Transition-Metal Catalysis of Triene 6π Electrocyclization: The π-Complexation Strategy Realized

Article abstract of DOI:10.1002/anie.202006992

Triene 6π electrocyclization, wherein a conjugated triene undergoes a concerted stereospecific cycloisomerization to a cyclohexadiene, is a reaction of great historical and practical significance. In order to circumvent limitations imposed by the normally

Full text of DOI:10.1002/anie.202006992

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
Chemie
International Edition
www.angewandte.org
Accepted Article
Title: Transition-Metal Catalysis of Triene 6π Electrocyclization: The
π-Complexation Strategy Realized
Authors: Joseph M. O'Connor, Pengjin Qin, Li-An Wang, Kim K.
Baldridge, Yifan Li, Burak Tufekci, Jiyue Chen, and Arnold L.
Rheingold
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202006992
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10.1002/anie.202006992
Angewandte Chemie International Edition
RESEARCH ARTICLE
p
p
Transition-Metal Catalysis of Triene 6 Electrocyclization: The -
Complexation Strategy Realized
Pengjin Qin[a], Li-An Wang[a], Joseph M. O'Connor*[a], Kim K. Baldridge*[b], Yifan Li[a], Burak Tufekci[a],
Jiyue Chen[a] & Arnold L. Rheingold[a]
[*]
P. Qin, L-A. Wang, Prof. J. M. O’Connor, Y. Li, B. Tufekci, J. Chen, Prof. A. L. Rheingold
Department of Chemistry and Biochemistry
University of California, San Diego
La Jolla, CA, 92093-0358 USA
E-mail: jmoconnor@ucsd.edu
[*]
Prof. K. K. Baldridge
School of Pharmaceutical Science and Technology
Tianjin University
92 Weijin Road, Nankai District
Tianjin, P. R. of China
Supporting information for this article is given via a link at the end of the document.
Abstract: Triene 6p electrocyclization, wherein a conjugated triene
undergoes concerted stereospecific cycloisomerization to
met with limited success.[10-12] Two fundamentally different
catalytic strategies have been described in the literature: a)
acceleration by interaction of a triene substituent with a Lewis
acid promoter (I, Figure 1b);[10-12] and b) acceleration by metal −
p-electron interaction(s).[13-15] The former approach was first
realized in 2008 when Bergman and Trauner described the use
of Lewis acid catalysts for accelerating electrocyclization of
trienes bearing ester or ketone substituents at the 2-position
(Figure 1b).[10a] Subsequent attempts to expand the range of
substituents and to develop enantioselective electrocyclizations
with chiral Lewis acids have revealed limitations to this
approach.[10b]
a
a
cyclohexadiene, is a reaction of great historical and practical
significance. In order to circumvent limitations imposed by the
normally harsh reaction conditions, chemists have long-sought to
develop catalytic variants based upon the activating power of metal
– alkene coordination. Herein we demonstrate the first successful
implementation of such a strategy by utilizing [(C5H5)Ru(NCMe)3]PF6
as a precatalyst for the disrotatory 6p electrocyclization of highly
substituted trienes that are resistant to thermal cyclization.
Mechanistic and computational studies implicate hexahapto
transition-metal coordination as responsible for lowering the
energetic barrier to ring closure. This work establishes a foundation
for the development of new catalysts for stereoselective
electrocyclizations.
a) Conrotatory (hν) and disrotatory (Δ) 6π electrocyclizations.
Δ
hν
b) Lewis acid catalyzed electrocyclization of 2-substituted trienes.
Ph
Introduction
Ph
Ph
AlMe2Cl
6π
Triene 6p electrocyclization is
a prominent molecular
50 °C
C6D6
Me
CO2Et
C
LA
rearrangement that involves the concerted formation of a s-bond
between the triene termini, with the corresponding loss of a p-
bond, or the reverse of that process (Figure 1a).[1,2] This
remarkable class of stereospecific transformations is found in
nature,[3] functions as a design element in stimuli responsive
materials,[4] and has been proposed as the ring-forming step in
the industrially desirable metal-catalyzed dehydrocyclo-
aromatization of alkanes to o-(n-alkyl)toluenes.[5] The reaction is
of great historical significance in that it provided an important
impetus for the development of the Woodward-Hoffmann
conservation of orbital symmetry theory.[2]
CO2Et
O
OEt
I
c) Can π-complexation accelerate 6π electrocyclization?
