formation of a 1-sila-3-cyclopentene ring fused with the indole ring. Further silylene transfer proceeds in the reaction of 2-(1-alkenyl)indoles, giving bissilylated tricyclic indoles in high yields.

The regioselectivity in the addition of silylboronic esters to terminal alkynes can be switched by the choice of phosphorus ligands on the palladium catalysts. The silaboration proceeds with normal regisoselectivity in the presence of (η3-C3H5)Pd(PPh3)Cl (1.0 mol %) to give 1-boryl-2-silyl-1-alkenes in high yields. In sharp contrast, selective formation of the inverse regioisomers, 2-boryl-1-silyl-1-alkenes, takes place when the reaction is carried out with a palladium catalyst bearing P(t-Bu)2(biphenyl-2-yl). A reaction mechanism for the change of regioselectivity that involves reversible insertion/β-boryl elimination steps is proposed.

A nickel-catalyzed intermolecular hydroacylation of methylenecyclopropanes (MCPs) has been developed. The reaction proceeds with stereospecific cleavage of the proximal C−C bond of the cyclopropane ring to give γ,δ-unsaturated ketones with high diastereoselectivities. A nickel catalyst generated in situ from Ni(cod)2 and P(n-Bu)3 with a P/Ni ratio of 1:1 is effective for the hydroacylation, in which benzaldehyde derivatives, heteroaryl aldehydes, and aliphatic aldehydes react with MCPs at 60−100 °C to afford the corresponding ketones in high yields.

1-Boryl- and 1,3-diborylisoindoles were synthesized by the palladium-catalyzed reaction of isoindolines with pinacolborane via sequential dehydrogenation and C−H borylation. Selective formation of the borylated isoindoles was achieved using Pd(dba)2/Me2S as the catalyst, in which Me2S suppressed the formation of 1-boryl-4,5,6,7-tetrahydroisoindoles. Diborylisoindole was cross-coupled with iodobenzene, giving 1,3-diphenylisoindole in high yield.

The first invertive B-alkyl Suzuki−Miyaura coupling has been achieved. The coupling of enantioenriched α-(acylamino)benzylboronic esters with aryl bromides and chlorides took place efficiently in toluene at 80 °C in the presence of Pd(dba)2 (5 mol %), XPhos (10 mol %), K2CO3 (3 equiv), and H2O (2 equiv). The reaction proceeded with inversion of configuration to give diarylmethanamine derivatives in high yields with high conservation of enantiomeric excesses.

Pyrazines and substituted pyrazines undergo addition of boron reagents having B–B, Si–B, and H–B bonds under transition-metal-free conditions, leading to high-yield synthesis of N-borylated 1,4-dihydropyrazines and 1,2,3,4-tetrahydropyrazine.

The C(sp3)–H bonds located on the methyl groups of an isopropyl group participate in iridium-catalysed C–H borylation with bis(pinacolato)diboron via a significant rate acceleration caused by a catalytic amount of t-BuOK.