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Catalysis

Coaxing chiral products from an SN 1 reaction

Quaternary stereocenters generated via combination of a chiral hydrogen-bond-donor catalyst and a Lewis acid

by Bethany Halford
April 30, 2018 | APPEARED IN VOLUME 96, ISSUE 18

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One of the first reactions organic chemistry students learn is the unimolecular nucleophilic substitution, or SN1. Yet SN1 reactions are unpopular among chemists, largely because chiral molecules made in this manner wind up as a racemic mixture of both enantiomers. But now, Harvard University chemists Eric N. Jacobsen, Alison E. Wendlandt, and Prithvi Vangal report a method to produce quaternary stereocenters via the SN1 reaction (Nature 2018, DOI: 10.1038/s41586-018-0042-1). The chemists use a chiral squaramide catalyst, which acts as a dual hydrogen-bond donor, in combination with the strong Lewis acid trimethylsilyl trifluoromethanesulfonate. This catalyst system promotes formation of a planar carbocation, as happens normally in SN1 reactions. However, it sidesteps elimination reactions that plague typical SN1 chemistry, and guides the nucleophile’s approach to just one face of the carbocat­ion, so that the chemists could transform racemic tertiary propargyl acetates into compounds with quaternary stereocenters (example shown). Although similar strategies have been deployed to make chiral products from cationic intermediates, such examples have been limited to heteroatom-stabilized carbocations, which are less likely to rearrange or generate elimination products.

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Comments
HAMISU MUSA JIMETA (Sun May 06 01:03:12 EDT 2018)
This is for the great minds. It is really inspiring. I commend your effort, keep it up ! Bravo !

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