The major industrial application of butyllithium is that of an anionic initiator for the manufacture of polyisoprene, polybutadiene and butadiene/styrene copolymers, all of which have gained considerable importance to the rubber and plastics market. Employing solution-phase polymerization with butyllithium, the molecular weight, molecular weight distribution, co-monomer ratio, sequence distribution of butadiene and styrene, the degree of branching and thus the microstructure can all be controlled effectively.
Moreover, butyllithium has become a very valuable tool in the organic synthesis of active pharmaceutical ingredients (APIs), agrochemicals, and electronic materials. Due to its strong base strength it can be used as versatile reagents for the metalation (deprotonation or halogen-metal exchange) of organic substrates.
n-butyllithium is one of the strongest bases that are applied in organic synthesis. With a pKa value of > 35 n-butyllithium is capable of deprotonating a wide range of acidic compounds, thus enabling further transformations like C-C coupling reactions. Mild reaction conditions, high yields, few by-products, and easy separation of the residual lithium salts are the major benefits of n-butyllithium compared to alternative reagents. For specific deprotonations more basic sec-butyllithium or even tert-butyllithium can be applied.
Another common application of butyllithium reagents in organic synthesis is the halogen-metal-exchange reaction. In this type of reaction, aliphatic or aromatic organic halide compounds (mostly iodides or bromides) are used as substrates. In equilibrium reactions with butyllithium, the less basic organolithium species is formed, thus enabling further transformations mediated by a reaction with a suitable electrophile. Compared to other classical methods like Mg insertion with the Mg metal, or the direct reaction with the Li metal, the halogen exchange with butyllithium is characterized by mild reaction conditions, high yields, and few by-products especially when applying substrates with functional groups sensitive to harsher reaction conditions.