Electron transfer photochemistry of chrysanthemol: An intramolecular S(N)2' reaction of a vinylcyclopropane radical cation

Article abstract of DOI:10.1021/ja953596c

The electron transfer photochemistry of optically pure (1R,3S)-(+)-cis-chrysanthemol (cis-2) results in the formation of (R)-5-(1-(p-cyanophenyl)-1-methylethyl)-2,2-dimethyl oxacyclohex-3-ene (4) with significant retention of optical activity. The product is rationalized via nucleophilic attack of the alcoholic function of the radical cation on the terminal carbon of the vinyl group with simultaneous replacement of an isopropyl radical as an intramolecular leaving group in an apparent S(N)2' reaction. This mode of attack is unprecedented in vinylcyclopropane radical cations and is interpreted as evidence for the significant role that relief of ring strain and its avoidance play in determining the course of nucleophilic capture in radical cationic systems.

Full text of DOI:10.1021/ja953596c

10954
J. Am. Chem. Soc. 1996, 118, 10954-10962
Electron Transfer Photochemistry of Chrysanthemol: An
Intramolecular SN2′ Reaction of a Vinylcyclopropane Radical
Cation
Torsten Herbertz and Heinz D. Roth*
Contribution from the Department of Chemistry, Rutgers UniVersity, Wright-Rieman Laboratories,
New Brunswick, New Jersey 08855-0939
ReceiVed October 26, 1995X
Abstract: The electron transfer photochemistry of optically pure (1R,3S)-(+)-cis-chrysanthemol (cis-2) results in
the formation of (R)-5-(1-(p-cyanophenyl)-1-methylethyl)-2,2-dimethyl oxacyclohex-3-ene (4) with significant retention
of optical activity. The product is rationalized via nucleophilic attack of the alcoholic function of the radical cation
on the terminal carbon of the vinyl group with simultaneous replacement of an isopropyl radical as an intramolecular
leaving group in an apparent SN2' reaction. This mode of attack is unprecedented in vinylcyclopropane radical
cations and is interpreted as evidence for the significant role that relief of ring strain and its avoidance play in
determining the course of nucleophilic capture in radical cationic systems.
Introduction
been carried out recently.10-14 Thus, the molecular ion of
vinylcyclopropane undergoes rearrangement to penta-1,3-diene
radical cation in the gas phase.11 Related results of two rigidly
linked vinylcyclopropane systems have been observed in
solution; the electron transfer induced rearrangements of sab-
inene to â-phellandrene (shown below), and of R-thujene to
R-phellandrene, were interpreted as novel examples of a
sigmatropic shift.12
Radical cations of molecules containing strained ring moieties
as well as olefinic fragments have been the focus of much
interest in recent years;1 the conjugative and homoconjugative
interactions between the two types of functions have been of
particular interest. Various substrates have been probed to
delineate changes in the molecular geometry upon one-electron
oxidation and to assess the spin and charge density distributions
in the resulting radical cations.2 Typically, the reactions of these
species proceed with release of ring strain;3-6 in some systems,
ring opening is assisted by a nucleophile.7-10
Vinylcyclopropane radical cation, 1•+, the simplest species
containing an olefinic moiety and a cyclopropane ring, has not
yet been characterized adequately, although several studies have
The spin density distributions of three radical cations contain-
ing syn-vinylcyclopropane systems, Viz., bicyclo[3.1.0]hex-2-
ene, bicyclo[4.1.0]hept-2-ene, and its 3,7,7-trimethyl-derivative
(2-carene) were characterized by CIDNP studies.14 Theoretical
calculations of the unsubstituted radical cation, 1•+, so far have
been limited to the STO-3G level of theory and a seriously
X Abstract published in AdVance ACS Abstracts, October 15, 1996.
(1) (a) Forrester, R. A.; Ishizu, K.; Kothe, G.; Nelsen, S. F.; Ohya-
Nishiguchi, H.; Watanabe, K.; Wilker, W. Organic Cation Radicals and
Polyradicals. In Landolt Bo¨rnstein, Numerical Data and Functional
Relationships in Science and Technology; Springer Verlag: Heidelberg,
1980; Vol. IX, Part d2. (b) Shida, T. Electronic Absorption Spectra of
Radical Ions; Elsevier: Amsterdam, 1988. (c) Shida, T.; Haselbach, E.;
Bally, T. Acc. Chem. Res. 1984, 17, 180. (d) Nelsen, S. F. Acc. Chem. Res.
1987, 20, 269. (e) Roth, H. D. Top. Curr. Chem. 1992, 163, 133-245.
(2) (a) Haddon, R. C.; Roth, H. D. Croat. Chem. Acta 1984, 57, 1165.
(b) Roth, H. D.; Schilling, M. L. M. Can. J. Chem. 1983, 61, 1027. (c)
Roth, H. D.; Schilling, M. L. M.; Schilling, F. C. J. Am. Chem. Soc. 1985,
107, 4152. (d) Roth, H. D.; Schilling, M. L. M.; Abelt, C. J. Tetrahedron
1986, 42, 6157. (e) Roth, H. D.; Schilling, M. L. M.; Abelt, C. J. J. Am.
Chem. Soc. 1986, 108, 6098. (f) Roth, H. D. Acc. Chem. Res. 1987, 20,
343-350. (g) Roth, H. D.; Du, X-M.; Weng, H.; Lakkaraju, P. S.; Abelt,
C. J. J. Am. Chem. Soc. 1994, 116, 7744-7752. (h) Weng, H.; Du, X.-M.;
Roth, H. D. J. Am. Chem. Soc. 1995, 117, 135-140.
(3) Quadricyclane to norbornadiene radical cation: (a) Roth, H. D.;
Schilling, M. L. M.; Jones, G., II J. Am. Chem. Soc. 1981, 103, 1246-
1248. (b) Roth, H. D.; Schilling, M. L. M. J. Am. Chem. Soc. 1981, 103,
7210-7217.
(4) Methylenecyclopropane rearrangement: (a) Takahashi, Y.; Mukai,
T.; Miyashi T. J. Am. Chem. Soc. 1983, 105, 6511-6513. (b) Miyashi, T.;
Takahashi, Y.; Mukai, T.; Roth, H. D.; Schilling, M. L. M. J. Am. Chem.
Soc. 1985, 107, 1079-1080.
(7) Cyclopropane systems: (a) Rao, V. R.; Hixson, S. S. J. Am. Chem.
Soc. 1979, 101, 6458-6459. (b) Mizuno, K.; Ogawa, J.; Kagano, H.; Otsuji,
Y. Chem. Lett. 1981, 437-438. (c) Mizuno, K.; Ogawa, J.; Otsuji, Y. Chem.
Lett. 1981, 741-744. (d) Mazzocchi, P. H.; Somich, C.; Edwards, M.;
Morgan, T.; Ammon, H. L. J. Am. Chem. Soc. 1986, 108, 6828. (e)
Dinnocenzo, J. P.; Todd, W. P.; Simpson, T. R.; Gould, I. R. J. Am. Chem.
Soc. 1990, 112, 2462-2464. (f) Hixson, S. S.; Xing, Y. Tetrahedron Lett.
1991, 32, 173-174. (g) Dinnocenzo, J. P.; Lieberman, D. R.; Simpson, T.
