On the Claisen rearrangement mechanism

Full text of DOI:10.1134/S0012500809020086

ISSN 0012-5008, Doklady Chemistry, 2009, Vol. 424, Part 2, pp. 52–55. © Pleiades Publishing, Ltd., 2009.
Original Russian Text © E.V. Klyuchareva, E.V. Ershova, R.G. Nigmatullina, M.Yu. Vozhdaeva, E.A. Kantor, 2009, published in Doklady Akademii Nauk, 2009, Vol. 424, No. 6,
pp. 777–780.
CHEMISTRY
On the Claisen Rearrangement Mechanism
E. V. Klyuchareva, E. V. Ershova, R. G. Nigmatullina,
M. Yu. Vozhdaeva, and E. A. Kantor
Presented by Academician N.S. Zefirov September 16, 2008
Received September 16, 2008
DOI: 10.1134/S0012500809020086
The Claisen rearrangement is one of the most group migration through a new cyclic transition state
important methods for the preparation of alkenylphe- to the para position at the second stage [3]. A disso-
nols from alkenyl aryl ethers [1]. The reaction proceeds ciative mechanism for the formation of these prod-
in high yield and is considered as a preparative method ucts has also been supposed [5]. Thus, there some
for the synthesis of phenols [2].
contradictions in the notion on the Claisen rearrange-
ment mechanism.
It is believed that the key stage of the reaction is a
concerted [3,3] sigmatropic shift accompanied by
inversion of the allyl fragment [3]. However, there are
data that the thermal ortho rearrangement can produce
alkenylphenols whose structure indicates the lack of
inversion of the allyl fragment (α rearrangement) [4].
The reaction products in some instances involve dialk-
enylphenols [5, 6], and their formation is explained by
intermolecular reactions via dissociation of C–O bond.
para-Substituted phenols are assumed to result from
To reveal possible routes for the formation of main
products and certain by-products of the Claisen rear-
rangement, we used quantum-chemical calculations
(B3PW91/6–31G(d), the GAMESS program package
[7]) to calculate the activation parameters of the rear-
rangement of ethers I–III: the activation energy (∆E,
∆EZPE) and free activation energy (∆G, ∆GZPE) with and
without inclusion of the zero point energy (ZPE).
The first stage of the rearrangement results in the
ortho migration at the first reaction stage and allyl formation of cyclohexadienones IV–VI:
O
OH
O
R
R
R
∆
H
R1
R1
R1
I–III
IV–VI
VII–IX
R and R1 = H (I, IV, VII) R=H, R1=CH3 (II, V, VIII) R=CH3, R1=H (III, VI, IX)
We calculated the possible variants of this reaction [3,3] sigmatropic shift; B, the formation of an
stage (Scheme 1): Ä, intramolecular concerted
intramolecular π complex followed by its rear-
rangement (or the migration of the allyl fragment);
C, heterolysis of the ether bond to form the allyl
cation followed by C-alkenylation of the phenolate
anion.
Ufa State Petroleum Technological University,
ul. Kosmonavtov 1, Ufa, 450062 Russia
52

ON THE CLAISEN REARRANGEMENT MECHANISM
53
O
R
TS1a
R1
O
R
TS1b
R1
A
O
R
B
–
+
I–III
IV–VI
TS2
R1
O
C
R
–
+
+
R1
Scheme 1.
The calculated energy of dissociation of the ether where allyl fragment is coordinated to the π system of
the aromatic ring (figure).
bond to form separated ion pair (route C) obtained as
prod
tot
reag
tot
It should be noted that this reaction can lead to
ortho-alkenyldienone without inversion of the allyl
fragment (IV, VI) or to para-alkenyldienone (X, XI).
The values of Etot, ZPE for the para-alkenyldienones are
slightly lower than for the ortho-alkenyldienones
(table), and this process is thermodynamically allowed.
Thus, the rearrangement through TS2 can produce both
ortho- and para-alkenylphenols (Scheme 2).
the difference Etot = E
– E
in the gas phase is
195 kcal/mol; it decreases to 82 kcal/mol when solva-
tion is taken into account (ethanol as a solvent). Thus,
this route is unlikely.
In the calculation of route A, we found transition
state TS1 with a chair (TS1a) or boat conformation
(TS1b), which agrees well with previous assumptions
[8–11].
TS1a
IV, VI
VII, IX
The formation of dienones IV–VI can proceed also
through transition state TS2 (route B) as a tight ion pair
O
OH
I, III
TS1b
TS2
R
R
C
C
H
C
R = H (I, IV, VII, X, XII)
2.26 Å
R = CH3 (III, VI, IX, XI, XIII) X, XI
XII, XIII
2.25 Å
Scheme 2.
C
C
C
O
The analysis of the structure and energy of complex
TS2 indicates that it can transfer the alkenyl fragment
to other molecules. Thus, the calculation shows the pos-
sibility of formation of stable complex TS2 with
allylphenol (XIV) where the allyl fragment can transfer
from TS2 to the monoalkenylphenol molecule and
attach to the free ortho and/or para position (Scheme 3).
C
C
C
TS2
The structure of transition state for the rearrangement of
allyl phenyl ether through route B.
DOKLADY CHEMISTRY Vol. 424 Part 2 2009

