Enantioselective Copper-Catalyzed Oxy-Alkynylation of Diazo Compounds

Article abstract of DOI:10.1021/jacs.7b04756

Enantioselective catalytic methods allowing the addition of both a nucleophile and an electrophile onto diazo compounds give a fast access into important building blocks. Herein, we report the highly enantioselective oxyalkynylation of diazo compounds usi

Full text of DOI:10.1021/jacs.7b04756

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Communication
Enantioselective Copper-Catalyzed Oxy-Alkynylation of Diazo Compounds
Durga Prasad Hari, and Jerome Waser
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 16 Jun 2017
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Durga Prasad Hari and Jerome Waser
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Laboratory of Catalysis and Organic Synthesis, Ecole Polytechnique Fédérale de Lausanne, EPFL SB ISIC LCSO,
BCH 4306, 1015 Lausanne, Switzerland.
Supporting Information Placeholder
reactions involving diazo compounds.10 Recently, our group
ABSTRACT: Enantioselective catalytic methods allowing the
developed a copper-catalyzed oxyalkynylation of diazo com-
pounds11 based on the use of EthynylBenziodoXolones (EBX)
reagents.12 Herein, we report the successful development of
an enantioselective variation of this transformation, which
constitutes the first asymmetric simultaneous introduction
of an alkyne and an ester onto a diazo compound (Scheme
1B). Importantly, the reaction required a single copper cata-
lyst bearing a broadly available BOX ligand, and gave prod-
ucts in high yield with up to 98% ee. The obtained propar-
gylic benzoyloxy esters are useful building blocks, which are
difficult to access using traditional methods, such as alkyne
addition to aldehydes,13 due to the sensitivity of the products
and required starting materials to basic conditions.14 Fur-
thermore, they could be easily transformed into other im-
portant building blocks, such as propargylic alcohols.
addition of both a nucleophile and an electrophile onto diazo
compounds give a fast access into important building blocks.
Herein, we report the highly enantioselective oxyalkynyla-
tion of diazo compounds using ethynylbenziodoxol-(on)e
(EBX) reagents and a simple copper bisoxazoline (BOX) cata-
lyst. The obtained -benzoyloxy propargylic esters are useful
building blocks, which are difficult to synthesize in enanti-
opure form using other methods. The obtained products
could be efficiently transformed into vicinal diols and α-
hydroxy propargylic esters without loss in enantiopurity.
Due to the different biological and optical properties of
enantiomers, the synthesis of enantiopure compounds is an
important field of research in organic chemistry. In this re-
spect, enantioselective metal-catalyzed reactions of diazo
compounds proceeding via carbenoid intermediates have
been highly successful.1 Asymmetric cyclopropanation and
insertion into carbon or heteroatom- hydrogen bonds are
now broadly used for the asymmetric synthesis of important
building blocks. The generation of ylides by reaction of elec-
trophilic carbenes with nucleophiles opened the way for [2,3]
sigmatropic rearrangements and cycloaddition reactions.2
Recently, researchers have focused on direct reactions of
ylides generated from diazo compounds with electrophiles,
allowing the introduction of more diverse functionalities on
the carbon center.3 The development of enantioselective
variations of such processes is highly challenging. Recent
breakthroughs have been realized based on elegant coopera-
tive catalytic systems involving late metal catalysts such as
rhodium/iridium,4 palladium5 and ruthenium,6 and either a
chiral phosphoric or Lewis acid (Scheme 1A). Nevertheless,
this approach is limited to electrophilic partners that can be
activated by Brønsted or Lewis acids, and it is based on a
relative complex dual catalyst system. Transformations rely-
ing on a single chiral catalyst remain extremely rare in this
new type of carbene transformations, including two exam-
ples of rhodium catalysts7 and an organocatalytic system
specific to diazo compounds derived from oxindoles.8
Scheme 1. Enantioselective transformations of diazo
compounds.
We started our investigations by screening various ligands,15
using
ethyl
diazoacetate
(1a)
with
1-[(triiso-
propylsilyl)ethynyl]-1,2-benziodoxol-3(1H)- one (TIPS-EBX,
2a) and Cu(OTf)2 as the copper source (Table 1).16 Several
classes of ligands, such as diimines, Salen, Phox or biphos-
Surprisingly, despite their success in enantioselective cy-
clopropanation and X-H insertion reactions,9 copper cata-
lysts have been used so far only in racemic multi-component
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phines gave either low selectivity or low conversion.15 tBu-
To further improve the enantioselectivity, we investigated
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BOX ligand 4a gave the desired propargylic ester 3a in excel-
lent yield with a promising 56% ee (entry 1). The use of cy-
clopropyl and cyclopentyl derived BOX ligands (4b and 4c)
didn’t improve the enantioselectivity (entries 2 and 3). In-
dane-BOX ligand 4d gave results identical to the ones ob-
tained with ligand 4a, whereas a slightly better enantioselec-
tivity was observed with cyclopropyl substituted ligand 4e
(entries 4 and 5). Among the solvents tested15 (entries 6-8),
chlorobenzene emerged as the best solvent (84% yield with
70% ee, entry 7). Generating a cationic complex in situ from
AgSbF6 and CuCl provided a slight improvement (entry 9).
No reaction was observed when using AgClO4 or NaB-
ARF(entries 10 and 11). AgNTf2 gave the desired product in
91% yield and 84% ee (entry 12). Without AgNTf2, no product
