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

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

References

(a) Brown

H. C.

, Spec. Publ. Chem. Soc., 1962, 16, 140; (b) H. C. Brown, Acc. Chem. Res., 1983, 16, 432; (c) H. C. Brown, The Nonclassical Ion Problem, comments P.v.R. Schleyer, Plenum Press, New York, 1977.

(a) Winstein

, and Trifan

D. S.

, J. Am. Chem. Soc., 1949, 71, 2953; (b) S. Winstein and D. S. Trifan, J. Am. Chem. Soc., 1952, 74, 1147; (c) S. Winstein, J. Am. Chem. Soc., 1965, 87, 381.

Raghavachari

, Haddon

R. C.

, Schleyer

P. v. R.

, and Schaefer

H. F.

III , J. Am. Chem. Soc., 1983, 105, 5915.

Yoshimine

, McLean

A. D.

, Liu

, DeFrees

D. J.

, and Binkley

J. S.

, J. Am. Chem. Soc., 1983, 105, 6815.

Goetz

D. W.

, Schlegel

H. B.

, and Allen

L. C.

, J. Am. Chem. Soc., 1977, 99, 8118.

Kohler

H. J.

, and Lischka

, J. Am. Chem. Soc., 1979, 101, 3479.

Dewar

M. J. S.

, Haddon

R. C.

, Komornicki

, and Rzepa

, J. Am. Chem. Soc., 1977, 99, 377.

Goddard

J. D.

, Osamura

, and Schaefer

H. F.

III , J. Am. Chem. Soc., 1982, 104, 3258.

(a) Walling

, Acc. Chem. Res., 1983, 16, 448; (b) G. A. Olah, G. K. S. Prakash and M. Saunders, Acc. Chem. Res., 1983, 16, 440; (c) G. A. Olah, G. K. S. Prakash, M. Arvanaghi and A. L. Anet, J. Am. Chem. Soc., 1982, 104, 7105; (d) W. Kirmse, Topics in Current Chemistry, Springer, Berlin, 1979. Vol. 80, p. 128; (e) V. A. Barkhash, Topics in Current Chemistry, Springer, Berlin, 1984. Vol. 116/117, pp. 1-249; (f) see ref. 8 for a comprehensive listing of lead references.

10.

Dewar

M. J. S.

, Healy

E. F.

, and Ruiz

J. M.

, J. Chem. Soc. Chem. Commun., 1987, 943.

11.

(a) Koch

, Bowen

, and DeFrees

D. J.

, J. Am. Chem. Soc., 1989, 111, 1527; (b) D. Lenoir, Y. Apeloig, D. Arad and P. v. R. Schleyer, J. Org. Chem., 1988, 53, 661, and refs. Therein; (c) P. v. R. Schleyer and S. Sieber, Angew. Chem. Int. Ed. Engl., 1993, 32, 1606-1608.

12.

Werstuik

N. H.

, and Muchall

H. M.

, J. Phys. Chem. A., 2000, 104, 2054–2060.

13.

All of the dihedral angles on the appropriate chlorides (rather than the tosylates ^12,15) were calculated using the MM2 program (courtesy of Professor Norman L. Allinger and Mr. Kuohsiang Chen, University of Georgia) cf. Charts 1, 2 and 3.

14.

Gassman

P. G.

, Cryberg

R. L.

, and Shudo

, J. Am. Chem. Soc., 1972, 94, 7600. In this system, θ₁ between the nitrogen lone pair electrons and the 2-exo-Cl leaving group is 42.6,°¹³ i.e. almost identical to θ₁ in norbornyl chloride. Consequently, Gassman's results and hypothesis are in agreement with the hypothesis and analysis presented herein.

15.

Grob

C. A.

, Acc. Chem. Res., 1983, 16, 426.

16.

Grob

C. A.

, Schlageter

M. G.

, and Schaub

, Helv. Chim. Acta, 1980, 63, 57.

17.

