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Magnesium-Catalyzed Mild Reduction of Tertiary and Secondary Amides to Amines
ACS Catalysis
  • Nicole L. Lampland, Iowa State University
  • Megan L. Hovey, Iowa State University
  • Debabrata Mukherjee, Iowa State University
  • Aaron D. Sadow, Iowa State University
Document Type
Article
Disciplines
Publication Version
Published Version
Publication Date
5-27-2015
DOI
10.1021/acscatal.5b01038
Abstract

The first example of a catalytic hydroboration of amides for their deoxygenation to amines is reported. This transformation employs an earth-abundant magnesium-based catalyst. Tertiary and secondary amides are reduced to amines at room temperature in the presence of pinacolborane (HBpin) and catalytic amounts of ToMMgMe (ToM = tris(4,4-dimethyl-2-oxazolinyl)phenylborate). Catalyst initiation and speciation is complex in this system, as revealed by the effects of concentration and order of addition of the substrate and HBpin in the catalytic experiments. ToMMgH2Bpin, formed from ToMMgMe and HBpin, is ruled out as a possible catalytically relevant species by its reaction with N,N-dimethylbenzamide, which gives Me2NBpin and PhBpin through C–N and C–C bond cleavage pathways, respectively. In that reaction, the catalytic product benzyldimethylamine is formed in only low yield. Alternatively, the reaction of ToMMgMe and N,N-dimethylbenzamide slowly gives decomposition of ToMMgMe over 24 h, and this interaction is also ruled out as a catalytically relevant step. Together, these data suggest that catalytic activation of ToMMgMe requires both HBpin and amide, and ToMMgH2Bpin is not a catalytic intermediate. With information on catalyst activation in hand, tertiary amides are selectively reduced to amines in good yield when catalytic amounts of ToMMgMe are added to a mixture of amide and excess HBpin. In addition, secondary amides are reduced in the presence of 10 mol % ToMMgMe and 4 equiv of HBpin. Functional groups such as cyano, nitro, and azo remain intact under the mild reaction conditions. In addition, kinetic experiments and competition experiments indicate that B–H addition to amide C═O is fast, even faster than addition to ester C═O, and requires participation of the catalyst, whereas the turnover-limiting step of the catalyst is deoxygenation.

Comments

Reprinted (adapted) with permission from ACS Catalysis 5 (2015): 4219, doi:10.1021/acscatal.5b01038. Copyright 2015 American Chemical Society.

Copyright Owner
American Chemical society
Language
en
File Format
application/pdf
Citation Information
Nicole L. Lampland, Megan L. Hovey, Debabrata Mukherjee and Aaron D. Sadow. "Magnesium-Catalyzed Mild Reduction of Tertiary and Secondary Amides to Amines" ACS Catalysis Vol. 5 Iss. 7 (2015) p. 4219 - 4226
Available at: http://works.bepress.com/aaron_sadow/26/