Use of the 2-Pyridinesulfonyloxy Leaving Group for the Fast Copper-Catalyzed Coupling Reaction at Secondary Alkyl Carbons with Grignard Reagents

Article abstract of DOI:10.1021/acs.orglett.9b00976

Investigation of the copper-catalyzed coupling reaction of 2-pyridinesulfonates with Grignard reagents revealed that reactions with catalytic Cu(OTf)₂ were completed in <40 min. The results differed from those of the previous CuI-catalyzed reactions of tosylates in the presence of additives (LiOMe and TMEDA) for 12-24 h. It was shown that the preferred coordination of the leaving group to the reagents accelerated the reaction. Successful reagents were MeMgCl and other RMgX. Complete inversion was established.

Full text of DOI:10.1021/acs.orglett.9b00976

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Use of the 2‑Pyridinesulfonyloxy Leaving Group for the Fast Copper-
Catalyzed Coupling Reaction at Secondary Alkyl Carbons with
Grignard Reagents
Riku Shinohara,† Masao Morita,† Narihito Ogawa,‡ and Yuichi Kobayashi*,†
†Department of Bioengineering, Tokyo Institute of Technology, Box B-52, Nagatsuta-cho 4259, Midori-ku, Yokohama 226-8501,
Japan
‡Department of Applied Chemistry, Meiji University, 1-1-1, Higashimita, Tama-ku, Kawasaki 214-8571, Japan
S
* Supporting Information
ABSTRACT: Investigation of the copper-catalyzed coupling reaction of
2-pyridinesulfonates with Grignard reagents revealed that reactions with
catalytic Cu(OTf)2 were completed in <40 min. The results differed
from those of the previous CuI-catalyzed reactions of tosylates in the
presence of additives (LiOMe and TMEDA) for 12−24 h. It was shown
that the preferred coordination of the leaving group to the reagents
accelerated the reaction. Successful reagents were MeMgCl and other
RMgX. Complete inversion was established.
Scheme 1. Background and Proposal of a Highly Reactive
Leaving Group
he metal-catalyzed coupling reaction of secondary alcohol
derivatives with organometallics is a promising method
T
for the construction of a secondary alkyl framework,1 Allylic,
propargylic, and benzylic electrophiles have been well studied,
and the stereochemistry has been established.2 The reactions
are promoted by the existing π-carbon moieties. In contrast,
coupling at a secondary carbon of inactivated alkyl electro-
philes has been published occasionally. Thus, the Ni- and Fe-
catalyzed versions3,4 seem to be a radical process.5 However,
the Ni-catalyzed coupling of racemic substrates possessing a
certain functional group proceeds enantioselectively in the
presence of a chiral ligand.6 Ag- and Cu-catalyzed coupling
reactions of secondary alkyl halides have also been reported,7,8
but their stereochemical aspect has not been investigated.
Meanwhile, the Cu-catalyzed coupling reaction of tosylates
reported by Burns9 was reinvestigated by Liu, who found
RMgBr/CuI/LiOMe/TMEDA for good yields of B from
tosylates A and established an inversion of the stereo-
chemistry10 (Scheme 1, eq 1). In this reagent system,
LiOMe is indispensable, whereas TMEDA was added to
suppress the formation of olefins,11 although the degree of
olefin formation was not reported. Later, the coupling was
applied by Negishi,12 who found that 2,2′-bipyridyl was a
better ligand than TMEDA. This protocol required a long
period of time (24 h), and MeMgX was not examined despite
the existence of attractive methyl-substituted bioactive
compounds.12,13 In this regard, we found that the reactivity
of MeMgX was lower than that of other RMgBr compounds
(vide infra). To break through these limitations, we focused
our attention on the design of a leaving group.
formation of the complex and/or the subsequent intra-
molecular reaction to produce B was reluctant and thus took
24 h. With this hitherto undiscussed kinetics in mind, we
selected 2-pyridinesulfonate C15 as an electrophile (Scheme 1,
eq 2). This group was once used for the bromination with
MgBr2.16 As stated, we expected the following effects: (1) the
native polarity of the pyridine ring would increase the leaving
potency; (2) two coordination sites by nitrogen and oxygen
atoms would strongly prefer the formation of complex D (eq
3); and/or (3) the coordination would result in electron-
withdrawal and, hence, further increase the leaving potency of
On the basis of the transition-state model for the coupling of
MeMgBr and Me2CuLi·LiCl,14 the reagent or its appendage
was considered to bind to the oxygen atom of the sulfonyl
group in tosylate A. Furthermore, we speculated that the
Received: March 19, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b00976
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Table 1. Preliminary Study of the Coupling
Table 2. Examination of Various Copper Salts
temp
time
(R)-9
10
entry
Cu cat.
