Dimesityldioxirane

Article abstract of DOI:10.1021/ja964280n

The preparative scale synthesis of dimesityldioxirane (5) via oxidation of dimesitylcarbene (6) is described. The dioxirane was obtained as colorless crystalline material, completely stable at -20 °C and characterized by spectroscopic methods, X-ray crystallography, and DFT calculations (B3LYP/6-31G(d)). The molecule shows approximately local C₂ symmetry with the mesityl rings twisted by ca. 60°. The experimental structural parameters, e.g., the O-O distance of 1.503 A?, are in good agreement with the theoretical values. The relief of steric strain caused by the mesityl groups is discussed in terms of the geometric distortion of the three-membered ring and the mesityl groups. Conformational features of 5 in solution are nicely reflected by measured and calculated (SOS-DFPT) NMR chemical shifts.

Full text of DOI:10.1021/ja964280n

J. Am. Chem. Soc. 1997, 119, 7265-7270
7265
Dimesityldioxirane
Wolfram Sander,*,† Kerstin Schroeder,† Sengodagounder Muthusamy,†
Andreas Kirschfeld,† Wilhelm Kappert,† Roland Boese,*,‡ Elfi Kraka,*,§
Carlos Sosa, and Dieter Cremer§
Contribution from the Lehrstuhl fu¨r Organische Chemie II der Ruhr-UniVersita¨t,
D-44780 Bochum, Germany, Institut fu¨r Anorganische Chemie der UniVersita¨t-GH Essen,
D-45117 Essen, Germany, Department of Theoretical Chemistry, UniVersity of Go¨teborg,
S-41296 Go¨teborg, Sweden, and Cray Research Inc., Eagan, Minnesota 55121
ReceiVed December 13, 1996. ReVised Manuscript ReceiVed April 22, 1997X
Abstract: The preparative scale synthesis of dimesityldioxirane (5) via oxidation of dimesitylcarbene (6) is described.
The dioxirane was obtained as colorless crystalline material, completely stable at -20 °C and characterized by
spectroscopic methods, X-ray crystallography, and DFT calculations (B3LYP/6-31G(d)). The molecule shows
approximately local C2 symmetry with the mesityl rings twisted by ca. 60°. The experimental structural parameters,
e.g., the O-O distance of 1.503 Å, are in good agreement with the theoretical values. The relief of steric strain
caused by the mesityl groups is discussed in terms of the geometric distortion of the three-membered ring and the
mesityl groups. Conformational features of 5 in solution are nicely reflected by measured and calculated (SOS-
DFPT) NMR chemical shifts.
Introduction
r(O-O) in dioxetanes (1.49-1.55 Å).17,18 The calculation of
reliable structural parameters of 1 requires the inclusion of
correlation effects (at least MP2 theory)19,20 and the use of large
basis sets. At the CCSD(T) level of theory an almost quantita-
tive agreement of theory and experiment was found (Table
1).20,21
The only other dioxirane for which experimental structural
data are reported is difluorodioxirane (4), which is stable in the
gas phase at room temperature.22 High-resolution IR spectros-
copy yields an O-O bond distance of 1.576 Å,23 and a recent
value determined by electron diffraction is 1.578 Å (Table 1).24
Thus, fluorine substitution results in a significant lengthening
of r(O-O), in agreement with theoretical predictions.24-27
During the last decade derivatives of dioxirane (1) have
evolved from laboratory curiosities, stable only at low
temperature,1-4 to versatile oxygen transfer reagents.5-10
A
number of dioxiranes were characterized by IR spectroscopy
in cryogenic matrices.1,3 Dimethyldioxirane (2),11,12 methyl-
(trifluoromethyl)dioxirane (3),13 and several alkyl-substituted
dioxiranes11 were prepared by the Caroate route and are fairly
stable in dilute solution. 2 and 3 are now frequently used in
preparative oxidations. Suenram and Lovas synthesized the
highly labile parent dioxirane (1) via the ozonolysis of ethene
at low temperature and determined the structure by microwave
spectroscopy.14 The O-O bond length is 1.516 Å (Table 1),
which is significantly longer than r(O-O) in H2O2 with 1.49
Å15,16 and other peroxides (1.44-1.50 Å),17 but in the range of
† Ruhr-Universita¨t.
‡ Universita¨t-GH Essen.
§ University of Go¨teborg.
A general route to dioxiranes is the oxidation of triplet
carbenes with molecular oxygen 3O2.3 Singlet carbenes are also
Cray Research, Silicon Graphics Inc.
X Abstract published in AdVance ACS Abstracts, June 15, 1997.
(1) Dunkin, I. R.; Shields, C. J. J. Chem. Soc., Chem. Commun. 1986,
154.
(16) Cremer, D. In The Chemistry of Functional Groups, Peroxides; Patai,
S., Ed.; Wiley: New York, 1983; p 1.
(17) Kalinowski, H.-O. In Struktur Von Peroxo-Verbindungen In Houben-
Weyl; Kropf, H., Ed.; Thieme: Stuttgart, 1988; Vol. E13, pp 5-16.
