Transition Structures of Epoxidation by CH3Re(O)2(O2) and CH3Re(O)(O2)2 and Their Water Adducts

Full text of DOI:10.1021/jo980002q

1
752
J . Org. Chem. 1998, 63, 1752-1753
Tr a n sition Str u ctu r es of Ep oxid a tion by
CH Re(O) (O ) a n d CH Re(O)(O a n d Th eir
Wa ter Ad d u cts
by about 0.3 Å. We attribute this discrepancy to hydrogen
bonding involving the water and diglyme in the crystal
structure, which increases the donating ability of the oxygen.
3
2
2
3
2 2
)
2
In the monoperoxo structures (2 and 4), the η -O2 unit is
Yun-Dong Wu*,† and J ian Sun†,‡
also nearly coplanar with the Re-C bond. One important
difference between the monoperoxo and bisperoxo structures
is in the Re-Oa and Re-Ob bond lengths. Although the two
bond lengths are quite similar, the Re-Oa bond in structures
2 and 4 is about 0.06 and 0.13 Å shorter than the Re-Ob
bond, respectively. On the other hand, the Re-Oa bond in
structures 3 and 5 is about 0.03 Å longer than the Re-Ob
Department of Chemistry, The Hong Kong University of Science
and Technology, Clear Water Bay, Kowloon, Hong Kong, and
Chengdu Institute of Organic Chemistry, Chinese Academy of
Sciences, Chengdu, 610041, China
Received J anuary 5, 1998
2
c
bond, in agreement with the X-ray crystal structure. These
indicate that the Ob in structures 2 and 4 might be
intrinsically more reactive while the Oa might be more
reactive in structures 3 and 5 (vide infra).
Methylrhenium trioxide (1, MTO) is one of the most
1
versatile catalysts, and its catalysis of alkene epoxidation
2
-5
has been studied extensively.
The active catalytic forms
are 2, 3, and/or their water adducts.6 In this paper, we
report a theoretical study that addresses several important
mechanistic aspects: (1) the relative reactivities of 2 and 3
toward alkenes, (2) the relative reactivities of the two
oxygens of the peroxo group(s), (3) the detailed geometrical
features of transition state of epoxidation,3 and (4) the
effect of H2O ligation on the reactivity.
,7
12
No π-complex can be located for ethene with 2-5.
2
2
Instead, ethene directly approaches the Re-η -O unit and
forms a three-membered-ring transition structure. Each
transition structure is in a spiro geometry.1
3,14
We were
The two
1
3,15
unable to locate a planar transition structure.
a,b
2
carbon atoms of ethene and the η -O unit are nearly in a
plane. The O-O bond lengthens by about 0.3-0.4 Å. The
2
nature of electrophilic addition by the peroxo group is
indicated by charge transfer of about 0.3 units from ethene.
Interestingly, the two forming O---C bonds are about 0.2 Å
different in length in most structures. To test the signifi-
cance of this distortion, 2A and 2B were optimized with the
two C-O bonds at the same length. These symmetrical
structures are higher in energy by within 0.4 kcal/mol. The
above geometrical features, along with the calculated activa-
tion enthalpies (Table 1), are quite similar to those calcu-
Geometries (Figure 1) were fully optimized with the
8
,9
density functional theory BLYP/HW3 method
using a
GAUSSIAN 94 program.10 Transition structures were con-
firmed by frequency calculations on the technetium conge-
ners with the BLYP/3-21G method.11
16
lated for the epoxidation by dioxanes.
