Hydrozirconation regiochemistry

Additional coordination upon hydrozirconation (see Scheme 8-13) but also during the subsequent transformations (see Schemes 8-14 and 8-15) has to be taken into account in the analysis of the regiochemistry of the hydrozirconation/trans-metalation reaction sequences. 

Note that hydrozirconation of 2-vinylfuran gives only the internal product [86] (Scheme 8-16) which probably is the result of the combination of the effects described in this section (i) O-coordination, (ii) aromatic stabihzation, (iii) reduced steric effect of the flat furan ring, which favors the reverse expected regiochemistry in the hydrozirconation reaction of alkenes with [Cp2Zr(H)Cl] (1). 

The regiochemistry of the hydrozirconation of disubstituted stannyl- [24, 167-170] and silyl- [171] acetylenes and boron- [118, 172-175] and zinc- [34, 126] alkynyl derivatives result in the formation of 1,1-dimetallo compounds. Hydrozirconation of alkynyliodonium salts affords alkenylchlorozirconocenes with the Zr-C bond geminal to the iodonium moiety [176]. These zirconocene complexes allowed the preparation of ( )-trisubstituted olefins (Scheme 8-20). 

Scheme 4.7. Variations in the regiochemistry of the hydrozirconation of alkynyl sulfones.

As an alternative to hydrozirconation of acetylenic tellurides or selenides, Dabdoub and co-workers have more recently described the first additions of the Schwartz reagent (one equivalent) to acetylenic selenide salts 51 (Scheme 4.30) [52]. Subsequent alkylation at selenium produces 1,1-dimetallo intermediates 52, which are cleanly converted in a one-pot process to stereodefined products 53. It is noteworthy that ketene derivatives 52 are of ( )-geometry, the opposite regiochemistry to that which results from hydrozirconation of acetylenic tellurides (vide supra). This new route also avoids the mixtures of regio-isomers observed when seleno ethers are used as educts. The explanation for the stoichiometric use of Cp2Zr(H)Cl in these reactions, in contrast to the requirement for two equivalents with seleno ethers, may be based on cyclic intermediates 54, in which Li—Cl coordination provides an additional driving force. Curiously, attempted hydrozirconation of the corresponding telluride salt 55 under similar conditions was unsuccessful (Scheme 4.31) (Procedure 12, p. 143). 

An overview of the hydrozirconation of aUcynyl sulfones reveals many variations in the regiochemistry. Starting from arylacetylenic sulfoxides, the hydrozirconation classically takes place in a cis fashion, with zirconium in the a-position... 

The regiochemistry of the hydrozirconation of acetylenic selenides with the Schwartz reagent [Cp2Zr(H)Cl] is dependent on the nature of the substituents. For simple acetylenic selenides (R = Ar, n-Bu), ( j-j9-zirconated vinyl selenides were formed exclusively. The reaction with electrophilic reagents has allowed the stereoselective synthesis of ( j-2-halovinylaryl selenides [75], (Ej-2-aryl-vinyl aryl selenides [76], ( , j-l-arylselanylbutadienes [77] and ( j-2-butyltel-luranylvinyl selenides [78] (Scheme 55). 

A method for the hydrocyanation of alkenes and alkynes is based upon hydrozirconation followed by isonitrile insertion (Scheme 13) overall yields range from 45-90%. Certain examples do not exhibit the clean regiochemistry represented here, and these were discussed in Section 3.9.3.3.2. The hydrozirco-nation/protonolysis of equation (53) results in high yield, stereoselective formation of C-methyldeoxy-sugars, an alternative to free radical deoxygenation. ... 

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