(a) Sunko

D. E.

, Starcevic

S. H.

, Pollack

S. K.

, and Hehre

W. J.

, J. Am. Chem. Soc., 1979, 101, 6163; (b) G. A. Olah, G. K. S. Prakash, J. G. Shih, V. V. Krishnamurthy, G. D. Mateescu, G. Liang, G. Sipos, V. Buss, T. M. Gund and P. v. R. Schleyer, J. Am. Chem. Soc., 1985, 107, 2764.

18.

(a) The DVLR analysis and accompanying statistical analysis was done utilizing the Statistical Analysis System (SAS) developed by the SAS Institute in Carey, North Carolina, 27511. Fig. 1 is a computer-generated plot utilizing the SAS program; (b) Of interest, as expected for the espoused hypothesis of solvolytic bridgehead anchimeric assistance, coefficient α for θ₁ is dominant, e.g. one order of magnitude higher than coefficient β for θ₂ in data sets 1-2 and 4-6, and a factor of −2.76 and −6.25 larger in data sets 7 and 8, respectively.

19.

(a) Grob

C. A.

, and Sawlewicz

, Tetrahedron Lett., 1984, 25, 2973; (b) For example, examination of α and β between sets 1 and 7 indicates that α in set 1 is a factor of 3.30 larger than in set 7. Also in set 1,β is a positive 0.252 in comparison to set 7 where β is negative, −0.370. Sets 7 and 8 are the only two data sets where the coefficient β is negative. This may mean that through-space inductive effects are occurring via the back lobes of the substituent bond orbitals. Since structure 4 appears to fit in the correlations of all the regression analyses, i.e. the data sets where the R group is on the same side and the opposite side of the ring in relation to the leaving group, may mean the R group of 4 could perhaps be involved in the transmission of electronic effects via both its front and back lobes. This concept is not out of line with the smaller negative β coefficient (—0.139) in set 8 relative to set 7 (β = −0.370) which does not include 4.

20.

Stock

L. M.

, J. Chem. Ed., 1972, 49, 400–404.

21.

(a) Olah

G. A.

, Tolgyesi

W. S.

, Kuhn

S. J.

, Moffat

M. E.

, Bastien

J. J.

, and Baker

E. B.

, J. Am. Chem. Soc., 1963, 85, 1328; (b) G. A. Olah, E. B. Baker, J. C. Evans, W. S. Tolgyesi, J. S. McIntyre and I. J. Bastien, J. Am. Chem. Soc., 1964, 86, 1360.

22.

Saunders

, Schleyer

P. v. R.

, and Olah

G. A.

, J. Am. Chem. Soc., 1964, 86, 5680.

23.

(a) Olah

G. A.

, and White

A. M.

, J. Am. Chem. Soc., 1969, 91, 3956; (b) J. Am. Chem. Soc., 1969, 91, 6883; (c) G. A. Olah, A. M. White, J. R. DeMember, A. Commeyras and C. Y. Lui, J. Am. Chem. Soc., 1970, 92, 4627.

24.

Hoffmann

, Acc. Chem. Res., 1971, 4, 1.

25.

Gleiter

, Hoffmann

, and Stohrer

W. D.

, Chem. Ber., 1972, 105, 8.

26.

Hoffmann

, Mollere

P. D.

, and Heilbronner

, J. Am. Chem. Soc., 1973, 95, 4860.

27.

Stohrer

W. D.

, and Hoffmann

, J. Am. Chem. Soc., 1972, 94, 779.

28.

Gleiter

, Stohrer

W.-D.

, and Hoffmann

, Helv. Chim. Acta, 1972, 55, 893.

29.

Kropp

P. J.

, Acc. Chem. Res., 1984, 17, 131.

30.

Keese

, Angew. Chem. Int. Ed. Engl., 1975, 14, 528.

31.

Olah

G. A.

, Liang

, Mateescu

G. D.

, and Reimenschneider

J. L.

, J. Am. Chem. Soc, 1973, 95, 8698. Originally, coupling of hydrogens assigned to the higher coordinate carbon (C₆) with the cyclopropane-like carbons (C₁ and C₂) was not observed in the proton coupled CMR spectrum.

