Ketones alkynylzinc addition

Alkynylzinc addition to ArCHO can also be carried out in the presence of 3, whereas the disilyl derivative 4 catalyzes enantioselective Reformatsky reaction on ketones (with ee up to 90%)" which operates by a ftee radical mechanism (requiring air to initiate the reaction). 

Enantioselective addition of alkynyl groups to aldehydes and ketones afford enantioen-riched alkynyl alcohols62. Early approaches to the catalytic enantioselective addition of dialkynylzincs and alkynylalkylzincs to aldehydes employed catalytic amounts of chiral amino alcohol63 and amino pyridine64. Stoichiometric enantioselective addition of alkynylzinc halide is reported using the lithium alkoxide of IV-methylephedrine65. 

Asymmetric additions of alkynylzinc reagents (pyridines and alkaloids as N-containing ligands) to aldehydes and ketones 03T9873. 

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. 

S)-BINOL-Ti was also shown to be an effective chiral catalyst for the catalytic asymmetric addition of alkynylzinc to inactivated simple ketones with good to excellent enantioselectivities [63] (Scheme 14.12). The same reaction has been furnished by using lithium phenylacetylide as the nucleophile under the catalysis of titanium complex prepared from BINOL and ClTi(O Pr)3 [64]. 

In the case of an intramolecular 1,4-addition, no activation is required [161]. The iodoenone 238 is readily converted into the corresponding alkylzinc iodide that undergoes an intramolecular addition at 25 °C in THF affording the hicycUc ketone 239 in 65-67% yield [161]. The addition of alkynylzinc halides to enones is hest performed in the presence of terl-butyldimethylsilyl triflate. Under these conditions, very high yields of the conjugated adducts, such as 240 are obtained (Scheme 7.71) [162]. 

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