Ruthenabenzene: A Robust Precatalyst

Article abstract of DOI:10.1021/jacs.1c02237

Metallaaromatics constitute a unique class of aromatic compounds where one or more transition metal elements are incorporated into the aromatic system, the parent of which is metallabenzene. One of the main concerns about metallabenzenes generally deals with the structural characterization related to their relative aromaticity compared to the carbon archetype. Transition metal-containing metallabenzenes are also implicated in certain catalytic processes such as alkyne metathesis polymerization; however, these transition metal-based metallaaromatic compounds have not been developed as a catalyst. Herein, we describe an effective strategy to generate diverse arrays of ruthenabenzenes and demonstrated them as an aromatic equivalent of the Grubbs-type ruthenium alkylidene catalysts. These ruthenabenzenes can be prepared via an enyne metathesis and metallotropic [1,3]-shift cascade process to form alkyne-chelated ruthenium alkylidene intermediates followed by spontaneous cycloaromatization. The aromatic nature of these complexes was confirmed by spectroscopic and X-ray crystallographic data, and the mechanistic pathways for the cycloaromatization process were studied by DFT calculations. These ruthenabenzenes display robust catalytic activity for metathesis and other transformations, which illustrates that metallabenzenes are not only compounds of structural and theoretical interests but also are a novel platform for new catalyst development.

Full text of DOI:10.1021/jacs.1c02237

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Ruthenabenzene: A Robust Precatalyst
Saswata Gupta, Siyuan Su, Yu Zhang, Peng Liu,* Donald J. Wink, and Daesung Lee*
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ABSTRACT: Metallaaromatics constitute a unique class of
aromatic compounds where one or more transition metal elements
are incorporated into the aromatic system, the parent of which is
metallabenzene. One of the main concerns about metallabenzenes
generally deals with the structural characterization related to their
relative aromaticity compared to the carbon archetype. Transition
metal-containing metallabenzenes are also implicated in certain
catalytic processes such as alkyne metathesis polymerization;
however, these transition metal-based metallaaromatic compounds
have not been developed as a catalyst. Herein, we describe an
effective strategy to generate diverse arrays of ruthenabenzenes and
demonstrated them as an aromatic equivalent of the Grubbs-type
ruthenium alkylidene catalysts. These ruthenabenzenes can be
prepared via an enyne metathesis and metallotropic [1,3]-shift cascade process to form alkyne-chelated ruthenium alkylidene
intermediates followed by spontaneous cycloaromatization. The aromatic nature of these complexes was confirmed by spectroscopic
and X-ray crystallographic data, and the mechanistic pathways for the cycloaromatization process were studied by DFT calculations.
These ruthenabenzenes display robust catalytic activity for metathesis and other transformations, which illustrates that
metallabenzenes are not only compounds of structural and theoretical interests but also are a novel platform for new catalyst
development.
membered aromatic system24−31 to generate a ruthenanaph-
thalene (or ruthenabenzene) by replacing the lone-pair
INTRODUCTION
■
After the theoretical consideration of the possibility of Mn- and
electron-containing O atom with a C−C double bond. On
the basis of the similarity between these isolobal analogies32 of
ruthenabenzofuran and ruthenanaphthalene, we predict that
the latter compound will be more stable than the
ruthenabenzofuran moiety of the Grubbs-Hoveyda catalyst
(GH-I/II), but it can still engage the ruthenium alkylidene (Ru
= C) moiety to participate in catalytic process. In theory, we
envisage that the ruthenabenzene framework33−40 can be
generated from the 1,3-enyne-conjugated ruthenium alkylidene
upon appropriate alkyne-chelation.34 In turn, the precursor
ruthenium alkylidene can be derived from multiynes via enyne
metathesis42−45 and metallotropic [1,3]-shift (Figure 1C).46−49
Upon forming the 14-electron ruthenium alkylidene species,
