Silyl Substituents Research Articles

Hydrogen/Deuterium Exchange Reactions and Transfer Hydrogenations Catalyzed by [C5Me5Rh(olefin)2] Complexes: Conversion of Alkoxysilanes to Silyl Enolates

A series of [C5Me5Rh(CH2CHR)2] complexes (1a−e) have been prepared in which the olefin bears a bulky silyl substituent, R = (a) SiMe3, (b) SiMe2OEt, (c) Si(OiPr)3, (d) SiMe(OSiMe3)2, (e) SiPh2OiPr. The solid-state structure of 1c has been determined by X-ray crystallography. When complex 1a is heated (50 °C) in deuterated solvents (C6D6, C6D5CD3, C6D5Cl, or (CD3)2CO), deuterium is incorporated into the olefinic sites. Thermolysis at higher temperatures results in further H/D exchange and deuteration of both the SiMe3 and C5Me5 groups. Heating 1a in C6D6 with added substrates (aniline, MeOtBu, MeOSiMe3, Cp2Fe, cyclopentene, or EtOAc) results in deuteration of these substrates via shuttling of deuterium from C6D6 to the olefinic sites and then into certain sites of the substrates. Thermolysis of 1a in the presence of vinyltrimethylsilane at higher temperatures results in C−Si bond cleavage and generation of a silacyclopentadiene complex (6) whose structure was determined by X-ray analysis. Thermolysis of 1c...

An ESR Study of a Tetrasilaspirobiindane Anion Radical. Stereoelectronic Electron-Accepting Effects of Trialkylsilyl Groups

Abstract ESR spectra of a tetrasilaspirobiindane anion radical (1−) showed a doublet of doublet hfs pattern at low temperatures and significant line-width alternation, indicating that there is remarkable difference in the electron-accepting effects between two silyl substituents at the benzene ring with the unpaired electron due to the ring conformation.

Nature of the Interactions between the β-Silyl Substituent and Allyl Moiety in (η3-Allyl)palladium Complexes. A Combined Experimental and Theoretical Study

Three new β-silyl-substituted (η3-allyl)palladium complexes were prepared (3−5) in order to study the substituent effects of the silyl functionality on the allyl−metal system. Complexes 3−5 show a downfield 13C NMR shift for the more substituted allylic terminus (C3). The deshielding effect of the silyl group on C3 is strongest when PPh3 ligands are coordinated to palladium. Both internal and external nucleophiles attack the less substituted allylic terminus (C1) with high selectivity. A theoretical analysis of the structure and stability of slightly simplified model compounds 7−10 was performed by applying density functional theory at the B3PW91 level. The theoretical results indicate hyperconjugative interactions between the π-allyl−metal system and the C−Si σ-bond. The intensity of these interactions depends on the conformation of the silyl substituent and on the σ-donor/π-acceptor character of the ancillary ligand on palladium. The β-substituent effects increase the thermodynamic stability of the comp...

Cycloadditions of Allylsilanes, Part 12. Regio- and Stereoselective Transformations of Silylbicyclo[n.3.0]alkanes

Lewis acid promoted [3 + 2] cycloaddition of 1-acetylcycloalkenes and allylsilanes provides the silylbicyclo[n.3.0]-alkanes 4 and 5. The sequence of regioselective Bayer-Villiger oxidation and elimination of acetic acid affords the 3-silylbicyclo[3.3.0]oct-1(5)-enes 8a and 9a and the 8-(triphenylsilyl)bicyclo[4.3.0]non-1(6)-ene (8b). Epoxidation and catalytic hydrogenation of these olefins proceeds with complete stereoselectivity anti relative to the silyl substituent. Ozonolysis of 8b leads to 3-(triphenylsilyl)cyclononane-1,5-dione (15).

Rhenium complexes of 2-triorganosilyl thiophenolate ligands. The syntheses and structures of [Bun 4nN] [ReO(2-Ph3SiC6H4S)4] and [Re(2-Ph2MeSiC6H4S)3(MeCN) (PPh3)

Reaction of the thiolates HSC 6H 4SiR 3-2 (R 3=Ph 3, Ph 2Me, PhMe 2) with a range of Re precursors leads to the new complexes [ReH 3(SC 6H 4SiPh 2Me-2) 2(PPh 3) 2], [ReO(SC 6H 4SiPh 2Me-2) 3(PPh 3)],[Re(OMe)(SC 6H 4SiPh 3-2) 2(MeCN)(PPh 3)] and[Re(SC 6H 4SiPh 3-2) 2(MeOH)(NO)(PPh 3)}]. The dependence of the product on the nature of the silyl substituents and the X-ray crystal structures of [ReO(SC 6H 4SiPh 3-2) 4][Bu n 4N] · Me 2CO and [Re(SC 6H 4SiPh 2Me-2) 3(MeCN)(PPh 3)] are discussed.

