Chemists are always searching for a finely tunable, robust catalyst. Often this is accomplished by the use of some type of external activation that not only allows control over the activation of the catalyst, but provides a more stable catalyst as well. Typically, catalyst activation has been accomplished thermally, but photoactivation has been another popular way to solve this problem. In 2009, Grubbs and coworkers tackled catalyst activation by introducing a catalyst that underwent photoactivation via a photoacid generator (PAG).1 Recently, Piers and coworkers combined their earlier work with Heppert’s catalyst (1)2, 3 with a photoactivated system via a PAG, [Ph3S]+[OTf]–, used by Grubbs and coworkers to produce a catalyst that can be readily activated with UV light at 254 nm (Scheme 1).
Scheme 1. Light activation of Heppert’s ruthenium carbide (1).
Similar to Piers and coworkers’ earlier studies, the acid generated by irradiation of [Ph3S]+[OTf]– readily initiated Heppert’s ruthenium carbide 1 via protonation.3 As expected, the coordinating ability of the anions has an effect on the efficiency of the reaction. The less coordinating anions provided a more active catalyst and the complex (2) obtained with the [Ph3S]+[OTf]– demonstrated the optimal level of activity. A series of substrates were examined to evaluate the efficiency of the photogenerated initiation as well as substrates scope by looking at a range of reaction types such as ring-opening metathesis polymerization (ROMP) and ring-closing metathesis (RCM). Gratifyingly, all displayed high amounts of conversion (>99%).
After examining the breadth of reactivity, Piers and coworkers wanted to understand the impact of having the photogenerating catalyst present during the metathesis reaction. To their delight, they found that the side products generated from the catalyst were not detrimental to the reaction. However, they did note that prolonged irradiation times caused a significant increase in the reaction temperature and subsequently noticeable decomposition of ruthenium complex 2. In order to avoid the thermal decomposition of 2, Piers and coworkers introduced an excess of isopropoxy-2-vinylbenzene in an effort to trap the more thermally stable Hoveyda-Grubbs catalyst 3 (Scheme 2). Unfortunately, UV light could only generate catalyst 3 in 50% yield with these conditions due the polymerization of the vinylbenzene.
Scheme 2. Formation of Hoveyda-Grubbs catalyst (3).
The quest for new methods to activate the metathesis catalyst will continue as new boundaries are pushed. The use of photoactivated catalysts is very exciting and hopefully will shine a light into new territory for chemists and scientists to venture.
1Keitz, B.K.; Grubbs, R.H. J. Am. Chem. Soc., 2009, 131, 2038-2039.
2Carlson, R.G.; Gile, M.A.; Heppert, J.A.; Mason, M.H.; Powell, D.R.; Velde, D.V.; Vilain, J.M. J. Am. Chem. Soc., 2002, 124, 1580-1581.
3(a) Romero, P.E.; Piers, W.E.; McDonald, R. Angew. Chem. Int. Ed., 2004, 43, 6161-6165. (b) Dubberley, S.R.; Romero, P.E.; Piers, W.E.; McDonald, R.; Parvez, M. Inorg. Chim. Acta, 2006, 359, 2658-2664.