Next, we focused our attention on studying the mechanism of the reaction. In an attempt to understand the dramatic effect of DMF as the reaction solvent, a variety of other polar aprotic solvents were tested. When the reaction of 1a, CO2, and diethylamine with 2c was run in MeCN, 2-bromo-1-phenylpropan-1-one ( ) was detected as the sole product (Scheme 3a). a-Bromo carbonyl compound 9 was also formed from 1a and 2c when the reaction was run in DMF in the absence of the nucleophile, in quantitative yield (Scheme 3b). We found that a-bromo carbonyl derivative 9 reacted with the carbamate anion to give 5a in quantitative yield (see Supporting Information). Therefore, it is reasonable to suggest that the reaction might proceed by umpolung bromination followed by a nucleophilic substitution step with the second nucleophile, in this instance the carbamate generated from CO2. The formation of a-substituted carbonyl derivatives as intermediates that can react with nucleophiles in SN2-type reactions has been previously studied by Maulide and Jorgensen among others, as a way to circumvent the inconvenience of using electrophilic reactants. Therefore, 2c seems to be an unusually mild brominating agent for the bromination of silyl enol ethers and 1,3-dicarbonyl compounds. Other brominating agents such as NBS or Br2 lead to the formation of polybrominated products in their reactions with silyl enol ethers. We also tested NBS and Br2 (see Scheme S2) under the standard conditions described above, but the desired umpolung products, the a-carbamoyl carbonyl compounds, were not detected in any case.