Radical Ion Probes. 8
J. Am. Chem. Soc., Vol. 120, No. 1, 1998 201
Infrared spectra were recorded on a Perkin-Elmer model 1600 FT-IR
spectrometer. GC/MS was performed on a Fisons 8060 gas chromato-
graph with a VG Quattro mass spectrometer detector. High-resolution
mass spectral data were obtained from a VG Analytical model 7070
E-HF double-focusing, magnetic sector, high resolution spectrometer
using electron impact (70 eV) ionization. GC analysis was performed
on a Hewlett Packard 5890A gas chromatograph equipped with an FID
detector and an HP 3393A reporting integrator. High performance
liquid chromatography (preparative and analytical scale) was performed
with a Beckman System Gold 128 model solvent pump system with a
166 model UV/vis detector. Samples were separated with Beckman
C-18 reverse phase columns (analytical: 4.6 mm × 250 mm; prepara-
tive: 21.2 mm × 150mm) with an 80/20 acetonitrile/water solvent
system. Preparative thin layer chromatography separations (PTLC,
Whatman, silica gel plates, 250 um layer, UV254) were performed with
hexane/ethylacetate solvent mixtures.
Strength of the cyclopropyl C-C in 1a, 1b, and 1c was
estimated according to Scheme 9, where ∆Hf°’s for the pertinent
species were obtained using semiempirical molecular orbital
theory (PM3, details are provided in the Supporting Informa-
tion),21 and literature values for the bond dissociation energies:
BDE(PhO-H) ) 90.4 kcal/mol,22 BDE(1°C-H) ) 100.0 kcal/
mol, BDE(2°C-H) ) 98.5 kcal/mol, and BDE(3°C-H) ) 95.6
kcal/mol.23 We assume that these calculated values for BDE-
(C-C) are the same in the gas phase and in solution.24 Results
of this analysis are summarized in Table 4.
D. Regiochemistry of Ring Opening. For unsymmetrical
radical anions 1b•- and 1c•-, ring opening occurs with modest
selectivity, favoring the more-substituted (stable) distonic radical
anion. On the basis of the yield of products observed in the
preparative-scale electrolyses, k3/k1 ) 9.7 (for 1c•-) and k2/k1
) 1.2 (for 1b•-). Consistent with the Hammond postulate, this
low selectivity suggests an early (reactant-like) transition state
for these ring openings, which is anticipated given their highly
exothermic nature.
Notably, the selectivity observed for ring opening of these
radical anions is remarkably similar to that observed for ring
opening of the ring-substituted cyclopropylcarbinyl (neutral) free
radicals. For example, ring opening of trans-7a leads to 2° and
1° radicals 8a and 9a in a 1.2:1 ratio (Scheme 10),25 a value
identical with that observed for 1b•-. Dimethyl-substituted
radical 7b leads to 3° and 1° radicals 8b and 9b in a 6.7:1 ratio
(Scheme 10),25 vs 9.7:1 observed for 1c•-. Ring opening of
neutral radicals 7a and 7b occurs with rate constants greater
than 108 s-1. We anticipate that the rate constants for ring
opening of 1b•- and 1c•- are of the same order of magnitude.
Electrochemical Measurements. The instrumentation and proce-
dures employed for cyclic and linear sweep voltammetry and preparative
electrolysis experiments have been previously described,26 with the
exception that in the direct electrochemical experiments the disk
electrode surface was polished between each CV to minimize fouling
between runs. Unless otherwise noted, typical measurements were
performed on solutions containing 0.5 M n-Bu4NclO4 (TBAP) in dry
N,N-dimethylformamide (DMF) at room temperature with use of a
5-mm-diameter glassy carbon electrode (GCE), a 0.1 M Ag+/Ag
reference electrode (+0.337 V vs SCE), and a Pt auxiliary electrode.
A conventional H-cell, with the two compartments separated by a
medium glass frit, was employed for preparative electrolysis experi-
ments. The substrate was added to the cathodic compartment exclu-
sively, and the cell was purged with dry, deoxygenated argon for several
minutes prior to electrolysis. All preparative solutions contained 0.2
M TBAP and were electrolyzed at ambient temperature with a constant
current of 30 mA, utilizing gold foil and coiled Cu wire as working
and auxiliary electrodes, respectively. After electrolysis, the cathodic
compartment was quenched with ca. 1 mL of 5% H2SO4, added to ca.
50 mL of water, and extracted with 4 × 50 mL of ether. Ether layers
were combined, washed with water and saturated NaCl, dried over
MgSO4, and concentrated. Product isolations, characterizations, and
quantitations were performed as noted.