the preference for the disposition of the chelated alkyne either
trans or cis relative to the N-heterocyclic carbene (NHC)50−53
would depend on the substituent R on the carbenic carbon and
the structure of the fused ring. While in equilibrium, the
Rh-metallabenzenes by Thorn and Hoffmann1 in 1979, Roper
and co-workers reported the first synthesis and characterization
of Os-metallabenzenes in 1982 (Figure 1A).2 Since then, this
unique class of organometallic compounds has burgeoned with
significant interests and has witnessed rapid theoretical and
experimental advances.3−12 While different criteria for
predicting the aromaticity of metallabenzenes can be used,
based on the calculated aromatic stabilization energies (ASEs),
metallabenzenes are considered to be aromatic even though
the aromatic stabilization is weaker than that of the parent
benzene.13−15 The extent of stabilization varies drastically
depending on the metal element in the framework, thus some
metalla-1,3,5-hexatrienes exist as transient intermediates and
cannot be isolated. For example, Schrock proposed tung-
stenabenzenes as transient intermediates in alkyne polymer-
ization although they were not isolated and characterized.16 In
recent olefin metathesis area, ruthenium-based Grubbs-type
alkylidene complexes (G-I−G-III) and their oxygen-chelated
congener Grubbs-Hoveyda catalyst (GH-I/II) are considered
to be the most user-friendly catalysts (Figure 1B).17−19
Received: February 27, 2021
Published: May 7, 2021
In terms of the structural characteristics, the oxygen-chelated
Grubbs-Hoveyda catalyst20,21 can be considered as the
ruthenium analog22,23 of benzofuran. We envisage that the 5-
membered aromatic furan moiety can be expanded to a 6-
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Figure 1. A new approach to Ruthenabenzenes: (A) Representative metallabenzenes containing Os, W, Ir, Pt, and Ru. (B) Isolobal analogy of the
Grubbs-Hoveyda complex and a ruthenabenzene. (C) Ruthenabenzenes from ruthenium alkylidenes generated via enyne metathesis and
metallotropic [1,3]-shift.
Figure 2. Selectivity in product distributions: (A) The crucial roles of a gem-dimethyl group (exo-Thorpe-Ingold effect) for the fate of propagating
alkylidenes to form trans-chelate or metathesis product. (B) The crucial role of an alkyl-substituted alkyne in ruthenabenzene formation. (C)
Favored aromatization to form ruthenabenzene over gem-dialkyl-driven trans-chelate formation.
aligned three π-systems on the alkylidene (Ru = C) and the
conjugated 1,3-enyne, of the cis-chelate, might undergo
structural reorganization to form the ruthenabenzene core
driven by the expected aromatic stabilization.
electron ruthenium intermediate I, and the trans-chelates could
be isolated if they are stabilized by a gem-dimethyl group (exo-
Thorpe-Ingold effect), and the size of the fused ring (Figure
2A).41 These results indicate that the 5-membered ring fused
chelate would have a smaller chance to be trapped as a trans-
chelate, thus the effect of the substituent on the carbenic
carbon was examined by employing ene-triyne 1a and 1b
(Figure 2B). Much to our delight, both the reactions of 1a and
1b with a stoichiometric amount of G-II provided
RESULTS AND DISCUSSIONS
■
Structural Requirements and Substituent Effects.
Previous study shows that the trans-chelate is more favorable
if the carbenic carbon (C1) is monosubstituted in the 14-
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Figure 3. Ruthenabenzene formation via ring-closing metathesis (RCM) → metallotropic [1,3]-shift (MTS) → RCM → aromatization sequence
with ene-triynes. # Reactions conditions: G-II or G-I (0.8 equiv) in CH2Cl2, 45 °C, 3h. ##2d was also obtained in 68% yield with G-III catalyst in
CDCl3, 45 °C, 3 h.
ruthenabenzenes 2a and 2b as green solids in 52 and 58%
yield, respectively, while none of the corresponding metathesis
products 2a′ and 2b′ were observed. These results clearly
indicate that the butyl group on the carbenic carbon (C1) of
intermediate I-1a and I-1b is crucial for populating the cis-
chelate that led to ruthenabenzene; while the gem-dimethyl
group seems to confer only a minimal effect in these reactions.
