Catalytic Formation of C-N Bond by Direct Condensation Between Non-Activated Amines and Alcohols

C-N bond forming cross-coupling reactions in a “atom economy” regime is among emerging issues in fine chemistry in order to attain highly sustainable processes. [1] As an innovative contribution in this field, the possibility to use amorphous silica as a catalyst able to promote the condensation between non-activated secondary amines and primary or secondary alcohols was carried out. Two types of commercial pyrogenic silicas (AEROSIL OX 50 and AEROSIL 300, by EVONIK), exhibiting two different density of surface silanols (1.4 and 5.1 OH/nm², respectively) were used as catalyst. Before the contact with reagents silicas were simply outgassed at room temperature under high vacuum, these treatments resulting in the complete removal of water molecules from their surface (as checked by in situ IR spectroscopy). Diethyl amine was then adsorbed on the catalyst and then the systems were contacted with ethanol or iso-propyl alcohol at room temperature. Once reached the equilibrium, a consumption of the amine nNH band was observed (by IR) and the formation of the expected products (triethyl amine or diethyl-N-isopropyl amine) were detected by HR-MS analysis of the solutions resulting from water washing of the reacted samples. Thus, silicas might be considered as potential catalyst for this kind of condensation reaction, in addition to the already known catalytic activity exhibited towards the formation of amides from non-activated carboxylic acids and amines, in actual catalytic tests. [2]

References
[1] Bariwal J., Van der Eycken E. Soc. Rev. 2013, 42, 9283.
[2] Comerford J.W., Clark J.H., Macquarrie D. J., Breeden S. W. Commun. 2009, 2562.