Alkynylzinc halide

Alkynylzinc chlorides are useful imermedtates in Pd°-catalysed couplings with vinyl, heteroaryl [56) and acyl halides [89] tsee Chaps. V and X). The conversion of lithium alkyn-vlides into alkynylzinc halides with anhydrous zinc chloride in THF proceeds smoothly and quantitatively under mild conditions. 

The acyl halide (0.10 mol, freshly distilled) is added dropwise over 15 min to a solution (sometimes two-layer system) of 0.10 mol of alkynylzinc halide (p. 36) in THF and hexane, maintained between 5 and 10 C. After the addition, die cooling bath is removed and the temperature allowed to rise. Stilting is continued for 45 min at 20"C, then the mixture is cooled to -10 C and a solution of 20 g of NH4Cl in 200 ml of water is added with vigorous stirring After separation of the layers, three extractions with Et20 are earned out. The combined organic solutions are washed four times with a saturated aqueous solution of... 

As usual, the reactivity order with respect to halogen in C=C-X is I > Br > Q. The organic halide in the couplings with alkynylzinc halides is usnaily an iodide, though also activated vinylic bromides (e.g. BrCH=CHCOOC2H5) have been found to react smoothly. We... 

The cross-coupling of alkynylzinc halides or fluorinated alkenylzinc halides with fluori-nated alkenyl iodides allows the preparation of a range of fluorinated dienes or enynes - Functionalized allylic boronic esters can be prepared by the cross-coupling of (dialkylbo-ryl)methylzinc iodide 428 with functionalized alkenyl iodides. The intramolecular reaction provides cyclized products, such as 429 (Scheme 109) ° °. In some cases, reduction reactions or halogen-zinc exchange reactions are observed. 

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. 

Other mixed gm-dimetalhc compounds containing zinc and either zirconium, aluminum, or titanium have been successfully prepared from alkynylzinc halide compounds via hydrozirconation by H(Cl)ZrCp2 (the Schwartz s reagent) or carbometalation by a combination of Me3AECp2TiCl2 (Scheme 15). 

Cychc enones were also found to react with alkynylzinc halides in the presence of tert-bnlyldimethylsilyl triflate (equation 71). 

The traditional scope for this reaction involved coupling alkenyl or aryl iodides or bromides with aryl, alkenyl, or alkynylzinc halides. However, recent modifications have allowed the scope to be extended to include additional electrophiles see Electrophile) such as aryl and vinyl chlorides, sulfonate esters, aryl ethers, and substrates with... 

Alkynylzinc halides yield alkynyl ketones, which are otherwise difficult to obtain. With acid chlorides they give yields of 30-70 %. 

Reagents of this type arc readily available by hydrozirconation of alkenyl- or alkynylzinc halides with Schwartz s reagent, H(Cl)ZrCp2, in CH1CI2 at 25° (equations I and 11). 

Scheme 4.71 Ni-catalyzed cross-coupling of alkynylzinc halides with vinylic tellurides [224].

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]. 

---