M(Ln-1
)
M(Ln)
M(Ln-2
)
II-η4
II-η6
II-η2
Figure 1. Thermal, photochemical and catalytic 6p electrocyclizations.
The synthetic power of 6p electrocyclization has been
demonstrated through applications for the synthesis of complex
cyclohexadienes,[6] aromatics,[7] and natural products;[8] however,
the high energetic barrier to ring closure has prevented this atom
economical transformation from reaching its full potential due to
competitive thermal rearrangements and decomposition
pathways for both starting trienes and cyclohexadiene
products.[9] Tantillo has discussed the challenges inherent in
catalyzing reactions that involve highly delocalized, nonpolar
transition-state structures.[12] It is therefore not surprising that
attempts to develop catalytic triene 6p electrocyclizations have
Despite a vast array of catalytic reactions facilitated by metal
− alkene coordination,[16] acceleration of 6p electrocyclization by
metal − p-complexation has not been previously demonstrated.
Herein we describe the first experimental evidence that metal −
p-coordination accelerates 6p electrocyclization of trienes,
including the first metal
−
catalyzed conversions of
unfunctionalized trienes to cyclohexadiene derivatives. A key
catalyst design element is the presence of three facial metal
coordination sites that permit new modes of transition-metal
coordination to acyclic conjugated trienes.
1
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10.1002/anie.202006992
Angewandte Chemie International Edition
RESEARCH ARTICLE
stereoisomeric 1,2-bis(prop-1-en-1-yl)cyclohex-1-enes, 7-ZZ, 7-
EZ, and 7-EE, as ideal substrates for mechanistic studies
(Figure 3a). Advantages of 7 include the presence methyl
groups that serve as convenient NMR spectroscopic reporters
for establishing the stereospecificity of ring closure, the
Results and Discussion
Acceleration of 6p electrocyclization by p-complexation could
reasonably take the form of dihapto (II-h2), tetrahapto (II-h4), or
hexahapto (II-h6) metal coordination (Figure 1c). Both h2 and h4
complexes of conjugated trienes are well known,[14,15,17] however
in no instance has conversion to a cyclohexadiene been
observed. Precisely the opposite, tetrahapto coordination has
been shown to decelerate triene electrocyclization.[14] Of the
presence of
a
fused six-membered ring to enforce
Z
stereochemistry at the central double bond, and the lack of
Lewis basic substituents in order to avoid mechanistic
ambiguities. The three stereoisomers of 7 were readily prepared
as described in the supporting information.
metal
– triene coordination modes shown in Figure 1c,
hexahapto structure II-h6 stands out given that there have been
no unambiguous examples of hexahapto coordination to an
acyclic conjugated triene.[18] Furthermore, computational studies
by Schleyer indicate that hexahapto coordination of an inorganic
cation, such as lithium ion, to (Z)-hexa-1,3,5-triene should lower
the activation energy for 6p electrocyclization.[19]
Under both thermal and photochemical conditions, the three
isomers of 7 failed to give a satisfactory yield of the anticipated
6p
electrocyclization
product,
6,7-dimethyl-1,2,3,4,6,7-
hexahydronaphthalene (8; Figure 3a). Thermolysis of 7-ZZ at
130 °C for one hour led to clean isomerization to 7-EZ and
continued heating at that temperature for 9 days gave a 22%
maximum yield of 8. Heating of the sample for an additional 29
days led to the formation of 2,3-dimethyl-1,2,3,5,6,7-
hexahydronaphthalene (9) in 52% yield. Both thermolysis of 7-
EE and photolysis of 7-EZ (acetone-d6, 254 nm broad-band
lamp, 7 h) led to decomposition of starting material with no
We previously observed that [(h5-C5R5)Ru(NCMe)3]PF6 (1-
Cp, R = H; 1-Cp*, R = Me) promotes the cycloaromatization of
conjugated enediynes[20,21a-c] and dienynes[21], possibly via the
intermediacy of hexahapto complexes wherein the metal is
coordinated to all six carbons of the p system. Based upon
those results, we speculated that the remarkable Chaudret and
Ito dehydrocycloaromatizations of (E)-hexa-1,3,5-triene (2-E)[22]
and hexa-1,5-diene (3)[22,23] to give ruthenium arene 4-Cp*
(Figure 2a) may involve the long sought-after p-complex –
accelerated 6p electrocyclization. We propose herein that 2-E
and 3 are converted in situ to (Z)-hexa-1,3,5-triene (2-Z) which
undergoes reaction with “Cp*Ru+” to generate a h6-hexatriene
intermediate (5-Cp*; Figure 2b). A 6p electrocyclization within
the coordination sphere of ruthenium would then give the
evidence for the formation of
8 or 9. Thus, commonly
encountered limitations of 6p electrocyclizations apply to 7.[9]
a) Thermolysis of trienes 7.
