Alkenylzinc

Regiocontrolled q- or / -alkenylation and arylation of cyclic enones are possible without protection of the ketone by applying the coupling reaction of the Q- or /3-halo enones 607 and 608 with aryl and alkenylzinc reagents[468,469]. 

Hydrozirconation of various alkenylboranes [118-121] and alkenylzinc halides [34] with 1 provides the corresponding 1,1-bimetalloalkanes, which can be selectively converted to functionalized organic compounds [122-125]. Interestingly, alkenylzinc halides (RCH=CHZnX) show remarkable chemoselectivity, and functional groups such as chloride, cyanide, or ester functionality are tolerated [126]. 

In 2004, Yang and Tseng reported the synthesis of a series of new chiral amino thiol ligands derived from L-valine, which were further employed (1 mol%) in the enantioselective alkenylzinc addition to aldehydes, providing an efficient route for chiral ( )-allylic alcohols with enantioselectivities of up to >99% ee, as shown in Scheme 3.67. ... 

Scheme 3.67 Amino thiol ligand for additions of alkenylzinc reagents to aldehydes.

Exchange with boranes can also be used to prepare alkenylzinc reagents.138... 

Alkenylzinc reagents can also be made from alkynes by (Cp)2TiCl2-catalyzed hydro-zincation (see Section 4.6).139 The reaction proceeds with high syn stereoselectivity, and the regioselectivity corresponds to relative carbanion stability. 

Organozinc compounds are also useful in palladium-catalyzed coupling with aryl and alkenyl halides. Procedures for arylzinc,156 alkenylzinc,157 and alkylzinc158 reagents have been developed. The ferrocenyldiphosphine dppf has been found to be an especially good Pd ligand for these reactions.159... 

A portion of the side chain of calyculin was prepared by a tandem reaction sequence that combined an alkenylzinc reagent with 2-bromoethenylboronate, followed by Suzuki coupling with a vinyl iodide in the same pot.230... 

Recently, Oshima has reported a cobalt-catalyzed allylzincation of internal alkyne derivatives (Scheme 10). Optimization of the reaction leads to utilization of cobalt(ll) chloride in THF at room temperature. No traces of regio- and stereoisomers are obtained. The resulting alkenylzinc species 34 can be trapped with a large number of electrophiles in order to generate stereoselectively the tri- and tetrasubstituted alkenes 35-37. 

Remarkably, the hydrozirconation can be applied to functionalized alkenylzinc reagents bearing an ester, chloride, or cyano functionality (Scheme 7.37) [230], The functionalized bimetallic species 129c—e gave significantly lower yields (55—64%) but excellent stereoselectivities (>94% ( ) Scheme 7.39). Although the (E)/(Z) ratio was always reproducible,... 

Alkenylzinc compounds have been prepared by a Te-Zn exchange reaction on vinyhc tellurides under halide-free conditions, with retention of the geometry of the starting tel-luride. ... 

A Pd-catalysed cross-coupling reaction takes place upon treatment of the formed alkenylzinc with p-iodotoluene giving the expected coupled product. 

Alkenylzinc reagents can also be made from alkynes.117... 

Earlier, [2.2]paracyclophane-based N,0-ligands had been employed in the dialkylzinc and the alkenylzinc addition to aldehydes a) S. Dahmen, S. Erase, Chem. Commun. 2002, 25 b) S. Hofener, F. Lauterwasser, S. Erase, Adv. Synth. Catal. 2004, 346, 755 c) review S. Erase,... 

The second generation of N,0-[2.2]paracyclophane ketimine ligands (Rp,S)-5b, 6b were investigated for their ability to catalyze the 1,2-addition of alkenylzinc... 

Over the last five years, we have designed, synthesized, and applied new ligands for asymmetric 1,2- and 1,4-addition reactions. Suitable ligands were found for the addition of alkyl-, aryl-, and alkenylzinc reagents to a,(3-unsaturated aldehydes and ketones, a-branched and unbranched aliphatic aldehydes, and imines. Although some substrates such as ketones and other carbonyl compounds have remained a challenge, we believe that this system provides an excellent entry into various classes of chiral intermediates. Application of these synthesized complex molecules is the current pursuit in our laboratories. 

SCHEME 25. Alkenylzinc species obtained from alkenylzirconium derivatives... 

A formal 13+2 -cycloaddi lion can be accomplished by adding bis(2-carbethoxyethyl) zinc to acetylenic esters . This reaction allows the construction of complex cyclopen-tenones, such as 388, which is a precursor of ( )-bilobalide (Scheme 101) . The allylzincation of tiimethylsilylacetylenes can be performed intramolecularly providing a functionalized alkenylzinc which cyclizes in the presence of Pd(PPh3)4 at 25 "C within 3.5 h, leading to the bicyclic product 389 in 84% yield (Scheme 102). The addition of allylic zinc halides to various alkynes occurs in the absence of copper salts. The related... 

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