Palladium Nanoparticle-Catalyzed Stereoretentive Cross-Coupling of Alkenyl Sulfides with Grignard Reagents

Article abstract of DOI:10.1021/acs.orglett.8b00208

Reaction conditions allowing a stereoretentive cross-coupling of alkenyl sulfides with Grignard reagents using ligand-free Pd catalysis are discussed here. The presence of an adequately positioned OH function is a key feature that allows a Mg-promoted Lewis acid activation of the mercaptide leaving group. This easy to implement procedure actually relies on an in situ generation of stable Pd nanoparticles by simply mixing Pd₂(dba)₃, the Grignard reagent, and the vinyl sulfide cross-coupling partner in THF. The efficiency of this procedure has been demonstrated in a natural product total synthesis context.

Full text of DOI:10.1021/acs.orglett.8b00208

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Palladium Nanoparticle-Catalyzed Stereoretentive Cross-Coupling of
Alkenyl Sulfides with Grignard Reagents
Louis Jeanne-Julien,† Eloi Astier,† Rene Lai-Kuen,‡ Gregory Genta-Jouve,† and Emmanuel Roulland*,†
́
́
†C-TAC, UMR 8638, CNRS/Universite
́
Paris Descartes, 4, avenue de l’Observatoire, 75006 Paris, France
‡Plateau Technique Imagerie Cellulaire et Moleculaire, UMR-S 1144, INSERM/Universite
́
Paris Descartes, 4, avenue de
l’Observatoire, 75006 Paris, France
S
* Supporting Information
ABSTRACT: Reaction conditions allowing a stereoretentive
cross-coupling of alkenyl sulfides with Grignard reagents using
ligand-free Pd catalysis are discussed here. The presence of an
adequately positioned OH function is a key feature that allows
a Mg-promoted Lewis acid activation of the mercaptide leaving
group. This easy to implement procedure actually relies on an
in situ generation of stable Pd nanoparticles by simply mixing
Pd2(dba)3, the Grignard reagent, and the vinyl sulfide cross-
coupling partner in THF. The efficiency of this procedure has been demonstrated in a natural product total synthesis context.
Scheme 1. Selected Examples of Ni-Catalyzed Cross-
Coupling of Vinyl Sulfides
tereodefined polysubstituted alkenes synthesis remains
Seven today a challenge despite the ever growing number
of transition-metal-catalyzed cross-coupling methods.1 The
abundance of the alkene functions in natural products2
stimulates the demand for new selective accesses to this
structural motive and offers many potential implementations.
Seminal works by Kumada and Takei3 that dealt with the cross-
coupling of organosulfides with Grignard reagents have
demonstrated the value of this approach to access polysub-
stituted alkenes.4 Since then, many similar conditions have been
described, but almost all remained in the Ni portfolio,5 except
two examples involving Fe6 or Pd.7
Although vinyl sulfides are readily accessed stereoselectively
and are stable compounds, their use in transition-metal-
catalyzed cross-coupling reactions remains scarce compared
with their vinyl halide analogues8 and are even more rarely used
in the total synthesis of natural products. This is attributable to
the drawbacks inherent to the Ni-catalyzed cross-coupling of
vinyl sulfides, which are a sensitivity to the steric hindrance
around the mercaptide leaving group and a lack of stereo-
retention with some substrates of cis configuration. Thus, in
many cases, it has been reported that nonterminal vinyl sulfides
are poor substrates. Even with a simple substrate such as (1-
phenylvinyl)alkylsulfane (Scheme 1, eq 1), yields remain at
around 50% despite the use of specially designed Ni ligands,5a
and obviously, intracatenary vinyl sulfides are equally mediocre
cross-coupling partners (Scheme 1, eqs 2 and 3).9 Conversely,
terminal vinyl sulfides typically react in very high yields
(Scheme 1, eq 4);3b,10 however, with terminal Z substrates the
stereoretention can be eroded.13e Single-electron-transfer
mechanism a priori involved in Ni catalysis is likely responsible
for this.
