Palladium-catalyzed synthesis of silyl-substituted enynes

Treatment of 1-ethynyl-1-phenyltetramethyldisilane, ethynyldimethylphenylsilane and ethynylmethyldiphenhlsilane with tetrakis(triphenylphosphine)palladium(0) at 100°C gave the corresponding enyne by head-to-head coupling, as the single regioisomer in high yield. Ethynyl-substituted mono- and di-silanes that have no phenyl group on the ethynylsilicon atom, however, afforded the enynes only in low yields.     

Palladium-catalyzed asymmetric hydrosilylation of 1,3-dienes

Advances in the palladium-catalyzed asymmetric hydrosilylation of 1,3-dienes are presented according to substrate types and chiral monophosphine ligands. Chiral monodentate phosphine ligands with a binaphthyl moiety have been proven to be the most efficient ligands for cyclic 1,3-dienes, and planar chiral ferrocenylmonophosphine ligands with two ferrocenyl moieties for linear 1,3-dienes. The ferrocenylmonophosphine ligands have expanded the substrate scope to 1,3-enynes in the asymmetric hydrosilylation. Palladium-catalyzed asymmetric hydrosilylation of 1,3-dienes and 1,3-enynes leads to the stereoselective synthesis of allylsilanes and allenylsilanes, respectively.     

Palladium-catalyzed regioselective reactions of silyl-substituted allylic carbonates and vinyl epoxide

Palladium-catalyzed reaction of α-substituted γ-trimethylsilylated allylic carbonates with carbonucleophiles took place regioselectively at the α-carbon. Reaction of formic acid took place at γ-carbon to afford allylic silanes. Methyl acetoacetate attacked regioselectively at 2-carbon of 1,2-epoxy-4-trimethylsilyl-3-butene.     

Palladium-catalyzed asymmetric hydrosilylation of styrenes with trichlorosilane

Asymmetric hydrosilylation of styrenes with trichlorosilane catalyzed by palladium complexes coordinated with chiral monodentate phosphorus ligands produces chiral benzylic silanes, which can be converted into enantiomerically enriched benzylic alcohols by a stereospecific oxidative cleavage of the carbonsilicon bond with retention of configuration. Due to its high catalytic activity and perfect regioselectivity without producing any by-products, it has recently become a potent methodology to prove the efficacy of newly-developed chiral ligands, especially monodentate ones, in asymmetric synthesis.     

Palladium-catalyzed regio- and stereoselective hydrosilylation of electron-deficient alkynes

Highly regio- and stereoselective hydrosilylation applicable to a broad range of electron-deficient alkynes has been established using palladium catalysis. The synthetic utility of the method has been demonstrated by further transformations of α-silylalkenes, particularly Hiyama coupling and stereoinverting iododesilylation followed by Suzuki-Miyaura coupling, which enables stereodivergent syntheses of α-arylenoates.     

Palladium-catalyzed asymmetric diene cyclization/hydrosilylation employing functionalized silanes and disiloxanes.

Pentasubstituted disiloxanes and silanes of the form HSiMe(2)CH(x)Ph(3-x)(x = 1 or 2) reacted with dimethyl diallylmalonate (1) and other functionalized 1,6-dienes in the presence of a catalytic 1:1 mixture of (N-N)Pd(Me)Cl [N-N = (R)-(+)-4-isopropyl-2-(2-pyridinyl)-2-oxazoline] [(R)-2] and NaBAr(4) [Ar = 3,5-C(6)H(3)(CF(3))(2)] to form the corresponding silylated cyclopentanes in good yield with high diastereoselectivity. The enantioselectivity of cyclization/hydrosilylation of 1 with disiloxanes and functionalized silanes at -20 degrees C increased in the following order: HSiMe(2)OSiMe(3) (75% ee) < HSiMe(2)OSiMe(2)-t-Bu (80% ee) < HSi(i-Pr)(2)OSiMe(3) (86% ee) = HSiMe(2)Bn (86% ee) < HSiMe(2)OSi(i-Pr)(3) (89% ee) < HSiMe(2)OSiPh(2)-t-Bu (91% ee) < HSiMe(2)CHPh(2) (93% ee). Silylated cyclopentanes derived from HSiMe(2)OSiMe(3) were oxidized with excess KF and peracetic acid at room temperature for 48 h to form the corresponding hydroxymethylcyclopentanes in good yield (82-95%). Silylated cyclopentanes derived from HSiMe(2)OSiPh(2)t-Bu were oxidized with a mixture of tetrabutylammonium fluoride and either H(2)O(2) or peracetic acid to form the corresponding alcohols in 48-76% yield. Silylated carbocycles generated from benzhydryldimethylsilane were oxidized with a mixture of TBAF/KHCO(3)/H(2)O(2) in 71-98% yield. Asymmetric cyclization/hydrosilylation/oxidation employing benzhydryldimethylsilane tolerated allylic and terminal olefinic substitution and a range of functional groups.     

