Bimolecular Cross-Metathesis of a Tetrasubstituted Alkene with Allylic Sulfones

Article abstract of DOI:10.1002/open.201800296

Exquisite control of catalytic metathesis reactivity is possible through ligand-based variation of ruthenium carbene complexes. Sterically hindered alkenes, however, remain a generally recalcitrant class of substrates for intermolecular cross-metathesis.

Full text of DOI:10.1002/open.201800296

DOI: 10.1002/open.201800296
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Bimolecular Cross-Metathesis of a Tetrasubstituted Alkene
with Allylic Sulfones
Rishi R. Sapkota,[a] Jacqueline M. Jarvis,[b] Tanner M. Schaub,[b] Marat R. Talipov,[a] and
Jeffrey B. Arterburn*[a]
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partners for ring-closing and cross-metathesis with challenging
1,1-disubstituted alkenes, by promoting catalyst turnover
through the avoidance of unstable methylidene complexes.[3,4]
Within the interplay of steric and electronic factors, substituted
alkenes remain reluctant partners for cross-metathesis and
impeded turnover kinetics can result in catalyst degradation
and competing secondary processes.[5] Modification of aryl
ligand substituents in ruthenium and molybdenum catalysts
opens the coordination environment to accommodate sterically
hindered alkene substrates, while substitution of the N-
heterocyclic carbene backbone provides catalysts with im-
proved efficiency for ring-closing metathesis to yield tetrasub-
stituted cycloalkenes.[6,7] Problems associated with this approach
include reduced catalyst stability and limited scope of effective
substrates, highlighting the need for alternative strategies.
Allylic sulfides and selenides have been identified as
“privileged substrates” that promote efficient cross-metathesis
coupling with the HG(II) catalyst.[8–11] Enhanced relative cross-
metathesis rates are particularly advantageous under challeng-
ing conditions such as aqueous media where high turnover
Exquisite control of catalytic metathesis reactivity is possible
through ligand-based variation of ruthenium carbene com-
plexes. Sterically hindered alkenes, however, remain a generally
recalcitrant class of substrates for intermolecular cross-meta-
thesis. Allylic chalcogenides (sulfides and selenides) have
emerged as “privileged” substrates that exhibit enhanced
turnover rates with the commercially available second-gener-
ation ruthenium catalyst. Increased turnover rates are advanta-
geous when competing catalyst degradation is limiting,
although specific mechanisms have not been defined. Herein,
we describe facile cross-metathesis of allylic sulfone reagents
with sterically hindered isoprenoid alkene substrates. Further-
more, we demonstrate the first example of intermolecular
cross-metathesis of ruthenium carbenes with a tetrasubstituted
alkene. Computational analysis by combined coupled cluster/
DFT calculations exposes a favorable energetic profile for
metallacyclobutane formation from chelating ruthenium β-
chalcogenide carbene intermediates. These results establish
allylic sulfones as privileged reagents for a substrate-based
strategy of cross-metathesis derivatization.
frequency overcomes competitive catalyst decomposition.[8]
A
mechanistic rationale for this enhanced reactivity was sug-
gested to involve a relay process in which sulfur coordinates
and positions the alkene proximal to the alkylidene complex.[10]
A subtle structural balance between stability and reactivity was
evident from observations that homologous butenyl and
pentenyl sulfides were inactive as metathesis substrates.[8] The
stabilizing effects of chelating benzylidene sulfide ligands are
evident in “latent” ruthenium carbene catalysts that require
thermal or photochemical activation to initiate metathesis
propagation.[12] In this work, we investigated HG(II)-catalyzed
cross-metathesis of allylic sulfide and sulfone reagents with a
panel of severely hindered alkene substrates. Product identi-
fication and quantitation of metathesis products was facilitated
using derivatives possessing a triazaborolopyridinium chromo-
phore, and unique features of reactivity and regioselectivity
were revealed. We report the unprecedented intermolecular
cross-metathesis of allylic chalcogenides with tetramethylethy-
lene. Extensive computer simulation with combined coupled
cluster/density functional theory calculations support a mecha-
nistic pathway that involves chelate-stabilized ruthenium β-
chalcogenide carbene complexes that provide an energetically
accessible pathway to π-complexes with highly substituted
alkenes, followed by rate-determining formation of metal-
lacyclobutane intermediates.
