Alkynylzinc Bromide

Alkynylzinc bromides 133 are also cleanly hydrozirconated with Cp2Zr(H)Cl leading to 1,1-bimetalloalkenes of zinc and zirconium (134), which react smoothly with aldehydes to afford allenes in satisfactory yields (Scheme 7.40) [232,233]. 

Alkynylzinc bromides, BrZnC=CR.6 Unlike magnesium and lithium ace-tylides, these reagents add to an a-alkoxy aldehyde with good to high syn-selectivity. Example ... 

Conjugate addition of alkynylzinc bromides.2 Alkynylzinc bromides in combination with 1 equiv. of a trialkylsilyl triflate undergo conjugate addition to cyclic and acyclic a,P-enones at -40° in THF or ether. A mixture of 1,2- and 1,4-adducts is formed from 3 disubstituted a,p-enones. 

The stereoselectivity of the reaction of 1 -alkynylzinc bromides 130 (R == heptyl or Ph) with various chiral a-benzyloxy aldehydes 131 (R = Me, /-Pr or PhCH20CH2) to yield the alcohols 132 depends on the nature of the substituents R and R Regiospecific propargylation with acylsilanes is exemplified by the reaction of the organozinc bromide 133 (from 2-octynyl bromide and zinc dust in THF) with benzoyltrimethylsilane, followed by desilylation, to yield only the alcohol 134 . 

Conjugate Addition of Alkynylzinc Bromides. Alkynylz-inc bromides undergo conjugate addition with a, 8-unsaturated ketones in the presence of ferf-butyldimethylsilyl trifluo-romethanesulfonate or TIPS triflate (trimethylsilyl trifluo-romethanesulfonate is also effective) in ether-THF at —40 °C to give the corresponding 1,4-adducts (54-96% yields). A representative example is illustrated in eq 7. 

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