Engineering pKa value of 3° amine for enhanced production of dialkyl carbonate via Se-catalyzed oxidative carbonylation

Unveiling the Role of DMAP for the Se-Catalyzed Oxidative Carbonylation of Alcohols: A Mechanism Study

“…The effect of DMAP (or pyridine) was also considered because hydrogen-bonded MEG···DMAP has been verified by 1 H NMR analysis in the previous work where DMAP played the role of an HB acceptor. 30 DFT calculations also confirmed the formation of strong HB between MEG and DMAP (−41.1 kJ mol –1 ; Figure S1d ). Pyridine also forms HB with MEG but with weaker interaction (−31.1 kJ mol –1 ; Figure S1e ) because of the lower p K a (5.2) compared to the p K a of DMAP (8.9).…”

“… 30 Moreover, the Se/DMAP catalytic system delivered excellent yields of DACs (over 40%) from C1–C4 alcohols even at milder reaction conditions [90 °C, n (substrate/catalyst) = 200], as compared to previously reported reaction conditions (120 °C) ( Scheme 2 ). 30 …”

“…Figure 1 a shows the energy diagram for the Se/DMAP-catalyzed oxidative carbonylation of MEG to produce BMEC. SeCO( II ) was chosen as the starting species ( Figure 1 a) in the DFT calculation due to the facile formation of II via the aid of a base (DMAP) with the copresence of Se( I ) and CO. 28 − 30 The first step is the approach of MEG to II . The effect of DMAP (or pyridine) was also considered because hydrogen-bonded MEG···DMAP has been verified by 1 H NMR analysis in the previous work where DMAP played the role of an HB acceptor.…”

“…From this fact, we envisioned DMAP to show dual functionality that acts as a nucleophile as well as an HB acceptor (i.e., an intrinsic base), and this work aims to provide a precise reaction mechanism in the Se/DMAP-catalyzed oxidative carbonylation of MEG ( Scheme 1 c). For this purpose, various density functional theory (DFT) calculations were used to calculate energy differences as well as energy barriers for each step, and the proposed intermediates were verified with in situ ATR-FTIR analysis under the actual carbonylation conditions [50 °C, 6.12 MPa (CO/O 2 = 7/3)] 30 to confirm the above hypothesis.…”

“… 29 In our recent study, we demonstrated a catalytic system comprising Se and DMAP [DMAP = 4-(dimethylamino)pyridine], which showed a remarkable catalytic activity (60.9% yield) and recyclability (5th runs) for the oxidative carbonylation of 2-methoxyethanol (MEG) to produce bis(2-methoxyethyl carbonate) (BMEC) under mild conditions [50 °C, n (substrate/catalyst) = 100 or 50)]. 30 Moreover, the Se/DMAP catalytic system delivered excellent yields of DACs (over 40%) from C1–C4 alcohols even at milder reaction conditions [90 °C, n (substrate/catalyst) = 200], as compared to previously reported reaction conditions (120 °C) ( Scheme 2 ). 30 …”