The reactivity of inert hydrocarbons can be transformed by a catalytic double act. With the ability to manipulate the lengths of the resulting carbon chains, this development opens up fresh vistas.
In chess, a knight's move can be formidable, because the implications of the combination of horizontal and diagonal motion are hard for an opponent to anticipate. Likewise, tandem reactions in chemistry are a combination of known reactions that provides an equally unexpected outcome. The power of this tandem strategy has been greatly enhanced by the use of homogeneous catalysis, in which soluble catalysts, typically metal compounds, are used to increase the rates and minimize the side-products of the individual reaction steps. In tandem catalysis1,2, two or more such catalytic reactions are combined.
Writing in Science, Goldman, Brookhart and co-workers3 describe a novel combination of two catalytic reactions that are each celebrated reactions in their own right. The first, C–H activation, which is initiated by cleavage of a normally unreactive carbon–hydrogen bond in an organic molecule, is proving increasingly influential in organic synthesis. The second is called metathesis, and involves reactive hydrocarbons known as alkenes, which contain carbon–carbon double bonds (C=C). Alkene metathesis is initiated by cleavage of such a bond, and has gained wider attention following the award of the 2005 Nobel chemistry prize to three of its pioneers, Robert Grubbs, Richard Schrock and Yves Chauvin4.
Both reactions have interesting histories. Initial work in the 1970s, now recognized, but at the time either ignored or even disbelieved, was carried out by Alex Shilov5 on C–H activation in the Soviet Union and by Chauvin6 on alkene metathesis in France. In each case, it has taken decades of further work by many groups to overcome the initial limitations of slow rates or lack of general applicability, until today the promise of these reactions is beginning to be more fully realized. Metathesis has moved forward farthest — in particular, the Grubbs7 and Schrock8 metathesis catalysts are widely used, because they are far more tolerant of other functional chemical groups in the reactant than were earlier catalysts.
The Goldman–Brookhart reaction3 is notable in being the first example of a combination of these two high-profile reactions. In the C–H activation step, which is catalysed by a specially chosen iridium catalyst, normally unreactive alkanes — hydrocarbons that contain no double bonds — can be converted to their reactive alkene counterparts by losing two hydrogen atoms to form a C=C double bond (Fig. 1, overleaf). A Schrock metathesis catalyst is also present, and this uses the alkene produced in the previous step as its starting point. Metathesis temporarily cleaves the strongest bond of the alkene — the C=C double bond — to form fragments that are bound briefly to the catalyst. If these fragments are not identical, new products are formed when the fragments switch partners and join together, in the key phase of the metathesis process. In the third and final stage of the tandem cycle, the hydrogen released in the initial C–H activation step is added back to the C=C bond of the metathesis product to give alkanes that have a different number of carbon atoms from that in the initial reactant. Because this step is simply the reverse of the first reaction in the sequence, it is catalysed by the same iridium catalyst, and no third catalyst is needed.
A surprising feature of this tandem reaction is the ability of the catalysts to work independently, without interfering with each other. This result will undoubtedly prompt other workers to try previously unconsidered catalyst combinations in the search for unexpected outcomes.
Many catalytic reactions leave the number of carbon atoms in a chain unaltered from starting material to products. Metathesis is noteworthy in that it converts carbon chains into a mixture of longer and shorter chains. Thanks to the metathesis component, the same is true of the Goldman–Brookhart reaction, which involves alkanes — also known as saturated hydrocarbons — and not just compounds with C=C bonds as in traditional metathesis.
Apart from its appealing novelty, this ability to manipulate the length of carbon chains is a potentially useful step in processing saturated hydrocarbon sources for fuel and chemical uses, particularly in a future regime aiming to use alternative feedstocks to oil, such as biomass or natural gas. The Fischer–Tropsch reaction converts mixtures of carbon mon-oxide and hydrogen, which may be derived from such alternative feedstocks, into a mixture of saturated hydrocarbons of various carbon-chain lengths. Once the medium-length chains needed for gasoline are removed, longer and shorter chains remain; the Goldman–Brookhart reaction could produce desirable chain lengths from this mixture by the route shown in Figure 1. Practical application is still a goal for the future, however, because the efficiency of the alkane dehydrogenation step needs to be improved.
As oil supplies are depleted in the coming decades, energy concerns will intensify. In this new era, we will need to fully harness the great power of chemical catalysis — including tandem versions — to carry us through what might otherwise be lean times.