AK-2054GP_published_mainmanuscript

AK-2054GP_published_mainmanuscript - Issue in Honor of Prof...

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Issue in Honor of Prof. Alain Krief ARKIVOC 2007 (x) 21-28 Stereoselective addition reactions with chalcogen electrophiles Franz W. Bürgler, a Gianfranco Fragale, a and Thomas Wirth a,b * a Universität Basel, Department Chemie, St. Johanns-Ring 19, 4056 Basel, Switzerland b Cardiff University, School of Chemistry, Park Place, Cardiff CF10 3AT, United Kingdom E-mail: [email protected] Dedicated to Professor Alain Krief on the occasion of his 65 th birthday Abstract The addition of chalcogen electrophiles to alkenes is investigated using sulfur and tellurium electrophiles. These reagents have been prepared in analogy to the known and well-investigated selenium counterparts. Keywords: Addition reactions, alkenes, chalcogens, electrophiles, sulfur reagents Introduction Electrophilic addition reactions to alkenes are known for a long time as key transformations in organic chemistry. Halogen- as well as chalcogen-based methods are widely explored and are now very useful tools in the hands of synthetic chemists. Before organoselenium reagents became important reagents it was discovered that electrophilic selenium compounds can add stereospecifically to alkenes. 1 Since that time this reaction has been an important tool in the portfolio of organic chemists and is now routinely used even for the synthesis of complex target compounds. Comprehensive reviews on this chemistry have appeared 2,3,4,5,6 and recently the synthesis of chiral selenium electrophiles and their application in asymmetric synthesis has emerged. Herein we describe the synthesis and use of enantiomerically pure sulfur- and tellurium-electrophiles and compare these reagents to the known chiral electrophilic selenium reagents. Results and Discussion The addition of chalcogen electrophiles to alkenes are usually stereospecific anti -additions. The additions proceed via the formation of a chalcogeniranium ion of type 1 , which are opened in the ISSN 1424-6376 Page 21 © ARKAT-USA, Inc.
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Issue in Honor of Prof. Alain Krief ARKIVOC 2007 (x) 21-28 presence of external nucleophiles to addition products 2 . Internal nucleophiles can lead to cyclic products via an endo or via an exo pathway. E R–E Nu Nu – X 12 X R–E–X E = S, Se, Te R We have investigated various enantiomerically pure selenium electrophiles of the general structure 5 and employed these in stereoselective addition reactions of alkenes. Common precursor molecules are the corresponding diselenides 4 which are easily transferred into the aryl selenenyl halides or into other selenium electrophiles with exchanged counterions. Under certain reaction conditions selectivities of up to 98:2 can be obtained. The right choice of the heteroatom Y and of the counterion X is important for obtaining high selectivities and some systematic studies of their influence have been published.
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This note was uploaded on 02/17/2011 for the course CHEMISTRY 101 taught by Professor Csr during the Spring '11 term at University of Louisville.

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AK-2054GP_published_mainmanuscript - Issue in Honor of Prof...

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