CuI
Cu(OTf)2
Cu(OTf)2
Cu(OMe)2
Cu(OAc)2
Cu(acac)2
(°C)
(min)
(%)
es
(%)
1
2
3
4
5
0
0
−20
−20
−20
−20
15
30
30
20
30
30
59
72
68
63
61
65
88.3
98.3
99.2
99.5
98.2
99.0
17
21
19
16
14
20
c
6
a
Treated as follows for obtaining er by chiral HPLC: (i) OsO4 (cat.)/
NMO, aq. acetone; (ii) p-TsOH·H2O, MeOH. Table 1, footnote b.
b
product, yield recovered, yield
c
MeMgCl (4 equiv).
entry substrate
RMgX
time
12 h
24 h
15 min
15 min
30 min
24 h
(%)
(%)
c
1
2
3
4
5
6
7
8
1
1
2
2
2
6
6
7
CyMgBr
MeMgCl
CyMgBr
MeMgCl
MeMgCl
CyMgBr
MeMgCl
MeMgCl
3, 76
4, 11
3, 76
4, 59
4, 77
8, 0
1, 0
1, 52
2, 0
2, 0
2, 0
6, 100
6, 100
7, 0
Scheme 3. Coupling of (S)-7 with MeMgX (X = Br, I)
c
d
d
e
24 h
20 min
9, 0
9, 73
f
a
Olefins 5: a mixture of PhCH2CHCHMe (5a), and Ph-
b
(CH2)2CHCH2 (5b). Olefins 10: a mixture of PhCH2CH
CH(CH2)2OTBS (10a) and Ph(CH2)2CHCHCH2OTBS (10b).
c
d
3/5a/5b = 85:10:5 (entry 1) and 88:7:5 (entry 3). 4/5a/5b =
e
77:14:9 (entry 4) and 88:9:3 (entry 5). Carried out with Cu(OTf)2
(0.1 equiv) without additives (LiOMe, TMEDA). 9/10a/10b =
f
76:13:11.
Scheme 4. Examination of Other Leaving Groups
Scheme 2. Synthesis of Enantioenriched 2-
Pyridinesulfonates
a
b
RuCl[(R,R)-TsDPEN](p-cymene). Determined by chiral HPLC.
c
Purity for (R)-16.
2-PySO3. Herein, we report the coupling reaction of 2-
pyridinesulfonates C.
tosylate 1 (footnote c). The coupling reaction with MeMgCl
was completed as well and produced 4 and olefins 5 in 59%
and 18% yields, respectively (entry 4). The finding that the
leaving potency of the 2-pyridinesulfonyloxy group was higher
than that of the tosyl group was also confirmed by a
competitive experiment using a 1:1 mixture of 1 and 2 (see
the SI). The reaction did not proceed without the catalyst (see
the SI).
Next, the coupling reaction of tosylate 6 with CyMgBr and
MeMgCl disclosed that the reactivity was sensitively affected
by the substituent that is bigger than methyl (entries 6 and 7).
The results suggest that the substituent interferes markedly in
the formation of the complex. A similar tendency could be read
from the literature coupling of tosylates possessing an Me or
First, the coupling reaction of tosylate 1 with CyMgBr (2
equiv) was carried out at 0 °C in the presence of CuI (0.1
equiv), LiOMe (1 equiv), and TMEDA (0.2 equiv) according
to the procedure described in the literature.10 Tosylate 1 was
indeed consumed in 12 h (shorter than the reported time of 24
h) to produce 3, but olefins 5 were produced as byproducts
(Table 1, entry 1, footnote c). In contrast, reaction of 1 with
MeMgCl (2 equiv) for 24 h produced 4 only in an 11% yield,
with recovery of tosylate 1 in 52% yield (entry 2), indicating
that the reactivity of MeMgCl was lower than that of CyMgBr.
On the other hand, the coupling of 2-pyridinesulfonate 2
with CyMgBr was remarkably fast (in 15 min) (entry 3). The
ratio of 3 and byproducts 5 was similar to that obtained with
B
DOI: 10.1021/acs.orglett.9b00976
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
alkyl substituent.10 In contrast, the reaction of 2-pyridinesul-
fonate 7 with MeMgCl afforded 9 in 73% yield (entry 8).
These results strongly suggest that the formation of complex D
is the important step. Further investigation led to the discovery
that the use of the additives (LiOMe and TMEDA) was not
essential for the coupling of 2-pyridinesulfonate 2 (Table 1,
entry 5).
Scheme 5. Coupling Reactions of Alkyl 2-
Pyridinesulfonates
a
The additive-free procedure found in Table 1, entry 5, was
applied to coupling of enantioenriched (S)-7 with MeMgX (X
= Cl, Br, I) to explore the stereochemical outcome and
catalytic activity of other copper salts. This substrate was
synthesized with 95.7% ee from ketone 11 via the asymmetric
transfer hydrogenation17 (Scheme 2). Propionaldehyde-
derived ketone 12 was also converted to (R)-17, which was
later subjected to the coupling reaction.
