Isotope-sensitive degenerate [1,3]-hydrogen migration versus competitive enol-keto tautomerization

Article abstract of DOI:10.1002/chem.200902176

A re-investigation of the dissociation of enol and keto forms of ionized acetone for a wide set of isotopologues, generated through dissociative electron ionization of the corresponding labeled 2-hexanones in a McLafferty reaction, was investigated. Enol

Full text of DOI:10.1002/chem.200902176

COMMUNICATION
DOI: 10.1002/chem.200902176
Isotope-Sensitive Degenerate [1,3]-Hydrogen Migration versus
Competitive Enol–Keto Tautomerization
[
a]
[a]
[b]
[c]
Xinhao Zhang, Waltraud Zummack, Detlef Schrçder, Frank A. Weinhold, and
[
a]
Helmut Schwarz*
In memoriam Chava Lifshitz
The question of intramolecular energy distribution in
formation which can be achieved from kinetic analysis of
[4]
polyatomic molecules or, more precisely, whether reacting
molecules display ergodic or non-ergodic behavior, is of cen-
multiple labeling data, here we report a re-investigation of
the dissociation of 1 for a wide set of isotopologues, gener-
ated via dissociative electron ionization of the corresponding
[1]
tral importance in reaction dynamics. For ionic gas-phase
systems, the unimolecular behavior of the enol and keto
forms of ionized acetone, 1 and 2, respectively, constitutes
[5,6]
labeled 2-hexanones in a McLafferty reaction.
To our sur-
prise, however, it turned out that the unimolecular dissocia-
tion of ionized acetone and its enol form offers additional
mechanistic puzzles which have not been recognized
[2,3]
2
13
one of the best studied systems.
Detailed H- and C-la-
beling experiments, energetic measurements, analysis of ki-
netic energy release distributions as well as electronic-struc-
ture calculations of the potential-energy surface with the in-
clusion of trajectory studies reveal the following (Scheme 1):
Direct dissociation of 1 to generate the hydroxyvinyl cation
[2,3]
before.
4
does not occur; rather, irreversible isomerization to a
highly excited, short-lived acetone ion 2 takes place, which
decomposes to the acylium ion 3 concomitant with loss of a
ꢀ
13
CH radical at a time-scale of 5ꢀ10 s. From 2, the newly
3
formed methyl group is eliminated preferentially as com-
pared to the one which is already present in 1, and the ki-
netic energy release associated with elimination of this pro-
cess is smaller than that for the loss of the newly generated
methyl group. All these findings clearly point to a non-stat-
istical (i.e., “non-ergodic”) behavior of the chemically acti-
vated acetone ion 2. Inspired by the detailed mechanistic in-
Scheme 1.
The experimentally obtained data for the unimolecular
dissociation of the enol ions 1–1h (Table 1) can be account-
ed for by a rate-determining, irreversible enol–keto tauto-
merization 1 ! 2 followed by a rapid dissociation of the
latter in which the two methyl groups do not have sufficient
time to completely equilibrate their originally different
[
a] Dr. X. Zhang, W. Zummack, Prof. Dr. H. Schwarz
Institut fꢁr Chemie, Technische Universitꢂt Berlin
Strasse des 17. Juni 135, 10623 Berlin (Germany)
Fax : (+49)30-314-21102
E-mail: Helmut.Schwarz@mail.chem.tu-berlin.de
energy content. As a consequence, both CꢀCH bonds are
3
[
b] Dr. D. Schrçder
cleaved with different rates. This view has been supported
Institute of Organic Chemistry and Biochemistry
Academy of Sciences of the Czech Republic
Flemingovo nꢃm. 2, 166 10 Prague 6 (Czech Republic)
[1b,2c,d,f,h,i,j]
by numerous computational studies.
Further, the
analysis of the data for 1–1h implies that a degenerate [1,3]-
hydrogen migration, by which the methyl and the methylene
groups of 1 equilibrate, cannot energetically compete with
the tautomerization 1 ! 2. This is in line with exploratory,
[
c] Prof. F. A. Weinhold
Department of Chemistry, University of Wisconsin
Madison, WI 53706 (USA)
Chem. Eur. J. 2009, 15, 11815 – 11819
ꢄ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
11815