Ni catalysis failed at introducing a methyl by substituting the
mercaptide group of an intracatenary (Z)-vinyl sulfide using
MeMgBr. Our innovative strategy of synthesis was thus
endangered in its whole by this failure, so to circumvent this
dead end we carried out a study with vinyl sulfide 2a (Scheme
2), a model substrate mimicking our intermediate of total
synthesis. For a more atom-economical total synthesis,12 as well
as for strategic convenience, we wanted to keep free of
In the course of our synthesis of the aglycone of tiacumicin
Received: January 19, 2018
B,11 a naturally occurring antibiotic drug of prime importance,
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b00208
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Letter
Scheme 2. Devising Conditions on Model Substrate 2a
Scheme 4. Exploration of the Scope with MeMgBr
protective groups the alcohol functions of model substrate 2a,
and this had surprisingly productive consequences. Vinyl sulfide
2a was prepared by regio- and trans-selective hydrosulfuration13
of enyne 1, itself readily synthesized using a Pd/Cu dual-
catalyzed allene/alkyne cross-coupling.14 As anticipated, despite
a large excess of MeMgBr 3a and a loading in Ni of 10 mol %,
whatever the ligand used the intracatenary hindered vinyl
sulfide 2a failed to give 4aa in a synthetically useful yield.
Inspired by a previous report,7 we decided then to shift to Pd
catalysis. Thus, the Pd[P(t-Bu)3]2 complex15 gave a cleaner
result than NiCl2(dppe), but the yield in 4aa remained at
around 35%. However, we found that using Pd2(dba)3 without
phosphane ligand improved the yield up to 84% with a total
stereoretention and with a loading of only 1 mol % (Scheme 2).
Hence, this cross-coupling reaction gathering the prerequisites
of stereoretentivity and efficacy, we included it in our synthetic
strategy of the aglycone of tiacumicin B (7) (Scheme 3). Thus,
a
b
Completed within 2 h, byproducts are formed. NiCl2(dppe) (10
c
mol %), MeMgBr (10 equiv)/THF, 120 °C, 4 h. The leaving group is
−SEt instead of −SBu. The leaving group is −SC8H17 instead of
−SBu.
d
4aa in 77%. Remarkably, with 0.1 mol % of Pd2(dba)3 the
cross-coupling still took place (4aa: 41%), even though 28% of
2a was recovered after 55 h of reaction. The reaction was also
performed with a series of analogues of 2a the mercaptide
leaving groups of which being −SC8H17, −SC12H25, or
−SC18H37, but with no impact. Much more interesting was
the behavior of 2b, the MOM-protected version of 2a, which
remained inert, thus revealing the importance of having a
vicinal free OH group. As expected, naphthyl derivative 2c gave
4ca in 72% yield, while nonconjugated vinyl sulfide 2d
remained unreactive. Although in Z vinyl sulfide 2e the
distance between the OH and the SBu groups has been
lengthened by one notch compared to 2a, trisubstituted alkene
4ea16 could still be obtained stereoretentively in only 2 h of
reaction. Similarly, the coupling of Z vinyl sulfide 2g gave
product 4ga, while in contrast, 2f, the E isomer of 2e, remained
inert. These observations suggested that the reactivity of
sulfides 2a, 2c, 2e, and 2g could result from the chelation of a
magnesium atom between the alcoholate and the proximal
sulfur atom, while such chelation is impossible with unreactive
2b (MOM-protected) and 2f (trans configuration). Therefore,
this chelation seems to activate the C−S bond, probably
favoring the Pd0 oxidative insertion. Indeed, theoretical
calculations we carried out on models predicted that the Mg/
S interaction actually lowers the C−S bond dissociation energy
by ca. 31 kcal.mol−1.17 These led us to imagine that the
inertness of 2f could possibly be circumvented by replacing the
−SBu by a “self-chelating” mercaptide leaving group such as
−S(CH2)2OH. This hypothesis was confirmed as 2f* was
transformed into 4fa16 (Scheme 5), although stereoretention
was slightly eroded. However, there is no such loss of
stereoretention with 2i*, which in addition provided 4ib with
a yield now improved to 55%. Csp2 Grignards were also
Scheme 3. New Strategy To Build Polysubstituted Dienes
we succeeded in transforming the polyfunctionalized and
enantioenriched vinyl sulfide 5 into fragment 6 with a yield
of 79% with no isomerization, and this enabled us to ultimately
reach our target.11 Note that although Ni catalyzed the
transformation of 2a into 4aa with a 31% yield, it led only to
the degradation of the precious vinyl sulfide 5. Note also that
this [allene−alkyne cross-coupling/selective hydrosulfuration/
PdNPs-catalyzed cross-coupling with MeMgBr] sequence is a
new and promising strategy of access to polysubstituted dienes,
and it can be seen as an alternative to more traditional Suzuki,
Negishi, or Stille-based strategies. Applied to tiacumicin B, this
three-step sequence took place with an overall yield of 48%
delivering a complex and enantioenriched chiral fragment, and
no additional protective-group installation was needed.
