Silylboronic Esters Research Articles

Copper-catalyzed silylation of aryl and alkenyl triflates with silylboronic esters avoiding base-mediated borylation.

Silylation of aryl and alkenyl triflates is found to occur readily with silylboronic esters as a silicon source under copper catalysis. The silyl moieties are exclusively installed into the organic frameworks through the preferential generation of a silylcopper species, wherein base-mediated direct borylation is totally suppressed. The combined use of tri-n-butylphosphine and 4,4'-diphenyl-2,2'-bipyridine as a ligand combination turned out to be indispensable for achieving the high catalytic activity.

Copper-Catalyzed Highly Enantioselective Addition of a Silicon Nucleophile to 3-Substituted 2H-Azirines Using an Si-B Reagent.

3-Substituted 2H-azirines can be considered strained cyclic ketimines, and highly enantioselective addition reactions of silicon nucleophiles to either acyclic or cyclic ketimines have been elusive so far. The present work closes this gap for those azirines by means of a copper-catalyzed silylation using a silyl boronic ester as a latent silicon nucleophile. The resulting C-silylated, unprotected (N-H) aziridines are obtained in high yields and with excellent enantioselectivities and can be further converted into valuable compounds with hardly any erosion of the enantiomeric excess.

Pd/Ming-Phos-Catalyzed Asymmetric Three-Component Arylsilylation of N-Sulfonylhydrazones: Enantioselective Synthesis of gem-Diarylmethine Silanes.

A Pd-catalyzed enantioselective three-component reaction of N-sulfonylhydrazones, aryl bromides, and silylboronic esters is developed, enabling the synthesis of chiral gem-diarylmethine silanes in high enantioselectivity with the use of a newly identified Ming-Phos. Compared with N-tosyl, the more easily decomposed N-mesitylsulfonyl is more suitable as the masking group of electron-rich hydrazone to improve the reaction efficiency. The reaction features a broad scope concerning both coupling partners, high enantioselectivity, and mild reaction conditions. The ready access to enantiomers and utility of this catalytic method are also presented.

Enantioselective, Copper-Catalyzed Addition of Nucleophilic Silicon to Alkenyl-Substituted Phosphine Oxides

AbstractAn enantioselective β-silylation of α,β-unsaturated phosphine oxide derivatives using a silylboronic ester as the silicon pronucleophile is reported. The reaction is catalyzed by copper salts in the presence of chiral pyridine–oxazoline (PyOx) ligands. Good to high enantioselectivities (≤95% ee) are obtained for β-aryl-substituted acceptors whereas alkyl residues in the β-position led to a lower ee value for 1° and no reaction for 2° and 3°. The new method represents another way of accessing α-chiral silanes and complements the known β-borylation.

HMPA-Free Generation of Trialkylsilyllithium Reagents and Its Applications to the Synthesis of Silylboronic Esters

AbstractTrialkylsilyllithium species have turned out to be facilely generated by treating hexaalkyldisilanes with methyllithium in the presence of tris(N,N-tetramethylene)phosphoric triamide (TPPA) as an activator, which can be trapped by boron electrophiles to afford silylboronic esters including long-awaited Me3Si–B(pin).

Copper-catalyzed regioselective trans-silaboration of internal arylalkynes with stereochemical switch to cis-addition mode.

Copper-catalyzed silafunctionalization of alkynes using a silylboronic ester as a silicon source has recently progressed rapidly. Generally, the reaction affords a product with cis-stereoselectivity. We herein describe trans-selective 1,2-addition of silylboronic esters to internal arylalkynes, which was promoted efficiently by the CuOt-Bu/RCy2P/NaOt-Bu catalysts. Moreover, we report a stereochemical switch to cis-addition in the reactions of Me2(i-PrO)Si-B(pin) in hydrocarbon solvents including cyclohexane.

Stereospecific and Chemoselective Copper-Catalyzed Deaminative Silylation of Benzylic Ammonium Triflates.

A method for the synthesis of benzylsilanes starting from the corresponding ammonium triflates is reported. Silyl boronic esters are employed as silicon pronucleophiles, and the reaction is catalyzed by copper(I) salts. Enantioenriched benzylic ammonium salts react stereospecifically through an SN2‐type displacement of the ammonium group to afford α‐chiral silanes with inversion of the configuration. A cyclopropyl‐substituted substrate does not undergo ring opening, thus suggesting an ionic reaction mechanism with no benzyl radical intermediate.

