Tautomerism in bis(oxazoline)s

Article abstract of DOI:10.1002/ejoc.201301282

Bis(oxazoline)s (BOXs) are a privileged ligand class and have found widespread use in catalysis. Herein, the tautomerism of selected BOX ligands was evidenced by X-ray diffractometry as well as by NMR and IR spectroscopy and supported by DFT calculations. In CDCl₃ solution at room temperature, the new 1,1-bis(4,4-dimethyl-1,3-oxazolin-2-yl)-1-phenylmethane ( ^Ph,HBOX-Me₂) ligand is present as a 1:1 mixture of the diimine and iminoenamine tautomers. Thermodynamic and kinetic data for the tautomeric equilibrium were determined, which allowed comparison with related bidentate ligand classes. The other BOXs studied, ^H,HBOX-Me ₂, ^Me,HBOX-Me₂, and ^tBu,HBOX-Me ₂, are largely present in the diimine form under similar conditions. IR spectroscopy was identified as a valuable tool for detecting the presence of the iminoenamine form as a minor component. Tautomerism in the prominent bis(oxazoline) ligand class is evidenced by X-ray diffractometry as well as by NMR and IR spectroscopy and supported by DFT calculations. Thermodynamic and kinetic data for the tautomeric equilibrium are determined for a specific example, which allows comparison with related bidentate ligand classes. Copyright

Full text of DOI:10.1002/ejoc.201301282

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201301282
Tautomerism in Bis(oxazoline)s
[a]
[a]
[a]
Adam Walli, Sebastian Dechert, and Franc Meyer*
Keywords: N ligands / Tautomerism / NMR spectroscopy / Kinetics / Structure elucidation
Bis(oxazoline)s (BOXs) are a privileged ligand class and have
found widespread use in catalysis. Herein, the tautomerism
of selected BOX ligands was evidenced by X-ray dif-
fractometry as well as by NMR and IR spectroscopy and sup-
Thermodynamic and kinetic data for the tautomeric equilib-
rium were determined, which allowed comparison with re-
lated bidentate ligand classes. The other BOXs studied,
H,H
Me,H
tBu,H
BOX-Me
2
,
BOX-Me
2
, and
2
BOX-Me , are largely
ported by DFT calculations. In CDCl
3
solution at room tem-
present in the diimine form under similar conditions. IR spec-
troscopy was identified as a valuable tool for detecting the
presence of the iminoenamine form as a minor component.
perature, the new 1,1-bis(4,4-dimethyl-1,3-oxazolin-2-yl)-1-
Ph,H
phenylmethane (
2
BOX-Me ) ligand is present as a 1:1
mixture of the diimine and iminoenamine tautomers.
Introduction
Since bis(oxazoline) (BOX) ligands were first developed
by several groups around 1990,[1–5] their metal complexes
have found use in a variety of metal-mediated catalytic reac-
tions, in particular in asymmetric synthesis. BOXs have
quickly developed into a high-performance ligand platform
and are now classified among the so called “privileged li-
gands”. Some BOXs are commercially available, such as
methylene- and isopropylidene-bridged ones (R = RЈ = H
or Me; Scheme 1a). Most common are metal complexes of
neutral bis(oxazoline) ligands[
6–12]
that have two alkyl sub-
Scheme 1. Structural motives of (a) bis(oxazoline)s (BOXs), (b) the
related β-diketimines, (c) methylenebis(imidazoline)s, and (d) the
stituents at the bridging backbone C atom (R = RЈ ϶ H),
but complexes with deprotonated monoanionic bis(oxazol- potential tautomeric BOX isomers. This work: R = Ph, tBu, Me,
inate) are also well established.[ The latter are closely re-
lated to the bidentate β-diketiminate ligands (Scheme 1b)
that are extremely popular in coordination chemistry.
In view of the widespread use of BOX ligands, it is some-
what surprising that little is known about the tautomerism
of BOXs containing a single substituent R at the backbone
bridge, or BOXs with the parent methylene spacer. In these
systems, the H atom may shift from the bridge C atom to
one of the oxazoline N atoms to give the iminoenamine
form; the latter may have E or Z configuration (Scheme 1d).
