Catalysis of 6π electrocyclizations

Article abstract of DOI:10.1002/anie.200803336

(Chemical Equation Presented) Filling the gap: Lewis acid catalysis ofa 6π electrocyclization has been achieved for the first time (see scheme). DFT calculations were performed on the coordination ofa Lewis acid to a Lewis basic group in the 2-position ofa triene, and experimentally, the rates ofthe electrocyclization increase in the presence of Me₂AlCl.

Full text of DOI:10.1002/anie.200803336

Communications
DOI: 10.1002/anie.200803336
Electrocyclization Reaction
Catalysis of 6p Electrocyclizations**
Lee M. Bishop, Jennifer E. Barbarow, Robert G. Bergman,* and Dirk Trauner*
The synthetic power of pericyclic reactions has greatly
increased with the emergence of catalytic variants. Indeed,
catalysis in cycloadditions[1] and sigmatropic rearrange-
ments[2] is now well established. General methods for the
catalysis of electrocyclizations, however, have remained
elusive, with the notable exception of the Nazarov cycliza-
tion.[3] The development of such methods would enable
electrocyclizations to occur under milder conditions and
create the possibility of catalytic asymmetric variants. Herein,
we report the first examples of catalytic 6p electrocyclizations
and provide a detailed investigation into the mechanism of
these reactions.
withdrawing groups located in the 2-position of hexatriene
systems have been observed to lower their electrocyclization
energy barriers,[6,7] sometimes by as much as 10 kcalmolÀ1.[7,8]
We envisioned exploiting this effect to catalyze 6p electro-
cyclizations by the coordination of a Lewis acid to a Lewis
basic electron-withdrawing group located in the 2-position of
a hexatriene system. This coordination should increase the
electron-withdrawing effect of the substituent, thereby
decreasing the electrocyclization energy barrier.
We began our investigations by computationally assessing
the viability of this approach in the catalysis of 6p electro-
cyclizations. Hexatriene systems with methyl ester substitu-
ents at all possible positions and orientations were modeled
by density functional theory (Figure 1). A proton, serving as
the simplest Lewis acid, was bonded to the carbonyl oxygen
Experimental and computational studies have shown that
the rate of 6p electrocyclizations can be influenced by varying
the electronics of the substituents on the triene.[4–7] Electron-
Figure 1. Relative electronic energies (kcalmolÀ1) of the thermal (numbers above line) and cprotonated carbonyl group (numbers below line)
electrocyclization pathways computed at the B3LYP/6-31G** level of theory. 1-substituted trienes (A and B); 2-substituted triene (C); 3-substituted
triene (D).[10]
[*] L. M. Bishop, J. E. Barbarow, Prof. R. G. Bergman, Prof. D. Trauner[+]
atom at the lone pair anti to the hexatriene.[9] As seen in
Figure 1, these calculations predict a slight increase and
decrease of the electrocyclization energy barrier for the
Department of Chemistry
University of California, Berkeley
Berkeley, CA 94720-1460 (USA)
(E,Z)- and (Z,Z)-1-carbomethoxy-substituted hexatriene sys-
tems (Figure 1A and B), respectively. Calculations addition-
ally predict a small decrease of the electrocyclization energy
barrier for the 3-substituted system (Figure 1D). However,
we found that the electrocyclization energy barrier is
predicted to decrease by 10 kcalmolÀ1 upon protonation of
the 2-carbomethoxy-substituted triene system (Figure 1C).
An intrinsic reaction coordinate search in both directions
from the protonated electrocyclization transition state of this
system suggests that the catalyzed pathway is a concerted
process, as no stationary points other than the transition state
were located between the protonated triene and protonated
cyclohexadiene.
Fax: (+1)510-642-7714
E-mail: rbergman@berkeley.edu
[+] Department of Chemistry
Ludwig Maximilians-Universität
Butenandstrasse 5-13, D-81377 Munich, Germany
E-mail: dirk.trauner@cup.uni-muenchen.de
[**] We acknowledge financial support from Novartis and Roche (to
D.T.), and from the National Science Foundation, grant no. CHE-
0345488 (to R.G.B.). L.M.B. thanks Jamin Krinsky and Kathleen
Durkin at the UC Berkeley Molecular Graphics Facility (NSF grant
no. CHE-0233882), and Michael Pluth, Jennifer Schomaker, Court-
ney Hastings, and Vincent Chan for helpful discussions.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200803336.
