Alkynylzinc

Similar observations have been made with the addition of alkynylzinc reagents to aldehydes in the presence of the lithium salt of Wmethylephedrine (26)28. Whereas addition of 2-phenyl-ethynylzinc bromide to an aromatic aldehyde led to the chiral alcohol in 88% ee, addition of ethyl(2-phenylcthynyl)zinc afforded the same alcohol with only 42% ee. 

Addition of Aryl-, Alkenyl- and Alkynylzinc Reagents to Aldehydes... 

Asymmetrical addition of alkynylzinc reagents to nitrones has been achieved in the presence of L-tartaric acid ester as a chiral additive (665). 

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 ... 

The cross-coupling reactions of allenes with components containing sp-carbon atoms are useful synthetic transformations since they provide yne-allenes and enyne-allenes, respectively. Due to the synthetic potential of these classes of carbon-rich unsaturated compounds, the scope and limitations were systematically investigated [1, 16-18]. The first synthetic application was reported in 1981, describing the preparation of alkynyl-substituted allenes by coupling of alkynylzinc chlorides with allenyl halides (Scheme 14.8) [11]. 

The cross-coupling reaction of vinylic tellurides with dialkylzinc or alkynylzinc reagents under Pd(PPh3)4-CuI catalyst was established as a useful method to synthesize different types of dienes, enynes or enediynes. ... 

Dialkylzincs are much less reactive than phenyl or alkynylzincs. In 2002, Kozlowski et al. developed a chiral salen-based catalyst 62 that can promote the diethylzinc addition to a-ketoesters in high yield, [Eq. (13.38)]. In their catalysis, titanium acts as a Lewis acid, and amine nitrogen acts as a Lewis base (63). The enantioselectivity was up to 78% ee ... 

Nucleophilic addition reactions to oxathiazinane N,0-acetals proceed smoothly with al-lylsilane, alkynylzinc, silyl enol ether, and silylketene acetal reagents (Scheme 17.22)... 

Additions of alkynylzinc compounds to nitrones possessing an easily recoverable mannose-derived chiral auxiliary of type 101 (equation 72) proceed with good steroeselectivity (93-98% dr). ... 

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. 

Reaction of Alkynylzinc Chloride with Acyl Halides THF-hexane... 

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... 

Sonogashira protocol.The alkynylzinc reagent can also be prepared in situ form terminal alkynes by addition of ZnCl2 as a co-catalyst. ... 

The zinc-cyanide complexes are on the interface of organic and inorganic chemistry. We include them here because of their Zn—C bond. (Perhaps this will encourage thermo-chemical investigations on the isoelectronic alkynylzinc species, much as there has been interest in both cyano and alkynyl silver-containing species. ) The species Zn(CN)2, Zn(CN)3 and Zn(CN)4 have all been thermochemically characterized in aqneons soln-tion by thermometric titration. The enthalpy of reaction values kJmoL are shown for reactions 9-11. 

Exchange with aryl, alkenyl and alkynylzinc compounds... 

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. 

Taken together, these observations suggest a reaction pathway involving an intermediate dialkyl alkynylzinc species which rearranges to the allenylzinc reagent by alkyl migration from zinc and internal Sjv2 displacement of the mesylate (equation 20). 

SCHEME 6. Pd-catalyzed reaction of alkynylzincs with alkenyl halides... 

Treatment of active H-containing compounds, such as terminal alkynes, with organozincs provides a convenient route to organozincs, as exemplified by the generation of alkynylzincs from terminal alkynes with alkylzinc derivatives62. 

---