Vinylzirconium

A valuable feature of the Nin/Crn-mediated Nozaki-Takai-Hiyama-Kishi coupling of vinyl iodides and aldehydes is that the stereochemistry of the vinyl iodide partner is reflected in the allylic alcohol coupling product, at least when disubstituted or trans tri-substituted vinyl iodides are employed.68 It is, therefore, imperative that the trans vinyl iodide stereochemistry in 159 be rigorously defined. Of the various ways in which this objective could be achieved, a regioselective syn addition of the Zr-H bond of Schwartz s reagent (Cp2ZrHCl) to the alkyne function in 165, followed by exposure of the resulting vinylzirconium species to iodine, seemed to constitute a distinctly direct solution to this important problem. Alkyne 165 could conceivably be derived in short order from compound 166, the projected product of an asymmetric crotylboration of achiral aldehyde 168. 

Ruen Chu Sun, Masami Okabe, David L. Coffen, and Jeffrey Schwartz 83 CONJUGATE ADDITION OF A VINYLZIRCONIUM REAGENT 3-(1 -OCTEN-1 -YL)CYCLOPENTANONE... 

Conjugate addition of vinylzirconiumsS (E)-Vinylzirconium reagents are readily prepared by reaction of 1 with 1-alkynes. The product undergoes conjugate addition to a, P-enones in the presence of a nickel catalyst. 

Depending on the nature of the R groups of the alkynyl selenides 99, vinylic species with different regio- and stereochemistry are obtained (Scheme 62). When R is an alkyl group, mixture of products of different regiochemistry is obtained,179 which makes this method unsuitable for the preparation of this class of compounds. A solution for this problem consists in the use of lithium alkynyl selenolates 102 as precursors of the vinylzirconium species 103, as shown in Scheme 63.1... 

When telluroalkynes 105 are submitted to the hydrozirconation, only the vinylzirconium intermediates 106 are formed.179,181 These are captured with organotellurium halides to give bistelluroketene acetals 107 (Scheme 64).179... 

Similarly, hydrozirconation of stannyl acetylenes 108 followed by treatment of the intermediate vinylzirconium species 109 with -butyltellurium bromide and then with sodium borohydride gives tintelluroketene acetals 110 (Scheme 65).182... 

Organometallic routes are frequently applied to the synthesis of vinylboranes. Vinyltin and vinylzirconium reagents, for instance, serve as precursors for the preparation of vinylboranes, in which the boron centers are not electronically stabilized by oxygen or nitrogen substituents. Moreover,... 

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). ... 

Vinylzirconium(IV) complexes, unlike their Ti analogs (Section 1.5.2.2), are rather stable and can be conveniently used to perform conjugate addition to a,3-unsaturated ketones, sometimes in preference to the more popular alkenylcopper(I) reagents. Vinylzirconium(IV) complexes, as such, are not very reactive and need catalytic amounts of Ni(acac)2 (Scheme 9). The reaction has been exploited in a prosta-... 

Regiospecific synthesis of phenols. These substrates, obtained by reaction of 4-hydroxycyclobutcnones with P(QH5)3/CCI4, undergo Pd-catalyzcd coupling with various vinyl- and arylstannancs and vinylzirconiums to form products that are transformed on thermolysis (10()°) to tri- and tetrasubstituted phenols. 

Ethynylnickel compounds, prepared from aluminum acetylides and nickel acetylace-tonate pretreated with diisobutylaluminum hydride, also undergo insertions with a, 8-unsaturated carbonyl compounds to form enolates which hydrolyze to 4-ynones . Similarly, vinylnickel derivatives from vinylzirconium compounds and nickel acetylace-tonate add to a, 8-unsaturated ketones to give enolates ... 

The hydrozirconation of alkynes with Cp2ZrHCl (Schwartz s reagent) provides ready access to vinylzirconium reagents, which have been shown to be suitable partners in palladium-catalyzed cross-couplings ( zirconium-Negishi reaction ) with aryl and vinyl electrophiles [1]. Recently, Fu disclosed the first... 

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