Silyl Transfer Research Articles

Iron-Catalyzed Electrophotochemical α-Functionalization of a Silylcyclobutanol.

Four-membered ring structure is important in organic chemistry, and selective cleavage and functionalization of these strained rings are of great interest. However, direct α-functionalization of cyclobutanols is rarely reported because of the high O-H bond dissociation energy and the occurrence of β-scission of C-C bonds in these alcohols. Recently, transition-metal catalysis has facilitated alkoxy radical generation. Herein, we report a method for electrophotochemical α-functionalization of a silylcyclobutanol via visible-light-induced LMCT reactions of M-alkoxy complexes. Introduction of the silyl group into the cyclobutanol structure favored fast [1,2]-silyl transfer over ring opening, thus allowing the generation of α-functionalized products.

Organophotocatalytic silyl transfer of silylboranes enabled by methanol association: a versatile strategy for C–Si bond construction

A visible light-induced organophotocatalytic silyl transfer strategy of silylboranes for the construction of C–Si bonds has been demonstrated, where methanol association with boron atoms enables the photocatalytic generation of silyl radicals.

Redox-Neutral Transformations of Carbon Dioxide Using Coordinatively Unsaturated Late Metal Silyl Amide Complexes.

Two-coordinate silylamido complexes of nickel and copper rapidly react with CO2 to selectively form a new cyanate ligand along with hexamethyldisiloxane byproducts. Mechanistic insight into these reactions was obtained from the synthesis of proposed intermediates, several silyl- and phenyl- substituted amido analogues, and their subsequent reactivity with CO2. These studies suggest that a unique intramolecular double silyl transfer step facilitates CO2 deoxygenation, which likely contributes to the rapid rates of reaction. The deoxygenation reactions create a platform for a synthetic cycle in which copper amido complexes convert CO2 to organic silylcarbamates.

1,3-Aza-Brook Rearrangement of Aniline Derivatives: In Situ Generation of 3-Aminoaryne via 1,3-C-(sp2)-to-N Silyl Migration.

The design, synthesis, and validation of 3-aminobenzyne precursors induced by C-(sp2)-to-N 1,3-aza-Brook rearrangement have been achieved, allowing access to diverse aniline derivatives. Through crossover experiments, we demonstrated the intramolecular mechanism of 1,3-C-to-N silyl transfer. To gain insight into the regioselectivity observed in the reactions, we performed density functional theory calculations. Finally, the method was applied to the synthesis of xylanigripones A in five linear steps in an overall yield of 30%.

Design, Synthesis, and Implementation of Sodium Silylsilanolates as Silyl Transfer Reagents

Design, Synthesis, and Implementation of Sodium Silylsilanolates as Silyl Transfer Reagents

Unanticipated Silyl Transfer in Enantioselective α,β-Unsaturated Acyl Ammonium Catalysis Using Silyl Nitronates.

The use of silyl nitronates is reported for the isothiourea-catalyzed synthesis of γ-nitro-substituted silyl esters containing up to two contiguous stereocenters in good yields with excellent enantioselectivities (up to 93% yield and 99:1 er). The serendipitously discovered formation of silyl ester products in this reaction demonstrates a novel platform for catalyst turnover in α,β-unsaturated acyl ammonium catalysis.

Investigation of Transfer Group, Tether Proximity, and Alkene Substitution for Intramolecular Silyloxypyrone-Based [5 + 2] Cycloadditions.

Systematic investigation of intramolecular silyloxypyrone-based [5 + 2] cycloadditions revealed three significant factors impacting conversion to cycloadduct: (1) the silyl transfer group has a substantial influence on the rate of reaction, and the robust t-butyldiphenylsilyl group was found to be more effective overall than the conventional t-butyldimethylsilyl group; (2) α,β-unsaturated esters were generally more reactive than terminal olefins and afforded appreciable quantity of cycloadduct even at room temperature; and (3) the proximity of the tether to the silyl transfer group revealed a critical alignment trend between the pyrone and the alkene. Taken together, these investigations provided insight regarding the steric and electronic parameters that impact the scope and limitation of these reactions.

High Stereocontrol in the Preparation of Silyl-Protected γ-Substituted Enoldiazoacetates

A robust and efficient synthesis of triisopropylsilyl (TIPS)-protected γ-substituted enoldiazoacetates with excellent Z stereocontrol by using lithium bis(trimethylsilyl)azanide (LiHMDS) as a base and TIPSOTf as a silyl transfer reagent is reported. Despite their increased size compared to previously tert-butyldimethylsilyl (TBS)-protected γ-unsubstituted enoldiazoacetates, a high product yield with exceptional stereocontrol has been achieved in copper-catalyzed [3+3] cycloaddition reaction with nitrones by using a chiral indeno bisoxazoline ligand.

