Silyl Substituents Research Articles

Theoretical Investigation on Intramolecular Friedel-Crafts Alkylation Of 2,2′-Disubstituted 1,3-Indandione Delivering Axially Chiral Biaryls Catalyzed by Chiral Phosphoric Acid

Our Density Functional Theory (DFT) calculations provide the first theoretical investigation on CPA-catalyzed intramolecular Friedel-Crafts alkylation of 1,3-indandione. The reaction is initiated by bis-coordination of CPA with carbonyl and phenol of indandione. The promotion of CPA lies in H-bridge and steric effect from big silyl substituents confirming less-hindered Re-face more favorable than more-shielded Si-face within central chiral alcohol. Next AlCl3-promoted process contains four steps. The dehydration gives positive carbocation, the rearrangement of which activates another carbonyl. The water reversely splits into hydrogen and hydroxyl to recover phenol. The naphthol is obtained from opening of five-membered ring. The central-to-axial chirality conversion is secured between methyl/phenol substituents and carboxylic moiety of axially chiral biaryls. The positive solvation effect is suggested by decreased absolute and activation energies in solution compared with in gas. These results are supported by Multiwfn analysis on FMO composition of specific TSs, and MBO value of vital bonding, breaking.

Diverse Reactivity of Amidinate-Supported Boron Centers with the Hypersilyl Anion and Access to a Monomeric Secondary Boron Hydride.

Diverse reactivity of the bulky tris(trimethylsilyl)silyl substituent [Si(SiMe3)3], also known as the hypersilyl group, was observed for amidinate-supported dichloro- and phenylchloroborane complexes. Treatment of the dichloroborane with potassium tris(trimethylsilyl)silyl led to the activation of the backbone β-carbon center and formation of saturated four-membered heterocyclic chloroboranes R'{Si(SiMe3)3}C(NR)2BCl [R' = Ph, R = Cy (3); R' = Ph, R = iPr (6); R' = tBu, R = Cy (8)], whereas the four-membered amidinate hypersilyl-substituted phenyl borane 4 {PhC(NCy)2B(Ph)[Si(SiMe3)3]} was observed for the case of an amidinate-supported phenylchloroborane. The highly deshielded 11B NMR spectroscopic resonance and the distinct difference in the 29Si NMR spectrum confirmed the presence of a σ-donating hypersilyl effect on compounds 3, 6, and 8. Reaction of 3 with the Lewis acid AlCl3 led to the formation of complex 11 in which an unusual cleavage of one of the C-N bonds of the amidinate backbone is observed. Nucleophilic substitution at the boron center of saturated chloroborane 3 with phenyllithium generated the phenylborane derivative 12, whereas the secondary monomeric boron hydride 13 was observed after treatment with alane (AlH3). All compounds (2-13) have been fully characterized by NMR spectroscopy and single-crystal X-ray structure determination studies.

Silyl‐Substituted ortho‐Terphenoxide Group 3–5 Complexes: Synthesis, Structure and Applications in Catalysis

AbstractThe complexation behavior of 3,3’‐silyl‐substituted ortho‐terphenoxide ligands o‐C6H4(C6H2‐2‐OH‐3‐SiR3‐5‐Me)2 (SiR3= SiPh3, SiMePh2, SiMe3, Sit‐BuMe2) with a range of yttrium, titanium, niobium and tantalum precursors was examined. The complexation of the ligands with [Y(N(SiHMe2)2)3(THF)2] yielded monomeric complexes, as confirmed by NMR spectroscopy and X‐ray crystallography for the methyldiphenylsilyl‐ and tert‐butyldimethylsilyl‐substituted complex. The triphenylsilyl‐substituted yttrium bis(dimethylsilyl)amido complex showed good activity in the intramolecular hydroamination reaction of aminoalkenes. Complexation with Ti(NMe2)4 yielded a mixture of products, depending on the silyl substituents and the solvent used for complexation. A mono‐ligated bimetallic titanium complex and a bisligated titanium complex were analyzed by X‐ray crystallography. Complexation with M(NMe2)5 (M=Nb, Ta) produced the corresponding monomeric ortho‐terphenoxide complexes as confirmed by NMR spectroscopy and X‐ray structure analysis of a niobium complex and 3 tantalum complexes.

Pd Nanoparticle-Catalyzed Stereospecific Mizoroki-Heck Arylation of cis-1,2-Disilylarylethylenes.

