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Abstract

Simple and dual variable linear regression equations are presented which can estimate the sensitivity constants ρ for the solvolysis reactions of hundreds of bicyclic and tricyclic compounds. These are the first QSARs which utilize dihedral angles to estimate/predict ρ constants. These QSARs and other analyses herein support the new theory that the bridgehead hydrogen bond orbital assists the displacement of the exo leaving group in the solvolyses of 2-norbornyl derivatives. Alternative interpretions of the ¹H NMR and ¹³C NMR spectra indicate that C₁ is hypercoordinated rather than C₆. Consequently, in normal hydroxylic solvents the 2-norbornyl cation is not symmetrical, does not require C—C σ-bond bridging and is not a single minimum energy system, i.e. it is a pair of rapidly equilibrating cations, eq 1, structure 2b. At ≤ −158°C, the 2-norbornyl cation is proposed to be an H—C₁—H σ-bond delocalized resonance hybrid structure 16/16’. Hypotheses are presented which suggest that the norbornane system can be viewed as a saturated counterpart of a conjugated π system, e.g. benzene. This new delocalization and bond concept, sigma aromaticity, can also help to explain the preferential “exo'’ reactions of norbornane and norbornene systems, the unusual stability of the 2-norbornyl cation, and perhaps provide new insight concerning the ubiquitousness of six-membered rings. Sigma aromaticity may also help account for the greater efficiency of singlet energy transfer between chromophores when the molecular spacer group is rigid rather than flexible, and electron transfer in DNA.

Keywords

QSARs sigma aromaticity electron transfer

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59.

The θ_1(exo) for a 1-methyl substituent is slightly larger than for the smaller, less sterically hindered hydrogen atom, i.e. 42.2° vs. 40.3°, Charts 3, 1, [13] respectively.

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63.

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64.

Traylor

T. G.

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65.

(a) Schleyer

P. v. R.

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66.

(a) Huisgen

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P. H. J.

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67.

(a) Inagaki

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68.

Kramer

G. M.

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69.

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70.

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Y. C.

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T. N.

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71.

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P. v. R.

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72.

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K. B.

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B. J.

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73.

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74.

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W. D.

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75.

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76.

(a) Karplus

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K. L.

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78.

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F. S.

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79.

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E. I.

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80.

(a) Emsley

J. W.

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L. H.

, High Resolution Nuclear Magnetic Resonance Spectroscopy, Pergamon Press, 1966. Vol. 2, pp. 16, 770-772; (b) at −80°C, under superacid conditions, the 4-bridgehead proton of the 2-norbornyl cation was shown to have a J = 0:4 and 1.6 (Hz) upon irradiation of the low field H_{1 2,6} peak and high field H_3,5,7 peak, respectively; in 2-methyl, 2-ethyl and 2-phenyl norbornyl cations, the 1-bridgehead proton is coupled to exo-H₆, J₁₂ = 6.5 (Hz).[37]

81.

Marchand

A. P.

, Stereochemical Applications of NMR Studies in Rigid Bicyclic Systems, Verlag Chemie, Weinheim, 1982. pp. 61–63. For comparison, benzene, cyclohexene, cyclopropane, cyclopentane and tetramethylpentane have J_CH values of 159, 159, 161, 128 and 120, respectively. W. W. Paudler, Nuclear Magnetic Resonance; General Concepts and Applications, Wiley Interscience, New York, 1987. p. 147. Of note, in benzene the internal ortho and meta J_CH coupling constants are positive, i.e. +1.1 and +7.6 whereas the para value is negative, −1.2, cf; J. L. Marshall, Carbon-Carbon and Carbon-Proton NMR Couplings: Applications to Organic Stereochemistry and Conformational Analysis, Verlag Chemie, Intnl., Deerfield Beach, Florida, 1983. p.43-44. In norbornane, the J_C1H4 value is 8.7, however, theoretical calculations indicate that the nonbonded interaction between the bridgehead carbons yields a coupling constant contribution of negative sign, cf. M. Barfield, S. E. Brown, E. D. Canada, Jr., N. D. Ledford, J. L. Marshall, S. R. Walter and E. Yakali, J. Am. Chem. Soc., 1980, 102, 3355.

82.

Pople

J. A.

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W. G.

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H. J.

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83.

Musher

J. I.

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84.

While the concept of ring currents due to “bent-bonds” is in accord with comparisons of C chemical shifts between norbornane (C₁, C₇ = 36:2, 38.7; C_2,3,5,6 = 30,7, ppm, TMS) and the more strained nortricyclene (C₁ = 9.9, C₄ = 29:7, C₇ = 33:2 ppm) they are not in accord with the ¹³C chemical shift of the most strained analogue, quadricyclane (C_1,4 = 23.2; C_2,3,5,6 = 15.0; C₇ = 32,2 ppm). In nortricyclene, the carbons are shifted upfield relative to their positions in norbornane. In quadricyclane, however, the C₇ shielding is almost unchanged (–1 ppm) while the cyclopropyl carbons are downfield relative to those of nortricyclene even though the presence of an appreciable ring-current effect would be expected to enhance these shieldings; cf. J. B. Stothers, C-13 NMR Spectroscopy, Academic Press, New York, 1972. p. 63.

85.

Jennings

W. B.

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D. R.

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C. G.

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E. D.

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R. B.

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D. M.

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86.

Shaik

S. S.

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P. C.

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J. M.

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A. M.

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88.

Of interest, in the 1-norbornyl cation, θ₁ between the formally vacant 1-p orbital and the 2-exo C–H bond is 46.3° whereas in the 2-norbornyl cation, θ₁ between the bridgehead C–H bond and the formally vacant exo lobe of the 2-p orbital is 37.2°, [13] Chart 3. Therefore, there is a greater potential for equatorial bond orbital interaction in the latter system relative to the 1-norbornyl cation.

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E. T.

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P. E.

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Gassman

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M. J. S.

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Schleyer

P. v. R.

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94.

Support for the latter suggestion is that the hydrolysis of 7-norbornyl chloride under superacid conditions yielded 7-norbornanol and exo-2-norbornanol.[32] The authors note that the results suggest that the 7- and 1-norbornyl cations are not interconvertible. Therefore, the exo-2-norbornanol may have formed as a result of an exo-2 hydride transfer to the syn position of C₇.

95.

Marshall

J. L.

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A. P.

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The 2-Norbornyl Cation is not a Single Minimum Energy System *

Abstract

Keywords

References