Organocuprates, addition alkynes

Highly stereoselective cis additions of organocuprates to alkynic esters have also been reported with alkylcopper-Lewis acid complexes.3 The choice of the proper Lewis acid was found to be important in determining the stereoselection as was demonstrated with enyne (32 Table 2).5- 8 While both the diorga-nocuprate and the alkylcopper-BF3 complex gave mixtures of stereoisomers, the Bu u-BEts provided... 

The reaction of carbon nucleophiles with ketones or aldehydes proceeds by acyl addition, as described in Chapter 18. The reaction of carbon nucleophiles with acid derivatives proceeds by acyl substitution, as described in Chapter 20. Carbon nucleophiles included cyanide, alkyne anions, Grignard reagents, organolithium reagents, and organocuprates. Alkyne anions are formed by an acid-base reaction with terminal alkynes (RC=C-H RCsCr). In this latter transformation, it is clear that formation of the alkyne anion relies on the fact that a terminal alkyne is a weak carbon acid. Other carbon acids specifically involve the proton on an a-carbon in aldehydes, ketones, or esters. With a siiitable base, these carbonyl compounds generate a new type of carbon nucleophile called an enolate anion. 

The lithium[2-tri- -butylstannyl-Z-l-alkenyl]-l-borates, prepared by copper catalyzed addition of Bu3SnB-(OCH3)-9-BBNLb to 1-alkynes (vide supra), are selectively coupled via organopalladium or organocuprate with allylbromide, exclusively at the vinyl boron bond. The process results into the exclusive or predominant formation of 5-tributylstannyl-l,4-(fi)-dienes (Chart 24.8) [38]. 

Ti -Alkenyliron complexes are obtained by addition of nucleophiles to cationic (ri -alkyne)iron complexes (Scheme 4-28). The nucleophiles range from thiolates, organocuprates, and malonates to cyanide. In most cases, /ra 5-addition is observed. 

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