In the presence of catalytic amounts of Pd nanoparticles, generated from Pd2dba3/Ag(I), cis-1,2-ditrimethylsilylarylethylenes undergo with aryl iodides a stereospecific Mizoroki-Heck arylation leading to trans-ditrimethylsilyldiarylethylenes. This chemoselectivity is in contrast to that of their trimethylgermyl analogues, which are arylated at the position of the C-Ge bonds. trans-1,2-Ditrimethylsilylarylethylenes are completely unreactive under the standard reaction conditions. The reaction tolerates the presence of boryl, silyl, or bromine substituents on the aryl iodides. From a mechanistic point of view, the process involves syn-arylpalladation followed by syn-dehydropalladation.

Access to C(sp3) borylated and silylated cyclic molecules: hydrogenation of corresponding arenes and heteroarenes.

This paper presents a simple and cost-effective hydrogenation method for synthesizing a myriad of cycloalkanes and saturated heterocycles bearing boryl or silyl substituents. The catalyst used are heterogeneous, readily available, bench stable, and recyclable. Also demonstrated is the application of the method to compounds that possess both boryl and silyl groups. When combined with Ir-catalyzed sp2 C-H borylation, such hydrogenations offer a two-step complementary alternative to direct sp3 C-H borylations that can suffer selectivity and reactivity issues. Of practical value to the community, complete stereochemical analyses of reported borylated compounds that were never fully characterized are reported herein.

Direct synthesis of conjugated tetraenes from 1,3-enynes with 1,3-dienes.

New direct access to conjugated tetraenes has been achieved. A Ru(0)-catalysed reaction of 1,3-enynes with 1,3-dienes gives 1,3,5,7-octatetraene derivatives by formal regioselective insertion of the alkynyl group of 1,3-enynes into the terminal C-H bond in 1,3-dienes. With a silyl substituent on the alkynyl side in 1,3-enynes, the reaction regioselectively proceeds to give the linear cross-dimerisation product having the silyl group at the internal position. Stoichiometric and DFT calculations support the oxidative coupling mechanism for the linear cross-dimerisation. Methyl (2E,4E,6E,8E)-10-hydroxy-2,4,6,8-decatetraenoate, a versatile polyene intermediate, is accessed by this method as a formal synthesis of biologically active compounds.

A tin analogue of propadiene with cumulated C[double bond, length as m-dash]Sn double bonds.

The synthesis, structure, and properties of a stable, linear 2-stannapropadiene are reported. The identical C[double bond, length as m-dash]Sn bonds in this 2-stannapropadiene are the shortest hitherto reported C-Sn bonds. This 2-stannapropadiene features a 119Sn NMR signal at 507 ppm for the central tin atom, indicative of an unsaturated Sn4+ oxidation state. Due to the inert-pair effect, the tin atom displays a pronounced preference for the +2 oxidation state over the +4 oxidation state. Nevertheless, by employing silyl substituents, it is possible to disrupt the inert-pair effect, leading to the formation of an isolable 2-stannapropadiene with a linear structure centered on a Sn4+ atom. Treatment of this 2-stannapropadiene with SnBr2·dioxane resulted in the formation of a novel four-membered cyclic 1,1-dibromo-1,3-distannetane, which was subsequently reduced to afford the corresponding stable four-membered cyclic bis(stannylene).

Fluoride-triggered phase transition of metallogels for on-demand in situ containment of fluids.

Sol-gel transition regulates mass transport in fluidic systems. We designed pre-gelators that react with fluoride anions to form a metallogel barrier. A combination of spectroscopic, rheological, and X-ray spectroscopic studies elucidated the mechanism of gelation involving desilylation followed by metal coordination-driven self-assembly, the kinetics of which can be finely controlled by the chemical structure of the silyl substituents. Protonation-induced degelation restores flow, allowing the metallogel to function as a reversible chemical valve.