R. J. Am. Chem. Soc. 1993, 115, 366-367.
(8) Bicyclobutane systems: (a) Gassman, P. G.; Olson, K. D.; Walter,
L.; Yamaguchi, R. J. Am. Chem. Soc. 1981, 103, 4977. (b) Gassman, P.
G.; Olson, K. D. J. Am. Chem. Soc. 1982, 104, 3740.
(9) Vinylcyclobutane systems: (a) Arnold, D. R.; Du, X. J. J. Am. Chem.
Soc. 1989, 111, 7666-7667. (b) Arnold, D. R.; Du, X. Can. J. Chem. 1994,
72, 403-414. (c) Zhou, D.; Skeik, M.; Roth, H. D. Tetrahedron Lett. 1996,
37, 2385-2388.
(10) Vinylcyclopropane systems: (a) Weng, H.; Sethuraman, V.; Roth,
H. D. J. Am. Chem. Soc. 1994, 116, 7021-7025. (b) Arnold, D. R.; Du,
X.; de Lijser, H. J. P. Can. J. Chem. 1995, 73, 522-530.
(11) Dass, C.; Peake, D. A.; Gross, M. L. Org. Mass. Spectrom. 1986,
21, 741-746.
(12) Weng, H.; Sheik, Q.; Roth, H. D. J. Am. Chem. Soc. 1995, 117,
10655-10661.
(13) Scott, L. T.; Erden, I.; Brunsvold, W. R.; Schultz, T. H.; Houk, K.
N.; Paddon-Row, M. N. J. Am. Chem. Soc. 1982, 104, 3659-3664.
(14) Roth, H. D.; Herbertz, T. J. Am. Chem. Soc. 1993, 115, 9804-
9805.
(5) Bicyclobutane to cyclobutene rearrangement: (a) Gassman, P. G.;
Hay, B. A. J. Am. Chem. Soc. 1985, 107, 4075. (b) Gassman, P. G.; Hay,
B. A. J. Am. Chem. Soc. 1986, 108, 4227. (c) Arnold, A.; Burger, U.;
Gerson, F.; Kloster-Jensen, E.; Schmidlin, S. P. J. Am. Chem. Soc. 1993,
115, 4271-4281.
(6) Vinylcyclopropane rearrangement: (a) Dinnocenzo, J. P.; Schmittel,
M. J. Am. Chem. Soc. 1987, 109, 1561-1562. (b) Dinnocenzo, J. P.; Conlon,
D. A. J. Am. Chem. Soc. 1988, 110, 2324-2326.
S0002-7863(95)03596-7 CCC: $12.00 © 1996 American Chemical Society

Electron Transfer Photochemistry of Chrysanthemol
J. Am. Chem. Soc., Vol. 118, No. 45, 1996 10955
restricted geometry.13 In addition, two conformers of 2-carene
radical cation were calculated recently, also at the STO-3G
level.10b
samples of (+)-cis- and (-)-trans-chrysanthemic acid were obtained
by multiple fractional crystallization of the diastereomeric salts prepared
from the acids and quinine (Janssen Chimica, 99%) from absolute
ethanol and, subsequently, ethanol-water {(+)-cis-chrysanthemic acid
In this publication we describe further attempts to illuminate
the interactions between the olefinic and cyclopropane moieties
of vinylcyclopropane radical cations. We selected three simple
systems, in which the rotation of the vinyl group is, at least
formally, unrestricted, a pair of geometric isomers, cis- and
trans-chrysanthemol, 2, and the bis-dimethyl-substituted deriva-
tive, 3. We have approached the structure of the corresponding
radical cations and their reactivity by two complementary
methods: we probed the electron spin density distribution of
the radical cations by chemically induced dynamic nuclear
polarization (CIDNP); and we studied their electron transfer
photochemistry with 1,4-dicyanobenzene and tetrachloro-p-
benzoquinone (chloranil) as acceptor/sensitizer.
25
quinine salt, mp 138-140 °C, [R]D -102.4 ( 5°; c 1.29 in EtOH;
25
lit. mp 127-135 °C, [R]D -103.5°; (-)-trans-chrysanthemic acid
25
quinine salt, mp 162 °C, [R]D -112.5 ( 5.5°; c 1.08 in EtOH; lit.
25
mp 159.5-161 °C, [R]D -115.4°; c 1.018 in EtOH}.17 The
enantiomerically enriched acids were obtained by decomposing the
purified salts with 2 M HCl and extracting with ether. In this fashion,
25
optically pure (+)-cis-chrysanthemic acid ([R]D +44.1 ( 2°; c 1.02
25
in EtOH; 100% ee; lit. [R]D +40.8°; c 1.775 in EtOH) and
enantiomerically enriched (-)-trans-chrysanthemic acid ([R]D25 -10.5
25
( 0.5°, 72 ( 3.5% ee, c 1.435 in EtOH; lit. [R]D -14.45°; c 1.211
in EtOH) were obtained.
The pure cis and trans acids were reduced with LiAlH4 in ether,
25
yielding (+)-cis-chrysanthemol ([R]D +26.0 ( 1°, c 1.56 in EtOH)
25
and (-)-trans-chrysanthemol ([R]D -39.1 ( 2°, c 1.15 in EtOH).
25
For natural (+)-trans-chrysanthemol a rotation, [R]D +49.7° (c 1.7
in methylcyclohexane), was measured.18
2,2-Dimethyl-1-(2-methyl-1-propenyl)cyclopropane. The crude
pyrazoline resulting from conjugate addition of hydrazine hydrate to
2,6-dimethylhepta-2,5-dien-4-one (phorone; Aldrich, 97%) was pyro-
lyzed over KOH/Pt black at 150 °C, giving rise to 2,2-dimethyl-1-(2-
methyl-1-propenyl)cyclopropane (bp 131 °C; ∼60% yield).19
CIDNP Experiments. Acetone-d6 solutions containing 0.02 M
donor and 0.01 M acceptor were irradiated in the probe of a Bruker
80-MHz NMR spectrometer. The collimated beam of a Hanovia
1000-W high-pressure Hg-Xe lamp was directed onto a fused silica
light pipe in the NMR tube. This setup limits the useful concentration
range of the light-absorbing sensitizer, as care has to be taken to ensure
essentially uniform absorption throughout the volume in the receiver
coil. Accordingly, the optical density of the sensitizer has to be kept
low, OD e 1.0. A total of 8 FIDs were collected before and during
irradiation. Because of the relatively low sensitizer concentration and
the relatively short irradiation time, there appeared to be no need to
pass the light beam through a water or other filter solution. A pulse
angle of 90° was used for all experiments.
Irradiation of an electron acceptor sensitizer-co-sensitizer
pair (DCB-Phen) in the presence of donors such as 2 or 3 (D)
leads to the generation of the radical cation (D•+) and the
sensitizer radical anion (DCB•-; eq 1); D•+ is captured by a
nucleophile (CH3OH; eq 2) and the resulting free radical reacts
with the radical anion by aromatic substitution (eq 3).7a-c,9 For
olefinic substrates, this reaction sequence has been termed a
“photoinduced nucleophile-olefin-combination-aromatic-substi-
tution” (photo-NOCAS).9
DCB-Phen + D
9
hν8 DCB•- + D•+ + Phen
(1)
(2)
D
•+ + CH3OH f •[D-OCH3] + H+
Results
•[D-OCH3] + DCB•- f p-CN-C6H4-D-OCH3 + CN-
(3)
Photoreaction of (1R,3S)-(+)-cis-Chrysanthemol (cis-2).