Table 1. Mass differences (Dm) observed in the unimolecular dissociation
of the enol form of the acetone radical cation 1 and its isotopologues.
are some differences, all methods agree on the following es-
sential features, that is:
[
a]
Dm
precursor ion
15
16
17
18
19
i) The enol form 1 is more stable than ionized acetone 2;
ii) the transition structure for the enol–keto tautomeriza-
tion TS1 is lower in energy than either of the transition
structures for the degenerate isomerizations 1 Q 1 pro-
ceeding via TS2 or TS2’;
iii) TS2 in which the migrating hydrogen atom interacts
strongly with the central carbon of the allylic carbon
framework is significantly more stable than the previ-
ously assumed in-plane TS2’, in which no allylic stabili-
zation occurs.
+
CH
CH
CH
3
3
C(OH)CH
C(OH) CH
2
C
(1)
98
45
57
58
46
44
48
65
2
1
3
+
C
C
(1a)
(1b)
(1c)
54
42
41
52
56
1
1
1
1
2
2
2
1
3
+
3
C(OH)CH
+
CH
CH
CH
CH
CD
CD
CH
2
3
3
3
3
3
3
DC(OH)CH
C(OH)CHDC (1d)
2
C
+
+
C(OD)CH
C(OH)CD
C(OH)CH
C(OH)CD
C(OD)CD
2
2
2
2
C
C
C
C
(1e)
(1 f)
(1g)
(1h)
+
+
+
+
50
2
59
14
1
2
31
40
30
1
<1
2C (1i)
36
19
[
a] Data are given as S=100.
DFT-based calculations per-
formed at the B3LYP/6-311++
G** level of theory, according
to which the barrier for the de-
generate hydrogen exchange 1
Q 1 proceeding via the classi-
cal, in-plane hydrogen transi-
tion structure TS2’ (Figure 2) is
ꢀ
1
1
0.2 kcalmol higher in energy
than the enol–keto tautomeri-
[
7]
zation 1 ! 2.
If this scenario constitutes a
complete picture for the unimo-
lecular dissociation of 1 and its
isotopologues,
expect that CH (OD)CD C (1i)
one
would
+
3
2
undergoes losses of only CH3C
and CD C; the eliminations of
3
either CH DC (Dm=16) or
2
CHD2C (Dm=17) should be
absent. As shown in Table 1,
however, this is not the case.
Obviously, the particular label-
ing pattern of 1i opens up a hy- Figure 1. Potential-energy surface for the isomerization and dissociation of the enol–keto forms of the acetone
radical cations, 1 and 2, respectively, calculated at the level of W1U theory.
drogen–deuterium
exchange
pathway that is almost or com-
pletely absent for all other iso-
topologues.
A solution to this mechanistic puzzle is provided by a de-
tailed analysis of the potential-energy surface (PES) depict-
ed in Scheme 1, which includes a degenerate [1,3]-hydrogen
migration 1 Q 1 as the central step. In view of the limita-
Thus, in the further discussion TS2’ is no longer consid-
ered and we focus on a comparison of DE=ETS2ꢀE
TS1
(Figure 1). The reaction via TS2 must not be confused with
a sequence of formal [1,2]-hydrogen migrations which we
could not locate and properly characterize on the PES. At
the best levels of theory, that is, G3B3, CBS-APNO and
[8]
tions of DFT based methods, in addition to B3LYP/6-
[
9]
3
11++G** calculations, the following electronic structure
[10]
ꢀ1
methods were employed: CCSD/6-311++G**, CASSCF-
W1U, this energy difference amounts to 2.8–3.8 kcalmol ,
[11,12]
[13]
[14]
A( 7,7)/aug-cc-pVTZ,
C
H
T
U
N
G
T
R
E
N
N
U
N
G
G3B3,
W1U, using the Gaussian 03 package.
The PES and the optimized geometries calculated at the
Weizmann-1U (W1U) level, which serves as a representa-
tive, are shown in Figures 1 and 2, respectively. The relative
energies as well as the free enthalpies (DG298) calculated
with all methods employed are listed in Table 2. While there
CBS-APNO,
and
thus explaining why the degenerate [1,3]-hydrogen migra-
tion 1 Q TS2 Q 1 cannot compete with the enol–keto tauto-
merization 1 ! TS1 ! 2, as long as isotope effects are not
included. The latter has been done for two relevant isotopo-
logues, and the data for DE are given in Table 3.
[15]
[16–18]
[15]
Direct evidence for the advantageous interaction favoring
the out-of-plane TS2 transition pathway is provided by Nat-
11816
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ꢄ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2009, 15, 11815 – 11819

