Allylic cuprate reagent

Lewis-acid catalysed methods and allyl cuprate reagents were found to 264... 

In Entry 5, the carbanion-stabilizing ability of the sulfonyl group enables lithiation and is then reductively removed after alkylation. The reagent in Entry 6 is prepared by dilithiation of allyl hydrosulfide using n-bulyl lithium. After nucleophilic addition and S-alkylation, a masked aldehyde is present in the form of a vinyl thioether. Entry 7 uses the epoxidation of a vinyl silane to form a 7-hydroxy aldehyde masked as a cyclic acetal. Entries 8 and 9 use nucleophilic cuprate reagents to introduce alkyl groups containing aldehydes masked as acetals. 

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

Low chemical yields and a-selectivity on the allyl copper reagent are observed with the phenylthio analogues. These observations suggest that the pyridine nitrogen facilitates transmetalation and or cuprate reactivity and plays a role in the regio-selectivity of the reaction. 

Obviously, the nature of the organocopper reagent is an important factor with respect to the stereochemical outcome of the cuprate addition. This is nicely illustrated for the cuprate addition reaction of enoate 75 (Scheme 6.15). Here, lithium di-n-butylcuprate reacted as expected by way of the modified Felkin-Anh transition state 77 (compare also 52), which minimizes allylic A strain, to give the anti adduct 76 with excellent diastereoselectivity [30]. Conversely, the bulkier lithium bis-(methylallyl)cuprate preferentially yielded the syn diastereomer 78 [30, 31]. It can be argued that the bulkier cuprate reagent experiences pronounced repulsive interactions when approaching the enoate system past the alkyl side chain, as shown in transition state 77. Instead, preference is given to transition state 79, in which repulsive interactions to the nucleophile trajectory are minimized. 

Scheme 6.41. Stereoselective construction of a quaternary stereocenter by allylic substitution of mesylate 195 with a boron trifluoride-modified cyano-Gilman cuprate reagent.

Silicon-based Lewis acids have been known for some time, and the related chemistry in catalysis has recently been reviewed [24]. Most examples in the literature are mainly based on achiral species and will be discussed only briefly in this section. In general, a broad variety of reactions can be catalyzed with compounds like MejSiOTf, MejSiNTf or MOjSiClO. One advantage over some metal Lewis acids is that they are compatible with many carbon nucleophiles like silyl enol ethers, allyl organometallic reagents and cuprates. 

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