Catalytic epoxidation of hindered olefins with dioxygen. Fast oxygen atom transfer to olefin cation radicals from nitrogen oxides

Article abstract of DOI:10.1021/ja953786n

Hindered olefins are efficiently converted to epoxides by dioxygen at 25°C in dichloromethane containing catalytic amounts of nitrogen oxides (NO₂, NO⁺, NO, etc.). Nitrogen dioxide also effects the direct (stoichiometric) epoxidation of the same hindered olefins. Olefin cation radicals are spectrally identified as the first observable intermediate, and separate experiments confirm the facile transfer of an oxygen atom from nitrogen dioxide to olefin cation radicals to produce epoxides. At low temperature (-78°C), the epoxidation is rapidly initiated by added 1-electron oxidants such as tris(2,4-dibromophenyl)amine cation radical and nitrosonium (NO⁺). Scheme 3 presents the complete sequence of redox changes that are mediated by the nitrogen oxides in the catalytic conversion of hindered olefins to epoxides via the cation radical. The deliberate irradiation of the charge-transfer absorption band of the corresponding olefin electron donor-acceptor complexes with tetranitromethane also establishes the formation of epoxides to occur via the same reactive pair, i.e., the olefin cation radical and NO₂. The mechanistic implication of rapid oxygen atom transfer to olefin cation radicals is underscored in the general consideration of catalytic epoxidations with dioxygen.

Full text of DOI:10.1021/ja953786n

J. Am. Chem. Soc. 1996, 118, 1319-1329
1319
Catalytic Epoxidation of Hindered Olefins with Dioxygen. Fast
Oxygen Atom Transfer to Olefin Cation Radicals from Nitrogen
Oxides
Eric Bosch and Jay K. Kochi*
Contribution from the Chemistry Department, UniVersity of Houston, Houston, Texas 77204-5641
ReceiVed NoVember 10, 1995X
Abstract: Hindered olefins are efficiently converted to epoxides by dioxygen at 25 °C in dichloromethane containing
catalytic amounts of nitrogen oxides (NO2, NO+, NO, etc.). Nitrogen dioxide also effects the direct (stoichiometric)
epoxidation of the same hindered olefins. Olefin cation radicals are spectrally identified as the first observable
intermediate, and separate experiments confirm the facile transfer of an oxygen atom from nitrogen dioxide to olefin
cation radicals to produce epoxides. At low temperature (-78 °C), the epoxidation is rapidly initiated by added
1-electron oxidants such as tris(2,4-dibromophenyl)amine cation radical and nitrosonium (NO+). Scheme 3 presents
the complete sequence of redox changes that are mediated by the nitrogen oxides in the catalytic conversion of
hindered olefins to epoxides Via the cation radical. The deliberate irradiation of the charge-transfer absorption band
of the corresponding olefin electron donor-acceptor complexes with tetranitromethane also establishes the formation
of epoxides to occur Via the same reactive pair, i.e., the olefin cation radical and NO2. The mechanistic implication
of rapid oxygen atom transfer to olefin cation radicals is underscored in the general consideration of catalytic
epoxidations with dioxygen.
Introduction
in 50% yield. Frei and co-workers irradiated a mixture of NO2
and ethylene in an argon matrix at 10 K and detected ethylene
oxide by low-temperature infrared spectroscopy.8,9
The direct oxidation of olefins by transfer of an oxygen atom
from the common reagent nitrogen dioxide is potentially a
straightforward transformation to epoxides. On the basis of the
readily available thermodynamic data,1 ethylene epoxidation by
NO2 shown in eq 1 is exothermic by 11.4 kcal mol-1 and the
We recently observed the facile reaction of nitrogen dioxide
with the sterically hindered olefin adamantylideneadamantane
in dichloromethane solution to afford a quantitative yield of
adamantylideneadamantane oxide at 25 °C (in the dark).10 We
now report that catalytic amounts of nitrogen dioxide are
sufficient to effect the epoxidation of related hindered olefins
with dioxygen. These studies will demonstrate that at low
temperatures the epoxidation is initiated by 1-electron oxidants,
and that the olefin cation radical can be spectrally identified as
the critical intermediate. In this regard, it should be noted that
an electron-transfer mechanism has been proposed11 for the
epoxidation of olefins by cytochrome P-450 analogues, although
no direct evidence for the electron-transfer process was pre-
sented. However, Bauld and Mirafzal12 recently showed that
tris(4-bromophenyl)amine cation radical was an effective initia-
tor in olefin epoxidation with selenium dioxide or selenic
anhydride as the oxygen atom donor.
driving force is -∆G ) 11.0 kcal mol-1 at 25 °C. Despite
these favorable energetics, the thermal epoxidation of ethylene
and various alkenes according to the stoichiometry in eq 1 is
unknown at ambient temperature. Instead, the treatment of
olefins with nitrogen dioxide generally results in a variety of
nitrogen oxide adductssincluding dinitro, nitro/nitrite, nitro/
nitrate and nitroso/nitrate derivatives.2,3 Allylic substitution
products are detected in very dilute solutions.4,5 The epoxidation
represented by eq 1 has only been achieved by photochemical
activation of the reactants. Thus, Jaffe6 irradiated a gaseous
mixture of nitrogen dioxide and ethylene diluted with dinitrogen
and found traces of ethylene oxide (0.004%). Nojima and co-
workers7 subsequently irradiated mixtures of cyclohexene and
NO2 in air with a xenon lamp and isolated cyclohexene oxide
Results
We reasoned that the epoxidation of olefins with NO2 would
predominate if the formation of nitrogen oxide adducts were
restricted by the presence of bulky substituents. Accordingly,
we prepared a series of adamantane-based olefins, 1-6,13 and
a series of sterically crowded tetraarylethylenes, 7-9.14
X Abstract published in AdVance ACS Abstracts, February 1, 1996.
(1) Weast, R. C., Lide, D. R. Eds. Handbook of Chemistry and Physics,
70th ed.; CRC Press: Boca Raton, FL, 1989.
(2) (a) Chatterjee, J.; Coombes, R. G.; Barnes, J. R.; Fildes, M. J. J.
Chem. Soc., Perkin Trans. 2 1995, 1031. (b) Shechter, H. Rec. Chem. Prog.
1964, 25, 55. (c) Reibsomer, J. L. Chem. ReV. 1945, 36, 157. (d) Bonetti,
G. A.; de Savigny, C. B.; Michalski, G.; Rosenthal, R. J. Org. Chem. 1968,
33, 237. (e) Wilkes, J. B.; Wall, R. G. J. Org. Chem. 1980, 45, 247.
(3) Larson, H. O. In The Chemistry of the Nitro and Nitroso Groups;
Feuer, H., Ed.; Interscience: New York, 1969; pp 319 ff.
(4) (a) Pryor, W. A.; Lightsey, J. W.; Church, D. F. J. Am. Chem. Soc.
1982, 104, 6685. (b) Giamalva, D. H.; Kenion, G. B.; Church, D. F.; Pryor,
W. A. J. Am. Chem. Soc. 1987, 109, 7059.
(5) Brand, J. C. D.; Stevens, I. D. R. J. Chem. Soc. 1958, 629.
(6) (a) Jaffe, S.; Keith, J. J. Chem. Phys. 1968, 48, 2805. (b) Jaffe, S.;
Grant, R. C. S. J. Chem. Phys. 1969, 50, 3477.
(7) Nojima, K.; Ohya, T.; Kanno, S.; Hirobe, M. Chem. Pharm. Bull.
1982, 30, 4500.
(8) (a) Nakata, M.; Frei, H. J. Am. Chem. Soc. 1989, 111, 5240. (b)
Nakata, M.; Frei, H. J. Phys. Chem. 1989, 93, 7670. (c) Fitzmaurice, D. J.;
Frei, H. J. Phys. Chem. 1992, 96, 10308. (d) Blatter, F.; Frei, H. J. Phys.
Chem. 1993, 97, 1178. (e) Frei, H. Chimia 1991, 45, 175.
(9) See also: (a) Loison, J. C.; Dedonder-Lardeux, C.; Jouvet, C.; Solgadi,
D. J. Phys. Chem. 1991, 95, 9192. (b) Tanaka, N.; Kajii, Y.; Shibuya, K.;
Nakata, M. J. Phys. Chem. 1993, 97, 7048.
(10) Bosch, E.; Kochi, J. K. J. Chem. Soc., Chem. Commun. 1993, 667.
(11) (a) Traylor, T. G.; Miksztal, A. R. J. Am. Chem. Soc. 1987, 109,
2770. (b) Traylor, T. G.; Xu, F. J. Am. Chem. Soc. 1988, 110, 1953. (c)
He, G.-X.; Arasasingham, R. D.; Zhang, G.-H.; Bruice, T. C. J. Am. Chem.
Soc. 1991, 113, 9828. See also: Kim, T.; Mirafzal, G. A.; Liu, J.; Bauld,
N. L. J. Am. Chem. Soc. 1993, 115, 7653.
(12) Bauld, N. L.; Mirafzal, G. A. J. Am. Chem. Soc. 1991, 113, 3613.
0002-7863/96/1518-1319$12.00/0 © 1996 American Chemical Society

1320 J. Am. Chem. Soc., Vol. 118, No. 6, 1996
Bosch and Kochi
a
Table 1. Epoxidation of Olefins with NO2
b
olefin
E°1/2
NO2
time convc epoxided
(amt, mmol) (V vs SCE) (amt, mmol) (h)
(%)
(%)
1 (0.26)
1 (0.26)e
1 (0.26)f
1 (0.26)g
1 (0.26)h
2 (0.06)
3 (0.25)
4 (0.20)
5 (0.20)
7 (0.20)
8 (0.09)
8 (0.09)n
1.52
1.52
1.52
1.52
1.52
1.56
1.56
1.43
1.16
1.37
1.12
1.12
0.30
0.30
0.30
0.30
0.30
0.14
0.30
0.22
0.22
0.22
0.10
0.10
1
1
1
1
1
100
13
9
12 (96)
12 (95)
12 (95)
12 (95)
12 (90)
13 (92)
15 (55)i
16 (35)j
17 (28)k
21 (<3)l
22 (69)m
22 (2)o
I. Stoichiometric Oxidation of Sterically Hindered Olefins
with Nitrogen Dioxide. Adamantylideneadamantane 1 was
treated with 1.1 equiv of nitrogen dioxide15 in dichloromethane
solution under an argon atmosphere for 1 h at 25 °C. The simple
removal of the solvent and nitrogen oxides in Vacuo led to
quantitative yields of epoxide 12, which upon spectral (NMR,
IR, and GC-MS) analysis was found to be free of organic
impurities (< 2%). UV-vis and IR analyses of the gaseous
products confirmed the concomitant formation of nitric oxide
by its characteristic spectral bands at λ ) 204, 214, and 226
9
<5
100
100
90
100
100
100
5
2
24
12
0.3
1
0.1
4.0
a All reactions with [NO2] ) 0.1 M in dichloromethane at 25 °C,
unless stated otherwise. b Reversible oxidation potential in dichlo-
nm and ν ) 1904, 1876, and 1851 cm-1 16
respectively, i.e.