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Encouraged by these results, we focused our synthetic
efforts on hexatriene systems with carbonyl-containing sub-
stituents at the 2-position—specifically triene substrates 1 and
2 (see Scheme 1 and the Supporting Information).[11] A Stork–
Zhao olefination furnished (Z)-vinyl iodides 3 and 4, which
were coupled with stannanes 5 and 6, respectively. Modified
Liebeskind coupling conditions allowed the synthesis of
triene substrates 1 and 2 under mild conditions.[12]
chloride (Me2AlCl) was an excellent catalyst for this reaction.
The addition of one equivalent of Me2AlCl resulted in a
significant rate increase, giving a half-life of 21 minutes at
508C. The reaction in the presence of Me2AlCl yields cis-
cyclohexadiene 7, indicating that the catalyzed reaction also
proceeds by the expected thermal disrotatory pathway.[13]
A plot of the observed first-order rate constant for the
electrocyclization of 1 to 7 in [D6]benzene at constant initial
substrate concentration versus varying Me2AlCl concentra-
tion is shown in Figure 3. For a reference, the thermal first-
Figure 3. Saturation in Me2AlCl. Conditions: [1]=40 mm, in C6D6 at
508C.
Scheme 1. Synthesis of triene substrates 1 and 2. DMF=N,N-dime-
thylformamide, HMPA=hexamethylphosphoramide, TC=thiophene-2-
carboxylate.
order rate constant in [D6]benzene at this temperature is
4.75(4) ” 10À5 sÀ1. The data in Figure 3 provide clear evidence
of catalytic turnover—the rate of the reaction is increased at
catalyst loadings greater than 17 mol% (kobs = 1.27(4) ”
10À4 sÀ1), and all the kinetic data fit a first-order exponential
model. A plot of the logarithm of the rate constant versus the
logarithm of the Me2AlCl concentration at sub-stoichiometric
catalyst loadings yields a straight line with a slope of 0.88(4),
indicating the reaction is first order in catalyst (see the
Supporting Information). Also evident in Figure 3 is the fact
that the rate increase begins to level off at approximately one
equivalent of the catalyst, suggesting tight binding of the
catalyst to the triene substrate.
Triene 1 cyclizes thermally to cis-substituted cyclohex-
adiene 7 (Figure 2). This reaction proceeds quantitatively (on
the basis of 1H NMR analysis) with a half-life of four hours at
508C. After investigating a variety of Lewis acids (see the
Supporting Information) we found that dimethylaluminum
The nature of the catalyst–substrate binding was addi-
1
tionally investigated by conducting a H NMR titration of 1
with Me2AlCl at 108C. A shift in all resonances of the
1H NMR spectrum of 1 is observed (see the Supporting
Information). This shift levels off at approximately one
equivalent of Me2AlCl for all resonances, which provides
additional evidence for an energetically favorable 1:1 binding
of Me2AlCl to 1.
A substrate-saturation curve was assembled to measure
the order in the substrate, to extract a Michaelis constant, and
to determine the rate acceleration for the catalyzed reaction
(Figure 4). A plot of the logarithm of the initial reaction
velocity versus the logarithm of the initial substrate concen-
tration (where [1] < [Me2AlCl]) yields a straight line with a
slope of 0.99(3), indicating the reaction is first order in triene
1 (see the Supporting Information). The substrate-saturation
curve also provides additional evidence for the high affinity of
Figure 2. Kinetic plots (with first order exponential decay fits) and half-
lives of the electrocyclization of 1 at 508C in the presence and absence
~
*
of Me2AlCl. : 1 equiv Me2AlCl, t1/2 =21 min; : 0.43 equiv Me2AlCl,
1/2 =41 min; &: thermal, t1/2 =4 h.
t
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Communications
data are in good agreement with the 13-fold rate acceleration
that is measured in the substrate-saturation experiments.
The rate of the electrocyclization of 2 to 8 is also
substantially increased in the presence of catalytic or stoi-
chiometric amounts of Me2AlCl (see Table 2 and the Sup-
Table 2: Activation parameters of the thermal and catalyzed (1 equiv
Me2AlCl) electrocyclizations.
Figure 4. Saturation in 1 (fit to the Michaelis–Menten equation).
Conditions: [Me2AlCl]=40 mm, in C6D6, at 508C.
Thermal
Catalyzed
DH° [kcalmolÀ1
DS° [e.u.]
]
20.3(4)
À12.4(5)
24.0(5)
18.1(1)
À11.6(1)
21.6(1)
Me2AlCl for 1, as this curve can be fit to the Michaelis–
Menten equation to give a Michaelis constant of 38(3) mm.