(18) Adam, W.; Schmidt, E.; Peters, E. M.; Peters, K.; Von Schnering,
H. G. Angew. Chem. 1983, 95, 566; Angew. Chem., Int. Ed. Engl. 1983,
22, 1546.
(19) Bach, R. D.; Andres, J. L.; Owensby, A. L.; Schlegel, H. B.;
McDouall, J. J. W. J. Am. Chem. Soc. 1992, 114, 7207.
(20) Cremer, D.; Gauss, J.; Kraka, E.; Stanton, J. F.; Bartlett, R. J. Chem.
Phys. Lett. 1993, 209, 547.
(21) Kim, S.-J.; Scha¨fer, H. F.; Kraka, E.; Cremer, D. Mol. Phys. 1996,
88, 93.
(22) Russo, A.; DesMarteau, D. D. Angew. Chem. 1993, 105, 956; Angew.
Chem., Int. Ed. Engl. 1993, 32, 905.
(23) Bu¨rger, H.; Weinrath, P.; Argu¨ello, G. A.; Ju¨licher, B.; Willner,
H.; DesMarteau, D. D.; Russo, A. J. Mol. Spectrosc. 1994, 168, 607.
(24) Casper, B.; Christen, D.; Mack, H.-G.; Oberhammer, H.; Argu¨ello,
G. A.; Ju¨licher, B.; Kronberg, M.; Willner, H. J. Phys. Chem. 1996, 100,
3983.
(2) Sander, W.; Kirschfeld, A. In Matrix-Isolation of Strained Three-
Membered Ring Systems; Halton, B., Ed.; JAI Press: London, 1995; Vol.
4, Chapter 2, pp 1-80.
(3) Sander, W. Angew. Chem. 1990, 102, 362; Angew. Chem., Int. Ed.
Engl. 1990, 29, 344.
(4) Bucher, G.; Sander, W. Chem. Ber. 1992, 125, 1851.
(5) Kafafi, S. A.; Martinez, R. I.; Herron, J. T. Mol. Struct. Energ. 1988,
6, 283.
(6) Adam, W.; Curci, R.; Edwards, J. O. Acc. Chem. Res. 1989, 22, 205.
(7) Murray, R. W. Mol. Struct. Energ. 1988, 6, 311.
(8) Adam, W.; Hadjiarapoglou, L. P.; Curci, R.; Mello, R. Dioxiranes,
three-membered ring cyclic peroxides; Ando, Ed. Org. Peroxides 1992,
195-219.
(9) Adam, W.; Hadjiarapoglou, L. Top. Curr. Chem. 1993, 164, 45.
(10) Curci, R.; Dinoi, A.; Rubino, M. F. Pure Appl. Chem. 1995, 67,
811.
(11) (a) Murray, R. W.; Jeyaraman, R. J. Org. Chem. 1985, 50, 2847.
(b) Murray, R. W.; Singh, M.; Jeyaraman, R. J. Am. Chem. Soc. 1992,
114, 1346.
(12) Adam, W.; Chan, Y. Y.; Cremer, D.; Gauss, J.; Scheutzow, D.;
Schindler, M. J. Org. Chem. 1987, 52, 2800.
(13) Mello, R.; Fiorentino, M.; Fusco, C.; Curci, R. J. Am. Chem. Soc.
1989, 111, 6749.
(14) Suenram, R. D.; Lovas, F. J. J. Am. Chem. Soc. 1978, 100, 5117.
(15) Cremer, D. J. Chem. Phys. 1978, 69, 4400.
(25) Cremer, D.; Schmidt, T.; Gauss, J.; Radhakrishnan, T. P. Angew.
Chem. 1988, 100, 431; Angew. Chem., Int. Ed. Engl. 1988, 27, 427.
(26) Catalan, J.; Escudero, F.; Laso, J.; Mo, O.; Yanez, M. J. Mol. Struct.
1980, 69, 217.
(27) Kraka, E.; Konkoli, Z.; Cremer, D.; Fowler, J.; Schaefer, H. F., III.
J. Am. Chem. Soc. 1996, 118, 10595.
S0002-7863(96)04280-1 CCC: $14.00 © 1997 American Chemical Society

7266 J. Am. Chem. Soc., Vol. 119, No. 31, 1997
Table 1. Structural Parameters of Dioxiranes
Sander et al.
R
O
O
R
method
r(O-O)
r(C-O)
θOCO
θRCR
1 R ) Ha
R ) Hb
microwave
CCSD(T)
1.516(3)
1.514
1.576
1.578(1)
1.576
1.569
1.521
1.503(5)
1.490
1.388(4)
1.385
1.349
1.348(8)
1.349
1.343
66.2(2)
66.2
71.5
71.7(5)
71.5
71.5
117.3(2)
116.6
109.05
108.8(7)
108.8
108.8
121.3
4 R ) Fc
R ) Fd
high-resolution IR
electron diffraction
MP2/6-311G(2d)
CCSD(T)
MP2/6-31G(d)
X-ray structure
B3LYP/6-31G(d)
R ) Fd
R ) Fe
f
2 R ) CH3
1.417
5 R ) mesitylg
R ) mesitylg
1.414(4)h
1.414
64.2(3)
119.2(4)
120.5
a Reference 14. b Ab initio calculation at the CCSD(T)(full)/cc-VTZ2P+f,d level, ref 21; see also ref 20. c Reference 23, best estimate with the
C-F bond length constrained to 1.315 Å. d Reference 24. e Reference 27. f References 12 and 50. g This work. h Mean value of 1.413(5) and 1.414(5)
Å.
