Roles of C-H...O=S and pi-stacking interactions in the 2-bromoacrolein complex with N-tosyl-(S)-tryptophan-derived oxazaborolidinone catalyst.
[structures: see text] Ab initio and density functional calculations were employed to examine the structures and binding energies of various complexes between 2-bromolacrolein and N-tosyl-(S)-tryptophan-derived B-butyl-1,3,2-oxazaborolidin-5-one (NTOB), a catalyst commonly used for Diels-Alder reactions. Our calculations show that the chiral oxazaborolidinone catalyst serves as a tridentate complexation agent via B...O donor-acceptor, C-H...O hydrogen-bonded, and pi-stacking interactions. The most stable complex (1TS) is predicted to have a binding energy of -93 kJ mol(-1) (deltaG(298) = -29 kJ mol(-1)). The formyl C-H...O hydrogen bond and pi-stacking interaction are the key factors governing the relative stabilities of the four acrolein-NTOB complexes examined. The calculated structure and binding properties of 1TS are consistent with the experimental results on the absorption spectrum of the acrolein-NTOB complex and the effects of substituents on the reactivity of Diels-Alder reactions. 1TS differs from Corey's proposed model of transition-state assembly in two aspects: (1) it involves the s-trans-acrolein and (2) it favors a C-H...O interaction via the sulfonyl oxygen (C-H...O=S), rather than the ring oxygen (C-H...O-B). This calculated structure of the acrolein-catalyst complex provides an alternate explanation of the origin of stereoselectivity in the NTOB-catalyzed Diels-Alder reactions.
Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543. chmwmw@nus.edu.sg
Ming Wah
MW
Wong
D000171:Acrolein / Q000031:analogs & derivatives* / Q000737:chemistry; D002244:Carbon / Q000737:chemistry*; D002384:Catalysis; D006859:Hydrogen / Q000737:chemistry*; D006860:Hydrogen Bonding; D008968:Molecular Conformation; D010100:Oxygen / Q000737:chemistry*; D013455:Sulfur / Q000737:chemistry*; D014364:Tryptophan / Q000031:analogs & derivatives* / Q000737:chemistry
C503207:N-tosyl-(S)-tryptophan; C041548:2-bromoacrolein; D013455:Sulfur; D002244:Carbon; D000171:Acrolein; D006859:Hydrogen; D014364:Tryptophan; D010100:Oxygen