The coupling reaction of (S)-7 with MeMgCl was followed
by oxidative removal of olefins 10 and TBS desilylation (Table
2, footnote a). The absolute configuration of the resulting
alcohol (R)-18 was determined by comparing the [α]D value
with the published data,18 and the enantiomeric ratio (er) of
(R)-18 determined by chiral HPLC was converted to the
enantiospecificity (es).19 CuI gave (R)-9 with a somewhat low
enantiospecificity (es) of 88.3% (entry 1). We deemed that the
SN2 reaction with the iodide ion originating from CuI took
place ahead of the coupling. Based on the result, copper salts
consisting of less nucleophilic counteranions were next
examined. The catalytic activity of CuCl and CuCN were
marginal and low, respectively (data not shown). The
Cu(OTf)2-catalyzed coupling shown in entry 8 of Table 1
was run with (S)-7 to afford (R)-9 in 72% yield with 98.3% es
(entry 2). A reaction at −20 °C gave similar yield and ee in 30
min (entry 3), indicating that strictly controlled reaction
conditions are not necessary. Similarly high levels of es were
recorded with Cu(OMe)2 and Cu(OAc)2, but the yields were
slightly low (entries 4 and 5). Cu(acac)2 required 4 equiv of
MeMgCl (entries 6). Olefins 10 produced in all cases in a
range of 14−22% were roughly a similar level to that observed
using the published reagent/catalyst system (Table 1, entry 8).
Consequently, the protocol using Cu(OTf)2 at temperatures
between −20 to 0 °C for 15−30 min was used for further
study.
Next, MeMgX (X = Br, I) were used for the coupling
reaction to determine the effect of the bromide and iodide
anions (Scheme 3). The coupling reaction with MeMgBr
proceeded with 99% es to afford (R)-18 in 55% yield.
However, MeMgI gave a mixture of 9 and iodide 19 in 16%
and 37% yields, respectively. The competing iodination might
account for the partial racemization observed for the CuI-
catalyzed coupling (Table 2, entry 1).
Structurally similar leaving groups or those used for allylic
substitution were subjected to Cu(OTf)2-catalyzed substitu-
tion with MeMgCl (Scheme 4). 3-Pyridinesulfonate 20 was
less reactive than 7 and afforded 9 in 54% yield after 4 h at rt.
A worse result was the production of alcohol 15 from 4-
pyridinesulfonate 21. These results support the notation that
the coordination of the leaving group to Mg2+ is a crucial step.
The picolinoxy leaving group developed previously for the
allylic substitution of secondary allylic substrates20 did not
promote coupling of 22 and instead afforded alcohol 15.
Phosphate 23 was completely unreactive.
a
Reactions were run with RMgX (2 equiv) and Cu(OTf)2 (0.1 equiv)
in THF at −20 or 0 °C for 20 min. The olefin byproducts were
removed by OsO4-catalyzed oxidation or by column chromatography.
b
c
The configuration was determined by [α]D value. Control
experiment with RMgX (2 equiv), Cu salt (0.1 equiv), LiOMe (1
equiv), and TMEDA (0.2 equiv) in THF. The stereochemistry was
established by comparison of the NMR data with those reported (see
the SI). The cis stereochemistry of 43 and 44 were determined by the
d
In order to evaluate the Cu(OTf)2-catalyzed coupling
reaction established above, we applied the reaction to
pyridinesulfonates delineated in Scheme 5. Several reactions
e
synthesis and NOESY analysis, respectively (see the SI).
C
DOI: 10.1021/acs.orglett.9b00976
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Letter
ORCID
were also examined in the presence of the additives (LiOMe,
TMEDA). In addition to the products, the olefins and in some
cases the corresponding alcohols were coproduced as
described in the Supporting Information. The olefins were
eliminated by OsO4-catalyzed dihydroxylation or chromatog-
raphy to afford pure coupling products. In all entries, the
coupling reactions at −20 or 0 °C were completed in 20−40
min. Coupling reaction of 7 with EtMgCl and i-BuMgBr gave
30 and 31 in moderate yields, whereas i-Pr was delivered in a
somewhat low yield (eq 1). The p-Tol group was attached in
52% yield even though generally being less nucleophilic than
alkyl reagents. Enantioenriched substrate (R)-17 afforded
somewhat volatile (S)-34 (eq 2) and the inversion reaction
was proven by the S configuration, which was confirmed by
Yuichi Kobayashi: 0000-0002-4385-9531
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by KAKENHI Grant No. 26410111.
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ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b00976.
Experimental procedures, compound characterization
data, chiral HPLC analysis, and NMR spectra (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: ykobayas@bio.titech.ac.jp.
D
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