oxidized; however, the reactivity is several orders of magnitude
lower.28,29 The primary products of these reactions are carbonyl
O-oxides, which on irradiation with visible light readily rear-
range to the dioxiranes. Several problems have to be solved to
utilize this procedure for a preparative scale synthesis: (i)
Carbenes are highly reactive, and thus the oxidation has to
compete with other inter- and intramolecular reactions. (ii) The
cyclization of carbonyl oxides to dioxiranes is a photochemical
reaction,3 and thus a long lifetime of the carbonyl oxide is
necessary to obtain high yields of dioxiranes. (iii) Dioxiranes
rearrange to esters on 400 nm irradiation. The photochemical
dediazoniation of the diazo compound and the cyclization of
the carbonyl oxide must occur at wavelengths at which the
dioxirane is stable. The ideal conditions are thus an inert solvent
with high O2 solubility at low temperature, a diazo compound
with long-wavelength absorption maxima, and a carbene which
is comparatively long-living in solution.
trapped to carbonyl oxide 8. The lifetime of 8 is several orders
of magnitude longer than that of benzophenone O-oxide.31 In
contrast to other diaryl carbonyl oxides, for which a second-
order decay was found, the steric hindrance in 8 causes first-
order decay.
These properties make the system dimesityldiazomethane/
O2 ideally suited for the preparative scale synthesis of the
sterically highly congested dimesityldioxirane (5).33 Here we
report on the synthesis, spectroscopic properties, and structural
characterization of the first crystalline dioxirane.
Results and Discussion
Synthesis in Argon Matrices. To explore the photochem-
istry and spectroscopic properties of the various intermediates
in the photooxidation of 7, matrix isolation experiments were
carried out. Irradiation (λ > 435 nm, 10 K) of 7 in solid argon
doped with 1% O2 produces carbene 6 and small amounts of
oxidation products of the carbene. In the UV spectrum the
carbene exhibits an absorption maximum at 274 nm with a
structured tailing to the red. The reported absorption with λmax
) 330 nm in organic glasses30 presumably is a shoulder of the
274 nm band. In the IR strong absorptions at 2977 (νCH), 1487,
1460, and 854 cm-1 are assigned to 6.
Annealing the O2-doped matrix for several minutes at 35 K
results in a yellow coloring of the matrix and formation of a
new absorption with λmax ) 394 nm, which is assigned to
dimesityl ketone O-oxide (8). All IR vibrations of 6 decrease
in intensity, and new absorptions of 8 at 1612 (83), 1451 (46),
1430 (100), 1358 (21), 984 (21), 877 (33), 850 (50), and 539
(17) cm-1 (rel. intens.) are grow in. With 18O2 only two
absorptions show a significant isotopic shift: the band at 877
cm-1 is shifted to 846 cm-1 and assigned to the O-O stretching
vibration. The frequency and the large isotopic shift are highly
characteristic for the O-O stretching vibration in carbonyl
oxides. The band at 1358 cm-1 is shifted to 1347 cm-1, which
indicates some contribution of C-O stretching.
Carbonyl oxide 8 is highly photolabile, and irradiation with
λ > 515 nm rapidly yields dimesityldioxirane (5) (Scheme 1,
Figure 1a). The dioxirane exhibits IR absorptions at 1457 (85),
1443 (31), 1423 (85), 1289 (100), 1096 (23), 973 (31), 887 (69),
733 (15), and 513 (23) cm-1 (rel. intens.). The only vibration
for which an 18O-isotopic shift is clearly observed is the 1096
cm-1 vibration (shifted to 1090 cm-1). Shorter wavelength
irradiation produces mesityl mesitoate (9) as the only detectable
product (Figure 1b), while dimesityl ketone (10) is not formed
during any of the photolysis steps. Both 9 and 10 were
independently synthesized and matrix isolated to prove the
identity of the oxidation products of 6.
A carbene that meets these requirements is dimesitylcarbene
(6), which is easily generated by irradiation of dimesityldiazo-
methane (7). UV-vis and ESR spectra of carbene 6 in organic
matrices at 77 K and kinetic data in solution from laser flash
photolysis (LFP) experiments were reported by several
authors.30-32 In organic glasses at 77 K carbene 6 is stable for
hours, and in cyclopentane or benzene at room temperature the
lifetime is still ca. 200 µs,30 compared to only 1.7 µs for
diphenylcarbene.
Scaiano et al. were able to generate dimesityl O-oxide (8) as
a transient with λmax ) 390 nm by laser flash photolysis of 7 in
O2-purged solvents.31 The reaction rate of 6 with O2 was
determined to be 1.3 × 108 M-1 s-1 (in benzene), only one
order of magnitude slower than the reaction of diphenylcarbene
with O2.31 Therefore, in the presence of oxygen 6 is efficiently
(28) Ganzer, G. A.; Sheridan, R. S.; Liu, M. T. H. J. Am. Chem. Soc.