The 13C NMR spectrum of 2a shows two carbene signals at
256.5 and 210.5 ppm, which are significantly shielded
compared to the carbene of the trans-chelate (298.1 ppm),
indicating that the π-bonds in 2a are delocalized such that both
the C1 and C5 carbons share the carbenic nature.54 The
competition between kinetically favored trans-chelate and
thermodynamically favored ruthenabenzene was tested by
employing alkene-tethered tetrayne 1c. If the gem-dialkyl effect
(Thorpe-Ingold effect)55−59 was large enough to promote
strong interaction between the alkyne and the ruthenium
center trans-chelate, 2c′ would be generated; otherwise,
aromatization of cis-chelate I-1c would provide ruthenaben-
zene 2c. Indeed, the reaction of 1c with G-II provided only
ruthenabenzene 2c in 58% yield as a 10:1 mixture of two
diastereomers (Figure 2C).
variations were introduced onto the aromatic system (Figure
3). The electronic effect of heteroatom substituents was
examined by introducing an NTs-linked ynamide, which ended
up as ruthenaindolines 2d and 2e in 62% and 63% yields,
where the phenyl and butyl substituent on the carbenic carbon
did not make any difference in the reaction efficiency. Also, by
employing G-I, the corresponding ruthenabenzene 2d-I was
obtained in 82% yield. The X-ray structure of 2d confirms the
expected ruthenabenzene framework which is a rare four-
ligand containing distorted tetrahedral 14-electron ruthenium
alkylidene60 rather than the typical five-ligand containing
square pyramidal 16-electron alkylidene. To reconstitute the
original square pyramidal ligand geometry around the
ruthenium center, an oxygen chelating group61−64 was
introduced in ruthenaindolines 2f, the X-ray structure of
which shows the ruthenabenzene core as a part of a square
pyramidal structure geometry where the isopropoxy group
chelated to the ruthenium center is cis to the NHC ligand.65 A
tricyclohexylphosphine ligand-containing complex 2f-I was
also obtained in excellent yield by employing G-I.
Surprisingly, introduction of a 4-nitro group in 2f makes the
resulting complex 2g too unstable to be isolated whereas 2g-I
could be obtained in 68% yield. Replacement of the ortho-
isopropoxyphenyl group in 2f with an acetoxymethyl group
provided the corresponding 6-membered carbonyl group-
Tolerance of Structural Variation on the Ruthena-
benzene Framework. Having defined the structural require-
ments for the formation of ruthenabenzenes, further structural
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Figure 4. Different modes of initiation and selectivity: (A) Cross metathesis (CM)-initiated pathway. (B) Relay metathesis allows for installation of
an alkyl group at the C5 carbenic carbon. (C) RCM-initiated pathway with enediynes and enetriynes. (D) Selectivity between cycloaromatization
#
and metallotropic [1,3]-shift. X-ray crystallographic data of 2z confirmed the structure.
chelated complex 2h in 94% yield.66 The moving of the NTs
group to the next position resulted in a slightly higher yield of
ruthenaisoindolines 2i (71%), 2j (78%), and 2k (89%) where
the substituents on the carbenic carbon are phenyl, ferrocenyl,
and isopropenyl, respectively. With G-I, the corresponding
complex 2i-I was also obtained in 78% yield. The X-ray
structure of 2j shows that the bulky ferrocenyl group does not
cause any structural perturbation compared with 2d.
Surprisingly, 2l was obtained in 78% yield without any
accompanied metathesis of the alkylidene intermediate with
the prenyl group. This suggests that the ruthenium center of
the intermediate alkylidene interacts with the alkyne moiety
more favorably than with the alkene.67,68 When an additional
NTs group was introduced at the carbenic carbon, the
sulphonyl oxygen-chelated complex 2m69 was obtained and
the overall ligand disposition of 2m around the ruthenium
center was quite similar to that of other oxygen-chelates 2f and
2h. Reducing the steric bulk of the substituent either on the
tether or on the carbenic carbon decreased the stability of the
resultant complexes, thus ruthenadihydroisobenzofuran 2n and
2o containing a methyl and phenyl group were obtained in 32
and 60% yield, respectively. Tricyclohexylphophine-containing
complexes 2o-I and 2o′-I were obtained in similar yield with
G-I, which suggests that the effect of gem-dimethyl group is
only minimal if at all for the formation of ruthenabenzenes.
The NICS calculations of aromaticity70−72 for several
ruthenabenzenes show that the order of aromaticity (ppm)
follows 2o-I (−19.6) < 2a (−20.1) < 2o (−21.2) < 2d
(−23.8) < 2e (−24.0). This result suggests the aromaticity of
ruthenabenzenes is affected by the ligand on the ruthenium
center to a higher extent than the substituent on the carbon
framework.
