REGIOSELECTIVE IODINATION OF ALKYLBENZENES
IN THE PRESENCE OF CuY ZEOLITE
Institute of Organic Chemistry, 9 Lavrentiev Ave., Novosibirsk 630090, Russia
Iodoarenes are useful reagents for fine organic synthesis. There are two main routes for their preparation. Indirect methods that involve the Sandmeyer reaction and iododemetallation, although selective, are often somewhat tedious or imply the use of highly toxic organomercury and organothallium compounds. Direct methods, being less complicated, do not always allow to control regioselectivity and are usually run in the presence of strong mineral acids and/or oxidants . Zeolites are reputed to increase regioselectivity in aromatic substitution reactions including halogenation . However, to our best knowledge, it has not been reported to date on the application of zeolites in direct aromatic iodination.
To a stirred mixture of an aromatic compound (4 mmol), precalcined zeolite (5 g) and hexane (30 mL) iodine (2.5 mmol) was added. The suspension was stirred for 24 hours at room temperature, then zeolite was filtered off and filtrate was washed with aqueous sodium thiosulfate and water. The organic layer was dried with sodium sulfate. Yields of crude products after evaporation of the solvent are given in the Table. The products were identified by 1H NMR and GLC.
3. RESULTS AND DISCUSSION
The reaction of alkylbenzenes with iodine has been studied in the presence of the zeolites HY, NaY, CaY and Cu(II)Y.
It is noteworthy that we have failed to detect any reaction with all the zeolites but Cu(II)Y (cf. data on the direct iodination of aromatics in the presence of alumina ). In mechanistic aspect it may be indicative of the electrophilic spieces formation via iodine oxidation rather than via iodine molecule polarisation by cations within the zeolite lattice (cf. ).
Table 1. Direct iodination of alkylbenzenes
|Alkylbenzene||Iodinating system||Products||Yield, (%)||Ref.|
|Toluene||I2/Hg(NO3)2||a mixture of para- and ortho-iodotoluenes (p/o = 1.9)||92||4|
|o-Xylene||I2/CuCl2||a mixture of 1,2-dimethyl-4-iodobenzene and 1,2-dimethyl-3-iodobenzene (isomer ratio 3 : 1)||13||5|
A new system for direct highly regioselective iodination of alkylbenzenes using Cu(II)Y zeolite has been proposed.