Me
Me
I2
130 °C
Me
1 h
C6D6
C6D6
Me
7-ZZ
Me
7-EZ (97%)
Me
7-EE (46%)
unsaturated
cyclohexadiene
complex
[(h5-C5Me5)Ru(h4-
9 days
decomposition
130 °C
130 °C
9 days
cyclohexadiene)]PF6 and/or saturated [(h5-C5Me5)Ru(h4,h2-
cyclohexadiene)]PF6 (6-Cp*), followed by dehydrogenation.[24]
Me
Me
Me
Me
130 °C
29 days
a) Literature reports of Ru − promoted dehydrocycloaromatizations.
9
8
(22% max)
(52% max)
Cp*Ru(C4H6)Cl
AgOTf
[Cp*Ru(OMe)]2
HOTf
b) Ruthenium − promoted dehydrocycloaromatization chemistry of trienes 7.
Ru
RT
90 °C
Me5
1-Cp
PF6
Me
Ru
3
2-E
4-Cp*
RT
CD2Cl2
Me
7-ZZ, 7-EE, 7-EZ
b) A mechanistic proposal for Ru − promoted dehydrocycloaromatizations.
Me
30 - 100 min
Me
10 (88 - 94%)
H
H
-H2
[Cp*Ru]
6π
Ru
4-Cp*
Ru
?
Figure 3. Cyclization chemistry of trienes 7.
Me5
Me5
2-Z
5-Cp*
6-Cp*
Although substituted hexatrienes have not previously been
observed to undergo [Cp*Ru]+
mediated dehydrocyclo-
–
Figure 2. Ruthenium − promoted dehydrocycloaromatizations may involve a
metal − accelerated 6p electrocyclization.
aromatization, we were pleased to find that the stereoisomers of
7 each underwent a clean reaction with 1-Cp in CD2Cl2 to give
the anticipated ruthenium – arene product 10 in 88 – 94% yields
(Figure 3b). Kinetic studies, carried out in the presence of
added NCMe to slow the reaction rate, established the half-lives
for disappearance of 7 to be: 7-ZZ (t1/2 = 0.88 h), 7-EZ (t1/2 = 1.60
h), and 7-EE (t1/2 = 0.64 h). Thus, there is a significant
stereochemical dependence on the rate of reaction.
Two defining characteristics of a p-complex – promoted 6p
electrocyclization are: a) concerted isomerization of a metal –
triene intermediate to a metal – cyclohexadiene intermediate;
and b) a high degree of stereospecificity; i.e. either conrotatory
or disrotatory ring closure. In an effort to observe metal – triene
In an effort to observe metal
– triene and metal –
and metal
– cyclohexadiene intermediates in cyclizations
cyclohexadiene intermediates, the reactions of 1-Cp with each
stereoisomer of 7 were independently monitored by variable
temperature 1H NMR spectroscopy (Figure 4, Table 1). The
mediated by 1-Cp, and to establish the stereospecificity for the
purported metal – cyclohexadiene formation, we chose the
2
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10.1002/anie.202006992
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RESEARCH ARTICLE
reaction of 7-ZZ and 1-Cp in acetone-d6 at –45 °C, resulted in
clean conversion to the chiral h4-triene complex 11-ZZ in 98%
yield (Figure 4a). Unfortunately, upon warming the sample to
ambient temperature, 11-ZZ underwent conversion to 10 without
observation of a cyclohexadiene intermediate. The structure of
11-ZZ was tentatively assigned on the basis of four relatively
upfield vinyl hydrogen resonances between d 4.02 – 4.75, and
associated methyl and cyclopentadienyl hydrogen resonances.