For β-diketones the enol isomer is usually far more stable
than the keto isomer, and β-diketimines are always regarded
as iminoenamines (Scheme 1b), but BOXs are mostly en-
countered and described in the form of the diimine. The
same is true for the closely related ligand class of azabis(ox-
13]
H; RЈ = H; RЈЈ = RЈЈЈ = Me.
azoline)s,[14] which also exist in the diimine form with the
CN
proton at the bridge N atom. Exceptions are the
BOXs
[
15]
and related semicorrins, which feature strongly electron-
withdrawing cyano groups at the bridge C atom. Some aza-
semicorrins have also been reported to exist in their imino-
[
16]
enamine form.
On the basis of UV/Vis spectra, Kova cˇ et al. assumed
that a small amount of an iminoenamine isomer was also
H,H
1
present in solutions of
BOX-tBu,H, but H NMR and
C NMR spectroscopy did not confirm this, and photo-
electron spectroscopy (PES) revealed no such tautomer in
13
[
17]
the gas phase. Just recently, the protonated bis(oxazoline)
Me,H
+[18]
Me,H
+ [19]
s [H{
BOX-Me }]
and [H{
BOX-Ph,H}] ,
2
which were serendipitously isolated, were found to exist in
[
a] Institut für Anorganische Chemie
the (Z)-enamine form in the solid state. Even more recently,
Georg-August-Universität Göttingen
3 6 4 2
p-CF C H CH ,H
Tammannstr. 4, 37077 Göttingen
the bis(oxazoline)
BOX-H was crystallized as
2
E-mail: franc.meyer@chemie.uni-goettingen.de
http://www.meyer.chemie.uni-goettingen.de/
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201301282.
the iminoenamine tautomer, and, furthermore, two tauto-
[
20]
mers were observed in solution.
Related methylene-
bridged bis(imidazoline)s were found to undergo tautomer-
7044
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2013, 7044–7049

Tautomerism in Bis(oxazoline)s
ism depending on the solvent and protonation state,[21,22]
and the conjugated iminoenamine tautomer is thermody-
namically more favorable (Scheme 1c).[ Herein, we report
a detailed NMR and IR spectroscopic and crystallographic
22]
study of the tautomeric equilibrium of the neutral bis(ox-
Ph,H
azoline)
BOX-Me , and we present relevant thermo-
2
dynamic and kinetic parameters, accompanied by DFT cal-
culations. This is important and fundamental information
for understanding and rationalizing the reactivity of this
prominent class of compounds.
Figure 1. X-ray solid-state molecular structures of (a) the iminoen-
Ph,H
H,H
amine
BOX-Me
2
and (b) the diimine
2
BOX-Me with se-
Results and Discussion
lected atoms labeled. Displacement ellipsoids are drawn at the 50%
probability level; hydrogen atoms except the enamine H1 (a) and
the H atoms at the backbone C1 (b) are omitted for clarity; sym-
metry transformation used to generate equivalent atoms (Ј) in (a):
Synthesis
The bis(oxazoline)s used in this work, R,HBOX-Me (R 1.5 – x, y, 1.5 – z. The dotted line represents the hydrogen-bonding
2
interaction between H1 and N1Ј. Selected interatomic distances
and angles are listed in Table 1.
=
Ph, tBu, Me, H), were prepared in close analogy to pro-
cedures reported previously (see the Supporting Infor-
mation). The simple methylene-bridged
H,H
BOX-Me and
BOX-Me2 and
BOX-Me were prepared for
2
Me,H
the homologous 1,1-ethylene-bridged
Table 1. Selected metric parameters (bond lengths [Å] and angles
tBu,H
1
,1-neopentylene-bridged
Ph,H
H,H
[a]
2
[°]) of the
BOX-Me and
BOX-Me bis(oxazoline)s.
2
2
Me,H
comparison.
BOX-Me is often applied as a ligand for
2
Ph,HBOX-Me
[b]
H,HBOX-Me
[
23]
H,H
2
2
several metal ions. Although the preparation of
BOX-
[24]
N1–C6, N2–C11
C1–C6, C1–C11
C6–C1–C6Ј/11
1.313(3)
1.403(3)
117.7(3)
1.264(1), 1.260(1)
1.493(1), 1.497(1)
113.88(8)
Me was previously reported, we found no published an-
2
Ph,H
tBu,H
alytical data.