Synthesis of silyl-substituted vinylallenes

The coupling between the 5-chloro-3-en-1-ynes $\text{1a}$ - $\text{1e}$ and trimetylchlorosilane gives rise to the vinylallenes $\text{2a}$ - $\text{2e}$ substituted by a trimethylsilyl group on the allenic moiety.     

Palladium-catalyzed hydrosilylation of ynones to access silicon-stereogenic silylenones by stereospecific aromatic interaction-assisted Si–H activation

Hydrosilylation is one of the most important reactions in synthetic chemistry and ranks as a fundamental method to access organosilicon compounds in industrial and academic processes. However, the enantioselective construction of chiral-at-silicon compounds via catalytic asymmetric hydrosilylation remained limited and difficult. Here we report a highly enantioselective hydrosilylation of ynones, a type of carbonyl-activated alkynes, using a palladium catalyst with a chiral binaphthyl phosphoramidite ligand. The stereospecific hydrosilylation of ynones affords a series of silicon-stereogenic silylenones with up to 94% yield, 20:1 regioselectivity and 98:2 enantioselectivity. The density functional theory(DFT) calculations were conducted to elucidate the reaction mechanism and origin of high degree of stereoselectivity, in which the powerful potential of aromatic interaction in this reaction is highlighted by the multiple C–H-π interaction and aromatic cavity-oriented enantioselectivitydetermining step during desymmetric functionalization of Si–H bond.     

Expedient, high-yielding synthesis of silyl-substituted salen ligands

Described is an efficient synthesis of silyl-substituted salen ligands, used for the preparation of enantioselective catalysts. The salicylaldehyde precursors are synthesized from the silyl ethers of 2,6-dibromophenols via a one-pot double lithium halogen exchanges, to induce an intramolecular retro-Brook rearrangement and allow introduction of the aldehyde group. Condensation of the salicylaldehyde products with a chiral diamine affords the silyl-substituted salen ligands in high yields. The use of other electrophiles allows easy access to silyl-substituted phenolic esters, ketones, and boronic acids.     

Proton magnetic resonance spectra of some silyl-substituted ethylenes

Conclusions The PMR spectra of a number of silyl-substituted ethylenes were obtained. An explanation was given for the positions of the resonance lines of the olefinic protons.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 4, pp. 875–876, April, 1971.     

Synthesis and neurotropic activity of silyl-substituted fufural thiosemicarbazones

5-Silyl-substituted-2-furaldehyde thiosemicarbazones were prepared by the condensation of the corresponding furfurals or furfural diethylace-tals with thiosemicarbazide. The neurotropic activity of the synthesized thiosemicarbazones has been studied. The majority of the compounds examined possess high or medium neurotropic activity of the depressant type. They also show antihypoxic activity and decrease the duration of phenamine stereotype behaviour. Structure-activity correlation has been found.     

Organometallic chemistry of fluorinated propadienes and butadienes

1,1-Difluoroallene (1,1-difluoropropadiene) and tetrafluoroallene are not only strong π acidic ligands but versatile starting materials for the generation of more complex fluoroorganic ligands by metal induced dimerization reactions, CF bond activation and fluorine migration. Palladium catalyzed CC coupling reactions of organometallic fluorovinyl compounds allow the systematic synthesis and study of the ligand properties of fluorinated buta-1,3-dienes. Furthermore, they are the key compounds for the synthesis of butatrienes like tetrafluorobutatriene which now can be studied in detail.     