Irradiation of acetonitrile solutions containing 0.2 M 1,4-
dicyanobenzene (DCB), 0.04 M phenanthrene (Ph), and 0.2 M
of optically pure (1R,3S)-(+)-cis-chrysanthemol ([R]D +26.0
( 1°, c 1.56 in EtOH) leads to the formation of one major
adduct, 5-(1-(p-cyanophenyl)-1-methylethyl)-2,2-dimethylox-
acyclohex-3-ene, 4, presumably the (R)-enantiomer, with sig-
nificant retention of optical activity ([R]D +26.3 ( 1°, c 1.255
in EtOH; 84% conversion after 7.5 h; 82% yield). Recovered
starting material, (1R,3S)-(+)-cis-2, has retained its optical purity
([R]D +26.4 ( 1°, c 0.74 in EtOH); no trans-chrysanthemol
The chrysanthemol isomers are of particular interest because
their alcohol function may serve as an intramolecular nucleo-
phile, intercepting the positive charge density on either the
strained ring or the CdC double bond. The two chiral carbons
of cis- and trans-2 appear to be uniquely suited to elucidate the
interactions between the three functional groups in any reaction
initiated by electron transfer to a photo-excited sensitizer. To
provide the proper perspective for the results obtained for 2,
we also discuss the electron transfer induced photoreactions of
substrate 3 in the presence of an external nucleophile.
1
was detected by H NMR.
Experimental Section
The sensitizers and solvents used in this study are commercially
available and were purified as described earlier.10a Similarly, the
photoreactions were performed and the products were isolated and
characterized according to previously described procedures.10a
Chrysanthemol. A mixture of cis- and trans-chrysanthemum
monocarboxylic acid ethyl ester (Sigma, 95%) was hydrolyzed with
aqueous KOH. Repeated fractional crystallization of the crude mixture
of acids from ethyl acetate yields pure cis-chrysanthemic acid (mp 112-
114 °C; lit.15 mp 113-116 °C). In contrast, trans-chrysanthemic acid16
was not obtained by crystallization; instead, it was prepared by
epimerization of the mixed ethyl esters with potassium tert-amylate in
benzene and subsequent alkaline hydrolysis. Enantiomerically enriched
The assignment of the oxacyclohexene structure to 4 is
supported by the presence of 2 olefinic protons, H-3 (δ 5.35
ppm; J ) 15.4 Hz) and H-4 (δ 5.70 ppm; J ) 15.4, 9.4 Hz),
one allylic proton (δ 2.45 ppm; ddd, J ) 9.4, 9.4, 4.1 Hz, “t”d)
coupled to H-4, H-6, and H-6′, and two alkoxy protons, H-6 (δ
3.4 ppm; dd, J ) 9.4, 9.4 Hz, “t”) and H-6′ (δ 3.5 ppm; dd, J
) 9.4, 4.1 Hz). The presence of the 1-(p-cyanophenyl)-1-
(17) Campbell, I. G. M.; Harper, S. H. J. Sci. Food. Agric. 1952, 3,
189-192.
(18) Epstein, W. W.; Alexander, K. J. Org. Chem. 1975, 40, 2576.
(19) Nelson, E. R.; Maienthal, M.; Lane, L. A.; Benderly, A. A. J. Am.
Chem. Soc. 1957, 79, 3467-3469.
(15) Campbell, I. G. M.; Harper, S. H. J. Chem Soc. 1945, 283-286.
(16) Julia, S.; Julia, M.; Linstrumelle, G. Bull. Soc. Chim. Fr. 1966, 11,
3499-3507.

10956 J. Am. Chem. Soc., Vol. 118, No. 45, 1996
Herbertz and Roth
methylethyl function is documented by the dominant base peak
of m/z 144 (C6H4CNC3H6).
tions containing 0.2 M 1,4-dicyanobenzene (DCB), 0.04 M
phenanthrene (Ph), and 0.2 M cis- or trans-2, respectively, in
two NMR tubes did not indicate an appreciable difference in
the rate of consumption of starting material by GC/MS
monitoring.
The DCB sensitized reaction also produced a photolabile
adduct, 5 (∼15% yield at 84% conversion), with significantly
higher GC retention time than 4. The adduct apparently is
formed by intermolecular capture of 2•+ at the quaternary
cyclopropyl carbon by another molecule of chrysanthemol.
Preparative Photoreaction of cis-2 with DCB-Phen in
CH3CN. A sample containing 3.08 g of cis-2 (0.4 M), 2.56 g
of DCB (0.4 M), and 0.72 g of Phen (0.08 M) in 50 mL of
CH3CN was irradiated in a 30-mm i.d. Pyrex tube for 41 h to
∼23% conversion of cis-2. The reaction mixture was concen-
trated to a volume of 5 mL, and the residue was loaded onto a
silica gel column and eluted with methylene chloride containing
ethyl acetate, gradually increasing from 0 to 20% v/v. The
components of the reaction mixture are eluted in the following
order: Phen, DCB, cis-2, and 4. The fractions containing
compound 4 were combined and concentrated; 4 was obtained
as a 0.74-g sample of ∼90% purity (55% yield).
The structure of 5 is supported by the presence of 3 olefinic
protons, H-3 (δ 5.42 ppm; J ) 15.8, 9.5 Hz), H-4 (δ 5.71 ppm;
J ) 15.8 Hz), and H-1′′′′ (δ 4.88 ppm; broad multiplet), a
tertiary allylic resonance, H-2 (δ 2.35 ppm; broad multiplet),
an allylic cyclopropyl resonance, H-3′′′ (δ 1.36 ppm; broad
multiplet), and four pairs of non-equivalent methyl resonances.
The two allylic methyl groups (δ) are most deshielded (∼1.70
ppm), followed by the two methyl groups attached to the allylic-
benzylic carbon, C-5 (R; ∼1.40 ppm). The pairs of methyl
groups attached to the alkoxy-substituted carbon, C-1′ (â; 1.20
1.16 ppm), and the cyclopropane ring (γ; 1.11, 1.02 ppm). In
essence, the spectrum of (E)-5-(p-cyanophenyl)-2-(1-(2,2-di-
methyl-3-(2-methyl-1-propenyl))cyclopropylmethoxy-1-methyl-
ethyl)-5-methylhex-3-enol, 5, is a superposition of the spectra
of 2 and 6 (vide infra).
Photoreaction of 2,2-Dimethyl-1-(2-methyl-1-propenyl)-
cyclopropane (3). Irradiation of acetonitrile/methanol (3/1 by
volume) solutions containing 0.333 M DCB, 0.083 M Ph, and
0.333 M 2,2-dimethyl-1-(2-methyl-1-propenyl)cyclopropane
leads to the formation of two NOCAS adducts, (E)-2-(p-
cyanophenyl)-2,6-dimethyl-6-methoxy-3-heptene, 7, and 4-(p-
cyanophenyl)-2,6-dimethyl-6-methoxy-2-heptene, 8, in yields
of 63 and 37%, respectively (7.5 h; 81% conversion). No
rearrangement of the vinylcyclopropane donor was observed.
Product 7 is stable under the irradiation conditions (0.25 M
DCB, 0.05M Ph, and 0.25 M 7, in 2 mL of acetonitrile/methanol,
3/1 by volume, 2 h); in particular, it does not undergo a
sigmatropic shift to 8.
