Metal catalyzed asymmetric [2+2+1] cycloaddition reactions

Abstract

Pauson-Khand-type reaction is a [2+2+1] carbonylative Cycloaddition which was firstly reported in 1971. This is one of the most powerful synthetic tools to produce cyclopentenones which are known to be versatile building blocks for natural products, pharmaceuticals and fine chemicals. Rhodium complex with chiral atropisomeric dipyidyl-diphosphine ligand, (S)-P-Phos was found to catalyze asymmetric Pauson-Khand-type reaction in aqueous medium to afford corresponding cyclopentenones in good yield and enantiomeric excess (up to 95% ee). Interestingly, a study on electronic effect of the enynes substrates revealed a correlation between the electronic property of the substrates and the ee of the cycloadducts in this reaction. A linear free energy relationship was observed from Hammett study. Besides (S)-P-Phos, (S)-BisbenzodioxanPhos was also found to be highly effective in the coorperative processes of decarbonylation of aldehyde and subsequent enantioselective Pauson-Khand-type reaction in alcoholic medium. Good yield and enantiomeric excess (up to 96% ee) of the products were obtained under these reaction conditions. Apart from rhodium metal precursor, iridium complex with (S)-BINAP was also effective in cascade decarbonylation of aldehyde and asymmetric Pauson-Khand-type reaction. Although their reactivity was inferior to rhodium catalyzed asymmetric Pauson-Khand-type reaction, they afforded the corresponding cyclopentenones with excellent enantiomeric excess (up to 98% ee), which is the highest ee achieved so far for this reaction. In addition to aldehyde as the CO source, we firstly found that formate esters were applicable as CO surrogate in Rh-catalyzed asymmetric Pauson-Khand-type reaction. Up to 94% ee cycloadducts were obtained. Microwave assisted organic syntheses have received increasing attentions in recent decades. In order to shorten the reaction time of our carbonylative cycloaddition, we applied microwave heating to our catalysis. To our delights, the asymmetric Pauson-Khand-type reaction proceeded smoothly within an hour to generate good yield of the products.