Steric hindrance organocuprates

The most frequently used organocuprates are those m which the alkyl group is pri mary Steric hindrance makes secondary and tertiary dialkylcuprates less reactive and they tend to decompose before they react with the alkyl halide The reaction of cuprate reagents with alkyl halides follows the usual 8 2 order CH3 > primary > secondary > tertiary and I > Br > Cl > F p Toluenesulfonates are somewhat more reactive than halides Because the alkyl halide and dialkylcuprate reagent should both be primary m order to produce satisfactory yields of coupled products the reaction is limited to the formation of RCH2—CH2R and RCH2—CH3 bonds m alkanes... 

Bicyclic or polycyclic enones may give slow or inefficient reaction with organocuprates. In particular, it is those enones which contain substituents arranged so as to create a steric hindrance on the carbon 3 to the carbonyl that see the greatest effect. The yields are often low to moderate, unless additional activation is provided. For example, when the bicyclic enone (23) was treated with R2C11U (R = 3-pentynyl), the -substituted product was formed in only 46% yield,69 while exposure of the same enone to RQ1BF3 (R = 3-pentynyl) gave the same (3-alkylated product in 76% yield (Scheme 9).70... 

Organocuprates can react with compounds containing an allylic leaving group even when there is steric hindrance, but if the steric encumbrance becomes too great it can also proceed by a Sn2 like pathway (sec. 2.7.A.iii).39i The reaction of dimethylcuprate and 409 to give the methyl derivative (410) illustrates this... 

The tendency for addition at the 5-carbon of the s-trans-diene carbon of 154a is attributed to the relative instability of the a-addition product 155b that experiences steric hindrance between the planar anti-eax-bomethoxyl and the CHCMe allylic n-bond. The tendency toward the a-carbon of organocuprates reflects its intrinsic preference for the con-... 

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