BOX-Me2 and
BOX-Me2 had not
been previously reported. We fully characterized all four
BOXs (see the Supporting Information). Contrary to other
[
a] Additional parameters are provided in the text and the Support-
ing Information. [b] Bonds are symmetrically equivalent.
bis(oxazoline)s, which were obtained as well-soluble color-
Ph,H
less liquids,
BOX-Me precipitated in the course of its
2
synthesis from the reaction solution. We attribute this to
of 2.15 Å and an angle of 131° (cf. Table 2 for additional
hydrogen-bonding parameters). The averaged C–C and
the presence of the iminoenamine tautomer in the case of
Ph,H
BOX-Me , which apparently is insoluble under the ap-
2
C–N bond lengths within the C1(–C6=N1···H1–N1Ј–
Ph,H
plied conditions.
C6=) ring of
BOX-Me suggest complete delocaliza-
2
tion, but the localized positions of the two enamine hydro-
gen atoms (H1, H1Ј) indicate that an overlay of an en-
amine–imine and an imine–enamine structure is more likely
(Scheme 2).
Solid-State Structure of Ph,HBOX-Me and H,HBOX-Me2
2
Small colorless needles of Ph,HBOX-Me were obtained
2
by slow concentration of a CH Cl /methanol (9:1) solution;
2
2
Table 2. Hydrogen-bonding parameters (bond lengths [Å] and
a single crystal was analyzed by X-ray diffraction. The mo-
Ph,H
angles [°]) of the iminoenamine form of
2
BOX-Me .
Ph,H
lecular structure of
BOX-Me , which crystallizes in the
2
D–H
D···A
H···A
2.726(5)
D–H···A
131(9)
monoclinic P2/n space group, is depicted in Figure 1a (see
Ph,H
also Table 1). Clearly,
BOX-Me exists in its (E)-imino- N1–H1···N1Ј
0.78(10)
2.15(10)
2
enamine form in the solid state.
Ph,H
The two oxazoline rings in
BOX-Me are roughly co-
2
planar (the torsion angle between the C6–N1 and C6Ј–N1Ј
distances of 1.40 and 1.31 Å differ strongly from those
found for oxazolines in their diimine form[ but are be-
tween typical values for C–C and C=C bonds and for C–N
7]
and C=N bonds, respectively, and thus reflect delocaliza-
Scheme 2. Autotrope interconverted tautomers disordered in the
tion or crystallographic disorder.[
25]
The cavity between solid-state structure of
Ph,H
BOX-Me2.
both oxazoline rings is widened at the front with an angle
between both C–N bonds of 14.3°; the N···N separation
The nature of the crystallographic disorder remains un-
is 2.73 Å. The backbone phenyl ring at C1 is rotated by clear: the structure is either statically or dynamically disor-
approximately 71° relative to the plane defined by the two dered. However, dynamic disorder seems reasonable, as re-
oxazoline rings. The enamine H1 proton lies in the latter lated compounds were found to show such phenomenon
plane [0.1(1) Å distance] and forms a moderate to weak, (see below). It should be emphasized that the two disor-
[
26]
resonance-assisted N–H···N hydrogen bond with a length dered structures are identical tautomers (autotrope in-
Eur. J. Org. Chem. 2013, 7044–7049
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
7045

A. Walli, S. Dechert, F. Meyer
SHORT COMMUNICATION
terconverted, Scheme 2).[27] Prototropic tautomerization in
the solid state is a relatively rare phenomenon found in,
for example, cyclic hydrogen-bonded pyrazole derivatives,
which have been intensively studied by solid-state CPMAS
(
cross polarization, magic angle spinning) NMR tech-
[
28]
Ph,H
niques. More closely related to
BOX-Me are dibenz-
2
oylmethane and analogous 1,3-diketones,[
29–31]
the degen-
erate, intramolecular hydrogen-bridged tautomers of which
resemble the situation found here. However, the solid-state
Ph,H
tautomerism of
BOX-Me was not further investigated
2
as part of the present study.
For comparison, we also determined the solid-state mo-
H,H
lecular structure of
BOX-Me . Large single crystals were
2
H,H
obtained by vacuum sublimation.
BOX-Me crystallizes
2
in the orthorhombic Pbca space group; the molecular struc-
ture is shown in Figure 1b. H,HBOX-Me is clearly present
in its diimine from; the two oxazoline rings are tilted by
2
Figure 2. (a) Equilibrium of diimine and iminoenamine tautomers
Ph,H
1
of
2
BOX-Me . (b) Parts of the H NMR spectrum of the pure
around 78.4°. The C2–C1–C7 angle at the bridging CH2 iminoenamine tautomer at –50 °C and an equilibrated mixture af-
3
group is 113.9°, which reflects sp hybridization. Accord- ter warming to 25 °C; the NH regions are magnified, and reso-
nance-free regions are omitted for the sake of clarity; the asterisks
indicate the residual chloroform solvent peaks. See text for details
ingly, the C1–C6/11 (1.49/1.50 Å) bonds are distinctly
longer and the C6/11=N1/2 (both 1.26 Å) bonds are shorter
and Figures S1–S5 in the Supporting Information for the
Ph,H
than those in
tautomer.
