DTBB-catalysed lithiation of 6-chloro-1-hexene and related systems: synthetically useful temperature-dependent behaviour

The reaction of 6-chloro-1-hexene 1 with lithium powder and a catalytic amount of 4,4′-di-tert-butylbiphenyl (DTBB, 5% molar) in THF at −78°C gives the corresponding organolithium intermediate 2, which by reaction with different electrophiles [ButCHO, PhCHO, Et2CO, (CH2)5CO, PhCOMe] affords, after hydrolysis with water, the expected products 3. The same reaction performed at −30°C gives cyclopentyl derivatives 5, probably by cyclisation of the open-chain intermediate 2 to give the cyclic organolithium compound 4. For the tertiary derivative 6, only the cyclic compound 10 could be isolated at −30°C due to the great instability of the corresponding tertiary organolithium intermediate 7, which undergoes a proton abstraction even at −78°C. From allyl 2-chloro-phenyl ether 11 only the corresponding cyclic compound 14 was isolated either at −78°C or at −30°C. In all cases a carbanionic cyclisation, better than a radical one, is postulated to occur.