The crystal structures of an unexpected carbon dioxide inserted carbamicdiphenylthiophosphinic anhydride and 1-[(4-nitrophenyl) sulfonyl]- trans-2, 5-pyrrolidinedicarboxylic acid methyl ester were determined by X-ray analysis. They crystallized in the space group P21(#4) with α=0.9550(2), b=0.9401(4), c= 1.2880(2) nm, β= 107.74°, V=1.1013(5) nm3, D calcd = 1.349 g/cm3, Z = 2 and P212121 ( # 19)with α= 1.4666(2), b = 0.7195(2), c = 1.6339(2) nm, V= 1.7240(7) nm3, Dcalcd= 1.434 g/cm3, Z=4, respectively.Through the investigation of these two crystal structures, the mechanistic insights into this unexpected carbon dioxide insertion in the reaction of trans-2,5-disubstituted pyrrolidine with diphenylthiophosphoryl chloride in the presence of potassium carbonate were disclosed.