BOX-Me , as expected for the diimine
1
13
15
1
2
H NMR, C NMR, and { N, H} HMBC NMR spectra for all
1
3
1
four BOXs as well as the { C, H} HSQC NMR spectrum of
Ph,H
2
BOX-Me .
A
search in the Cambridge Structural Database
[
32]
(
CSD) revealed over 100 deposited structures containing
a bis(oxazoline) moiety, most of which have BOXs as coor-
dinating ligands. Only about 10 structures of free bis(ox-
azoline)s are known, including the iminoenamine and the The enamine NH proton is observable at –50 °C as a broad
two protonated (Z)-iminoenamines mentioned above. How- peak (δ ≈ 9.0 ppm); no NH resonance signal could be de-
H
ever, the rest are diimines, and only one of them has a CH
tected at 25 °C. The diimine tautomer, seen at elevated tem-
2
spacer between the oxazoline rings,[ whereas all of the peratures after equilibration, is C -symmetric, and, hence,
33]
s
others are bridged by a dialkylmethylene group.
its oxazoline CH protons are diastereotopic (and aniso-
2
chronous) and give a typical AX spin system. Also, the
methyl carbon atoms at each oxazoline are diastereotopic,
1
13
3
Characterization of Ph,HBOX-Me in Solution
which leads to two H and C resonances. The sp hybrid-
ization of the bridge C atom in the diimine tautomer is
2
NMR spectroscopy is the method of choice for studying reflected in its 13C NMR spectrum by a shift of δC
tautomerism. Its main drawback is its limited sensitivity, 46 ppm and a cross-peak with the C-bound proton that ap-
as it is not possible to detect minority tautomers that are pears at δH = 4.74 ppm in the { C, H} HSQC spectrum
present in less than 1–2%. In chloroform solution, in which (cf. Figure S3). { N, H} HMBC NMR analysis (Figure S4)
=
[
34]
13
1
15
1
Ph,H
BOX-Me is only slightly soluble, it exists as a mixture confirmed the assignment and the identity of both isomers:
2
of both tautomers, (E)-iminoenamine (N-protonated) and only two distinct 15N resonances are observed in the ex-
diimine (C-protonated, Figure 2a), with a ratio of about 1:1 pected regions at δN = –132 (ketimine N of the diimine iso-
at around room temperature. Although the formation of a mer) and –216 ppm (averaged signal for the iminoenamine
[
35]
Ph,H
(
Z)-iminoenamine tautomer is, in principle, feasible, we did isomer).
The obvious tautomerism of
BOX-Me ap-
2
not observe any indication of its presence during the course pears to be unique so far, as NMR spectroscopy of
H,H
Me,H
tBu,H
of this work.
BOX-Me2,
BOX-Me , and
BOX-Me bis(oxa-
2
2
1
13
Ph,H
The H and C NMR resonances of the different tauto- zoline)s did not show any indication of the presence of an
meric forms of
BOX-Me are well resolved at 25 °C iminoenamine tautomer in solution (Figures S1, S2, and
2
(Figure 2b; see also Figure S2 in the Supporting Infor- S5).
mation), and this is evidence of rather slow interconversion.
IR spectroscopy can be used as a complementary tool to
Unambiguous assignment was facilitated by the isolation of examine the tautomeric state of bis(oxazoline)s. Whereas
the pure iminoenamine tautomer: by dissolving an isomer- the diimine tautomer gives rise to a single broad distinctive
–1
ically pure crystalline sample at low temperature, we were band at around 1660 cm , which we assigned to the asym-
able to record the NMR spectra of the pure (E)-iminoen- metric and symmetric stretching modes (ν and ν ) of the
a
s
amine tautomer. A single set of signals and apparent C2v (N=C–C–C=N) group, the iminoenamine tautomer in the
Ph,H
symmetry reflect rapid proton exchange between both oxaz- crystalline material of
BOX-Me is identified by two
2
oline N atoms and the degenerate autotrope tautomerism. prominent and very strong bands at 1646 and 1558 cm–
1
7046
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© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2013, 7044–7049

Tautomerism in Bis(oxazoline)s
(
Figure 3). We assigned both bands to ν and ν of the Table 3. Thermodynamic data of the equilibrium of iminoenamine
a
s
and diimine tautomers.[ See Table S4 for additional DFT-calcu-
a]
C=C–C=N group, accompanied with in-plane bending (δ)
of the N–H bond (Figure S7 represents the characteristic
computed modes); they resemble a similar mode found in
the enol form of acetylacetone.