,
-
romethane solution with 0.1 M (n-Bu)4N+PF6 at V ) 0.1 V s-1 and
25 °C. c Consumed olefin determined by quantitative GC. d Yields based
on consumed olefin determined by quantitative GC (see the Experi-
mental Section). e Reaction in carbon tetrachloride solvent. f Reaction
-
in cyclohexane solvent. g Nitric oxide (0.3 mmol) added. h Bu4N+NO3
(0.3 mmol) added. i 18 (45%) also formed. j 19 (55%) also formed.
k 20 (62%) also formed. l 1-(4-Nitrophenyl)triphenylethylene (95%)
formed. m 25 (16%) and a nitro product also formed (see the Experi-
mental Section). n PPN+NO3- (0.2 mmol) added. o 26 (2%) also formed.
The oxidation of 1 was severely retarded in tetrachloromethane
and cyclohexane solutions, and after 1 h, the epoxide 12 was
isolated in only 13% and 9% yield, respectively. The oxidation
of the series of hindered olefins 2-9 was thus uniformly
performed in dichloromethane solutions containing 1.1-1.2
equiv of NO2. Under these standard conditions, even the less
rigid analogue 2 was quantitatively transformed to epoxide 13
in less than 2.5 h. The treatment of benzhydrylideneadamantane
3 with NO2 yielded a mixture of epoxide 15 (55%) and the
spiroketone 18 (45%), i.e.
Tetraphenylethylene (7) was nitrated almost quantitatively
by NO2, and (p-nitrophenyl)triphenylethylene was isolated in
92% yield, but only traces of tetraphenylethylene oxide (21)
(< 3%) were detected by GC-MS analysis of the crude reaction
product.17 By contrast, the reaction of tetra-p-tolylethylene (8)
with NO2 yielded tetra-p-tolylethylene oxide (22) as the major
product in 69% yield, together with minor amounts of tri-p-
tolyl-4-methylacetophenone (25) (16%) and an unidentified
nitroarene (10%), i.e.
The combination of NO2 with the related unsymmetrical
olefins 4 (Ar ) p-MeC6H4) and 5 (Ar ) p-MeOC6H4) also
yielded mixtures of the corresponding epoxides and rearranged
ketones (see Table 1). The rates of oxidation of 3, 4, and 5
were directly correlated with the donor strength of the olefin,
as assessed by the reversible 1-electron oxidation potentials
recorded at a platinum electrode (see Table 1, column 2). For
example, 3 (E°1/2 ) 1.56 V vs SCE) required more than 24 h
for 100% conversion, while 4 (E° ) 1.43 V) afforded 22 after
12 h, and 5 (E° ) 1.16 V) was completely consumed after only
30 min, under otherwise identical reaction conditions. Treat-
ment of the less hindered isopropylideneadamantane 6 with NO2
resulted in a complex mixture of products which was not fully
characterized.
where Ar ) p-MeC6H4.
The reactivity of olefins with NO2 was not amenable to a
direct kinetics analysis owing to strong retardation by the
gaseous product NO. For example, the reaction of adamantyl-
ideneadamantane with NO2 in the presence of added nitric oxide
(under anaerobic conditions) led to a sharply diminished
conversion of adamantylideneadamantane oxide (see entry 4,
Table 1). The epoxidation of 1 and 8 with NO2 was also
severely retarded by the presence of nitrate anion (see entries 5
and 12 in Table 1).
II. Spectral Observation of the Olefin Cation Radical as
a Reactive Intermediate. The oxidations of all the olefins in
Table 1 were characterized by the formation of highly colored
solutions. For example, immediately upon mixing NO2 and 8
in dichloromethane, a transient purple color was observed, and
the solution then faded to pale yellow. Since the purple color
(13) (a) Belluci, G.; Bianchini, R.; Chiappe, C.; Marioni, F.; Ambrosetti,
R.; Brown, R. S.; Slebocka-Tilk, H. J. Am. Chem. Soc. 1989, 111, 2640.
(b) Nugent, W. A. J. Org. Chem. 1980, 45, 453. (c) Garratt, D. G.
Tetrahedron Lett. 1978, 1915. (d) Olah, G. A.; Schilling, P.; Westerman,
P. W.; Lin, H. C. J. Am. Chem. Soc. 1974, 96, 3581. (e) Wieringa, J. H.;
Strating J.; Wynberg, H. Tetrahedron Lett. 1970, 4579. (f) Strating, J.;
Wieringa, J. H.; Wynberg, H. J. Chem. Soc., Chem. Commun. 1969, 907.
Note that the allylic hydrogens in olefins 1-5 are coplanar with the olefin
and are thus not susceptible to abstraction or deprotonation (see: Nelsen,
S. F. Acc. Chem. Res. 1987, 20, 269).
(14) The aromatic rings are tilted with respect to the central double bond
sufficient to cause tetraarylethylenes to be severely hindered. (a) Suzuki,
H. Bull. Chem. Soc. Jpn. 1960, 33, 389. (b) Suzuki, H.; Koyano, K.; Shida,
T.; Kira, A. Bull. Chem. Soc. Jpn. 1979, 52, 2794. See also: Bock, H.;
Nather, C.; Havlas, Z. J. Chem. Soc., Chem. Commun. 1995, 1111.
(15) For brevity, we do not distinguish between the monomeric nitrogen
dioxide (NO2) and the dimeric dinitrogen tetraoxide (N2O4) which coexist
in a highly mobile equilibrium.
(16) (a) Marmo, F. F. J. Opt. Soc. Am. 1953, 43, 1186. (b) Fateley, W.
G.; Bent, H. A.; Crawford, B., Jr. J. Chem. Phys. 1959, 31, 204. (c) Keck,
D. B.; Hause, C. D. J. Mol. Spectrosc. 1980, 27, 465.
(17) For aromatic nitrations with NO2, see: Bosch, E.; Kochi, J. K. J.
Org. Chem. 1994, 59, 3314.

Catalytic Epoxidation of Hindered Olefins with Dioxygen
J. Am. Chem. Soc., Vol. 118, No. 6, 1996 1321
Table 2. Epoxidation of Olefins with Dioxygen Catalyzed by
Nitrogen Oxidesa
amt
(mmol)
amt
(mmol)
amt
(mmol)
olefin
catalyst
epoxide
1
1
1
2
4
5
8
2.00
0.26
0.26
0.36
0.50
0.50
1.50
NO2
NO
0.10
0.05
0.10b
0.04
0.05
0.05
0.10
12
12
12
13
16
17
22
1.94
0.26
0.24
0.34
0.17c
0.14d
0.93e
NO+
NO2
NO2
NO2
NO2
a All reactions in dichloromethane at 25 °C for 14 h (see the
Experimental Section). b Reaction at 25 °C for 40 min. c 19 (0.34 mmol)
also formed. d 20 (0.32 mmol) also formed. e 25 (0.31 mmol) and a
nitroarene (∼0.010 mmol) also formed.
Table 3. Electron-Transfer Catalysis of Olefin Epoxidation with
a
NO2
olefin
NO2
oxidantb
time
productc
(amt, mmol) (amt, mmol) (concn, mol %) (min) (yield, %)
Figure 1. Spectral changes attendant upon the mixing of 0.003 M
tetratolylethylene and 0.004 M NO2 in dichloromethane at -5 °C: (a)
immediately upon mixing, (b) after 2 min, and (c) prior to workup at
15 min. The dotted curve is the residual (low-energy) tail of the nitrogen
dioxide absorption. Inset: Absorption spectrum of authentic tetratolyl-
ethylene cation radical hexachloroantimonate in dichloromethane
solution.
1 (0.24)
1 (0.20)
2 (0.06)
3 (0.15)
4 (0.20)
1.00
1.00
0.14
0.30
0.40
DB (1)
NO+ (5)
DB (8)
DB (3)
DB (4)
20
50
40
90
5
12 (93)
12 (94)
13 (97)
15 (55)d
16 (36)e
a All reactions in dichloromethane at -78 °C. b DB ) tris(2,4-di-
bromophenyl)aminium hexachloroantimonate; NO+ ) NO+BF4-
.
c Yields
determined by GC-MS using internal standard; in each reaction, the
olefin was totally consumed (see the Experimental Section). d 18 (40%)
also formed. e 19 (58%) also formed.
was more persistent at lower temperatures, the reaction profile
could be readily examined by UV-vis absorption spectroscopy
at -5 °C, as shown in Figure 1. The initial intense color was
spectrally characterized as a broad absorption band centered at
λmax ) 516 nm, which was identical with that obtained from
an authentic crystalline sample of the hexachloroantimonate salt
of tetra-p-tolylethylene cation radical18 (Vide infra). We
concluded therefore that the cation radical 8‚+ was the reactive
intermediate in epoxide formation, i.e.
2. The other olefins listed in Table 2 were also subjected to
these catalytic conditions, and while 2 was quantitatively
converted to epoxide 13 in high yield, the olefins 4, 5, and 8
yielded mixtures of epoxides and ketones. Although the reaction
conditions were not optimized, catalytic turnovers of up to 40
were readily obtained under these mild conditions.
In order to gain further understanding of both the stoichio-
metric and catalytic oxidations, we investigated the selective
activation of the epoxidation at low temperature as follows.
IV. Electron-Transfer Catalysis of Olefin Epoxidation
with Nitrogen Dioxide at -78 °C. Olefin epoxidation was
severely retarded merely by lowering the reaction temperature,
as readily indicated by a mixture of 1 and 2 equiv of NO2, which
was stable at -78 °C for extended periods. However, when a
small amount (0.01 equiv) of tris(2,4-dibromophenyl)amine
cation radical as the hexachloroantimonate salt (E°red ) 1.6 V
vs SCE)21 was added to the stable colorless solution at -78
°C, a rapid series of color changes ensued. The green color
due to the amine cation radical was immediately replaced by a
purple-colored solution, which turned brown and then pale blue
within 20 min.22 After removal of the nitrogen oxides and
solvent in Vacuo, the epoxide 12 was isolated in 93% yield (see
Table 3). IR spectral analysis of the head gas led to the
identification of nitric oxide (νNO ) 1876 cm-1),16 i.e.
where Ar ) p-MeC6H4.