In a separate experiment, addition of hexanes to a small-scale
reaction of 7-ZZ and 1-Cp maintained at –45 °C led to isolation
of crystalline 11-ZZ, and an X-ray crystallographic analysis
confirmed the proposed structure (Figure 5, upper panel). The
3.885(6) Å C1××××C6 nonbonded distance and the position of C1
and C6 on opposite faces of the Ru,C2,C3,C4,C5 mean plane
(1.203(6) and -1.273(6) Å displacement, respectively) indicate
that a concerted 6p electrocyclization to a ruthenium – h4-
cyclohexadiene intermediate is geometrically prohibited. The h4-
triene coordination mode observed for chiral 11-ZZ is
unprecedented.[25]
sample to –20 °C led to clean conversion to the h6-triene
complex 12 (92% yield), and further warming to ambient
temperature led to formation of ruthenium – arene 10 in 88%
yield.
Table 1. 1H NMR spectroscopic data for trienes 7 and transient [CpRu]+
triene complexes 11 and 12.[a,b]
–
Cmpd
CH3
=CH-
C5H5
−
NCCH3
−
7-ZZ
1.63 (d, J = 6.0)
6.03 (d, J = 10.8)
(C6D6)
5.43 (m)
7-EE
(C6D6)
1.73 (d, J = 6.4)
6.94 (d, J = 15.4)
5.64 (m)
−
−
−
−
7-EZ
(CDCl3)
1.53 (d, J = 7.2)
1.76 (d, J = 6.4)
6.31 (d, J = 15.6)
5.87 (d, J = 15.6)
5.65 (m), 5.55 (m)
11-ZZ
((CD3)2CO,
−45 °C)
1.48 (d, J = 6.5),
1.77 (d, J = 5.5)
4.02 (d, J = 9.0)
4.51 (d, J = 9.5)
4.75 (m), 5.13 (m)
5.26
(s)
2.55
(s)
a) Reactions of 1-Cp with 7-ZZ and 7-EE.
Me
1
11-EE
(CD2Cl2,
−30 °C)
1.48 (d, J = 6.5),
1.88 (d, J = 6.5)
3.92 (m), 4.00
(d, J = 12.0)
4.45 (d, J = 11.0)
5.01
(s)
2.46
(s)
2
1-Cp
10 (92%)
Ru
5
Me
Me
RT
−45 °C
acetone-d6
45 min
NCMe
6
11-EZ
(CD2Cl2,
−20 °C)
1.45 (d, J = 6.0),
1.82 (d, J = 6.0)
3.75 (d, J = 13.0)
4.38 (d, J = 10.0)
4.62 (m, 2H)
4.97
(s)
2.44
(s)
7-ZZ
Me
Me
RT
11-ZZ (98%)
12
(CD2Cl2,
−20 °C)
1.66 (d, J = 6.0),
2.07 (d, J = 6.0)
2.22 (d, J = 13.2)
3.61 (m), 6.07 (m)
5.10 (d, J = 10.0)
5.13
(s)
−
NCMe
1-Cp
Ru
6
Ru
1
1
Me
Me
CD2Cl2
−30 °C
30 min
−20 °C
150 min
2
2
4
[a] Chemical shifts in d. [b] Coupling constants (J) in Hz.
5
5
6
Me
Me
Me
7-EE
12 (92%)
11-EE (25%)
The 1H NMR chemical shifts for the vinyl hydrogens of 12 are
markedly different than those observed for h4-trienes 11-ZZ and
11-EE, in that one resonance is shifted far upfield (d 2.22; d, J =
13.2 Hz, C2-H) and one far downfield (6.07, m, C1-H) relative to
those for the h4-complexes (Table 1). Complex 12, contaminated
with ～ 5% of 10, was isolated by addition of hexanes to a
reaction solution that was maintained at –20 °C. Unexpectedly,
isolated 12 proved to be stable in CD2Cl2 at room temperature,
suggesting that conversion to 10 requires the presence of
acetonitrile. The stability of 12 in CD2Cl2 at ambient
temperatures permitted the collection of HMBC, HSQC and
b) Reaction of 1-Cp and 7-EZ.
