The
excited radical anion of rhodamine 6G reduces heteroaryl bromides
and chlorides to generate heteroaryl radicals that were successively
trapped by pyrroles for the synthesis of heteroaromatic biaryls in
moderate to excellent yields. The synthetically important photoredox
catalytic C–H heteroarylation reaction works for a broad range
of brominated electron-rich heteroarenes and chlorinated heteroarenes
bearing electron withdrawing groups. In addition, this methodology
was applied to the formal synthesis of a benzimidazole derivative
II with interesting pharmacological properties.
An organocatalytic
strategy for the synthesis of tetrasubstituted
pyrrolidines with monoactivated azomethine ylides in high enantiomeric
excess and excellent exo/endo selectivity is presented. The key to
success is the intramolecular activation via hydrogen bonding through
an o-hydroxy group, which allows the dipolar cycloaddition
to take place in the presence of azomethine ylides bearing only one
activating group. The intramolecular hydrogen bond in the azomethine
ylide and the intermolecular hydrogen bond with the catalyst have
been demonstrated by DFT calculations and mechanistic proofs to be
crucial for the reaction to proceed.
Dihydroarylfuran skeletons are efficiently synthesized from (Z)-bromonitroalkenes and naphthol derivatives in good yields and excellent enantioselectivities by using squaramide catalysis.
1,3-Dipolar cycloadditions of C,N-cyclic azomethine imines with α,β-unsaturated aldehydes can be performed with complete control of the regio-, exo-, and enantioselectivity under aminocatalytic conditions. The so far never studied competence of the iminium-dienamine reactivity inherent to β-alkyl α,β-unsaturated aldehydes was studied, which was possible by allowing achievement of complete control of the chemoselectivity in reactions of the β-arylmethyl derivatives with azomethine imines by using different additives and organocatalysts, whose role has been rationalized by DFT calculations and chemical proofs. Thus, it has been possible to selectively obtain the pyrazolidines resulting from both the attack to the C2-C3 (via iminium) and the C3-C4 (via dienamine) bonds at the starting enals, which can be used as precursors of interesting tetrahydroisoquinolinic compounds.
The first asymmetric cycloaddition of nonstabilized azomethine ylide and N-sulfinylimines is presented. In reactions with aryl-alkyl and heteroaryl-alkyl ketimines, excellent diastereoselectivities and good yields are obtained in all cases, regardless of the electronic character of the substituents at the aromatic rings. Moreover, the cycloadducts obtained can easily be deprotected in acid media, giving access to free 1,2-diamines which are prevalent in many natural and pharmaceutical products.
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