Organocopper, allylic reagents

Tab. 6.7. Results of allylic organocopper reagents. substitution of y-silyl- substituted allyl ic carbamates 181 with ...

Compound 33 reacts with PhgSnCl (Scheme 7-27) to give mainly the stannylated product at the exocyclic carbon, whereas its transmetallation into the allylic organocopper reagent followed by addition of 2-cyclohexenone in the presence of TMSCl gives the exo-methylenecyclobutyl product. 

Allylic organocopper reagents.4 The activated copper species (Cu ), produced by reduction of CuCN LiX, reacts with atlyl chlorides and acetates to afford allylic organocopper reagents, which couple with various electrophiles in good yields. 

Table 5.5 Reactions of thienyl-based allylic organocopper reagents with electrophiles.

Typical Experimental Details of Copper Chemistry 233 Table 5.14 1,4-Conjugate additions with copper anion-based allyl organocopper reagents. 

Reaction of Allyl Organocopper Reagents Derived from CuCN 2LiBr with Benzoyl Chloride... 

Substituted epoxides are attacked by organocopper reagents at the least hindered carbon atom and form alcohols (C.R. Johnson, 1973A). With a, 9-unsaturated epoxides tram-allylic alcohols are produced selectively by 1,4-addltion (W. Carruthers, 1973 G.H. Posner, 1972). 

Treatment of allylic substrates ISO, possessing suitable leaving groups X in tlieir allylic positions, witli organocopper reagents may result eitlier in an S 2-type process fa-attack) or alternatively in an S 2 one fy-attack), giving tlie substitution products 151 and 152, respectively fSclieme G.30) [Ij]. 

To explain tlie stereodieniistiy of tlie allylic substitution reaction, a simple stereoelectronic model based on frontier molecular orbital considerations bas been proposed fl55. Fig. G.2). Organocopper reagents, unlike C-nudeopbiles, possess filled d-orbitals fd - configuration), wbidi can interact botli witli tlie 7t -fC=C) orbital at tlie y-carbon and to a minor extent witli tlie cr -fC X) orbital, as depicted... 

Scheme 2.30 Ring-opening reaction with organocopper reagents to form (E)-allyl 111 amines.

There is generally little or no competition from 1,2 addition (to the C=0). However, when R is allylic,l,4 addition is observed with some substrates and 1,2 addition with others. The compound R2CuLi also add to a,P-unsaturated sulfones but not to simple aP-unsaturated nitriles. Organocopper reagents RCu (as well as certain R2CuLi) add to ocP-unsaturated and acetylenic sulfoxides. ... 

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 6.30. Potential reaction products from allylic substitution with organocopper reagents (= Nu).

Tab. 6.6. Results of allylic substitution of styrene system 163 with organocopper reagents.

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