Reductive Coupling Reactions Ti, Zr

The group 4 metals, especially Ti and Zr, are most stable in their (IV) oxidation states. In their (II) oxidation states they act as reducing agents, but in a very spe- 

The reductive coupling of two carbonyl compounds using reduced Ti reagents is called McMurry coupling. The products may be 1,2-diols or alkenes. 

In another mechanism for McMurry coupling, a Ti(IV) ketyl forms from Ti(III) and the carbonyl. Radical-radical combination then gives the 1,2-diol in a pina-col coupling (Chapter 5). 

Reductive coupling of alkynes or alkenes may be carried out with Cp2ZrCl2, Ti(0-i-Pr)4, or other Ti(IV) and Zr(IV) complexes. The reaction is often carried out in an intramolecular fashion to achieve better control of regiochemistry. The diyne or other polyunsaturated compound is added to a mixture of the Zr(IV) or Ti(IV) complex and a reducing agent such as BuLi. After formation of the intermediate metallacycle, the C-M bonds are cleaved by treatment with an electrophile such as H+ or I2 to give the organic product. 

The reductive coupling requires that the Zr(IV) be reduced to Zr(II). Metals such as Mg may be used to reduce Zr(IV) to Zr(II) by electron transfer, but the most widely used method today involves the addition of two equivalents of BuLi to Cpo/rCh. Transmetallation to give Cp2ZrBu2 is followed by /3-hydride abstraction to give the Zr(IV) metallacyclopropane, which can also be described as a Zr(II)-butene complex. 

The group 4 metals, especially Ti and Zr, are most stable in their (IV) oxidation states. In their (II) oxidation states they act as reducing agents, but in a very specific way. They use the two excess electrons to create a new C—C bond between two C=X or C=X species to give a new X—C—C—X or X—C—C=X compound. (X may be N, O, or C.) 

In the reductive coupling part of the reaction, one alkyne displaces 1-butene from Zr(II) to give a new tt complex then a second alkyne coordinates to the Zr(IV) metallacyclopropene and inserts into the C—Zr bond to give a Zr(IV) metallacyclopentadiene. 

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