Vinylsilane Moiety Research Articles

From Alkynes to Heterocycles through Metal-Promoted Silylformylation and Silylcarbocyclization Reactions

Oxygen and nitrogen heterocyclic systems are present in a large number of natural and synthetic compounds. In particular, oxa- and aza-silacyclane, tetrahydrofuran, benzofuran, cycloheptadifuranone, cycloheptadipyrrolone, pyrrolidine, lactone, lactam, phthalan, isochromanone, tetrahydroisoquinolinone, benzoindolizidinone, indoline and indolizidine scaffolds are present in many classes of biologically active molecules. Most of these contain a C=O moiety which can be easily introduced using carbonylative reaction conditions. In this field, intramolecular silylformylation and silylcarbocyclization reactions may afford heterocyclic compounds containing a carbonyl functional group together with a vinylsilane moiety which can be further transformed. Considering these two aspects, in this review a detailed analysis of the literature data regarding the application of silylformylation and silylcarbocyclization reactions to the synthesis of several heterocyclic derivatives is reported.

Facile synthesis of multifunctionalized allenes by magnesium-promoted reductive silylation of aromatic conjugated ynones

Mg-promoted reductive silylation of 4-phenyl-3-butyn-2-one in N,N-dimethylformamide (DMF) in the presence of chlorotrimethylsilane brought about double silylation at the β-carbon and oxygen atoms of the carbonyl group to give a multifunctionalized allene with a vinylsilane moiety and a silyl enol ether moiety. A variety of allenes can be synthesized through this simple methodology. Acid-catalyzed hydrolysis of an allene derived from the trimethylsilylation of 4-phenyl-3-butyn-2-one resulted in the formation of the corresponding benzalacetone with a trimethylsilyl group at the β-position of the carbonyl group in high yield.

ChemInform Abstract: Formation of Vinyl‐, Vinylhalide‐ or Acyl‐Substituted Quaternary Carbon Stereogenic Centers Through NHC—Cu‐Catalyzed Enantioselective Conjugate Additions of Si‐Containing Vinylaluminums to β‐Substituted Cyclic Enones.

A catalytic method for enantioselective conjugate addition (ECA) of Si-containing vinylaluminum reagents to β-substituted cyclopentenones and cyclohexenones is described. Reactions are promoted by 1.0−5.0 mol % of a bidentate NHC−Cu complex, which is prepared from air-stable CuCl2•2H2O and used in situ, and typically proceed to completion within 15−20 min. The requisite vinylmetals are generated efficiently by a site-selective hydroalumination of an alkyne with dibal-H. The desired products, containing a quaternary carbon stereogenic center, are obtained in 48−95% yield after purification and in 89:11 to >98:2 enantiomer ratio (er). The vinylsilane moiety within the products can be functionalized to afford acyl, vinyliodide, or desilylated alkenes in 67% to >98% yield and with >90% retention of the alkene’s stereochemical identity. The utility of the catalytic process is illustrated in the context of a concise enantioselective synthesis of riccardiphenol B.

Formation of vinyl-, vinylhalide- or acyl-substituted quaternary carbon stereogenic centers through NHC-Cu-catalyzed enantioselective conjugate additions of Si-containing vinylaluminums to β-substituted cyclic enones.

A catalytic method for enantioselective conjugate addition (ECA) of Si-containing vinylaluminum reagents to β-substituted cyclopentenones and cyclohexenones is described. Reactions are promoted by 1.0-5.0 mol % of a bidentate NHC-Cu complex, which is prepared from air-stable CuCl(2)•2H(2)O and used in situ, and typically proceed to completion within 15-20 min. The requisite vinylmetals are generated efficiently by a site-selective hydroalumination of an alkyne with dibal-H. The desired products, containing a quaternary carbon stereogenic center, are obtained in 48-95% yield after purification and in 89:11 to >98:2 enantiomer ratio (er). The vinylsilane moiety within the products can be functionalized to afford acyl, vinyliodide, or desilylated alkenes in 67% to >98% yield and with >90% retention of the alkene's stereochemical identity. The utility of the catalytic process is illustrated in the context of a concise enantioselective synthesis of riccardiphenol B.

ChemInform Abstract: Vinylsilane-Terminated Cycloacylation: A General Synthetic Approach to Four- to Six-Membered Cyclic Ketones and Its Regiochemical Features.

