The peroxy acid dioxirane equilibrium: Base-promoted exchange of peroxy acid oxygens [24]

Full text of DOI:10.1021/ja005648q

11272
J. Am. Chem. Soc. 2000, 122, 11272-11273
Table 1. Relative B3LYP Electronic Energies of Isomers 1
The Peroxy Acid Dioxirane Equilibrium:
Base-Promoted Exchange of Peroxy Acid Oxygens
(RC(O)OOH) and 3 and Their Anion Isomers 2 (RC(O)OO-) and 4
neutral
E1-E3
anion
E2-E4
anion
E2-E4
anion
G2-G4
Ned A. Porter,* Huiyong Yin, and Derek A. Pratt
R
(6-31G(d))a (6-31G(d))b (6-31+G(d,p))b (6-31+G(d,p))b,c
Department of Chemistry, Vanderbilt UniVersity
NashVille, Tennessee 37235
H
CH3
CF3
Ph
m-ClPh
p-NO2Ph
C6F5
17.5
18.6
14.4
21.4
20.9
20.1
19.0
7.6
8.7
-3.6
3.7
3.1
3.5
3.6
11.8
12.5
1.2d
7.4
6.3
7.0
4.1
5.8
-2.0e
5.5
ReceiVed September 27, 2000
5.4
1.6
1.9
Peroxy acids, 1, are among the most versatile oxidation reagents
in organic chemistry. Olefin epoxidation1 and Baeyer-Villiger
oxidation of ketones2 are signature reactions of peroxy acids but
other oxidative transformations utilizing peroxy acids also have
frequent synthetic application.3 Even oxygen insertion into
carbon-hydrogen bonds, a reaction difficult to achieve by
7.7
a Peroxy acids (RC(O)OOH, 1, and the dioxirane isomers 3.
b (RC(O)OO-) 4 and their anions, 2. c Solution phase free energies of
the anions in dichloromethane using the COSMO solvation model.
d With the triple-ú, 6-311+G(d,p) basis set, this value is 2.4 kcal/mol.
e When this calculation is done in water using the COSMO algorithm,
this value is -4.0 kcal/mol.
differences between the two isomers are relatively large in the
neutral species, but much smaller in the anions, suggesting that
under basic conditions the dioxirane isomer is in equilibrium with
the peroxy acid. Indeed, the dioxirane isomer of trifluoroperacetate
(R ) CF3) is lower in energy than the open form by 3.6 kcal/
mol.
When calculations of the relative electronic energies of the
anions are done with the larger 6-31+G(d,p) basis set, which
includes diffuse functions to better describe the electron density
around the anionic oxygen, there is a dramatic increase in the
∆E values between the anionic isomers. None of the dioxirane
isomers are lower in energy than their corresponding peroxy acids
with this larger basis set. When the structures were re-optimized
by the same method, but now including the conductor-like
solvation model (COSMO),10 with a dielectric constant of 8.3
(that of dichloromethane), the substituent-anion interactions are
attenuated, as the hydroxydioxirane is better solvated than the
peroxy acid for all R, e.g. for R ) CF3, ∆Gsolv ) -52.5 kcal/
mol for the hydroxydioxirane versus -49.6 kcal/mol for the
peroxy acid.
nonradical methods, has been reported to result from the reaction
of alkanes with trifluoroperoxyacetic acid.4 The mechanisms of
the reactions of peroxy acids have been of longstanding interest5
and recently there have been several computational studies
reported on the mechanism of peroxy acid reactions. The
chemistry of peroxy acid conjugate bases, 2, has also been the
focus of several mechanistic studies although these species appear
to have received little, if any, theoretical consideration.6
Mimoun7 has suggested the possible intervention of a peroxy
acid dioxirane isomer, 3, in the chemistry of peroxy acids although
there is no experimental evidence for and little theoretical
consideration of this species. Given the importance of peroxy acids
in synthesis and the fact that dioxiranes are now established as
one of the most potent of oxidants, we have undertaken a
computational and experimental study to assess the relevance of
hydroxydioxirane 3 and its conjugate base 4 in the chemistry of
peroxy acids and their conjugate bases. Our computations suggest
that an alkoxy dioxirane is readily accessible from the peroxy
acid conjugate base, and consistent with this, we observe a base-
promoted interchange of peroxy acid peroxide oxygens.
Transition state optimizations for the isomerizations of the
anions using the quadratic synchronous transit-2 (QST2) algo-
rithm11 reveal structures in which the terminal (anionic) oxygen
is raised out of plane to close the distance between it and the
carbonyl carbon. The barriers to ring-closure are small for each
of the peroxy acid anions studied, with values of 9.9, 11.3, and
4.6 kcal/mol for performate, peracetate, and trifluoroperacetate,
respectively. The imaginary vibrational modes corresponding to
The hydroxydioxirane forms of 7 peroxy acids are found to
be true isomers at the B3LYP/6-31G(d)8 level of theory9 (Table
1). The corresponding conjugate bases of the dioxirane and the
percarboxylates are also found to be isomers. The energy
(1) Sawaki, Y. In Organic Peroxides; Ando, W., Ed.; John Wiley:
Chichester, 1992; pp 425-477.
these ring-closings were -160.71, -144.65, and -162.98 cm-1
,
(2) Hassall, C. H. Org. React. 1957, 9, 73.