The ligand-based design of ruthenium metathesis catalysts has
achieved remarkable efficiency with broad tolerance of func-
tional groups under a wide range of reaction conditions. The
versatile second generation HoveydaÀ Grubbs(II) precatalyst [HG
(II)]
incorporates
imidazolin-2-ylidene
and
chelating
benzylidene-ether ligands.[1] Intermolecular cross-metathesis is
most successful with matched alkene pairs where one exhibits
high reactivity, characterized by rapid homodimerization, and
the second substrate is less reactive and dimerizes with
reluctance due to electronic or steric factors.[2] Recently,
trisubstituted prenyl derivatives have been employed as
[a] R. R. Sapkota, Dr. M. R. Talipov, Dr. J. B. Arterburn
Department of Chemistry and Biochemistry, New Mexico State University,
Las Cruces, NM, 88003
E-mail: jarterbu@nmsu.edu
[b] Dr. J. M. Jarvis, Dr. T. M. Schaub
Chemical Analysis and Instrumentation Laboratory, College of Agricultural,
Consumer and Environmental Sciences, New Mexico State University, Las
Cruces, NM, 88003
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/open.201800296
©201x The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
This is an open access article under the terms of the Creative Commons
Attribution Non-Commercial NoDerivs License, which permits use and dis-
tribution in any medium, provided the original work is properly cited, the
use is non-commercial and no modifications or adaptations are made.
The three fluorescent metathesis probes shown in Figure 1
with vinyl (1b), allylic sulfide (1c), and sulfone (1d) groups were
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Figure 1. Structures of spectrophotometric probes
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synthesized (Supplemental Scheme 1). Sulfoxides were not
included in this study because they are reported to be poor
substrates in crossÀ metathesis reactions, although they readily
participate in ring closing metathesis.[13,14] Complete experimen-
tal procedures, compound characterization data, photophysical
properties of (1b–1d), and computational details are provided
in the Supplementary Information.
Comparative homodimerization reactions of the alkenyl-
probes (vinyl 1b, allylic sulfide 1c, and sulfone 1d were
investigated to establish their relative metathesis reactivity
respectively) after 8 hours (see SI for details Figure S11). While it
is clear that homodimer 2d re-entered the catalytic cross-
metathesis reaction, poor solubility limited conversion under
these reaction conditions. Additional control experiments with
the isolated prenylsulfide 4c and prenylsulfone 4d demon-
strated that these trisubstituted alkenes were unable to initiate
metallacyclobutane formation with the HG(II) catalyst precursor
(data not shown), consistent with expectations based on steric
influences.[3,6, 19]
Catalytic metathesis of terpenoids is of interest for the
production of fine chemicals and polymers.[18] In light of the
efficient labeling and high isolated yields of prenyl products
from sulfone 1d, we investigated the possible electronic effects
of the substrate on the relative reactivity of this probe. The
terpenoid substrate citral (2S), which consists of an E/Z isomeric
mixture of citral A/B (61/31), was selected as a model system to
evaluate possible electronic factors that control regioselectivity
of 1d for different trisubstituted alkene groups. For this
experiment the stoichiometry was modified by using a 1:1 ratio
of allyl sulfone 1d to citral in the presence of the catalyst HG(II)
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°
using the HG(II) catalyst (5 mol%), in dichloromethane at 40 C
(see SI for details). The allylic sulfide 1c and sulfone probes 1d
produced the homodimers 2c and 2d in high yields, respec-
tively. In contrast, the vinyl substrate 1b produced only a trace
quantity of homodimer 2b even after extended reaction time
(12 hours), consistent with expectations for substituted styr-
enes.[15]
The alkene 2-methyl-2-butene (1 S) is a convenient sub-
strate for the preparation of isoprenoid derivatives due to its
°
volatility (35–38 C), that enables the use of excess reagent to
°
drive a cross-metathesis event towards complete conver-
sion.[16,17] Comparative reactions of the alkenyl probes 1b, 1c,
and 1d were conducted using 100 equivalents of 2-methyl-2-
butene and 5 mol% of the HG(II) catalyst in dichloromethane at
(5 mol%) in dichloromethane at 40 C (Equation 2). After 1 h
cross-metathesis occurred exclusively at the isolated isoprenyl
tail to yield disubstituted alkene 5d (43%) and trisubstituted
prenyl product 4d (33%), accompanied by the homodimeriza-
tion product 2d (24%) and trace amounts of unreacted 1d (<
1%). No products from cross-metathesis of the electron
deficient conjugated enal group (6d, 7d) were observed, which
illustrates the preferential reactivity of 1d with electron-rich
alkene substrates. Authentic samples of compound 7d were
synthesized by an alternative route (see SI) and used to verify
that 7d was undetectable as a product under these conditions.