This success in a natural product synthesis is a testimony to
the usefulness and the relevance of this synthetic pathway, and
it encouraged us to further explore the scope and the
limitations of this cross-coupling method. First, we centered
the study on MeMgBr (Scheme 4). As already mentioned, 4aa
was obtained in 84% yield from 2a with 1 mol % of Pd2(dba)3,
but a loading of 2.5 mol % led to the formation of byproducts
and a lower yield (64%), while a loading of 0.5 mol % yielded
Scheme 5. Using − S(CH2)2OH a Self-Chelating Mercaptide
Leaving Group, Instead of −SBu
B
DOI: 10.1021/acs.orglett.8b00208
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Letter
evaluated (Scheme 6): thus, Z vinyl sulfides 2i reacted with
phenylmagnesium bromide 3b to give known 4ib18 in only 35%
distribution of these NPs ranges mainly from 1.5 to 2.0 nm with
a mean of 1.85 nm for the main population (skewness = 0.52),
which is rather classical for catalytically active PdNPs.29b,30 The
PdNPs formed under these conditions being stable, this raised
the question of the nature of the stabilizer because unless
stabilized by quaternary ammonium salt,29b,20 polymers27,21 or
heteroatom-containing ligands,22 PdNPs are thermodynami-
cally unstable with respect to agglomeration and tend to form
clusters with lower or even no catalytic activity.23 Platinum-
group metals have a strong affinity for soft sulfur-based donors,
which makes S-containing molecules highly efficient NPs
stabilizers.22 Thus, PdNPs can be efficiently stabilized by
sulfur-doped graphene24 or by thiol-treated silica that allows
Heck and Suzuki reactions to take place.25 Simple lipophilic n-
alkanethiols have been used to prepare “alkanethiol-protected”
PdNPs; however, their catalytic properties were not studied.26
Therefore, in our case, the BuS moiety being a reaction
product, it is reasonable to think that it also plays this key role
of PdNP stabilizer. The exact nature of the Pd-active species in
PdNP-catalyzed cross-couplings has given rise to intense
debates,27 but now it is unequivocally established that PdNPs
can be highly active catalytic entities.28 Research in this field is
gaining in intensity. Thus, the role of PdNPs in the Heck
reaction (Jeffery conditions)29 or in ligand-free Suzuki−
Miyaura cross-couplings have been thoroughly studied, and in
2017, Feringa and co-workers30 demonstrated that their
ultrafast cross-coupling of aryllithium with halogenated Csp2
electrophiles is in fact catalyzed by PdNPs. Therefore, it has
appeared particularly important to verify if our cross-coupling
belongs or not to the family of the PdNP-catalyzed trans-
formations because if yes it would mean that the Kumada−
Corriu reaction also belongs to the PdNP portfolio. Test
experiments were carried out to prove it:
Scheme 6. Exploration of the Scope with Other Grignard
Reagents
a
(3E,5E)-4-Methylocta-3,5-diene-1,7-diol 8 was isolated in 32% yield.
b
Not separable from starting material 2h; ratio evaluated by 1H NMR;
c
no other products are formed. 49% of the starting material 2i was
recovered. The leaving group is SMe instead of SBu. The leaving
d
e
group is −SC8H17 instead of −SBu.
yield but with full stereoretention.19 In contrast, the same
transformation catalyzed by Ni yielded a 14:86 E/Z mixture
(Scheme 1, eq 4).10 (E)-Vinyl sulfide 2h also reacted
stereoretentively with PhMgBr 3b.19 Vinyl sulfide 2a reacted
with PhMgBr 3b or with 2-naphthylmagnesium bromide 3c to
give 4ab (50%) and 4ac (54%), respectively. However,
vinylmagnesium bromide 3d led to degradation, and aromatic
sulfides 2j remained inert under our reaction conditions. Other
types of Csp3 Grignard reagents were tested on vinyl sulfide 2a
(Scheme 6). Thus, EtMgBr 3e led to the desired diene 4ae in
only 6% yield, desulfurated diene 8 being the main product.