of both tautomers of
good agreement with experiment (see the Supporting Infor-
mation for details). Whereas
convert into the planar iminoenamine form because of the
bulky tBu group) exists solely as the diimine tautomer as
expected, the IR spectra of
lated parameters.
298
ΔH°
[kcalmol ]
ΔS°
[calK mol ]
ΔG°298
[kcalmol ]
K
T
–
1
–1
–1
–1
[
36]
DFT-calculated spectra
H,H
Ph,H
Ph,H
[b]
BOX-Me and
BOX-Me show
BOX-Me
BOX-Me
2
2.33Ϯ0.06
3.8
8.5Ϯ0.2
–
–0.21Ϯ0.08
0.7
1.43
0.31
2
2
Me,H
[c]
2
tBu,H
BOX-Me (which cannot [a] Parameters refer to the equilibrium (E)-iminoenamine i di-
2
3
imine. [b] In CDCl solution, derived from the van’t Hoff plot
shown in Figure S9; this work. [c] DFT calculation at the BP86/
TZVP level; ref.[
18]
H,H
Me,H
BOX-Me and
BOX-
2
Me interestingly suggest the presence of some iminoen-
2
amine tautomer as a minor component. See Figure S6 for
a comparison of all experimental and computed spectra.
In the H NMR spectrum of Ph,HBOX-Me
1
, no signal
2
broadening owing to proton exchange was observed over
the examined temperature range, and this reflects rather
slow exchange (k Ͻ 1 s ). Kinetic parameters of the tauto-
merization were determined by the equilibration method.
After dissolving the crystalline material of the pure imino-
enamine tautomer, the time dependence of the equilibration
–1
1
was monitored by H NMR spectroscopy in the range be-
tween 25 and –5 °C (Figure 4, see also Figure S8). Values
for the first-order rate constant kobs = k + k (with k1
1
–1
and k–1 denoting the forward and the backward reaction,
–
3
respectively) at 298 K are (6.90Ϯ0.62)ϫ10 , with k =
1
4.14Ϯ0.37)ϫ10–3 and k–1 = (2.76Ϯ0.25)ϫ10
–3
s
–1
(see
(
Table S1). Activation parameters were determined from the
temperature dependence of the respective rate constants
BOX-Me
2
. Spectra were (Eyring plot shown in Figure S10) and are compiled in
Figure 3. Experimental FTIR spectrum (black line) and DFT-cal-
culated vibrational spectra (grey) of
Ph,H
computed from the geometry-optimized structures of the diimine
grey dashed line) and iminoenamine structure (grey solid line) at
Table 4.
(
the B3LYP/def2-TZVPP level of theory. See the Supporting Infor-
mation for details and spectra of all BOXs (Figure S6). The charac-
teristic modes of the iminoenamine are represented in Figure S7.
Thermodynamic and Kinetic Parameters of the Equilibrium
From the ratio of the two tautomers of Ph,HBOX-Me2
1
(
determined by integration of the signals in the H NMR
spectrum) we estimated equilibrium constants K (pK ) be-
T
T
tween 0.93 (0.03) and 1.92 (–0.28) in the temperature range
68–323 K [KT = c(diimine)/c(iminoenamine)]. Thermo-
dynamic parameters were derived from a van’t Hoff plot
2
(
Figure S9, Table 3) and reveal only small energy differences Figure 4. Time course of the equilibration of the iminoenamine
Ph,H
form of
tures (top). Linearized form with solid lines showing fits to the
integrated rate law to obtain rate constants from the slope kobs
2
BOX-Me to the respective diimine at various tempera-
between the two tautomers: the diimine has a ΔH° of
–
1
2
8
.33(6) kcalmol
and
a
weakly positive ΔS° of
=
ϱ
–
1
–1
.5(2) calK mol . These parameters are comparable to
those of recent DFT calculations
BP86/TZVP level, Table 3). However, under the conditions
ϱ
0
k
1
+ k–1 (bottom). The asterisk stands for ln[(χ
BOX-Me2 in which χ is the fraction of iminoenamine at time t, χ
at equilibrium.