1986, 108, 1517.
(29) Gould, I. R.; Turro, N. J.; Butcher, J. J.; Doubleday, C. J.; Hacker,
N. P.; Lehr, G. F.; Moss, R. A.; Cox, D. P.; Guo, W.; Munjal, R. C.; Perez,
L. A.; Fedorynski, M. Tetrahedron 1985, 41, 1587.
(30) Nazran, A. S.; Griller, D. J. Am. Chem. Soc. 1984, 106, 543.
(31) Scaiano, J. C.; McGimpsey, W. G.; Casal, H. L. J. Org. Chem.
1989, 54, 1612.
(32) Nazran, A. S.; Lee, F. L.; Gabe, E. J.; Lepage, Y.; Northcott, D. J.;
Park, J. M.; Griller, D. J. Phys. Chem. 1984, 88, 5251.
(33) Kirschfeld, A.; Muthusamy, S.; Sander, W. Angew. Chem. 1994,
106, 2261; Angew. Chem., Int. Ed. Engl. 1994, 33, 2212.

Dimesityldioxirane
J. Am. Chem. Soc., Vol. 119, No. 31, 1997 7267
Scheme 1
These experiments clearly demonstrate that the photooxidation
of diazo compound 7 produces high yields of dioxirane 5 if the
reaction conditions are carefully chosen. Principally this
reaction should allow for a preparative scale synthesis of 5.
mixture of CFCl3 and (CF2Br)2 (1:1),34 and cooled to 77 K to
produce a transparent glass. Irradiation with λ > 515 nm results
in an intense yellow coloring of the glass, caused by the broad
and intense absorption of carbonyl oxide 8 at λmax ) 398 nm.
Melting the glass at 160 K results in a yellow solution of 8
which is stable at 195 K (-78 °C) in the dark for several hours.
In these solutions 8 was unequivocally characterized by 13C-
and 1H-NMR spectroscopy.34 Exposure to daylight or irradia-
tion with λ > 455 nm rapidly causes the complete bleaching of
the glass or solution. If a glass after bleaching the carbonyl
oxide is slowly warmed to the melting point (160 K), some
carbonyl oxide is formed back, indicating the presence of
unreacted carbene even in oxygen-saturated glasses at 77 K.
After being warmed to room temperature, the stable products
were separated by HPLC (reversed phase/methanol-water) and
characterized by conventional spectroscopic methods and
independent synthesis. The major products are unreacted
diazomethane 7, ketone 10, ester 9, and dioxirane 5. Minor
products are dimesitylmethane (traces from a contamination of
starting material 7) and 2-(hydroxymethyl)-4,6-dimethylphenyl
mesityl ketone (11). If the reaction mixture is irradiated with
λ > 420 nm prior to the HPLC separation, the dioxirane peak
decreases in intensity and more ester 9 is formed. These results
confirm the photochemistry observed in solid argon. The
ketones 10 and 11, which are not formed in solid argon, are
thermal products of carbonyl oxide 8. Intermolecular oxygen
transfer results in the formation of 10, while intramolecular
oxygen transfer produces 11. The ester 9 is solely the
photoproduct of dioxirane 5 and neither thermally nor photo-
chemically formed from 8.
Synthesis in Organic Glasses. Diazomethane 7 (1.5 × 10-3
M) is dissolved in an oxygen-saturated (1 bar of O2, -20 °C)
Synthesis in Solution. For the preparative scale synthesis
of 5 the use of inert gas matrices or organic glasses is
impractical, and we thus investigated the photooxidation of 7
in solution. Initial experiments were carried out in a cooled 10
mL cuvette at -90 °C irradiated with a high-pressure mercury
arc lamp and cutoff filters (λ > 475 and 495 nm). CFCl3,
(CF2Br)2, pentane, isopentane, and n-heptane were used as
solvents. With 495 nm irradiation the yield of 5 is 40-50%,
and the major byproducts are ketone 10 (30-35%), ester 9 (10-
20%), and unreacted diazomethane 5. In isopentane, the yield
Figure 1. Difference IR spectra showing the photochemical reaction
sequence 8 f 5 f 9 in argon at 10 K. (a) Bottom: bands of carbonyl
oxide 8 disappearing on λ > 515 nm irradiation. Top: bands of
dioxirane 5 appearing. (b) Bottom: bands of dioxirane 5 disappearing
on λ > 420 nm irradiation. Top: bands of ester 9 appearing.
(34) Sander, W.; Kirschfeld, A.; Kappert, W.; Muthusamy, S.; Kiselews-
ky, M. J. Am. Chem. Soc. 1996, 118, 6508.

7268 J. Am. Chem. Soc., Vol. 119, No. 31, 1997
Sander et al.
Scheme 2
IR spectra of 5 were recorded in solid argon, KBr, and CCl4
solution. The absence of any absorption in the carbonyl range
proves the purity of 5. The IR spectrum of 5 is dominated by
the mesityl groups, and the only absorption that shows a
significant 18O-isotopic shift is found at 1096 cm-1. Presumably
this isotopic shift is caused by some contribution of the C-O
vibration to this mode. The O-O vibration of dioxiranes is
expected in the range 730-800 cm-1 21
, and a weak absorption
at 733 cm-1 that shows a small 18O-isotopic shift might be
attributed to this vibration.
