Zirconium compounds alkenylation

During the last ten years, the chemistry of 1,1-boriozirconocene complexes has been studied. Both hydrozirconation and hydroboration reactions are well established, and are widely applicable to a wide variety of vinyl and acetylene derivatives [1], Alkenylboranes and alkenyl zirconium compounds can also be readily prepared. Therefore, hydrometalla-tion of the corresponding alkenyl metals should offer a convenient method for preparing gem-boriozirconocenes [24]. 

In addition to alkenylboron compounds, alkenylalane, alkenyl zirconium, alkenyltin, - alkenyl copper, and alkenylmagnesiura reagents are reported to undergo a related alkenyl-alkenyl coupling reaction to give 1,3-alkadienes. 

Many of the palladium- and nickel-catalysed reactions of alkenyl-aluminium or -zirconium compounds which fail or give low yields of cross-coupled products can be promoted to give the desired product in high yield by the addition of catalytic amounts of zinc chloride. " Trisubstituted olefins (40), for example, can be synthesized in good yield (ca. 70%) from the metal complex (38) in the presence of a palladium or nickel phosphine complex and zinc chloride. This reaction is particularly attactive for the one-pot synthesis of natural products containing... 

The first example of a stable 1,1-bidentate Lewis acid based on boron and zirconium has been reported [35]. The synthesis of 22 is outlined in Scheme 7.12. Treatment of hex-l-yne with HBBr2 Me2S followed by conversion of the dibromoboronic ester to the corresponding alkenyl boronic acid and esterification with propane-1,3-diol provided the alkenyl boronic ester. Hydrozirconation of this compound with 3 equivalents of the Schwartz reagent, Cp2Zr(H)Cl [57], afforded the desired product 22 in 86% yield. 

Casey was able to prepare related zirconocene alkenyl complexes according to Scheme 8.18. Alkene coordination was established by a number of NMR techniques. While zwitterionic compounds 38 allowed the determination of the alkene dissociation energy, AG = 10.5 kcal mol , very similar to that of 35. Thermally more stable complexes were obtained by protonation of 37 with [HNMePh2][B(C5F5)4[. Dynamic NMR spectroscopy and line shape analysis allowed the measurement of the barriers of alkene dissociation (AG = 10.7 and 11.1 kcal mol ), as well as for the site epimerisation ( chain skipping ) at the zirconium center (AG = 14.4 kcal mol" ) (Scheme 8.19) [77]. 

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. 

Alkenyl and dienyl groups can be present in Ti and Zr complexes as well. Vinyltitanium compounds cannot be exploited for nucleophilic additions, however, due to their propensity to undergo oxidative coupling reactions,likewise transition metal derivatives. Vinylzirconium compounds (20) can be prepared from alkynes (hydrozirconation equation 12) by action of zirconium derivatives like (19). ... 

Lithiated chloromethyltrimethylsilane as a stable carbenoid could insert into the alkenyl zirconium bond of zirconacyclopentadienes. The zirconacyclohexadienes 48 formed are valuable precursors of such heteroaromatic compounds as phosphinines (phosphabenzenes) 49, as shown in Scheme 11.19 [21],... 

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