ACHTUNGTRENNUNG[ 1,3]-Hydrogen Migration
COMMUNICATION
of such incremental C(3)···H(1)
transition-state bonding is also
reflected in the significant NRT
bond
order
(bNRT =0.473,
level)
B3LYP/6-311++G**
found at TS2 geometry. Al-
though partial bond formation
with the migrating hydrogen
competes with allylic-like con-
jugative stabilization of the rad-
ical cation, it nevertheless
lowers the overall barrier to
out-of-plane hydrogen transfer
compared to the seemingly
more “direct” in-plane pathway.
Inspection of these data re-
Figure 2. Geometries of relevant points of the PES (Figure 1) calculated at the W1U level. Distances are given veals that the barriers of both
in ꢅ.
[1,3]-hydrogen shifts via TS1
and TS2 increase by about
ꢀ
1
1
kcalmol when the migrating
ꢀ
1
Table 2. Relative energies (in kcalmol ) of the structures given in
Figure 1. Numbers in parentheses refer to relative free enthalpies
hydrogen atom is replaced by deuterium. As a consequence,
for all enol ions 1a–1h the degenerate isomerization 1 Q
TS2 Q 1 has no chance to compete with the anyhow slightly
favored enol–keto tautomerization 1 ! TS1 ! 2 ! 3 +
(
DG298).
Method
1
TS1
TS2
TS2’
2
3 + CH
3
+
B3LYP/6-311++G**
0.0
43.3
48.2
53.8
7.4
28.4
ACHTUNGTRENNUNG
CH C. However, in the case of CH C(OD)CD C (1i)—and
3
3
2
A
T
N
R
N
N
A
H
U
G
R
N
N
A
H
U
G
R
N
U
G
A
T
N
R
N
N
A
H
U
G
R
N
G
almost exclusively only for this very isotopologue—a new
situation arises such that the enol–keto tautomerization gets
disfavored because the migrating unit corresponds to a deu-
terium atom, whereas the degenerate 1,3-shift involves a hy-
CCSD/6-311++G**
A
H
N
T
E
N
G
A
T
G
E
N
N
A
H
N
T
E
N
G
A
H
U
G
R
N
N
(58.9)
A
H
U
T
E
N
G
A
H
U
G
R
N
U
G
[
a]
[b]
CAS
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
–
–
–
–
[
a]
A
H
N
T
E
U
G
A
H
U
G
E
N
N
A
H
U
T
E
N
G
ACHTUNGTRENNUNG
G3B3
55.8
26.1
(16.5)
27.3
(16.7)
27.9
(18.3)
drogen atom. As
a
consequence, the isomerization
A
H
U
G
R
N
U
G
A
H
U
G
R
N
N
A
H
N
T
E
U
G
A
H
U
G
R
N
N
(56.7)
A
T
N
T
E
N
G
A
H
U
G
E
N
N
+
+
CH C(OD)CD C (1i) ! CH C(OD)CD HC (1j) can con-
[
a]
3
2
2
2
CBS-APNO
W1U
–
–
[
a]
tribute to the overall reactivity. The newly formed enol in 1j
has then the option to isomerize back to 1i or to move on
to produce CH DC(O)CD HC . From the latter, the methyl
A
H
U
G
R
N
N
A
H
U
G
R
N
U
G
A
T
N
R
N
U
A
H
N
T
E
N
N
ACHTUNGTRENNUNG
55.2
(55.9)
+
A
H
U
G
E
N
G
A
H
N
R
N
N
A
T
N
T
E
N
N
A
H
U
G
R
N
U
G
A
H
N
R
N
U
G
A
H
U
G
R
N
U
G
2
2
[
a] Converged to TS2. [b] Not calculated due to the active orbitals sepa-
groups CH D and CHD can be eliminated thus explaining
2
2
ration problem.
the rather significant amount of hydrogen scrambling in the
+
case of CH C(OD)CD C .
A
3
2
comparison of the DE and
DDG298 values demonstrate that
the inclusion of entropy effects
does not affect the basic conclu-
sions. To much smaller extents,
also the data of some other iso-
topologues indicate the occur-
rence of very little hydrogen
scrambling via TS2; for exam-
ꢀ
1
Table 3. Energy differences of the transition structures TS1 and TS2 (DE=ETS2ꢀETS1 in kcalmol ) for the
isotopologous enol radical cations 1, 1g, and 1i. In parentheses, the DDG298 values are given.
Enol Ion
B3LYP/
-311++G**
CCSD/
6-311++G**
CAS
aug-cc-pVTZ
A
H
U
G
R
N
N
(7,7)/
G3B3
CBS-
W1U
6
APNO
+
+
+
CH
CD
CH
3
3
3
C(OH)CH
C(OH)CH
C(OD)CD
2
2
2
C
C
C
(1)
4.9
(5.4)
5.8
(6.3)
3.9
(4.2)
3.5
1.6
3.4
3.8
2.8
ACHTUNGTRENNUNG
A
H
U
G
R
N
U
G
A
H
U
T
N
U
A
H
U
T
N
U
A
H
U
T
N
U
G
A
H
U
T
N
U
G
(1g)
(1i)
A
H
U
G
R
N
U
G
A
H
U
T
E
U
G
A
H
U
T
E
U
G
A
H
U
T
E
U
G
A
H
U
T
E
U
G
ACHTUNGTRENNUNG
A
H
U
G
R
N
N
A
H
U
G
E
U
A
H
U
G
E
U
A
H
U
G
R
N
N
A
H
U
G
R
N
N
ACHTUNGTRENNUNG
ple, losses of CH DC from 1 f
2
and 1g. Compound 1i is unique,
however, in that about a third
[
19]
ural Bond Orbital (NBO) and Natural Resonance Theory
(
of the methyl radicals lost can only be accounted for by the
occurrence of the degenerate rearrangement 1 Q TS2 Q 1.
The origin of this particular situation is a manifestation of
[20]
NRT) analysis. The optimal NBO Lewis structure for the
equilibrium radical cation exhibits the expected out-of-plane
p-type orbital vacancy at the central carbon (LP* NBO, sim-
isotopically-sensitive branching, a phenomenon usually
[21]
ilar to that of BH ) in the minority b-spin set. The vacant
known in biochemical processes. Specifically, most of the
isotopologous radical cations of 1 do not explore TS2,
whereas the [O-D] labeling in 1i rises the barrier for the iso-
merization via TS1, such that the pathway via TS2 can start
3
precursor LP* orbital can stabilize out-of-plane hydrogen
migration by forming a C(3)–H(1) b-spin “half-bond” NBO
in the TS2 geometry, as displayed in Figure 3. The presence
Chem. Eur. J. 2009, 15, 11815 – 11819
ꢄ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
11817