Bis‐Silyl‐triazenide ligands in alkaline‐earth metal chemistry

AbstractMonoanionic N,N‐chelating triazenide ligands, [R‐NNN‐R]−, are compared to formamidinate or β‐diketiminate ligands considerably less electron‐donating and therefore potentially suitable for stabilizing electron rich metals in low oxidations states. A feature reported for silyl‐substituted triazenide ligands, is their ability to eliminate N2 resulting in (R3Si)2N− anions. Here we describe a series of group 1 and 2 metal complexes with the very bulky bis‐silyl‐triazenide ligand [tBu3Si‐NNN‐SitBu3]−. Heteroleptic complexes could be isolated for Mg: {[(tBu3Si)2N3]MgnBu}2 or {[(tBu3Si)2N3]MgI}2, including its ether adducts. Despite bulky silyl substituents, the ligand did not stabilize heteroleptic [(tBu3Si)2N3]AeN(SiMe3)2 complexes for Ca, Sr and Ba and only homoleptic Ae[(tBu3Si)2N3]2 complexes were isolated. Thermal decomposition of these complexes did result in N2 elimination and formation of the expected amide complexes. Reduction of [(tBu3Si)2N3]MgI ⋅ (Et2O) with KC8 in Et2O led to the formation of a MgI complex with a Mg−Mg bond length of 2.902(1) Å. As only one of the Mg centres shows coordination with a Et2O ligand, this is a rare example of an asymmetric Mg−Mg bond. This MgI complex is rather unstable in benzene solution, likely due to reduction of the ligand system. Triazenide ligands are therefore not suitable for stabilization of group 2 metals in low oxidation states.

Combining Ligand Deuteration with Ligand Bulkiness in Non‐Heme Iron Oxidation Catalysis: Enhancing Catalyst Lifetime and Site‐Selectivity

AbstractBulky tri‐isopropyl silyl (TIPS) substituents and deuterium atoms in the ligand design have been shown to enhance the site‐selective oxidation of aliphatic C−H bonds and the epoxidation of C=C bonds in non‐heme iron oxidation catalysis. In this work, a series of non‐heme iron complexes were developed by combining TIPS groups and deuterium atoms in the ligand. These bulky deuterated complexes show a significant increase in catalytic performance compared to their counterparts containing only TIPS groups or deuterium atoms. A broad range of substrates was oxidized with excellent yields, particularly, using [Fe(OTf)2((S,S)‐TIPSBPBP‐D4)] (1‐TIPS‐D4) (0.1 mol % to 1 mol %) via a fast or slow oxidant addition protocol, resulting in an overall improvement in catalytic performance. Notably, in the oxidation of the complex substrate trans‐androsterone acetate, the use of a slow addition protocol and a lower catalyst loading of 1‐TIPS‐D4 resulted in significant increases in reaction efficiency. In addition, kinetic and catalytic studies showed that deuteration does not affect the catalytic activity and the secondary C−H site‐selectivity but increases the catalysts’ lifetime resulting in higher conversion/yield. Accordingly, the yield of selectively oxidized secondary C−H products also increases with the overall yield by using the bulky deuterated iron complexes as catalysts. These catalytic improvements of the bulky deuterated complexes exemplify the enhanced design of ligands for homogeneous oxidation catalysis.

Synthesis and Transformation of Bis(silyl)-Substituted Conjugated Vinylallene Compounds.

We develop a copper-catalyzed disilylation of polysubstituted pent-1-en-4-yn-3-yl acetate derivatives, which in one pot furnishes the bis(silyl)-substituted vinylallenes in moderate yields under mild reaction conditions. The reaction shows broad substrate scope and single stereoselectivity. Besides, we develop a method to decrease the electrocyclization temperature of vinylallenes for the synthesis of methylenecyclobutenes by installing bis(silyl) substituents. And the effect of a silyl substituent on electrocyclization of vinylallenes was studied via DFT computation. The synthetic method features broad substrate scope and excellent stereoselectivity.

Synthesis and Properties of Backbone Silylated Imidazol‐2‐thiones

AbstractImidazole‐2‐thiones have attracted considerable interest in the past as materials for potential applications in the pharmaceutical and chemical industries. Herein, the synthesis of a series of backbone silylated 1,3‐dialkylimidazol‐2‐thiones is reported. The developed synthesis protocol involves the silylation of N,N‐dimethylimidazol‐2‐thione 1 followed by the addition of organochlorosilanes RnSiCl4–n (R=Me, Ph; n=0–4) and enabled the synthesis of the C‐silylated derivatives with monocyclic, silyl‐bridged or fused tricyclic structures. Reactivity studies performed with N,N‐dimethyl‐4,5‐bistrimethylsilylimidazole‐2‐thione as a model substance showed surprisingly stable silicon‐vinyl bonds and reactivity patterns closely related to those observed for the unsilylated species 1. Combined UV‐spectroscopic and computational studies revealed only minor impact of the silyl substituents on the electronic structure of the imidazol‐2‐thione ring.