A
– χ
A
)/(χ
A
A
– χ
A
)]
[18]
Me,H
0
for
A
at t = 0
ϱ
and χ
A
(
applied in that study, only the diimine tautomer was ob-
served experimentally. In comparison, our DFT calcula-
The first-order rate and the relatively small and positive
tions resulted in similar thermodynamic parameters for entropy of activation for the iminoenamine Ǟ diimine
Ph,H
H,H
‡
–1
–1
BOX-Me2 and
BOX-Me2 (B3LYP/def2-TZVPP transformation, ΔS = (12Ϯ3) calK mol , suggest a uni-
[39]
level, Table S4). Finally, these parameters are reminiscent of molecular reaction in the rate-determining step.
those of acetylacetone (equilibrium ca. 90% on the enol experimental
side) and also of acetoacetates.
The
of
‡
activation
barrier
(ΔG )
[
37]
[38]
–1
Ph,H
(20.7Ϯ1.3) kcalmol for
BOX-Me is in the expected
2
Eur. J. Org. Chem. 2013, 7044–7049
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
7047

A. Walli, S. Dechert, F. Meyer
SHORT COMMUNICATION
Table 4. Activation parameters of the equilibration of iminoen- Acknowledgments
[
a]
amine tautomers. See Table S5 for additional parameters and
DFT-calculated values.
This work was supported by the international PhD programme Ca-
talysis for Sustainable Synthesis (CaSuS) at the Georg August Uni-
versity Göttingen.
ΔH‡
kcalmol ]
ΔS‡
[calK mol ]
ΔG‡298
[kcalmol ]
–1
–1
–1
–1
[
Ph,H
[b]
[c]
BOX-Me
BOX-Me
2
24.1Ϯ0.9
55.7
12Ϯ3
20.7Ϯ1.3
Me,H
2
[
1] R. E. Lowenthal, A. Abiko, S. Masamune, Tetrahedron 1990,
[
a] Parameters refer to the reaction: (E)-iminoenamine Ǟ diimine
46, 6005–6008.
(
k
1
). [b] In CDCl solution, derived from the Eyring plot shown in
3
[2] D. A. Evans, K. A. Woerpel, M. M. Hinman, M. M. Faul, J.
Am. Chem. Soc. 1991, 113, 726–728.
[3] E. J. Corey, N. Imai, H.-Y. Zhang, J. Am. Chem. Soc. 1991,
Figure S10; this work. [c] Zero-point energy corrected energies
from DFT calculations at the BP86/TZVP level; ref.[
18]
113, 728–729.
[
4] D. Müller, G. Umbricht, B. Weber, A. Pfaltz, Helv. Chim. Acta
991, 74, 232–240.
[5] J. Hall, J.-M. Lehn, A. DeCian, J. Fischer, Helv. Chim. Acta
991, 74, 1–6.
‡
1
range if tautomer interconversion can be observed (ΔG Ͻ
–
1
2
5 kcalmol ). However, it is significantly lower than the
1
‡
–1
barrier (zero-point energy corrected, ΔE ) of 55.7 kcalmol
that was calculated for the analogous intramolecular tauto-
merization reaction of
[
6] A. K. Ghosh, P. Mathivanan, J. Cappiello, Tetrahedron: Asym-
metry 1998, 9, 1–45.
Me,H
[18]
BOX-Me2. The latter value is [7] M. Gómez, G. Muller, M. Rocamora, Coord. Chem. Rev. 1999,
1
93–195, 769–835.
likely too high, as possible solvent assistance or tunneling
effects were not considered.
[
8] K. A. Jørgensen, M. Johannsen, S. Yao, H. Audrain, J. Thor-
hauge, Acc. Chem. Res. 1999, 32, 605–613.
Ph,H
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2
2
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0 kcalmol (Figure S14, Table S5). This is quite compar-
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It is likely that the
[
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2
[19] H. Arii, F. Nakadate, K. Mochida, Organometallics 2011, 30,
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[
[
[
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7048
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© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Received: August 26, 2013
Published Online: October 2, 2013
Eur. J. Org. Chem. 2013, 7044–7049
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
7049