°
40 C (Equation 1). The cross-metathesis of 1b yielded a product
ratio favoring disubstituted/trisubstituted alkene 3b/4b=2.4/
1.0 and trace amounts of homodimer 2b after 0.5 hours.
Extension of the reaction period to 8 hours increased the
amount of trisubstituted alkene 3b/4b=1.3/1.0. In contrast,
allylic sulfide 1c reacted rapidly with regioselectivity for the
trisubstituted prenylsulfide 3c/4c=1.0/2.4 at 0.5 hours. This
product distribution and the amount of homodimer 2c varied
little after an extended reaction time (8 hours), although small
amounts of oxidized compounds (sulfoxide and sulfone) were
detected and isolated yields for preparative-scale reactions
decreased. Sulfone 1d reacted rapidly and regioselectively to
preferentially produce the prenylsulfone 3d/4d=1.0/2.9 at
0.5 hours. Extension of the reaction period of 1d to 8 hours
resulted in complete conversion to the trisubstituted prenyl
sulfone.
To further evaluate steric influences, we investigated the
cross-metathesis of 1d with the triterpene squalene (3S). This
symmetrical polyisoprenoid substrate consists of two head-to-
head linked farnesyl units, and can produce six different
products of intermolecular cross-metathesis. The reaction
stoichiometry was modified to a 5-fold excess of squalene so
that 1d was the limiting reactant, while maintaining standard
conditions and reaction time (1 hour). The product distribution
was analyzed by HPLC- PDA-HRMS, and quantified results are
shown in Figure 2. The product mixture contained all of the
These results demonstrate that the disubstituted sulfone-
functionalized alkene 3d remained a competent substrate in
the catalytic cycle with 1 S to produce the thermodynamically
favored trisubstituted prenyl product 4d. Only trace amounts of
the allylsulfone homodimerization product 2d were observed
under these conditions that involved a large excess of 1S.
Control experiments using homodimer 2d and excess 1S in the
presence of HG(II) catalyst proceeded slowly to form the
expected di- and trisubstituted products 3d/4d (8% and 23%,
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4c (24%), although the yield was lower relative to the sulfone,
and minor amounts of unreacted 1c, and oxidized products
were also detected (SI Figure S15). Additional allylic sulfone
substrates were evaluated to assess the scope of this surprising
reaction and to determine if any specific features of the
reactivity were associated with the triazaborolopyridinium dye.
The HG(II) catalyzed cross metathesis of tetrasubstituted 4S was
successful with aliphatic substrate methylallylsulfone 12, pro-
ducing the trisubstituted prenyl derivative 13 (45%) and
homodimer 14 (38%). In a similar way, the phenylallylsulfone
analog 15 underwent efficient cross-metathesis with 4S to
produce the alkene 16 (51%) and homodimer 17 (40%).
Additional cross-metathesis reactions of 4S with the electron
deficient vinyl sulfone 18, and the allylic chalcogenide allyl p-
anisyl ether 19 were attempted (see SI). No cross metathesis
products of the vinylsulfone 18 with 4S were detected under
these conditions over 12 h. Previous reports have shown cross
metathesis of 18 with type I alkene substrates.[20] Only trace
amounts of cross metathesis products from the allylic ether 19
with 4S were detected, similar to published reports demon-
strating increased reactivity of allylic sulfides compared to
ethers.[9] These results demonstrate that unique structural
features of the allylic sulfone and sulfide moieties are associated
with the enhanced cross-metathesis reactivity of sterically
obstructed alkenes. With experimental evidence documenting
cross metathesis of allylic chalcogenides with tetrasubstituted
alkene substrates, computational studies were initiated to
elucidate potential structural features and the energetics of the
interaction with activated ruthenium N-heterocyclic carbene
catalysts.