III. Nitrogen Dioxide-Catalyzed Epoxidation of Hindered
Olefins with Dioxygen. The well-known aerial oxidation of
the nitric oxide19 formed in the stoichiometric reaction of NO2
with olefins (eq 2) provided the basis for the nitrogen dioxide-
catalyzed oxidation of olefins with dioxygen. Thus, a dichlo-
romethane solution of adamantylideneadamantane 1 (2 mmol)
was saturated with dioxygen and cooled to 0 °C and the Schlenk
flask connected to an O2-filled gas buret. Nitrogen dioxide (0.1
mmol) was added and the consumption of dioxygen monitored
volumetrically. The uptake of dioxygen ceased after 20 mL
(0.89 mmol) of O2 was consumed, and epoxide 12 was isolated
in quantitative yield simply upon the removal of the nitrogen
oxides and the solvent in Vacuo, i.e.
Control experiments confirmed that the amine cation radical
under these conditions quantitatively oxidized adamantyli-
deneadamantane 1 to its purple radical cation 1‚+ which was
readily identified by its diagnostic (broad) absorption band with
Spectral analysis of the gaseous products indicated the presence
of unchanged NO2 (νNO ) 1600 and 1620 cm-1).20
The epoxidation with dioxygen was also catalyzed by small
amounts of other nitrogen oxides such as nitrosonium cation
(NO+) and nitric oxide (NO), as listed in Table 2, entries 3 and
(21) Schmidt, W.; Steckhan, E. Chem. Ber. 1980, 113, 577. See also:
Bell, F. A.; Ledwith, A.; Sherrington, D. C. J. Chem. Soc. C 1969, 2719.
(22) The final blue color is characteristic of N2O3 (λmax ) 690 nm)
formed by combination of NO with the excess NO2. See: (a) Shaw, A.
W.; Vosper, A. J. J. Chem. Soc., Dalton Trans. 1959, 31, 204. (b) Ashmore,
P. G.; Tyler, B. J. Chem. Soc. 1961, 1017.
(18) See also: Shida, T. Electronic Absorption Spectra of Radical Ions;
Elsevier: New York, 1988.
(19) Smith, J. H. J. Am. Chem. Soc. 1943, 65, 74.
(20) Laane, J.; Ohlsen, J. R. Prog. Inorg. Chem. 1980, 27, 465.

1322 J. Am. Chem. Soc., Vol. 118, No. 6, 1996
λmax ) 530 nm,23 i.e.
Bosch and Kochi
The low-temperature epoxidation of olefin 1 with NO2 at -78
°C was also catalyzed by nitrosonium cation as the 1-electron
oxidant. Thus, the addition of 5 mol % NO+ (as the tetrafluo-
roborate salt) to a dichloromethane solution of 1 and NO2 at
-78 °C resulted in the quantitative formation of epoxide 12
within 50 min. The low-temperature oxidation of the olefins
2, 3, and 4 with NO2 in dichloromethane solution was also
catalyzed by the amine cation radical (see Table 3). In
particular, the low-temperature (-78 °C) aminium-catalyzed
oxidation of 3 with NO2 resulted in the epoxide 15 and ketone
18 in excellent yield after only 90 min, i.e.
Figure 2. Bathochromic (and hyperchromic) shifts of the charge-
transfer absorption bands of the 1:1 EDA complexes of 0.15 M
tetranitromethane and 0.04 M 3 (- ‚ -), 0.04 M 4 (- - -), and 0.04 M 5
(‚‚‚) in dichloromethane solution at 25 °C. The solid curve is the tail
absorption of TNM (0.15 M) alone.
By comparison, the same transformation at 25 °C in the absence
of the amine cation radical required more than 24 h to yield
the same mixture of 15 and 18 (see Table 1, entry 7).
V. Stoichiometric Epoxidation of Adamantylideneada-
mantane with Nitryl Cation. When a cold (-40 °C) dichlo-
romethane solution of 1 was added to a slurry of 1 equiv of
nitryl tetrafluoroborate in acetonitrile at -40 °C, the olefin was
rapidly consumed. After 5 min, the colorless slurry was diluted
with ether and washed with water, and epoxide 12 was isolated
in quantitative yield. In a separate experiment, the crystalline
salt was verified as the nitrosonium salt NO+BF4- spectroscopi-
cally (see the Experimental Section), i.e.
donors (D) and tetranitromethane to yield a reactive triad that
was comprised of the donor cation radical, NO2, and trinitro-
methide, i.e.
Thus, the complementary means of generating the olefin cation
radicals in the presence of NO2 was the charge-transfer
activation of analogous olefin donors (D), as follows.26
A. Formation of EDA Complexes of Hindered Olefins
with Tetranitromethane. The observation of orange-brown
solutions immediately upon mixing colorless solutions of olefins
and tetranitromethane (TNM) was indicative of the formation
of 1:1 electron donor-acceptor (EDA) complexes.26
In another experiment, the reaction mixture was allowed to stir
at -30 °C for a prolonged period, and the products were
monitored by periodic removal of aliquots and analysis by
quantitative GC. The results showed that 1 was completely
transformed to epoxide 12 within 5 min. However, the yield
of 12 steadily decreased with the passage of time, and this
change was accompanied by a corresponding increase in the
yield of spiroketone 14. After 1 h, the rearranged ketone 14
was the sole organic product,24 i.e.
The colored solutions remained unchanged over a period of
several hours at room temperature when protected from room
light. UV-vis spectral analysis of the yellow-brown solutions
formed on mixing each of the olefins in Table 1 with TNM
revealed a broad tailing charge-transfer absorption band which
precluded a precise determination of the spectral band energies
(hνCT). The variation of the spectral band energy with the donor
strength of the olefin was however apparent in Figure 2 from
the progressive bathochromic shift of the charge-transfer absorp-
tion bands formed between TNM and the structurally similar
olefins 3, 4, and 5 with increasing donor strengths that were
evaluated from the reversible oxidation potentials of E1/2 ) 1.56,
1.43, and 1.16 V vs SCE, respectively.
B. Selective Photochemical Activation of the Olefin EDA
Complexes with Tetranitromethane. The EDA complexes
formed from TNM and each of the olefins 1, 3, 4, 5, and 8
were selectively activated at 25 °C by the deliberate irradiation
of charge-transfer absorption bands with actinic radiation from
a 250-W mercury lamp. The lamp was fitted with a sharp cutoff
filter so that only light with λmax > 410 nm was transmitted.
Under these controlled conditions (compare Figure 2), there
could be no ambiguity about the adventitious excitation of either
the uncomplexed olefin or TNM. The selective photoactivation
VI. Charge-Transfer Oxidation of Hindered Olefins with
Tetranitromethane by Selective Activation of the EDA
Complex. The critical role of olefin cation radicals and NO2
in the formation of epoxides was probed by the selective
photochemical generation of this reactive pair. Earlier flash
photolysis studies25 established the specific photoactivation
(hνCT) of the charge-transfer absorption bands of electron
donor-acceptor (EDA) complexes between various electron
(23) Nelsen, S. F.; Teasley, M. F.; Kapp, D. L.; Kessel, C. R.; Grezzo,
L. A. J. Am. Chem. Soc. 1984, 106, 791.
(24) (a) Olah and co-workers13d reported the exclusive formation of
ketone 14 on reaction of 1 with nitryl cation, but they did not detect epoxide
12. (b) However, our control experiments showed that epoxide 12 readily
rearranged to 14 in the presence of NO+ (Compare Lopez et al. in ref 31).
(25) Masnovi, J. M.; Kochi, J. K.; Hilinski, E. R.; Rentzepis, P. M. J.
Am. Chem. Soc. 1986, 108, 1126.
(26) (a) Masnovi, J. M.; Kochi, J. K. Recl. TraV. Chim. Pays-Bas 1986,
105, 286. (b) Rathore, R.; Kochi, J. K. J. Org. Chem., in press. (c) Bosch,
E.; Kochi, J. K. Res. Chem. Intermed., in press.

Catalytic Epoxidation of Hindered Olefins with Dioxygen
J. Am. Chem. Soc., Vol. 118, No. 6, 1996 1323
stable dilute solutions (10-3 M) at -78 °C in dichloromethane.
However, the solutions were not stable at higher temperatures,
and the crystalline salt could not be isolated in pure form. Since
the tetra-p-tolylethylene cation radical was more stable even at
25 °C, it was isolated as the hexachloroantimonate salt as
follows.
Tetra-p-tolylethylene Cation Radical Hexachloroanti-
monate. The addition of antimony pentachloride (2 equiv) to
a dichloromethane solution of tetratolylethylene (1 equiv, cooled
in a dry ice/acetone bath) resulted in a deep purple solution.
The addition of a cold mixture of ether and hexane led to a
heavy purple precipitate. The microcrystalline solid was filtered,
washed sequentially with cold ether and hexane, dried in Vacuo,
and stored under an argon atmosphere. The purity of the salt
was established by elemental analysis, and by titrimetric
oxidation of iodide and N-phenylphenothiazine (see the Ex-
perimental Section). The hexachloroantimonate salt of tetra-
p-tolylethylene cation radical was readily soluble in dichlo-
romethane to afford a deep purple solution with λmax 516 nm
(ꢀ ) 14 000 Μ-1 cm-1).