Abstract Intramolecular acylations of m-trimethylsilyl-m-alkenoyl chlorides (m=4 and 5) are described which afford the expected α-alkylidenecycloalkanone and/or the unexpected cycloalkenone, depending markedly upon the substitution pattern on the vinylsilane moiety and/or the chain length (m).

Synthesis of a new oxazoline macromonomer for dispersion polymerization

We synthesized new macromonomers containing vinylsilane moiety by cationic ring-opening polymerization of 2-ethyl-2-oxazoline. Kinetic studies proved that initiation with vinylsilane derivative, bearing chloride counterion, is a fast process followed by a slow propagation and absence of termination. We used the dispersion polymerization of styrene as test for the stabilizing efficiency of the newly synthesized macromonomer. The dispersion polymerization tests allowed the investigation of the effect of the macromonomer concentration on the number average diameter of the prepared polystyrene microspheres and its polydispersity index. In addition, we compared the stabilizing efficiency of commercial poly(N-vinylpyrollidone) with that of our macromonomer. Copolymerization of styrene with 10 wt% oxazoline macromonomer yielded stable and monodisperse microspheres, having the number average diameter of 1.8 μm and a good size polydispersity index of 1.05.

Thiol-ene chemistry of vinylsilanes

The thiol-ene addition reaction is a clean, effective method for the introduction of the thioether functionality into unsaturated compounds. Though widely used in organic, polymer and materials chemistry, the application of the thiol-ene reaction in organosilicon chemistry has been relatively minimal. The literature discussed here emphasizes the synthetic utility of thiol-ene reactions of vinylsilanes by highlighting the following areas: (1) the tolerance of various vinylsilane derivatives, (2) the reactivity of various substituents on the vinylsilane moiety, (3) the simplicity of reaction conditions, and (4) the versatility of reaction products as intermediates.

Synthesis of 2,6-disubstituted dihydropyrans via an efficient BiBr 3-initiated three component, one-pot cascade

The rapid synthesis of cis-2,6-disubstituted dihydropyrans is achieved in a three-component, one-pot cascade reaction. BiBr 3-mediated addition of ketene silyl acetals or silyl enol ethers to β,γ-unsaturated cis-4-trimethylsilyl-3-butenal provides a Mukaiyama aldol adduct containing a vinylsilane moiety tethered to a silyl ether. Addition of a second aldehyde initiates a domino sequence involving intermolecular addition followed by an intramolecular silyl-modified Sakurai (ISMS) reaction. Isolated yields of this one-pot reaction vary from 44 to 80% and all compounds were isolated as the cis-diastereomers (10 examples).

On the Synthesis and Reactivity of Enantiopure Azetidines

Abstractβ‐Amino alcohols with an (E)‐vinylsilane moiety were cyclized in the presence of N‐bromosuccinimide to afford diastereomerically pure azetidines. The chemo‐ and stereoselectivities of this bromocyclization are discussed. AM1 calculations support the observed chemoselectivity. The reactivity of the azetidines towards fluorinated reagents was studied. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

1,2,3-Triols: Partial Oxidation of γ-Silyl Allylic Alcohols with Ozone

In a unique oxidation of an alkene with ozone, the vinylsilane moiety does not undergo the normal cleavage of the double bond. The solvent is the key to the unusual reaction since a 1:1 mixture of 2:(usual ozonolyzed product) was observed in CH2Cl2. The reaction is chemoselective since an alkene was oxidized in presence of an alkyne group. The authors noted that the silyl peroxides are stable and require no special handling (slow degradation above 80 °C).

Synthesis of Enantiopure Azetidines: A Route to New β‐Amino Alcohols.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access the actual ChemInform Abstract, please click on HTML or PDF.

Synthesis of enantiopure azetidines: a route to new β-amino alcohols

β-Amino alcohols possessing an E vinylsilane moiety were cyclized in the presence of N-bromosuccinimide to afford diastereoisomerically pure polyfunctional azetidines. These azetidines were then transformed into enantiopure β-amino alcohols with a Z vinylic bromide moiety.

Silylcuprates from Allene and Their Reaction with α,β-Unsaturatedd Nitriles and Imines. Synthesis of Silylated Oxo Compounds Leading to Cyclopentane and Cycloheptane Ring Formation

The silylcupration of allenes and the subsequent capture of the intermediate cuprate with alpha,beta-unsaturated nitriles is reported. The influence of the substitution of the nitrile, the nature of the silylcopper species, and the temperature on the selectivity of the reaction is studied. An interesting diaddition process was observed (1,2-addition and 1,4-addition), leading to oxo compounds which simultaneously have an allylsilane and a vinylsilane group. The different reactivity of these two units has been employed in the intramolecular allylsilane-terminated cyclization of these adducts, where the vinylsilane moiety remains unchanged. To shed some light on the reaction pathway, the behavior of alpha,beta-unsaturatedd imines was also checked and a new and convenient way for cycloheptane annulation presented. In light of the former results a feasible mechanism is proposed.

