Copper-mediated Substitution Reactions of Extended Substrates

Copper-mediated Substitution Reactions of Extended Substrates 

Stereoselective substitution reactions of chiral dienyl electrophiles have also been carried out. In analogy to the copper-promoted 8 2 reactions of simple allylic electrophiles [3], the corresponding 8 2 (1,3) substitutions of dienyl carbonates [43] have been reported to proceed with high anti selectivity. Interestingly, treatment of chiral dienyl acetal 63 with the Yamamoto reagent PhCu BFs gave rise to the formation of a 1 3 mixture of the anti-S l substitution product 64 and the syn-Sn2 (1,5) substitution product 65 (Eq. 4.28) [44]. A mechanistic explanation of this puzzling result has yet to be put forward, however. 

Highly enantioselective 1,5-substitution reactions of enyne acetates are also possible under carefully controlled conditions (Eq. 4.31) [46]. For example, treatment of enantiomerically pure substrate 70 with the cyano-Gilman reagent tBu2CuLi-LiCN at —90 °C provided vinylallene 71 as a 1 3 mixture of E and 2 isomers with 20% and 74% ee, respectively. This mediocre selectivity might be attributable to race-mization of the allene by the cuprate or other reactive copper species formed in the reaction mixture. The use of phosphines as additives, however, can effectively prevent such racemizations (which probably occur by one-electron transfer steps) [47]. Indeed, vinylallene 71 was obtained with an ee of 92% for the E isomer and of 93% for the 2 isomer if the substitution was performed at —80 °C in the presence of 4 eq. of nBusP. Use of this method enabled various substituted vinylallenes (which are interesting substrates for subsequent Diels-Alder reactions Sect. 4.2.2) to be prepared with 90% ee. 

Over the last 30 years, organocopper reagents have been utilized with great success in organic synthesis. The results presented in this chapter highlight the excellent 

Taylor (Ed.), Organocopper Reagents, Oxford University Press, Oxford, 1994 m) B. H. Lipshutz in Organometallics in Synthesis, ScHLOSSER, M. (ed.), Wiley, 

5 Reviev/s sj R. A. Smith, A. S. VEttonoot, in Adiancei in Mailed Reacdon Medianinni, J. M. Coxopt Ed.j, AI Piess, (irieeuv/idi, 1994, 

Modem Organocoppcr Chemistry. Edited by Novben Kvause Copyrigbt Z/ 2002 WUey-VCH Vevlag GmbH I.SbNs 3-527-29773-1 IHavdcovev 3-527-1)0008-6 lEleaionic  

Reduction of Me- CuLi2 wiiii LAH was described by Asliby and crr-workers as a o produce tlie powerful reducing reagent Li2CuH- [10], wbidi can be n temperature for conjugate reductions lEq. 5.3). 

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Copper-mediated Substitution Reactions of Extended Substrates 161... 

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