32.

(a) Schleyer

P. v. R.

, Watts

W. E.

, Fort

R. C.

Jr. , Comisarow

M. B.

, and Olah

G. A.

, J. Am. Chem. Soc., 1964, 86, 5679; (b) G. A. Olah, C. S. Lee, G. K. S. Prakash, R. M. Moriarty and M. S. C. Rao, J. Am. Chem. Soc., 1993, 115, 10728; (c) Ionization of 7-chloronorbornane under the same superacid conditions is also reported to exhibit the ¹H NMR spectrum of the 2-norbornyl cation at −60°C. [32a] However, hydrolysis of this solution at −70°C, after warming to −50°C and then cooling back down to −70°C, yielded 70% 7-norbornanol and 30% 2-exo-norbornanol. Here also, warming the solution to higher temperatures than −50° C, prior to recooling back down to −70°C, increased the amount formed of the 2-exo-norbornanol.

33.

Olah

G. A.

, and Liang

, J. Am. Chem. Soc., 1971, 93, 6873.

34.

(a) Siehl

H. U.

, Adv. Phys. Org. Chem., 1987, 23, 123; (b) G. M. Kramer and C. G. Scouten, in Advances in Carbocation Chemistry, ed. Creary

, 1989, 1, 93.

35.

(a) Sorensen

T. S.

, and Ranganayakula

J. Org. Chem., 1984, 49, 4310; (b) However, there is no explanation provided for the mechanism of stereospecificity. If the factors of 2/3 and 1/3 are not used (an explanation for their use is not provided) the ratio would be 3.3.

36.

Saunders

, and Kates

M. R.

, J. Am. Chem. Soc., 1983, 105, 3571.

37.

(a) Olah

G. A.

, White

A. M.

, DeMember

J. R.

, Commeyras

, and Lui

C. Y.

, J. Am. Chem. Soc., 1970, 92, 4627; (b) G. A. Olah and A. M. White, J. Am. Chem. Soc, 1969, 91, 5801.

38.

Laszlo

, and Schleyer

P. v. R.

, J. Am. Chem. Soc., 1964, 86, 1171 and refs. cited therein.

39.

Wiberg

K. B.

, Baily

W. F.

, and Jason

M. E.

, J. Org. Chem., 1976, 41, 2711.

40.

Olah

G. A.

, and Liang

, J. Am. Chem. Soc., 1975, 97, 6803.

41.

(a) Masamune

, Sakai

, Jones

A. V. K.

, and Nakashima

, Can. J. Chem., 1974, 52, 855; (b) S. Masamune, M. Sakai and A. V. K. Jones, Can. J. Chem., 1974, 52, 858.

42.

Coates

R. M.

, and Fretz

E. R.

, J. Am. Chem. Soc., 1975, 97, 2538.

43.

Olah

G. A.

, Chimica Scripta, 1981, 18, 97–125.

44.

Shiner

V. J.

Jr. , and Humphrey

J. S.

Jr. , J. Am. Chem. Soc., 1963, 85, 2416.

45.

Murr

B. L.

, and Conkling

J. A.

, J. Am. Chem. Soc., 1970, 92, 3466.

46.

Houk

K. N.

, Rondan

N. G.

, and Brown

F. K.

, Isr. J. Chem., 1983, 23, 3.

47.

Dewar

M. J. S.

, and Reynolds

C. H.

, J. Am. Chem. Soc., 1984, 106, 1744.

48.

Howe

, Friedrich

E. C.

, and Winstein

, J. Am. Chem. Soc., 1965, 87, 379.

49.

(a) Schleyer

P. v. R.

, and Nicholas

R. D.

, J. Am. Chem. Soc., 1961, 83, 2700; (b) Also considered surprising was that both 1-chloro and 7-chloronorbornane, “legendary in their inertness toward normal ionization conditions,” would yield the 2-norbornyl cation under superacid conditions.[32]

50.

