Intramolecular carbolithiation promoted by a DTBB-catalysed chlorine–lithium exchange

The reaction of 6-chlorohex-1-ene 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 , which by reaction with different electrophiles affords, after hydrolysis with diluted hydrochloric acid, the expected products . The same reaction performed at −30°C gives cyclopentyl derivatives , probably by cyclisation of the open-chain intermediate to give the cyclic organolithium compound . When the double bond in the starting material contains an alkyl substituent, for instance compounds and , the corresponding cyclisation is inhibited, so the corresponding acyclic products and are respectively, obtained. However, when the substituent at the same positions is a phenyl group, like in starting materials and , the cyclised products and were respectively, isolated. In the case of the secondary starting chlorinated material , the reaction can be directed to both, the acyclic products or the cyclic ones , working at −78 or −30°C, respectively, as it happens in the case of the unsubstituted chlorinated material . For the tertiary chloro derivative , only the cyclic compound could be isolated at −30°C due to the great instability of the corresponding tertiary organolithium intermediate , which undergoes a proton abstraction even at −78°C. From allyl 2-chlorophenyl ether or N,N-diallyl-2-chloroaniline , only the corresponding cyclic compounds and , respectively, are isolated either at −78 or at −30°C. In all cases a carbanionic cyclisation, better than a radical one, is postulated to occur as mechanistic pathway.