323 research outputs found
Decarboxylative AlkylâAlkyl CrossâCoupling Reactions
Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/137566/1/anie201605593_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/137566/2/anie201605593.pd
Hydrophobically directed aldol reactions: polystyrene-supported L-proline as a recyclable catalyst for direct asymmetric aldol reactions in the presence of water
A simple synthetic methodology for the preparation of a polystyrene-
supported L-proline material is reported, and this
material has been used as catalyst in direct asymmetric aldol
reactions between several ketones and arylaldehydes to furnish
aldol products in high yields and stereoselectivities.
Screening of solvents showed that these reactions take place
only in the presence of water or methanol, at lower levels of
conversion in the latter case. This solvent effect, coupled
with the observed high stereoselectivities, has been ex-
Introduction
In the last decade organocatalysis has became a field of
great interest.[1] Organocatalysts are metal-free small organic
molecules that are able to function as efficient and
selective catalysts for a large variety of enantioselective
transformations. In this context, -proline and its derivatives
have emerged as powerful organocatalysts.[2] -Proline
can be regarded as the simplest âenzymeâ and it has been
successfully applied in many reactions, such as Robinson
annulations,[3] aldol reactions,[4] Mannich reactions,[5]
Michael reactions,[6] direct electrophilic α-aminations,[7] Dielsâ
Alder reactions,[8] BaylisâHillman reactions,[9] aza-Morita-
BaylisâHillman reactions,[10] α-selenenylation,[11] oxidation,[
12] chlorination,[13] and others.[14]
Among all these processes, -proline-mediated aldol reactions
affording ÎČ-hydroxy ketones have been investigated
in great depth. Indeed, the aldol reaction is one of the most
important CâC bond-formation methods in organic synthesis.[
15] Proline and its derivatives operate by bifunctional catalysis
and play the role of a simplified version of the type I
[a] Dipartimento di Chimica Organica âE.PaternĂČâ, UniversitĂ di
Palermo,
Viale delle Scienze, Pad. 17, 90128 Palermo, Italy
Fax: +39-091-596825
E-mail: [email protected]
© 2007 Wiley-VCH Verlag GmbH 4688 & Co. KGaA, Weinheim Eur. J. Org. Chem. 2007, 4688â4698
plained in terms of the formation of a hydrophobic core in
the inner surface of the resin, whereas the hydrophilic proline
moiety lies at the resin/water interface. Such a microenvironment
both promotes the aldol reaction and increases the
stereoselectivity. Recycling investigations have shown that
this material can be reused, without loss in levels of conversion
and stereoselectivity, for at least five cycles
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