Different Modes of Initiation and Pathway Selectiv-
ity. In the RCM-initiated formation of ruthenabenzenes, the
incorporation of a cyclic alkene such as dihydrofuran at the C5
position is unavoidable. To alleviate this limitation, a cross
metathesis (CM)-initiated approach was implemented with
terminal 1,3-diyne 1p (Figure 4A).73−75 In this approach,
installation of a different alkene functionality at the C5 position
in ruthenaindolines 2pa−2pc could be achieved by employing
different terminal alkenes to generate the intermediate I-1p.
For example, while 2pa can be obtained directly from 1p and
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Figure 5. Mechanisms of cycloaromatization to form ruthenabenzene: (A) Plausible pathways for ruthenabenzene formation from the ruthenium
alkylidene intermediate. (B) Computed energy profiles at the M06/SDD-6-311+G(d,p)/SMD(CH2Cl2)//B3LYP-D3(BJ)/SDD-6-31G(d)
/SMD(CH2Cl2) level of theory. H atoms are omitted in the 3D structures for clarity. aThe geometry of TS1 was optimized using the growing string
method (GSM).
G-II, 2pb and 2pc were obtained in the presence of 1-octene
and ethylene, respectively. Cross metathesis between 2pa and
1-octene or ethylene did not produce 2pb or 2pc, which
suggest that the alkylidene exchange between the catalyst and
the alkene happened before the formation of ruthenabenzene.
This CM-initiated approach, although effective for diversifying
the substituent at C5, provided relatively low yield of the
products. To improve the efficiency of delivering the
ruthenium alkylidene onto the mainframe to generate
intermediate I-1r, a relay metathesis strategy was also designed
(Figure 4B),76,77 where intramolecular delivery of the metal
center can be achieved by a concomitant removal of the relay
tether (dihydrofuran). This strategy allows for installation of
an alkyl group at the C5 position, thus generating a methyl
group-containing ruthenadihydroisobenzofuran 2r in 67%
yield. Another tactic to get access to the 1,3-enynyl ruthenium
alkylidene relies on enediynes, a minimum structural element
required for ruthenabenzene formation (Figure 4C). In this
approach, the chelating alkyne is preinstalled with a different
terminal substituent, thus providing flexibility at the C5
position. RCM of enetriyne 1s with G-II generated an
intermediate I-1s, which provided 2s after a metallotropic
[1,3]-shift, while enediyne 1t with G-I delivered 2t through an
aromatization of intermediate I-1t. Finally, tetraynes 1u−z
were employed to examine the selectivity for forming different
end products from a common precursor, which should depend
on the relative rate of alkyne chelation-induced aromatization
versus metallotropic [1,3]-shift from the initially formed
alkylidene I-1u-z (Figure 4D). Treatment of terminal tetrayne
1u (R = H) with G-II resulted in the formation of complex
2u′78 instead of ruthenabenzene 2u, which suggest that the
metallotropic [1,3]-shift of I-1u to monosubstituted alkylidene
I-1u′ should be faster than aromatization in this case. Tetrayne
1v (R = SiMe3) provided ruthenabenzene 2v in 95% yield,
which is the consequence of a preferred aromatization of
intermediate I-1v over [1,3]-shift because the silyl group
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Figure 6. Catalytic activity of ruthenabenzenes: (A) Structure-dependent reactivity difference of ruthenabenzenes in the RCM of dimethyl 2,2-
diallylmalonate 3a. (B) Comparison of catalytic activity of ruthenabenzenes with G-I and G-II in the RCM of 3a. (C) Reactivity of ruthenabenzene
precatalyst in other metathetic and nonmetathetic reactions. (D) Catalyst recoverability experiment indicating % conversion and % catalyst
recovered after each cycle for the conversion of 3e to 4e.
