On the Development of Catalytic Carba-6π Electrocyclizations

Abstract

Hexatriene substrates substituted in the 2-position with carbonyl groups were studied in the context of catalytic 6π electrocyclizations. The nature of the carbonyl group and the substitution pattern on the hexatriene have significant effects on the ability of these substrates to succumb to catalysis. A novel 2-formyl hexatriene dimerization was observed. The first example of a catalytic asymmetric carba-6π electrocyclization is reported along with the discovery of an unusual kinetic resolution via a catalytic photochemical electrocyclic ring-opening.Key words: electrocyclic reactions, catalysis, dimerization, asymmetric catalysis, kinetic resolution.The ability to catalyze pericyclic reactions has greatly increased their synthetic utility and has allowed for the development of asymmetric versions. Among the different types of these reactions, Diels-Alder cycloadditions, Claisen rearrangements, and Nazarov cyclizations have been rendered asymmetric by using a variety of chiral Lewis acids, Brønsted acids and aminocatalysts. 1,2 Our group has contributed to this topic by introducing 2-alkoxy-1,4-pentadien-3-ones as excellent substrates for Nazarov cyclizations, which have yielded the first highly enantioselective examples of these 4π electrocyclizations. 2 More recently, 6π electrocyclizations have come into focus. In 2009, List and Smith independently reported asymmetric catalysis in aza-6π electrocyclizations. 3 Prior to this, we demonstrated that carba-6π electrocyclizations could be catalyzed with Lewis acids (Scheme 1). 4,5 Using esters (e.g., 1) and ketones (e.g., 2) as Lewis bases and Me 2 AlCl as a Lewis acid, we showed that rate accelerations up to 55-fold could be achieved. Our experimental results were accompanied by theoretical calculations on simple aldehydes, such as 3, which indeed showed that placement of a formyl group in the 2-position should result in significant rate accelerations upon binding of a Lewis acid or a proton.It is generally believed that aldehydes are preferred substrates for asymmetric Lewis acid catalyzed reactions, since the geometry of their complexation is well-defined. In addition, unsaturated aldehydes, such as 3, allow for aminocatalysis, which is more problematic with ketones due to their decreased electrophilicity and increased steric hindrance. We have therefore investigated 2-formyl hexatrienes in the context of catalytic 6π electrocyclizations. Herein, we report on our experiences with these substrates and provide an update on our continued studies of catalytic and asymmetric catalytic carba-6π electrocyclizations. In the course of these investigations, we have established that certain substituents on the hexatriene system are required to promote electrocyclizations and prevent side reactions, such as double bond isomerizations or Prins-type cyclizations. Powerful new Lewis acid catalysts have been identified that have led to rate accelerations an order of magnitude higher than previously described. In addition, we have launched preliminary investigations ...

Application of aluminum triiodide in organic synthesis

“…183 The electrocyclization proceeded at 9 o C with an acceleration rate of 600 (t 1/2 =7 minutes) compared to thermocyclization that occurs at 52 o C.…”