(a) Bartlett

P. D.

, and Knox

L. H.

, J. Am. Chem. Soc., 1939, 61, 3184; (b) W. P. Whelan, Jr., Dissertation, Columbia University, 1952; (c) W. v. E. Doering, M. Levitz, A. Sayigh, M. Sprecher and W. P. Whelan, J. Am. Chem. Soc., 1953, 75, 1008; (d) G. J. Gleicher and P. v. R. Schleyer, J. Am. Chem. Soc., 1967, 89, 582.

51.

Wiberg

K. B.

, and Lowry

B. R.

, J. Am. Chem. Soc., 1963, 85, 3188.

52.

Wiberg

K. B.

, and Williams

V. Z.

Jr. , J. Am. Chem. Soc., 1967, 89, 3373.

53.

(a) Applequist

D. E.

, and Roberts

J. D.

, Chem. Revs., 1954, 54, 1065; (b) U. Schollkopf, Angew. Chemie., 1960, 72, 147.

54.

(a) Sita

L. R.

, and Bickerstaff

R. D.

, J. Am. Chem. Soc., 1989, 111, 6454; (b) K. B. Wiberg, R. F. W. Bader and C. D. H. Lau, J. Am. Chem. Soc., 1987, 109, 985; (c) K. B. Wiberg, Acc. Chem. Res., 1984, 17, 379; (d) K. B. Wiberg and F. H. Walker, J. Am. Chem. Soc., 1982, 104, 5239. All of these strained compounds have been synthesized either as parent hydrocarbons or as derivatives and have received considerable attention regarding the extent of bonding between the bridgehead carbon atoms. In [1.1.1] propellane, the appreciable accumulation of charge in the bridgehead bond has invoked the term “fat bond”.[54b] Also, it is known that the addition of a proton to [1.1.1] propellane may yield either the bridgehead carbonium, bicyclo [1.1.1] pentyl cation, or bicyclo [1.1.0] butyl-1-carbinyl cation.[54c]

55.

Jensen

F. R.

, and Smart

B. E.

, J. Am. Chem. Soc., 1969, 91, 5686.

56.

Lin

, and Nickon

, J. Am. Chem. Soc., 1970, 92, 3496.

57.

Schleyer

P. v. R.

, Stang

P. J.

, and Raber

, J. Am. Chem. Soc., 1970, 92, 4725.

58.

Hartman

G. D.

, and Traylor

T. G.

, J. Am. Chem. Soc., 1975, 97, 6147.

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.

60.

Schleyer

P. v. R.

, Lenoir

, Mison

, Liang

, Prakash

G. K. S.

, and Olah

G. A.

, J. Am. Chem. Soc., 1980, 102, 683.

61.

Goering

, and Humski

, J. Am. Chem. Soc., 1968, 90, 6213.

62.

Myrhe

P. C.

, McLaren

K. L.

, and Yannoni

C. S.

, J. Am. Chem. Soc., 1985, 107, 5294.

63.

Maskill

, J. Am. Chem. Soc., 1976, 98, 8482. Maskill has found that 1-deuterio-and 1,2-dideuterio-exo-2-norbornyl bromo-benzene-p-sulfonates yielded deuterium kinetic isotope effects (kies) in 80% aqueous ethanol (k_H/k_D = 1.081, 25.5°C and 1.192, 25.4°C, respectively) and buffered acetic acid (1.051 and 1.173, respectively, 29.4°C). The hypothesis presented herein is further supported by the lack of a deuterium kie for 1-deuterio-endo-2-norbornyl bromobenzene-p-sulfonate in 80% aqueous ethanol (k_H/k_D = 1:000, 54.5°C) and in buffered acetic acid (0.996, 64.7°C). The dideuterated endo-2-norbornyl brosylate has kies of 1.191 (80% ethanol, 54.5°C) 1.194 (buffered acetic acid, 64.7°C) which are also supportive of the present proposal.