A new silyl substituted arachno-tetraborane(8) derivative

The reaction of nido-B 5H 9 with MeHSi(NMe 2) 2 yields a new silyl substituted tetraborane(8) derivative (Me 2N) 2BH 2MeSiB 4H 8 ( 1). The silyl substituent is part of a new four membered ring system SiR 2(NR′ 2) 2BH 2 + · The reaction product has been characterized by X-ray crystallography, multinuclear and multidimensional NMR spectroscopy, mass spectrometry and elemental analyses.

Synthesis, Properties, and Structure of Poly(silyl)pyridines. The Phantom of Intramolecular Si−N Bonding

2- and 3-silylpyridine (2, 3) have been prepared from the corresponding bromopyridines either directly by metathesis reaction with KSiH3 or via the lithiated pyridines and their reaction with halosilanes (in poorer yield). 3-Silylquinoline 4 was obtained from 3-lithioquinoline and tetra(ethoxy)silane followed by reduction with LiAlH4. 4-Silylpyridine (5), 2,6-disilylpyridine (9), 5/6-methyl-2-silylpyridine (6, 7), and 2-bromo-6-silylpyridine (8) were also obtained via the KSiH3 route, and traces of pentasilylpyridine (15) have been detected among the products of the analogous reaction with pentabromopyridine. These compounds and the di- and tripyridylsilanes 10−13 have been characterized by their analytical and spectroscopic data. The crystal structures of 6, 9, and 5-methyl-2-trimethylsilylpyridine (16) have been determined by low-temperature (“in situ”) X-ray diffraction techniques. In all cases the silyl substituents in the 2-position show bending toward the nitrogen atom of the ring (with angles N−C−S...

The Twofold Heck Reaction on 1,2-Dihalocycloalkenes and Subsequent 6π-Electrocyclization of the Resulting (E, Z, E)-1,3,5-Hexatrienes: A New Formal {2+2+2}-Assembly of Six-Membered Rings

1,6-Disubstituted (E,Z,E)-1,3,5-hexatrienes (4 and 5) were prepared by vicinal twofold Heck coupling reactions from 1,2-dibromocyclopentene (1), 1,2-dibromocyclohexene (2), 1,2-diiodocyclopentene (8), 1,2-diiodocyclohexene (9), 1-bromo-2-trifluoromethanesulfonyloxycyclohex-1-ene (11), or 1-chloro-2-nonafluorobutanesulfonyloxycyclohex-1-ene (19) with alkenes 3, e. g. methyl, tert-butyl, menthyl, 8-phenylmenthyl acrylate, styrene, and alkenylsilanes, respectively, in moderate to mostly good and very good yields (20-92 %). The coupling of alkenylsilanes 3f-k could only be achieved with the 1,2-diiodocycloalkenes 8 and 9, respectively. The corresponding hexatrienes 5 with two different substituents in the 1- and 6-positions were prepared by a sequence of two coupling reactions with different alkenes from 1-chloro-2-nonafluorobutanesulfonyloxycyclo-hex-1-ene (19) or by Wittig-Horner-Emmons olefination of 2-bromocyclohexene-1-carbaldehyde (24) and subsequent Heck reaction of the resulting (E,Z)-bromodienes 25. Several of the hexatrienes (4aa, ee, 5aa, ee, al, am, el) readily underwent a 6-electrocyclization upon heating in an inert atmosphere to give the 5- or 6-ring-annelated cis-5,6-disubstituted cyclohexadienes 26, 27 (50-95 %). Starting from 5am 2,3-disubstituted tetrahydronaphthalene 28am was formed under oxidative conditions (air) in the reaction and in the work-up procedure. The bissilyl-substituted derivatives 4ff, jj, 5ff did not cyclize under thermal conditions, apparently due to the steric demand of the two silyl substituents which would have to end up cis with respect to each other in the cyclohexadiene products.

Ab Initio Calculations of the Effects of Geminal Silyl Substituents on the Stereomutation of Cyclopropane and on the Singlet−Triplet Splitting in Trimethylene

(10/10)CASSCF and CASPT2N/6-31G* calculations predict that geminal silyl substituents at C(2) of the (0,0)-trimethylene diradical (2c) should strongly stabilize the lowest singlet state by hypercon...