H. Schwarz et al.
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[
[
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5
957.
[
5] The mass spectrometric experiments were performed by using a
modified VG instruments ZAB mass spectrometer; this is a four
sector BEBE instrument whose performance has been described in
Figure 3. b-spin NBO for C(3)ꢀH(1) “half-bond” in the TS2 transition-
state geometry (B3LYP/6-311++G** level).
[6]
detail previously. Briefly, enol cation radicals were generated by
7
0 eV dissociative ionization (McLafferty-type g-hydrogen rear-
rangement) of appropriately labeled 2-hexanones, the relevant enol
ions 1–1i were mass-selected by B(1)E(1) and their unimolecular
dissociations recorded by scanning B(2). The labeled 2-hexanones
were synthesized by standard laboratory procedures, purified by
preparative gas chromatography, and characterized by spectroscopic
means. Particular attention was paid to the integrity of the labeling
in the 2-hexanones deuterated in the a-and a’-positions because
keto–enol tautomerization may lead to scrambling during workup.
[
22]
to compete. The influence of this additional pathway on
the dynamics of the dissociation of energized 1 may form a
challenging subject for future theoretical trajectory studies.
A take-home message is that even for seemingly exhaustive-
ly investigated molecules, systematic labeling studies may
not only provide additional mechanistic insight, but can also
reveal completely novel facets.
1
However, detailed H NMR analysis revealed >98% recovery of
the labels in the original a-and a’-positions.
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[
[
[
Acknowledgements
Support by the Fonds der Chemischen Industrie, the Deutsche For-
schungsgemeinschaft (Cluster of Excellence: “Unifying Concepts in Cat-
alysis“), and the Academy of Sciences of the Czech Republic
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(
Z40550506) is acknowledged. X.Z. is grateful to the Alexander von
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tions by Dr. Marija Semialjac are appreciated.
[
Keywords: hydrogen migration · isotope effects · keto–enol
tautomerism · nonergodic behavior · reaction mechanisms
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ꢄ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2009, 15, 11815 – 11819

ACHTUNGTRENNUNG[ 1,3]-Hydrogen Migration
COMMUNICATION
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[
17] Frequency calculations were carried out for all optimized structures
with the same method to confirm the nature of the stationary point
on the PES and to obtain the zero-point energies (ZPE) as well as
to estimate the isotope effects. Relative energies (corrected for
[22] Note that for the only other [O-D]-labeled compound studied (1e),
occurrence of the degenerate rearrangement is “invisible” in the
mass spectrometric experiments because the resulting ions are iden-
ꢀ
1
+
+
ZPE) are reported in kcalmol . Intrinsic reaction coordinate calcu-
lations were performed to follow the reaction pathway and link
transition structures (TS) with the respective intermediates. In the
tical, that is, CH
3
C(OD)CH
2
C
! CH
2 3
C(OD)CH
C .
CAS calculations, the three 2p
z
orbitals of the carbon atoms, the 2p
z
Received: August 5, 2009
Published online: October 6, 2009
and 2p orbitals of oxygen, and the two 1s orbitals of the two hydro-
x
Chem. Eur. J. 2009, 15, 11815 – 11819
ꢄ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
11819