Highly Efficient Rhodium-Catalyzed Asymmetric Ring-Opening Reactions of Oxabenzonorbornadiene with Amine Nucleophiles
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Highly Efficient Rhodium-Catalyzed Asymmetric Ring-Opening Reactions of Oxabenzonorbornadiene with Amine Nucleophiles Yuhua Long • Shuangqi Zhao • Heping Zeng Dingqiao Yang
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Received: 23 April 2010 / Accepted: 25 May 2010 / Published online: 10 June 2010 Ó Springer Science+Business Media, LLC 2010
Abstract Rhodium-catalyzed asymmetric ring-opening reactions of oxabenzonorbornadienes with aliphatic primary amines and substituted N-phenylpiperazines were investigated. These reactions are featured with the formation of a new carbon-nitrogen bond via an intermolecular allylic displacement of the bridgehead oxygen with amine nucleophiles, which offered the corresponding products in good to excellent yields (up to 97%) with excellent enantioselectivities (up to 99%ee). Keywords Rhodium catalysis Asymmetric ring-opening Oxabenzonorbornadienes Chiral bisphosphine ligand
1 Introduction To explore efficient methods for creating carbon-carbon bond and carbon-heteroatom bonds via asymmetric catalysis is among the hot topics in field of organic synthesis, and has been attracted increasing interests in recent years due to their great potential to offer practical methods for the synthesis of complicated molecules with biological interests. Asymmetric ring-opening of oxabenzonorbornadiene mediated by rhodium-catalyst are particularly remarkable, which offers products with more than one chiral center in a single step. In general, this type of reaction is highly efficient and characterized with low catalyst loading, high yield and enantioselectivities [1–7]. Lautens’s group has extensively investigated the rhodiumY. Long S. Zhao H. Zeng D. Yang (&) School of Chemistry and Environment, South China Normal University, Guangzhou 510006, People’s Republic of China e-mail: [email protected]
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catalyzed asymmetric ring-opening reactions of oxabenzonorbornadiene with many nucleophiles including alcohols [8, 9], phenols [10], amines [11], carboxylates [12], 1,3-dicarbonyl nucleophiles [13], and sulfur nucleophiles [14] which demonstrated the great promising on providing novel and simple routes to optically active compounds [15–18]. We herein report a novel extension to Lautens’s method, in which the ring of oxabenzonorbornadiene 1a was opened by aliphatic primary amines or substituted N-phenylpiperiazines secondary amines as nucleophiles in the presence of rhodium complex catalyst to afford the corresponding trans-1,2-dihydronaphth-1-ols derivatives with excellent enantioselectivities. In this paper, we used six different ligands to form complexes with rhodium for ring-opening reaction to explore the rhodium-catalyzed asymmetric ring-opening reaction of oxabenzonorbornadiene with amine nucleophiles.
2 Results and Discussion By using appropriate rhodium catalysts, aliphatic primary amines, despite their low activities in this type of reactions, can efficiently attach the ring to give the corresponding trans-1,2-dihydronaphth-1-ols 2a–2e in good yields with reasonable enantioselectivities (Table 1). Aliphatic primary amine
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