Use of gem-Borazirconocene Alkanes in Regioselective Synthesis

The different reactivities of the carbon—boron and carbon—zirconium bonds toward electrophiles are a consequence of the different bond polarities and the different electronegativities of boron and zirconium. Moreover, zirconium is a transition metal, while boron exhibits intriguing transition metal-like chemistry [55], It is thus reasonable to presume that the combined use of boron and zirconium in organic chemistry should be synergistic, affording products and chemistry not attainable with the individual organo-metallics alone. 

The alkenylboronic esters were synthesized according to a literature procedure [56]. Hydrozirconation of alkenylboronic esters with zirconocene hydrochloride, Cp2Zr(H)Cl, prepared by Buchwald s procedure [57], took place smoothly in CH2C12, providing in each case the corresponding borazirconocene 1,1-alkane 18 [34], Addition of propargyl bromide and a catalytic amount of copper(I) cyanide was accompanied by the disappearance of the yellow color associated with these compounds and by carbon—carbon bond 

Iodination of terminal allenes has been reported to occur on the end carbon of the al-lenic system to give cis/trans mixtures of 1,2-diiodo adducts [64—66]. Reaction of a nucleophile with these adducts proceeds solely with displacement of the allylic iodine [65]. Iodi- 

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