Copper-catalyzed diastereo- and enantioselective desymmetrization of cyclopropenes: Synthesis of cyclopropylboronates

Article abstract of DOI:10.1021/ja510419z

A novel Cu-catalyzed diastereo- and enantioselective desymmetrization of cyclopropenes to afford nonracemic cyclopropylboronates is described. Trapping the cyclopropylcopper intermediate with electrophilic amines allows for the synthesis of cyclopropylaminoboronic esters and demonstrates the potential of the approach for the synthesis of functionalized cyclopropanes.

Full text of DOI:10.1021/ja510419z

Communication
pubs.acs.org/JACS
Copper-Catalyzed Diastereo- and Enantioselective
Desymmetrization of Cyclopropenes: Synthesis of
Cyclopropylboronates
Alejandro Parra, Laura Amenos,† Manuel Guisan-Ceinos,† Aurora Lopez, Jose Luis García Ruano,
́
́
́
́
and Mariola Tortosa*
Departamento de Química Organ
́
ica, Universidad Auton
́
oma de Madrid, Cantoblanco, 28049 Madrid, Spain
S
* Supporting Information
Scheme 1. Metal-Catalyzed Synthesis of Nonracemic
Cyclopropylboronates
ABSTRACT: A novel Cu-catalyzed diastereo- and
enantioselective desymmetrization of cyclopropenes to
afford nonracemic cyclopropylboronates is described.
Trapping the cyclopropylcopper intermediate with electro-
philic amines allows for the synthesis of cyclopropyl-
aminoboronic esters and demonstrates the potential of the
approach for the synthesis of functionalized cyclopropanes.
yclopropanes are an important class of compounds widely
Cpresent in biologically active natural products and
pharmaceuticals.1 In recent years, cyclopropylboronates have
gained increasing attention as useful building blocks for the
synthesis of functionalized cyclopropanes. In particular, they are
suitable partners for metal-catalyzed cross-coupling reactions
allowing the synthesis of interesting intermediates such as
arylcyclopropanes,2 cyclopropylketones,3 or vinylcyclopro-
panes.4 Although there are a number of methods available for
the synthesis of racemic cyclopropylboronic esters,5 the
preparation of enantiomerically enriched derivatives remains a
distinct challenge. The classic approach for the synthesis of
optically active cyclopropylboronates requires the use of
stoichiometric amounts of chiral auxiliaries on the boron
atom.6,5d,g There are only two approaches that take advantage
of the use of asymmetric transition-metal catalysis (Scheme 1).7,8
In an elegant study, Gevorgyan et al. described the Rh-catalyzed
asymmetric hydroboration of cyclopropenes (Scheme 1, eq 1).7
They prepared cyclopropylboronates with high levels of
diastereo- and enantioselectivity, although a coordinating
group (CO2Me) was necessary to achieve those levels of
stereocontrol. More recently, Ito and Sawamura reported the
transformation of (Z)-3-silyl- and aryl-allylic phosphates to
nonracemic 1,2-disubstituted cyclopropylboronates through a
Cu(I)-catalyzed reaction with a diboron compound (Scheme 1,
eq 2).8
limited to aryl-substituted olefins.11 Second, despite the excellent
work on diastereoselective functionalization of cyclopropenes
using copper,12 there are no reports on Cu-catalyzed
enantioselective desymmetrizations of these intermediates.
Moreover, most of the diastereoselective Cu-catalyzed carbo-
metalations of cyclopropenes require the presence of a directing
group to control the stereoselectivity.12,13 Third, trapping the
cyclopropylcopper intermediate A generated through this
approach (Scheme 1, eq 3) with electrophiles other than a
proton would allow access to highly functionalized cyclopropyl-
boronates that would be difficult to synthesize by known
methods. This last feature highlights the synthetic potential of
Cu-catalyzed borylations and is in contrast to other metal-
catalyzed hydroborations where the C−B bond is formed
through a reductive elimination step.14 Herein, we describe the
first Cu-catalyzed enantioselective desymmetrization of cyclo-
propenes to produce cyclopropylboronates with high diastereo-
and enantioselectivity and without the need of a directing group.
Additionally, we have shown that racemic cyclopropylcopper
intermediates related to A can react with electrophilic amines to
give unprecedented cyclopropylaminoboronates with high levels
of diastereoselectivity.
