Lithium silyl cuprates

Keywords Reaction Intermediates / Tris(trimethylsilyl)silyllithium / Lithium Silyl Cuprates / Disilagermirane... 

Although ethereal solutions of methyl lithium may be prepared by the reaction of lithium wire with either methyl iodide or methyl bromide in ether solution, the molar equivalent of lithium iodide or lithium bromide formed in these reactions remains in solution and forms, in part, a complex with the methyllithium. Certain of the ethereal solutions of methyl 1ithium currently marketed by several suppliers including Alfa Products, Morton/Thiokol, Inc., Aldrich Chemical Company, and Lithium Corporation of America, Inc., have been prepared from methyl bromide and contain a full molar equivalent of lithium bromide. In several applications such as the use of methyllithium to prepare lithium dimethyl cuprate or the use of methyllithium in 1,2-dimethyoxyethane to prepare lithium enolates from enol acetates or triraethyl silyl enol ethers, the presence of this lithium salt interferes with the titration and use of methyllithium. There is also evidence which indicates that the stereochemistry observed during addition of methyllithium to carbonyl compounds may be influenced significantly by the presence of a lithium salt in the reaction solution. For these reasons it is often desirable to have ethereal solutions... 

Regiospecific route to silyl enol ethers.11 Silyl enol ethers arc formed regio-specifically by reaction of lithium dialkyl cuprates or diaryl cuprates with 2-trimcthylsiloxyallyl halides. An allylic rearrangement occurs in reactions with hindered halides. 

Other addition reactions Additions involving carbon Copper(I) chloride, 85 Lithium bis(dimethylphenyl-silyl)cuprate, 161 Manganese(III) acetate, 171 Mercury(II) chloride, 175 2-(Phenylseleno)acrylonitrile, 244 Threophos, 298 Additions involving nitrogen Benzeneselenenyl halides, 26 Additions involving oxygen Bis(Tj5-cyclooctadienyl)ruthe-nium(II), 35... 

N-Phenylselenophthalimide, 245 Additions involving selenium Benzeneselenenyl halides, 26 N-Phenylselenophthalimide, 245 Miscellaneous reagents Aluminum chloride, 15 Lithium bis(dimethylphenyl-silyl)cuprate, 161 Organotin reagents, 211 Addition reactions to carbonyl... 

Similarly, a./l-unsaturated bornyl aldehyde undergoes silyl cupration with dimethylphenylsilyl-lithium and copper(I) cyanide to give, after column chromatography, the endo,endo-isomer 7 as the sole product. The silylcuprate attacks the double bond from the endo-face and the addition of the proton from the opposite face leads to the endo,endo-product242. 

Allylation of a-thio-35), a-seleno-35) and a-silyl- 35,77) cyclopropyllithiums was not very successful35) but addition at —78 °C of 0.5 equivalent of copper (I) iodide-dimethylsulfide complex 35,106, W7> prior to the allylhalide leads 35,106,107) to a very high yield of homoallyl cyclopropyl sulfides or selenides (Scheme 24). Similar observations have been made on cyclobutyl derivatives3S). It is not clear at present whether a cuprate is involved in the process but we have evidence ( Se-NMR) that a new species is transiently being formed, at least in the seleno series. The synthesis of homoallyl cyclopropylsilanes was also reported 78) and involves the allylation of a postulated cuprate formed by the addition of lithium dibutyl cuprate to a-lithiocyclopropylsilane (Scheme 26). 

The leaving group X may be only an OH group, inserted by hydroxylation of a silyl enol ether 136 (chapter 33) formed by conjugate addition of a silyl cuprate (chapters 9 and 10) to an enone 135 and protected by the robust silyl group TBDMS (f-BuMe2Si-) 137 for reaction with the lithium derivative of 117. 

Charge acceleration has also been observed in the rearrangement of 2-allyloxy-2-cyclo-hexenones. In comparison to the silyl enol ether derivative of 2-allyloxy-3-methyl-2-cyclo-hexenone, the lithium cuprate derivative obtained by conjugate addition of lithium dimethyl-cuprate rearranges much faster. This effect has been explained by the intermediacy of an allyl radical/oxyoxaallyl radical anion (semi-dione) pair84. 

