Trimethylsilyl Substituents Research Articles

Synthesis and van Alphen–Hüttel rearrangement of substituted 3H-pyrazoles, adducts of 1,3-dipolar cycloaddition of 9-diazofluorene and diphenyldiazomethane to trimethyl(tosylethynyl)silane

Trimethyl(tosylethynyl)silane reacted in ethyl ether at 20°С with diphenyldiazomethane affording 3Н-pyrazole, a product of 1,3-dipolar cycloaddition against Auwers rule. The reaction with 9-diazofluorene is less selective, but its regioselectivity is also governed by the steric effect of the bulky trimethylsilyl substituent at the triple bond С≡С. The adduct with diphenyldiazomethane at boiling in methanol or keeping in glacial acetic acid in the presence of a catalytic quantity of conc. H2SO4 undergoes the Van Alphen–Huttel rearrangement by the migration of the phenyl substituent to the atom N2 in the 1Н-pyrazole. The same 1Н-pyrazole together with a product of nitrogen elimination, trimethylsilyl substituted cyclopropane, is formed in the 2: 1 ratio at boiling in benzene. A similar behavior is observed in the glacial acetic acid for the anti-Auwers adduct of 9-diazofluorene. It suffers nitrogen elimination at boiling in benzene converting in spirocyclic cyclopropene. The Auwers adduct of 9-diazofluorene at boiling in methanol transforms due to the van Alphen–Huttel rearrangement into the corresponding 4Н-pyrazole that undergoes a hydrodesilylation to give a derivative of 1Н-pyrazole, 3-tosyl-1(2)H-dibenzo[e,g]indazole.

New Regio- and Stereoselective Cascades via Unstabilized Azomethine Ylide Cycloadditions for the Synthesis of Highly Substituted Tropane and Indolizidine Frameworks.

Multisubstituted tropanes and indolizidines have been prepared with high regio- and stereoselectivity by the [3+2] cycloaddition of unstabilized azomethine ylides generated from readily prepared trimethylsilyl-substituted 1,2-dihydropyridines via protonation or alkylation followed by desilylation. Starting from 1,2-dihydropyridines bearing a ring trimethylsilyl substituent at the 6-position, an intermolecular alkylation/desilylation provides endocyclic unstabilized ylides that successfully undergo cycloaddition with a range of symmetrical and unsymmetrical alkyne and alkene dipolarophiles to afford densely substituted tropanes incorporating quaternary carbons in good yields and with high regio- and stereoselectivity. Additionally, an intramolecular alkylation/desilylation/cycloaddition sequence provides convenient and rapid entry to bridged tricyclic tropane skeletons, allowing for five contiguous carbon stereocenters to be set in a single experimental operation and under mild conditions. Starting from 1,2-dihydropyridines with trimethylsilylmethyl groups on nitrogen, protonation followed by desilylation generates exocyclic unstabilized ylides that undergo cycloaddition with unsymmetrical alkynes to give indolizidines with good regio- and stereoselectivity. N-Trimethylsilylmethyl-1,2-dihydropyridines can also be alkylated and subsequently desilylated to give endocyclic unstabilized ylides that undergo intermolecular cycloadditions with carbonyl compounds to give bicyclic oxazolidine products in good overall yields. Moreover, an intramolecular alkylation/desilylation/cycloaddition sequence with the N-trimethylsilylmethyl-1,2-dihydropyridines affords tricyclic indolizidines that incorporate quaternary carbons and up to five stereocenters with good to excellent regio- and diastereoselectivity.

Crystal structure of the inverse crown ether tetra-kis-[μ2-bis-(tri-methyl-sil-yl)amido]-μ4-oxido-dicobalt(II)disodium, [Co2Na2{μ2-N(SiMe3)2}4](μ4-O).

The title compound, [Co2Na2{μ2-N(SiMe3)2}4](μ4-O), (I), represents a new entry in the class of inverse crown ethers. In the mol-ecule, each Co atom is formally in the oxidation state +II. The structure contains one half of a unique mol-ecule per asymmetric unit with the central μ4-oxido ligand residing on an inversion center, leading to a planar coordination to the Na and Co atoms. In the crystal, bulky tri-methyl-silyl substituents prevent additional inter-actions with cobalt. However, weak inter-molecular Na⋯H3C-Si inter-actions form an infinite chain along [010]. The structure is isotypic with its Mg, Mn and Zn analogues.