prohibits the [1,3]-shift. A sterically less hindered prenylox-
ymethyl group-containing tetrayne 1w provided ruthenaben-
zene 2w in 61% yield along with RCM product 2w′ in 15%
yield. It is not clear whether the ratio of these products reflects
the equilibrium ratio of I-1w and I-1w′ or their respective rates
of aromatization and RCM while the two intermediates rapidly
interconvert. Tetrayne 1x that contains an ortho-isopropoxy
phenyl substituent provided ruthenabenzene 2x in 86% yield
without forming isopropoxy-chelate 2x′. With consideration of
the possibility of interconversion of I-1x and I-1x′, oxygen-
chelate 2x′ could be formed but the reversible nature of the
chelation funnels the equilibrium of these hemilabile species to
ruthenabenzene 2x. Both tetraynes 1y and 1z led to the
formation of 2y and 2z in 40 and 78% yield, respectively. Even
though there is a possibility of metallotropic [1,3]-shift of I-1y
and I-1z to I-1y′ and I-1z′ and generate alternative
ruthenabezenes 2y′ and 2z′, these were not observed.
Mechanism of Ruthenabenzene Formation. The
mechanisms of cycloaromatization79 of the alkyne-chelated
ruthenium alkylidene to form a ruthenabenzene core were
investigated using DFT calculations.80−82 The Ru-alkylidene
Int-I was used in the calculations as a representative system,
which is formed from triyne 1d through a cascade process
involving an RCM−metallotropic [1,3]-shift−RCM sequence.
Among other possibilities, two most probable pathways for the
conversion of Int-I to ruthenabenzene 2d were considered
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(Figure 5A). Path A involves the dissociation of a chloride
anion to form the ion-pair Int-II followed by its recombination
via nucleometalation (TS2) of the π-alkyne complex. Path B
proceeds via a cis-trans isomerization to form the intermediate
Int-I′ followed by an intramolecular [1,5]-Cl shift. The energy
profiles of these pathways were computed at the M0683
/SDD84 -6-311+G(d,p)/SMD(CH2Cl2)85 //B3LYP-D3-
(BJ)86−88 /SDD-6-31G(d)/SMD(CH2Cl2) level of theory
(Figure 5B).
In Path A, the Cl− ion dissociation from Int-I to form the
ion pair Int-II is endergonic by 7.6 kcal/mol. The resulting π-
alkyne complex with the cationic Ru center is highly reactive
toward anti-nucleometalation with the Cl− ion.89 The
nucleometalation transition state TS2 requires an activation
free energy of 10.9 kcal/mol with respect to Int-I and leads
directly to the ruthenabenzene (2d). Path B involves a cis-trans
isomerization to place one of the chloride ligands trans to the
alkylidene to form Int-I′ (ΔGsol = 5.0 kcal/mol). The trans
effect of the alkylidene weakens the Ru−Cl bond in Int-I′ (the
Ru−Cl bond distance is 2.47 Å compared to 2.44 Å in Int-I)
and promotes the subsequent [1,5]-Cl shift to form 2d.
Because the potential energy surface of the [1,5]-Cl shift is
nearly flat in the transition state region (see SI for details), the
transition state (TS1) could not be located using Gaussian
after multiple attempts. Therefore, we applied the growing
string method (GSM)90−95 to locate this transition state
structure and calculate the activation barrier. The GSM
calculations indicate the [1,5]-Cl shift is a concerted process
via TS1, with relatively long bond lengths of the cleaving Ru−
Cl bond and the forming C−Cl bond (4.02 and 3.29 Å,
respectively). The activation energy (ΔE‡sol) of TS1 is 24.2
kcal/mol with respect to Int-I, which is 13.8 kcal/mol higher
Figure 7. Possible catalytic cycles. Computed activation free energies
(in toluene) of reversion of precatalyst 2a to alkyne-chelated
alkylidene I-1a (TS3), initiation by [2 + 2] cycloaddition of I-1a
with diene (TS4), and initiation by [2 + 2] cycloaddition of 2a with
diene (TS5). All Gibbs free energies are in kcal/mol with respect to
2a.
zene-based precatalysts, the reactivities of 2a, 2n, and 2r were
compared with G-I and G-II at 45 °C in RCM of 3a to
generate 4a (Figure 6B). Because the ruthenium alkylidene
moiety is a part of an aromatic system and in a more sterically
congested environment, these ruthenabenzene-based precata-
lysts showed much lower reaction rates than G-I and G-II.
Thus, while complete conversion was achieved within few
minutes by G-I and G-II catalyst, roughly, 45, 17, and 12%
conversions were achieved by precatalysts 2a, 2n, and 2r
respectively after 1 h. It is interesting to note that a sterically
less hindered methyl group-containing precatalyst 2n turned
out to be slightly less active in comparison to 2a with a butyl
substituent. RCM of 3a could be promoted effectively with
lower catalyst loading (2 mol %), but it took longer time.