Photoreaction of (()-cis-Chrysanthemol (cis-2) in the
Presence of Methanol. Irradiation of acetonitrile/methanol (3/1
by volume) solutions containing 0.167 M DCB, 0.033 M Ph,
and 0.167 M (()-cis-chrysanthemol (4 h, 50% conversion) leads
to the formation of a single NOCAS-adduct, (E)-5-(p-cyano-
phenyl)-2-(1-methoxy-1-methylethyl)-5-methylhex-3-enol (6).
The structure of 7 rests on the presence of 2 olefinic protons,
H-3 (δ 5.62 ppm, d, J ) 15.6 Hz) and H-4 (δ 5.52 ppm, dt,
15.6, 6.0 Hz), and 2 allylic protons, H-5 (δ 2.24 ppm, d, J )
6.0 Hz). The 2D-COSY spectrum shows cross peaks between
one olefinic (δ 5.52 ppm) and one methyl signal (δ 1.4 ppm),
which also interacts with the (ortho) aromatic protons (δ 7.45
ppm). Product 8 shows 1 olefinic proton, H-3 (δ 5.25 ppm, d,
J ) 9.5 Hz), 1 allylic-benzylic proton, H-4 (δ 3.76 ppm, dt, J
) 9.5, 6.6 Hz), and 2 allylic methyl groups (δ 1.66, 1.67 ppm).
Photoreaction of 3 in the Absence of Methanol. Irradiation
of 3 (0.333 M) with DCB (0.333 M) and Ph (0.083 M) in
acetonitrile solutions not containing methanol failed to show
any rearrangement of the vinylcyclopropane donor; instead, an
acetonitrile adduct, presumably 3-(2-methyl-1-propenyl)-2,5,5-
trimethyl-1-pyrroline, 9, is formed slowly (66% yield; 120 h;
35% conversion). The acetonitrile adduct has a very weak
molecular ion peak (m/z 165; C9H16 + CH3CN); the base peak
(m/z 109; C8H13; C11H19N - CH3CN - CH3) and a second
strong peak (m/z 124; C9H16, M+• - CH3N) document the
incorporation of a labile acetonitrile function.
The structure of 6 is supported by the presence of 2 olefinic
protons, H-4 (δ 5.73 ppm; J ) 15.7 Hz) and H-3 (δ 5.38 ppm;
J ) 15.7, 9.5 Hz), one allylic proton, (δ 2.39 ppm; ddd, J )
9.5, 6.7, 5.1 Hz), and two diastereotopic alkoxy protons (δ 3.64
ppm; ddd, J ) 11.2, 5.6, 5.1 Hz; and δ 3.80 ppm; ddd, 11.2,
6.7, 5.6 Hz).
At lower methanol concentrations products 4 and 6 are formed
competitively. CH3CN solutions of 0.1 M DCB, 0.1 M cis-2,
0.02 M Ph, and varying concentrations of methanol (0.001, 0.01,
0.033, and 0.1 M) were irradiated simultaneously for 2 h. At
0.001 and 0.01 M methanol the relative yield of 4 is 5.5 and
0.75 times that of 6, respectively.
Photoreaction of (1S,3S)-(-)-trans-Chrysanthemol (trans-
2). Irradiation of acetonitrile solutions containing 0.175 M
DCB, 0.035 M Ph, and 0.175 M (1S,3S)-(-)-trans-2 ([R]D
-39.1 ( 2°, c 1.15 in EtOH, ∼72 ( 3.5% ee) generates the
enantiomeric adduct (4; [R]D -12.4 ( 0.5°, c 1.125 in EtOH,
90% yield; 100% conversion after 6 h). A photolabile adduct,
analogous to 5, was formed and identified by its mass spectrum.
Starting material, (1S,3S)-(-)-trans-2, recovered after 77%
conversion, showed reduced optical activity ([R]D -16.8 ( 1°,
c 0.715 in EtOH). However, cis-chrysanthemol is not detected
1
by H NMR.
Upon chromatography on silica gel, the primary adduct
rearranges to 2,5,5-trimethyl-3-(2-methylpropenylidene)-1-pyr-
Comparative Irradiation of cis- and trans-2 under Identi-
cal Conditions. Simultaneous irradiation of acetonitrile solu-
roline, 10. The stable molecular ion of 10 (m/z 165; C9H16
+

Electron Transfer Photochemistry of Chrysanthemol
J. Am. Chem. Soc., Vol. 118, No. 45, 1996 10957
Table 1. Summary of Electron Transfer Reactions
donor ([R]D)
solvent
time, conversion
7.5 h, 82%
products (yield; [R]D)
(+)-cis-2 (+26.0°)
CH3CN
4 (82%; +26.3°)
5 (15%)
cis-2 (+26.4°)
(()-cis-2
(()-cis-2
CH3CN:CH3OH, 3:1 v/v
CH3CN; 0.001 M CH3OH
4.0 h, 50%
2.0 h
6 (100%)
4 (85%)
6 (15%)
(()-cis-2
CH3CN; 0.01 M CH3OH
CH3CN
2.0 h
4 (43%)
6 (57%)
4 (90%; -12.4°)
5 (10%)
(-)-trans-2 (-39.1°)
6.0 h, 100%
trans-2 (-16.8°)
7 (63%)
3
CH3CN:CH3OH, 3:1 v/v
7.5 h, 81%
8 (37%)
10 (66%)
3
3
CH3CN
CH3NO2
120 h, 35%
120 h, 0%
Figure 1. 1H NMR spectra (80 MHz) of acetone-d6 solutions containing
30 mM cis-chrysanthemol (cis-2) and 5 mM chloranil in the dark
(bottom) and during UV irradiation (top). Only the resonances of the
non-equivalent pairs of methyl groups (allylic, ∼1.6 ppm; cyclopropane,
∼1.1, 1.0 ppm) show significant polarization.
Figure 2. 1H NMR spectra (80 MHz) of an acetone-d6 solution
containing 30 mM 2,2-dimethyl-1-(2-methyl-1-propenyl)cyclopropane
(3) and 5 mM chloranil during UV irradiation. The non-equivalent pairs
of methyl groups (allylic, ∼1.65 ppm; cyclopropane, ∼1.05, 1.0 ppm)
show strong emission; in addition, the geminal protons, H-3trans and
H-3cis, trans and cis to the isobutenyl group, show strong and moderate
emission, respectively. The identity of the CIDNP signals is assigned
in the figure.
CH3CN) and its base peak (m/z 150; C10H16N) document facile
loss of CH3 and insignificant loss of CH3CN. The structure of
10 rests on the presence of 1 olefinic proton (δ 5.56 ppm, dt, J
) 9.15, 2.5 Hz; H-1′) which is coupled to three allylic protons
(δ 2.34 ppm, d, J ) 2.5 Hz, H-4, 2H; and δ 2.37 ppm, dsep, J
) 9.15, 6.77 Hz; H-2′, 1H) and on the fact that one allylic proton
(H-2′) is coupled to two equivalent methyl groups (δ 1.02 ppm,
d, J ) 6.77 Hz).
cationic systems, 2•+ and 3•+, and the stereochemical details of
their nucleophilic capture, which occurs in intramolecular or
intermolecular fashion, respectively. The structural aspects are
based on the CIDNP results.