New chelating silylamido ligands: syntheses and X-ray crystal structures of lithium and magnesium derivatives of [ t-Bu–HN–SiMe 2– o-C 6H 4–X] (X=OMe, NMe 2, CH 2NMe 2, CF 3)

New chelating silylamido ligands with four `directing metalation donor groups' (DMGs), OMe, NMe 2, CH 2NMe 2 and CF 3 on aryl moieties have been synthesized. The X-ray crystal structures of the dimeric lithium derivatives [{ t-BuN–SiMe 2– o-(C 6H 4)–DMG}Li] 2 ( 1) 2 (OMe), ( 2) 2 (NMe 2), ( 3) 2 (CH 2NMe 2) and ( 4) 2 (CF 3) reveal Li–DMG contacts in all four cases and decreased lone pair-aryl conjugation for OMe and NMe 2. In plane distortions are apparent for the alkyl and silyl substituents of the central (LiN) 2 rings in ( 1) 2–( 4) 2; these give rise to short `agostic' Li⋯H 3C– interactions with the t-Bu moieties. While the OMe, NMe 2 and CH 2NMe 2 groups exhibit `side on' lithium–heteroatom contacts, lithium coordinates to a CF 3 fluorine atom significantly more `end on'. The ability of the chelating silylamido ligands to coordinate metal ions other than lithium is demonstrated by the X-ray crystal structures of the magnesium complexes [{ t-BuN–SiMe 2– o-(C 6H 4)–OMe} 2Mg] ( 9) and [{ t-BuN–SiMe 2– o-(C 6H 4)–CH 2NMe 2}(OMe) 2Mg 2] ( 10).

Hochreaktive Trialkylsilylierungsreagentien aus Bis(trifluormethansulfonyl)imid - Silylierung von funktionellen Gruppen, alkinen und reaktiven Aromaten

Highly Reactive Trialkylsilylation Reagents Derived from Bis(trifluoromethanesulfonyl)imide – Silylation of Functional Groups, Alkines and Reactive Aromatics The synthesis of trialylsilyl-bis(trifluormethane-sulfonyl)imides 3 is described. Compounds 3 with bulky trialkylsilyl groups 3a,b only exist in the silatautomeric form 3a′, b′ under usual conditions, 3c in the N-trimethylsilyl structure. Despite of the bulky silyl substituents in 3a′, b′ their reactivity is higher than of trimethylsilyltriflate. Alcohols, carbonyl compounds, nitroalkanes and carboxylic acid esters are silylated in good yields, especially by the more reactive triisopropylsilyl derivative 3b′ in presence of tertiary amines. tert.-Butyl carboxylates and benzylcarboxylates are cleaved. Monosubstituted alkines and electron-rich (hetero) aromatics are carbosilylated in presence of N-ethyl-diisopropylamine.

Synthesis of a cis-5-cyclodecenone and cis fused hydronaphthalenols through control of the stereochemistry of the oxy-Cope rearrangement with the tri- n-propylsilyl substituent

In the course of preparing a trimethylsilyl substituted 5-cyclodecenone through anionic oxy-Cope rearrangement of a trans-1,2-divinylcyclohexanol, it was discovered that the silyl substituent in the divinylcyclohexanol is positioned so that it destabilizes [3,3]-sigmatropic rearrangement through the normally observed chair-like transition state. In the case of the tri- n-propylsilyl derivative, oxy-Cope rearrangement was observed to take place exclusively through the boat-like transition state to give ( E)-5-(tri- n-propylsilyl)-5-cyclodecenone with the cis double bond with respect to the ring. Acid-induced transannular cyclization of the E isomer led to 1,6-cyclization and generation of the cis-fused hydronaphthalenol.

Synthesis of 1-oxacephems from D-arabinal and L-rhamnal

Readily available from d-arabinal and l-rhamnal 2-C:1-N-carbonyl-2-deoxy-β- d-arabino- and -α- l-gluco-pyranosylamines 5, 25 and 26 were transformed into cephems 45 and 49 via a sequence of reactions consisting of alkylation of the β-lactam nitrogen atom, deprotection of pyranoid hydroxy groups, glycolic cleavage of the vic diol grouping, discrimination of carbon atoms which were separated by periodate oxidation, and formation of the 1,3-oxazine six-membered ring fused to the β-lactam fragment. Discrimination of two aldehyde groups obtained during the glycolic cleavage step was achieved after their reduction and protection of one of the hydroxymethyl group by a bulky silyl substituent or by lactonization.