The 1H-NMR spectrum of 5 in CDCl3 shows three signals at
δ ) 2.19 (s, 12 H), 2.24 (s, 6 H), and 6.82 (s, 4 H) assigned to
the o-methyl, p-methyl, and m-aromatic protons of the mesityl
rings, respectively. These signals are not broadened on lowering
the temperature to -40 °C, which indicates that the two C2
symmetric conformers are rapidly equilibrating by hindered
rotation or libration of the mesityl groups.
The seven signals in the 13C-NMR spectrum of 5 could be
assigned by recording a DEPT spectrum, by specific 13C-labeling
of the three-membered ring, and by H-C long-range correlation.
The signal of the four o-methyl groups is found at δ ) 21.6
and that of the two p-methyl groups at δ ) 20.9. The secondary
aromatic carbon atom (meta position) is observed at δ ) 130.6,
and the three quarternary carbon atoms are observed at δ )
130.5 (ipso), 137.5 (ortho), and 139.0 (para). 13C-labeling (99%
13C) of the dioxirane carbon atom C(1) dramatically increases
the intensity of the quarternary carbon atom at δ ) 103.0, and
thus unequivocally allows this signal to be assigned. The
chemical shift of C(1) is in excellent agreement with the
chemical shifts found in other dioxiranes such as dimethyldiox-
irane (2, δ ) 102.3),12,35,36 methyl(trifluoromethyl)dioxirane (3,
δ ) 97.3),37 or ethylmethyldioxirane (δ ) 103.9).7
of 5 is significantly lower and larger amounts of 10 are formed,
indicating the oxidation of the solvent. Shorter wavelength
irradiation (λ > 475 nm) in CFCl3 reduced the time required
for the complete consumption of 7 drastically; however, the yield
of 5 dropped to 30%, and both the yields of 10 and 9 increased.
A special photoreactor was constructed that allowed up to
500 mg of 7 in 300 mL of O2-purged CFCl3 at -78 °C to be
irradiated with the filtered light (CuCl2/HCl filter with 50%
transmission at 435 nm) of a Hg/Cd medium-pressure lamp.
Due to the fire hazard of O2-saturated hydrocarbon solutions,
only CFCl3 was used in the preparative scale syntheses of 5.
After 10 h of irradiation most of the diazo compound 7 is
consumed. The highest yield of 5 (>50%) is obtained if the
concentration of 7 is kept low by adding it in several small
portions to the photolysis mixture. At higher concentration of
7 substantial amounts of ketone 10 are formed, indicating the
oxidation of 7 by carbonyl oxide 8 (7 is not oxidized by 5).
Dioxirane 5 is purified after evaporation of the solvent by
preparative scale HPLC and recrystallized from pentane to give
colorless crystals.
Solutions of pure 5 or the crystalline material can be stored
in the dark at -20 °C for many months without decomposition,
while crude photolysis mixtures are less stable. At room
temperature crystalline 5 slowly (14% loss within two weeks)
decomposes to an approximately 1:2 mixture of ester 9 and
ketone 10. The melting point is 62-64 °C with partial
decomposition. The half-life of 5 in CDCl3 at room temperature
is on the order of one day, and the major product detected by
1H-NMR spectroscopy is ester 9, while only traces of 10 are
formed. The stability of 5 in halogenated solvents such as CCl4
or CFCl3 is similar, while in methanol within several hours
considerable amounts of both ester 9 and ketone 10 are formed.
This indicates that methanol is slowly oxidized by dioxirane 5.
The photolysis (λ > 360 nm) of purified solutions of 5
quantitatively produces ester 9, in agreement with the results
from the matrix studies.
Dioxirane 5 is a powerful oxygen transfer reagent. Addition
of 5 in CDCl3 to equimolar amounts of triphenylphosphine (in
CCl4), phenyl methyl sulfide, or cyclohexene in the same solvent
results within several minutes in the quantitative consumption
of 5 and formation of the expected oxidation products (Scheme
2).
Spectroscopic Characterization. The UV spectrum of 5 in
methanol exhibits a very strong absorption at λmax ) 206 nm
(log ꢀ ) 4.6) and a strong absorption at λmax ) 274 nm (log ꢀ
) 3.2). Alkyldioxiranes form pale yellow solutions, caused by
weak n-π* transitions around 320-350 nm extending to the
red.6 In contrast to this, crystals of 5 are completely colorless,
and even in 10-2 M solutions of 5 a transition in the near UV
or blue range of the spectrum could not be unequivocally
detected, although the photochemistry of 5 on irradiation with
blue light requires some absorption in this range.
The experimental NMR spectrum is nicely reproduced by the
calculated (SOS-DFPT)38,39 chemical shifts (Figure 2a). The
calculation allows the NMR chemical shifts of a fixed confor-
mation of 5, which is experimentally not accessible, and of the
17O nuclei to be discussed. The calculated 17O shift agrees well
with known and calculated shift values of other dioxiranes.