64.

Traylor

T. G.

, Acc. Chem. Res., 1969, 2, 152. Cis exo additions are also known to occur with bicyclo [2.2.2] octene, bicyclo [2.1.1] hexene and bicyclo [3.1.0] hexene. While the dihedral angles θ_1(exo) and θ_2(endo) in the corresponding hydrocarbon parent systems may be equal, e.g. in bicyclo [2.2.2] octane, θ_1(exo) = β_1(endo) = 58:28, [13] substitution of hydrogen adjacent to the bridgehead position, as well as generation of a carbonium ion can alter θ_1(exo) and θ_1(endo) thereby making them nonequivalent. For example, see structures 8 and 9, Chart 1, where θ_1(exo) = 46:88, whereas θ_1(endo) = 59:0°.[13] Consequently the postulated stabilizing effects described in the discussion may also be operative in these and other cyclic systems.

65.

(a) Schleyer

P. v. R.

, J. Am. Chem. Soc., 1967, 89, 699; (b) P. v. R. Schleyer, J. Am. Chem. Soc., 1967, 89, 701.

66.

(a) Huisgen

, Ooms

P. H. J.

, Mingin

, and Allinger

N. L.

, J. Am. Chem. Soc., 1980, 102, 3951; (b) R. Huisgen, Pure Appl. Chem., 1981, 53, 171.

67.

(a) Inagaki

, Fujimoto

, and Fukui

, J. Am. Chem. Soc., 1976, 98, 4054; (b) J. Spanget-Larsen and R. Gleiter, Tetrahedron Lett., 1982, 23, 2435; (c) N. G. Rondan, M. N. Paddon-Row, P. Caramella, J. Mareda, P. H. Mueller and K. N. Houk, J. Am. Chem. Soc., 1982, 104, 4974.

68.

Kramer

G. M.

, Scouten

C. G.

, Kastrup

R. V.

, Ernst

E. R.

, and Pictroski

C. F.

, J. Phys. Chem., 1989, 93, 6257–50.

69.

(a) Gassman

P. G.

, and Cryberg

R. L.

, J. Am. Chem. Soc., 1968, 90, 1355; (b) P. G. Gassman and R. L. Cryberg, J. Am. Chem. Soc., 1969, 91, 2047; (c) Since J_BE and J_BG are identical within experimental error and the through-bond paths are not identical, the results, as noted, [69a] support the through-space transmission concept for W-form coupling which may occur via the back lobes. However, a front lobe transmission concept cannot be excluded.

70.

Meinwald

, Meinwald

Y. C.

, and Baker

T. N.

, J. Am. Chem. Soc., 1964, 86, 4074.

71.

Laszlo

, and Schleyer

P. v. R.

, J. Am. Chem. Soc., 1964, 86, 1171.

72.

Wiberg

K. B.

, Lowry

B. R.

, and Nist

B. J.

, J. Am. Chem. Soc., 1962, 84, 1594.

73.

Meinwald

, and Lewis

, J. Am. Chem. Soc., 1961, 83, 2769.

74.

Kumler

W. D.

, Schoolery

J. N.

, and Brutcher

F. V.

Jr. , J. Am. Chem. Soc., 1958, 80, 2533.

75.

(a) Karplus

, J. Chem. Phys., 1959, 30, 11; (b) M. Karplus, J. Am. Chem. Soc., 1963, 85, 2870.

76.

(a) Karplus

, J. Chem. Phys., 1960, 33, 1842; (b) E. D. Becker, High Resolution NMR. Theory and Applications, 2nd ed., Academic Press New York, 1980.

77.

Williamson

K. L.

, J. Am. Chem. Soc., 1963, 85, 516.

78.

Mortimer

F. S.

, J. Mol. Spectry., 1959, 3, 528.

79.

Snyder

E. I.

, and Franzus

, J. Am. Chem. Soc., 1964, 86, 1166.

80.

