Addition reactions hydrozirconation

A variety of other addition reactions occurring regioselectively are also known. These include hydrocyanation, hydroalumination, hydrosilylation, and hydrozirconation. 

Another type of mixed cyanocuprate has both methyl and alkenyl groups attached to copper. Interestingly, these reagents selectively transfer the alkenyl group in conjugate addition reactions.16 These reagents can be prepared from alkynes via hydrozirconation, followed by metal-metal exchange.17... 

A highly creative method for ZnMe2-mediated addition reactions of alkenylzirconocenes to aldimines was recently proposed by the Wipf group.345,346 The protocol includes a three-step sequence (i) alkyne hydrozirconation,... 

Alkenylzirconium(iv) coihplexes, prepared by hydrozirconation of the corresponding acetylenes with Cp2Zr(H)Cl, can be utilized in conjugate addition to a,/8-enones catalysed by Ni(acac)2. Applications of this conjugate addition reaction to the synthesis of prostaglandin analogues are also described. "... 

Alternatively, the hydrozirconation of 4 can be performed with i-BuZrCp2Cl generated in situ by the treatment of Cp2ZrCl2 with i-BuMgCl in benzene at 50 °C [5c], When the sequence described in Scheme 12.4is applied to 5-hexynoic acid ethyl or methyl ester (3c,d), no transmetallation-1,4-addition reaction was observed. These failures to transmetallate may be a consequence of intra- and/or in-termolecular chelation of the ester carbonyl group with the Zr(IV) present (Scheme 12.5) [5]. 

The selective one-pot synthesis of ketones from acid halides and functionalized alkynes based on a hydrozirconation-copper(I)-catalyzed addition reaction has also been reported as a very efficient transformation even in the presence of very labile functionality (Scheme 12.12) [14]. 

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. 

Little is known about the tolerance of 1 with unsaturated (poly)halogen compounds. Hydrozirconation of chloroalkenes can lead to competitive cycUzation, and simple reduction of both C=C and C-Cl bonds [98, 222], However addition of 1 to an alkenyl- or propargyl bromide led to the expected product as the sole product of the reaction in excellent yield (Scheme 8-30) [134, 223]. 

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). 

The use of Cp2Zr(H)Cl in hydrozirconation reactions has been referred to (80), and consists of addition to the Zr-H moiety to unsatu-... 

Transmetallations starting from alkenylzirconium species, which are obtained by hydrozirconation using H(Cl)ZrCp2, are readily accomplished.98,99 "3 Ichikawa, Minami, and co-workers have elegantly shown that diflurovinylzinc iodide is obtained by the addition of ZrCp2 to the corresponding alkenyl tosylate (Equation (36)). In situ transmetallation reactions have also been reported. 

---