Recently, we have been involved in the development of
different Cu-catalyzed borylation reactions.9 Logically, we were
intrigued by the possibility of synthesizing enantioenriched
cyclopropylboronates from cyclopropenes using a chiral copper-
(I) boryl complex, ideally without the help of a coordinating
group (Scheme 1, eq 3).10 Along with the importance of the
products, there were three additional factors that made this
project challenging and interesting: First, Cu-catalyzed asym-
metric hydroborations of nonpolarized alkenes have so far been
Received: July 29, 2014
Published: October 23, 2014
© 2014 American Chemical Society
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Journal of the American Chemical Society
Communication
We began by examining the reactivity of cyclopropene 1a
under Cu-catalyzed borylation conditions, using achiral
phosphine ligands (Table 1). In the presence of CuCl (10 mol
Table 2. Effect of the Ligand and Copper Source on the
Enantioselective Hydroboration of Cyclopropenes
Table 1. Effect of the Ligand and Temperature on the
Diastereoselective Hydroboration of Cyclopropenes
a
b
c
entry
L
T (°C)
dr
yield (%)
e
yield
(%)
1
2
xantphos
xantphos
xantphos
23
−20
−20
95:5
≥98:2
≥98:2
40
60
90
ab
,
c
d
entry
Cu(I)
L*
dr
er
1
2
3
4
5
6
7
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
L1
L2
L3
L4
L5
L6
L6
L6
L6
L6
96:4
70:30
91:9
95:5
94:6
≥98:2
96:4
97:3
95:5
−
78:22
77:23
42:58
60:40
82:18
92:8
49
70
65
15
70
71
58
74
50
0
d
3
a
Reaction conditions: 1a (0.2 mmol), B2pin2 (0.22 mmol), NaOt-Bu
(0.1 mmol), CuCl (10 mol %), xantphos (11 mol %), MeOH (0.8
mmol), THF (0.33 M). Determined by H NMR analysis. Yield of
isolated ( )-2a. Cyclopropane 1a and MeOH were added at −78
°C; the reaction mixture was then warmed up to −20 °C.
b
c
1
d
[Cu(CH3CN)4]PF6
[Cu(CH3CN)4]PF6
[Cu(CH3CN)4]PF6
−
82:18
95:5
90:10
−
f
8
%), xantphos (11 mol %), B2pin2 (1.1 equiv), NaOt-Bu (0.5
equiv), and MeOH15 (4 equiv) in THF, we observed the
formation of cyclopropylboronate ( )-2a with excellent
diastereoselectivity (entry 1, Table 1). However, the yields
were consistently low due to formation of variable quantities of
an inseparable mixture of dimers B and C. This problem was not
completely unexpected since dimerization is one of the most
common undesired pathways in transition-metal-catalyzed
reactions with cyclopropenes.13b Trying to minimize the
formation of these dimeric structures therefore became one of
the major challenges of this project.16 The yield of ( )-2a
increased when we carried out the reaction at lower temperature
(entry 2, Table 1), but significant amounts of B and C were still
produced. After extensive experimentation,17 we observed that
addition of cyclopropene 1a and MeOH to a −78 °C solution of
the preformed xantphos−copper−boryl complex, followed by
warming to −20 °C, afforded ( )-2a7 in excellent yield as a single
diastereomer.
Once we minimized the dimerization pathway for the
diastereoselective Cu-catalyzed hydroboration, we looked at
the possibility of developing an asymmetric variant (Table 2).
We started testing several commercially available phosphines
with different steric and electronic properties using the
conditions previously optimized for xantphos (entries 1−6,
Table 2).17 We soon realized that the yields and stereo-
selectivities were highly dependent on the ligand. (R)-DTBM-
Segphos L6 was superior to other chiral ligands affording
cyclopropane (R,R)-2a in 71% yield and 92:8 enantiomeric ratio.
However, we found that these values were poorly reproducible
with enantiomeric ratios varying inconsistently from 85:15 to
92:8. Trying to solve this problem, we searched for a different Cu
source.17 A first attempt using [Cu(CH3CN)4]PF6 (entry 7,
Table 2) afforded (R,R)-2a with high diastereoselectivity but
only moderate yield and enantioselectivity. Gratifyingly, when
acetonitrile was removed in vacuo after phosphine−copper
complex formation (entry 8, Table 2), the desired compound was
consistently obtained in high yield with excellent diastero- and
enantioselectivity (dr = 97:3, er = 95:5). This result significantly
improves upon the 58% ee found in the Rh-catalyzed
hydroboration of 1a.7 The use of 5 mol % of [Cu(CH3CN)4]PF6
resulted in a lower yield and enantioselectivity (entry 9, Table 2).