Chromocene, ferrocene, cobaltocene, and nickelocene derivatives with isopropyl and methyl or trimethylsilyl substituents

From sodium 2,3-diisopropyl-1,4-dimethyl-cyclopentadienide and MBr2(dme) (M = Fe, Co, Ni) the corresponding ferrocene (1), cobaltocene (2) and nickelocene (3) could be obtained as pure, crystalline solids. From an analogous reaction of sodium 2,3,5-triisopropyl-1,4-dimethyl-cyclopentadienide with FeCl2, only the mixed-substituted ferrocene (C23p)(C22p)Fe (4) could be crystallized (C23p = 2,3,5-triisopropyl-1,4-dimethyl-cyclopentadienyl, C22p = 2,3-diisopropyl-1,4-dimethyl-cyclopentadienyl). The trimethylsilyl substituted ferrocene (5) and nickelocene (6) were synthesized from the corresponding metal halides and potassium 2,3-diisopropyl-1,4-dimethyl-5-trimethylsilyl-cyclopentadienide as well as a series of chromocenes with silylated or alkylated cyclopentadienyl ligands (7–10) from chromium(II) acetate. Octaisopropylnickelocene (11) has been obtained and converted to octaisopropylnickelocenium hexafluorophosphate (12). 1,2,3,4-tetraisopropylnickelocenium tetrabromoaluminate (13) was obtained from tetraisopropylcyclopentadienylnickel(II) tetrabromoaluminate and nickelocene. Crystal structures have been obtained for metallocenes 1–5, 7–9, and 11.

Synthesis of Alkynyl‐Glycinols by Lewis Acid Catalyzed Propargylic Substitution of Bis‐Imidates

AbstractRacemic and enantioenriched alkynyl‐glycinols can be synthesized by Lewis acid catalyzed cyclization reaction of bis‐trichloracetimidates derived from alkynyl‐glycols. The cyclization proceeds selectively to give 4‐alkynyl‐oxazolines as the propargylic substitution products. Enantioenriched bis‐imidates that contain an alkyl or trimethylsilyl substituent at the acetylene gave oxazolines with complete inversion of configuration. In turn, considerable racemization was observed in the cyclization of bis‐imidates that contain a phenyl substituent. The racemization for these substrates can be suppressed by introduction of the electronegative substituent at the phenyl ring. Oxazolines prepared by bis‐imidate cyclization reaction can be readily transformed to protected alkynyl‐glycol derivatives.

Highly efficient and selective probes based on polycyclic aromatic hydrocarbons with trimethylsilylethynyl groups for fluoride anion detection

Two polycyclic aromatic hydrocarbons bearing trimethylsilylethynyl groups were designed and synthesized as new colorimetric and ratiometric fluorescent probes for fluoride anion. Both probes exhibit high specific selectivity and sensitivity for F− compared to other anions. When F− is added to the solution of the probe, the trimethylsilyl substituents can be removed immediately, and the color of the solution changes from yellow to colorless under ambient light. Moreover, test papers based on the probes are prepared, which could be applied to detect F− in organic phase.

Reaching for two new stable ambiphilic quinoline-derivedN-heterocyclic carbenes at DFT level

A detailed investigation on the thermodynamic and kinetic stability of four carbenic tautomers of quinoline 1, including quinoline-2-ylidene 2, quinoline-3-ylidene 3, quinoline-4-ylidene 4, and 3,4-dihydroquinoline-4-ylidene 5, reveals that singlet planar six-membered ring N-heterocyclic carbenes (NHCs) 2 and 4 have less stability than Arduengo type NHC but seems to have enough conceivably for reaching at B3LYP/aug-cc-pVTZ//B3LYP/6–31+G* and B3LYP/6–311++G**//B3LYP/6–31+G* levels. All these six-membered NHCs are extremely ambiphilic with the more nucleophilic and electrophilic characters compared to the Arduengo type one. The aromaticity of singlet 2 and 4 is a significant contributor to their stability which is confirmed through their Nucleus-independent chemical shift(1)zz values. Finally, among 2–5, the normal NHC 2 is thermodynamically preferred but the remote NHC 4 is kinetically proffered over the other isomeric carbenes. The effects of different N- or C-substituted NHCs of 2 are studied using appropriate isodesmic reactions. The trimethylsilyl substituent exhibits slightly larger carbene stabilization in quinoline-derived NHCs than the pyridine analogue. © 2014 Wiley Periodicals, Inc.