Me
NCMe
NCMe
Ru Me
Ru
1-Cp
Me
H
1
CD2Cl2
−20 °C
30 min
−
10 °C
2
H
Me
50 min
5
6
Me
Me
13 (24%)
7-EZ
11-EZ (20%)
[1,5]-H
shift
0 °C
20 min
NCMe
1
RT
COSY spectral data and a complete assignment of the H and
Ru
Ru
H
10
Me
H
13C NMR spectroscopic resonances, and an X-ray
crystallographic analysis confirmed 12 to be the first hexahapto
complex of an acyclic conjugated triene (Figure 5, lower
panel).[26] All previously reported hexahapto metal complexes of
conjugated trienes (e.g. cycloheptatriene and cyclooctatriene
complexes) involve cyclic triene ligands which enforce an s-
cis,s-cis triene conformation.[27] The C1=C2–C3=C4 diene unit
in 12 adopts an s-trans conformation with a dihedral angle of –
116.1(9)° while the C3=C4–C5=C6 diene unit adopts an s-cis
conformation with a dihedral angle of 6.4(15)°. The 2.362(5) Å
Ru – C1 distance is significantly longer than the remaining Ru –
C distances of the triene ligand (ave. 2.206(5) Å), which is
consistent with the downfield chemical shift of the C1 hydrogen
resonance in the 1H NMR spectrum. It is clear from the structure
of 12 that concerted ring closure to give a metal – h4-
cyclohexadiene derivative is geometrically prohibited since it
would require inversion of configuration at C2.
Me
10 °C
20 min
H
Me
Me
14 (38%)
15 (22%)
Figure 4. Observation of reactive intermediates in triene cyclizations promoted
by 1-Cp.
The reaction of 7-EE and 1-Cp was also examined by low
temperature H NMR spectroscopy (CD2Cl2; Figure 4a). Upon
1
warming the sample from –60 to –30 °C in the spectrometer
probe, the resonances for 7-EE and 1-Cp were gradually
replaced by a new set of resonances attributed to formation of
the h4-triene complex 11-EE (25% yield after 30 min). The
structural assignment for 11-EE is based on the similarity of its
1H NMR spectroscopic data to that of 11-ZZ. Warming the
3
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10.1002/anie.202006992
Angewandte Chemie International Edition
RESEARCH ARTICLE
spectroscopy. At ambient temperature there was no reaction,
and even upon heating at 50 °C there was no evidence for the
formation of cyclization product. Thus, three labile ligands
appear to be necessary for cyclization.
The involvement of ruthenium – cyclohexadienes 13 – 15 in
the conversion of 7-EZ to 10 suggested that aromatization would
be impeded by the presence of a quaternary carbon in the h4-
cyclohexadiene intermediate(s).[30] We therefore prepared the
trimethyl-substituted trienes 16-Z and 16-E and independently
monitored their reactions with 5 mol% 1-Cp by 1H NMR
spectroscopy (CD2Cl2; Figure 6a). Remarkably, within 100 min
of reaction, the metal-free cyclohexadiene 17 was formed from
16-Z in 71% yield at 83% conversion. At longer reaction times
decomposition of 17 was observed; however, at 20% catalyst
loading, 16-Z was observed to convert quantitatively to 17. In a
similar fashion, 16-E underwent reaction with 1-Cp (5 mol%) to
give 17 in 67% yield, and once again a 20% catalyst loading
resulted in a quantitative yield of 17. Kinetic studies on the
reaction of 16-E and 1-Cp in CD2Cl2 at ambient temperature
demonstrated a first-order disappearance of triene 16-E, and
catalytic – turnover was manifested by pronounced differences
in reaction rate (t1/2 = 385 – 3466 s) at different catalyst loadings
(0.36 – 0.04 equivalents). A plot of ln[kobs] vs. ln[1-Cp] at sub-
stoichiometric catalyst loadings gave a straight line with a slope
as 0.998, indicative of a first-order rate dependence on [1-Cp].
In a separate experiment, 16-Z underwent reaction with one
equivalent of 1-Cp over the course of 90 min to form
cyclohexadiene complex 18 in 76% NMR yield (Figure 6a). The
structure of 18 is based on the observation of an agostic
hydrogen resonance at d –8.03 (d, 1H, J = 18.0 Hz), a
methylene hydrogen resonance at 2.13 (d, J = 18.0 Hz, 1H), a
vinyl hydrogen resonance at 5.33 (s, 1H), and three methyl
singlets at 0.50, 1.30, and 1.61. Over the course of 72 h, 30% of
18 was converted to 10 (Figure 6a). In a related NMR-tube-
scale reaction, cyclohexadiene 19 was cleanly liberated from 18
in 95% NMR yield by simple replacement the CD2Cl2 solvent
with NCCD3. Notably, heating a C6D6 solution of 16-Z for 15
days at 130 °C failed to form cyclohexadiene 17 or 19, thereby
demonstrating the ability of [CpRu]+ to catalyze electrocyclization
of trienes that fail to cyclize under thermal conditions.