Reductive Silylation Using a Bis-silylated Diaza-2,5-cyclohexadiene.

1,4-Bis(trimethylsilyl)-1,4-diaza-2,5-cyclohexadiene, 1, was tested as a reagent for the reductive silylation of various unsaturated functionalities, including N-heterocycles, quinones, and other redox-active moieties in addition to deoxygenation of main group oxides. Whereas most reactions tested are thermodynamically favorable, based on DFT calculations, a few do not occur, perhaps giving limited insight on the mechanism of this very attractive reductive process. Of note, reductive silylation reactions show a strong solvent dependence where a polar solvent facilitates conversions.

Selective deoxygenation of nitrate to nitrosyl using trivalent chromium and the Mashima reagent: reductive silylation.

1,4-Bis(trimethylsilyl)-1,4-diaza-2,5-cyclohexadiene is an effective silyl transfer reagent towards the oxygen of nitrate coordinated to Cr(iii) in a pincer complex. Two nitrate oxygens are removed to give the 17 valence electron octahedral complex (H2L)Cr(NO3)2(NO). This is shown by a variety of spectroscopic methods, together with DFT, to be a Cr(i) complex with a linear CrNO unit. This work also identifies future applications of this reductive silylation process.

Diastereoselective silylene transfer reactions to chiral enantiopure alkenes: effects of ligand size and substrate bias.

Silylenes are useful reactive intermediates for the stereoselective construction of compounds containing carbon-silicon bonds. Despite their synthetic utility, the development of either an enantioselective or diastereoselective metal-catalyzed silylene transfer reaction, in which ligands on the metal catalyst control stereoselectivity, has not been achieved. In this article, we report that the structure of the alkene is the most important for controlling stereoselectivity in these reactions. The stereochemical course of kinetically controlled silacyclopropanation reactions was not affected by the nature or chirality of the ligands on the metal. When silylene transfer reactions were reversible, however, products can be formed with a high degree of diastereoselectivity (90 : 10 d.r.).

ChemInform Abstract: Organocatalytic Silyl Transfer from Silylborane to Nitroalkenes for the Synthesis of β‐Silyl Nitroalkanes and β‐Silyl Amines.

AbstractNovel Si—B activation by acid/base organocatalysis, which leads to a generally efficient silyl transfer to nitroalkenes is developed.

Rhodium-Catalyzed Dehydrogenative Silylation of Acetophenone Derivatives: Formation of Silyl Enol Ethers versus Silyl Ethers.

A series of rhodium-NSiN complexes (NSiN=bis (pyridine-2-yloxy)methylsilyl fac-coordinated) is reported, including the solid-state structures of [Rh(H)(Cl)(NSiN)(PCy3 )] (Cy=cyclohexane) and [Rh(H)(CF3 SO3 )(NSiN)(coe)] (coe=cis-cyclooctene). The [Rh(H)(CF3 SO3 )(NSiN)(coe)]-catalyzed reaction of acetophenone with silanes performed in an open system was studied. Interestingly, in most of the cases the formation of the corresponding silyl enol ether as major reaction product was observed. However, when the catalytic reactions were performed in closed systems, formation of the corresponding silyl ether was favored. Moreover, theoretical calculations on the reaction of [Rh(H)(CF3 SO3 )(NSiN)(coe)] with HSiMe3 and acetophenone showed that formation of the silyl enol ether is kinetically favored, while the silyl ether is the thermodynamic product. The dehydrogenative silylation entails heterolytic cleavage of the Si-H bond by a metal-ligand cooperative mechanism as the rate-determining step. Silyl transfer from a coordinated trimethylsilyltriflate molecule to the acetophenone followed by proton transfer from the activated acetophenone to the hydride ligand results in the formation of H2 and the corresponding silyl enol ether.

Mechanism of Boron-Catalyzed N-Alkylation of Amines with Carboxylic Acids.

Mechanistic study has been carried out on the B(C6F5)3-catalyzed amine alkylation with carboxylic acid. The reaction includes acid-amine condensation and amide reduction steps. In condensation step, the catalyst-free mechanism is found to be more favorable than the B(C6F5)3-catalyzed mechanism, because the automatic formation of the stable B(C6F5)3-amine complex deactivates the catalyst in the latter case. Meanwhile, the catalyst-free condensation is constituted by nucleophilic attack and the indirect H2O-elimination (with acid acting as proton shuttle) steps. After that, the amide reduction undergoes a Lewis acid (B(C6F5)3)-catalyzed mechanism rather than a Brønsted acid (B(C6F5)3-coordinated HCOOH)-catalyzed one. The B(C6F5)3)-catalyzed reduction includes twice silyl-hydride transfer steps, while the first silyl transfer is the rate-determining step of the overall alkylation catalytic cycle. The above condensation-reduction mechanism is supported by control experiments (on both temperature and substrates). Meanwhile, the predicted chemoselectivity is consistent with the predominant formation of the alkylation product (over disilyl acetal product).