Pyridine‐Based Organocatalysts for Regioselective syn‐1,2‐Silaboration of Terminal Alkynes and Allenes

AbstractAn organocatalytic silaboration of terminal alkynes and allenes was established using pyridine‐based catalysts. In the presence of 4‐cyanopyridine (1–2 mol%), alkyl propiolates underwent regio‐ and stereoselective addition of silylboronic esters in toluene at 135 °C to afford (Z)‐3‐boryl‐2‐silylacrylates in good yields. 2,6‐Dichloro‐4,4′‐bipyridine and 4‐(3,5‐dichlorophenyl)pyridine also exhibited high catalyst efficiency for the 1,2‐silaboration of ethyl propiolate, whereas 1,1‐silaboration was induced by P(n‐Bu)3, t‐BuOK, and ICy to afford ethyl 3‐boryl‐3‐silylacrylates as Z/E mixtures. The silaboration of ethynylbenzenes and terminal allenes was also catalyzed by 4‐cyanopyridine to afford (Z)‐β‐boryl‐α‐silylstyrenes and β‐borylallylsilanes in a regioselective manner.

Palladium-Catalyzed β-Elimination of Aminoboranes from (Aminomethylsilyl)boranes Leading to the Formation of Silene Dimers

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.

Site- and Regioselective Silaborative C–C Cleavage of 1-Alkyl-2-Methylenecyclopropanes Using a Platinum Catalyst with a Sterically Demanding Silylboronic Ester

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.

CuI-Catalyzed Conjugate Addition of Silyl Boronic Esters: Retracing Catalytic Cycles Using Isolated Copper and Boron Enolate Intermediates

Copper(I)-catalyzed conjugate additions of silyl boronic esters to α,β-unsaturated aldehydes, ketones, and esters are synthetically well-established reactions. For the first time central reactive intermediates as well as the boron enolates as the primary reaction products are isolated and employed in order to deduce catalytic cycles on an experimental basis. Employing an NHC CuI complex as a model catalyst, it is possible to perform efficient catalytic transformations as well as to isolate and characterize the formed copper enolate complexes as the key intermediates. It is shown that for this catalytic system the nature of this enolate—O- or C-enolate—is crucial for the catalytic process. For α,β-unsaturated aldehydes and ketones the O-enolate is formed predominantly, while for α,β-unsaturated esters the C-enolate is the major product. Catalytic turnover is only facile for copper O-enolates, as they react efficiently with the silyl boronic ester under (re)formation of the catalytically active Cu–Si species and a thermodynamically favored boric acid ester. Thus, the formation of copper C-enolates is inhibiting the catalytic process, and effective turnover is possible only after solvolysis by an alcohol additive. The individual catalytic processes were retraced by performing stepwise stoichiometric reactions monitored by in situ NMR spectroscopy.

ChemInform Abstract: Synthesis of Multi‐Substituted Vinylsilanes via Copper(I)‐Catalyzed Hydrosilylation Reactions of Allenes and Propiolate Derivatives with Silylboronates.

AbstractMulti‐substituted allenes with electron‐withdrawing groups, e.g. (I), (IV), and (VI) and propiolate derivatives (IX) react with silyl boronic acid ester (II) to afford silyl‐substituted butenoate derivatives (III), (V), and (VII)/(VIII) or β‐silyl‐substituted arylates (X), respectively.

ChemInform Abstract: Palladium‐Catalyzed Regioselective Silaboration of Pyridines Leading to the Synthesis of Silylated Dihydropyridines.

AbstractDepending on the pyridine used, 1,4‐ or 1,2‐silaboration takes place.

Palladium-Catalyzed Regioselective Silaboration of Pyridines Leading to the Synthesis of Silylated Dihydropyridines

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.

2-Vinylindoles As the Four-Atom Component in a Catalytic [4+1] Cycloaddition with a Silylene-Palladium Species Generated from (Aminosilyl)boronic Ester

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.

Switch of Regioselectivity in Palladium-Catalyzed Silaboration of Terminal Alkynes by Ligand-Dependent Control of Reductive Elimination

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 regioselectivity in the presence of (eta(3)-C(3)H(5))Pd(PPh(3))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/beta-boryl elimination steps is proposed.

Palladium-Catalyzed Silylene-1,3-Diene [4 + 1] Cycloaddition with Use of (Aminosilyl)boronic Esters as Synthetic Equivalents of Silylene

Silylboronic esters bearing a dialkylamino group on the silicon atoms reacted with 1,3-dienes in the presence of a palladium catalyst to give silacyclopent-3-enes (i.e., 2,5-dihydrosiloles) in high yields via efficient silylene transfer from the silylboronic ester to the 1,3-dienes. The [4 + 1] cycloaddition was applicable to the parent 1,3-butadiene and various mono-, di-, and trisubstituted 1,3-dienes having silyloxy, cyano, and ester groups. Stereospecific ring formation took place in the reaction with either stereoisomer of 5,7-dodecadiene: the (E,E)-diene gave the cis product, whereas selective formation of the trans product was observed in the reaction of the isomeric (E,Z)-diene. The [4 + 1] cycloaddition followed by dehydrogenation with DDQ or chloranil afforded 2,4- and 2,5-diarylsiloles.