Fe-BPsalan Complex-Catalyzed Asymmetric [4 + 2] Cycloaddition of Cyclopentadiene with α,β-Unsaturated Heterocycles

Abstract

An efficient iron-catalyzed asymmetric [4 + 2] cycloaddition of cyclopentadiene with α,β-unsaturated acyl imidazoles or 2-cinnamoylisoindoline-1,3-dione derivatives was developed to afford the addition products in high yield and selectivity. Interestingly, the absolute structures of the addition products were controlled by the auxiliaries via different coordination modes with the same type of catalyst.

Fe-BPsalan complex-catalyzed asymmetric 1,3-dipolar [3 + 2] cycloaddition reaction of N,N′-cyclic azomethine imines with α,β-unsaturated acyl imidazoles

“…Although the reaction catalyzed by 1a alone didn't give any cycloaddition product (Table 1, entry 1), the addition of AgClO 4 improved the reaction and the corresponding addition product 4a was obtained in 83% yield with >20 : 1 dr and 91% ee (Table 1, entry 2). Such a dramatic improvement in both the activity and selectivity of the catalyst is consistent with our previous findings in asymmetric Fe-BPsalan catalyzed reactions, 13–15 which could be attributed to the stronger Lewis acidity of the cationic iron complexes generated upon the addition of AgClO 4 . Subsequently, the effect of silver salts was further investigated (Table 1, entries 3–7); the ones with a non-coordinative anion significantly improved the reaction with AgSbF 6 giving the best result affording 4a in 86% yield with >20 : 1 dr and 92% ee (Table 1, entry 3).…”

“…The iron( iii )-BPsalan complexes 1a–1h were prepared by following the literature method. 13–15 The Fe-catalyzed asymmetric [3 + 2] cycloaddition was initially investigated by the reaction of α,β-unsaturated acyl imidazole 2a and N , N ′-cyclic azomethine imine 3a in the presence of complex 1a (5 mol%) with dichloromethane (DCM) as solvent at room temperature (Table 1). Although the reaction catalyzed by 1a alone didn't give any cycloaddition product (Table 1, entry 1), the addition of AgClO 4 improved the reaction and the corresponding addition product 4a was obtained in 83% yield with >20 : 1 dr and 91% ee (Table 1, entry 2).…”

“…To this end, iron catalysis is appealing as iron is Earth-abundant and biocompatible, 11 and there has been growing interest in the development of iron catalysis in recent years. 12 In our endeavour to develop practical iron catalysis for organic transformation reactions, we have reported a series of Lewis acidic Fe-BPsalan complexes that can efficiently catalyze asymmetric fluorination, 13 hydroxylation, 13 chlorination 14 and Diels–Alder cycloaddition reactions 15 to afford the corresponding products in high yields and stereoselectivities. To further broaden the application scope of Fe-BPsalan complexes in organic synthesis, herein we have described their application in catalyzing the asymmetric [3 + 2] cycloaddition reaction of N , N ′-cyclic azomethines with α,β-unsaturated 2-acyl imidazoles at room temperature, through which the corresponding N , N ′-bicyclic pyrazolidine derivatives with three contiguous tertiary stereocenters were obtained in moderate to good yields (up to 98%) and excellent stereoselectivities (up to 98% ee and >25 : 1 dr, Scheme 1b).…”