Minimizing Isobutylene Emissions from Large Scale tert-Butoxycarbonyl Deprotections

Isobutylene off-gas amounts liberated during the methanesulfonic acid-catalyzed deprotection of N-BOC−pyrrolidine in THF, methanol, ethanol, 2-propanol, toluene, and dichloromethane were measured using on-line gas-phase mass spectroscopy. While one full equivalent of isobutylene was released as an off-gas when THF was used as the reaction solvent, emissions were reduced by 65−95% in other solvents. In alcohol solvents, the corresponding alkyl tert-butyl ethers are formed as byproducts of the reaction as expected. In dichloromethane and toluene, oligomers of isobutylene can be formed under the reaction conditions. These results provided the basis for developing an effective acid/toluene scrubber for isobutylene that was successfully employed on the pilot plant scale.