Alkenylzinc compounds

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. ... 

The alkenylzinc compounds 12 prepared by the described methodology can be used in palladium-catalysed cross coupling reactions using Pd(PPh3)2 (prepared in situ by... 

The stereochemical outcome was in agreement with the formation of a chelated allyl alkenylzinc compound that underwent a metalla-Claisen rearrangement with delivery of the allyl moiety anti to the homoallylic substituent. Although the stereocenter was more remote from the alkenylmetal, excellent induction was still observed152,153. 

Acylation of organozinc reagents. Reaction of acyl chlorides with organozinc compounds catalyzed by palladium-phosphine complexes provides a general synthesis of ketones. The organozincs are readily available by treatment of the corresponding organo-lithium with ZnCh. Alkenylzinc compounds and a,p-unsaturated acyl chlorides react with retention of the stereochemistry. Isolated yields of ketones are 55-90%. 

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 reaction of 3-methylcyclopentane-l,2-dione with Tf20/Et3N affords the vinyl triflate in 53% yield (eq 23). The reaction takes place probably through the enol form. The product was coupled with alkenylzinc compounds in the presence of a palladium catalyst. ... 

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]. 

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... 

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. 

Combining substrate-induced diastereoselection and mutual diastereoselectivity, as illustrated for the crotylzincation of the alkenyllithium derived from 209, led to excellent results as the gewt-dimetallic species 217 was obtained in a highly stereoselective fashion. The stereochemical outcome was explained by the addition of the kinetically reactive cisoid metallotropic form of the crotylzinc reagent anti to the propyl group in the chelated allyl alkenylzinc intermediate. After hydrolysis, compound 218 was obtained as a single diastereomer (equation 106)148,149. 

The coupling of aryl- and alkenylzinc reagents with various halides has widespread use in the cross-coupling of aromatic rings [101]. The reactions of zinc derivatives of aromatic and heteroaromatic compounds with aryl and heterocyclic halides have wide synthetic applications [103,104],... 

Zinc and zirconium 1,1-bimetallic reagents 4.104, prepared by the hydrozirconation of alkenylzinc halides 4.103, react with carbonyl compounds to produce alkenes with high R-stereoselectivity (Scheme 4.51). Ketones give an E/Z mixture of stereoisomers . 

The other organometallic compounds, such as alkynylcoppers, alkenylzincs, and alkenylzirconocenes, have also been utilized for the tran -selective couphng reaction of 1,1-dihaloalkenes (Scheme 4). The monoalkynylation of 1,1-dichloroethylene in the presence of palladium and copper catalysts is troublesome. Since dialkynylation is not easily suppressed, an excess amount of the substrate, 1,1-dichloroethylene, is required to produce the desired product. 

---