Table 4. Charge-Transfer Epoxidation of Olefins with
Tetranitromethanea
olefin
(amt, mmol)
TNM
(amt, mmol)
time
(h)
convb
(%)
epoxidec
(yield, %)
1 (0.17)
3 (0.25)
4 (0.20)
4 (0.20)
4 (0.20)
5 (0.25)
5 (0.25)
8 (0.20)
0.41
0.33
0.25
0.25
0.25
0.28
0.28
0.30
0.5
4.0
0.2
1.0
2.2
0.2
0.5
0.7
45
70
17
35
65
37
78
70
12 (87)d
15 (45)e
16 (35)f
16 (34)g
16 (32)h
17 (27)i
17 (23)j
22 (59)k
a All oxidations performed in dichloromethane solution at 25 °C by
irradiation with a medium pressure Hg lamp and a 410-nm cutoff filter
(see the Experimental Section). b Based on recovered olefin, determined
by quantitative GC. c Yield of epoxide based on consumed olefin.
d Adamantanone (4%) also formed. e 18 (40%) also formed. f 19 (53%)
also formed. g 19 (57%) also formed. h 19 (46%) and traces of 4,4′-
dimethylbenzophenone (<5%) also formed. i 20 (71%) also formed.
j 20 (70%) also formed. k 25 (35%) also formed.
of the EDA complex of 1 and TNM for 30 min at 25 °C resulted
in the formation of epoxide 12 in 87% yield. UV-vis spectral
analysis of the aqueous washing confirmed the formation of
trinitromethide (λmax ) 350 nm; ꢀ ) 1.4 × 104 M-1 cm-1 27
) in
80% yield based on the amount of 1 converted, i.e.
Tetra-p-tolylethylene cation radical was also readily obtained
by reaction with 1 equiv of nitrosonium cation according to eq
17.28
A control dark experiment with an identical solution (protected
from light) showed no apparent (visible) change after 30 min,
and quantitative GC-MS analysis confirmed the complete
recovery of 1. The charge-transfer oxidations of the related
olefins 3, 4, 5, and 8 with TNM resulted in good yields of the
respective epoxides and ketones as shown in Table 4. In each
case, there was no reaction in the dark control over the period
of the irradiation. Furthermore, there was no change in the
degree of conversion or the relative distribution of the products
whether the reaction mixture was worked up immediately or
left in the dark for 6 h prior to workup and analysis. These
controls showed that the olefin oxidation did not proceed Via a
photochemically initiated chain reaction of the type previously
observed with N-vinylcarbazole.26
The selective irradiation of the charge-transfer absorption
band of the EDA complex formed between tetranitromethane
and tetra-p-tolylethylene (8) at 25 °C resulted in the formation
of a deep purple coloration which was persistent throughout
the irradiation. However, the color faded rapidly when the
solution was removed from the light, but it persisted when the
irradiated solution was cooled to -78 °C. UV-vis spectral
analysis of the purple solution revealed the characteristic
absorption spectrum of the tetra-p-tolylethylene cation radical
(8‚+) with λmax ) 516 nm (compare Figure 1). We thus
concluded that the cation radical 8‚+ was the direct intermediate
in the charge-transfer oxidation of 8 with TNM, i.e.
VIII. Reactivity of Olefin Cation Radicals toward Various
Nitrogen Oxides. If cation radicals were intermediates in the
oxidation of olefins, they must have been transformed to the
oxidized products by another species extant in the reaction
mixture. Accordingly, authentic samples of the cation radicals
1+. and 8+. were directly treated with various nitrogen oxides
as follows.29
Nitric Oxide. Tetratolylethylene cation radical did not react
with NO in dichloromethane solution at 25 °C in the absence
of oxygen. Thus, a purple solution of the tetratolylethylene
cation radical and NO in dichloromethane persisted indefinitely
when protected from air.
Nitrogen Dioxide. A purple solution of 1+.SbCl6- (10-3 M)
was prepared in dichloromethane at -78 °C according to eq 8.
When a dichloromethane solution of NO2 (1 equiv) was added,
the purple color bleached immediately. Quantitative GC-MS
analysis of the colorless solution indicated that epoxide 12 was
formed in 84% yield, and the concomitant production of the
nitrosonium salt was confirmed spectroscopically (see the
Experimental Section), i.e.
In a separate experiment, a dark purple dichloromethane solution
-
of 8+.SbCl6 was added dropwise into a stirred solution of
nitrogen dioxide in dichloromethane at 25 °C. The purple color
VII. Isolation of Crystalline Salts of Olefin Cation
Radicals. The consistent observation of olefin cation radicals
in each of the above oxidation procedures prompted us to
prepare and isolate olefin cation radicals in order to directly
assess their role in the epoxidation. Adamantylideneadamantane
cation radical hexachloroantimonate was prepared in situ as
(28) Our attempts to isolate 8‚+ as the tetrafluoroborate salt have not
been successful. Concentrated solutions of 8‚+ decolorized, and 3,6-dimethyl-
9,10-bis(4′-methylphenyl)phenanthrene was formed.
(29) Reactions with 1‚+ were performed at -78 °C owing to the instability
of the cation radical at higher temperatures. The reactions with 8‚+ were
performed at -5 to +10 °C since the cation radical was persistent in the
presence of nitrogen oxides at -78 °C.
(27) Bielski, B. H. J.; Allen, A. O. J. Phys. Chem. 1967, 71, 4544.

1324 J. Am. Chem. Soc., Vol. 118, No. 6, 1996
Bosch and Kochi
faded, and after 5 min, quantitative GC-MS analysis indicated
that epoxide 22 and ketone 25 were the major products.
Nitrate. A solution containing 1 equiv of PPN+NO3- 30 was
added dropwise to a dark purple dichloromethane solution of
oxygen atoms of dioxygen are utilized.35 Such an autooxidation
is however not general for all olefins since the less congested
analogues yield little or no epoxide (but other products result
from 1,2-addition or allylic hydrogen abstraction). We empha-
size that the hindered nature of olefins 1-11 is designed to
discourage multiple access to the double bond and to render
hydrogen abstraction unfavorable.36 Although the epoxidation
of these hindered olefins is also promoted by nitric oxide and
nitrosonium cation, nitrogen dioxide is unique in its ability to
also be separately used as a stoichiometric reagent for the
epoxidation. Indeed, the direct reduction of NO2 by adaman-
tylideneadamantane in eq 2, coupled with the facile aerial
reoxidation of NO to nitrogen dioxide19 provides the initial basis
for the formulation of NO2 catalysis (see Scheme 1). However,
-
8+.SbCl6 at 10 °C. The color bleached within 5 min, and
quantitative GC-MS analysis indicated that epoxide 22 and
ketone 25 were the major products, i.e.
where Ar ) p-MeC6H4. No reaction was apparent when the
solutions were cooled to -78 °C prior to mixing. In a separate
-
-
experiment, PPN+NO3 reacted with 1+.SbCl6 at -78 °C to
Scheme 1
afford the epoxide 12 and ketone 14 in good yields.
IX. Stability of the Epoxides under the Oxidation Condi-
tions. In the oxidation of olefins under the various conditions
described above, rearranged ketone was consistently isolated
along with the epoxide. In order to determine whether the
ketone was derived from the epoxide, we tested the stability of
epoxides to nitrogen dioxide, nitrosonium cation, and the
intermediate olefin cation radicals as follows.31,32
Nitrogen Dioxide. Epoxide 16 was slowly transformed into
ketone 19 on treatment with 1 equiv of NO2 in dichloromethane
at 25 °C (see the Experimental Section), i.e.
this simple scheme does not explain the catalysis by nitrosonium
cation in Table 2, and it does not account for all the
characteristics of the stoichiometric epoxidation with NO2, such
as (a) the retardation of the oxidation by the NO product (see
entry 4, Table 1) and by added nitrate (entries 5 and 12) and
(b) the spontaneous appearance of the olefin cation radical (see
Figure 1). Clearly any mechanistic formulation must also take
into account the effective initiation of the epoxidation at low
temperatures by 1-electron oxidants (see eqs 7 and 9). In order
to reconcile these diverse observations, let us first focus on how
NO2 in stoichiometric amounts effects the epoxidation of
olefinsswith particular regard to the transient formation of
olefin cation radicals and their subsequent conversion to
epoxides.
I. Oxidative Transformation of Olefins to their Cation
Radicals by Nitrogen Dioxide. The direct observation in
Figure 1 of the cation radical, immediately on mixing tetra-p-
tolylethylene and NO2, provides definitive identification of the
first observable intermediate in the epoxidation. Since NO2
itself is a weak oxidant (E° ≈0.25 V vs SCE),37 the oxidation
of the olefins listed in Table 1 is prohibitively endergonic with
∆GET > 20 kcal mol-1. Therefore, the effective oxidant must
be another species (that is derived from NO2). Indeed, NO2 is
known to exist in solution in equilibrium with its dimeric forms,
namely, the predominant N-N bonded dimer O2N-NO238 and
the N-O bonded minor isomer O2N-ONO,39 i.e.
Nitrosonium Cation. Tetra-p-tolylethylene oxide (22) was
rapidly and quantitatively transformed to ketone 25 on treatment
with 2 mol % nitrosonium hexachloroantimonate in dichlo-
romethane at 0 °C,33 i.e.
Olefin Cation Radicals. Epoxide 22 was also quantitatively
transformed to ketone 25 within 10 min by catalytic amounts
-
of the olefin cation radical 8+.SbCl6 (1 mol %) in dichlo-
romethane at 0 °C.
The efficacy of the rearrangement in eq 22 was further evaluated
by varying both the 1-electron oxidant and the epoxide as
detailed in the Experimental Section.34
Significantly, the minor dimer has been independently shown
to afford nitrosonium nitrate as the ion pair salt.40 Although
the heterolysis of the N-O bond in O2N-ONO is unlikely to
be important in nonpolar media such as dichloromethane, we
Discussion
The novel catalysis of olefin epoxidation by nitrogen dioxide
as described in eq 6 represents a rare example in which both
(35) (a) Compare: Groves, J. T.; Quinn, R. J. Am. Chem. Soc. 1985,
107, 5790. See also Groves, J. T.; Stern, M. K. J. Am. Chem. Soc. 1987,
109, 3812. (b) For a review, see: Sheldon, R. A.; Kochi, J. K. Metal-
Catalyzed Oxidations of Organic Compounds; Academic: New York, 1981.
(36) Indeed, the least hindered olefin in this study, isopropylideneada-
mantane 6, yielded a complex mixture of products which did not include
the epoxide. Note that the allylic hydrogen atoms of 6 are susceptible to
abstraction/deprotonation. See: Nelsen, S. F.; Kapp, D. L.; Teasley, M. F.
J. Org. Chem. 1984, 49, 579.
(37) Eberson, L.; Radner, F. Acta Chem. Scand., Sect. B 1985, 39, 343.
(38) (a) James, D. W.; Marshall, R. C. J. Phys. Chem. 1968, 72, 2963.
(b) Redmond, T. F.; Wayland, B. B. J. Phys. Chem. 1968, 72, 1626. (c)
Brunning, J.; Frost, M. J.; Smith, I. W. M. Int. J. Chem. Kinet. 1988, 20,
957. (d) Vosper, A. J. J. Chem Soc. A 1970, 2191.
