Alkenylzirconium compounds

Hard carbon nucleophiles of organometallic compounds react with 7r-allyl-palladium complexes. A steroidal side-chain is introduced regio- and stereo-selectively by the reaction of the steroidal 7T-allylpalladium complex 319 with the alkenylzirconium compound 320[283]. 

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 alkenylzirconium compounds can also be readily prepared. Therefore, hydrometalla-tion of the corresponding alkenyl metals should offer a convenient method for preparing gem-boriozirconocenes [24]. 

Scheme 2.62. Michael addition of an alkenylzirconium compound", by successive transmetalation into zinc and copper intermediates.

Other similar approaches have used alkenylsilanes and PhI0/Et30+ BF4 and also alkenylstannanes and PhI+CN TfO [123,124]. The reactions proceed also stereoselectively. The parent ethenyl(phenyl)iodonium triflate as well as several trisubstituted alkenyl members were obtained in this way. In an analogous manner E-alkenylzirconium compounds upon reaction with (diacetoxyiodo)ben-zene afforded -alkenyl(phenyl)iodonium salts stereoselectively [125]. 

The alkenylzirconium compound 314 is prepared by hydrozirconation of 1-heptyne (312) with hydrozirconocene chloride (313), and reacts with alkenyl iodide to afford the 1,3-diene 315 [138]. The Zr reagent can be used even in the presence of carbonyl group, which is sensitive to A1 or Mg reagents. 

Alkenylzirconium compounds 34 obtained by the hydrometallation of alkynes were transformed to 1-alkenylboron compounds via transmetallation with B-halo-9-BBN or B-chlorocatecholborane62,63 (Scheme 5). Mercuration followed by transmetallation to BH3 was advantageous over the lithiation route in the synthesis of indole-3-boronic acid 35.64... 

Alkenylzirconium compounds couple efficiently with alkenyl and aryl halides in the presence of Pd(0) catalysts, maintaining the stereochemical integrity of both... 

This cross coupling can also be used with alkenylzirconium compounds derived from internal alkynes if metal salts of Zn or Cd, such as ZnCl2, are added in catalytic amounts. For example, the reaction of 1 with 2 under the above conditions fails, but in the presence of 0.2 equiv. of ZnCl2 it results in formation of 3 in 72% yield (2 hours). ... 

The intermediate alkenylzirconium compounds can be used in a cross-coupling reaction with alkenyl, aryl, or alkynyl halides in the presence of Pd or Ni catalysts and ZnCU. ... 

Alkenylzirconium compounds, prepared from acetylenes and normally inert to a,/3-unsaturated ketones, have been activated towards conjugate addition by [Ni(acac)2], or by [Ni(acac)2] pre-reduced by di-isobutylaluminium hydride. Alkylzirconium reagents were not activated by the nickel complexes. 1-Alkynyltrialkylborates require the presence of a Lewis acid, preferably titanium tetrachloride, to add to methyl vinyl ketone (Scheme 60). The product isolated after oxidation is the 1,5-diketone. 

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