Rhodium(I)-catalyzed asymmetric carbene insertion into B-H bonds: Highly enantioselective access to functionalized organoboranes

Article abstract of DOI:10.1021/jacs.5b00892

A unique rhodium(I)-catalyzed asymmetric B-H insertion of α-diazo carbonyl compounds with easily available amine-borane adducts was achieved using a newly developed C₁-symmetric chiral diene as ligand. This first Rh(I)-carbene-directed B-H insertion example represents an attractive and promising approach for synthesis of highly enantioenriched organoboron compounds, allowing for the efficient construction of α-boryl esters and ketones with excellent enantioselectivities (up to 99% ee) under exceptionally mild conditions.

Full text of DOI:10.1021/jacs.5b00892

Communication
pubs.acs.org/JACS
Rhodium(I)-Catalyzed Asymmetric Carbene Insertion into B−H
Bonds: Highly Enantioselective Access to Functionalized
Organoboranes
Diao Chen,† Xu Zhang,‡ Wei-Yi Qi,† Bin Xu,‡ and Ming-Hua Xu*,†
†State Key Laboratory of Drug Research, Shanghai Institute of Materia Medica, Chinese Academy of Sciences, 555 Zuchongzhi Road,
Shanghai 201203, China
‡Department of Chemistry, Shanghai University, Shanghai 200444, China
S
* Supporting Information
intermediates to engage in C−H or X−H insertion reactions,
ABSTRACT: A unique rhodium(I)-catalyzed asymmetric
B−H insertion of α-diazo carbonyl compounds with easily
available amine−borane adducts was achieved using a
newly developed C1-symmetric chiral diene as ligand. This
first Rh(I)−carbene-directed B−H insertion example
represents an attractive and promising approach for
synthesis of highly enantioenriched organoboron com-
pounds, allowing for the efficient construction of α-boryl
esters and ketones with excellent enantioselectivities (up
to 99% ee) under exceptionally mild conditions.
even in non-asymmetric fashion, has not yet been disclosed.
Organoborons are among the most frequently used
compounds in organometallic chemistry and organic synthesis.
Typically, chiral boron compounds can be prepared by Brown’s
hydroboration using chiral borane reagents or transition-metal-
catalyzed asymmetric hydroboration and boration of alkenes.6
Although much progress has been made in recent years, the
development of new strategies for efficient catalytic enantio-
selective synthesis of versatile organoboron compounds
remains highly desirable. In fact, the carbene-directed B−H
insertion approach could represent another ideal way for this
purpose.7 However, there have been no reports of catalytic
variants of this type of process. A breakthrough has only
recently been achieved by two research groups. Curran
successfully demonstrated a rhodium(II)-catalyzed insertion
reaction between NHC−borane and diazocarbonyl com-
pounds, providing diverse α-NHC−boryl carbonyl compound-
s.8a Almost at the same time, Zhou and co-workers established
a copper(I)-catalyzed carbene insertion into the B−H bonds of
phosphine−borane adducts, and further achieved the first
asymmetric B−H insertion using chiral spirobisoxazoline
ligands (91−94% ee).8b
ransition-metal-catalyzed C−H or X−H (X = O, N, S, Si)
Tinsertion of metal−carbene represents an excellent and
powerful approach for the construction of C−C or C−X
bonds.1 Among them, catalytic asymmetric carbene insertion
into C−H bonds has been well-defined,1a−c while asymmetric
C−X bond formation via metal−carbene intermediate was
established only recently.1d−m Rh(II),1e Cu(I),1f−i Cu(II),1j,k
Ir(III),1l and Fe(II)1m with suitable chiral ligands are the most
regularly used catalysts for these transformations.
It has been demonstrated that rhodium(I) complexes with
diverse chiral ligands are efficient catalysts for various
asymmetric reactions, such as addition of organoboron reagents
to electron-deficient olefins, imines, and carbonyl compounds.2
In contrast, the Rh(I)−carbene chemistry is far less studied.
Although many examples of Rh(I) complexes with N-
heterocyclic (NHC) carbene ligands have been known, their
use as catalysts in asymmetric reactions is still an underexplored
area of research.3 On the other hand, Rh(I)−carbenes have
proved to be useful intermediate species possessing novel and
interesting reactivity in a few catalytic C−C bond-forming
processes.4,5 Nevertheless, asymmetric variants of related
transformations are extremely rare. In 2010, Hayashi and co-
workers succeeded in asymmetric cyclopropanation of alkenes
with dimethyl diazomalonate using a cationic chiral diene−
Rh(I) complex as catalyst.5a Very recently in 2014, Murakami
reported enantioselective synthesis of cyclopentanols via
Rh(I)−carbene insertion promoted by diphosphine ligands;5b
Hu and co-workers achieved an enantioselective three-
component reaction through trapping of Rh(I)-associated
ammonium ylides generated from chiral Rh(I)−carbene.5c To
the best of our knowledgement, the potential of Rh(I)−carbene
In recent years, we have been particularly interested in
developing chiral olefin-based ligands for Rh(I)-catalyzed