The metal – catalyzed conversion of 16 to cyclohexadiene
17 does not reveal the stereochemistry of ring closure. We
therefore examined the ambient temperature reaction of a 1.0 :
1.5 ratio of 20-ZE : 20-ZZ with a catalytic amount of 1-Cp in
CD2Cl2 (Figure 6b). The observed formation of a 1.0 : 1.5 ratio
of cyclohexadienes 21-cis and 21-trans is consistent with a
metal – catalyzed disrotatory 6p electrocyclization reaction.
A catalytic cycle for the conversion of trienes 16 and 20 to
cyclohexadienes 17 and 21, respectively, is shown in Figure 6c,
using 20-ZZ as a representative substrate. We propose that
metal – p-complexation triggers the key carbon – carbon bond
formation, as shown for the conversion of III-hn to
cyclohexadiene 22-L. In principle, the metal – p-complex could
take the form of dihapto (III-h2), tetrahapto (III-h4) or hexahapto
(III-h6) coordination (Figure 6c, boxed equilibria). The trans
stereochemistry in the metal – cyclohexadiene intermediate (22-
L) is consistent with a disrotatory ring closure process. A
concerted ring closure from III-h6 would directly form agostic
complex 22-h2-CH (vide infra) whereas a concerted ring closure
from III-h4 would lead to 22-NCMe or 22-Solv. In the case of
trienes 16-Z and 16-E, the termination step, which is slow
Figure 5. Solid-state structures of 9-ZZ and 12 with ellipsoids at 50%
probability level. Solvent molecules, hydrogen atoms except vinyl hydrogens,
and counter ions are omitted for clarity. Upper panel: Selected bond lengths
for 9-ZZ: Ru–C1 2.273(4) Å, Ru–C2 2.308(4) Å, Ru–C5 = 2.204(4) Å, Ru–C6
= 2.217(4) Å, C1–C2 = 1.382(6) Å, C2–C3 = 1.487(6) Å, C3–C4 = 1.316(6) Å,
C4–C5 = 1.482(6) Å, C5–C6 = 1.411(6), C1××××C6 = 3.885(6) Å. Selected bond
angles are: C1–Ru–C6 = 119.83(18)°, C2–Ru–C5 74.85(16)°. Lower panel:
One of two independent molecules of 12 in the unit cell. Selected bond
lengths: Ru–C1 = 2.362(5) Å, Ru–C2 = 2.196(5) Å, Ru–C3 = 2.177(5) Å, Ru–
C4 = 2.254(5) Å, Ru–C5 2.178(5) Å, Ru–C6 2.224(6) Å, C1–C2 = 1.368(7),
C2–C3 = 1.450(7) Å, C3–C4 = 1.437(7) Å, C4–C5 = 1.428(9) Å, C5–C6 =
1.418(10) Å, C1××××C6 = 3.431(8) Å. Selected bond angles: C1–Ru–C6 =
96.8(2)°, C1-Ru-C5 77.7(2)°, H6-C6-C5-H5 –143.8(7) Å, H1-C1-C2-H2
137.0(5)°.
–
Monitoring the reaction of 1-Cp and 7-EZ by 1H NMR
spectroscopy proved to be more revealing than in the case of
the ZZ and EE isomers (Figure 4b). Most notably, in addition to
formation of h4-triene complex 11-EZ, the ruthenium – h4-
cyclohexadiene complex 13 was formed at –10 °C. The
structural assignment for 13 is consistent with the observation of
two methyl doublets (d 0.92, J = 6.5 Hz; 1.24, J = 6.5 Hz) and a
singlet at 2.57 (3H, RuNCCH3). In addition, a broad vinyl
hydrogen singlet at d 5.37 (2H) fits the typical pattern for a h4-
cyclohexadiene substructure.[28] Upon further warming of the
sample to 0 °C, the appearance of an upfield resonance at d –
7.9 (app t, 1H, J = 16.0 Hz) indicated the presence of an agostic
hydrogen,[29] as shown for 14. The anti relationship of the agostic
hydrogen and the tertiary ring hydrogen is consistent with the 16
Hz coupling constant. Finally, warming the sample to 10 °C led
to observation of h4-diene complex 15, whose structure is
tentatively assigned on the basis of 1H NMR resonances at d
0.92 (d, J = 7.0 Hz, 3H, CH3), 1.75 (s, 3H, CH3), 2.63 (s, 3H,
NCCH3), and 4.60 (s, 1H, =CH-). The rapid extrusion of H2 from
cyclohexadiene 14 to form ruthenium – arene 10 has precedent
in Cp*Ru – mediated dehydrocycloaromatization reactions.[24]
In an effort to determine if three metal coordination sites are
required for cyclization, we examined the reaction of 7-EZ and
[(C5H5)Ru(CO)(NCMe)2]PF6
(1-Cp-CO)
by
1H
NMR
4
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10.1002/anie.202006992
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RESEARCH ARTICLE
relative to the rate of conversion to cyclohexadiene 17, is
and s-trans/s-trans (5-Cp-trans/trans) isomers 2.45 kcal/mol and
16.84 kcal/mol higher in energy, respectively. Proceeding from
5-Cp-cis/cis to 5-Cp-trans/cis to 5-Cp-trans/trans, the C1××××C6
non-bonded distance increases from 2.795 Å to 3.450 Å to 4.240
Å. The geometric constraints imposed by the s-trans
configurations in 5-Cp-trans/cis and 5-Cp-trans/trans prevent a
proposed to involve demethanation of the ruthenium
–
cyclohexadiene intermediate via a multistep process that has
precedent in the chemistry of other methyl – substituted
ruthenium – cyclohexadiene complexes.[31]
concerted 6p electrocyclization to
a
ruthenium
–
h4-
a) Catalytic conversion of 16 to 17, and observation of 18.