The calculations give a 0.9 kcal/mol preference for 2A over
The calculated geometry of 5 is very similar to the X-ray
crystal structure.2c The two peroxo units are nearly coplanar
with the methyl group. The calculated bond lengths are
systematically longer than the experimental values by about
2
B but a 2.5 kcal/mol preference for 4B over 4A. On the
other hand, for the bisperoxo species 3 and 5, the Oa is much
more reactive than Ob toward the olefin. Thus, 3A and 5A
are 7.2 and 6.0 kcal/mol more stable than 3B and 5B,
respectively. Both geometrical factors and steric effects
contribute to these interesting patterns of reactivity. Geo-
metrically, the Oa-Ob in 2 are in such an environment that
Oa is more strongly bound to Re than Ob, as indicated by
the shorter Re-Oa distance. Thus, Ob is intrinsically the
better transferring oxygen. The steric effect, on the other
hand, disfavors the attack of ethene on the Ob. In structure
2B, two of the methyl hydrogens are about 2.2 Å away from
the two “endo” hydrogens of ethene, respectively, causing
severe steric interactions. Because of the two opposite
effects, the calculations predict similar stabilities for transi-
tion structures 2A and 2B. The coordination of water in
structure 4 has two important consequences: (1) the C-Re-
Ob angle is opened up by about 3°. This results in reduced
crowding in transition structure 4B; (2) the Re-Ob bond
becomes considerably longer than the Re-Oa bond, making
Ob more reactive. Thus, 4B is predicted to be more favorable
0
.02-0.05 Å except for the Re-O(H2) bond which is longer
†
The Hong Kong University of Science and Technology.
Chinese Academy of Sciences.
‡
(1) For recent reviews, see: (a) Rem a˜ o, C. C.; K u¨ hn, F. E.; Herrmann,
W. A. Chem. Rev. 1997, 97, 3197. (b) Herrmann, W. A.; K u¨ hn, F. E. Acc.
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5
00, 149.
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(
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M. U.; Scherer, W. J . Mol. Catal. 1994, 86, 243. (c) Herrmann, W. A.;
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1
993, 32, 1157.
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(
(
(
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(
b) Murray, R. W.; Singh, M.; Williams, B. L.; Moncrieff, H. M. Tetrahedron
Lett. 1995, 36, 2437. (c) Adam, W.; Mitchell, C. M. Angew. Chem., Int. Ed.
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Chem. Soc. 1997, 119, 6189. (b) Cop e´ ret, C.; Adolfsson, H.; Sharpless, K. B.
Chem. Commun. 1997, 1565.
(
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(12) (a) Trost, M. K.; Bergman, R. B. Organometallics 1991, 10, 1172.
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J . D., Ed.; Academic Press: New York, 1985; Vol. 5, p 247.
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Soc. 1997, 119, 12982. (b) Bach, R. D. Andr e´ s, J . L.; Owensby, A. L.; Schlegel,
H. B.; McDouall, J . J . W. J . Am. Chem. Soc. 1992, 114, 7207.
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Parr, R. G. Phys. Rev. 1988, B37, 785.
9) The HW3 basis set is equivalent to the 6-31G* basis set, see: J ohns,
V.; Frenking, G.; Reetz, M. T. Organometallics 1993, 12, 2111.
10) Gaussian 94, Revision B.3: Frisch, M. J .; Trucks, G. W.; Schlegel,
(
(
(
H. B.; Gill, P. M. W.; J ohnson, B. G.; Robb, M. A.; Cheeseman, J . R.; Keith,
T. A.; Petersson, G. A.; Montgomery, J . A.; Raghavachari, K.; Al-Laham,
M. A.; Zakrzewski, V. G.; Ortiz, J . V.; Foresman, J . B.; Cioslowski, J .;
Stefanov, B. B.; Nanayakkara, A.; Challacombe, M.; Peng, C. Y.; Ayala, P.
Y.; Chen, W.; Wong, M. W.; Andres, J . L.; Replogle, E. S.; Gomperts, R.;
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(11) The transition structures for the Tc and Re systems are very similar.
S0022-3263(98)00002-4 CCC: $15.00 © 1998 American Chemical Society
Published on Web 02/20/1998

Communications
J . Org. Chem., Vol. 63, No. 6, 1998 1753
F igu r e 1. Calculated structures of monoperoxo- and bisperoxomethylrhenium trioxide and their H2O adducts and the transition structures
for the epoxidation of ethene with these oxidants. The values of angle C-Re-Ob in 2-5 are given in parentheses.