A Linear C=Ge=C Heteroallene with a Di-coordinated Germanium Atom.

Allenes (>C=C=C<) are classified as cumulated dienes with a linear structure and an sp-hybridized central carbon atom. We have synthesized and isolated a stable 2-germapropadiene with bulky silyl substituents. The 2-germapropadiene allene moiety adopts a linear structure both in the solid state and in solution. An X-ray diffraction electron-density-distribution (EDD) analysis of this 2-germapropadiene confirmed the linear C=Ge=C geometry with a formally sp-hybridized germanium atom that bears two orthogonal C=Ge π-bonds. Based on detailed structural and computational studies, we concluded that the linear geometry of the isolated 2-germapropadiene most likely arises from the negative hyperconjugation of the silyl substituents at the terminal carbon atoms. The 2-germapropadiene reacts rapidly with nucleophiles, indicating that the linearly oriented germanium atom is highly electrophilic.

A Linear C=Ge=C Heteroallene with a Di‐coordinated Germanium Atom**

AbstractAllenes (&gt;C=C=C&lt;) are classified as cumulated dienes with a linear structure and an sp‐hybridized central carbon atom. We have synthesized and isolated a stable 2‐germapropadiene with bulky silyl substituents. The 2‐germapropadiene allene moiety adopts a linear structure both in the solid state and in solution. An X‐ray diffraction electron‐density‐distribution (EDD) analysis of this 2‐germapropadiene confirmed the linear C=Ge=C geometry with a formally sp‐hybridized germanium atom that bears two orthogonal C=Ge π‐bonds. Based on detailed structural and computational studies, we concluded that the linear geometry of the isolated 2‐germapropadiene most likely arises from the negative hyperconjugation of the silyl substituents at the terminal carbon atoms. The 2‐germapropadiene reacts rapidly with nucleophiles, indicating that the linearly oriented germanium atom is highly electrophilic.

Phosphole aromaticity enhancement by electron pumping through Schleyer hyperconjugative aromaticity: A comprehensive DFT study

Combination of aromaticity and hyperconjugation as two fundamental concepts in organic chemistry results hyperconjugative aromaticity. Due to trigonal pyramidal P (III) atom in phosphole ring, it is only slightly more aromatic than cyclopentadiene. In the present work, the hyperconjugative aromaticity of phosphole derivatives, bearing an electron donating group, have been studied at the DFT-B3LYP/6–311++G(d, p) level of theory using various aromaticity indices such as geometric, magnetic, electronic and energetic criteria. Our DFT calculations reveal that an electropositive substituent can enhance phosphole aromaticity through Schleyer hyperconjugative aromaticity. This hyperconjugative electron donation by the substituents results in a partially anionic phosphole ring. Among the electropositive groups, silyl and germanium substituents have the best performance. The excellent correlations are observed between all aromaticity indices. Our findings could help chemists to design novel hyperconjugative aromatic phospholes.

Cu/Pd-catalyzed arylboration of a 1-silyl-1,3-cyclohexadiene for stereocontrolled and diverse cyclohexane/ene synthesis.

The synthesis and Cu/Pd-catalyzed arylboration of 1-silyl-1,3-cyclohexadiene is described. This diene is significant as it allows for synthesis of polyfunctional cyclohexane/enes. To achieve high levels of diastereoselectivity, the use of a pyridylidene Cu-complex was employed. In addition, through the use of a chiral catalyst, an enantioselective reaction was possible. Due to the presence of the silyl and boron substituents, the products can be easily diversified into a range of valuable cyclohexane/ene products.

Metathesis between E−C(sp n ) and H−C(sp 3 ) σ‐Bonds (E=Si, Ge; n =2, 3) on an Osmium‐Polyhydride