(3) Krow, G. R. In ComprehensiVe Organic Synthesis; Trost, B. M. Ed.;
with the most negative corresponding to the earliest, lowest barrier
transition state (R ) CF3), and the most positive corresponding
to the latest, highest barrier transition state (R ) CH3). The barrier
heights are easily rationalized by considering that as the carbonyl
carbon becomes more electron-deficient, the barrier to ring-closing
is lowered.
Pergamon Press: New York, 1991; Vol. 7, pp 671-688.
(4) (a) Deno, N. C.; Jedziniak, E. J.; Messer, L. A.; Meyer, M. D.; Stroud,
S. G.; Tomezsko, E. S. Tetrahedron 1977, 33, 2503. (b) Camaioni, D. Personal
communication.
(5) (a) Bartlett, P. D. Rec. Chem. Prog. 1950, 11, 47. (b) Kwart, H.;
Hoffman, D. M. J. Org. Chem. 1966, 31, 419. (c) Bach, R. D.; Glukhovtsev,
M. N.; Gonzalez, C. J. Am. Chem. Soc. 1998, 120, 9902. (d) Yamabe, S.;
Kondou, C.; Minato, T. J. Org. Chem. 1996, 61, 616. (e) Freccero, M.;
Gandolf, R.; Amade, M. S.; Ratelli, A. J. Org. Chem. 1999, 64, 3853. (f)
Singleton, D. A.; Merrigan, S. R.; Liu, J.; Houk, K. N. J. Am. Chem. Soc.
1997, 119, 3385. (g) For a recent theoretical discussion of dioxirane
epoxidation see: Armstrong, A.; Washington, I.; Houk, K. N. J. Am. Chem.
Soc. 2000, 122, 6297-6298.
(6) Base-promoted epoxidation reactions of alkene by peroxy acids: (a)
Querci, C.; Ricci, M. J. Chem. Soc., Chem. Commun. 1989, 14, 889. (b)
Ishikawa, K.; Charles, H. C.; Griffin, G. W. Tetrahedron. Lett. 1977, 5, 427.
(c) Moyna, G.; Williams, H. J.; Scott, H. I. Synth. Commun. 1996, 26, 2235.
(7) Mimoun, H. Angew. Chem., Int. Ed. Engl. 1982, 21, 734.
(8) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Lee, C.; Yang,
W.; Parr, R. G. Phys. ReV. B 1988, 37, 785.
(9) Implemented in the Gaussian 98 suite of programs, Gaussian 98
(Revision A.9), Gaussian, Inc., Pittsburgh, PA, 1998 (compiled to run on a
Silicon Graphics Origin).
Given the importance of the substitutent/anion interactions in
the alkoxy dioxirane clear from the results in Table 1, the
transition structures were re-optimized using the 6-31+G(d,p)
basis set, to yield barriers of 12.8, 13.6, and 7.0 kcal/mol for R
) H, CH3, and CF3, respectively. However, when the COSMO
solvation model is included, the substituent/anion interactions are
again attenuated, and the barriers to isomerization are lowered,
e.g. 6.8 kcal/mol for performate.12 Since the relative energies of
the hydroxydioxirane isomers were much higher than the neutral
(10) Barone, V.; Cossi, M. J. Phys. Chem. A 1998, 102, 1995.
(11) Peng, C.; Ayala, P. Y.; Schlegel, H. B.; Frisch, M. J. J. Comput. Chem.
1996, 17, 49.
10.1021/ja005648q CCC: $19.00 © 2000 American Chemical Society
Published on Web 11/01/2000

Communications to the Editor
J. Am. Chem. Soc., Vol. 122, No. 45, 2000 11273
Scheme 1
Scheme 2
peroxy acids, we did not pursue extensive calculations of the
transition state structures of their isomerization.
Labeled peroxybenzoic acid was prepared according to Scheme
1. Thus, acetyl chloride labeled with 18O in the carbonyl carbon13
was reacted with peroxybenzoic acid and the labeled unsymmetric
7. The -20 °C solution of 7 was treated with acetyl chloride or
propionyl chloride and the diacyl peroxide 8 was recovered by
chromatography. Analysis of 8 (R ) Me) by ESI-MS/MS showed
that f (18OTotal) ) 0.115, f (18O1+2) ) 0.050, and f (18O3+4) ) 0.065.
We interpret these experiments to indicate that during the
methanolysis of 6 the label scrambled from O3 to O2 in the peroxy
anion such that the ratio 18O2:18O3 ) 1:1.3. If, rather than reaction
with acid chloride the sodium perbenzoate 7 was converted to
the peroxy acid and then to 8 by reaction with acid chloride and
pyridine, the f (18OTotal) and f (18O1+2) were identical to those
determined for the direct conversion of the peroxy anion to 8.
