Heptanal, self-condensation

Jasminaldehyde can be obtained classically from heptanal and benzaldehyde in 70% yield within 3 days at room temperature (Eq. 41). By use of a 600-W domestic micro-wave oven, however, an enhanced yield of 82% was achieved in only 1 min. The amount of side-products (self condensation of w-heptanal) decreased from 30 to 18% when this technique was used [62],... 

Several cross-aldol condensations have been performed with alkaline earth metal oxides, including MgO, as a base catalyst. A general limitation of the cross-aldol condensation reactions is the formation of byproducts via the self-condensation of the carbonyl compounds, resulting in low selectivities for the cross-aldol condensation product. For example, the cross-condensation of heptanal with benzalde-hyde, which leads to jasminaldehyde (a-K-amylcinnamaldehyde), with a violet scent... 

Pd-containing aluminophosphate molecular sieves have been used to carry out crossed aldol condensations between an aldehyde and a ketone by using a 0.5 % Pd/ MnAPSO-31 catalyst in a vapour-phase fixed bed reactor.[14] Thanks to the excess of the ketone with respect to the aldehyde (4 1), it is possible to get high selectivity to the desired product, i.e. 70 % of heptan-2-one from n-butyraldehyde and acetone and 89 % of pentan-2-one from acetaldehyde and acetone, the major by-product being, in both cases, MIBK from acetone self-condensation. 

This is a typical example of aldol condensation leading to a very important product in perfume chemistry with some impurities due to self-condensation of n-heptanal (Table 15). The best result is achieved using the last system described, i. e. with MW within one minute and giving only 18% of self-condensation. 

Liquid phase aldol condensation reaction between heptanal and benzaldehyde is studied over two series of oxynitride catalysts aluminium phosphate oxynitrides AlPON and mixed aluminium gallium phosphate oxynitrides AlGaPON , with increasing nitrogen contents (0-14 wt.% for AlPON and 0 - 16 wt. % for AlGaPON ). The main products are jasminaldehyde and 2-pentyl-2-nonenal. Jasminaldehyde is formed via the cross-aldol condensation reaction between heptanal and benzaldehyde and 2-pentyl-2-nonenal is formed via the self-condensation reaction of heptanal. 

Liquid samples were regularly withdrawn with a filtering syringe and analyzed by a gas chromatograph (Perkin Ehner) equipped with an FID detector and using a capillary column (CPSilSCB, 25 m). All the isomers (Z, E) of crosscondensation and self condensation reactions have been separated and identified by H-NMR and GC-MS. Quantitative determinations were based on the measured response factors of the reactants and reaction products. We checked that reactions were carried without transfer limitations. The selectivity for a particular product was defined as the mmol of this product divided by the total mmol of heptanal converted. 

We showed, over two different series of Al(Ga)PON oxynitrides catalysts, that the nitridation of phosphate precursors has a positive effect on the selectivity to jasminaldehyde for the mixed aldol condensation reaction of heptanal with benzaldehyde. The influence of nitridation on the product distribution was interpreted in terms of changes in the relative density of acid and basic sites on the catalyst surface. Decreasing the acidity and increasing the basicity through nitridation enhances the simultaneous activation of benzaldeyde and heptanal and favors the cross condensation reaction between those two aldehydes, rather than the self-condensation of heptanal. 

Scheme 1.10 Cross-aldol reaction of benzaldehyde and heptanal (top) and self-condensation of...

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