g
9
10
a
Reaction conditions: 1a (0.2 mmol), B2pin2 (0.22 mmol), NaOt-Bu
(0.1 mmol), Cu(I) (10 mol %), L (11 mol %), MeOH (0.8 mmol),
b
THF (0.33 M). Cyclopropane 1a and MeOH were added at −78 °C;
c
the reaction mixture was then warmed up to −20 °C. Determined by
d
e
1H NMR analysis. er determined by chiral SFC. Yield of isolated
f
(R,R)-2a. CH3CN was removed in vacuo after phosphine−copper
complex formation. 5% of [Cu(CH3CN)4]PF6 was used.
g
To rule out a possible organocatalytic activation of B2pin2,18 we
carried out the reaction in the absence of a Cu salt (entry 10,
Table 2). Under these conditions, formation of (R,R)-2a was not
observed.
Next, we applied the Cu-catalyzed hydroboration conditions
to different (3,3-disubstituted)-cyclopropenes (Table 3). In
some cases, the reaction was carried out at either −50 or −78 °C
to optimize the enantiomeric ratio. Compounds bearing an
electron-rich aromatic substituent afforded the corresponding
cyclopropylboronates [(R,R)-2b−2d] with similar efficiency to
model (R,R)-2a (dr up to ≥98:2, er up to 96:4). Cyclopropenes
with electron-deficient aryl groups underwent hydroboration
with good yields and high diastereoselectivities although slightly
lower enantiomeric ratios [(R,R)-2e−2f]. Moreover, substitu-
tion at the meta position of the aryl group also seemed to affect
the enantioselectivity [(R,R)-2g]. Compounds (R,R)-2h and
(R,R)-2i, with a thiophene and a furan ring, respectively, were
also prepared with good yields and high enantioselectivities.
Starting from dideuterated 1a (1a-d2), compound (R,R,R)-2j
with three contiguous stereocenters was successfully obtained.
Additionally, coordinating groups (CH2OMe) were also
compatible with the hydroboration conditions and compound
(R,S)-2k was obtained with excellent results (er = 96.5:3.5, dr ≥
98:2).19
Cyclopropane (R,R)-2l, bearing a spiro-quaternary stereo-
center, was also prepared with high levels of stereocontrol.
Gratifyingly, bulkier groups on the cyclopropene (R2 = i-Pr,
(R,R)-2m) maintained high levels of enantiocontrol although the
diastereoselectivity was moderately decreased (80:20 vs 97:3).
Similarly, cyclopropane (R,S)-2n bearing two alkyl groups (R1 =
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Journal of the American Chemical Society
Communication
a b c
, ,
products would be cyclopropylaminoboronates (6) with three
contiguous stereocenters, which would be difficult to obtain by
known methods. We started our study using achiral phosphines.
Unfortunately, the conditions found for the diastereoselective
hydroboration of cyclopropene 1a17 (entry 3, Table 1) were not
optimal for the aminoboration reaction.24 After significant
optimization, we were pleased to find that a CuCl/dppf catalyst
system (5 mol %) and LiOt-Bu in THF afforded cyclopropyl-
aminoboronates ( )-6 in good yields and excellent diastereo-
meric ratios (Table 4). Interestingly, the reactions were carried
Table 3. Substrate Scope
ab c
, ,
Table 4. Copper-Catalyzed Aminoboration
a
Reaction conditions: 1 (0.4 mmol), B2pin2 (0.44 mmol), R2NOBz
(0.6 mmol), LiOt-Bu (1.2 mmol), CuCl (5 mol %), dppf (5 mol %),
a
Reaction conditions: 1 (0.2 mmol), B2pin2 (0.22 mmol), NaOt-Bu
(0.1 mmol), CuCl (10 mol %), (R)-DTBM-Segphos (11 mol %),
b
c
THF (0.2 M). Determined by 1H NMR analysis. Yield of isolated 6.
b
c
MeOH (0.8 mmol), THF (0.33 M). Yield of isolated 2. er
determined by chiral SFC; dr determined by 1H NMR analysis.
d
out at rt without observing significant amounts of dimerization
products B and C. These conditions worked well for a variety of
O-benzoyl-N,N-dialkylhydroxylamines. Piperidine, pyrrolidine,
tetrahydroisoquinoline, morpholine, and piperazine derivatives
(( )-6a−6e, Table 4) were easily prepared using the conditions
described above. In all cases, a cyclopropane with the methyl,
nitrogen, and boron substituents in a syn orientation was
obtained with high diastereoselectivity.25 Cyclopropylamino-
boronates bearing an acyclic N,N-dialkyl amine moiety were also
successfully obtained through this method (( )-6f−6h, Table
4). Additionally, we performed the reaction with cyclopropenes
bearing different substituents on the aromatic ring, obtaining the
desired compounds in good yields as nearly single diastereomers
(( )-6i−6k, Table 4). Unfortunately, none of the chiral
phosphines used in the optimization of (R,R)-2a were
compatible with the aminoboration conditions. In all cases, we
obtained inseparable mixtures of the desired compounds and
unknown byproducts with diastereomeric ratios significantly
lower than in the case with dppf.17
MeOD (0.8 mmol) instead of MeOH was used.
