Chemo- and Regioselective Functionalization of Polyols through Catalytic C(sp3)-C(sp3) Kumada-Type Coupling of Cyclic Sulfate Esters

Article abstract of DOI:10.1021/acs.orglett.6b01745

This contribution describes a copper-catalyzed, C(sp³)-C(sp³) cross-coupling reaction of cyclic sulfate esters, a distinct class of electrophilic derivatives of polyols, with alkyl Grignard reagents to afford functionalized alcohol products in good yields. The method is operationally simple and highlights the potential of cyclic sulfate esters as highly reactive substrates in catalytic, chemoselective polyol transformations.

Full text of DOI:10.1021/acs.orglett.6b01745

Letter
pubs.acs.org/OrgLett
Chemo- and Regioselective Functionalization of Polyols through
Catalytic C(sp3)−C(sp3) Kumada-Type Coupling of Cyclic Sulfate
Esters
Rodrigo Ramírez-Contreras and Bill Morandi*
Max-Planck-Institut fur Kohlenforschung, Kaiser-Wilhelm-Platz 1, 45470 Mulheim an der Ruhr, Germany
̈
̈
S
* Supporting Information
ABSTRACT: This contribution describes a copper-catalyzed,
C(sp3)−C(sp3) cross-coupling reaction of cyclic sulfate esters,
a distinct class of electrophilic derivatives of polyols, with alkyl
Grignard reagents to afford functionalized alcohol products in
good yields. The method is operationally simple and highlights
the potential of cyclic sulfate esters as highly reactive substrates
in catalytic, chemoselective polyol transformations.
he selective transformation of polyol derivatives is at the
by the groups of Kambe,14 Liu,15 and Hu,16 enantiospecific, Zn-
catalyzed variant of the reaction using triflates developed by
Breit,17 and a coupling of sulfamidates very recently reported by
Lee.18 Suzuki and Negishi-type reactions of tosylates have also
been reported.19
T
frontier of our ability to transform easily accessible,
biomass-derived building blocks into useful synthetic inter-
mediates.1 In a recent contribution, we reported the develop-
ment of a catalytic reaction that addresses the challenge of
regioselective deoxygenation of 1,2-diols at the primary
position.2 It was found that the siloxane intermediate involved
in the transformation was endowed with unique reactivity as a
result of its cyclic structure, which facilitated the reductive
cleavage of the terminal C−O bond under mild conditions. The
observation of high regioselectivity and enhanced reactivity of
such intermediate made us consider the possibility of using the
structurally analogous 1,2-cyclic sulfate esters as electrophiles in
catalytic, chemoselective cross-coupling reactions of polyols.
Cyclic sulfates are a class of electrophiles that are analogous to
epoxides in terms of electrophilic reactivity3,4a and display
selectivity for nucleophilic attack at the least hindered position to
yield substituted alcohols after hydrolysis.3a Cyclic sulfates are
very versatile and useful building blocks that have found a wide
variety of applications in organic synthesis,3−6 as well as in larger
scale pharmaceutical processes.7 Cyclic sulfates provide a
potential way to obtain high-value fine chemicals through
selective C−O bond reductive functionalization of diols and
other biomass-based platform chemicals.8 In addition, chiral,
enantioenriched 1,2-cyclic sulfates are also readily accessible
through asymmetric dihydroxylation of olefins,9 or by kinetic
resolution of epoxides or diols.10
We envisaged that the development of metal-catalyzed cross-
coupling reactions of cyclic sulfate esters would provide a
conceptually new entry into the catalytic, chemoselective
transformation of polyols. We started our investigations by
using decane-1,2-diol cyclic sulfate (1) as the test substrate in
control reactions with ethyl, isopropyl, and tert-butylmagnesium
chloride in the absence of a metal salt, to find that no
transformation had occurred. Once ascertained that cross-
coupling with alkyl Grignard reagents requires a catalyst to
occur, we proceeded to evaluate a set of metal systems that are
known to catalyze cross-coupling reactions of chemically related
alkyl sulfonates.20,21 Platinum-group metal salts performed
poorly, forming large amounts of decenes and no, or almost
no, coupling product (Table S1). We found that a high yield of
the regioselective coupling product was obtained using both
Cu(I) and Cu(II) chloride salts in the absence of any additives
(Table S1).18,20 Rapid addition of 1.5 equiv of the nucleophile at
room temperature is necessary in order to obtain the highest
possible yield with all Grignard reagents studied (Table S2).20
Zn(II) was also found to be a competent precatalyst, although it
performed less efficiently than Cu.17
With a set of optimum reaction conditions in hand (1.5 equiv
of RMgCl, 1% Li2CuCl4, overnight hydrolysis using 20%
H2SO4), we explored the scope of Grignard reagents (Table
1). Initially, a set of simple primary alkyl Grignards were used in
coupling reactions with 1, to obtain high yields of the
corresponding coupling products (entries 1−4). Next, we
examined coupling reactions with nucleophiles bearing addi-
tional functionality, such as a double bond (entry 5), a trialkylsilyl
While cyclic sulfates have found many applications in organic
synthesis, examples of their use in catalytic C−C bond-forming
reactions11 are scant.4 Moreover, the original report of cross-
coupling between (−)-diisopropyl tartrate-2,3-cyclic sulfate and
benzylmagnesium chloride has been found to proceed in the
absence of catalyst.4,12 The lack of broader studies using these
easily accessible electrophilic partners stands in stark contrast to
the many elegant studies reported using other alcohol-derived
electrophiles, such as powerful C(sp3)−C(sp3) Kumada-type13
reactions using Cu and Ni catalysts and alkyl tosylates reported
Received: June 16, 2016
© XXXX American Chemical Society
A
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Table 1. Scope of Grignards
Table 2. Scope of Substrates
a
b
Isolated yields. 1.0 mmol scale reactions. Ethylmagnesium bromide
was used.