Precatalyst 2a also demonstrated an effective cross metathesis
reaction involving allyl benzene and (Z)-but-2-ene-1,4-diyl
diacetate (see SI, page S-32).
The catalytic activity for the most active precatalyst was also
tested for other processes (Figure 6C). The ring-rearrange-
ment metathesis of cyclopentenyl derivative 3b with catalyst 2a
provided cyclohexenyl derivative 4b in 84% yield with 10:1 dr,
which is nearly the same result as with G-II (89% yield, > 10:1
dr).101 A nonmetathetic catalytic activity of 2a was also
examined for the hydroboration102,103 of 5-decyne 3c, which
provided E-alkenyl boronate 4c in 69% yield while G-II was
found to be nearly unreactive for this transformation. The
catalytic activity of 2a was also examined for the hydrosilylative
cyclization104,105 of 1,6-enyne 3d in comparison with G-I and
G-II (Figure 6C). Under identical conditions, these catalysts
showed significant difference in forming products 4d and 4d′.
While precatalyst 2a exclusively provided hydrosilylation/
cyclization product 4d in 68% yield, G-I and G-II generated
enyne metathesis product 4d′ as the predominant product.
Precatalyst 2a was found to be recoverable,106−108 and its
than that of TS2 (ΔE‡ = 10.4 kcal/mol). These computa-
sol
tional results suggest that Path A involving the stepwise
chloride anion dissociation and nucleometalation is the most
favorable pathway to form the ruthenabenzene.
Catalytic Activity of Ruthenabenzenes. The catalytic
activity of ruthenabenzenes was examined by using a standard
substrate 3a for RCM reaction to generate 4a (Figure 6A). The
relative reactivities of ruthenabenzenes containing different
substituents on the carbenic carbons (C1−Ru−C5) and the
fused ring were compared. DFT calculations revealed that
these ruthenabenzenes are in fact precatalysts, which release
small amounts of active alkyne-chelated ruthenium alkylidene
species that catalyze the RCM reaction (Figure 7). Since most
of these complexes show only latent catalytic behaviors,96−100
the reactions were performed in toluene-d8 at 70 °C. From
their reaction profiles, it is evident that the sterically less
hindered oxygen-containing ruthenadihydroisobenzofurans are
more reactive than NTs-containing ruthenaindolines and
ruthenaisoindolines; for example, 2a (97% conv. at 70 min)
and 2o (99% conv. at 18.3 h) are more reactive than 2d (36%
conv. at 45 h) and 2i (30.5% conv. at 42.5 h).
Ruthenabenzenes containing an alkyl group at C1 are more
reactive than the corresponding phenyl-containing complexes;
a butyl-containing 2a is more reactive than a phenyl-containing
2o. Also, NHC and PCy3 ligands have a significant impact on
the catalytic activity; 2o with NHC (99% conv. at 18.3 h) is
more reactive than 2o-I with PCy3 (81% conv. at 43.5 h). As
expected, chelated complex 2f showed a slower initial rate than
nonchelated congener 2d but gave a higher conversion (56%
vs 36% at 45.0 h) probably because of its higher longevity.
After identifying the general reactivity trend of ruthenaben-
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longevity was demonstrated by five rounds of recycling of 2a,
with similar efficiency in each cycle for the RCM of dienyl
alcohol 3e to 4e (Figure 6D). The robust nature of
ruthenabenzenes compared to G-II is demonstrated by higher
stability of precatalyst 2a under air or ethylene atmosphere
(Figures S4 and S5 of Supporting Information).
Experimental details for all reactions and character-
ization data for all products (PDF). Crystallographic
data have been deposited at the Cambridge Crystallo-
graphic Data Centre, under deposition numbers
2050605 (2d), 2050606 (2f), 2050607 (2h), 2050608
(2j), 2050609 (2m), 2050611 (2r), 2050610 (2z)
(PDF)
The recyclability of the precatalyst might have two
independent origins. The first path, because of the stable
aromatic nature of 2a, involves only a small fraction of the
ruthenabenzene precatalyst entering the Catalytic Cycle I via
reversion to alkyne-chelated alkylidene I-1a (Figure 7). The
other path initiated by direct participation of the ruthena-
benzene precatalyst entails the Catalytic Cycle II or III,
wherein catalytically active species are constantly regenerated.