ChemInform Abstract: Desilylative Carboxylation of Aryltrimethylsilanes Using CO2 in the Presence of Catalytic Phosphazenium Salt.

AbstractArylsilanes bearing a triethoxysilyl or trichlorosilyl group instead of the trimethylsilyl substituent do not react to the desired benzoic acid derivatives.

Steric Effects in Reactions of Decamethyltitanocene Hydride with Internal Alkynes, Conjugated Diynes, and Conjugated Dienes

Titanocene hydride [Cp*2TiH] (Cp* = η5-C5Me5) (1) readily inserts simple internal alkynes R1C≡CR2 into its Ti–H bond, yielding titanocene alkenyl Ti(III) compounds of two structural types. The less sterically congested products [Cp*2Ti(R1C═CHR2)] (2a–e) contain a σ1-bonded alkenyl group, whereas the products bearing at least one trimethylsilyl substituent and other bulky substituents (R1 = SiMe3; R2 = SiMe3, 4a; CMe3, 4b; and Ph, 4c) possess a remarkable Ti–H agostic bond of the σ1-bonded alkenyl group. This feature is consistent with solution EPR spectra of 4a–4c showing a doublet due to coupling of the hydrogen nucleus with the Ti(III) d1 electron. Compound 1 reacts with one molar equivalent of conjugated buta-1,3-diynes (RC≡C)2 to give η3-butenyne complexes (R = SiMe3, 5a; CMe3, 5b). The Ti(III) complexes 2a–2e and 5a and 5b were oxidatively chlorinated with PbCl2 to give Ti(IV) chloro-alkenyl complexes [Cp*2TiCl(R1C═CHR2)] 3a–3e and chloro-alkenynes 6a and 6b, respectively. 1H and 13C NMR spectra of 3a–3e and 6a and 6b revealed that these compounds form equilibria of two atropisomers differing by the anti- and syn-position of the chlorine and the alkenyl hydrogen atoms. Such atropisomers are denoted by appended (a) and (b), respectively. Compound 1 reacted with 1,3-butadiene to give a thermally stable π-bonded 1-methylallyl complex (7) and with penta-1,3-diene to give a thermally labile 1,3-dimethylallyl complex (8). In toluene-d8 solutions 7 dissociated at 80 °C and 8 at room temperature to give [Cp*Ti(C5Me4CH2)] and corresponding alkenes. Other methyl-substituted dienes, isoprene, 4-methylpenta-1,3-diene, and 2,3-dimethylbuta-1,3-diene, did not yield observable π-bonded allyl products; the dienes were, however, hydrogenated to olefins with concomitant formation of [Cp*Ti(C5Me4CH2)]. Compound 1 was shown to catalyze the hydrogenation of the alkynes and dienes to olefins and ultimately to alkanes under lower than atmospheric hydrogen pressure at room temperature. Single-crystal structures were determined for 3d(a), 3e(a), 4a–4c, 5a, 6b, and 7.

The effect of trimethylsilyl substituents in the monomer unit on the energy characteristics of surfaces of polynorbornenes obtained via metathesis polymerization

The surface properties of polynorbornenes obtained via metathesis polymerization are investigated via the wetting method. The incorporation of trimethylsilyl groups into the monomer unit causes decreases in the specific free surface energies of the investigated polymers. It is found that the dispersion terms of the specific free surface energies correlate with the free-volume and gas-permeability values of the polymers. As shown through the methods of differential scanning calorimetry and FTIR spectroscopy, during heating in air, the considered polynorbornenes undergo oxidative crosslinking, which increases their specific free surface energies. The interphase energies of crosslinked polynorbornenes are determined at their interfaces with liquids modeling polar and nonpolar phases, and the values of the work of adhesion of the crosslinked polynorbornenes to the model liquids are calculated. It is shown that the work of adhesion of the polymer to the polar phase decreases after introduction of trimethylsilyl groups, while the combination of polar and nonpolar phases give rise to an increase in the work of adhesion.