54
KLYUCHAREVA et al.
OH
OH
R
C
•
•
O
+
•
•
•
•
C
C
C
C
•
R1
R1
C
•
•
•
•
•
•
C
R1
•
XV, XVI
XIX, XX
C
C
•
•
•
OH
OH
•
C
•
•
•
O
•
R
•
•
•
R
+
R
C
•
C
C
•
•
C
C
•
C
•
•
•
•
•
•
C
C
•
•
R1
XVII, XVIII
XXI, XXII
R = H (XVII, XXI) R1 = H (XV, XIX)
R = CH3 (XVIII, XXII) R1 = CH3 (XVI, XX)
Scheme 3.
The calculated activation energy for the formation ionizing voltage, 70 eV), the products of the thermal
of disubstituted alkenylphenols is 25–28 kcal/mol. rearrangement of ether I contain 2,4-di(2-prope-
Therefore, the products of the Claisen rearrangement,
along with monoalkenylphenols, should involve dialk-
enylphenols and trialkenylphenols when free ortho and
para positions of the aromatic ring are available.
According to chromatography/mass spectrometry
(an Agilent chromatograph/mass spectrometer; HP-
5MS column, 30 m; temperature programming from 30
nyl)phenol XV (8.7%), 2,6-di(2-propenyl)phenol XVII
(5.4%), and 2,4,6-tri(2-propenyl)phenol XXIII (0.7%);
the rearrangement products of ether II contain 2,6-di(2-
propenyl)-4-methylphenol XVI (5.7%), while those of
ether III include 2-methyl-4-(2-propenyl)phenol XIII
(19.5%)
and
4,6-di(2-propenyl)-2-methylphenol
XVIII (9.4%). MS (m/z (Irel, %)): XIII (retention
to 60°C at 20 K/min and from 60 to 250°C at 6 K/min; time, 8.43 min), 148 (100), 133 (84), 121 (36), 105 (27);
Activation parameters for the rearrangement of ethers I–III, kcal/mol
Etot, ZPE
State (species)
Etot
∆E
∆G
ZPE
∆EZPE
∆GZPE
I
–266064.3
–266027.9
–266023.2
–266023.2
–290727.9
–290692.0
–290687.6
–290686.0
–290728.6
–290692.6
–290688.7
–290688.9
–266053.0
–290718.8
–266054.2
–290720.0
104.8
103.7
103.3
103.3
122.1
121.1
120.6
120.6
122.4
121.1
120.7
120.8
104.2
121.7
104.3
121.8
–265959.5
–265924.2
–265919.9
–265919.9
–290605.8
–290570.9
–290567.0
–290565.4
–290606.2
–290571.5
–290568.0
–290568.1
–265948.8
–290597.1
–265949.9
–290598.2
TS1a
TS1b
TS2
II
TS1a
TS1b
TS2
III
TS1a
TS1b
TS2
IV
36.5
41.2
41.2
38.4
42.5
42.9
35.3
39.6
39.7
37.2
41.0
41.4
35.9
40.3
41.9
38.5
42.3
44.6
34.9
38.8
40.4
37.5
40.7
43.1
36.0
39.9
39.7
38.6
41.8
42.4
34.7
38.3
38.2
37.3
40.2
40.9
VI
X
XI
DOKLADY CHEMISTRY Vol. 424 Part 2 2009

ON THE CLAISEN REARRANGEMENT MECHANISM
55
3. Dewar, M.J.S. and Dougherti, R.C., The PMO Theory of
Organic Chemistry, New York: Plenum Press, 1975.
Translated under the title Teoriya vozmushchenii mole-
kulyarnykh orbitalei v organicheskoi khimii, Moscow:
Mir, 1977.
4. Bunina-Krivorukova, L.I., Yagodin, V.G., Marty-
nova, V.P., and Bal’yan Kh.V., Zh. Org. Khim., 1970,
vol. 6, pp. 991–995.
XV (retention time, 9.08 min), 174 (100), 145 (51), 131
(45), 133 (31); XVI (retention time, 9.85 min), 188
(100), 145 (44), 173 (30), 159 (34); XVII (retention
time, 9.69 min), 174 (100), 133 (58), 131 (37), 145
(19); XVIII (retention time, 10.11 min), 188 (100), 147
(50), 173 (34), 131 (23); XXIII (retention time,
11.19 min), 214 (100), 173 (53), 145 (50), 115 (32).
Thus, the calculation results indicate that the
Claisen rearrangement can proceed through the cyclic
six-centered transition state in the chair or boat confor-
mation or the π complex. On the one hand, the π com-
plex is a transition state that can result in the classical
products of the Claisen rearrangement, ortho-alke-
nylphenols, and on the other hand, the π complex seems
to be a key transition state for the formation of by-prod-
ucts of the Claisen rearrangement: para-alkenylphenols
and di- and trialkenylphenols. The process via a sol-
vent-separated ion pair and the release of the alkenyl
fragment to the bulk appears to be unlikely.
5. Bunina-Krivorukova, L.I., Rossiiskii, A.P., and
Bal’yan, Kh.V., Zh. Org. Khim., 1974, vol. 10, pp. 2461–
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Krivorukova, L.I., Zh. Org. Khim., 1984, vol. 10,
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Nguyen, K.A., Su, S.J., Windus, T.L., Dupuis, M., and
Montgomery, J.A., J. Comput. Chem., 1993, vol. 14,
pp. 1347–1363.
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Helv. Chim. Acta, 1968, vol. 51, pp. 1457–1461.
9. Hansen, H.-J. and Schmid, H., Tetrahedron, 1974,
vol. 30, pp. 1959–1969.
10. Bernardi, F., Epiotis, N.D., andYates, R.L., J. Am. Chem.
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DOKLADY CHEMISTRY Vol. 424 Part 2 2009