CHIRALITY 26:327 (2014)

Editors’ Note ASY MME T RIC A LK ENE H A L OGENATION The halogenation of alkenes is often one of the first reactions taught in organic chemistry courses, both because of its long history and ability to readily transmit core concepts of reactivity. Yet among reactions involving the addition of both an electrophile and a nucleophile to a π-system, it remains one of the most underdeveloped processes in terms of effective enantioselective solutions. Recent studies by a number of teams are beginning to change this paradigm. It was in that general spirit that a special issue of Chirality dedicated to the topic of chiral halogenation of alkenes, in all of its varied forms, was deemed timely. The goal was not only to give a sense of the current state of the art, but also to help inspire the next generation of solutions for this deeply challenging area of research, one with significance for pharmaceutical science, materials research, and complex natural product total synthesis (among others). The ultimate result, as can be gleaned from the ensuing pages, is an excellent synopsis due to the gracious acceptance of five leaders of the field to contribute their thoughts, perspectives, and recent findings in this growing and vibrant area of inquiry. Our first contribution is both a review as well as a personal account of various alkene bromofunctionalization reactions from Y.-Y. Yeung. It focuses on recent developments at large, as well as provides an interesting perspective on his own group’s efforts to affect enantioselective bromocyclization of alkenes and to develop multicomponent reactions initiated by N-bromosuccinimide. The latter processes, in particular, can access a diverse array of valuable molecular frameworks in short order. In addition to novel strategies, it is also clear that unique tools are required for advances to be made beyond current abilities. In this regard, we have an exciting contribution from S. Denmark on the development of a novel binary catalyst system consisting of an achiral Lewis base and a chiral Brønsted acid to achieve controlled bromoetherifications. Preliminary kinetic studies shed much light on future directions that will hopefully be reported in due course. In addition, we have an interesting article from W. Tang disclosing that the use of

unique N-acylhemiaminal nucleophiles can affect enantioselective halocyclization of several alkenes, affording products with up to three new stereogenic centers. Finally, we close with two articles devoted to another longstanding problem in halogenation chemistry: haloniuminduced polycyclizations. In the first, C. Braddock and his group report on the ability to utilize enantiomerically pure bromohydrins as starting materials to effect enantiospecific polyene cyclizations. Critical is the use of dimethylaluminum triflate to activate tetrafluorobenzoate derivatives of these starting materials, thereby preventing the racemization pathways typical of past approaches. In the second, K. Ishihara and members of his team shows that achiral 2-geranylphenols can be cyclized directly with high enantioselectively using certain nucleophilic phospite-urea catalysts. This work is an important follow-up to their ground-breaking 2007 article in Nature describing the first example of such an enantioselective process. Globally, it is hoped that this special issue will be of value to students and practitioners alike. As early evidence for the impact of these works, many are among the most read and cited articles within the journal for the first 4 months of 2014. We sincerely wish to thank each of these investigators and their collaborators for their contributions to this special issue; we could not have hoped for a more successful array of research articles and perspectives, and we hope that you, the reader, will enjoy them as well. Prof. Scott A. Snyder Guest Editor Department of Chemistry The Scripps Research Institute Jupiter, FL Prof. Nina Berova Editor Department of Chemistry Columbia University New York, NY

DOI: 10.1002/chir.22345 Published online in Wiley Online Library (wileyonlinelibrary.com). © 2014 Wiley Periodicals, Inc.

Asymmetric alkene halogenation.

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