(30) Where PPN+ ) bis(triphenylphosphoranylidene)ammonium pre-
pared according to Ruff, J. K.; Schlientz, W. J. Inorg. Synth. 1974, 15, 84.
(31) The cation radical catalyzed rearrangement of epoxides to ketones
has been reported.32
(32) (a) Lopez, L.; Troisi, L. Tetrahedron Lett. 1989, 30, 3097. (b) Lopez,
L.; Mele, G.; Finandanese, V.; Cardellicchio, C.; Nacci, A. Tetrahedron
1994, 50, 9097.
(33) The reaction was heterogeneous since nitrosonium salts are only
sparingly soluble in dichloromethane.
(34) The epoxides were also rapidly isomerized to the corresponding
ketones during cyclic voltammetric measurements as described in the
Experimental Section. For the electron-transfer chain isomerization at the
electrode during cyclic voltammetric studies, see: Lebouc, A.; Tallee, A.;
Simonet, J. J. Chem. Soc., Chem. Commun. 1982, 387. Uneyama, K.;
Isimura, A.; Fujii, K.; Torri, S. Tetrahedron Lett. 1983, 24, 2857.
(39) Pinnick, D. A.; Agnew, S. F.; Swanson, B. I. J. Phys. Chem. 1992,
96, 7092.
(40) (a) Parts, L.; Miller, J. T., Jr. J. Chem. Phys. 1965, 43, 136. (b)
Givan, A.; Loewenschuss, A. J. Chem. Phys. 1991, 94, 7562.

Catalytic Epoxidation of Hindered Olefins with Dioxygen
J. Am. Chem. Soc., Vol. 118, No. 6, 1996 1325
previously demonstrated that the strong complexation of the
nitrosonium moiety by electron-rich aromatic hydrocarbons
promotes ion pair formation,41,42 i.e.
potentials in Table 1) increases in the order 5 > 4 > 3.
Although no direct kinetics analysis was performed, the qualita-
tive rate data in Table 1 show that 5 reacts very quickly (30
min), an intermediate rate is observed for 4 (12 h), and 3 reacts
very slowly (24 h). Such a distinctive trend suggests that
electron transfer is rate-limiting. This conclusion is also
consistent with the equilibria in eqs 27 and 28, since the varied
donor strength of the olefin will exert two synergistic effects
on the overall rate of the electron-transfer oxidation with respect
to (a) the degree of autoionization of NO2, which will be greatest
for 5 and least for 3 owing to a progressively enhanced
complexation of NO+ to stronger donors, and (b) the driving
force for electron transfer, which is exergonic for 5, moderately
so for 4, and almost thermoneutral for 3, as given by -∆G )
F[E°ox(CdC) + E°red(NO+)], where F is the Faraday constant
and E° is the redox potential.
Indeed, the extent of disproportionation of NO2 according to
eq 26 is correlated with the donor strength of the aromatic
hydrocarbon. For example, hexamethylbenzene which is a
strong donor (IP ) 7.85 eV) promotes the ionization of NO2 to
an extent of 80%. On the other hand, the use of the weaker
donor durene (IP ) 8.05 eV) results in less than 25% ion pair
formation. Since all the olefins in Table 1 are uniformly strong
donors, as evidenced by the relatively low oxidation potentials
(E°ox < 1.6 V vs SCE), they are also expected to promote an
analogous ionic disproportionation of NO2, i.e.43
B. Electron-Transfer Initiation of the Epoxidation at Low
Temperature. The rate of the epoxidation of olefins with NO2
is subject to temperature controlsmerely lowering the temper-
ature to -78 °C being sufficient to suppress the epoxidation.
The addition of the strong 1-electron oxidant tris(2,4-dibro-
mophenyl)amine cation radical (E°red ) 1.60 V vs SCE)21 at
this low temperature results in the rapid consumption of olefin
and a quantitative yield of epoxide. It is independently known45
that at -78 °C the aminium salt rapidly oxidizes each of the
olefins in Table 3. Thus, the first-formed cation radical can be
formulated as the critical intermediate in the reaction with NO2,
i.e.
Furthermore, the reduction potential of nitrosonium in dichlo-
romethane (E°red 1.50 V vs SCE)44 is sufficient to effect the
rapid oxidation of the olefins in Table 1, since the redox reaction
in eq 28 is predicted to be moderately exothermic (compare eq
17). The EDA complex is the putative (unobserved) precursor
to the electron-transfer oxidation of the olefin,26 i.e.
Most importantly, the mechanistic formulation in eqs 27 and
28 accounts for (a) the immediate formation of the olefin cation
radical and (b) the marked retardation of olefin epoxidation by
added nitrate and by nitric oxide. Moreover, the facile (alterna-
tive) initiation of the epoxidation by added 1-electron oxidants
circumvents the electron-transfer step in eq 28, and such
oxidants can thus effectively generate higher steady-state
concentrations of olefin cation radicals at low temperature Via
the alternative electron transfer, i.e.
C. Photochemical Epoxidation Wia the Olefin Cation
Radical in the Presence of NO2. An independent verification
of the reactivity of the olefin cation radical in eq 28 is provided
in the charge-transfer irradiation of the EDA complexes of
olefins and tetranitromethane. The latter is based on earlier
studies25,26 that demonstrated the direct irradiation of the charge-
transfer (CT) absorption bands of the EDA complexes of
tetranitromethane and olefins to generate the reactive triad in
eq 32. The isolation of the trinitromethide salt (following the
II. Olefin Cation Radical as the Critical Reactive Inter-
mediate. There are four independent lines of evidence that
identify the olefin cation radical as the critical reactive
intermediate in epoxidation, namely (a) the strong correlation
between the rate of epoxidation and the oxidation potential of
the olefinic substrate, (b) the initiation of epoxidation at low
temperatures by added 1-electron oxidants, (c) the ready
transformation of the olefin cation radical to the epoxide by
nitrogen dioxide, and (d) the alternative formation of epoxides
Via photoactivation of the olefin complex with tetranitromethane.
Let us consider each of these separately in the following way.
A. Structure/Activity Relationship. The effect of the donor
strength on the reactivity of the olefins in Table 1 is best
addressed with the structurally similar olefins 3, 4, and 5, since
they only differ in the remote para-substituent. For these
olefins, the donor strength (as assessed by the oxidation
charge-transfer irradiation in Table 4) indicates that the triad in
eq 32 leads directly to the epoxide Via the collapse of the olefin
cation radical with NO2 according to eq 18.46 Such an olefin
cation radical paired with NO2 must also be common to the
thermal epoxidation and the aminium-catalyzed (low-temper-
ature) epoxidation in eqs 2 and 7, respectively, since the product
distributions in Table 4 are identical to those in Tables 1 and 3.
D. Direct Transformation of Olefin Cation Radicals to
Epoxides with Nitrogen Oxides. Any formulation in which
the olefin cation radical is the critical reactive intermediate in
epoxidation with NO2 requires that it be quantitatively converted
to epoxide after it is formed according to the sequence of
disproportionation/electron-transfer steps in eqs 27 and 28.
Indeed, this requirement for the reactive triad is met by
independent studies that establish the facile transfer of an oxygen
(41) (a) Bontempelli, G.; Mazzochin, G.-A.; Magno, F. Electroanal.
Chem. Interfacial Electrochem. 1974, 55, 91. (b) Wartel, A.; Boughriet,
A.; Fischer, J. C. Anal. Chim. Acta 1979, 19, 811.
(42) Bosch, E.; Kochi, J. K. Res. Chem. Intermed. 1993, 19, 811.
(43) The ionic disproportionation of NO2 is, however, expected to be
less favored in nonpolar solvents, and it will result in a slower reaction.
Several authors have proposed that the reaction of NO2 with olefins in
nonpolar solvents such as carbon tetrachloride and cyclohexane proceeds
Via a radical (addition) mechanism. See Chatterjee et al. in ref 2a, Shechter
in ref 2b, and Pryor in ref 4.
(45) Nelsen, S. F.; Akaba, R. J. Am. Chem. Soc. 1981, 103, 2096. See
also: Nelsen, S. F. Acc. Chem. Res. 1987, 20, 269.
(46) Nitrosonium trinitromethide (NO)C(NO2)3 is presumably the other
product of the photolysis, but the nature or subsequent reactivity of this
ion pair is not known. However, any nitrosonium cation thus formed can
intervene in the reaction pathway.
(44) Lee, K. Y.; Kuchynka, D. J.; Kochi, J. K. Inorg. Chem. 1990, 29,
4196.

1326 J. Am. Chem. Soc., Vol. 118, No. 6, 1996
Bosch and Kochi
atom to the olefin cation radicals 1‚+ and 8‚+ from nitrogen
dioxide in eq 18, i.e.
Scheme 3
and nitrate in eq 19, i.e.
III. Electron Transfer in the Stoichiometric Epoxidation
of Olefins with Nitrogen Dioxide. The formation and reaction
of olefin cation radicals (as described in eqs 28 and 33) provide
the mechanistic basis for the facile transfer of oxygen atom from
NO2 to olefin, as originally outlined in eq 23. The complete
sequence of steps from olefin to epoxide, including the
preequilibrium disproportionation of NO2 in eq 27, is presented
in Scheme 2.47
catalytic epoxidations with dioxygen as the terminal oxidant,
we note that nitrogen oxides in three different oxidation states,
Viz., nitrogen(IV), -(III), and -(II) as NO2, NO+, and NO,
respectively, can be employed with comparable catalytic ef-
ficiencies (see Table 2). Such an interchangeability of NOx
species is consistent with their ready interconversion during the
course of the catalytic cycle. Accordingly, let us initially
proceed from the combination of the simple catalytic process
outlined in eqs 23 and 24 with the mechanistic details provided
by eqs 35-38 (see Scheme 3). According to the catalytic
mechanism in Scheme 3, olefin oxidation by nitrosonium (eq
36) is driven to completion by the efficient interception of nitric
oxide (NO) by dioxygen,19 and the accompanying olefin cation
radical is rapidly converted to epoxide by oxygen atom transfer
from nitrogen dioxide Via eq 37.47,51 Since the active species
in NO2 catalysis is the disproportionated nitrosonium nitrate,
the separate initiation by nitrosonium salt NO+BF4- (see entry
3, Table 2) simply circumvents the initial disproportionation of
NO2 (compare eq 17).52
The catalytic mechanism for olefin epoxidation as presented
in Scheme 3 underscores the importance of electron transfer in
the formation of paramagnetic intermediates. The role of
nitrogen oxides in mediating the electron-transfer processes also
extends to byproduct formation. Indeed, the appearance of
rearranged ketones 18-20, 24, and 25 is common to both the
stoichiometric epoxidation (Table 1) and the catalytic epoxi-
dation (Table 2). Furthermore, its ubiquity extends to the
aminium-induced (low-temperature) epoxidation in Table 3, as
well as to the charge-transfer epoxidation with tetranitromethane
in Table 4. Separate experiments confirm that the isolated
epoxides are rapidly transformed into the corresponding ketones
by catalytic amounts of strong 1-electron oxidants, including
the triarylamine cation radical 8.+ and NO+. [Note that NO2
itself is somewhat less efficient.] On this basis, we conclude
that an electron-transfer mechanism31 for the rearrangement of
the epoxides to ketones is compatible with the results in Table
Scheme 2
According to Scheme 2, the rate of olefin epoxidation is
limited by electron transfer in eq 36, which occurs subsequent
to the rapid disproportionation of NO2. The marked suppression
of epoxidation at low temperatures relates to electron-transfer
reversibility in eq 36, which favors the free olefin at low
temperatures (-78 °C), but the cation radical at 25 °C.48 The
addition of the aminium salt at low temperatures then increases
the concentration of olefin cation radicals, and it results in a
higher steady-state concentration of nitrosonium by virtue of
eq 37.