Ta ble 1. Ca lcu la ted Ga s-P h a se Rea ction or Activa tion
in 5 reduces the Re-Ob binding and slightly shortens the
Oa-Ob bond length. This reduces its reactivity, and 5A and
-
1
En th a lp y (k ca l‚m ol
)
5
B are computed to have 3.8 and 2.6 kcal/mol higher
∆H°
∆H°
activation enthalpy than 3A and 3B, respectively, in accord
with the experimental observation2b and calculation that
amine coordination reduces the reactivity of 3. This effect
is smaller for 4 and compensated by the favorable geo-
metrical effects aforementioned in 4B.
1
2
2
2
4
4
+ H2O2 f 2 + H2O
+ H2O f 4
+ CH2dCH2 f 2A
+ CH2dCH2 f 2B
+ CH2dCH2 f 4A
+ CH2dCH2 f 4B
-3.0
-6.5
12.2
13.1
13.3
10.8
2 + H2O2 f 3 + H2O
3 + H2O f 5
3 + CH2dCH2 f 3A
3 + CH2dCH2 f 3B
5 + CH2dCH2 f 5A
5 + CH2dCH2 f 5B
1.0
-13.9
6.6
13.8
10.4
16.4
21
In summary, transition structures of epoxidation of ethene
by 2- 5 have been located. Each transition structure is in
a spiro geometry. The two peroxo oxygens have different
reactivities: For 4, the oxygen adjacent to the methyl group
is more reactive, while the oxygen that is away from the
methyl is more reactive for the other species. It is predicted
that 3 is much more reactive than 2, and its reactivity is
reduced by water complexation. We are currently studying
a more detailed mechanism in an effort to understand the
than 4A by over 2 kcal/mol. The situation for the bisperoxo
species is different. In structure 3, the Ob is in a better
position to form a double bond with Re and the Oa is a better
transferring group. Since the C-Re-Ob angle in 3 is even
smaller than that in 2, an even larger steric interaction
would be expected for 3B, as indicated by the ∼30° distortion
2
of the reactive η -O2 unit in 3B. Both the geometrical and
5
unusual pyridine acceleration effect found by Sharpless and
steric factors disfavor the electrophilic attack of Ob to
ethylene. The coordination of water in 5 has a similar effect
as in 4, but to a lesser extent.
to design ligands for possible stereoselective version of
reaction.
Without water complexation, calculations indicate that the
bisperoxo species 3 is much more reactive than the mono-
peroxo species 2 (by about 6.3 kcal/mol). This is due at least
to two reasons: (1) in 2, the reactive oxygen is Ob, and 2B
is severely destabilized by steric interactions, while for 3,
the reactive oxygen is Oa and the transition structure 3A
does not suffer from steric interaction; (2) the Re center is
more acidic in 3 than in 2. Thus, the Oa-Ob unit is more
tightly bound in 3, resulting in a weaker Oa-Ob bond and
higher reactivity.
Ack n ow led gm en t. We thank the Research Grants
Council of Hong Kong for financial support.
J O980002Q
(17) Herrmann, W. A.; Correia, J . D. G.; K u¨ hn, F. E.; Artus, G. R. J .;
Rom a˜ o, C. C. Chem. Eur. J . 1996, 2, 167.
(
2 2
18) The calculated complexation entropies for 2 + H O and 3 + H O
are -37.8 and -37.7 eu, respectively. If the loss of entropy in solution is
corrected to be -27.3 and -27.2 eu, respectively, according to the free-
volume theory,19 the calculated complexation free energy would be 1.6 and
-
5.8 kcal/mol for the two reactions.
19) Benson, S. W. The Foundations of Chemical Kinetics; Krieger,
Florida, 1982.
Calculations give a complexation enthalpy of -6.5 kcal/
mol for 4 and -13.9 kcal/mol for 5, indicating much stronger
acidity for 3 over 2. While the complexation of H2O with 3
(
(20) (a) Abu-Omer, M. M.; Hansen, P. J .; Espenson, J . H. J . Am. Chem.
Soc. 1996, 118, 4966. (b) Yamazaki, S.; Espenson, J . H.; Huston, P. Inorg.
Chem. 1993, 32, 4683.
(21) Sun, J .; Wu, Y.-D. Unpublished results.
2
c,17,18
is apparently strong, in agreement with experiments,
4
is predicted to be unstable.1
8,20
The water complexation