Tropylium Salt-Promoted Vinylogous Aza-Michael Addition of Carbamates to para-Quinone Methides: Elaboration to Diastereomerically Pure α,α′-Diarylmethyl Carbamates

Abstract

Carbocation catalysis is emerging as an important subarea of Lewis acid catalysis. Some stable and isolable carbocations have been successfully utilized as Lewis acid catalysts and promoters in many synthetic transformations. In this manuscript, we report a tropylium cation-promoted vinylogous aza-Michael addition of carbamates to para-quinone methides (QMs) to access a wide range of unsymmetrical α,α′-diarylmethyl carbamates. This mild protocol was effective for the vinylogous conjugate addition of (−)-menthyl carbamate to p-QMs, and the respective diastereomerically pure α,α′-diarylmethyl carbamate derivatives could be obtained in excellent yields and diastereoselectivities (up to >20:1 de).

Aza‐Michael Reaction: A Decade Later – Is the Research Over?

“…For example, reactions with carbamates proceed in the presence only 5 mol % of tropylium salt under mild conditions and target adducts 87 are obtained in good to quantitative yields. [196] Finally, the 1,6-addition of para-quinones methides 86 with chiral amino acid esters in the presence of potassium carbonate leading to target adducts 87 in moderate to good yields has been reported for the first time. [198] Surprisingly, no 1,2-or 1,6 adducts are observed when acyclic dienones 88 are treated with benzotriazole.…”

“…Very recently, enantioselective addition of oxazolin‐5‐ones to Michael acceptors 86 has been reported [194] . Moreover, Zn(OTf) 2 , [195] tropylium salt, [196] and Sc(TfO) 3 [197] are Lewis acid catalyst for the chemoselective 1,6‐addition of various N‐nucleophiles to para‐ quinones methides 86 . For example, reactions with carbamates proceed in the presence only 5 mol % of tropylium salt under mild conditions and target adducts 87 are obtained in good to quantitative yields [196] .…”

“…Moreover, Zn(OTf) 2 , [195] tropylium salt, [196] and Sc(TfO) 3 [197] are Lewis acid catalyst for the chemoselective 1,6‐addition of various N‐nucleophiles to para‐ quinones methides 86 . For example, reactions with carbamates proceed in the presence only 5 mol % of tropylium salt under mild conditions and target adducts 87 are obtained in good to quantitative yields [196] . Finally, the 1,6‐addition of para‐ quinones methides 86 with chiral amino acid esters in the presence of potassium carbonate leading to target adducts 87 in moderate to good yields has been reported for the first time [198] …”

Acrylonitrile Adducts: An Efficient Adsorbent Media for Removal of Iron from Water

“…[47][48][49] Aza-Michael addition reaction constitutes a simple and convenient chemical reaction for synthesis of novel compounds through CÀ N bond formation. [50] We have developed a novel adsorbent for adsorption of Fe 2 + ions in aqueous media. It is cost-effective, easy to synthesize, easy availability reflects the novelty of the work.…”

“…Michael additions are facilitated by the addition of an electron withdrawing group to the nitrile [47–49] . Aza‐Michael addition reaction constitutes a simple and convenient chemical reaction for synthesis of novel compounds through C−N bond formation [50] …”