group (entry 6), and an aromatic ring (entry 7), to obtain
moderate to high yields of the respective products. While
secondary Grignards consistently afforded good yields of the
desired products (entries 8−10), tert-butylmagnesium chloride
afforded only a low yield of the target product (entry 11), along
with significant amounts of 1-decene and 2,2,3,3-tetramethylbu-
tane. Both of these side products were detected in the reaction
mixture in a 1:1 ratio.22
When 1,3 instead of 1,2-cyclic sulfates were used with tertiary
Grignards, yields improved significantly (Table 2, entries 1 and
2). In addition, 1,3-cyclic sulfates were also efficiently coupled
with a primary Grignard to give the corresponding product in
high yield (Table 2, entry 3). Remarkably, a 14-membered PEG-
derived macrocyclic sulfate was cross-coupled in high yield as
well (Table 2, entry 4). In particular, this type of cyclic sulfates
has been used in larger scales for the preparation of PEG
derivatives, pointing out the possibility of using cyclic sulfates on
process scales.24 Our system also displayed good tolerance to a
wide range of functional groups. Electrophilic substrates known
to react with Grignard reagents were well tolerated (nitrile and
ester groups, Table 2, entries 5 and 6). Moreover, substrates
bearing functional groups known to be reactive under cross-
coupling conditions (OTs and Cl, Table 2, entries 7 and 8), were
selectively cross-coupled with the cyclic sulfate moiety at the
primary position. The high reactivity of cyclic sulfate esters is
demonstrated by its chemoselective cross-coupling in the
presence of a tosylate group, which attests the utility of cyclic
sulfates as cross-coupling partners (Table 2, entry 8).
Furthermore, both unprotected hydroxyl and carboxylic acid
groups were tolerated, which is an outcome that considerably
improves the synthetic applicability of the protocol (Table 2,
entries 9 and 10). A relevant result in the context of biomass
transformation was the coupling of an acetal-protected
mannopyranoside (Table 2, entry 12). In this case, a single C−
O bond could be site-selectively transformed to afford a
monocatenary glycolipid product that has surfactant proper-
ties25a and known liquid-crystalline behavior.25b Finally, (R)-
decane-1,2-diol cyclic sulfate underwent cross-coupling with
retention of the chiral information (Table 2, entry 13).
Interestingly, substrates with double bonds two positions away
from the cyclic sulfate moiety (Scheme 1, 2a and 2b) only
afforded the corresponding parent diols 3a and 3b.
a
b
c
Isolated yield on 1.0 mmol scale. 0.4 mmol scale reaction. 2.5 equiv
d
e
of Grignard reagent used. 0.16 mmol scale reaction. 99.7% ee
starting material, 96.7% ee product.23
Scheme 1. Observation of Desulfuration of Cyclic Sulfates
Bearing 5−6 Unsaturation
Dihexyl sulfoxide and unreacted starting material were
recovered from the reaction mixture of 2b.20 We speculate that
B
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intramolecular double bonds can act as π ligands for the
organocuprate catalyst and, in the case of 2a and 2b, this π
complex can adapt a conformation that favors R group transfer to
sulfur instead of carbon. Normal reactivity was observed,
however, when an additional spacer was present between the
site of unsaturation and the cyclic sulfate moiety (Table 2, entry
11).
