Alkenylzinc bromide

Enantioselective alkenylation was first reported by Oppolzer and Nadinov. Divinylzinc adds to aldehydes in the presence of the chiral diamino alcohol catalyst 37 with high enantioselectivity (>96% ) (equation 16)51. Alkenylzinc bromide adds to aldehydes in the presence of the lithium alkoxide of /V-methylephcdrinc (>98% )52. Ti-TADDOL is also an effective chiral catalyst in the addition of divinylzinc53. 

Alkenylzinc bromides add to aldehydes to afford optically active allyl alcohols with 88% ee in 80% yield using a stoichiometric amount of the lithium salt of (15,27 )-(1) (eq 9). ... 

The pure E stereochemistry of the vinylsilane 21 clearly shows that the carbocylization proceeds in a totally stereoselective fashion to give, under these conditions (20 °C, 5 min) the configurationally stable (Z)-alkenylzinc bromide [21] 20 via a syn addition of the allenic zinc bromide to the acetylenic moiety. This carbocyclization represents a new desymmetrization of diynes such as 18 which serves to deliver functionalized exo-... 

Coupling of organozincs with mryi synthesis of biaryls,including heterc and even Zn(CN)2 are reactive toward alkenylzinc bromides and cyclopropet iodoarenes. 

Coupling of organozincs with aryl halides. The method is very useful for the synthesis of biaryls,including heterobiaryls.Vinylic, " aliphatic zinc reagents, and even Zn(CN>2 are reactive toward ArX in the presence of (Ph3P)4Pd. Perfluoro-alkenylzinc bromides and cyclopropenylzinc chlorides also couple efficiently with iodoarenes. 

Betzemeier and Knochel performed cross-couplings of aryl iodides with arylzinc bromides in a toluene/l-bromoperfluorooctane mixture in the presence of a Pd catalyst derived from fluorous phosphine la and Pd2(dba)3 (Scheme 1) [15]. The reaction yielded the desired coupling products in excellent yields (87-99%). By using 1.5 mol% of catalyst it was possible to re-use the catalyst up to four times, yielding biphenyl derivative 2. This cross-coupling reaction could be extended to benzyl- and alkenylzinc bromides forming 3 (76%) and 4 (92%). 

Scheme 7.27 Preparation of alkenylzinc bromides from organomercurials.

Coupling reactions. Cobalt(II) bromide catalyzes the coupling of diorganozincs RjZn, allylic chlorides and phosphates without disturbing the configuration of the double bond." Other couplings useful for skeletal construction feature alkenylzinc compounds with... 

The range of nucleophiles can be extended to alkenylzinc reagents. These species are subjected to alkyl chlorides, bromides, or iodides in an iron-catalyzed Negishi coupling to efficiently form olefins (Scheme 4-235). Iron(III) chloride fiinctions as a precatalyst in the presence of TMEDA. ... 

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