cyclohexadiene intermediate. However, the structure of 5-Cp-
cis/cis is ideally predisposed for a concerted disrotatory 6p
electrocyclization to give to the ruthenium – cyclohexadiene
Me
Me
cat.
1-Cp
R1
R2
Me
Me
RT
CD2Cl2
Me
agostic
complex
[(C5H5)Ru(h4-C6H7-µ-H)]+ (6-Cp).
The
17 (100%)
torquoselectivity for this process involves rotation of the triene
termini (and H1s/H6s) toward the metal.[32] The conversion of 5-
Cp-cis/cis to 6-Cp is found to be exothermic by 5.9 kcal/mol, with
a 17.7 kcal/mol energetic barrier (Eb). For comparison, the
calculated barrier for thermal 6p electrocyclization of (Z)-hexa-
1,3,5-triene to cyclohexadiene via transition-state structure 23-
TS is 27.7 kcal/mol at the same level of theory, thus [(C5H5)Ru]+
– complexation lowers the barrier by 10.0 kcal/mol (Figure 7).
16-Z, R1 = Me, R2 = H
16-E, R1 = H, R2 = Me
Me
Me
1 equiv
NCMe
Me
1-Cp
16-Z
Ru
H
19 (95%)
Me
Me
RT
CD2Cl2
Me
10 (30%)
18 (76%)
b) Ruthenium − catalyzed disrotatory 6π electrocyclizations.
Et
Me
Me
Et
Me
Me
Me
21-cis
20-ZE
cat.
1-Cp
+
+
Me
Et
RT
CD2Cl2
15 min
Et
Me
Me
21-trans
20-ZZ
1.00 : 1.47
1.00 : 1.47
20-ZE : 20-ZZ
21-cis : 21-trans
c) Catalytic cycle for ruthenium − catalyzed 6π electrocyclizations.
H
Me
[Cp*Ru(η6-triene)]+
III-η6
III-ηn
n = 2, 4, or 6
Et
Me
21-trans
+L
-L
[Cp*Ru(η4-triene)(L)]+
III-η4
disrotatory
6π
-L
+L
[Cp*Ru(η2-triene)(L)2]+
III-η2
L
Ru
-L
+L
Me
Et
H
Me
Et
20-ZZ + 1-Cp
Me
Me
20-ZZ
L = NCMe, Solv
22-L, L = η2-CH, NCMe, Solv
Figure 6. Ruthenium – catalyzed 6p electrocyclizations.
The most intriguing mechanism for the ring closure step in
Figure 6c is a 6p electrocyclization triggered by formation of a
hexahapto triene intermediate. In order to evaluate the feasibility
of such
a reaction, the structures of the three possible
[(C5H5)Ru(h6-hexatriene)]+ complexes, 5-Cp, were established
by BP86/Def2-TZVPP(THF) computation (Figure 7, Table S1).
The minimum energy isomer is found to be the s-cis/s-cis isomer,
5-Cp-cis/cis, with the alternative s-trans/s-cis (5-Cp-trans/cis)
Figure 7. Computational studies support the feasibility of a ruthenium –
accelerated concerted disrotatory 6p electrocyclization.
