Describe the mechanism of the McMurry reaction.

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The McMurry reaction is a chemical reaction used to form an alkene from two carbonyl compounds (aldehydes or ketones) through a reductive coupling process. It typically employs a low-valent titanium reagent, such as $\text{TiCl}_3$ or $\text{TiCl}_4$ with a reducing agent like zinc or magnesium.

The mechanism proceeds as follows:

Generation of Low-Valent Titanium
A low-valent titanium species, often $\text{Ti(0)}$ or $\text{Ti(II)}$ , is generated from a titanium halide (e.g., $\text{TiCl}_3$ ) and a reducing agent (e.g., $\text{Zn}$ ).
$\text{TiCl}_3 + \text{Zn} \rightarrow \text{Ti(0)} \text{ species}$
Formation of Ketyl Radicals
The carbonyl oxygen of an aldehyde or ketone coordinates to the titanium species. Electron transfer from titanium to the carbonyl group occurs, breaking the $\pi$ -bond and forming a ketyl radical.
$\text{R}_2\text{C=O} + \text{Ti(0)} \rightarrow \text{R}_2\text{C}^{\cdot}\text{-O-Ti}$
Dimerization
Two ketyl radicals combine to form a 1,2-diol (pinacol) intermediate, which remains coordinated to the titanium.
$2 \text{R}_2\text{C}^{\cdot}\text{-O-Ti} \rightarrow \text{Ti-O-CR}_2\text{-CR}_2\text{-O-Ti}$
Deoxygenation to Alkene
The titanium removes the oxygen atoms from the pinacol intermediate, leading to a syn-elimination and the formation of a carbon-carbon double bond (an alkene). This step is highly favorable due to the formation of stable titanium oxides.
$\text{Ti-O-CR}_2\text{-CR}_2\text{-O-Ti} \rightarrow \boxed{\text{R}_2\text{C=CR}_2} + \text{TiO}_x$

Describe the mechanism of the McMurry reaction.

Answer

Step-by-step solution

Generation of Low-Valent Titanium

Formation of Ketyl Radicals

Dimerization

Deoxygenation to Alkene

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