Summary
Radical anions derived from 1a, 1b, and 1c undergo facile
ring opening, with rate constants g107 s-1. Based upon the
values of R observed in the direct electrochemistry of these
compounds, the reorganization energy (λ) derived from the
mediated reductions, and the fact that the derived E°’s closely
match model compound 2, we conclude that these radical anions
have a finite lifetime (i.e., electron transfer and ring opening
are not concerted). For unsymmetrical radical anions 1b•- and
1c•-, ring opening yields preferentially the more substituted
(stabilized) distonic radical anion. These results also provide
evidence for stabilization of these radical anions via conjugative
interactions with the cyclopropyl group, decreasing in the order
1c•- > 1b•- > 1a•-. Both the rapid rate and selectivity
associated with the ring opening of these radical anions can be
exploited in the utilization of these substrates as “probes” for
single electron transfer.
Materials. N,N-Dimethylformamide (DMF, EM Science, 98%) was
stirred over copper(II) sulfate (Aldrich, 98%) and activated alumina
(Aldrich, neutral, Brockman activity I) for several days and vacuum
distilled just prior to use. Tetra-n-butylammonium perchlorate (TBAP)
was prepared by the method of House27 and recrystallized 4× from
ethyl acetate/hexane and vacuum oven dried before use. 5,7-Di-tert-
butylspiro[2.5]octa-4,7-dien-6-one (1a),28 1-methyl-5,7-di-tert-butylspiro-
[2.5]octa-4,7-dien-6-one (1b),28 and 1,1-dimethyl-5,7-di-tert-butylspiro-
[2.5]octa-4,7-dien-6-one (1c),28 and 2,6-di-tert-4,4-dimethylcyclohexa-
2,5-dien-1-one29 were prepared through modification of previously
published syntheses. 2,4,6-Tri-tert-butylphenol (Aldrich, 96%) was
used as received. All catalysts used in this study except fluoranthene
(Agros Organics, >98%) and anthracene (Matheson, Coleman & Bell,
>98%) were obtained from Aldrich and used as received.
Electrolysis (Specific). Products of bulk electrolysis are all known
compounds. Characterization was confirmed as needed (spectroscopic
data available in Supporting Information), for compounds which were
not commercially available.
Experimental Section
General. Nuclear magnetic resonance spectra (1H, 13C, 2D NMR)
were obtained on a WP 270 MHz Bruker, an AM 360 MHz Bruker, or
a 400 MHz Varian Unity FT NMR spectrometer. All chemical shifts
are reported in δ units relative to TMS (δ ) 0.00 ppm) in CDCl3.
5,7-Di-tert-butylspiro[2.5]octa-4,7-dien-6-one (1a). Electrolysis of
80 mg (0.34 mmol) of 1a for 37 min at 30 mA (2 equiv of electrons)
and subsequent workup and separation of crude oil via PTLC (1%
EtOAC/hexane) yielded the following pure compounds: 2,6-di-tert-
(21) For a series of representative cyclopropane derivatives, PM3 was
found to accurately reproduce experimentally determined ∆Hf°’s with an
average error of (4 kcal/mol. Details are provided in the Supporting
Information.
(22) Bordwell, F. G.; Liu, W.-Z. J. Am. Chem. Soc. 1996, 118, 10819.
(23) Tsang, W. In Energetics of Organic Free Radicals; Simo˜es, J. A.
M., Greenberg, A., Liebman, J. F., Eds.; Blackie Academic and Profes-
sional: London, 1996; pp 22-58.
(24) Bordwell, F. G.; Cheng, J.-P.; Ji, G.-Z.; Satish, A. V.; Zhang, X. J.
Am. Chem. Soc. 1991, 113, 9790. Bordwell, F. G.; Zhang, X.-M. Acc. Chem.
Res. 1993, 26, 510.
(26) Tanko, J. M.; Drumright, R. E. J. Am. Chem. Soc. 1992, 114, 1844.
(27) House, H. O.; Feng, E.; Pert, N. P. J. Org. Chem., 1971, 36, 2371.
(28) (a) Portnykh, N. V.; Volodkin, A. A.; Ershov, V. V. Bull. Acad.
Sci. USSR, DiV. Chem. Sci. 1966, 2181. (b) Schwartz, L. H.; Flor, R. V. J.
Org. Chem. 1969, 34, 1499. (c) Volodkin, A. A.; Belostotskaya, I. S.;
Ershov, V. V. Bull. Acad. Sci. USSR, DiV. Chem. Sci. 1967, 1328. (d)
Schwartz, L. H.; Flor, R. V.; Gullo, V. P. J. Org. Chem. 1974, 39, 219.
(29) Tarkhanova, M. V.; Volod’kin, A. A., Ershov, V. V., Rasuleva, D.
Kh.; Malievskii, A. D. Bull. Acad. Sci. USSR DiV. Chem. Sci. (Engl. Transl)
1968, 2642.
(25) Beckwith, A. L. J.; Bowry, V. W. J. Org. Chem. 1989, 54, 2681.