The reaction of π-cyclopentadienyldicarbonylcobalt with silyl-substituted acetylenes

The reaction of (π-C₅H₅)Co(CO)₂ with PhCCSiMe₂R (R = Me, SiMe₃) gave two isomeric cyclobutadiene complexes, cis- and trans-(π-C₅H₅)Co[Ph₂C₄(SiMe₂R)₂], in almost quantitative yields. However, the reaction with RMe₂SiCCSiMe₂R (R = Me, Ph) led to the formation of new dinuclear cobalt complexes. For example, with bis(trimethylsilyl)acetylene, (π-C₅H₅)₂Co(CO)[(Me₃Si)₂C₂] was obtained quantitatively. The latter was further converted to (π-C₅H₅)Co(Ph₄C₄) and (πC₅H₅)Co[cis-Ph₂C₄(Me₃Si)₂] by treatment with PhCCPh. The physical properties and spectroscopic characteristics of these new compounds are described.     

Cyclic voltammetry and theoretical calculations of silyl-substituted 1,4-benzoquinones

Cyclic voltammograms of 2,3,5,6-tetrakis(trimethylsilyl)-1,4-benzoquinone (1a), 2,3,5,6-tetrakis(dimethylvinylsilyl)-1,4-benzoquinone (1b), 2,3,5,6-tetrakis(dimethylsilyl)-1,4-benzoquinone (1c), 4,4,6,6,10,10,12,12-octamethyl-4,6,10,12-tetrasilatricyclo[7.3.0.0^3,7]dodeca-1(9),3(7)-diene-2,8-dione (1d), and 5-t-butyl-2-(pentamethyldisilanyl)-1,4-benzoquinone (5h) showed that the first reduction step was reversible and that the second step was irreversible. The first half-wave reduction potentials of 1a, 1b, 1c, and 1d shifted negatively relative to 1,4-benzoquinone by −0.31, −0.24, −0.03, and −0.18 V, respectively. These results demonstrated that the electron-accepting ability of the chair-form quinones 1a and 1b was lower than that of the planar quinones 1c and 1d. The of 5h (−0.93 V vs. Ag/Ag⁺) was quite similar to that of 5-t-butyl-2-trimethylsilyl-1,4-benzoquinone (5a, −0.94 V). A cyclic voltammogram of dimethylsilylene-bridged 1,4-benzoquinone dimer 7 showed two kinds of (−0.76 and −0.94 V). The electrochemical behavior of 7 would be interpreted in terms of near-neighbor interactions between reduced and non-reduced quinone units. Theoretical calculations of the silyl-1,4-benzoquinones reproduced well the solid state structures of the compounds. Also, the computed vibrational frequencies of the silyl-1,4-benzoquinones were in good agreement with the IR absorption frequencies of the CO units in the compounds. The LUMO energy levels of the silyl-1,4-benzoquinones were quantitatively proportional to the .     

The hydrosilylation of propylene

The reactions of separate and competitive hydrosilylation of propylene with HSiCl₃, MeSiHCl₂, Me₂SiHCl, and MePh₂SiH in the presence of the Speier catalyst (SC) with different additives and a catalyst obtained from SC and propylene were studied. A mutual influence of the hydrosilanes in the competitive reactions was found. The influence of various additives to SC on the process was considered. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2048–2051, October, 1998.     

Chemoselective hydrosilylation of hydroxyketones

A chemoselective method for the hydrosilylation of ketones has been developed, using the combination of triphenylsilane and a catalyst prepared from Ni(COD)₂ and the simple N-heterocyclic carbene IMes. The most notable feature of this method is that free hydroxyls are largely unaffected, thus providing a simple one-step procedure for the conversion of hydroxyketones to mono-protected diols, wherein the protecting group is exclusively installed on the ketone-derived hydroxyl. The process is typically high yielding with both simple ketones and more complex hydroxyketone substrates.     

Silver-catalyzed hydrosilylation of aldehydes

Silver triflate, either alone or in the presence of an appropriate phosphine or NHC ligand, has been shown to catalyze the chemoselective hydrosilylation of aromatic and aliphatic aldehydes to yield silyl ethers, thus representing the first systematic application of silver species as catalysts for the hydrosilylation of unsaturated organic substrates.     

New nitrogenated siloxy butadienes from 1,3-dichloroacetone

New 1-formamido-2-siloxy-1,3-butadienes have been generated from 1,3-dichloroacetone by means of phosphorane formation, formamido substitution, Wittig olefination and silylation. The procedure demonstrates the utility and versatility of this methodology for the formation of polysubstituted dienes, designed as useful building blocks for the synthesis of polycyclic alkaloids and related analogues.