Halogen-metal exchange reaction, acidic proton

Since Li2Zn Bu4 could weU tolerate substrates bearing an acidic proton (Fig. 12), the protection-free synthesis of poly[3-(6-hydroxyhexyl)thiophene] (P3HHT), with hydroxyhexyl side chains, was also successful (Fig. 29). After halogen-metal exchange reaction of 2-bromo-3-(6-hydroxyhexyl)-5-iodothiophene with... 

The debromination of gem-dibromocyclopropanes by dimsyl anion (73) involves removal of a soft halogen by the carbon base. The formation of allenes from the reaction of g cm-dibromocyclopropanes with alkyllithiums (74) must proceed at its early stages by a similar mechanism. Halogen-metal exchange of a e/n-dibromocyclopropanecarboxylic acid (75) is 2.5 times faster than proton abstraction from the acid This is a remarkable demonstration of the HSAB axiom. 

With few exceptions arynes used in the nucleophilic additions discussed so far are generated either by removing with base an aryl proton adjacent to a leaving group (Type A2, Section III.A.2), or by decomposing an anthranilic acid derivative (Type Bl, Section III.A.3). Reactions described in the remainder of this section are initiated instead by metal-halogen exchange (Type Al, Section IILA.l). 

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