A Mild Method for Regioselective Labeling of Aromatics with Radioactive Iodine

AbstractA novel technique to label ortho‐, meta‐, and para‐trimethylsilyl‐substituted aryl substituents with radioactive iodide is described. The method takes advantage of the ipso‐directing and activating properties of trimethylsilyl substituents on the arenes. The method was demonstrated on a griseofulvin analogue with promising anticancer properties and on lidocaine, a widely used local anesthetic drug. Treatment of a trimethylsilyl precursor with Tl(OCOCF3)3 followed by Na125I consistently afforded radioactive purities over 95 % in all cases.

Hydroboration of Alkyne‐Functionalized 1,3,2‐Benzodiazaboroles

AbstractReactions of equimolar amounts of dicyclohexylborane (DCB) with a series of 2‐alkynyl‐1,3‐diethyl‐1,3,2‐benzodiazaboroles R–C≡C–B(NEt)2C6H4 (1: R = nBu, 2: R = tBu, 3: R = Ph, 4: R = p‐Me2NC6H4, 5: R = p‐MeOC6H4) regioselectively afforded the 1,1‐diborylalkenes (Z)‐R(H)C=C{(B{NEt}2C6H4)B(c‐C6H11)2} (7a–11a) as the result of a cis‐addition of the BH bond of the borane to the C≡C triple bond in compounds 1 to 5. In contrast to this, reaction of Me3Si–C≡C–B(NEt)2C6H4 (6) with HB(c‐C6H11)2 yielded a 2.5:1 mixture of the 1,2‐diborylated alkene (Z)‐Me3Si{(c‐C6H11)2B}C=CHB(NEt)2C6H4 (12b) and the 1,1‐regioisomer (Z)‐Me3Si(H)C=C{B(NEt)2C6H4}{B(c‐C6H11)2} (12a). These results were rationalized by the differing π‐acceptor qualities of the benzodiazaborolyl and trimethylsilyl substituents at the C≡C triple bond in compounds 1 to 6. The novel products 7–12 were characterized by elemental analysis, mass spectrometry and NMR spectroscopy (1H, 11B, and 13C NMR). The molecular structures of 9b and 12b were substantiated by single‐crystal X‐ray diffraction analysis.

DFT studies on the mechanisms of palladium-catalyzed intramolecular arylation of a silyl C(sp3)–H bond

Detailed mechanisms of the intramolecular C(sp3)–C(sp2) bond formation via C(sp3)–H bond activation of a trimethylsilyl (TMS) substituent catalyzed by palladium(0) complexes have been investigated with the aid of density functional theory (DFT) calculations. The results reveal that the favorable catalytic cycle includes oxidative addition, ligand substitution, concerted metalation deprotonation (CMD) and reductive elimination steps. The CMD was found to be the rate-determining step with an overall free energy barrier of 26.4 kcal mol−1. For the analogous CMe3-substituted substrate, the C(sp3)–H bond activation of the CMe3 substituent was calculated to have a high free energy barrier of 29.9 kcal mol−1. Our calculation results show that during the deprotonation process of the TMS C(sp3)–H bond, the adjacent Si atom stabilizes the charge accumulated on the C(sp3)–H carbon and facilitates the C(sp3)–H bond activation due to the ability of Si to engage in Si–C hypervalent bonding.

