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Microwave promoted synthesis of dibenzofulvenes |
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8th International Electronic Conference on Synthetic Organic Chemistry. ECSOC-8. 1-30 November 2004. http://www.lugo.usc.es/~qoseijas/ECSOC-8/
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Microwave promoted synthesis of dibenzofulvenes |
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Julio A. Seijas,* M. Pilar Vázquez-Tato,* Francisco J. Amigo-Fuertes, M. Raquel Carballido-Reboredo
Dibenzofulvenes are a kind of interesting compounds since some of them have medicinal aplications as antimalaric[1] or antiinflamatory[2] and others are used in electrophotography as photoconductors.
Although several approaches to dibenzofulvenes have been developed like those based on: Pd catalyzed[3], Grignard reagents[4], Peterson[5] and Wittig[6] olefination. Most of methologies employed rely on the formation of an anion on the methylene group of fluorene[7], however none of them have used microwaves to enhance the reactivity of fluorene anion towards benzaldehydes.
The reaction of fluorene with benzaldehyde was firstly tried with NaOH in ethanol and heated at reflux under microwave irradiation (800W), giving 30% yields (Table 1). Similar yield was obtained using KOH as base. When BMIMCl (butylmethyl imidazolium chloride) was used instead of solvent the yield raised to 49%, but no significant improvement in the yield was obtained with longer reaction times. Changing the ionic liquid to diglyme, a polar solvent with a high boiling point and weaker interaction with microwaves, in the presence of a small amount of water to make possible the solution of KOH, gave the coupled product 1 in 98% yield after 2.5 minutes of irradiation.
Scheme 1
| Base | Solvent | Irradiation | Yield |
| NaOH | EtOH | 5 | 30 |
| NaOH | EtOH | 15 | 49 |
| KOH | EtOH | 5 | 30 |
| NaOH | BMIMCl | 5 | 48 |
| KOH | Diglyme/water | 2.5 | 98 |
Table1
Several substituted benzaldehydes (anisaldehyde, veratraldehyde and 3,5-dimethoxybenzaldehyde) were used to obtain several 9-arylidenefluorenes (85-98%) yields in very short irradiation times (scheme 2)
Scheme 2
In summary, a new microwave promoted organic synthesis is described to obtain substituted dibenzofulvenes, in very short reaction times and very good reaction yields, becoming a competitive method with the ones previously described for this type of dibenzofulvenes.
Acknowledgements
XUNTA DE GALICIA for financial support: PGIDT01PXI26203PR, PR405 A
098/59-0 and a predoctoral fellowship to M.R.C-R.
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