Figure 2. Regioselective cross-metathesis of 1d with polyisoprenoid squa-
lene.
expected trisubstituted (4d, 6d, 10d) and disubstituted (8d,
9d, 11d) products. The predominant products resulted from
cross-metathesis at the terminal alkene that produced the
disubstituted alkene 8d (20%) and the regioisomeric trisubsti-
tuted C5 prenyl compound 4d (45%). Similar ratios (~1:2) of
di-/trisubstituted products were observed for each of the other
internal alkene positions, which resulted in similar amounts of
the C10 geranyl 6d (6%) and C15 farnesyl 10d (9%) analogs.
These results demonstrate efficient catalyst turnover for cross-
metathesis of 1d with electron-rich trisubstituted alkenes and
suggest the operation of sulfone-associated structural features
that poise the incipient ruthenium β-sulfonyl carbene com-
plexes for enhanced reactivity with sterically hindered sub-
strates.
A review of the literature was unable to identify examples
of metathesis events for ruthenium carbene complexes incorpo-
rating
a tetrasubstituted alkene, and previously reported
attempts to engage 2,3-dimethyl-2-butene (4S) in cross-meta-
thesis were unsuccessful.[16]
The facile reactivity of allylic sulfone 1d with isoprenoid
substrates encouraged us to investigate the possibility of cross-
metathesis with 4 S. Remarkably, the allylic sulfone 1d
efficiently produced the cross-metathesis product 4d (59%),
accompanied by homodimer 2d (31%), see Equation 3. The allyl
sulfide 1c was also converted to the cross-metathesis product
The energetic profiles of the reaction of 2,3-dimethyl-2-
butene with HG(II) and the incipient carbenes derived from
methylallyl sulfide 12 (that is, [Ru]=CHÀ CH2À SÀ CH3) and sulfone
15 (that is, [Ru]=CHÀ CH2À SO2À CH3), were determined using
electronic structure calculations. We analyzed the intermediates
and transition states following the established three-step
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Figure 3. Top: Free energy profile of interaction of 2,3-dimethyl-2-butene 4S with HG(II), [Ru]=CHÀ CH2À SÀ CH3 and [Ru]=CHÀ CH2À SO2À CH3, shown using red,
black, and blue colors, respectively [DLPNO-CCSD(T)/def2-TZVP//M06-L/def2-SV(P)+PCM(MeCN)]. Bottom: Calculated initial chelate structures of the HG(II)
precatalyst and ruthenium β-chalcogenide carbene complexes are aligned along the left edge, the corresponding key intermediates in the reaction coordinate
pathways are correlated within columns directly below the composite free energy profiles, and selected bond lengths are associated with the computed
structures. It is noted that the transition state free energy of the π-complex formation involving HG(II) lies below the free energy of the resulting π-complex
due to the entropy correction (no such effect is present on the corresponding potential energy surface diagram).
reaction course according to the Chauvin mechanism that
involves formation of the ruthenium carbene π-complex,
followed by the formation of metallacyclobutane, and finally
dissociation to the product π-complex.[21] Previous computa-
tional studies on the mechanisms of HG(II) initiation reveal that
an interchange mechanism involving synchronous departure of
the chelating ether ligand with incoming alkene was the most
common route leading to π-complex formation with α-olefins,
compared with the alternative dissociative or associative
mechanisms.[22] The extreme steric parameters associated with a
tetrasubstituted alkene component have not previously been
explored by computation. Electronic structure calculations were
performed using a state-of-the-art combined coupled cluster/
DFT approach, in which the geometry structures of interest
were optimized at the M06-L/def2-SV(P)+PCM(dichlorome-
thane) level of theory [see Figure S18 and Table S3 in the
Supporting Information for computational details and compar-
ison of the computed and X-ray structures of HG(II)], and the
electronic energies were further refined by the single-point
calculations at the DLPNO-CCSD(T)/def2-TZVP level of theory.[23]
The calculated free energy profile for the interaction of 2,3-
dimethyl-2-butene 4S with HG(II) showed that formation of the
corresponding metallocyclobutane is
a prohibitively high
endothermic step (ΔG=100 kJ/mol, see Figure 3 (red lines).