The labeling data can be understood by the mechanism
presented in Scheme 2. Thermolysis of the diacylperoxide gives
a radical pair that partially scrambles the carbonyl label into the
peroxide bond.14 The total label in the molecule is not depleted
but the distribution of label between O4 and O3 changes.
Methanolysis16 of the diacyl peroxide gives the peroxy anion
which scrambles the label via the dioxirane anion, 4 with R )
Ph. Reacylation of the peroxy anion gives a diacyl peroxide and
MS analysis of that peroxide indicates that the label is now
scrambled into the benzoyl side of the peroxide. Scrambling of
peroxide label occurs readily in the anion, even at -20 °C.
We note that there are several examples of base-promoted
reactions of peroxy acids reported in the literature. These include
the decomposition of peroxy acids to oxygen and carboxylic
acids,17 the epoxidation of electron-deficient alkenes,4 and the
oxidation of sulfoxides to sulfones.18 The computational and
experimental studies reported here suggest that dioxiranes are
accessible intermediates from peroxy acids, particularly in reac-
tions involving peroxy acid conjugate bases, and it would seem
advisable to keep this possibility in mind when considering
mechanisms for peroxy acid reactions.
diacyl peroxide 5 was heated in Nujol at 80 °C for 3 h (τ1/2
)
2.8 h) to partially scramble the oxygen label into the peroxide
oxygen (O4 f O3).14 The position of the oxygen label in 5 was
monitored by electron spray ionization tandem mass spectrometry
(ESI-MS/MS). ESI-MS/MS of 5 labeled at O4 gave an [M +
NH4]+ adduct at m/z 198 for unlabeled and m/z 200 for the 18O-
labeled species.15 In fact, comparison of these two adducts
permitted determination of total label present in 5 labeled at any
of the oxygens, f (18OTotal) ) I200/(I198 + I200). Collision-induced
dissociation (CID) of the labeled adduct, m/z 200, gave charac-
teristic fragments at m/z 141 and 139 for the labeled and unlabeled
species, [PhCOO/NH4]+•. Thus, CID on adduct m/z 200 from 5
labeled at either position O3 or O4 yielded only m/z 139, with no
m/z 141 fragment, while 5 labeled at either O1 or O2 gave no m/z
139 adduct and only m/z 141. In this way, a decision about
whether the label was on the benzoyl (O1 and O2) or acetyl side
(O3 and O4) of 5 was determined by analysis of the CID spectrum,
f (18O1+2) ) I141/(I139 + I141)‚f( 18OTotal) defining this distribution.15
Heating 5 having f (18OTotal) ) 0.45 for 2 h at 80 °C resulted in
substantial decomposition of the peroxide. Recovered 6 from this
experiment (40% recovery) had the same f (18OTotal) as starting 5.
None of the label on the acetyl side of 5 scrambles to the benzoyl
side of the peroxide during thermolysis since f (18O1+2) is neglible
for both 5 and 6.15 Treatment of this recovered peroxide with
sodium methoxide16 at -20 °C for 5 min led to complete
consumption of 6 and formation of labeled sodium perbenzoate
(12) Attempts to find the transition structures for the isomerization of
peracetate and trifluoroperacetate using the COSMO solvation model were
unsuccessful. The result was usually a second-order saddle point, with relative
energies of 8.4 and 8.9 kcal/mol for the peracetate and trifluoroperacetate,
respectively. Thus, the barriers for the true transition structures must be less
than these values. For these calculations, force constants were calculated
numerically, as analytical second derivatives are not available with Gaussian
98 for this type of calculation.
(13) Fujimori, K.; Oae, S. J. Chem. Soc., Perkin Trans. 2 1989, 1335.
(14) (a) Martin, J. C.; Hargis, J. H. J.Am. Chem. Soc. 1969, 91, 5399. (b)
Taylor, J. W.; Martin, J. C. J. Am. Chem. Soc. 1965, 88, 3650. (c) Goldstein,
M. J.; Haiby, W. A. J. Am. Chem. Soc. 1974, 96, 7358.
Acknowledgment. Support from the NSF is gratefully acknowledged.
D.A.P. thanks NSERC Canada for support.
Supporting Information Available: Experimental procedures, mass
spectral data for labeled compounds, transition state structures, and
Cartesian Coordinates for the rearrangement of 3 to 4 (PDF). This material
is available free of charge via the Internet at http://pubs.acs.org.
JA005648Q
(15) Yin, H.; Hachey, D. L.; Porter, N. A. Rapid Commun. Mass. Spectrom.
2000, 14, 1248. The ESI-MS/MS of 5 labeled at each of the oxygens confirms
the fragmentation assignments.
(17) Swern, D. In Organic Peroxides; Swern, D., Ed.; Wiley-Interscience:
New York, 1971; Vol. 1, pp 249-255.
(18) Curci, R.; Modena, G. Tetrahedron. Lett. 1965, 863-866 and
references cited.
(16) Koenig, T. W.; Martin, J. C. J. Org. Chem. 1964, 29, 1520.