Journal article

Authors list: Schreiner, PR; Schleyer, PV; Schaefer, HF

Publication year: 1997

Pages: 4216-4228

Journal: The Journal of Organic Chemistry

Volume number: 62

Issue number: 13

ISSN: 0022-3263

DOI Link: https://doi.org/10.1021/jo9613388

Publisher: American Chemical Society

Abstract: 
In order to analyze the Solvolysis behavior of epimeric norbornyl derivatives, the dissociative mechanisms of protonated 2-exo- (1, X = OH2+) and 2-endo-norbornanol (2, X = OH2+), 1-methyl-2-exo- (7) aad 2-endo-norbornanol (8), and 1-phenyl-2-exo- (9) and 2-endo-norbornanol (10) were studied ab initio at the B3LYP/6-311+G*//B3LYP/6-3iG* level. in agreement with the experimental salvolysis data, the activation energy (including the 1.2 kcal mol(-1) ground state energy difference) for dissociation of exo-1 (X = OH2+) is 3.7 kcal mol(-1) lower than that of endo-2 (X = OH2+). This value is much smaller than the 14 kcal mol(-1) energy difference favoring the isolated nonclassical (3) over the classical (5) 2-norbornyl cation. That the rate acceleration reflects only a small part of the driving force available poses a general interpretative problem in neighboring group participation. Winstein's hypothesis, that ''bridging lags behind ionization is not the full explanation for this discrepancy. Brown's hypothesis, that there is ''steric hindrance to ionization from the (norbornyl) cndo face'', is not correct as the interaction of the (partially positively charged) endo-hydrogen (C6) and the leaving group is attractive in the transition state. Although the structure of the C7H11+ moiety in the exo-transition state is unsymmetrical, its energy is only 1.3 kcal mol(-1) higher than that of the fully relaxed nonclassical norbornyl cation(3). The norbornyl cation moiety in the 2-endo transition structure (also computed by removing the water molecule and retaining the C7H11+ geometry) is 4.3 kcal mol(-1) more stable than the classical 2-norbornyl cation but 8.8 kcal mol(-1) less stable than the fully bridged ion. Hence, the changes in geometry and charge distribution in the solvolysis transition structures reduce the energy difference of the classical and nonclassical cation moieties in the endo and exo transition structures to 7.5 kcal mol(-1). This is reduced further by the stronger leaving group interaction in the 2-endo over the 2-exo transition structure. The leaving group interaction with the developing carbocation in the 2-endo-norbornyl transition structure is stronger than in the 2-exo-transition structure, This difference (which exemplifies the general behavior of participating systems) arises since the stabilizing interactions of the neighboring group and of the leaving group must compete. Consequently, the effectiveness of both the neighboring group and the leaving group interactions is reduced relative to anchimerically unassisted solvolysis, and only a fraction of the potential driving force is reflected in the stabilization of the transition structure of participating systems. This is shown even more dramatically by the very modest effect (which Nas confirmed computationally) of a 1-methyl or a 1-phenyl substituent an the rate of 2-exo-norbornyl solvolyses (less than 100-fold acceleration), despite the huge increase in driving force.

Harvard Citation style: Schreiner, P., Schleyer, P. and Schaefer, H. (1997) Why the classical and nonclassical norbornyl cations do not resemble the 2-endo- and 2-exo-norbornyl solvolysis transition states, The Journal of Organic Chemistry, 62(13), pp. 4216-4228. https://doi.org/10.1021/jo9613388

APA Citation style: Schreiner, P., Schleyer, P., & Schaefer, H. (1997). Why the classical and nonclassical norbornyl cations do not resemble the 2-endo- and 2-exo-norbornyl solvolysis transition states. The Journal of Organic Chemistry. 62(13), 4216-4228. https://doi.org/10.1021/jo9613388