Compared to the 17O chemical shift of dimethyldioxirane (δ(17O)
) (exp) 338, (calcd) 334 ppm),40 the 17O nuclei in 5 (δ(17O) )
321 ppm) are somewhat more shielded. While the protons of
the p-methyl and the C(14) and C(17) o-methyl groups possess
typical values found also for toluene (two more deshielded
1
protons, one more shielded proton), the H shifts of the C(16)
and C(19) o-methyl groups suggest that two protons rather than
one are shielded (Figure 2). This is a result of methyl group
rotation, which places one H atom above the benzene ring in
the shielding region of the ring current.
Structural Characterization. In the crystal (Figure 3a)
dioxirane 5 possesses approximately local C2 symmetry with
the two mesityl groups twisted by 61.6° and 53.7° from the
C(2)-C(1)-C(8) plane, respectively. B3LYP41-43 calculations
(35) Murray, R. W.; Jeyaraman, R.; Pillay, M. K. J. Org. Chem. 1987,
52, 746.
(36) Cassidei, L.; Fiorentino, M.; Mello, R.; Sciacovelli, O.; Curci, R.
J. Org. Chem. 1987, 52, 699.
(37) Mello, R.; Fiorentino, M.; Sciacovelli, O.; Curci, R. J. Org. Chem.
1988, 53, 3890.
(38) Ault, B. S. J. Mol. Struct. 1985, 129, 287.
(39) Olsson, L.; Cremer, D. J. Phys. Chem. 1996, 100, 16881.
(40) Kraka, E.; Sosa, C. P.; Cremer, D. Chem. Phys. Lett. 1996, 260,
43.
(41) Becke, A. J. Chem. Phys. 1993, 98, 5648.
(42) Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J. J.
Phys. Chem. 1993, 98, 11623.
(43) Bauschlicher, Jr., C. W.; Partridge, H. Chem. Phys. Lett. 1995, 240,
533.
(44) Hariharan, P. C.; Pople, J. A. Chem. Phys. Lett. 1972, 66, 217.

Dimesityldioxirane
J. Am. Chem. Soc., Vol. 119, No. 31, 1997 7269
Figure 3. X-ray crystal structure of dimesityldioxirane (5).
Figure 2. BPW91/VTZ+P SOS-DFT calculated NMR and structural
data of dimesityldioxirane (5) compared to experimental values. (a)
SOS-DFPT NMR chemical shifts (ppm) relative to TMS (1H and 13C)
and gaseous H2O (17O) for 5. For methyl groups and C atoms that are
equilibrated by hindered rotation, average values (ave) are compared
with the corresponding experimental values (in italics). (b) B3LYP/6-
31G(d) geometry of 5 compared to the crystal structure. Since the
calculated C2 symmetrical structure is slightly distorted in the crystal
(see Figure 3b), the experimental values given here (in italics) are
averaged values over corresponding bond lengths and angles in the
two mesityl rings.
terms, the widening of the external ring angle leads to a
stabilization of orbital π2 of the ring, which is OO bonding
and CO antibonding. This is in line with a larger negative
charge at the O atoms (the dioxirane ring accepts 0.151 e from
the two mesityl groups) and a stronger shielding of the O nuclei
(see above). Since the strength of the OO bond determines the
reactivity of a dioxirane, which is characterized by OO bond
rupture, formation of a methylenebis(oxy) biradical, and sub-
sequent rearrangements and decompositions of this biradical, 5
should be more stable than other dioxiranes.
with a 6-31G(d) basis set44 lead to C2 symmetry and a twisting
of the mesityl groups by 54.2° in reasonable agreement with
the crystal structure (Figure 2b), where one has to consider that
packing forces in the unit cell probably lead to deviations from
C2 symmetry and somewhat different twist angles. Steric strain
caused by the mesityl groups is relieved in different ways
(Figures 2b and 3b): (a) Rotation of the mesityl groups. (b)
The central C(2)-C(1)-C(8) angle is widened to 119.2°
(B3LYP 120.5°), which is clearly larger than the corresponding
value for dioxirane (1; 117°, Table 1)20,21 or difluorodioxirane
(4; 109°).27 (c) The o-methyl groups are slightly bent backward
away from the dioxirane ring (widening of the C(17)-C(9)-
C(8) angle to 126°). (d) The benzene hexagon is distorted in a
way that the distances C(9)-C(13) (2.430 Å, B3LYP 2.446 Å)
and C(3)-C(7) (2.422 Å) are larger than the distances C(10)-
C(12) (2.354 Å, B3LYP 2.383 Å) and C(4)-C(6) (2.367 Å),
respectively. (e) The o-methyl groups are rotated by 20.2° and
22.6° (averaged B3LYP values) relative to the methyl group in
toluene (p-methyl rotation 3.8°) so that steric interactions
between the mesityl groups and the dioxirane ring become
smaller. As a result of (a)-(e), there are no close contacts in
5 and all H-H and O-H distances are within the region of
typical van der Waals values.
