Bromination and iodination of donor–acceptor cyclopropanes. Evidence for an ET mechanism

Ethyl 2,2-dimethoxycyclopropanecarboxylates 1a–d react easily with Br2 and I2 in CCl4 or CH2Cl2 leading, in high yields, to 1-ethyl-4-methyl 2-halobutanedioates 2 and 3, respectively. Bromination in the presence of pyridine, NBS, trimethyl phosphate, and iodination with ICl and ICl/pyridine has been also performed. A common SET mechanism may be proposed for both halogenations; depending on the reaction conditions, bromination can also occur via acid-catalysed or SE2 routes. The reaction of the 2-ethoxyanalogues cis-12 and trans-12 with the same halogens proceeds in a similar manner, giving 3-formyl-2-haloesters along with the corresponding diethylacetals as main products. Iodination of 12 with the catalytic system NaI/m-CPBA/18-crown-6 has also been investigated.