Zinc compounds alkenylation

The addition of a large excess of bis(cj-alkenyl)zinc compounds to the TiC -catalyzed polymerization of propene resulted in an increased polymer yield, but a reduction in the molecular weights of the polymers.64 This suggests that the diorganozinc compounds are both co-catalysts and chain-transfer agents in this polymerization. The catalyst activity decreased in the order bis(3-butenyl)zinc < bis(7-octenyl)zinc < chlorodiethylaluminum. Bis(7-octenyl)zinc was co-polymerized with propene to afford hexylzinc side chains, whose zinc-carbon moieties were converted to vinyl groups by the addition of allyl bromide. 

In the early 1990s, Soga studied several bis(co-alkenyl)zinc compounds, such as bis(3-butenyl)zinc and bis(4-methyl-3-pentenyl)zinc, as CTAs in propylene polymerization in an attempt to prepare polypropylenes having a terminal alkylzinc group on one chain end and an alkenyl group on the other. As expected for a heterogeneous TiCb catalyst system with multiple active sites, the reaction forms a mixture of polymer structures with zinc groups both in the side chains and on the chain ends. In fact, bis(m-alkenyl)zinc serves three roles in the reaction as coinitiator, comonomer... 

Shiono, T. Kurosawa, H. Soga, K. Isospecific polymerization of propene over TiCl3 combined with bis(oo-alkenyl)zinc compounds. Macromolecules 1995, 28,437-443. 

In an early example of this reaction aryl and benzyl zinc halides were coupled with aryl halides (equation 40) (102). Alkynyl zinc compounds cross-couple with aryl halides (iodides or bromides) in the presence of a Pd catalyst (equation 41) (103) or alkynyl iodides can be coupled with aryl and alkenyl zinc compounds (equation 42) (100). 

I.3.3.3.3.1.1.3. From Alkenyl Halides and Zinc-Boron Compounds... 

As shown in Scheme 6, the addition forms two stereogenic centers via the favorable chair-like transition state. The diastereoselective construction of stereogenic centers has been studied extensively by Marek and Normantla. For the control of stereochemistry, one should think about the configuration of allylzinc compounds and the alkenyl metal. Interestingly, a comparison of four possible transition states (Scheme 7) by calculation concludes that Z-crotylzinc bromide is the most favorable transition state. This means that it is not necessary to think about the stereochemistry of crotylzinc bromide as its configuration changes via 1,3-transposition of the zinc atom (Scheme 6)12. 

A more comprehensive study on the behavior of 5-acetylenic iodides in the presence of zinc was later reported21 and clearly revealed that when iodide 14 was treated with acid-washed zinc (pre-treated with Mel for activation and removal of traces of moisture) in a mixture of benzene and DMF, the acyclic organozinc iodide 21 and the cyclic vinyl iodide 22 were both produced. The amount of 22 increased significantly when the reaction mixture was sonicated rather than stirred or if a zinc-copper couple was used. The accumulation of compound 22 was consistent with its inability to be converted to an alkenyl organozinc species by reaction with metallic zinc under these conditions. However, no substantial cyclization of the open-chain organozinc 21 was observed. The formation of the five-membered ring compound 22 was attributed to a free-radical process... 

Treatment of a variety of a,/i-unsaturated carbonyl compounds with zinc and 1,2-bis(chlorodimethylsilyl)ethane leads to the formation of organozinc carbenoids which may be trapped with olefins to give alkenyl- and arylcyclopropanes (equation 78)131. With... 

Before adding aldehyde 14 a transmetalation from zirconium to zinc is necessary because of low reactivity of the sterically hindered organozirconocene compounds like 18 toward most organic electrophiles.9 Resulting alkenylethylzinc 19 reacts in a 1,2-addition with the cr,y3-unsaturated aldehyde 14 transferring exclusively the alkenyl moiety. The formation of Z -allylic alcohol 20 reveals stereochemical retention of the double bond configuration in the transmetalation and addition steps. 

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