The results are summarized in Table 1.
Nuclear Spin Polarization: Structure of Vinylcyclopro-
pane Radical Cations. In order to assign the structures of
vinylcyclopropane radical cationic systems, we consider the
frontier molecular orbital (FMO) of the cyclopropane unit and
consequences for the interaction between the two moieties. MO
calculations suggest that the vertical ionization of cyclopropane
occurs from a degenerate pair of in-plane e′ orbitals (s, a)
CIDNP Results. Irradiation of chloranil in the presence of
cis- and trans-2 as well as 3 resulted in strong CIDNP effects
for the respective donor molecules. The CIDNP spectra of cis-2
(Figure 1) and trans-2 support radical cations of very similar
spin density distributions. The two pairs of non-equivalent
methyl groups at 1.6 and 1.1 and 1.0 ppm, respectively, of 2
show strong emission; the single olefinic resonance appears in
weak emission (not shown). Similarly, the two pairs of non-
equivalent methyl groups of 3 (Figure 2) show strong emission,
as does the doublet of doublets of H-3trans (0.65 ppm) and, to a
lesser extent, the doublet of doublets of H-3cis (0.18 ppm). The
allylic cyclopropane proton (1.25 ppm) and the single olefinic
proton show negligible polarization.
2
resulting in a doubly degenerate E′ state;20 first-order Jahn-
Teller distortion leads to two non-degenerate electronic states,
2
2
2A1 and B2 (C2V symmetry).20 The A1 component (orbital s
singly occupied) relaxes to a “one-electron-bonded trimethylene”
(20) (a) Haselbach, E. Chem. Phys. Lett. 1970, 7, 428. (b) Rowland, C.
G. Chem. Phys. Lett. 1971, 9, 169. (c) Collins, J. R.; Gallup, G. A. J. Am.
Chem. Soc. 1982, 104, 1530. (d) Bouma, W. J.; Poppinger, D.; Radom, L.
Isr. J. Chem. 1983, 23, 21. (e) Wayner, D. D. M.; Boyd, R. J.; Arnold, D.
R. Can. J. Chem. 1985, 63, 3283; Can. J. Chem. 1983, 61, 2310. (f) Du,
P.; Hrovat, D. A.; Borden, W. T. J. Am. Chem. Soc. 1988, 110, 3405. (g)
Krogh-Jespersen, K.; Roth, H. D. J. Am. Chem. Soc. 1992, 114, 8388-
8394.
Discussion
The results discussed in this paper elucidate several diverse
aspects, including the structure of the vinylcyclopropane radical

10958 J. Am. Chem. Soc., Vol. 118, No. 45, 1996
Herbertz and Roth
Scheme 1
between the strained ring and the vinyl group. They are
formally related to structure type A, i.e., one cyclopropane bond
participates in delocalizing spin and charge; the steric integrity
of the three-membered ring is conserved in the radical cations.
The participating bond is determined by stereoelectronic effects
and the degree of substitution at the two allylic cyclopropane
bonds. Thus, in the case of 12•+, the internal cyclopropane bond
is involved, whereas for 11•+ and 13•+, the lateral allylic bond
is weakened.
structure (type A) with one lengthened (but not broken) C-C
bond. This structure type has been assigned to many cyclo-
propane radical cations, based on either CIDNP2a,b,21 or low-
temperature ESR spectra,22 and they are consistent with various
electron transfer induced ring opening reactions.7
The CIDNP effects observed upon irradiation of chloranil
(CA) in the presence of 2 and 3, respectively, allow the
unambiguous assignment of the respective radical cation
structures. Photoinduced electron transfer from cis- and trans-2
3
or 3 to CA* generates radical ion pairs (eq 5); hyperfine-
induced intersystem crossing (eq 6) forms singlet pairs which
recombine regenerating the reactants (eq 7). Essentially identi-
cal polarization patterns are observed for cis- and trans-2; strong
emission of the four methyl groups supports positive spin density
for the cylopropane and olefinic carbons, C-2 and C-2′,
respectively; weakly enhanced absorption for the olefinic proton
(H-1′) indicates weak positive spin density at the adjacent carbon
(C-1′). These results suggest radical cations, cis- and trans-
2•+, in which spin and charge are delocalized between the
olefinic p-orbital and the Walsh orbital of the tri-substituted
cyclopropane bond, with significant spin density at C-2 and C-2′
and very weak spin density at C-1′. The suggested structures
are analogous to that of 13•+.
2
In the alternative B2 component the antisymmetrical mo-
lecular orbital a is singly occupied. The resulting structure has
two lengthened C-C bonds; its cyclopropane fragment re-
sembles a “π-complex” (type B).1e,2a,b,f This structure type may
be stabilized by substitution at a single carbon as in the parent
vinylcyclopropane radical cation.23 The substrates discussed
here are 1,1,2-trisubstituted cyclopropanes; precedent2a,b and ab
initio MO calculations20g suggest that this substitution pattern
favors a structure of the “one-electron-bonded trimethylene” type
(A).
The structures of three vinylcyclopropane radical cations have
been assigned previously based on (chemically induced) nuclear
spin polarization (CIDNP) effects.14 These effects may be
induced, when photoinduced electron transfer reactions (eq 5)
are carried out in the probe of an NMR spectrometer. The
competing processes (eq 6-8), delineated in Scheme 1, give
rise to a pattern of enhanced absorption or emission lines in
the resulting NMR spectrum.
The polarization patterns observed for the reaction products
are determined by four parameters, including the sign and
magnitude of the hyperfine coupling constants (A) of the nuclei
in the paramagnetic intermediate, the relative magnitude of the
g-factor of the intermediate (∆g), the initial spin multiplicity
of the radical ion pair (which is dictated by the precursor spin
multiplicity; µ), and the mode of product formation (ꢀ).24-27
For three radical cations, 11•+-13•+, in which the two
functionalities are held rigidly in a syn orientation, three of the
polarization-determining parameters are known with certainty
(µ, ꢀ > 0; ∆g < 0), the hyperfine coupling pattern can be
derived from the observed polarization pattern, revealing the
spin density distribution and, hence, the structure of the radical
cationic intermediate.2f The three radical cations have in
common that spin and charge are significantly delocalized
The structure of radical cation, 3•+, is also quite similar.
Strong emission of all methyl signals supports major spin density
on C-2′ and C-1; spin and charge must be delocalized between
the olefinic and the trisubstituted cyclopropane bond. The
polarization of the geminal protons, H-3syn,anti, supports spin
density on an adjacent carbon, C-1. The weak emission of H-2
rules out significant spin density at the allylic cyclopropane
carbon, C-2.
Intramolecular Nucleophilic Capture. The intramolecular
reactivity of cis- and trans-2•+, particularly its regiochemistry,
are of significant interest. The reactions of several radical
cations of terpenoid vinylcyclopropane systems with (external)
nucleophiles10a,12 reveal three factors that may influence the
regiochemistry of capture: (1) the attack occurs at a carbon
bearing spin and charge density; (2) the attack occurs with relief
of ring strain; and (3) the attack generates a delocalized (allylic)
free radical. This precedent and the fact that the spin density
distribution of cis-2•+ is similar to that of other vinylcyclopro-
pane radical cations may lead to the conclusion that the
hydroxymethyl moiety attacks the quaternary carbon of the
three-membered ring, forming the oxetane system, 14•. This
reaction is an intramolecular SN2 process with an allyl radical
as a leaving group, tethered to the newly formed ring.