Bn, R2 = Me) was successfully obtained with somewhat lower
diastereoselectivity than (R,R)-2a but with excellent enantiose-
lectivity (er = 98:2).20 Finally, in the presence of MeOD,
compound (R,R,S)-2o was obtained in good yield and high
stereoselectivity (er = 93:7, dr = 96:4, >98% D incorporation).
This experiment demonstrates the syn insertion of the cyclo-
propene in the copper−boryl complex and reveals the configura-
tional stability of the cyclopropylcopper intermediate A (Scheme
1, eq 3). Overall, the results described in Table 3 suggest that the
stereoselectivity is mostly controlled by steric factors.21
To demonstrate the versatility of the cyclopropylboronate
species, (R,R)-2a was easily transformed into cyclopropane
(R,R)-3a through a Suzuki−Miyaura coupling with iodobenzene
(eq 4).22 Aditionally, cyclopropanol derivative (S,R)-4a was
prepared through an oxidation−benzoylation sequence.
In summary, we describe here the first diastereo- and
enantioselective Cu-catalyzed hydroboration of cyclopropenes.
This method allows for the synthesis of enantiomerically
enriched cyclopropylboronates with a quaternary stereocenter
and represents the first enantioselective Cu-catalyzed desymmet-
rization of cyclopropenes. Our approach nicely complements the
few existing methods to synthesize nonracemic cyclopropyl-
boronates and gives new insights into the enantioselective metal-
catalyzed desymmetrization of cyclopropenes. Additionally, the
Finally, we wanted to explore the trapping of A with
electrophiles other than a proton. We focused on the use of O-
benzoyl-N,N-dialkylhydroxylamines23 due to the importance of
cyclopropylamines in biologically active compounds. The
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Journal of the American Chemical Society
Communication
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capture of the cyclopropylcopper intermediate with electrophilic
amines highlights the synthetic potential of this approach and
opens a new way to synthesize functionalized cyclopropanes.
Further applications toward the enantioselective synthesis of
cyclopropylaminoboronates as well as functionalization of
cyclopropenes with different electrophiles are underway.
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ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedures, spectral data, and crystallographic CIF
files. This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
mariola.tortosa@uam.es
J. Am. Chem. Soc. 2000, 122, 978.
Rh-catalyzed: Reference 7.
■
(b) Sherrill, W. M.; Rubin, M. J. Am. Chem. Soc. 2008, 130, 13804.
(c) Phan, D. H. T.; Kou, K. G. M.; Dong, V. M. J. Am. Chem. Soc. 2010,
132, 16354. Pd-catalyzed: (d) Rubina, M.; Rubin, M.; Gevorgyan, V. J.
Author Contributions
Am. Chem. Soc. 2004, 126, 3688. (e) Kramer, K.; Leong, P.; Lautens, M.
̈
†L.A. and M.G.-C. contributed equally.
Org. Lett. 2011, 13, 819.
(14) (a) Crudden, C. M.; Edwards, D. Eur. J. Org. Chem. 2003, 4695.
(b) Carroll, A. M.; O’Sullivan, T. P.; Guiry, P. J. Adv. Synth. Catal. 2005,
347, 609.
Notes
The authors declare no competing financial interest.
(15) Mun, S.; Lee, J.-E.; Yun, J. Org. Lett. 2006, 8, 4887.
(16) We believe B is formed through a Cu-catalyzed formal [2 + 2]
cycloaddition. C could be formed from B through an electrocyclic ring
opening.
(17) For a full account on all the ligands used and other parameters see
the Supporting Information.
ACKNOWLEDGMENTS
■
We thank the European Research Council (ERC-337776) and
MINECO (CTQ2012-35957) for financial support. M.T. and
A.P. thank MICINN for RyC and JdC contracts. We acknowl-
edge Dr. Josefina Perles for X-ray structure analysis.
(18) Selected examples of organocatalyzed activation of diboron
compounds: (a) Lee, K.-s.; Zhugralin, A.; Hoveyda, A. J. Am. Chem. Soc.
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