Dioxiranes are in general about 20 kcal/mol (1, 25.6; R )
H, R ) CH3, 21.0;45 2, 19.846), in the case of difluorodioxirane
(4) even 42.3 kcal/mol,27 more stable than the corresponding
carbonyl oxides. In 5 the energy difference between the two
isomers is just 9.9 kcal/mol, indicating that dimesityl ketone
oxide is strongly stabilized relative to 5.40 There seems to be
no particular increase in the thermodynamic stability of the
dioxirane ring of 5 due to the mesityl groups, which can be
understood in view of the twisting of the mesityl groups. The
twisting angle of about 55° prevents strong interactions of the
phenyl ring MOs with either the Walsh or the π-MOs of the
dioxirane ring. On the other hand, steric repulsion between the
mesityl rings guarantees the kinetic stability of 5 since ring
opening implies an increase in steric interactions.
Conclusion
The preparative scale synthesis of dimesityldioxirane (5) was
optimized on the basis of preliminary matrix isolation studies.
The results nicely demonstrate that matrix isolation not only is
useful for the spectroscopic characterization of reactive inter-
mediates but also is a powerful tool for the optimization of larger
scale syntheses. By carefully controlling the reaction condi-
tionssexpecially the light source and filterssmore than 50%
yield of 5 based on consumed dimesityldiazomethane (7) was
obtained.
Compared to that in the parent compound 1, r(O-O) in 5
(exp, 1.503 Å; B3LYP, 1.490 Å; Table 1, Figure 3) is shortened
by 0.013 Å while the CO bonds are slightly longer (exp, 1.414;
B3LYP, 1.414; 1, exp, 1.388 Å; Table 1). Obviously, a
widening of the central C2-C1-C8 angle implies a decrease
of the OCO angle and a shortening of the OO bond. In MO
(45) Gutbrod, R.; Kraka, E.; Schindler, R. N.; Cremer, D. J. Am. Chem.
Soc., in press.
(46) Gutbrod, R.; Schindler, R. N.; Kraka, E.; Cremer, D. Chem. Phys.
Lett. 1996, 252, 221.

7270 J. Am. Chem. Soc., Vol. 119, No. 31, 1997
Sander et al.
range 77-300 K. Experiments at 77 K were also conducted in a simple
quartz cuvette and a quartz Dewar vessel with optical windows. In a
typical experiment a 1.53 × 10-3 M solution of dimesityldiazomethane
(7) in a 1:1 mixture of CFCl3 and (CF2Br)2 was saturated with oxygen
at -20 °C for 20 min and cooled to 77 K to form a transparent glass.
After irradiation of the glass with λ > 475 nm for 3 h the reaction
mixture was warmed to room temperature, and the solvent was removed
under reduced pressure. The yield of 5 in the crude photolysis solution
was 55%.
Dioxirane 5 is a sterically highly congested dioxirane that is
completely stable at -20 °C and even at room temperature, as
crystalline material decomposes only slowly. The stabilization
of 5 compared to other dioxiranes is a kinetic effect caused by
the steric shielding of C(1) and a thermodynamic effect related
to the short OO bond distance. The major route of the thermal
or photochemical decomposition is rearrangement to ester 9.
This rearrangement proceeds via cleavage of the OO bond and
[1,2]-migration of a mesityl group in the methylenebis(oxy)
biradical. The OO distance in 5 is the shortest of all known
dioxiranes, and the OO bond is comparatively strong, which
explains the high thermal stability of 5. However, the oxygen
atoms are still accessible in bimolecular reactions, and thus
oxygen transfer to double bonds or hetero atoms is fast and
highly efficient. We are currently investigating the chemistry
of 5 to further explore the relation of structure and reactivity in
dioxiranes.
Synthesis of 5 in Solution. The small-scale synthesis of 5 was
carried out at -80 °C in a 20 mL quartz cuvette equipped with a cooling
jacket and a laboratory cryostat (irradiation with λ > 475 nm, (1-3)
× 10-3 M 7 in CFCl3, O2-purged at -20 °C). For the preparative
scale synthesis of 5 a specially designed photoreactor with a 150 W
Hg/Cd medium-pressure arc lamp (Heraeus-Nobelight TQ 150 Z3) was
used. Compared to a Hg arc lamp, the irradiation times were drastically
reduced by using the Cd-enhanced lamp. The lamp with a water cooling
jacket, a filter solution jacket (2 cm), and a vacuum jacket was immersed
into a 500 mL flask cooled to dry ice temperature. A CuCl2‚2H2O/
HCl filter solution gave the highest yields (CuCl2‚2H2O, 86.31 g; aqua
dest, 600 mL; concentrated HCl, 60 mL; 50% transmission at 435 nm).
The flask was filled with 300 mL of CFCl3 and purged with O2 for 10
min. A 500 mg sample of dimesityldiazomethane (7) was added in
small portions during the irradiation time (10 h). After merely complete
consumption of 7 the solvent was distilled off in vacuum into a trap
cooled with liquid nitrogen while the photolysis mixture was kept
between -50 and -30 °C. The photolysis process was observed with
analytical HPLC (RP-18 column, methanol as eluent). The yield of 5
in the crude reaction mixture was 50-60%. 5 was purified by
preparative scale HPLC (Polygosil 60-10 CN at -20 °C with ether/
pentane (1:100) as eluent) and recrystallized from pentane to give
colorless crystals.
