Fig. 5. (a) Comparison of free energies for acylammonium salt formation (I, DTBP; II, 2,6-lutidine; III, Hünig's base; IV, Et3N; V, (S)-(–)-BTM; VI, pyridine) between ethyl fumaroyl chloride (5) and various achiral amines and (S)-(–)-BTM. Free energies of transition state structures (TSSs) and products (shown in kcal mol–1 relative to free energies of separated reactants) computed using SMD(DCM)-M06-2X/6-31G(d). (b) Calculated TSSs (I–VI) for the formation of acylammonium salts with various Brønsted bases optimized at the M06-2X/6-31G(d) level with an implicit solvent model [SMD (dichloromethane)]. Selected bond distances are shown (Å). (c) Section of the 1H–13C gHMQC NMR spectrum of the acylammonium salt 17 in CD2Cl2 formed from a 1 : 1 mixture of (S)-(–)-BTM and ethyl fumaroyl chloride (5).
