Superconductivity continues to dominate the Hot Ten of chemistry, occupying five positions in the list at #1, #2, #3, #9 and #10. Of those remaining, only #7 and #8 are new to the list. The former reports the structure of a nanoparticle of gold, while the latter reports a way of computing and predicting large polypeptide structures.

Paper #7, from the departments of Structural Biology and Applied Physics at Stanford University, reports the structure of a nano-sized agglomerate of 102 gold atoms sheathed in an outer layer of 44 p-mercaptobenzoic acid molecules. Its structure was determined by X-ray crystallography. Large thiolate-protected gold monolayers have been produced by the decomposition of gold(I) benzenethiolate, but paper #7 is the first time that the structure of such a nanoparticle of gold has been determined.

The crystals were grown from a solution in which the gold thiolate had to be soluble, and this was achieved with 40% methanol, plus small amounts of sodium chloride and sodium acetate. Fifteen crystals were prepared by this method and, surprisingly, all were found to have the same arrangement of 120 gold atoms. According to the authors of #7, it is electronic forces within the cluster which explains why Au₁₀₂ forms and they suggest that such an arrangement completes a particularly stable 58-electron shell.

The nature of the outer layer of thiolates poses interesting questions also, and these are addressed by Robert Whetten and Ryan Price of the Georgia Institute of Technology in the same issue of Science (318[5849]: 407-8, 19 October 2007). While there are various ways in which thiolate ligands could bind to gold(I), the most likely one involves the formation of covalent bonds to two thiolates and these then arrange themselves like a shell around the positively charged core of gold atoms.

Gold also features in a paper which only just failed to make the current list and is at position #12. This reports oxidative rearrangements catalyzed by gold and is the work of a group led by Dean Toste of the University of California at Berkeley. It was published in the Journal of the American Chemical Society (C.A. Witham, et al., 129[18]: 5838, 2007; 13 citations this period).

The Berkeley group have demonstrated that oxidative rearrangement reactions of alkynes with sulfoxides as the oxidizing agents can be successfully catalyzed by gold(I) compounds to give yields as high as 94%. The products from such rearrangement reactions contain carbonyl groups and as such offer routes to other molecules. A typical catalyst was triphenylphosinegold(I) (Ph₃PAuCl) and it was used in conjunction with silver antimony hexafluoride (AgSbF₆).

So how do gold(I) catalysts effect their magic? In their early investigations the group focused on intercepting cyclopropane-gold intermediates which should be present in the formation of ring compounds from alkynes. These intermediates then reacted further with dimethylsulfoxide, but yields were disappointing. However, by a judicious change of reagent to diphenylsulfoxide, and by varying the ligands on the gold catalyst, the yields of desired products were boosted to more than 90%. Further research provided further support for the team's belief that the intermediates which the catalyst was forming were carbenoid in nature.

Subsequent research by the Berkeley group has focussed on a wider range of reactions providing additional support for the carbenoid nature of the gold(I) species in the formation of pyrrolone from an azide. Toste has also studied the effect of ligands in homogeneous gold catalysis (D.J. Gorin, et al., Chem. Rev., 108[8]: 3351, 2008).

In 2008 Toste also showed just how versatile these new catalysts were. They performed well in a ring-expanding cyclo-isomerization that was part of the total synthesis of ventircosene (S.G. Sethofer, et al., Org. Lett., 10[19]: 4315, 2008). They were used in an intermolecular ring formation in the synthesis of azepines (N.D. Shapiro, et al., J. Am. Chem. Soc., 130[29]: 9244, 2008), and in the cyclo-isomerization of 1.5-allenyes (P.H.Y. Cheong, et al., J. Am. Chem. Soc., 130[13]: 4517, 2008). Fluorenes and styrenes were produced by a gold(I)-catalyzed annulation of enynes and alkynes (D.J. Gorin, et al., J. Am. Chem. Soc., 130[12]: 3736, 2008).

Gold, once the Cinderella of metals as far as chemical reactivity was concerned, is now finding itself being wooed and won for all kinds of synthetic and nanotech applications. For example see Science Watch January/February 2008 (19[1]: 7), in which other gold catalyst papers made the Hot Ten, reporting equally impressive yields.

Dr. John Emsley is based at the Department of Chemistry, Cambridge University, U.K.

KEYWORDS: GOLD, GOLD NANOPARTICLES, GOLD(I), GOLD CATALYSIS, DEAN TOSTE, ROBERT WHETTEN, RYAN PRICE.