We believe that the olefin cation radical is uniquely suited
to effect rapid oxygen atom transfer from the paramagnetic NO2
‚
in eq 37, since the intermolecular interaction of +C-C‚ and NO2
is reminiscent of other radical/radical coupling processes with
low activation barriers.49 As such, it can occur in a single step
by the direct transfer of oxygen anion radical (O‚-) or in two
successive steps via a cationic (+C-C-ONO) adduct followed
by oxirane ring closure and loss of NO+.50
IV. Catalytic Mechanism for Olefin Epoxidation with
Dioxygen. In order to gain insight into the nature of the
(47) An alternative formulation involves the reaction of the olefin cation
radical with nitrate to generate the epoxide and NO2 according to eq 34.
However, the actual nitrogen oxide species (NO2 or NO3-) which reacts
with the olefin cation radical will depend on their relative concentrations
and the relative rates of reaction in eqs 33 and 34. For the sake of clarity,
we only present the formulation with NO2, but we do not intend to exclude
the direct involvement of nitrate.
(48) Such a temperature-dependent electron transfer is reminiscent of
that previously observed during aromatic nitration with NO2. We noted then
that the electron transfer between NO+ and dimethoxybenzene (DMB) is
reversible. At ambient temperatures, the radical pair [DMB‚+, NO]
predominates, but at low temperatures the charge-transfer complex [DMB,
NO+] is favored. See: Rathore, R.; Bosch, E.; Kochi, J. K. Tetrahedron
1994, 50, 6722. Note that the temperature dependence of the preequilibrium
disproportionation in eq 35 is expected to follow the reverse trend (compare
Bosch, E.; Kochi, J. K. J. Org. Chem. 1994, 59, 3314).
(50) For examples of concerted and stepwise atom transfers in free radical
chemistry, see: Ingold, K. U.; Roberts, B. P. Free-Radical Substitution
Reactions; Wiley: New York 1971.
(51) Since the aerial oxidation of NO to NO2 is likely to be a multistep
process, the reviewer has suggested that an intermediate peroxynitrite radical
(‚OONO) may also enter separately into the catalytic chain process in
Scheme 3 in indeterminant amounts. Compare the epoxidation of olefins
by alkylperoxy radicals. Marnett, L. J. Carcinogenesis 1987, 8, 1365. See
also: Mayo, F. R. Acc. Chem. Res. 1968, 1, 193.
(52) The cation radical catalyzed chain oxygenation of adamantyli-
deneadamantane with dioxygen has been reported by Nelsen et al. in ref
45 to yield adamantylideneadamantane-dioxetane. In oxygenations cata-
lyzed by nitrosonium and nitryl salts, these authors found adamantyli-
deneadamantane oxide to be the major (isolated) product. They suggested
that epoxide was formed by a nitrosonium-catalyzed decomposition of the
dioxetane. However, we believe that the epoxide formed during the
oxygenations induced by nitrosonium can be ascribed to eq 37. We further
note that the reaction of adamantylideneadamatane cation radical with
dioxygen is generally competitive with its reaction with either nitrogen
dioxide or nitrate. Since the competition (quantitatively) will be a function
of the respective concentrations of reactants, the absolute rates of reactions
and the reversibile nature of the reactions cannot be evaluated at this
juncture.
(49) (a) Ingold, K. U. In Free Radicals; Kochi, J. K., Ed.; Wiley: New
York 1973; Vol. 1, pp 37 ff. (b) Note that the facile (paramagnetic)
interactions of NO2‚ in bimolecular coupling to N2O4 and ionic dispropor-
tionation to NO+NO3 in eqs 25 and 26 also bear a strong parallel to the
-
paramagnetic behavior of alkyl radicals (R‚) in bimolecular coupling (R-
R) and disproportionation [RH + R(-H)] with uniformly low activation
barriers (see: Benson, S. W. AdV. Photochem. 1964, 2, 1).

Catalytic Epoxidation of Hindered Olefins with Dioxygen
J. Am. Chem. Soc., Vol. 118, No. 6, 1996 1327
Table 5. Electron-Transfer-Induced Rearrangement of Epoxides to
at λ ) 204, 214, and 226 nm due to nitric oxide were observed.16a The
solvent and remaining nitrogen oxides were removed in Vacuo, and
the solid residue was recrystallized from ethanol to yield adamantyl-
ideneadamantane oxide 12 as colorless crystals in 96% yield.
Effect of Solvent on the Rate of Epoxidation with NO2. Reactions
were carried out as described above, except the solvent was tetrachlo-
romethane. The progress of the reaction was monitored by quantitative
GC-MS: after 1 h there was 13% conversion to epoxide 12, after 2.5
h there was 30% conversion to 12, and after 24 h 12 was formed
quantitatively. In cyclohexane solution, a 9% conversion of 1 to 12
was observed after 1 h, 27% after 2.5 h, and 100% conversion after 24
h.
Ketonesa
epoxide
reagentb
ketone
conv (%)c
-
-
21
22
22
22
22
22
TT‚+SbCl6
TT‚+SbCl6
24
25
25
25
25
25
20
100
100
100
2
-
TH‚+BF4
-
-
BA‚+SbCl6
MP‚+SbCl6
-
NO+SbCl6
100
a All reactions with epoxide at 1.4 × 10-2 M and oxidant at 1.3 ×
10-4 M in dichloromethane at 0 °C for 10 min. b TT ) tetra-p-
tolylethylene; TH ) thianthrene; BA ) tris(4-bromophenylamine; MP
) N-methylphenothiazine; NO+ ) nitrosonium. c Based on recovered
olefin. In all reactions, the ketone was formed in quantitative yield
based on consumed olefin (see the Experimental Section).
Effect of Additives on the Rate of Epoxidation with NO2. A.
Nitric Oxide. A solution of nitrogen dioxide in dichloromethane (3
mL, 0.1 M) was prepared under an argon atmosphere in a Schlenk
tube capped with a rubber septum. The solution was cooled to -10
°C, and NO (7 mL) was bubbled into the solution with the aid of a
gas-tight syringe. Adamantylideneadamantane 1 (70 mg, 0.26 mmol)
was added, the flask capped with a Teflon stopcock, and the solution
stirred at 25 °C for 1 h. The solvent and nitrogen oxides were removed
in Vacuo, and the solid residue was analyzed by quantitative GC-MS.
Unreacted 1 (91%) and 12 (8.5%) were the only organic products. The
oxidation of 8 (0.09 mmol) with NO2 (0.10 mmol) in dichloromethane
(3 mL) was also severely retarded by added NO (0.10 mmol). After
2 h, there was only 11% conversion of 8 to a mixture of 22 and 25.
B. Nitrate. A solution of nitrogen dioxide in dichloromethane (3
mL, 0.1 M) was prepared under an argon atmosphere in a Schlenk
tube capped with a rubber septum. Tetrabutylammonium nitrate (0.3
mmol) and adamantylideneadamantane 1 (70 mg, 0.26 mmol) were
added sequentially. The flask was capped with a Teflon stopcock and
the solution stirred at 25 °C for 1 h. The solution was diluted with
dichloromethane, washed with water, and dried with anhydrous MgSO4.
Quantitative GC-MS analysis indicated that 1 was largely recovered
(95%) along with traces of 12 (∼4.5%).
5, i.e.
Thus, the best epoxide donors (e.g., 17) afford the highest yields
of rearranged ketones. By the same token, the weakest epoxide
donors (e.g., 21) are the least likely to produce rearranged
ketones. Furthermore, weak 1-electron oxidants do not catalyze
the rearrangement (see Table 5, entry 5). With the weakest
epoxide donors (i.e., 12 and 13 with E° > 2 V vs SCE), the
amount of rearrangement is minor, and olefin oxidation leads
only to epoxides in high yields.53
Summary and Conclusions
Oxygen atom transfer from nitrogen dioxide directly to
olefins, according to the stoichiometry in eq 1, is (disappoint-
ingly) ineffective. In marked contrast, the corresponding oxygen
transfer to the olefin cation radical is very fast and produces
epoxides efficiently. Such a facile oxygen transfer (eq 33) forms
the basis for the electron-transfer formulation of catalytic
epoxidations with dioxygen as presented in Scheme 3. Thus,
the olefin cation radical and NO are formed by an initial
oxidation of the olefin by nitrogen dioxide (as the dispropor-
tionated ion pair NO+NO3-) in eqs 35 and 36. The subsequent
combination of the olefin cation radical with NO2 generates the
epoxide, together with the chain-transfer oxidant NO+. The
fast aerial oxidation of nitric oxide to NO2 then completes the
catalytic cycle. We predict that the facile oxygen atom transfer
from nitrogen dioxide to olefin cation radicals in eq 33 will
prove to be the prototypical example of a general case in which
catalytic epoxidations are considerably facilitated by the electron-
transfer generation of olefin cation radicals as the reactive
intermediate by other types of (oxidizing) catalytic species.11,12
The other olefins listed in Table 1 were treated according to the
general procedure in dichloromethane solution. The oxidation products
were isolated by TLC or flash chromatography, and the characteristic
physical data are included in the supporting information.