5
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10.1002/anie.202006992
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RESEARCH ARTICLE
For rate acceleration, metal coordination in the transition
state structure must provide greater stabilization than does
metal coordination in the triene ground state structure. The
2.221 Å average Ru-(C2,C3,C4,C5) bond distance in 5-Cp-
cis/cis is 0.045 Å longer than that in transition state structure
These proof-of-principle studies provide a foundation for the
development of new electrocyclization catalysts with three
accessible facial coordination sites for substrate binding. It is
anticipated that chiral analogues of 1-Cp[35] may afford
advantages over chiral Lewis acids for asymmetric catalysis of
electrocyclic reactions since the stereogenic center will be in
closer proximity to the site of carbon – carbon bond formation
(e.g. I vs. II-h6; Figure 1). Future studies will be directed toward
establishing the scope and limitations of this chemistry, and
developing catalysts that more generally favor cyclohexadiene
ligand dissociation over dehydrogenation.
5/6-Cp-TS
– indicative of much tighter binding between
ruthenium and those carbon atoms in the transition state
structure. The C1××××C6 non-bonded distance in transition state
5-Cp-TS (2.101 Å) is significantly shorter (D = 0.694 Å) than in
precursor 5-Cp-cis/cis (2.795 Å). It is clear that [CpRu]+
coordination draws the triene termini into close proximity, and
the tilting of the terminal alkenes toward ruthenium mimics the
beginning of the disrotatory ring closure process. As the
disrotatory motion occurs upon going from 5-Cp-cis/cis to 5/6-
Cp-TS, the syn-hydrogens (H1s and H6s) move 0.09 Å closer to
ruthenium. Notably, there is no computational evidence for the
formation of an unsaturated [(C5H5)Ru(h4-cyclohexadiene)]PF6
(6-Cp-h4) intermediate on the path to 6-Cp. A concerted ring
closure leading from 5-Cp-cis/cis directly to 6-Cp would avoid a
high energy unsaturated intermediate[33] by donation of C-H –
bond electron density to ruthenium. A comparison of the
transition state structure for the ruthenium – mediated process to
that for a thermal hexatriene electrocyclization (23-TS) reveals
that h6-coordination shortens the C1××××C6 non-bonded distance
at the transition state by 0.175 Å. In addition, the fold-angle
between the C1-C2-C3 and C1-C3-C4-C6 mean planes is 27.0°
in 23-TS compared to 19.2° in 5/6-Cp-TS. Thus, metal –
complexation flattens the triene boat conformation and moves
the triene termini into closer proximity relative to the metal – free
process.
Finally, we note that the stability of s-cis/s-trans h6-triene
complex 12 at ambient temperature in the absence of
acetonitrile, and its rapid conversion to 10 in acetone-d6,
suggests that the presence of an external coordinating ligand
(NCMe or acetone) facilitates cyclization (Figure 4a).[34]
Accordingly, we speculate that an external ligand may catalyze
isomerization of 12 to its s-cis/s-cis isomer by formation of a
[(C5H5)Ru(h4-triene)L]+ intermediate capable of rotation about
the C2 – C3 bond prior to loss of L and recoordination on the
opposite alkene face. Furthermore, the conversion of 7-EZ to
ruthenium – cyclohexadiene 13 with trans methyl groups is
consistent with a disrotatory ring closure from an unobserved
hexahapto triene intermediate of s-cis/s-cis stereochemistry
(Figure 4c). Taken together, the computational results and the
experimental observation of 13 implicate, but do not prove, the
involvement of s-cis/s-cis hexahapto triene complexes in the ring
Acknowledgements
We acknowledge financial support from the U. S. National
Science Foundation (JMO; CHE-1465079). KKB acknowledges
the National Basic Research Program of China (2015CB856500),
the Qian Ren Scholar Program of China, and the Synergetic
Innovation Center of Chemical Science and Engineering
(Tianjin) for support of this work.
Keywords: electrocyclization • triene • catalysis •
cycloaromatization • transition metals
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10.1002/anie.202006992
Angewandte Chemie International Edition
RESEARCH ARTICLE
Entry for the Table of Contents
Metal − p-complexation is shown for the first time to catalyze disrotatory 6p electrocyclization of conjugated trienes. Unprecedented
hexahapto metal coordination to the acyclic triene is implicated as the key mechanistic feature responsible for lowering the energetic
barrier to disrotatory ring closure.
9
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