Bromo( t-butyldimethylsilyl)fluoromethyllithium: A fluorinated nucleophilic carbenoid reagent stabilized by a silyl substituent

The title carbenoid reagent was generated by treatment of dibromofluoromethyl( t-butyl)dimethylsilane with butyllithium in THF at −78 °C and was allowed to react with aldehydes and ketones to give 1-fluoro-1-silyloxiranes in good yields. Alkylation of the silyl-substituted carbenoid was also achieved efficiently in good yields.

Novel Lewis Acid Promoted Reactions of Allylsilane Bearing Bulky Silyl Substituents and Aldehydes

Abstract Reported herein are two novel modes of reactions that were mediated by the proper choice of Lewis acid. Thus, by the influence of SnCl4, allyl-t-butyldimethylsilane reacted with aldehyde in 2:1 stoichiometry to afford a ketone derivative. In contrast, use of BF3•OEt2 led to the formation of a 1,3-dioxane derivative, which is a 1:2 adduct.

Palladium-Catalyzed Additions of Terminal Alkynes to Acceptor Alkynes

The development of addition reactions wherein the product is the simple sum of the reactants plus anything else (only needed catalytically) constitutes an important goal for enhanced synthetic efficiency. The C−H bond of terminal alkynes (the donor alkynes) can be added to either terminal alkynes (self-coupling) or activated internal alkynes (cross-coupling) (the acceptor alkynes) in the presence of a catalytic amount of palladium acetate and an electron rich sterically encumbered ligand, tris(2,6-dimethoxyphenyl)phosphine. The activated internal alkynes for cross-coupling (the acceptor alkyne) include alkynes bearing an ester, sulfone, and ketone. Self-coupling is completely overwhelmed by cross-coupling, even at 1:1 ratios of donor and acceptor alkynes. The reaction exhibits extraordinary chemoselectivity with free carboxaldehydes, alcohols, ketones (saturated and conjugated), esters (saturated and conjugated), sulfones (saturated and conjugated), malonates, and silyl ethers all proving to be compatible. A 1:2 donor/acceptor alkyne adduct can also be optimized. Ethyl propiolate fails as an acceptor but its C-silylated analogue serves with the proper choice of silyl substituent. The products of the latter serve as useful precursors to β-keto esters. An iterative sequence is readily performed and led to a novel conformationally rigid retinoid analogue. The mechanism of this mild method for construction of conjugated enynes, versatile building blocks, is discussed.

The First Carborane with a Distorted Cuboctahedral Structure

As intermediates in the rearrangement of icosahedral 1,2-carboranes to give the corresponding 1,7-carboranes, carboranes with cuboctahedral structure were postulated by Lipscomb in the 1960s. The first example of this type of carborane, (CSiMe3)4B8H8, has now been prepared and characterized (X-ray crystal structure on the right). The stability of this compound and the minor deviation of the C4B8 frame from cuboctahedral geometry can be explained by the steric demand of the silyl substituents.

Conformational preference of the silyl group. Ab initio and molecular mechanics studies

The energy difference between the equatorial and axial conformations (the A-value) for the methyl and silyl substituents in monocyclohexane has been estimated by ab initio molecular orbital and molecular mechanics methods. MP4(SDTQ)/6-31G ∗//MP2/6-31G ∗ results predict the A-values to be 1.90 kcal mol −1 for the methyl group, and 1.69 kcal mol −1 for the silyl group, respectively. After including zero-point vibrational energies, thermal vibrational energies, and entropy corrections, the A-values become 2.14 kcal mol −1 for the methyl group and 1.90 kcal mol −1 for the silyl group, respectively. These values are about 0.5 kcal mol −1 higher than those observed by NMR experiments. However, MM3 estimates the A-values of the methyl and silyl groups as 1.78 and 1.16 kcal mol −1, respectively. The slightly smaller A-value of the silyl group with respect to the methyl group appears to be ascribed to the reduced steric interactions due to the longer CSi bond compared to the CC bond. A perfect correlation between the A-values and the gauche/anti energy differences may indicate that the MM3 calculated A-values entirely depend on the selected values of the torsional parameters.

1995 R.U. Lemieux Award Lecture Ketenes and bisketeness: organic chemistry in microcosm

Silyl substituents stabilize ketenes, and permit the preparation of persistent bisketenes, which show characteristic 13C, 17O, and 29Si NMR chemical shifts. The structures and conformations of bisketenes have been examined, as well as their interconversion with cyclobutenediones. A tetraketene has been prepared, and the hydration reactivity of carbon suboxide has been measured. Key words: bisketenes, structures and conformations; cyclobutenedione interconversion.

Synthesis and Structure of a Digallane with Tris(trimethylsilyl)silyl and Chloro Substituents

Synthesis and Structure of a Digallane with Tris(trimethylsilyl)silyl and Chloro Substituents