These results suggest that a dissociative mechanism would be
required with HG(II) due to steric constraints of the tetrasub-
stituted substrate. In contrast, the free energy diagram for the
intermolecular cross-metathesis of 4S with the β-sulfonyl-
ruthenium carbene complex derived from allylsulfone reveals
that an efficient route is available with a net free energy of
activation ΔG� =42 kJ/mol as shown in Figure 3 (green lines).
The free energy diagram for the β-sulfidyl-ruthenium carbene
complex shown in Figure 3 (black lines) exposes a significantly
lower reaction profile than that for the HG(II) catalyst, but still
higher than the corresponding potential energy surface of the
β-sulfonyl-ruthenium carbene complex. Chelation of the sulfone
and sulfide appendages are important structural features
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involved in both of the computed lowest energy ruthenium
carbene complexes derived from the allylic chalcogenides, but
in presence of alkene 4S each of these intermediates proceeds
to form the non-chelated π-complexes. These results suggest
that chelation of the β-chalcogenide groups provides anchi-
Computational resources were provided by ICT Supercomputing of
NMSU and by the Extreme Science and Engineering Discovery
Environment (XSEDE) award TG-CHE170004.
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meric stabilization of the activated ruthenium carbene inter- Conflict of interest
mediates and may thereby avoid competitive catalyst degrada-
tion that often accompanies stalled catalytic cycles. The
coordination sphere then opens to accommodate ligation of
tetramethylethylene that initiates the cross-metathesis process.
The fine balance involved in the energetics of chelate
stabilization in this case contrasts with the well-known exam-
ples of latent metathesis catalysts in which the stability of the
chelated carbene requires significant thermal or photochemical
initiation of metathesis. Strategies that involve exogenous
addition of Lewis acids have been implemented to overcome
impeded metathesis with ester-functionalized substrates, but
this approach does not extend the steric tolerance for hindered
substrates.[24] The increased energetic cost of the sulfide-based
Ru catalyst as compared with the sulfone-based Ru catalyst
develops steadily as the reaction progresses and is therefore
likely related to the total endothermicity of the reaction (i.e.
ΔG=54 and 30 kJ/mol for the profile with the sulfide- and
sulfone-based catalyst). The larger energetic cost associated
with the β-sulfidyl-ruthenium carbene complex in the reaction
with 4S is consistent with the experimental observations of
reduced product yields (SI Figure S15), and the detection of
oxidized byproducts that likely result from catalyst degradation
in the corresponding reactions of 1c that were discussed
previously.
These results reveal favorable energetics for the production
of π-complex and metallocyclobutane intermediates from
ruthenium carbene complexes that possesses β-chalcogenide
ligands, and provide a rationale for the observed intermolecular
cross-metathesis reactions of allylic sulfones with sterically
hindered substrates, including the first example involving a
tetrasubstituted alkene. The β-sulfonyl chelate structure
achieves a functional balance between a stabilized resting state
for the intermediate carbene catalyst to promote turnover and
avoid the energetic traps that result in latent or stalled catalysts.
Additionally, the β-sulfonyl chelate structure provides access to
coordination by alkenes with severe steric constraints. These
results support the classification allylic sulfone reagents as
privileged substrates for intermolecular cross-metathesis, and
we anticipate that this method will have broad utility for the
derivatization of isoprenoids, recycling of polyisoprenoid rubber
materials,[25] and for bioconjugation and bioorthogonal labeling
applications.
The authors declare no conflict of interest.
Keywords: tetrasubstituted
squalene · coupled cluster
· chalcogenide · terpenoid ·
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Acknowledgements
This research was supported by the National Science Foundation
(NSF CMI 1507070) and mass spectrometry data was recorded on
instrumentation obtained through the National Science Founda-
tion Major Research Instrumentation Program (NSF MRI 1626468).
Manuscript received: December 29, 2018
Revised manuscript received: January 23, 2019
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