(21) (a) Roth, H. D.; Schilling, M. L. M. J. Am. Chem. Soc. 1980, 102,
7956-7958. (b) Roth, H. D.; Schilling, M. L. M. J. Am. Chem. Soc. 1983,
105, 6805.
(22) (a) Iwasaki, M.; Toriyama, K.; Nunome, K. J. Chem. Soc., Chem.
Commun. 1983, 202. (b) Qin, X. Z.; Snow, L. D.; Williams, F. J. Am. Chem.
Soc. 1984, 106, 7640-7641. (c) Qin, X. Z.; Williams, F. Tetrahedron 1986,
42, 6301-6313.
(23) Herbertz, T.; Roth, H. D. Unpublished results.
(24) Closs, G. L. AdV. Magn. Reson. 1974, 7, 157-229.
(25) Kaptein, R. AdV. Free Radical Chem. 1975, 5, 319-480.
(26) Adrian, F. J. ReV. Chem. Intermed. 1979, 3, 3-43.
(27) Freed, J. H.; Pederson, J. B. AdV. Magn. Reson. 1976, 8, 2-84.

Electron Transfer Photochemistry of Chrysanthemol
J. Am. Chem. Soc., Vol. 118, No. 45, 1996 10959
nature of the vinylcyclopropane radical cation allows the
substitution to proceed even with a weak nucleophile such as
methanol. The stereochemical and chiral features of our
substrate clearly document the stereochemical course of the
reaction. Since the key chiral carbon atom (C-1) acts as a pivot
in the conversion of 2 to 4, the observed significant optical
rotation of the SN2′ product is expected.
The geometry of the newly formed double bond of free
radical, 18• (and product 4), is of interest because it is expected
to reflect the conformation of the vinyl group relative to the
cyclopropane moiety in 2•+. Since the terminal olefinic carbon
and the cyclopropane ring are sterically congested (the syn
conformer lies ∼80 kJ mol-1 higher than the anti conformer;
MM3), 2 should exist predominantly as the anti conformer. This
may lead to the expectation that 2•+ should form a free radical
with a trans double bond.
Intramolecular nucleophilic capture has precedent in the
trapping of acyclic,28,29 monocyclic,30 or bicyclic31,32 radical
cations by carboxylic acid28 or alcohol functions.29-32 For
example, the radical cationic site of 15•+ or 16•+ is captured by
an alcohol function tethered to the system.31,32 However, there
is no precedent for nucleophilic capture via a four-membered
transition state; thus, cis-verbenol, 17, failed to undergo in-
tramolecular capture upon electron transfer.33 Given the strain
inherent in the potential trapping product 14• and that a four-
membered transition state is required for its formation, we
consider attack of the internal nucleophile at the terminal olefinic
carbon, C-2′.
The olefinic coupling constant of 4 (J ) ∼15.4 Hz) lies near
the upper limit of the range (5 Hz < Jcis < 14 Hz) typical for
cis olefins,36 and no unusual coupling constants have been
reported for known derivatives of 5,6-dihydro-2H-pyran, includ-
ing the 2,2-dimethyl derivative.37 On the other hand, trans-
cyclohexene systems assigned on the basis of emission spectra38
or invoked as intermediates39 have low barriers to cis-trans
isomerization (∼30 kJ mol-1) and very short lifetimes (∼10
µs).38 Thus, it is unlikely that the trans olefin can be isolated
at room temperature, even if it is formed initially.
Since the preferred conformer of cis-2 is not accessible via
intramolecular trapping, we investigated the electron transfer
photochemistry of cis-2 in the presence of methanol as an
external nucleophile. Only one new product, 6, is formed in
this reaction; its formation can be explained via the acyclic allyl
radical (19•) and exclusive coupling of the sensitizer radical
anion at the tertiary allyl terminus. Apparently, the -CH2OH
and 2-methoxypropyl moieties of 19• impede access to the
monosubstituted allyl terminus at C3. The (trans) geometry of
product 6 reflects the anti species as the preferred conformer
of cis-2•+, because the allyl radical (19•) should be configura-
tionally stable for its limited lifetime.40
The reaction of cis-2 with 1,4-dicyanobenzene and phenan-
threne in acetonitrile generates a 1-(p-cyanophenyl)-1-methyl-
ethyl-substituted dihydro-2H-pyran (4) as the major product.
No evidence for oxetane (14-C6H4-p-CN) formation was
observed. Apparently, nucleophilic attack via a four-membered
transition state is significantly disfavored relative to a six-
membered transition state. The final step in this reaction, an
aromatic substitution of free radical 18• on the sensitizer radical
anion, is unexceptional.7a-c,9,34 The significant finding concerns
the structure of the oxygen heterocycle formed in the preceding
step. To our knowledge, the intramolecular capture of 2•+ is
the only example in which a nucleophile attacks a vinylcyclo-
propane10 (or vinylcyclobutane)9 system at the terminal olefinic
carbon. The unprecedented course of this reaction demonstrates
that the reaction favors release of ring strain and avoids
introducing additional strain. This reaction is an intramolecular
SN2′ process with a tertiary carbon radical as a leaving group
tethered to the newly formed ring.
The diamagnetic prototype of the SN2′ reaction has been
documented most elegantly by the stereochemical consequences
in isotopically labeled or chiral substrates.35 The intramolecular
nucleophilic capture discussed here is the first SN2′ reaction of
a radical cation. The oxacyclohex-3-ene (dihydro-2H-pyran)
structure clearly establishes the SN2′ reaction as a viable
mechanism in radical cation chemistry. The electropositive
The competitive formation of products 4 and 6 at low
methanol concentrations makes it possible to estimate a rate
(36) (a) Friebolin, H. Basic One- and Two-Dimensional NMR Spectros-
copy, 2nd ed.; VCH Weinheim, 1993. (b) Gu¨nther, H. NMR-Spectroscopy;
Wiley & Sons: New York, 1980.
(37) (a) Olsson, L.-I.; Claesson, A. Synthesis 1979, 743-745. (b)
Delaunay, J.; Lebouc, A.; Riobe, O. Bull. Soc. Chim. Fr. Pt. 2 1979, 547-
551.
(38) (a) Bonneau, R.; Joussot-Dubien, J.; Salem, L.; Yarwood, A. J. J.
Am. Chem. Soc. 1976, 98, 4329-4330. (b) Bonneau, R.; Joussot-Dubien,
J.; Yarwood, A. J.; Pereyre, J. Tetrahedron Lett. 1977, 235-238.
(39) (a) Kropp, P. J. J. Am. Chem. Soc. 1966, 88, 4091-4092. (b) Kropp,
P. J. J. Am. Chem. Soc. 1967, 89, 1126-1134. (c) Kropp, P. J.; Krauss, H.
J. J. Am. Chem. Soc. 1967, 89, 5199-5208. (d) Kropp, P. J. Organic
Chemistry, Pawda, A., Ed., Marcel Dekker, New York, 1979; Vol. 4, p 1.
(40) Korth, H.-G.; Trill, H.; Sustmann, R. J. J. Am. Chem. Soc. 1981,
103, 4483.
(28) (a) Gassman, P. G.; Bottorff, K. J. J. Am. Chem. Soc. 1987, 109,
7547-7548. (b) Gassman, P. G.; De Silva, S. A. J. Am. Chem. Soc. 1991,
113, 9870-9872.
