Breaking benzene

Industrially to break benzene aromaticity in order to add an alkyl group using the Friedel Crafts reaction requires fairly hairy conditions — e.g. pressure to keep everything liquid and temperatures of 130 – 160 Centigrade.

A remarkable paper [ Nature vol. 512 pp. 413 – 415 ’14 ] uses a Titanium hydride catalyst and mild conditions (22 C — room temperature) for little over a day to form a titanium methylcyclopentenyl complex from benzene (which could be isolated) and studied spectroscopically.

The catalyst itself is rather beautiful. 3 titaniums, 6 hydrides and 3 C5Me4SiMe3 groups.

Benzene is the aromaticity workhorse of introductory organic chemistry. If you hydrogenate cyclohexene 120 kiloJoules is given off. Hydrogenating benzene should give off 360 kiloJoules, but because of aromatic stabilization only 208 is given off — implying that aromaticity lowers the energy of benzene by 152 kiloJoules. Clayden uses kiloJoules. I’m used to kiloCalories. To get them divide kiloJoules by 4.19.

What other magic does transition metal catalysis have in store?

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  • Rhenium  On September 4, 2014 at 5:45 pm

    Typo in the page numbering I think.

  • luysii  On September 4, 2014 at 6:15 pm

    Yes, thanks — corrected

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