The isolation of the latter suggests that β-hydride elimination
on a [Pd]−Et species gave reactive hydridopalladium. This
hypothesis was verified by using a Csp3 nucleophile bearing no
β-eliminable H such as benzylmagnesium chloride 3f, which led
to the efficient formation of 4af (73%) with no trace of 8.
These reaction conditions being ligand-free, we came to
suspect that palladium nanoparticles (PdNPs) could form and
therefore be the catalyst in this reaction. To gain some insights
into this hypothesis, we submitted a sample of our reaction
medium (2a → 4aa) to transmission electron microscopy
(TEM), which revealed the presence of a multitude of particles
strongly diffracting the electrons (Figure 1). The diameter
Experiment A (eq 6): Mercury (Hg0) is a selective poison for
surface chemical processes,31 and addition of Hg0 to PdNPs led
to encapsulated by a monolayer of Hg atoms.28 Some Hg0
drops were thus added in the reaction medium at t = 1 h (TLC
indicating 40% of conversion of 2a into 4aa), and at t = 68 h we
saw that the reaction was blocked, ca. 20% of starting material
remaining nonconverted, while this reaction normally reaches
completion in 10−15 h. This marked reaction rate diminution
suggests that the catalytic activity could be exclusively due to
PdNPs and not to soluble Pd species.
Experiment B (eq 7): Rigid large-bite-angle bisphosphines32
are known to strongly slow down the speed of the reductive
elimination step, impacting negatively the catalytic cycle in
homogeneous Pd-catalyzed reactions and putting two large-
bite-angle bisphosphines on a Pd atom just impair all reaction.
Thus, at t = 1 h, we added 1.5 molecules of Xantphos33 per
atom of Pd so there was considerably more Xantphos than
possibly remaining dissolved Pd. At t = 1 h, TLC indicated 40%
of conversion of 2a into 4aa, and at t = 18 h the reaction
reached completion. This nondiminished transformation rate
Figure 1. TEM image of a sample of our reaction medium. (A) TEM
image of a sample of monodispersed PdNPs formed during the
reaction 2a + 3a → 4aa (1 mol % of Pd2(dba)3). (B) PdNP diameter
distribution.
C
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(2) The Dictionary of Natural Products indicates that more than 1500
compounds contain a structural motive closely related to 4aa, 4ea, or
4ga.
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of this reaction.
Experiment C (eq 8): PdNPs were prepared as described in
the literature from Pd(NO3)2 and Bu4N(OAc)20 and used as
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with the same overall amount of Pd, which allowed us to
transform 2a into 4aa in 64% yield after 42 h of reaction. This
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We have devised reaction conditions allowing the PdNP-
catalyzed stereoretentive cross-coupling of vinyl sulfides with
Grignard reagents. High stereoretention, even with Z
substrates, and good yields with hindered vinyl sulfide are
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situ through an operationally simple and robust procedure. This
reaction involves an interesting activation of the mercaptide
leaving group by chelation and therefore tolerates protective
group free OH. More efficient at transferring a methyl group
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b00208.
■
S
J.; Dabdoub, V. B.; Lenardao, E. J.; Hurtado, G. R.; Barbosa, S. L.;
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Experimental procedures and analytical data for all new
compounds (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: emmanuel.roulland@parisdescartes.fr.
ORCID
Emmanuel Roulland: 0000-0002-8012-7946
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
This work was supported by the Agence Nationale pour la
Recherche (ANR-14-CE16-0019-02, SYNTIA project), the
■
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CNRS, and the Universite Paris-Descartes.
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