Further Studies on Palladium‐Catalyzed Bismetallative Cyclization of Enynes in the Presence of Bu3SnSiMe3.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Further studies on palladium-catalyzed bismetallative cyclization of enynes in the presence of Bu 3SnSiMe 3

Bismetallative cyclization of enynes with Bu 3SnSiMe 3 catalyzed by Pd(0) complex was realized for the first time, which gives cyclized products containing a vinylsilane moiety and a homoallyltin moiety in good yield. In this cyclization, Pd 2(dba) 3·CHCl 3 or Pd(OH) 2 on charcoal is effective as a Pd(0) catalyst and the addition of a phosphine ligand increased the formation of alkyne bismetallation by-product. On the other hand, it was found that a nucleophilic N-heterocyclic carbene could be utilized as a ligand for this cyclization. The utility of the cyclized products obtained from this cyclization in synthetic organic chemistry have been proven by transformation into cyclopropanol derivatives.

Pd(0)‐Catalyzed Bismetallative Cyclization of Enynes in the Presence of Bu3SnSiMe3 Using N‐Heterocyclic Carbene as a Ligand.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Pd(0)‐Catalyzed Bismetallative Cyclization of Enynes in the Presence of Bu3SnSiMe3 Using N‐Heterocyclic Carbene as a Ligand

AbstractIt was found that a nucleophilic N‐heterocyclic carbene could be utilized as a ligand for Pd(0)‐catalyzed bismetallative cyclization of enynes in the presence of Bu3SnSiMe3. An imidazolium salt having a bulky alkyl group attached to both nitrogen atoms of its imidazol‐2‐ylidene skeleton or an imidazolinium salt is suitable as a ligand precursor. The cyclized products obtained from this cyclization have both a vinylsilane moiety and a homoallylstannane moiety, whose utility in synthetic organic chemistry has been proven by transformation into cyclopropanol derivatives.

Cyclizative radical carbonylations of azaenynes by TTMSS and hexanethiol leading to alpha-silyl- and thiomethylene lactams. Insights into the E/Z stereoselectivities.

Free-radical mediated cyclizative carbonylations of azaenynes were carried out using TTMSS as a radical mediator to compare the efficiency and the stereochemistry with those using tributyltin hydride. Using a substrate concentration of 0.1 M, the reactions gave good yields of alpha-silylmethylene lactams having four to seven-membered rings. The observed E-diastereoselectivity of the resulting vinylsilane moiety is in sharp contrast to the Z-selectivity observed during the analogous carbonylation using tributyltin hydride. When hexanethiol was used as the radical mediator, alpha-thiomethylene lactams were formed with E-favoring stereoselectivity again. Ab initio and DFT molecular orbital calculations on the stability of E and Z products were carried out for a set of five-membered methylene lactams bearing SnH3, SiH3, and SMe groups. The distinct thermodynamic preference for the Z-isomer was only predicted for the Sn-bearing lactam. A steric effect due to the bulky (TMS)3Si group is proposed for the E-selectivity observed in the TTMSS-mediated reaction.

The Intriguing Reactivity of Functionalized β‐Amino Alcohols with Glyoxal: Application to a New Expedient Enantioselective Synthesis of trans‐6‐Alkylpipecolic Acids.

AbstractFor Abstract see ChemInform Abstract in Full Text.

The intriguing reactivity of functionalized beta-amino alcohols with glyoxal: application to a new expedient enantioselective synthesis of trans-6-alkylpipecolic acids.

New beta-amino alcohols possessing a vinylsilane moiety were reacted with glyoxal to produce lactones that were transformed in three steps in enantiopure pipecolic acid derivatives. The key step was a totally diastereoselective ene-iminium cyclization, whose mechanism can be viewed as a direct cyclization of the vinylsilane moiety onto the iminium ion. The reactivity of two beta-amino alcohols having an allylsilane terminator was also examined. Their difference of reactivity toward glyoxal can be ascribed to the intervention of a carbocation, which presents a behavior that differs according to the position of the trimethylsilyl group.