A model for the mechanism that best fits all of the
experimental data is presented in Scheme 3. We propose that
Scheme 3. Proposed Mechanism
Finally, we delved into the mechanism of our system. To
examine the possibility of nanoparticle catalysis, excess Hg was
added to a reaction of 1 with ethylmagnesium chloride, to obtain
identical results as in the absence of Hg, thus rendering this
possibility unlikely.20 Next, we turned our attention to a Cu(I)−
Cu(III) mediated cycle. We subjected the diastereomerically
pure labeled compound 4 to our reaction conditions using
primary, secondary, and tertiary Grignards. Analysis of the
stereochemistry of the coupling products, directly from the crude
reaction mixtures, revealed that inversion of configuration at the
primary site had occurred, as evidenced by the magnitude of the
3Jab coupling constant of 1.8 Hz (Scheme 2a, syn 5a and 5b),19a
Scheme 2. Mechanistic Experiments
the catalytically active diorganocuprate(I) species, most likely an
oligomeric aggregate,26 is formed in solution from either the
Cu(I) or Cu(II) chloride precursor in the presence of excess
Grignard. Once the diorganocuprate(I) has formed, the next step
is nucleophilic attack on the cyclic sulfate at the sterically least-
hindered position through one of the filled d orbitals of Cu, to
form the Cu(III) organometallic intermediate 7 with inversion of
configuration at the electrophilic carbon center. At this point 7
can follow a path of productive C−C bond-forming reductive
elimination to yield the desired coupling product, or a path of
unproductive reductive elimination to form the Grignard
−
homocoupling product, followed by elimination of Cu(I)SO4
to form the olefin of the respective substrate. The relative rates of
the productive versus unproductive paths will be determined by
the net steric bulk around 7.
In conclusion, we have developed an operationally simple,
highly chemoselective catalytic method for the C(sp3)−C(sp3)
cross-coupling of polyol derivatives with alkyl Grignard reagents
to yield functionalized alcohols. The tolerance of our method to a
broad set of functional groups, and the ability to use cyclic
sulfates of various ring sizes, should allow for its implementation
in target-oriented synthesis. Furthermore, the transformation
reported herein highlights the potential of cyclic sulfate ester
electrophiles in catalytic chemoselective transformations of
polyols.
while no traces of syn 5c were detected. In all cases, anti 5
products were not observed, within the NMR detection limit.
This information is consistent with a catalytic cycle operating
through Cu(I)−Cu(III) species.26 Moreover, the need for quick
addition of 1.5 equiv of the Grignard reagent to the reaction
mixture also seems to indicate that a diorganocuprate(I) species
is the active catalyst. Dropwise addition of Grignard would create
a condition of constant deficiency of the reagent, which precludes
formation of the catalytically active diorganocuprate.27,28 It was
also found that olefins 6Z and 6E had formed in all reactions as
byproducts. These olefins are formed in a competing side
reaction, where unproductive reductive elimination of the alkyl
substituents takes place to form the observed homocoupling (R−
R) product and the organometallic Cu(I) salt of the ring-opened
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.6b01745.
sulfate,20,22 which could then eliminate Cu(I)SO4 , by either the
−
Ei or E2 mechanism. Finally, although epoxides bear considerably
Experimental details, NMR spectra, HPLC chromato-
graphic analyses (PDF)
more ring strain than 5-membered cyclic sulfate esters,29,30
a
competition experiment using a limited amount of Grignard
reagent led to the formation of similar amounts of the respective
coupling products. The better leaving group ability of the sulfate
group, compared to that of alkoxide, appears to be sufficient to
compensate for the difference in ring strain (Scheme 2b).31
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: morandi@kofo.mpg.de.
C
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D.; Osborne, C. A.; Jarvo, E. R. J. Am. Chem. Soc. 2014, 136, 14951.
Notes
(g) Gartner, D.; Stein, A. L.; Grupe, S.; Arp, J.; Jacobi von Wangelin, A.
̈
The authors declare no competing financial interest.
Angew. Chem., Int. Ed. 2015, 54, 10545. (h) Li, B.-J.; Xu, L.; Wu, Z.-H.;
Guan, B.-T.; Sun, C.-L.; Wang, B.-Q.; Shi, Z.-J. J. Am. Chem. Soc. 2009,
131, 14656. (i) Joshi-Pangu, A.; Wang, C.-Y.; Biscoe, M. R. J. Am. Chem.
ACKNOWLEDGMENTS
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Soc. 2011, 133, 8478. (j) Soler-Yanes, R.; Guisan
́
-Ceinos, M.; Bunuel, E.;
̃
Generous funding from the Max-Planck-Society and the Max-
Planck-Institut fur Kohlenforschung is acknowledged. We thank
Cardenas, D. J. Eur. J. Org. Chem. 2014, 2014, 6625.
́
̈
(14) (a) Terao, J.; Watanabe, H.; Ikumi, A.; Kuniyasu, H.; Kambe, N. J.
Am. Chem. Soc. 2002, 124, 4222. (b) Terao, J.; Todo, H.; Begum, S. A.;
Kuniyasu, H.; Kambe, N. Angew. Chem., Int. Ed. 2007, 46, 2086.
(c) Iwasaki, T.; Imanishi, R.; Shimizu, R.; Kuniyasu, H.; Terao, J.;
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Kuniyasu, H.; Kambe, N. Chem. Lett. 2003, 32, 890. (e) Terao, J.; Todo,
H.; Watanabe, H.; Ikumi, A.; Kambe, N. Angew. Chem., Int. Ed. 2004, 43,
6180.
Prof. Dr. Benjamin List at the Homogenous Catalysis depart-
ment of the Max-Planck-Institut fur Kohlenforschung for sharing
analytical equipment, as well as the HPLC and MS Departments
for technical assistance.
̈
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