Alkenyl zinc

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. 

SCHEME 20. Preparation of an alkenyl zinc reagent bearing an azide function... 

SCHEME 24. Preparation of fluoro-substituted alkenyl zincs via lithium intermediates... 

Fluorinated alkenyl zinc reagents can be prepared by two different routes 1) exchange of the corresponding alkenyl lithium reagents at low temperature with zinc chloride 2) direct insertion of zinc into the carbon-halogen bond of fluorinated alkenyl halides. 

A variety of fluorinated alkenyl zinc reagents such as CF2=CFZnCl, CF2=CHZnCl, E and Z-RCF = CFZnCl have been prepared by the first method (Scheme 54). In the presence of palladium catalyst, these fluorinated alkenyl zinc reagents undergo cross-coupling reactions with aryl iodides, vinyl iodides, acid chlorides and 1-iodo-l-alkynes to give the corresponding fluorinated alkenyl derivatives [127, 146-153], which have been utilized in the synthesis of fluorinated codlemones [154]. Typical examples are outlined below (Scheme 55). 

The second method for the preparation of fluorinated alkenyl zinc reagents involves the reaction of alkenyl bromides or iodides with acid-washed zinc in a... 

One particular example of carbometallation of alkenes is the addition of 2-aUcenylmagnesium halides and bis(2-alkenyl)zinc to ethylene under pressure (20-100 bar) to give the corresponding 4-alkenylmetal compounds [95] (Scheme 7-84). 

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). 

Solvents play a pivotal role in the Negishi coupling, as demonstrated in the coupling of alkenyl zinc reagents with iodo alkenes. In this case, DMF is superior to THF, one of the standard solvents (Scheme 5-77). 

Highly congested alkenyl zinc reagents can also be used for cross-coupling reactions (Scheme 5-78). In the case of the total synthesis of the nakienones, the same building block having a sensitive dienyl structure was coupled in good yields. 

Scheme 5-78. Highly congested alkenyl zinc reagents in Negishi coupling reactions applications in total syntheses of nakienones. " ...

Several other examples of the hydrozirconation of functionalized alkyne were further described and the reactivity of the corresponding vinyl zirconocene derivatives was explored in detail either by addition of a Lewis acid [6] or by a transmetallation reaction into alkenyl zinc derivatives [7-10] as described in Scheme 12.6. 

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