'Half-sandwich' rare-earth catalysts provide unprecedented directional control over aromatic insertion reactions

March 8, 2013
A cationic ‘half-sandwich’ rare-earth complex (center, yellow) catalyzes the addition of anisoles (blue structures) to alkenes (red structures) with high site-specificity. Credit: 2012 Zhaomin Hou, RIKEN Advanced Science Institute

Methoxybenzene, commonly known as 'anisole' because of its aniseed aroma, is an important compound in organic chemistry. Anisole-type frameworks are found in numerous molecules ranging from pheromones to fluorescent dyes. New findings from Zhaomin Hou and colleagues from the Organometallic Chemistry Laboratory at the RIKEN Advanced Science Institute now promise to make the construction of these substances more efficient thanks to unique rare-earth catalysts that direct synthetic reagents to specific sites on anisole rings.

One of the simplest ways to modify anisole molecules is through a process known as 'C–H alkylation', where a approaches the methoxy–ether group of the anisole and activates a carbon–hydrogen bond on the . After the metal bonds to the activated position, unsaturated reagents, such as double-bonded alkenes, can insert themselves into this bond, producing an anisole with a new alkyl substituent. Although promising, this procedure has had mixed success thus far due to the between metals and the anisole, which results in the generation of a mixture of possible products.

Hou and his team took a different approach to this problem by using 'half-sandwich' rare-earth complexes to catalyze the C–H alkylation. They recently discovered that these distinctly shaped compounds, which contain elements like scandium and yttrium, could polymerize olefins with . A key step in this polymerization involved selective alkene insertion into metal–—a process the researchers anticipated could be reproduced with anisole reagents.

The team first attempted the C–H alkylation with a neutral rare-earth complex to encourage the initial activation step. However, this catalyst could not promote the critical alkene insertion step. By switching to a cationic half-sandwich complex—a positively charged material with high affinity for alkenes—the researchers were successful in attaching a new alkyl group to the anisole ring.

The rare-earth complex also proved exceptionally adept at controlling product selectivity. Experiments with several alkene reagents and anisole derivatives revealed that alkylation occurred at a single specific location on the aromatic ring, and the rare-earth catalyst continued to promote alkylation at this site even if already occupied by a methyl substituent.

Hou notes that this site selectivity for anisole alkylation arises from the strong affinity of rare-earth metals towards oxygen. This interaction directs the initial coordination of the catalyst to the anisole, as well as subsequent alkene insertion, to proceed in a specific orientation. The team continues to investigate further applications of this novel process.

Explore further: Economizing chemistry, atom by atom

More information: Oyamada, J. et al. Regioselective C–H alkylation of anisoles with olefins catalyzed by cationic half-sandwich rare earth alkyl complexes. Angewandte Chemie International Edition 51, 12828–12832 (2012). dx.doi.org/10.1002/anie.201206233

Nishiura, M. et al. Novel polymerization catalysts and hydride clusters from rare-earth metal dialkyls. Nature Chemistry 2, 257–268 (2011). dx.doi.org/10.1038/nchem.595

Related Stories

Economizing chemistry, atom by atom

February 3, 2012

In chemistry, downsizing can have positive attributes. Reducing the number of steps and reagents in synthetic reactions, for example, enables chemists to boost their productivity while reducing their environmental footprint. ...

Rare-earth half-sandwiches prove rewarding

May 18, 2012

The chemical frameworks of ’natural products’—molecules generated by biological organisms—have inspired many of today’s most potent pharmaceuticals. But the complexity of these compounds makes time-consuming ...

The new kid on the block

June 3, 2011

In synthetic chemistry, ‘carbene’ species—compounds bearing a carbon atom with two unpaired electrons—have a ferocious reputation. Left uncontrolled, they will react with almost any molecule ...

Putting the squeeze on rare earth metals

April 15, 2011

(PhysOrg.com) -- Rare-earth metals are a series of elements that represent one of the final frontiers of chemical exploration. The vigorous reactivity of these substances, however, has made it difficult for researchers to ...

Recommended for you


Adjust slider to filter visible comments by rank

Display comments: newest first

1 / 5 (1) Mar 08, 2013
Bet they were using the alkenes as ligands and discovered it on accident.
1 / 5 (1) Mar 08, 2013
Hardly novel or new. This yet another variation on the fridels-crafts type reaction with a lone pair coordinating with a metal to direct the chemistry. This has been done for years.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.