Non-symmetrical secondary allyl acetates and urethanes react in a regiospecific manner with lithium(dimethylphenylsilyl)cuprate to afford allylsilanes. Thus, -pent-3-en-2-ylacetate (140) is converted into -2-dimethylphenylsilylpent-3-ene (141) (yield 70%), by the silylating agent in THF/ether/pentane at 0°C (equation 66)90. 

Whatever the exact mechanism of the conjugate-addition reaction, it seems clear that enolate anions are formed as intermediates and they can be trapped as the silyl enol ether or alkylated with various electrophiles. For example, addition of lithium methylvinyl cuprate (a mixed-cuprate reagent) to cyclopentenone generates the intermediate enolate 166, that can be alkylated with allyl bromide to give the product 167 (1.161). The trans product often predominates, although the transxis ratio depends on the nature of the substrate, the alkyl groups and the conditions and it is possible to obtain the cis isomer as the major product. Examples of intramolecular trapping of the enolate are known, as illustrated in the formation of the ds-decalone 168, an intermediate in the synthesis of the sesquiterpene valerane (1.162). 

Lithium bis(dimethylphenylsilyl)cuprate and lithium methyl(dimethylphenyl-silyl)cuprate have been used to effect a conjugate addition of the dimethylphenyl-silyl moiety to some carbohydrate enones. Use of trimethylstannyl lithium and tributylstannyl lithium also afforded conjugate addition products. In a new approach to carbohydrate organotin compounds, tributylstannylcopper has been used as a source of nucleophilic tributyltin to effect displacements of a primary tosylate, an allylic bromide and addition to an aldehyde. ... 

Lithium bis-silyl cuprates have been used in the diastereoselec-... 

Related Reagents. Dimethylphenylsilyllithium Dimethyl-phenylsilyl(methyl)magnesium Lithium Cyano(dimethylphenyl-silyl)cuprate. 

Conjugate Addition Reactions. The cuprate and zincate reagents lithium bis[dimethyl(phenyl)silyl]cuprate (75583-57-2) and lithium dimethyl[dimethyl(phenyl)silyl]-zincate (100112-20-7) are usually used in conjugate addition reactions, but PhMe2 SiLi can be used with catalytic amounts of dimethylzinc (eq More resistant substrates may employ the zincate produced in situ, with catalytic amounts of copper(I) additives, and aided by the cocatalyst Sc(OTf)3 (eq 18). ... 

Related Reagents. Dimethylphenylsilyl(methyl)magnesium lithium bis[dimethyl(phenyl)silyl]cuprate trimethylsilylcopper trimethylsilyllithium. 

Preparative Method the silyl cuprate is prepared from the corresponding silyllithium reagent (dimethylphenylsilyllithium) commercially available chlorodimethylphenylsilane is stirred with lithium shot, wire, or powder under Ar or N2 in THF at 0 °C for 4—12 h the silyllithium solution may also be prepared, free of halide ion, by cleaving tetramethyldiphenyldisilane with lithium and ultrasound irradiation the silyllithium solution (2 equiv), after assay, is transferred by syringe on to anhyd copper(I) iodide, copper(I) bromide, or copper(I) cyanide (1 equiv), kept under argon or nitrogen at 0°C, stirred at this temperature for 20 min, and then used immediately. 

Reaction with Allenes. Allenes react with the silyl cuprate at low temperature. The regiochemistry with allene itself places the silyl group on the central carbon atom and the added electrophile at the terminus (eq 4). One surprising exception to this rule is with iodine as the electrophile, when the product (5) has the opposite regiochemistry even from that of the reaction with chlorine. Since the iodide (5) can be converted into a lithium reagent [and a cuprate that is not identical with 4], it is possible to achieve... 

DilithiumCyanobis(dimethylphenylsilyl)-cuprate Dimethylphenylsilyllithium Lithium Bis[dimethyl(phenyl)silyl]cuprate Lithium Cyano(dimethylphenylsilyl)cuprate E) 1-Trimethylsilyl-1,3-butadiene... 

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