Experimental Section
Materials and General Methods. 1H and 13C NMR were recorded
with a Bruker AM 400 at 400.1 and 100.6 MHz, respectively, in CDCl3
as solvent. IR spectra were obtained with a Bruker IFS66 FTIR
spectrometer with a standard resolution of 1 cm-1 in the range from
400 to 4000 cm-1. UV spectra were taken on a Hewlett-Packard 8452A
diode array spectrophotometer with a resolution of 2 nm. Preparative
scale HPLC was carried out with a Knauer photometer (detection at λ
) 254 nm), a Knauer HPLC pump 64, a Polygosil 60-10 CN column
(250 mm), and ether/pentane as eluent at -20 °C with a flow rate of
9.5 mL min-1. Analytical HPLC was carried out with a Merck-Hitachi
diode array detector L-4500, a Knauer HPLC pump 64, a RP-18 column
(125 mm; particle size 5 µm) and methanol as eluent with a flow rate
5: mp 62-64 °C; UV (CH3OH) λmax [nm] 206 (log ꢀ ) 4.6), 274
(log ꢀ ) 3.2); IR (CCl4) 2923 (m), 1612 (m), 1571 (w), 1456 (m),
of 0.5 mL min-1
.
1295 (w), 1242 (m), 881 (m), 852 (s) cm-1; H NMR (-40 °C) δ )
Irradiations were carried out using Osram HBO 500 W mercury high-
pressure arc lamps in Oriel housings equipped with quartz optics. IR
irradiation of the lamps was absorbed by a 10 cm path of water and by
a Schott KG1 filter. For broad-band irradiation Schott cutoff filters
were used (50% transmission at the wavelength specified), and for
narrow-band irradiation, interference filters in combination with dichroic
mirrors and cutoff filters (420, 435, 455, 475, and 495 nm) were
employed.
1
2.19 (s, 12H, o-CH3), 2.24 (s, 6H, p-CH3), 8.82 (s, 4H, aromatic) ppm;
13C NMR (-30 °C) δ ) 20.9 (p-CH3), 21.6 (o-CH3), 103.0 (quaternary
C), 130.6 (m-C), 130.5 (ipso-C), 137.5 (o-C), 139.0 (p-C) ppm; MS
(70 eV) m/z (%) 282 (M+, 1), 267 (2), 251 (7), 236 (2), 164 (3), 147
(100), 119 (26), 91 (16), 77 (10); HRMS calcd 282.161 980, found
282.162 500.
Dimesityl ketone (10)47 and mesityl mesitoate (9)48 were synthesized
according to literature procedures. The procedure of Zimmermann and
Paskowich was used to synthesize dimesityldiazomethane (7).49 The
synthesis of [13C]7 labeled at the diazo carbon atom started from K13CN
(99% 13C, Isotec Inc.) via Cu13CN and [13C]mesitonitrile. [13C]7: IR
(KBr) ν˜ ) 2915 (w), 2045 (CdN2, s), 1608 (w), 1478 (m), 1379 (w),
1011 (w), 883 (w), 852 (w) cm-1 (rel. intens.); 13C-NMR (100 MHz,
CDCl3) δ ) 20.6 (CH3), 20.9 (CH3), 58.0 (13CdN2), 126.0, 129.7, 136.9,
137.1; HRMS calcd 279.181 654, found 279.181 800.
Matrix Spectroscopy. Matrix isolation experiments were performed
by standard techniques with an APD CSW-202 Displex closed-cycle
helium cryostat. Matrices were produced by deposition of argon (Linde,
99.9999%) at 30 K on top of a CsI (IR) or sapphire (UV-vis) window
at a rate of approximately 0.15 mmol/min.
Acknowledgment. This paper is dedicated to Professor
Waldemar Adam on the occasion of his 60th birthday. This
work was financially supported by the Deutsche Forschungs-
gemeinschaft and the Fonds der Chemischen Industrie. We
thank Dr. F. Scheidt for his help in the chromatography of the
dioxirane and D. Bla¨ser for the experimental X-ray analysis.
At the University of Go¨teborg, this work was supported by the
Swedish Natural Science Research Council (NFR) and the
Nationellt Superdatorcentrum (NSC), Linko¨ping, Sweden. The
Corporate Computing and Networking Division at Cray Re-
search is kindly acknowledged for providing computational
resources to carry out some of the calculations.
Organic Glasses. An Oxford instruments liquid nitrogen bath
cryostat (DN1714) equipped with four quartz windows and a quartz
cuvette (5 mL) was used for variable temperature experiments in the
Supporting Information Available: Tables of crystal-
lographic data, atomic coordinates, and anisotropic and isotropic
displacement parameters and the package diagram of dimesi-
tyldioxirane (5) (7 pages). See any current masthead page for
ordering and Internet access instructions.
(47) Kohler, E. P.; Baltzly, R. J. Am. Chem. Soc. 1932, 54, 4015.
(48) Thiessen, G. W.; Farr, W. N. J. Org. Chem. 1959, 24, 559.
(49) Zimmerman, H. E.; Paskovich, D. H. J. Am. Chem. Soc. 1964, 86,
2149.
(50) Cremer, D.; Schindler, M. Chem. Phys. Lett. 1987, 133, 293.
JA964280N