The silylation of a phosphine of OsH6(PiPr3)2 is performed via net-metathesis between Si−C(spn) and H−C(sp3) σ-bonds (n=2, 3). Complex OsH6(PiPr3)2 activates the Si−H bond of Et3SiH and Ph3SiH to give OsH5(SiR3)(PiPr3)2, which yield OsH4{κ1-P,η2-SiH-[iPr2PCH(Me)CH2SiR2H]}(PiPr3) and R−H (R=Et, Ph), by displacement of a silyl substituent with a methyl group of a phosphine. Such displacement is a first-order process, with activation entropy consistent with a rate determining step occurring via a highly ordered transition state. It displays selectivity, releasing the hydrocarbon resulting from the rupture of the weakest Si-substituent bond, when the silyl ligand bears different substituents. Accordingly, reactions of OsH6(PiPr3)2 with dimethylphenylsilane, and 1,1,1,3,5,5,5-heptamethyltrisiloxane afford OsH5(SiR2R′)(PiPr3)2, which evolve into OsH4{κ1-P,η2-GeH-[iPr2PCH(Me)CH2SiR2H]}(PiPr3) (R=Me, OSiMe3) and R′−H (R′=Ph, Me). Exchange reaction is extended to Et3GeH. The latter reacts with OsH6(PiPr3)2 to give OsH5(GeEt3)(PiPr3)2, which loses ethane to form OsH4{κ1-P,η2-GeH-[iPr2PCH(Me)CH2GeEt2H]}(PiPr3).

Metathesis between E-C(spn ) and H-C(sp3 ) σ-Bonds (E=Si, Ge; n=2, 3) on an Osmium-Polyhydride.

The silylation of a phosphine of OsH6(PiPr3)2 is performed via net‐metathesis between Si−C(sp n ) and H−C(sp3) σ‐bonds (n=2, 3). Complex OsH6(PiPr3)2 activates the Si−H bond of Et3SiH and Ph3SiH to give OsH5(SiR3)(PiPr3)2, which yield OsH4{κ1‐P,η2‐SiH‐[iPr2PCH(Me)CH2SiR2H]}(PiPr3) and R−H (R=Et, Ph), by displacement of a silyl substituent with a methyl group of a phosphine. Such displacement is a first‐order process, with activation entropy consistent with a rate determining step occurring via a highly ordered transition state. It displays selectivity, releasing the hydrocarbon resulting from the rupture of the weakest Si‐substituent bond, when the silyl ligand bears different substituents. Accordingly, reactions of OsH6(PiPr3)2 with dimethylphenylsilane, and 1,1,1,3,5,5,5‐heptamethyltrisiloxane afford OsH5(SiR2R′)(PiPr3)2, which evolve into OsH4{κ1‐P,η2‐GeH‐[iPr2PCH(Me)CH2SiR2H]}(PiPr3) (R=Me, OSiMe3) and R′−H (R′=Ph, Me). Exchange reaction is extended to Et3GeH. The latter reacts with OsH6(PiPr3)2 to give OsH5(GeEt3)(PiPr3)2, which loses ethane to form OsH4{κ1‐P,η2‐GeH‐[iPr2PCH(Me)CH2GeEt2H]}(PiPr3).

Synthesis of Silacyclohexanones from Divinylsilanes and Allylamines by a Rh-Catalyzed Cyclization.

An efficient synthesis of silacyclohexanones bearing a variety of silyl substituents has been developed by a [Rh(coe)2Cl]2/PCy3-catalyzed cyclization of divinylsilanes with Jun's allylamine. The silacyclohexanones can be oxidized with DDQ to give the corresponding silacyclohexadienones, which are further transformed into silicon analog of 2-deoxystreptamine or exo-alkylidenesilacyclohexadienes.

Tuning the metal-ligand bond in the σ-complexes of stannylenes and azabenzenes.

The metal-ligand bond in a set of 60 σ-complexes has been investigated by electronic structure computations. These σ-complexes originate from the unique combination of 12 stannylenes (SnX2 ) with five azabenzene ligands (pyridine, pyrazine, pyrimidine, pyridazine, and s-triazine), where the nitrogen center of the ligand acts as σ-donor and the tin(II) center as σ-acceptor in a 1:1 fashion. The Sn ← N bond and the total interaction between the stannylene and azabenzene moieties of the σ-complexes are characterized in depth to relate the Sn ← N strength to the substitution pattern at SnX2 and to the number and the positioning of N atoms in the azabenzenes. Such X substituents as (iso)cyano and trifluoromethyl groups enhance the interaction strength, while the presence of alkyl, phenyl, and silyl substituents in SnX2 diminishes the stability of σ-complexes. A gradual weakening of the total interaction is associated with the growing number of N atoms in the azabenzenes, while the N-atom positioning in pyridazine is particularly effective in strengthening the interaction with stannylenes. Variations in the Sn ← N bond strength usually follow those in the total interaction between the moieties but the interacting quantum atoms picture of Sn ← N reveals certain intriguing exceptions.