Improved performance of solution-processable OLEDs by silyl substitution to phosphorescent iridium complexes

Development of new materials for solution-processable organic light-emitting diodes (OLEDs) has been gaining much attention recently. Although many results have been reported about new OLED materials, most of them are aiming for vacuum-deposition. The OLED materials developed for vacuum-deposition cannot be easily applied to solution processing due to their limited solubility and processability. We therefore designed and synthesized two kinds of iridium complexes, Ir(Si-bppy)2(acac) and Ir(bppy)2(acac), with similar chemical structures but with or without silyl substitution. From the optical and electrochemical characterization, Ir(Si-bppy)2(acac) showed an emission maximum at 537nm (ET1=2.33eV) and HOMO energy level of −5.13eV, while Ir(bppy)2(acac) showed an emission maximum at 531nm (ET1=2.31eV) and HOMO energy level of −5.14eV. While the optical and electrochemical properties are almost the same, we found out that OLED device performance can differ pretty fair. The emitting layer of our OLEDs consists of PVK, TPD, PBD and the iridium complexes. With the configuration of [ITO/PEDOT:PSS/emitting layer/CsF/Al], the maximum current efficiency of Ir(Si-bppy)2(acac) reached 24.5cd/A, while that of Ir(bppy)2(acac) was 18.0cd/A. This enhancement is attributed to the existence of the trimethylsilyl substituents at the pyridine ring of the ligand, which increased solubility and processability of the compounds, and reduced intermolecular interaction.

A study on the properties of Zr-doped γ-Al2O3 xerogels hybridized with α-Al2O3 whiskers synthesized by solvothermal drying

Hydrophobic zirconium doped alumina xerogels and zirconium doped alumina xerogels hybridized with α-Al2O3 whisker were fabricated by Si(CH3)3 (trimethylsilyl substituent) modification of alcogels via ambient pressure solvothermal drying procedure. One-step solvent exchange and surface modification were simultaneously progressed by immersing alumina alcogels in hexamethyldisilazane/hexane solution. The synthesized Zr doped Al2O3 xerogels have shown increased phase transition temperature from γ-phase to α-phase over 200°C. Although phase transition temperature was increased, the porous structure of Zr-doped Al2O3 xerogel was collapsed over 1200°C losing its specific surface area. To maintain their porous structures at the elevated temperatures, the Zr-doped Al2O3 xerogels were hybridized with α-Al2O3 whiskers. The α-Al2O3 whiskers were grown at 1400°C and thus sustained its phase and structure over 1400°C. The α-Al2O3 whiskers in xerogels act as a pillar at elevated temperatures. As a result, Zr doped Al2O3 xerogels hybridized with α-Al2O3 whiskers maintained their porous structure over 1400°C even though there was a phase transition from γ-Al2O3 to α-Al2O3.

Meso-Bis{η5-1-[1-(dimethylamino)ethenyl]-3-(trimethylsilyl)cyclopentadienyl}iron(II) and the cobalt(II) analogue

The two title crystalline compounds, viz. meso-bis{η(5)-1-[1-(dimethylamino)ethenyl]-3-(trimethylsilyl)cyclopentadienyl}iron(II), [Fe(C(12)H(20)NSi)(2)], (II), and meso-bis{η(5)-1-[1-(dimethylamino)ethenyl]-3-(trimethylsilyl)cyclopentadienyl}cobalt(II), [Co(C(12)H(20)NSi)(2)], (III), were obtained by the reaction of lithium 1-[1-(dimethylamino)ethenyl]-3-(trimethylsilyl)cyclopentadienide with FeCl(2) and CoCl(2), respectively. For (II), the trimethylsilyl- and dimethylaminoethenyl-substituted cyclopentadienyl (Cp) rings present a nearly eclipsed conformation, and the two pairs of trimethylsilyl and dimethylaminoethenyl substituents on the Cp rings are arranged in an interlocked fashion. In the case of (III), the same substituted Cp rings are perfectly staggered leading to a crystallographically centrosymmetric molecular structure, and the two trimethylsilyl and two dimethylaminoethenyl substituents are oriented in opposite directions, respectively, with the trimethylsilyl group of one Cp ring and the dimethylaminoethenyl group of the other Cp ring arranged more closely than in (II).