(a) Emsley

J. W.

, Feeney

, and Sutcliffe

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.

, Schneider

W. G.

, and Bernstein

H. J.

, High Resolution Nuclear Magnetic Resonance, McGraw Hill, New York, 1959. p. 183, 392.

83.

Musher

J. I.

, Mol. Phys., 1963, 6, 93.

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.

, Boyd

D. R.

, Watson

C. G.

, Becker

E. D.

, Bradley

R. B.

, and Jerima

D. M.

, J. Am. Chem. Soc., 1972, 94, 8501.

86.

Shaik

S. S.

, Hiberty

P. C.

, Lefour

J. M.

, and Ohanessian

, J. Am. Chem. Soc., 1987, 109, 363.

87.

Jug

, and Koster

A. M.

, J. Am. Chem. Soc., 1990, 112, 6772.

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.

89.

Winstein

, Gadient

, Stafford

E. T.

, and Klinedinst

P. E.

Jr. J. Am. Chem. Soc., 1958, 80, 5895.

90.

Gassman

P. G.

, and Hornback

J. M.

, J. Am. Chem. Soc., 1967, 89, 2487.

91.

(a) Miles

F. B.

, J. Am. Chem. Soc., 1967, 89, 2488; (b) J. Am. Chem. Soc., 1968, 90, 1265.

92.

Dewar

M. J. S.

, and Schoeller

W. W.

, Tetrahedron, 1973, 27, 4401.

93.

Schleyer

P. v. R.

, J. Am. Chem. Soc., 1964, 86, 1856.

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.

, Walter

S. R.

, Barfield

, Marchand

A. P.

, Marchand

N. W.

, and Segre

A. L.

, Tetrahedron, 1976, 32, 537.

96.

Ref. 76, Marshall

J. L.

1983, pp. 107–114.

97.

Schleyer

P. v. R.

, J. Am. Chem. Soc., 1957, 79, 3292.

98.

Fort

R. C.

Jr. , Adamantane, The Chemistry of Diamond Molecules, Dekker, New York, 1976.

99.

Engler

E. M.

, Farcasiu

, Sevin

, Cense

J. M.

, and Schleyer

P. v. R.

, J. Am. Chem. Soc., 1973, 95, 5769.

100.

Farcasiu

, Hagaman

E. W.

, Wenkert

, and Schleyer

P. v. R.

, Tetrahedron Lett., 1981, 1501.

101.

Salomon

R. G.

, Acc. Chem. Res., 1985, 18, 294 and refs. therein.

102.

(a) Huber

, Angew. Chemie, Int. Ed. Engl., 1989, 28, 848; (b) Progress in Inorganic Chemistry: Bioinorganic Chemistry, 1990, 38, 259-323, ed. Stephen J. Lippard, John Wiley & Sons.

103.

Closs

G. L.

, and Miller

J.R.

, Science, 1988, 240, 440–447.

104.

(a) Kroon

, Oliver

O. M.

, Paddon-Row

M. N.

, and Verhoeven

J. W.

, J. Am. Chem. Soc., 1990, 112, 4868–4873; (b) H. O. Oevering, J. W. Verhoeven, M. N. Paddon-Row, E. Cotsaris and N. S. Hush, Chem. Phys. Lett., 1988, 143.

105.

(a) Closs

G. L.

, Piotrowiak

, MacInnis

J. M.

, and Fleming

G. R.

, J. Am. Chem. Soc, 1988, 110, 2652–2653; (b) G. L. Closs, L. T. Calcaterra, N. J. Green, K. W. Penfield and J. R. Miller, J. Phys. Chem., 1986, 90, 3673.

106.

Delaney

, and Barton

J. K.

, J. Org. Chem., 2003, 68, 6475–6483.

107.

Smith

D. P.

, Bryant

, Quate

C. F.

, Rabe

J. P.

, Gerber

, and Swalen

J. D.

, Proc. Natl. Acad. Sci. USA, 1987, 84, 969–972.