Chloromethylene Dibenzoate and Dichloro Alkyl Ethers

Wenzel reported the formation of mesitylaldehyde on the addition of chloromethylene dibenzoate and anhydrous aluminum chloride to a benzene solution of mesitylene followed by a hydrolytic work-up (Eq 1.14). Anisole reacts in a similar manner with chloromethylene dibenzoate to give anisaldehyde (80% yield) while reaction with benzene produces only diphenylcarbinol benzoate 

Rieche, Gross and Hdft found that dichloromethyl alkyl ethers interact with aromatics and some heterocyclic compounds in the presence of Friedel-Crafts catalysts to give aldehydes in good yields. The intermediates in these reactions are relatively unstable a-alkoxybenzyl chlorides that decompose thermally or on the addition of water to yield aldehydes (Eq 1.15).  

The dichloromethyl alkyl ether is generally added to a solution or suspension of the aromatic substrate and the catalyst (TiCU, SnCU, SnBr4, FeCIs or AICI3) in methylene chloride, n-hexane or carbon disulfide. Gross, Rieche and Matthey prepared phenolic aldehydes in good yields using this method. 

As the Gattermann-Koch reaction fails to produce aldehydes from phenol and phenolic ethers, Gattermann and co-workers developed an alternative method that produces aldehydes from phenols, phenol ethers and heterocyclic compounds successfully. The method involves the addition of HCN and HCI to the aromatic substrate, with or without the presence of an acidic halide catalyst (Eq 1.17) 

Benzene can be used as solvent as it is un-reactive under the reaction conditions. 

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