(29) (a) Jiang, Z. Q.; Foote, C. S. Tetrahedron Lett. 1983, 24, 461-
464. (b) Dai, S.; Wang, J. T.; Williams, F. J. Chem. Soc., Perkin Trans. 1
1989, 1063-1064.
(30) (a) Du, X.; Arnold, D. R. Unpublished results, quoted in ref 9b. (b)
Arnold, D. R.; McManus, K. A.; Coskun, D. Private communication.
(31) Weng, H.; Roth, H. D. Unpublished results.
(32) Adam, W.; Sendelbach, J. J. Org. Chem. 1993, 58, 5316-5322.
(33) Zhou, D.; Roth, H. D. Unpublished results.
(34) Arnold, D. R.; Snow, M. S. Can. J. Chem. 1988, 66, 3012-3026.
(35) Magid, R. M. Tetrahedron 1980, 36, 1901-1930.

10960 J. Am. Chem. Soc., Vol. 118, No. 45, 1996
Herbertz and Roth
constant for intramolecular capture from the respective yields,
æ4 and æ6, viz.,
failed to show any reaction, whereas in acetonitrile the radical
cation, 3•+, forms an adduct with the solvent.
kintra ) kinter[MeOH](æ4/æ6)
(9)
A rate constant, k ) 3.2 × 108 M-1 s-1, has been measured
for the intermolecular capture of 1,1-diphenyl-2,2-dimethyl-
cyclopropane radical cation by methanol.7g Assuming a similar
rate constant for the intermolecular capture of cis-2•+, the
intramolecular capture should occur with an estimated rate
constant of kintra ≈ 2 × 106 s-1; apparently, intramolecular
capture is not very favorable, most likely because of steric
factors.
Electron Transfer Photochemistry of 2,2-Dimethyl-1-(2-
methyl-1-propenyl)cyclopropane. We extended our studies
to the electron transfer photochemistry of 2,2-dimethyl-1-(2-
methyl-1-propenyl)cyclopropane, 3. The nucleophilic capture
of 3•+ should occur at a carbon bearing spin and charge density,
i.e., at C-1 or C-2′. Attack at either carbon would break the
cyclopropane ring, generating either an allylic free radical (20•,
attack at C-1) or a tertiary one (21•, attack at C-2′). The
intramolecular capture of cis-2•+ suggests that reaction at C-2′
may be competitive; the precedence of several bicyclic vinyl-
cyclopropane systems10,12 and the intermolecular capture of cis-
2•+ lead us to expect attack at C-1.
The capture of radical cations by weak nucleophiles, such as
acetonitrile, has been documented previously for vinylcyclo-
propane12 and vinylcyclobutane9 systems. The available pre-
cedent leads one to expect an adduct of structure 9. However,
the NMR data of the product suggest a different structure, 2,5,5-
trimethyl-3-(2-methylpropenylidene)-1-pyrroline, 10. The for-
mation of this product is readily rationalized by a prototropic
shift that moves the isobutenyl double bond into conjugation
with the imine function during chromatography on silica gel.
The NOCAS products, 7 and 8, resulting from the reaction
of 3 show that the radical cation, 3•+, captures methanol
exclusively at C-1, generating 20•; this result provides yet
another example of a highly regiospecific nucleophilic capture.
The subsequent aromatic substitution with DCB•- occurs
preferentially at the tertiary terminus of 20•, generating 7. The
fact that 8 is formed as a minor product shows that the reaction
of 20• is less regiospecific than that of 19•. Apparently, the
1-methoxy-1-methylethyl substituent shields the secondary
terminus of 20• less effectively than the pair of substituents
shields that of 19•.
The structure of 7 rests on the 2D-COSY spectrum; cross
peaks between one olefinic signal (δ 5.52 ppm) and the methyl
resonance (δ 1.4 ppm) which also interacts with the o-aryl
protons (δ 7.45 ppm) clearly establish the connectivity of these
nuclei. The COSY spectrum is incompatible with structure 22
and, thus, rules out the alternative intermediate, 21•. The olefinic
coupling constant (J ) 15.6 Hz) identifies 7 as a trans alkene,
suggesting that 3•+ exists predominantly as the anti conformer.
The rearranged acetonitrile adduct, 10, is tentatively identified
as the E isomer because of the relatively large coupling constant
between H4 and H1' (4J4,1′ ) 2.5 Hz); this value lies near the
upper limit for allylic coupling constants. Given that the protons
H-4 have torsional/dihedral angles near (30° relative to the
olefinic p-orbitals, their coupling lies closer to the values
expected for a transoid (4J ∼ 2.5-3.0 Hz) than a cisoid
geometry (4J ∼ 1.5-2 Hz).41 Molecular mechanics (MM3)
calculations indicate that the energy of the E isomer lies ∼2
kcal/mol below that of the Z isomer. Nevertheless, the
4
assignment remains tentative, because J couplings offer less
insight into the stereochemistry of double bonds than 3J coupling
constants.
In summary, the vinylcyclopropane radical cation 3•+ reacts
with nucleophiles, even as weak as acetonitrile, by nucleophilic
capture at the quaternary cyclopropane carbon. Neither a
vinylcyclopropane rearrangement nor a sigmatropic shift are
competitive. The absence of the valence isomerization is readily
explained by the preferred transoid geometry of the radical
cation and by the apparently significant steric hindrance barring
the approach of the dimethyl substituted vinyl group toward
the cyclopropane ring.
Electron Transfer Photochemistry of trans-Chrysan-
themol. Interestingly, the dicyanobenzene-sensitized photo-
reaction of trans-chrysanthemol gives rise to the same product
formed from the cis isomer. The apparent parallel reactivity
of the geometric isomers is interesting; however, optically active
samples show important differences. The electron transfer
reaction of (1R,3S)-(+)-cis-2 produces 4 with substantial optical
rotation and the recovered starting material has fully retained
its optical purity; the steric integrity of the three-membered ring
of cis-2 is conserved in its radical cation. In contrast, the
reaction of (1S,3S)-(-)-trans-2 produces 4 of lower optical
purity; also, recovered starting material has suffered ∼30%
racemization (77% conversion); both results offer significant
insight.
We also probed the potential involvement of intramolecular
reactions, such as a vinylcyclopropane rearrangement,6 or a [1,3]
sigmatropic shift.10a,12 A vinylcyclopropane rearrangement of
3 is expected to produce 3,3,4,4-tetramethylcyclopentene (23)
whereas a [1,3] sigmatropic shift would generate 2,6-dimeth-
ylhepta-2,4-diene (24). The involvement of these reactions was
probed by carrying out the photoreaction between DCB and 3
in the absence of an alcoholic nucleophile, i.e., in pure
acetonitrile or nitromethane. The irradiation in nitromethane
The conversion of trans-2 to 4 requires rotation around one
C-C bond, before capture by the internal nucleophile (-CH2-
OH). This reaction can be accommodated in a radical cation
(41) Sternhell, S. Q. ReV. 1969, 23, 236-270.

