Figure 2. The ratio of deuterated and nondeuterated products for bromination (left) and iodination (right) of cyclohexane as well as of the
28
tertiary and secondary positions of adamantane under PTC conditions.
ing28 the ratio of the deuterated and nondeuterated products
at early stages of the reaction (conversion < 7%).3,30-32 The
KIEs were extrapolated from plots of the product ratios (H/
D) vs time (Figure 2).
7), the TSs are roughly located halfway along the reaction
path. This is confirmed by the computed geometries (Figure
1) which indicate rather similar bond lengths between the
hydrogen and the respective carbon atom.
3
As both bromination and iodination under PT conditions
are accompanied by a considerable primary H/D KIE,
hydrogen abstraction must be rate-determining. As found in
These findings are clearly different from other radical
halogenation processes such as Gif-type reactions (KIE )
36
2.1-2.4 for cyclohexane (3) vs d -cyclohexane (4)),
1
2
24-26
36
earlier studies using the B3LYP method
for hydrogen
Fenton-type reactions (KIE ) 1.5 for 3 vs 4), or free radical
abstraction reactions,3
3-35
the computed KIEs for the
23
5
bromination (KIE ) 2.4 for 3 vs C H D). The computed
6
11
adamantane reactions are in good to excellent agreement with
experiment (Table 1). Therefore, the computed TSs are likely
to be good representations of the key bond breaking/forming
processes. In view of this, the experimental KIEs for the
cyclohexane reactions may be slightly underestimated, pos-
sibly due to inevitable side reaction such as elimination. As
the KIEs are around 5 (the theoretical maximum is about
H/D KIE for the hydrogen abstraction from cyclohexane is
2.3 which is also in excellent agreement with experiment.5
This is another indication that bromine radicals cannot be
responsible for the hydrogen abstraction. Note that bromine
radicals also show a much lower selectivity for the two
1
9
different positions in adamantane.
The observed high tertiary over secondary selectivities in
the adamantane halogenations may be explained by minor
but decisive differences in the hydrocarbon C-H bond
(
17) Orvik, J. A. J. Org. Chem. 1996, 61, 4933.
18) Tabushi, I.; Aoyama, Y.; Kojo, S.; Hamuro, J.; Yoshida, Z. J. Am.
(
Chem. Soc. 1972, 94, 1177.
19) Minisci, F.; Fontana, F.; Zhao, L.; Banfi, S.; Quici, S. Tetrahedron
Lett. 1994, 35, 8033.
20) Kruppa, G. H.; Beauchamp, J. L. J. Am. Chem. Soc. 1986, 108,
162.
(
(23) All computations were performed with the GAUSSIAN98 program
suite (Gaussian98, Rev. A.7: Frisch, M. J.; Trucks, G. W.; Schlegel, H.
B.; Gill, P. M. W.; B. G. Johnson; Robb, M. A.; Cheeseman, J. R.; Keith,
T.; Petersson, G. A.; Montgomery, J. A.; Raghavachari, K.; Al-Laham, M.
A.; Zakrzewski, V. G.; Ortiz, J. V.; Foresman, J. B.; Cioslowski, J.; Stefanov,
B. B.; Nanayakkara, A.; Challacombe, M.; Peng, C. Y.; Ayala, P. Y.; Chen,
W.; Wong, M. W.; Andres, J. L.; Replogle, E. S.; Gomperts, R.; Martin,
R. L.; Fox, D. J.; Binkley, J. S.; Defrees, D. J.; Baker, J.; Stewart, J. P.;
Head-Gordon, M.; Gonzalez, C.; Pople, J. A. Gaussian, Inc.: Pittsburgh,
(
2
(
21) Aubry, C.; Holmes, J. L.; Walton, J. C. J. Phys. Chem. A 1998,
1
02, 1389.
(
22) Trapping experiment: 200 mg (1.35 mmol) of 3,7-dimethylenobicyclo-
[
3.3.1]nonane (1) and 1.20 g of CBr4 (3.6 mmol) were dissolved in 10 mL
of CH2Cl2, and 4 g of NaOH (solid) was added. This mixture was stirred
in a closed 25 mL vial for 30 h at room temperature. After removing solvents
under reduced pressure, 0.7 g of crude products were obtained. Column
chromatography (silica gel Merck 60, n-hexane, Rf ) 0.30) and recrystal-
lization from n-hexane gave 400 mg (60%) of 3-(1′-bromomethyl)-7-
PA, 1995) utilizing analytical first and second energy derivatives, employing
the three-parameter hybrid functional (B3LYP).2
4,25
We used 6-31G(d,p)
2
6,27
basis sets for hydrogen and carbon and a 3-21G(d) basis set for iodine.
Vibrational frequencies and zero-point vibrational energies (ZPVE) were
not scaled; tunneling effects were not considered. All computed structures
are available from the authors upon request.
(24) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
(25) Parr, R. G.; Yang, W. Density Functional Theory of Atoms and
Molecules; Oxford University Press: New York, 1989.
3
,
7
1
(
2′′,2′′,2′′-tribromoethyl)tricyclo[3.3.1.0 ]nonane (2). H NMR (δ ppm,
37 13
CDCl3) is identical to that described the literature.
C NMR (δ ppm,
CDCl3): 33.46 (C9); 35.57 (C1, C5); 36.72 (C2"); 41.58 (C1"); 48.22 (C6,
C8); 49.52 (C2, C4); 53.61 (C7); 54.13 (C3); 64.83 (C1′). Anal. (C, H).
Calcd: 30.20, 3.41. Found: 30.03, 3.36. MS (70 eV): m/z (%) ) 403,
3
8
21, 265, 214, 133, 105, 91, 41. IR (KBr): 2910, 2849, 1454, 1244, 1071,
(26) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785.
(27) Hariharan, P. C.; Pople, J. A. Theoret. Chim. Acta 1973, 28, 213.
-
1
99, 785, 736, 664, 635, 581 cm
.
Org. Lett., Vol. 2, No. 15, 2000
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