Atom Economy—A Challenge for Organic Synthesis: Homogeneous Catalysis Leads the Way

Enhancing the efficiency of the synthesis of complex organic products constitutes one of the most exciting challenges to the synthetic chemist. Increasing the catalogue of reactions that are simple additions or that minimize waste production is the necessary first step. Transition metal complexes, which can be tunable both electronically and sterically by varying the metal and/or ligands, are a focal point for such invention. Except for catalytic hydrogenation, such methods have been rare in complex synthesis and virtually unknown for CC bond formation until the advent of cross-coupling reactions. These complexes may orchestrate a variety of CC bond-forming processes, important for creation of the basic skeleton of the organic structure. Their ability to insert into CH bonds primes a number of different types of additions to relatively nonpolar π-electron systems. Besides imparting selectivity, they make feasible reactions that uncatalyzed were previously unknown. The ability of these complexes to preorganize π-electron systems serves as the basis both of simple additions usually accompanied by subsequent hydrogen shifts and of cycloadditions. The ability to generate “reactive” intermediates under mild conditions also provides prospects for new types of CC bond-forming reactions. While the examples reveal a diverse array of successes, the opportunities for new invention are vast and largely untapped.