Synthesis of heterocycles: from simple to complex structures in the golden age of catalysis

We cannot say that gold was an unknown element of the Periodic Table. In its ground state, gold has been used over the past thousands of years for jewels, coins, fabrics, chinaware and many other precious applications. However, only very recently chemists took a hand in this dig for gold and, eccentrically, decided to focus on different oxidation states. Due to its stability, gold was considered to be catalytically inactive and for this reason was neglected for a long time. Actually a few reports existed about its catalytic properties though they were described as very poor when compared to those of other well-known catalysts. Things started to change in the early Seventies of the last century when for the first time the hydrogenation of olefins was carried over supported gold catalyst and, in homogeneous catalysis, the unexpected thermal [2+2+2] cycloreversion of diademan to snoutene represented an unwanted but serendipitous result. More than fifteen years later, the low-temperature oxidation of CO and the hydrochlorination of ethyne were reported, showing that gold was the best heterogeneous catalyst for these reactions. A few years later the breakthrough in homogeneous catalysis was made first by reporting the addition of nucleophiles to alkynes, employing Au(I) as a catalyst and, a handful of years later, by proving that cationic Au(I) species gave excellent turnover numbers (TONs) and turnover frequencies (TOFs) in similar experiments. These pioneering reports set the stage for all the following publications on homogeneous cationic gold catalysis whose yearly number increased more and more rapidly over the past ten years till the nowadays weekly appearance. It would be a titanic enterprise to cover every application of gold catalysis in the field of organic chemistry. Therefore we focus only on the synthesis of heterocyclic compounds by means of homogeneous gold-catalyzed reactions and report on the most recent results.