was obtained (entry 13). Finally, the enantioselectivity could
be improved to 90% by lowering the concentration of the
reaction (entry 14).
the influence of the structure of the α-diazo ester (Scheme
2A). Aliphatic diazoesters of different steric bulk afforded
products 3a-h in 85-92% ee. Hindered aryl diazo esters18 pro-
vided higher enantioselectivities (up to 97%) (products 3i-k).
The reaction was not limited to α-diazo esters. Both ethyl
diazomethanesulfonate
and
diethyl
(diazome-
thyl)phosphonate gave the desired product 3l and 3m in high
yield and moderate to high selectivity (Scheme 2B). Finally,
diazo Weinreb amide also delivered the product 3n, which
open up possibilities for further derivatization.19
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Scheme 2. Scope of diazo compounds with TIPS-EBX
(2a).
Table 1. Optimization of the reaction conditions.a
Catalyst Ligand
Time (h) Yieldb (%) eec (%)
En-
try
Sol-
vent
1
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
4a
4b
4c
4d
4e
DCE
DCE
DCE
DCE
DCE
0.5
0.5
0.5
2
97
98
98
98
97
98
84
79
89
<5
<5
91
56e
55e
54e
54
2
3
4
5
6
7
8
9
0.5
0.5
2
62
40
70
We next examined the scope of R-EBX reagents using 2,6-
di-tert-butyl-4-methylphenyl 2- diazoacetate (1k) (Scheme
3). Electron-donating and -withdrawing groups were well
tolerated on the aryl ring of TIPS-EBX (2a) (products 3o-t).
The reaction was successful with a triethyl silyl group (prod-
uct 3u), whereas no product could be isolated with a trime-
thylsilyl group. Aliphatic EBX reagents bearing substituents
such as long alkyl chain, chloro, TMS-alkyne and a cyclopro-
pyl group worked efficiently in this transformation, giving
products 3v-y. Finally, EBX reagents bearing aryl substituents
on the alkyne led to the desired products 3z-c’ in excellent
yields and good enantioselectivities.
4e DCM
4e PhCl
4e xylene
2
69
72
CuCl/AgSbF6 4e
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
1
10 CuCl/AgClO4 4e
11 CuCl/NaBARF 4e
12 CuCl/AgNTf2 4e
24
24
18
24
18
ndf
nd
84
nd
90
13
CuCl
4e
<5
95
14d CuCl/AgNTf2 4e
The absolute configuration of 3o could be determined by
X-ray analysis (Figure 1A).20 The observed stereochemistry
would be in agreement with an attack of the carboxylate of
the reagent in the free quadrant opposite to the ester group
on a three coordinate copper carbene complex with a 90°
angle between the ligand and the carbene plan,9g followed by
stereospecific alkynylation with retention of configuration (I,
Figure 1B). Further studies will be needed to support the
proposed stereoinduction model.
aReaction conditions: 0.30 mmol ethyldiazoacetate (1a),
0.15 mmol TIPS-EBX (2a), copper catalyst (2.0 mol%), ligand
(2.5 mol%), solvent (0.05 M), for entries 9-12 and 14: AgX or
NaBARF (2.0 mol%). bYield after purification by column
chromatography. cObtained by chiral HPLC. d0.025 M in-
e
stead of 0.05 M. The opposite enantiomer of the product
was obtained. fnd = not determined.
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thermore, vicinal diol 1222 could be synthesized by reduction
Scheme 3. Scope of R-EBX reagents. Ar = 2,6-di-tert-
butyl-4-methylphenyl.
1
2
of 3k using LiAlH4. Such alkynyl diol building blocks are use-
ful in synthetic chemistry, but are usually accessed via longer
multi step procedures.23
3
4
5
6
Scheme 4. Reactions with menthol and steroids es-
ters.
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Reaction conditions: 0.30 mmol diazoacetate (1), 0.15
mmol TIPS-EBX (2a), CuCl (2.0 mol%), ligand (2.5 mol%),
AgNTf2 ( 2 mol%), PhCl (0.025 M), 25 °C. R = 2-iodobenzoyl.
When (-) menthol diazoacetate 1o was subjected to the
standard conditions, the desired product 5 was obtained in
good yield with 95:5 d.r. (Scheme 4A).21 The use of the S-
enantiomer ent-4i of ligand 4i afforded the other diastere-
omer 6 in good yield with 6:94 d.r, demonstrating that the
configuration at the new stereocenter could be controlled by
the chiral catalyst. Similar results were obtained with (+)
menthol diazoacetate 1p. All four diastereomers can there-
fore be obtained in good yield and selectivity. Good ligand
control over the stereoselectivity could also be achieved with
more complex diazo compounds 1q and 1r derived from ster-
oids (Scheme 4B).
Scheme 5. Scale up and product modifications.
In summary, we have developed a highly enantioselective
oxyalkynylation of diazo compounds. This transformation is
the first example of copper-catalyzed addition of both a nu-
cleophile and an electrophile onto a carbenoid intermediate.
A broad range of EBX reagents and diazo compounds were
well tolerated. The reaction proceeds under mild conditions,
giving highly functionalized products with excellent yields
and selectivities. The obtained products were efficiently
transformed into useful building blocks, such as α-hydroxy
propargylic esters and vicinal diols, in a single step without
loss of enantioselectivity. Further extending this methodolo-
gy to other diazo compounds and hypervalent iodine rea-
gents is currently under investigation in our laboratory.
Figure 1. Absolute configuration and stereoinduction
model.
Experimental procedures and analytical data for all new
compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
The obtained products were then further transformed into
useful building blocks for organic synthesis (Scheme 5).
Compound 3k was synthesized on gram scale in 98% yield
with 95% ee. The benzoyl group could be readily removed
using DIBAL-H, thus affording the α-hydroxy propargylic
ester 11 in 99% yield with retention of enantiopurity. Fur-
jerome.waser@epfl.ch
The authors declare no competing financial interests.
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We thank the European Research Council (ERC; Starting
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