To gain more insight into these mechanisms, the energy
profiles of the different initiation pathways were calculated
using DFT (see SI for details, page S-110, S-111). A direct [2 +
2] cycloaddition with ruthenabenzene precatalyst 2a to form a
dearomatized ruthenacyclobutane intermediate requires very
high activation free energy (TS5, ΔG‡ = 38.3 kcal/mol),
suggesting both the Catalytic Cycles II and III are less likely
operative. The Catalytic Cycle I requires a lower barrier (TS4,
ΔG‡ = 14.9 kcal/mol with respect to I-1a) for the [2 + 2]
cycloaddition with alkyne-chelated alkylidene I-1a (ΔG = 6.8
kcal/mol) that is generated by ring-opening of the
ruthenabenzene (TS3, ΔG‡ = 30.8 kcal/mol with respect to
2a in toluene). Involvement of Catalytic Cycle I is further
supported by mass analysis of the reaction of 1-octene with 2a
wherein the organic component formed from dearomatization
of 2a followed by exchange with methylene ([M + H]+ =
245.1665) was detected unambiguously in the crude reaction
mixture. The fact that the ruthenabenzene coordinated with
NHC ligand shows higher productivity than that with PCy3
(2o vs 2o-I) is an indication of a mechanism similar to that of
G-I and G-II. In this mechanism, with the trans-disposed
chlorides, the donor ligand is positioned trans to the Ru-bound
alkene leading to the corresponding metallacyclobutane, and
this geometry can occur only with Catalytic Cycle I.
(XYZ)
Accession Codes
CCDC 2050605−2050611 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Authors
■
Daesung Lee − Department of Chemistry, University of Illinois
at Chicago, Chicago, Illinois 60607, United States;
orcid.org/0000-0003-3958-5475; Email: dsunglee@
uic.edu
Peng Liu − Department of Chemistry, University of Pittsburgh,
Pittsburgh, Pennsylvania 15260, United States; orcid.org/
0000-0002-8188-632X; Email: pengliu@pitt.edu
Authors
Saswata Gupta − Department of Chemistry, University of
Illinois at Chicago, Chicago, Illinois 60607, United States
Siyuan Su − Department of Chemistry, University of Illinois at
Chicago, Chicago, Illinois 60607, United States
Yu Zhang − Department of Chemistry, University of
Pittsburgh, Pittsburgh, Pennsylvania 15260, United States
Donald J. Wink − Department of Chemistry, University of
Illinois at Chicago, Chicago, Illinois 60607, United States;
orcid.org/0000-0002-2475-2392
Complete contact information is available at:
https://pubs.acs.org/10.1021/jacs.1c02237
CONCLUSION
■
Notes
In conclusion, we developed an effective method to prepare a
diverse array of ruthenabenzenes and explored their structural
characteristics and catalytic activities. A tandem enyne
metathesis and metallotropic [1,3]-shift process was resorted
to form ruthenium alkylidene intermediates, which sponta-
neously cycloaromatize if an alkyne is appropriately chelated.
This approach to form a ruthenabenzene framework is general
and tolerant to broad structural variations. The aromatic nature
of these complexes was confirmed by spectroscopic and X-ray
crystallographic data, and the mechanistic pathways for the
cycloaromatization process were studied by DFT calculations.
As for reactivity, these stable ruthenabenzenes display robust
catalytic activity.109−111 Even though the ruthenium alkylidene
moiety is a part of an aromatic system, it can still show
sufficient reactivity for olefin metathesis with increased
longevity, which illustrates that metallabenzenes not only are
compounds of structural and theoretical interests but also are
novel platforms for new catalyst development.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to NSF (CHE-1764141, D. L.) and NIH
(R35GM128779, P. L.) for financial support. The Mass
Spectrometry Laboratory at UIUC is acknowledged. DFT
calculations were performed at the Center for Research
Computing at the University of Pittsburgh, the Frontera
supercomputer at the Texas Advanced Computing Center, and
the Extreme Science and Engineering Discovery Environment
(XSEDE) supported by the NSF.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
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■
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