Molecules 2012, 17(6), 7121-7150; doi:10.3390/molecules17067121
Article

Cooperative Al(Salen)-Pyridinium Catalysts for the Asymmetric Synthesis of trans-Configured β-Lactones by [2+2]-Cyclocondensation of Acylbromides and Aldehydes: Investigation of Pyridinium Substituent Effects

1 Institut für Organische Chemie, Universität Stuttgart, Pfaffenwaldring 55, 70569 Stuttgart, Germany 2 Institut für Theoretische Chemie, Universität Stuttgart, Pfaffenwaldring 55, 70569 Stuttgart, Germany
* Author to whom correspondence should be addressed.
Received: 26 April 2012; in revised form: 23 May 2012 / Accepted: 4 June 2012 / Published: 12 June 2012
(This article belongs to the Special Issue Asymmetric Catalysis)
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Abstract: The trans-selective catalytic asymmetric formation of β-lactones constitutes an attractive surrogate for anti-aldol additions. Recently, we have reported the first catalyst which is capable of forming trans-β-lactones with high enantioselectivity from aliphatic (and aromatic) aldehyde substrates by cyclocondensation with acyl bromides. In that previous study the concepts of Lewis acid and organic aprotic ion pair catalysis were combined in a salen-type catalyst molecule. Since a pyridinium residue on the salen periphery is essential for high trans- and enantioselectivity, we were interested in the question of whether substituents on the pyridinium rings could be used to further improve the catalyst efficiency, as they might have a significant impact on the effective charges within the heterocycles. In the present study we have thus compared a small library of aluminum salen/bispyridinium catalysts mainly differing in the substituents on the pyridinium residues. As one result of these studies a new catalyst was identified which offers slightly superior stereoselectivity as compared to the previously reported best catalyst. NBO calculations have revealed that the higher stereoselectivity can arguably not be explained by the variation of the effective charge.
Keywords: aluminum; cooperative catalysis; ion pair catalysis; ketene; 2-oxetanone; pyridinium

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MDPI and ACS Style

Meier, P.; Broghammer, F.; Latendorf, K.; Rauhut, G.; Peters, R. Cooperative Al(Salen)-Pyridinium Catalysts for the Asymmetric Synthesis of trans-Configured β-Lactones by [2+2]-Cyclocondensation of Acylbromides and Aldehydes: Investigation of Pyridinium Substituent Effects. Molecules 2012, 17, 7121-7150.

AMA Style

Meier P, Broghammer F, Latendorf K, Rauhut G, Peters R. Cooperative Al(Salen)-Pyridinium Catalysts for the Asymmetric Synthesis of trans-Configured β-Lactones by [2+2]-Cyclocondensation of Acylbromides and Aldehydes: Investigation of Pyridinium Substituent Effects. Molecules. 2012; 17(6):7121-7150.

Chicago/Turabian Style

Meier, Patrick; Broghammer, Florian; Latendorf, Katja; Rauhut, Guntram; Peters, René. 2012. "Cooperative Al(Salen)-Pyridinium Catalysts for the Asymmetric Synthesis of trans-Configured β-Lactones by [2+2]-Cyclocondensation of Acylbromides and Aldehydes: Investigation of Pyridinium Substituent Effects." Molecules 17, no. 6: 7121-7150.

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