Hydrozirconation chemoselectivity

Hydrozirconation of various alkenylboranes [118-121] and alkenylzinc halides [34] with 1 provides the corresponding 1,1-bimetalloalkanes, which can be selectively converted to functionalized organic compounds [122-125]. Interestingly, alkenylzinc halides (RCH=CHZnX) show remarkable chemoselectivity, and functional groups such as chloride, cyanide, or ester functionality are tolerated [126]. 

The chemoselectivity of Schwartz s reagent (1) toward alkynes, alkenes, nitriles, and carbonyl groups, and thus its general functional group compatibility, can be modulated. However, it is important to keep in mind that the presence of functional groups may have regiochemical consequences on the hydrozirconation reaction. 

As predicted from the comparative rates for C=C over C=C hydrozirconation cited earlier, a (poly)enyne is selectively hydrozirconated at the alkyne moiety, whatever the position of the alkene function [138, 210] in the molecule. It can be exempUfied by the chemoselective hydrozirconation of 1,3-butenyne. One exception to this chemoselectivity has been reported, which showed the terminal alkene to react with 1 but leaving the TMS-substituted alkyne function intact (Scheme 8-25). 

The rate of hydrozirconation of a terminal olefin is much faster than the reduction of the oxirane ring. This chemoselectivity was used in the preparation of various cycloalkylmethanols by hydrozirconation of alkenyloxiranes (Scheme 8-28) [217]. 

In terms of scope and chemoselectivity, hydrozirconation takes its place between hydroboration and hydroaiumination. However, the synthetic applications of organozirconocene complexes have been considerably expanded over these last few decades, and it can be expected that they will become more and more attractive in the future. Beside the direct substitution sequences, indirect reaction pathways involving transmetalation or activation by ligand abstraction have been successfully applied in a number of cross-coupling and C-C bond-forming reactions. 

In 1997, Taguchi, Hanzawa and coworkers described the formation of cyclopropyl carbinols from alkenyl oxiranes through chemoselective hydrozirconation followed by intramolecular attack of the generated alkylzirconocene on the epoxide ring (Scheme 18) [27]. Both the reactivity and the selectivity patterns are worthy of note. The sequence was made possible owing to the chemoselective hydrozirconation of the alkene in the presence of oxirane. Despite the poor nu-... 

The availability of the Schwartz s reagent Cp2Zr(H)Cl (see Schwartz s Reagent) and its versatile behavior explain its extensive use in orgaiuc chemistry. The scope of hydrozirconation (see Hydrozirconation) with respect to both substrate structure and chemoselectivity lies somewhere between those of hydroboration (see Hydroboration) and hydroalumination. 

Despite the low intrinsic reactivity of alkylboranes, Pd-catalyzed alkylation with alkylboron compounds holds considerable promise as a unique and chemoselective alkylation method. It is uifique in part because hydroboration of alkenes is by far the most general and dependable method for stoichiometricaUy converting alkenes into organometals and in part because alkylboron compounds thus generated can satisfactorily participate in Pd-catalyzed alkylation under basic conditions, hi this connection, it should be noted that the current scopes of hydroalumination and hydrozirconation of alkenes are much more Umited than... 

Cp2ZrH(Cl) = ([Zr]-H) reacts with unsaturated C—C bonds to give isolable addition products (in opposition to boranes and alanes, organo-Zr are rather stable in dry air) that can be hydrolyzed with diluted acidic solutions. The scope of hydrozirconation with respect to chemoselectivity and substrate structure lies between those of hydroalumination and hydroboration. The facile isomerization of secondary alkylzirconium derivatives into primary alkyl derivatives occurs very readily (even below room temperature, note the difference with A1 and B ). 

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