Theoretical Study of Ir-Catalyzed Chemoselective C1–O Reduction of Glucose with Silane

Density functional theory (DFT) calculations have been performed to study the mechanism of Ir(III) pincer complex (POCOP)Ir(H)(acetone)+ (POCOP = 2,6-bis(dibutylphosphinito)phenyl) catalyzed chemoselective C1–O hydrosilylative reduction of glucose. The mechanisms for reduction of the external and internal C1–O (i.e., C1–Oext and C1–Oint) on the C1-MeO-substituted glucose (i.e., 1Me) and C1–Me2EtSiO-substituted glucose (i.e., 1Si) have been investigated. The calculation results show that both mechanisms proceed with the first concerted silyl transfer and the subsequent C1–Oext or C1–Oint bond cleavage and hydride transfer steps. In the hydride transfer step, the Ir-H moiety acts as the hydride source. The C1–O cleavage is the rate-determining step of the overall mechanism. The C1–Oext reduction is more favorable than C1–Oint reduction for the substrate 1Me, while the C1–Oint reduction is more favorable for 1Si. These results are consistent with the recent experimental outcomes. Analyzing the origin of chem...

Enantioselective Catalysis by Chiral Isothioureas

This review covers recent developments relating to organocatalyzed transformations by chiral isothioureas (ITUs) since their original introduction by Birman in 2006. This class of nucleophilic heterocycles was first involved in anhydride activation in enantioselective acyl transfer reactions, but it was more recently shown that activation of other reagents was possible, considerably enlarging their number of catalytic enantioselective transformations. Four main modes of activation as Lewis bases can currently be listed: (1) acylisothiouronium intermediates involved in acyl transfer, (2) silylisothiouronium species involved in silyl transfer, (3) acylisothiouronium enolates involved in several concerted and formal pericyclic transformations, and (4) α,β‐unsaturated acylisothiouronium species involved in domino transformations. This review is organized according to these different modes of activation of chiral isothioureas.

Enantioselective Visible‐Light‐Induced Radical‐Addition Reactions to 3‐Alkylidene Indolin‐2‐ones

The title compounds underwent a facile and high-yielding addition reaction (19 examples, 66-99% yield) with various N-(trimethylsilyl)methyl-substituted amines upon irradiation with visible light and catalysis by a metal complex. If the alkylidene substituent is non-symmetric and if the reaction is performed in the presence of a chiral hydrogen-bonding template, products are obtained with significant enantioselectivity (58-72% ee) as a mixture of diastereoisomers. Mechanistic studies suggest a closed catalytic cycle for the photoactive metal complex. However, the silyl transfer from the amine occurs not only to the product, but also to the substrate, and interferes with the desired chirality transfer.

Organocatalytic silyl transfer from silylborane to nitroalkenes for the synthesis of β-silyl nitroalkanes and β-silyl amines.

The silylation of a wide array of nitroalkenes is achieved by organocatalysts in yields up to 93%. The reaction is carried out in a toluene/water biphasic solvent under operationally simple conditions. Reduction of the nitro group provides efficient access to functionalized β-silyl amines.

Computational Picture of Silyl Transfer from Silylsilatranes to Arylpalladium Chloride

Abstract The previously reported palladium-catalyzed silylation of aryl chlorides with silylsilatranes was theoretically reinvestigated by DFT calculations with a focus on the effect of the transannular Si–N coordination on silyl transfer. The Si–N coordination proved to be not so influential as to accelerate the silyl transfer. However, it should be of benefit to use silylsilatranes as silylating agents because of their easy handling and facile preparation.

ChemInform Abstract: Chemoselective Silyl Transfer in the Mukaiyama Aldol Reaction Promoted by Super Silyl Lewis Acid.

In the silyl Lewis acid-promoted Mukaiyama aldol reaction, the steric and electronic properties of the silyl group on the silyl Lewis acid influence the reaction mechanism and product distribution. When super silyl triflates such as (TMS)3SiOTf and (TES)3SiOTf are used as Lewis acids, the silyl group of the silyl enol ether chemoselectively transfers to the product. The mechanistic details have been investigated using density functional theory (DFT) calculations.