Nitrogen Dioxide-Catalyzed Epoxidation of Olefins with Oxygen.
General Procedure. A solution of adamantylideneadamantane 1 (536
mg, 2 mmol, 20 mL) in dichloromethane (20 mL) was prepared in a
flask fitted with a side arm. The solution was cooled to 0 °C in an ice
bath and saturated with oxygen and the flask sealed with a Teflon
stopper. An O2-filled gas buret was attached to the side arm, and a
dichloromethane solution of nitrogen dioxide (1.0 mL, 0.1 mmol) was
added with the aid of an all-glass syringe. The temperature of the
solution was carefully controlled at 0 °C, and the uptake of oxygen
was monitored. After 12 h, 20 mL (0.89 mmol) of oxygen was
consumed, and the uptake of oxygen ceased. The head gas was
transferred to an evacuated 5-cm IR gas cell and the presence of NO2
confirmed by the characteristic absorptions at ν ) 1629 and 1604
cm-1 20
The reaction was worked up as described above and adaman-
.
tylideneadamantane oxide 12 (556 mg, 97%) isolated as a colorless
solid.
Experimental Section
Electron-Transfer Catalyzed Oxidation of Olefins with Nitrogen
Dioxide. General Procedure. A dichloromethane solution of ada-
mantylideneadamantane 1 (180 mg, 0.67 mmol, 60 mL) was prepared,
under an argon atmosphere, in a Schlenk flask and cooled to -78 °C
in a dry ice/acetone bath. A green solution of tris(2,4-dibromophenyl)-
aminium hexachloroantimonate (7 mg, 0.006 mmol, 5 mL) in dichlo-
romethane was prepared in a separate Schlenk flask and cooled to -78
°C under an argon atmosphere. The solution of the aminium cation
radical was rapidly transferred by cannula into the cold solution of 1
which immediately became purple. A cold (-78 °C) dichloromethane
solution of NO2 (0.5 mmol, 10 mL) was then rapidly added with the
aid of a cannula. The resultant mixture was stirred at -78 °C for 20
Epoxidation of Olefins with Nitrogen Dioxide. General Proce-
dure. A solution of nitrogen dioxide in dichloromethane (3 mL, 0.1
M) was prepared under an argon atmosphere in a Schlenk tube fitted
with a Teflon stopcock. Adamantylideneadamantane 1 (70 mg, 0.26
mmol) was added, the flask sealed with the aid of a Teflon stopcock,
and the solution stirred at 25 °C for 1 h. The solution was degassed,
and the gases were transferred to an evacuated 5-cm gas IR cell. The
IR spectrum thus recorded exhibited the characteristic absorbances at
1852, 1875, and 1904 cm-1 of nitric oxide.16b The gases from the IR
cell were then transferred to an evacuated 1-cm quartz cuvette, and
the UV absorption spectrum was recorded. The three sharp absorbances
min, whereupon the solution turned blue [the absorbance at λmax
)
(53) However, we hasten to add the caveat that the mechanistic distinction
been electron-transfer and electrophilic (acid) catalysis54 is difficult to
rigorously delineate. See: Rathore, R.; Kochi, J. K. J. Org. Chem. 1995,
60, 7479.
(54) The acid-catalyzed pinacol rearrangement of epoxides, in particular
adamantylideneadamantane oxide, is well known (see: Wynberg, H.;
Boelema, E.; Wieringa, J. H.; Strating, J. Tetrahedron Lett. 1970, 3613).
690 nm corresponded to the formation of N2O3 by combination of NO
and NO2].22 The gaseous products were transferred to an evacuated
5-cm IR cell, and the characteristic absorbances of NO (νNO ) 1876
16b
20
cm-1
)
and NO2 (νNO ) 1629, 1601 cm-1
)
were observed. After
complete removal of the nitrogen oxides in Vacuo, the solution was
slowly warmed to 0 °C and the solvent evaporated in Vacuo. The crude

1328 J. Am. Chem. Soc., Vol. 118, No. 6, 1996
Bosch and Kochi
residue was analyzed by quantitative GC-MS and found to contain
adamantylideneadamantane oxide 12 (0.629 mmol, 94%), adamantyl-
ideneadamantane 1 (< 0.004 mmol), and adamantanone (∼0.003
mmol). A control (“uncatalyzed”) reaction was performed as follows:
An identical solution of 1 and NO2 in dichloromethane was prepared
in a separate Schlenk flask (but without the electron-transfer catalyst)
and stirred at the same temperature for the same time and then worked
up in the same way. The solution remained colorless throughout the
reaction time, and there was no evidence for the formation of either
NO or N2O3 [IR and UV-vis spectroscopic analysis]. The olefin 1
(0.636 mmol, 95%) was recovered, and less than 0.004 mmol of epoxide
12 was detected by quantitative GC-MS analysis. The other olefins
listed in Table 3 were treated according to the above procedure at the
concentrations and times listed. In all cases, the olefin was recovered
intact (>95%) from the uncatalyzed (control) reaction.55
of 5 and TNM (0.05 M) was irradiated for 25 min (see Table 4, entry
7) and the solution then divided into two portions. One portion was
immediately worked up and analyzed. The second portion was stored
in the dark for 5 h before analysis. There was no significant difference
in the final analyses. During the irradiation of 8 and TNM (see Table
4, entry 8), an intense purple color developed which dissipated after
the solution was stirred for a few minutes in the dark. On cooling the
solution to -78 °C, the purple color persisted. The UV-vis absorption
spectrum confirmed the formation of 8‚+ (λmax ) 516 nm). Indeed,
when the irradiation was performed at -78 °C, a dark purple colored
solution was formed. However, the conversion was <5% even after 4
h.
Reaction of Tetra-p-tolylethylene Cation Radical Hexachloro-
antimonate with Nitrogen Oxides. Nitrate. A Schlenk flask was
charged with tetratolylethylene cation radical hexachloroantimonate
(110 mg, 0.152 mmol) in the drybox and the flask capped with a rubber
septum. The flask was removed from the drybox and dichloromethane
(60 mL) added with the aid of a syringe. The resultant dark purple
solution was cooled to 0 °C in an ice bath. A cold (0 °C) solution of
PPN+NO3- (91 mg, 0.152 mmol) in dichloromethane (5 mL) was added
with the aid of a Teflon cannula under argon pressure. The purple
color was quenched over the course of 5 min, the resultant pale yellow
solution was washed with water (3 × 25 mL) and dried, and the solvent
was removed in Vacuo. Quantitative GC-MS and NMR analysis of
the crude reaction product indicated that tetratolylethylene oxide was
formed in 62% yield. In addition, the rearranged ketone (20%) was
observed together with traces of the nitroarene (previously detected),
three chlorinated products (M+ m/z 433), and a product tentatively
identified as 3,6-dimethyl-9,10-bis(4′-methylphenyl)phenanthrene which
was >95% pure by 1H and 13C NMR analysis (see the supporting
information). In a separate reaction, a dark purple solution of 8‚+
Epoxidation of Adamantylideneadamantane with Nitryl Tet-
rafluoroborate. A Schlenk flask was charged with nitryl tetrafluo-
roborate (130 mg, 1 mmol) in a drybox and the flask capped with a
rubber septum. The flask was removed from the drybox and dry
acetonitrile (5 mL) added under a flow of argon with the aid of a
hypodermic syringe. A dichloromethane solution of adamantylidene-
adamantane 1 (268 mg, 1 mmol, 5 mL) was prepared under an argon
atmosphere in a separate Schlenk flask, and both flasks were cooled to
-40 °C in a dry ice/acetone bath. The dichloromethane solution of 1
was then rapidly transferred by cannula into the acetonitrile solution
-
of NO2+BF4 and the mixture stirred at this temperature for 5 min. A
mixture of aqueous bicarbonate and ether was added and the mixture
vigorously stirred as it warmed to room temperature. The organic layer
was washed with water and dried and the solvent removed in Vacuo.
Adamantylideneadamantane oxide 12 was obtained as a colorless
crystalline solid (264 mg, 93%). A second identical reaction was diluted
with dichloromethane after 5 min, an excess of 18-crown-6 (5 mmol)
was added, and the UV-vis absorption spectrum of the dilute solution
was recorded. The characteristic absorption spectrum of the complex
of nitrosonium with 18-crown-6 (λmax ) 300 nm)56 confirmed the
-
SbCl6 was cooled to -78 °C under an argon atmosphere, and a
precooled (-78 °C) dichloromethane solution of PPN nitrate was added
with the aid of a cannula. There was no apparent color change over
the period of 1 h, but the color rapidly bleached upon warming to 0
°C.
formation of NO+ (0.9 mmol) by comparison with a standard solution
-
of 18-crown-6 and NO+BF4
.
-
Nitrogen Dioxide. A Schlenk flask was charged with 8‚+SbCl6
A separate experiment was performed as described above at -40
°C with 134 mg (0.5 mmol) of 1 and 65 mg of NO2+BF4- (0.5 mmol),
but the reaction mixture was stirred at -30 °C for a prolonged period.
During this time, a series of aliquots were extracted and analyzed by
quantitative GC. The yield of epoxide 12 steadily decreased after 5
min, with a concomitant increase in the yield of a second product. After
1 h, there was no epoxide, and the spiroketone 14 was isolated in
quantitative yield (147 mg, 0.49 mmol) as a white solid.
(72 mg, 0.10 mmol) in the drybox and the flask capped with a rubber
septum. Dichloromethane (20 mL) was added, and the resultant purple
solution was added dropwise, with the aid of a cannula, to a cool (5
°C) dichloromethane solution of nitrogen dioxide (0.15 mmol, 80 mL).
The purple color completely faded after the addition was complete.
The solution was washed with water and dried and the solvent removed
in Vacuo. The pale yellow residue, analyzed by quantitative GC-MS,
contained epoxide 22 (19.6 mg, 0.048 mmol), acetophenone 25 (9.1
mg, 0.022 mmol), and 3,6-dimethyl-9,10-bis(4′-methylphenyl)phenan-
threne. The relative yield of the phenanthrene increased when the
reaction was performed at higher concentrations.
Photochemical Oxidation of Olefins with Tetranitromethane Wia
the Selective Activation of the EDA Complex. General Procedure.
