Silyl cupration

Allyl acetates can similarly be transformed into allylsilanes by treatment with bis(silyl)cuprates (5). 

SN displacement reactions, 27-9 a-Selenocyclohexanones, 77 Senecioyl chloride, 33 Silmagnesiation, platinum-catalysed, 8 Silametallation of terminal alkynes, 7-9 Silver acetylide, 49 Silver trifluoroacctate, 42,127 Silyl cuprates, 7... 

Jin and Weinreb reported the enantioselective total synthesis of 5,11-methano-morphanthridine Amaryllidaceae alkaloids via ethynylation of a chiral aldehyde followed by allenylsilane cyclization (Scheme 4.6) [10]. Addition of ethynylmagnesium bromide to 27 produced a 2 1 mixture of (S)- and (R)-propargyl alcohols 28. Both of these isomers were separately converted into the desired same acetate 28 by acetylation or Mitsunobu inversion reaction. After the reaction of 28 with a silyl cuprate, the resulting allene 29 was then converted into (-)-coccinine 31 via an allenylsilane cyclization. 

Fleming et al. reported that allenes 48 and 50 undergo highly stereoselective silyl-cupration with (PhMe2Si)2CuLi. LiCN, allylic silanes 49 and 51 being formed [29]. 

For the addition of silicon nucleophiles to activated enynes, silyl cuprates can be utilized. For example, treatment of ethyl 5-phenyl-2-penten-4-ynoate with (MejSi CuLi gave the 1,4-addition product with 76% yield (equation 77)38. 

Scheme 3.27. Regioselectivity in reactions of silyllithium and silyl cuprate reagents with allylic carbamates and epoxides...

A similar explanation may also hold for the result of conjugate addition to y-phthalimido enoate 80 (Scheme 6.16). Thus, addition of the bulky cyano-Gilman silyl cuprate gave the syn diastereomer 81 (dr = 96 4) [32, 33]. Preference for the sterically least hindered nucleophile trajectory seems to dictate the overall stereochemical outcome (transition state 82). 

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... 

Anionic ring-opening polymerization of l,2,3,4-tetramethyl-l,2,3,4-tetraphenylcyclo-tetrasilane is quite effectively initiated by butyllithium or silyl potassium initiators. The process resembles the anionic polymerization of other monomers where solvent effects play an important role. In THF, the reaction takes place very rapidly but mainly cyclic live- and six-membered oligomers are formed. Polymerization is very slow in nonpolar media (toluene, benzene) however, reactions are accelerated by the addition of small amounts of THF or crown ethers. The stereochemical control leading to the formation of syndiotactic, heterotactic or isotactic polymers is poor in all cases. In order to improve the stereoselectivity of the polymerization reaction, more sluggish initiators like silyl cuprates are very effective. A possible reaction mechanism is discussed elsewhere49,52. 

Silyl cuprates give high yields of acylsilanes with acid chlorides (equation 58)77. Both lower-order Gilman trimethylsilylcuprates and h.o. cyanocuprates accomplish the transformation. 

Conjugate additions. The silyl cuprate rcapcnt (PhMc,Si),CuLi (1) undergoes conjugate addition to a,(3-unsaturated aldehydes, ketones, or esters to form (3-silyl carbonyl compounds. Methylation of the intermediate enolate in acyclic systems is highly dia-stereoselective in favor of the isomer in which the silyl and methyl groups have the anti-... 

Addition of trialkyisilyllithium and silyl-cuprates to Michael acceptors occurs in a 1,4-fashion with high stereoselectivity. 

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. 

As discussed, conjugate addition reactions involving chiral y-alkyl-substituted unsaturated carbonyl derivatives usually occur with low levels of diastereoselectivity. In accord with this general trend, the benzyloxy and silyloxy derivatives 103 and 104 (Scheme 6.23) both reacted with a silyl cuprate in non-selective fashion, to give the conjugate adducts 108 and 109, respectively (entries 1 and 2, Tab. 6.3) [39]. Conversely, hi levels of diastereoselectivity were found for the corresponding carbamates, and even better results were obtained for carbonates, giving the anti esters 110-112 as the major diastereomers (entries 3-5) [39]. 

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