Electron Transfer Photochemistry of Chrysanthemol
J. Am. Chem. Soc., Vol. 118, No. 45, 1996 10961
with one weakened bond (trans-2•+(C2‚‚‚C3)), whose structure is
supported by our CIDNP results. The isomerization of trans-
2•+ may proceed also by a two-bond rotation, around C1-C3
and C1-C2, as established for 1-phenyl-2-vinylcyclopropane
radical cation.6a In the case of trans-2•+, we cannot distinguish
one-bond from two-bond rotation since the rotation around C1-
C2 interchanges two identical groups.
Figure 3. Stereoview of trans-3-ethenyl-1-hydroxymethyl-2,2-di-
methylcyclopropane radical cation (25•+) calculated at the MP2/6-
31G* level of theory. The intermediate has cyclopropane bonds of three
distinctly different bond lengths (Å).
Therefore, either a single rotation around C1-C3 or a two-
bond rotation around C1-C3 and C1-C2 is expected to produce
optically pure 4 because in either case the stereochemical
integrity of C1 is conserved. The reduced optical purity of
product 4 and the partial racemization of trans-2 require at least
one competing pathway.
is expected to alter the FMO energy of the vinyl fragment and,
thus, affect the extent of interaction with the cyclopropane
function. However, the relative contribution of the C1---C3 and
C2---C3 bonds to delocalizing spin and charge in 25•+ vs trans-
2•+ should be less significantly affected.
The partial racemization of the starting material requires two
rotations; it can be explained, for example, by concurrent (two-
bond) rotation around C1-C2 and C2-C3 in a bifunctional
radical cation, in which the C1-C3 bond is broken or weakened
(2•+(C1‚‚‚C3)). On the other hand, two consecutive single-bond
rotations can be eliminated. One-bond rotation around C1-
C2 or C2-C3, respectively, would convert trans-2 to cis-2 with
either inversion or retention at C1. If one of these reactions
were prominent, the reverse reaction, forming trans-2, should
be faster because the heat of formation calculated for cis-2•+
(HF/6-31G*) is 1.9 kcal mol-1 less negative than that of trans-
2•+. Yet, neither conversion of cis-2 to trans-2 nor racemization
of recovered cis-2 is observed. Clearly, the most likely pathway
for the partial racemization involves concurrent rotations around
the C1-C2 and C2-C3 bonds.
Geometry optimization of 25•+ at the MP2/6-31G* level of
theory supports an unsymmetrical (scalene) triangular structure
(Figure 3) with two lengthened (and weakened) bonds, C2-
C3 (1.674 Å) and C1-C3 (1.571 Å). Inasmuch as the calculated
structure can be extrapolated to trans-2•+, it is compatible with
all experimental results in the electron transfer photochemistry
of trans-2 and reconciles any potential discrepancies. The
longest (weakest) bond (C2-C3) determines the major electron
spin density distribution; accordingly, it is reflected in the
CIDNP results (Figure 2). This bond also allows concurrent
rotations about C1-C2 and C1-C3, converting trans-2•+ to
cis-2•+. The second weakened cyclopropane bond facilitates
concurrent rotations about C1-C2 and C2-C3, explaining the
observed racemization of trans-2•+.
Finally, in the light of the results observed for trans-2•+ and
the calculations for 25•+ and in the interest of “mechanistic
unity”, we note that cis-2•+ may very well also have a second
lengthened bond. However, we emphasize that the experimental
results neither support nor eliminate this assignment. The
CIDNP data (Figure 1) support a structure with electron spin
density primarily at the two dimethyl-substituted carbons and
a weakened bond between the allylic and the quaternary carbon
(cis-2•+(C2‚‚‚C3)). The high retention of optical activity in 4
requires that intramolecular nucleophilic capture is faster than
any rearrangement of the radical cation. The bond rotation
observed for trans-2•+ becomes competitive only because the
option of intramolecular nucleophilic capture has been removed.
In this sense, the loss of optical purity of trans-2 and 4 has
provided significant insight.
In summary, three different experimental results observed for
trans-2•+ require the existence of two different weakened
cyclopropane bonds. The partial racemization of trans-2 is
readily accommodated by a lengthened C1---C3 bond, whereas
the reduced optical purity of 4 as well as the CIDNP results
suggest that the C2---C3 bond is weakened. Combined, these
conclusions suggest that trans-2•+ has an unsymmetrical
(scalene) triangular structure.20g This possibility was probed
by ab initio calculations42 using an extended basis set, including
p-type polarization functions on C (6-31G*), and appropriate
levels of electron correlation (MP2). In order to limit the
significant computing resources required for calculations at this
level, we chose a slightly simpler model, in which the two
methyl groups at the vinyl terminus are omitted, i.e., we
optimized the structure of trans-3-ethenyl-1-(hydroxymethyl)-
2,2-dimethylcyclopropane radical cation (25•+). This change
Conclusion
(42) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.;
Johnson, B. G.; Robb, M. A.; Cheeseman, J. R.; Keith, T.; Peterson, G. A.;
Montgomery, J. A.; Raghavachari, K.; Al-Laham, M. A.; Zakrzewski, V.
G.; Ortiz, J. V.; Foresman, J. B.; 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. P.; Stewart, J. P.; Head-
Gordon, M.; Gonzalez, C.; Pople, J. A. GAUSSIAN 94; Gaussian, Inc.,
Pittsburgh, PA.
The electron transfer photochemistry of three vinylcyclopro-
pane systems illustrates the reactivity and structure of the
respective radical cations and reveals the stereochemistry of their
reactions. The radical cation of 2,2-dimethyl-1-(2-methyl-1-
propenyl)cyclopropane, 3, is captured by methanol at the
quaternary cyclopropane carbon, a result anticipated from

10962 J. Am. Chem. Soc., Vol. 118, No. 45, 1996
Herbertz and Roth
available precedent. This reaction is favored by release of ring
strain and by the formation of a delocalized (allylic) free radical.
Optically pure (1R,3S)-(+)-cis-chrysanthemol (cis-2) forms
an oxacyclohexene, 4, with high optical purity. The alcohol
function of cis-2•+ reacts by nucleophilic attack on the terminal
vinyl carbon, presumably because the alternative intramolecular
attack on the quaternary cyclopropane carbon has an unfavorable
(four-center) transition state. The conversion of cis-2•+ to 4•
amounts to an SN2′ process, unprecedented in all of radical
cation chemistry. The intermolecular capture of cis-2•+ by
methanol competes with the intramolecular capture, yielding
an estimated rate constant for this process.
The electron transfer photochemistry of (1S,3S)-(-)-trans-
2) generates 4, apparently via cis-2•+; both 4 and recovered
trans-2 are partially racemized. The results are rationalized by
an unsymmetrical (scalene) triangular structure, allowing con-
current rotations around two different pairs of cyclopropane
bonds of trans-2•+. The assignments are supported by ab initio
MO calculations (MP2/6-31G*) on a somewhat simplified
model compound, trans-3-ethenyl-1-(hydroxymethyl)-2,2-di-
methylcyclopropane radical cation (25•+).
Acknowledgment. Financial support of this work by the
National Science Foundation through grant NSF CHE-9414271
and equipment grants NSF CHE-9107839 and CHE-9520633
is gratefully acknowledged; the authors are indebted to Dr. G.
Sluggett for technical assistance with the CIDNP spectra.
Supporting Information Available: NMR spectral assign-
ments and MS data for products 4-8 and 10; procedures for
purification of solvents, photoreactions, isolation, and charac-
terization of products (9 pages). See any current masthead page
for ordering and Internet access instructions.
JA953596C