Tetranitromethane (50 µL, 0.41 mmol) was added, under a flow of
argon, to a dichloromethane solution of 1 (45 mg, 0.17 mmol, 5 mL)
in a quartz cuvette. The cuvette was sealed with a Teflon stopcock
and the solution irradiated at 25 °C with light from a medium-pressure
250-W Hg lamp passed through a water filter and a 410 nm ESCO
sharp cutoff filter. This ensured that only the charge-transfer absorption
band was irradiated. After 30 min, the solution was diluted with
dichloromethane, washed with water, dried, and analyzed by quantitative
GC-MS. The epoxide 12 (18.6 mg, 0.065 mmol), 1 (24.8 mg, 0.093
mmol), and adamantanone (2 mg, 0.013 mmol) were observed as the
only organic products [the conversion of 1 was thus 45%]. The
combined aqueous washings were diluted to 2500 mL, and the UV-
vis absorption spectrum was measured. The concentration of trini-
tromethide was estimated to be 5.2 × 10-5 mol (λmax ) 350 nm, ꢀ )
14 000 M-1 cm-1),27 and it corresponded to 80% of the conversion of
1. A second identical solution of TNM and 1 was prepared, under an
argon atmosphere, in a separate Schlenk tube and stored at 25 °C in
the dark for the duration of the irradiation. This “dark” control was
worked up simultaneously with the irradiated solution and the olefin 1
recovered quantitatively. Analysis of the aqueous washings indicated
that less than <0.1% of trinitromethide was formed.
Reaction of Adamantylideneadamantane Cation Radical with
Nitrogen Oxides. Nitrogen Dioxide. A dichloromethane solution of
antimony pentachloride (0.6 mL, 0.5 M, 0.3 mmol) was added, under
a flow of argon, to a vigorously stirred dichloromethane solution of
tris(2,4-dibromophenyl)amine (85 mg, 0.11 mmol, 2 mL) in a Schlenk
flask. The resultant deep green solution was stirred at 25 °C for 10
min. Hexane (10 mL) was added with the aid of a hypodermic syringe
and the resultant green precipitate allowed to settle. The supernatant
solution was carefully removed by cannula using argon pressure and
the solid washed twice in this manner with hexane (2 × 10 mL). The
dark green solid was dissolved in 20 mL of dichloromethane and cooled
to -78 °C and the solution transferred dropwise by cannula into a cold
(-78 °C) dichloromethane solution of adamantylideneadamantane 1
(27.2 mg, 0.10 mmol, 80 mL) under an atmosphere of argon. The
mixture which immediately became purple was stirred at -78 °C for
10 min, and a cold (-78 °C) dichloromethane solution of nitrogen
dioxide (1.1 mL, 0.11 mmol) was added by cannula under a flow of
argon. The color bleached to pale green within 1 min. The solution
was immediately diluted with ether and washed with water (2 × 25
mL). The organic layer was separated and dried and the solvent
removed in Vacuo. The residue was analyzed by quantitative GC-
MS: Adamantylideneadamantane oxide 12 (23.3 mg, 0.08 mmol, 81%),
adamantylideneadamantane 1 (0.5 mg, 2%), spiroketone 14 (0.6 mg,
2%), and an unidentified chlorinated product [GC-MS m/z 302
(M+,35Cl, 100), 304 (M+,37Cl, 34]. A second reaction was performed
Reaction of the other olefins was similarly performed at the
concentrations and times listed in Table 4. A dichloromethane solution
(55) The low conversions (<5%) observed in the control reactions may
have occurred adventitiously during workup.
(56) Elsenbaumer, R. L. J. Org. Chem. 1988, 53, 437.

Catalytic Epoxidation of Hindered Olefins with Dioxygen
J. Am. Chem. Soc., Vol. 118, No. 6, 1996 1329
in the same manner, but the reaction was not worked up. The colorless
solution obtained after addition of NO2 was further diluted and 18-
crown-6 (0.5 mmol) added. The UV-vis absorption spectrum con-
firmed the formation of nitrosonium (0.08 mmol) by the characteristic
absorbance at λmax ) 300 nm.56
Nitrate. A dichloromethane solution of adamantylideneadamantane
1 radical cation (0.102 mmol, 80 mL) was prepared with tris(2,4-
dibromophenyl)aminium hexachloroantimonate at -78 °C, as described
-
above. A cold (-78 °C) solution of PPN+NO3 (65 mg, 0.11 mmol)
in dichloromethane (20 mL) was rapidly added with the aid of a cannula,
and the purple color bleached immediately. The colorless reaction
mixture was worked up and analyzed as described above. Adaman-
tylideneadamantane oxide 12 (13.1 mg, 0.05 mmol, 46%), adamantyl-
ideneadamantane 1 (5.9 mg, 21%), the spiroketone 14 (4.1 mg, 14%)
and an unidentified chlorinated product [GC-MS m/z 302 (M+, 35Cl,
100), 304 (M+, 37Cl, 34] were formed.
Stability of Epoxides. Nitrogen Dioxide. A dichloromethane
solution of epoxide 16 and 1 equiv of NO2 was stirred at 25 °C, and
aliquots were periodically removed for GC-MS analysis. Rearranged
ketone 19 was detected in 15% conversion after 2 h. Under similar
conditions, adamantylideneadamantane oxide 12 underwent <3%
conversion to the rearranged ketone 14.
Figure 3. Square wave voltammograms of (a) epoxide 22 and (b)
ketone 25 (both at 5 × 10-3 M) in dichloromethane containing 0.2 M
tetra-n-butylammonium hexafluorophosphate as the supporting elec-
trolyte.
Nitrosonium. A Schlenk flask was charged with nitrosonium
hexachloroantimonate (3 mg, 0.008 mmol) in the drybox, and the flask
was sealed with the aid of a rubber septum. The flask was removed
from the glovebox and dichloromethane (8 mL) added to the flask under
a flow of argon. A dichloromethane solution of tetratolylethylene oxide
22 (50 mg, 0.124 mmol, 3 mL) was prepared, under an argon
atmosphere, in a separate Schlenk flask and transferred with the aid of
a cannula into the stirred slurry of nitrosonium salt. After 10 min, the
slurry was diluted with dichloromethane (25 mL). The dichloromethane
extract was washed with aqueous sodium bicarbonate and water and
dried over magnesium sulfate, and the solvent was removed in Vacuo
to yield the rearranged ketone 25 in 91% yield. Adamantylideneada-
mantane oxide 12 underwent a 20% conversion to ketone 14 when
treated with 20 mol % NO+BF4- at 25 °C in dichloromethane for 2 h.
Cation Radicals. A dichloromethane solution of 22 (50 mg, 0.124
mmol, 5 mL) was prepared, under an argon atmosphere, in a Schlenk
evaluated by measuring the oxidation potential of some representative
epoxides at a platinum electrode as 5 × 10-3 M solutions in
dichloromethane containing 0.2 M tetrabutylammonium hexafluoro-
phosphate as the supporting electrolyte. It was particularly noteworthy
that significant amounts of the rearranged ketone were observedseven
on simply recording the cyclic voltammograms at scan rates of 1 V
s-1. The cyclic voltammograms (CV) were comprised of two irrevers-
ible anodic waves, the more positive of which was unequivocally
assigned to that of the rearranged ketone by CV comparison with that
of the authentic ketone.34 The first anodic wave was assigned to the
epoxide. The ratio of the two anodic waves was dependent on the CV
scan ratesepoxide predominanting at higher scan rates and the ketone
predominanting at lower scan rates, as shown in Figure 3. The anodic
peak potentials for the irreversible oxidation of epoxides ranged from
1.3 V vs SCE for epoxide 21 to more than 2.0 V for 12. [Note the
latter did not undergo rearrangement in either the stoichiometric or
catalyzed oxidation described above.]
-
flask sealed with a rubber septum. A purple solution of 8‚+SbCl6
(9.5 mg, 0.013 mmol) in dichloromethane (2 mL) was added by cannula
and the resultant purple solution stirred at room temperature for 20
min. Since there was no visible color change, the reaction was
quenched by addition of zinc dust (50 mg) into the vigorously stirred
mixture. After the purple color bleached, the slurry was diluted with
dichloromethane (25 mL), washed sequentially with saturated aqueous
sodium bicarbonate, water, and brine, and dried over magnesium sulfate.
The solvent was removed in Vacuo. The residue consisted of rearranged
ketone 25 (93%) and tetratolylethylene oxide 22 (7%), in addition to
tetratolylethylene 8 (6%). Rearrangement of tetraphenylethylene oxide
21 yielded R,R,R-triphenylacetophenone (24), mp 179 °C (lit.57 mp
179-180 °C).
Acknowledgment. We thank the National Science Founda-
tion, R. A. Welch Foundation, and Texas Advanced Research
Project for financial assistance.
Supporting Information Available: Text describing the
materials and instrumentation used, the preparation and char-
acterization of unsymmetrical adamantylidene analogues, tet-
raarylethylenes, and their oxidation products, the preparation
of tetra-p-tolylethylene cation radical hexachloroantimonate, the
spectral characterization of the precursor EDA complexes
formed from olefins and tetranitromethane, and the cyclic
voltammetry of olefins and epoxides and figures showing the
1H and 13C NMR spectra for 2-(4-methylphenyl)-2-(4-methyl-
benzoyl)adamantane (19) and 3,6-dimethyl-9,10-bis(4′-meth-
ylphenyl)phenanthrene (15 pages). This material is contained
in many libraries on microfiche, immediately follows this article
in the microfilm version of the journal, can be ordered from
the ACS, and can be downloaded from the Internet; see any
current masthead page for ordering information and Internet
access instructions.
The efficacy of the cation radical catalyzed rearrangement was further
evaluated by varying both the 1-electron oxidant and the epoxide. For
example, when tetraphenylethylene oxide 21 was treated with 1 mol
-
% 8‚+SbCl6 under otherwise identical conditions, there was only a
20% conversion of the epoxide to R,R,R-triphenylacetophenone after
10 min. [After 3 h, the rearranged ketone was formed in quantitative
yield.] The rearrangement of epoxide 22 to ketone 25 was efficiently
catalyzed by both thianthrene cation radical58 (E°red ) 1.20 V vs SCE)
and tris(4-bromophenyl)aminium21 (E°red ) 1.16 V vs SCE). However,
N-methylphenothiazine cation radical58 (E°red ) 0.70 V vs SCE) was
not effective, and only 2% rearrangement of 22 to ketone 25 was
observed after 10 min (see Table 5). The thermodynamics were further
(57) Depovere, P.; Devis, R. Bull. Chim. Soc. Fr. 1968, 2470.
(58) Bandlish, B. K.; Shine, H. J. J. Org. Chem. 1977, 42, 561.
JA953786N