Furthermore, a first-order rate constant (kcat) of 6.2(2) ”
10À4 sÀ1 for the electrocyclization of the Me2AlCl-bound
substrate at 508C can be extracted, which is in excellent
agreement with the data obtained under catalyst-saturation
conditions (Figure 3). This data represents a 13-fold rate
increase of the catalyzed 6p electrocyclization over that of the
thermal reaction (thermal kobs = 4.75(4) ” 10À5 sÀ1 at 508C).
An Eyring plot of the thermal reaction reveals activation
parameters typical of a carba-6p electrocyclization (see
Table 1 and the Supporting Information).[5] The Eyring plot
of the catalyzed reaction was assembled under saturation
conditions (2 equiv Me2AlCl) to assure that the parameters
being measured are those for the electrocyclization of the
catalyst-bound triene, without any effect from the pre-
equilibrium of the catalyst–triene complex. Under these
conditions, a 1.7 kcalmolÀ1 decrease in the Gibbs free
energy of activation is measured (Table 1) for the catalyzed
electrocyclization relative to that of the thermal reaction. This
value corresponds to a 2.5 kcalmolÀ1 decrease in the enthalpy
of activation, and a 0.8 kcalmolÀ1 decrease in TDS° (298 K),
which indicates that the catalysis is primarily enthalpic.
Additionally, the data obtained from the Eyring plots can
be compared to the data obtained in the substrate-saturation
experiments. An 11-fold rate acceleration would be expected
at 508C based on the measured activation parameters. These
DG°298 [kcalmolÀ1
]
Conditions: [2]=40 mm, in C6D6.
porting Information).[14]
A 55-fold rate acceleration is
observed for this substrate in the presence of 1 equiv of the
Lewis acid at 288C, and Eyring analysis reveals a 2.4 kcal
molÀ1 decrease in the Gibbs free energy of activation for the
catalyzed process. Again, this catalysis is primarily enthalpic,
exhibiting a 2.2 kcalmolÀ1 decrease in the enthalpy of
activation and a 0.2 kcalmolÀ1 increase in TDS° (298 K).
This result demonstrates that ketones as well as esters are
suitable Lewis-basic groups for catalytic 6p electrocycliza-
tions, and is an indication of the broad synthetic scope of this
reaction.
In conclusion, the catalysis of 6p electrocyclizations has
been achieved for the first time. Our experimental work
confirms the predictions of the density functional theory
calculations, which suggest that such catalysis is possible with
hexatriene systems substituted in the 2-position with suitable
functional groups. We have synthesized trienes 1 and 2 and
found that the rates of their electrocyclizations are increased
in the presence of Me2AlCl. The reaction is catalytic in
Me2AlCl, first order in both catalyst and substrate, and
exhibits saturation behavior for both catalyst and substrate.
The Gibbs free energy of activation for the catalyzed pathway
is 1.7 kcalmolÀ1 lower than that of the thermal pathway for 1,
and 2.4 kcalmolÀ1 lower for 2. Efforts towards catalytic
asymmetric 6p electrocyclizations, as well as organocatalysis,
are currently underway in our laboratories.
Table 1: Activation parameters of the thermal and catalyzed (2 equiv
Me2AlCl) electrocyclizations.
Experimental Section
Experimental procedures, kinetic data, and characterization data for
products are available in the Supporting Information.
Thermal
Catalyzed
Received: July 9, 2008
Published online: September 10, 2008
DH° [kcalmolÀ1
DS° [e.u.]
]
22.4(5)
À9.2(4)
25.2(5)
20.0(2)
À11.8(2)
23.5(2)
DG°298 [kcalmolÀ1
]
Keywords: cyclization · density functional calculations ·
homogeneous catalysis · pericyclic reaction · transition states
Conditions: [1]=40 mm, in C6D6.
.
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[8] Relative to the hexatriene analogue where the electron-with-
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[9] Proton coordination to the carbonyl oxygen atom, syn to the
triene, as well as proton coordination on the alkoxy oxygen atom
were also modeled. However, these coordination sites were
found to have less pronounced effects on the electrocyclization
energy barriers.
[10] Calculations were carried out at the B3LYP/6-31G** level by
using GAUSSIAN 03: M. J. Frisch, et al. Gaussian 03; Gaussian,
Inc.: Pittsburgh PA, 2003. See the Supporting Information
section for complete citation. Energies shown are zero-point-
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also performed using MACROMODEL: F. Mohamadi, N. G. J.
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1
[14] This reaction is first order in Me2AlCl, and a H NMR titration
reveals binding that is similar to that for triene 1 (see the
Supporting Information). Product decomposition was observed
with superstoichiometric amounts of Me2AlCl.
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