Titanium hydromagnesiation

Dicyclopentadienyltitanium dichloride is possibly superior to titanium tetrachloride as a catalyst. Nickel(ll) compounds are also active, but with these catalysts concurrent addition of the organomagnesium compounds to carbon-carbon multiple bonds (see Section 4.1) causes complications. Examples of hydromagnesiation by Grignard reagents are listed in Table 3.8. As will be seen from entries in Table 3.8, the stereochemistry of addition to alkynes is syn. The regiochemistry is also usually predictable in the relevant examples in Table 3.8, one regioisomer is obtained mainly or exclusively. In other cases, however, mixtures are formed [54]. 

As far as the role of the transition metal catalysts in the hydromagnesiation is concerned, other hydrometallation reactions such as hydroa-lumination 1108] and the recently reported hydro-zincation 1109) are catalyzed by the same class of titanium complexes, so all of these reactions can be grouped together. In fact, these reactions show quite similar applicabilities to particular unsaturated compounds and similar rcgioselcctivi-ties with unsymmetrical substrates (eqs. 3.64 and 3.65). 

Grignard reagent (107) by titanium-mediated hydromagnesiation, and (107) can then be transformed into a range of ( )-3-trimethylsilyl-alk-2-en-l-ols (108)... 

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