Hydrozirconation mechanism

Besides hydrozirconation of terminal triple bonds Cp2Zr(H)Cl (16) also reacts with double bonds.8 The mechanism is similar to that described for alkynes. After coordination of alkene 5 to the Zr center giving -complex 23 the terminal double bond inserts into the Zr-H bond to form the stable (T-alkyl complex 24. The bulky zirconocene moiety again adds to the end-position of the terminal double bond. 

As mentioned earlier, hydrozirconation of internal alkenes gives terminal alkylzir-conium compounds. The isomerization occurs so quickly that no intermediates are observed. The mechanism of the reaction is a series of insertions and /3-hydride eliminations. 

In the presence of ZrCLt or HfCLt, alkynes react with Bu3SnH or Bu2SnH2 at 0 °C by anti addition.81 Plausible mechanisms involve direct electrophilic hydrostannation via a tin hydride-Lewis acid complex, or initial electrophilic hydrozirconation followed by transmetallation. 

Heterosubstituted alkenes undergo hydrozirconation. Hydrozirconation of aUcenyl-zirconiums is a suggested mechanism for isomerization of alkenylzirconiums, and 1,2-dizirconiums are known . Hydrozirconation of alkenylboranes and alkenyl-aluminums leads to both 1,1- and 1,2-dimetalated derivatives ... 

Stoichiometric studies of M-H additions to alkynes also show mixed stereochemical results. The more common cw-addition is typified by the CO-promoted transformation of Cp2Nb(H)(RCECR) to ds-Cp2Nb(CO)(Ti -CR=CHR) [90]. In mononuclear systems where frans-additions have been found, radical-type mechanisms have been implicated [91] or cis/trans isomerization of the intermediate vinyl species [92] has been found. Although the intermediacy of alkyne complexes has not been established, Schwartz s hydrozirconation of alkynes [93] by Cp2ZrHCl represents a general entry to vinyl-metal species which can be transformed stereoselectively to alkenes, vinyl halides, and/or carboxylic acids. 

The regioselective hydrozirconation of internal unsymmetrical alkenes remains a challenge, as it could considerably expand the use of zirconocene complexes. Little is known about the mechanism of zirconium migration along an alkyl chain. 

Regiospecific deuterium labeling can be achieved by quenching organozir-conocenes with D2O or by using Cp2ZrDCl for hydrozirconation. A nice demonstration of this concept is the synthesis of three deuterium-substituted analogues of dimethyl hept-l,6-dienyl-4,4-dicarboxylate [42] (Scheme 6). These compounds were used for the study of the mechanism of the transition metal-... 

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