Bridging Amines with CO2: Organocatalyzed Reduction of CO2 to Aminals

Xavier Frogneux 1 Enguerrand Blondiaux 1 Pierre Thuéry 1 Thibault Cantat 1
1 LCMCE - Laboratoire de Chimie Moléculaire et de Catalyse pour l'Energie (ex LCCEF)
NIMBE UMR 3685 - Nanosciences et Innovation pour les Matériaux, la Biomédecine et l'Energie (ex SIS2M)
Abstract : The 4-electron reduction of CO2 in the presence of secondary aromatic amines is described for the first time to access aminals. Under metal-free hydrosilylation conditions, the four CO bonds of CO2 are cleaved and the organocatalysts are able to balance the reactivity of CO2 to promote the selective formation of two C-N and two C-H bonds. The methodology enables the formation of various symmetrical and unsymmetrical aminals. Because CO2 is a renewable, cost-efficient and non-toxic resource, it is a desirable carbon feedstock for the production of value-added chemicals and a lot of groups have focused their attention on designing new transformations involving CO2 over the last few years. 1 In particular, the reductive func-tionalization of CO2 with nitrogen reagents has known tremendous developments using various types of reductants such as hydrosilanes, hydroboranes and dihydrogen. 2 These reactions have enabled the conversion of CO2 into formamides, 2b, 3 formamidines 4 and methylamines. 5 Notably, the carbon oxidation state in these products is either +2 or-2 and the formation of C 0 organic functional groups from CO2 remains a challenge. This trend reflects the higher electrophilicity of C 0 groups compared to C +II functions in carbonyl derivatives. It is indeed well established that upon hydrogenation of CO2, formate derivatives can be accumulated while formaldehyde is an elusive species because its reduction to methanol is more rapid than the hydrogenation of formic acid. 1a As a consequence of this limitation, only a few reports have tackled the formation of C 0 species from CO2. Under hydrosilylation conditions, the selective reduction of CO2 into a bis(silyl)acetal species with triethylsilane has been revealed. 6 Using a hydroborane reduct-ant, Bontemps, Sabo-Etienne et al. successfully trapped transient formaldehyde, obtained from CO2, with 2,6-diisopropylaniline, yielding the corresponding imine. 7 To unlock new 4-electron reduction transformations of CO2, one should focus on the use of well-balanced catalysts, able to finely control the kinetics of CO2 reduction. In nature, aceto-genic bacteria are able to produce over 10 9 tons of acetic acid annually, following the Wood-Ljungdahl pathway (Scheme 1). 8 In this biochemical cycle, CO2 is anchored to a diamine moiety and undergoes successive 2-electron reduction steps to yield a methylamine (C-II) after formation of the corresponding formamide (C +II), formamidine (C +II) and aminal (C 0) intermediates. While CO2 conversion to formamides, formaminides and methylamines has been described, the synthesis of aminals 9 directly from CO2 remains unknown and was only suggested as a possible intermediate in the Ru-based methylation of amines. 5b To open up the variety of products accessible from CO2, we describe herein the first catalytic synthesis of aminals by intermolecular coupling of two amines using CO2 as a C1-bridge. Scheme 1. Simplified mechanism of the Wood-Ljungdahl pathway for acetogenesis with CO2 Hydrosilanes are mild reductants, cheap, non-toxic with a redox potential well poised for CO2 reduction. Additionally, their slightly polar Si-H bond can be activated with metal-free catalysts, using either Lewis bases or Lewis acids. 10 Using phenylsilane as reductant, the reactivity between N-methylaniline (1a) and CO2 has been explored using a variety
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Xavier Frogneux, Enguerrand Blondiaux, Pierre Thuéry, Thibault Cantat. Bridging Amines with CO2: Organocatalyzed Reduction of CO2 to Aminals. ACS Catalysis, American Chemical Society, 2015, 5, pp.3983-3987. ⟨10.1021/acscatal.5b00734⟩. ⟨hal-01228582⟩



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