Influence of the Acetylenic Substituent on the Intramolecular Carbolithiation of Alkynes

AbstractThe intramolecular carbolithiation of a series of propargylic ethers has been performed to evaluate the influence of the terminal substituent on the efficiency and the stereochemical outcome of the cyclization. Our results show that only 5‐exo‐dig cyclizations are observed, and dihydrobenzofurans are obtained exclusively. Depending on the nature of the terminal substituent, two cases can be considered. If the terminal substituent carried by the acetylenic carbon atom is itself a carbon atom, the cyclization can occur provided the terminal propargylic position bears a coordinating element and is at least disubstituted. When the cyclization occurs, it follows an anti‐carbolithiation pathway and thus leads to the E isomer of the exocyclic double bond. Only in one case (Ph) was a mixture of the E and Z isomers of the resulting olefin recovered. The cyclization can also take place if the alkyne is directly substituted by S or Si, provided the cyclization conditions are tuned. In the case of the trimethylsilyl substituent, a syn‐carbolithiation was observed. If the double bond is recovered, in most cases, in the exocyclic position, the products can aromatize directly for SPh‐substituted substrate 24. Furthermore, in the two latter cases, when alkylation of the vinyllithium intermediate is performed, isomerization of the double bond seems instantaneous.

Conformational stability and rotational energy barrier of RC60–C60R dimers: hyperconjugation versus steric effect

Open image in new window The conformational stability of 4, 4′ disubstituted HC60–C60R and RC60–C60R dimers were calculated at ONIOM approach (AM1:B3LYP/6-31+G**) and density functional theory (B3LYP/6-31G**). The new evidences for stability and rotational energy barriers of these dimers were obtained by natural bond orbital, natural steric and molecular orbital analyses. Based on B3LYP/6-31G** calculations, except for RC60–C60R (R = hydrogen, tert-butyl and trimethylsilyl) where gauche is the most stable conformer, trans is a global energy minimum. The greater stability of the gauche conformer of HC60–C60H over trans is the result of hyperconjugation, which dominates the instability caused by the steric effect. By increasing the size of the substituent of HC60–C60R dimers, the trans becomes sterically unstable but the hyperconjugation of bulky substituents dominates (the trans is global energy minimum). The hyperconjugation stability of RC60–C60R dimers dominates until R = iso-propyl (higher stability of trans). In the case of bulky tert-butyl and trimethylsilyl substituents, the steric energy of trans is large and overweighs the hyperconjugation effect. This favors gauche as the most stable conformer. The calculated rotational energy barrier for HC60–C60R and RC60–C60R dimers is less than 7.3 and more than 10 kcal/mol, respectively (depending on substitution).

Ortho‐Selective Nucleophilic Addition of Primary Amines to Silylbenzynes: Synthesis of 2‐Silylanilines

Cause and effect: The first ortho-selective nucleophilic addition reaction of amines to 3-substituted benzynes has been achieved. Despite a large trimethylsilyl substituent, primary amines attack the C2 position of 3-silylbenzynes to produce 2-silylanilines (see scheme). This outcome is likely to result from the inductive electron-donating effect of the silyl group, which overrides its steric repulsion with the approaching amines.

Synthesis and Olfactory Characterization of Novel Silicon‐Containing Acyclic Dienone Musk Odorants

AbstractWith an odor threshold value of 0.54 ng L–1 air, (3E,5E)‐7,7‐dimethyl‐5‐tert‐butylocta‐3,5‐dien‐2‐one (1a) constitutes the most potent member of a new family of acyclic musk odorants with ionone aspects. Replacement of the quaternary carbon atoms of 1a with silicon atoms leads to the (di)sila analogues 1b (replacement of the carbon atom C‐7), 1c (replacement of the quaternary carbon atom of the 5‐tert‐butyl group), and 1d (replacement of both quaternary carbon atoms). Compounds 1b–1d were prepared in multistep syntheses, and their olfactory properties were characterized. The disila analogue 1d turned out to be the weakest compound of the series (197 ng L–1 air), with only very faint musky aspects. The 7‐sila analogue 1b was very floral, of iononeand rose character, and also had a distinct musky note (1.63 ng L–1 air). The 5‐trimethylsilyl analogue 1c was the most musky sila odorant of the series (10.6 ng L–1 air), with rosy but not ionone‐like facets. Thus, exchange of one or both tert‐butyl groups of 1a for a trimethylsilyl substituent had quite dramatic effects on the odor, and steric bulk at the carbon atom C‐5 seems to be important for creating a musk odor impression.