asymmetric reactions.9 Intrigued by the fact that Rh(I)−
carbenes can serve as reactive intermediate species for a series
of C−C bond-forming transformations,4,5 we envisaged the
possibility of performing the asymmetric metal−carbene
insertion into B−H bonds using Rh(I) complex with proper
olefin-chelating ligand as catalyst. Herein, we describe our
efforts toward the development of the first Rh(I)−carbene-
mediated asymmetric B−H insertion reaction using unprece-
dented C1-symmetric chiral diene ligands. This Rh(I)-catalyzed
process allows practical construction of stereogenic C−B bonds
in a highly enantioselective manner at room temperature,
providing a promising new approach to the synthesis of
enantiomerically enriched functionalized organoboranes.
Our initial experiments began with the reaction of methyl α-
diazophenylacetate 1a and easily available amine−borane
adduct10 2a in dichloromethane at room temperature. When
Received: January 27, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/jacs.5b00892
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Journal of the American Chemical Society
Communication
Rh2(OAc)4 was used as catalyst, the B−H insertion occurred
smoothly to afford the corresponding borane product 3a
(Table 1, entry 1). Gratifyingly, we observed that, in the
L6) loading was found to be sufficient to achieve satisfactory
results, albeit a decrease in yield (entry 14).
With the optimized reaction conditions established, we then
investigated the substrate scope of this process. As summarized
in Table 2, various tert-butyl α-diazoarylacetates with diverse
a
Table 1. Optimization of Reaction Conditions
Table 2. Substrate Scope of Asymmetric B−H Insertion
a
Reactions of α-Diazoesters
b
c
entry
Ar
time (h)
3
yield (%)
ee (%)
1
Ph
3
2
2
3
6
2
4
3
3
4
6
6
3f
92
86
87
96
87
84
80
83
82
82
75
80
99
99
99
98
99
97
98
98
98
97
97
96
2
4-FC6H4
3g
3h
3i
b
c
entry
R
adduct
ligand
3
yield (%)
ee (%)
3
4-ClC6H4
4-MeC6H4
2-naphthyl
3-CF3C6H4
3,5-Me2C6H3
3-BrC6H4
3-MeC6H4
4-MeOC6H4
2-MeC6H4
2-FC6H4
d
4
1
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Ph
2a
2a
2a
2a
2a
2b
2c
2d
2c
2c
2c
2c
2c
2c
3a
3a
3a
3a
3a
3b
3c
3d
3c
3c
3c
3e
3f
70
75
6
e
5
3j
2
6
3k
3l
3
L1
L2
L3
L3
L3
L3
L4
L5
L6
L6
L6
L6
−80
1
7
4
12
83
90
98
52
42
98
98
94
92
70
8
3m
3n
3o
3p
3q
5
66
60
60
60
54
72
84
95
99
98
9
6
10
11
12
7
8
9
a
10
11
12
13
The reaction was performed with 1 (0.1 mmol), 2c (0.15 mmol),
[Rh(C2H4)2Cl]2 (1.5 mol%), and L6 (3.3 mol%) in 2 mL of CH2Cl2
b
c
at room temperature. Isolated yield. Determined by chiral HPLC.
tBu
tBu
f
steric and electronic properties were examined in the Rh(I)/
L6-catalyzed asymmetric B−H insertion reaction. In all cases,
products 3 were obtained in good to excellent yields with
uniformly high enantioselectivities (96−99%), regardless of the
substitution pattern of the diazo substrates. In comparison with
the only reported copper-catalyzed B−H insertion reaction,8b
the enantioselectivities are generally superior. Moreover, the
use of cheap and easily accessible amine−borane 2c instead of a
phosphine−borane adduct as boron reagent is also an
advantage of our reaction.13 The absolute configuration at the
newly created stereocenter was determined to be R by X-ray
crystallographic analysis of 3m (Figure 1).
14
3f
a
The reaction was performed with 1 (0.1 mmol), 2 (0.15 mmol),
[Rh(C2H4)2Cl]2 (1.5 mol%), and ligand (3.3 mol%) in 2 mL of
b
CH2Cl2 at room temperature unless otherwise noted. Isolated yield.
c
d
e
Determined by chiral HPLC. 2.5 mol% Rh2(OAc)4 was used. 2.5
f
mol% [Rh(cod)Cl]2 was used. 0.8 mol% Rh(I) catalyst was used.
presence of 2.5 mol% [Rh(cod)Cl]2, the reaction also worked
well to give the expected borane 3a in an even better yield
(75%, entry 2), clearly indicating that Rh(I) could act as the
center metal for this insertion reaction of metal−carbene. Next,
several representative chiral olefin ligands were chosen to
investigate the potential of asymmetric catalysis. Interestingly,
our linear sulfur−olefin ligand L19c showed good enantio-
selectivity but with very poor reactivity in this transformation
(entry 3). While another previously developed bicyclo[3.3.0]-
diene9b was not effective, promising results (83% yield, 66% ee)
were obtained using Hayashi’s bicyclo[2.2.2]octadiene L311
(entry 5). Examination of other amine−borane adducts and the
NHC−borane adduct (entries 6−8) revealed that 2c was the
most reactive borane that maintained enantioselectivity (entry
7). To achieve better enantiocontrol, elaboration of chiral
ligands for further screening was conducted. To our delight, the
use of easily prepared C1-symmetric dienes,12 which are
analogues of bicyclo[2.2.2]octadiene L3, gave the product in
excellent yields with much improved enantioselectivities
(entries 10 and 11). More interestingly, when phenyl α-
diazophenylacetate was employed as substrate, it was quite
encouraging that the use of L6 furnished the insertion product
3e in both excellent yield and ee (94% yield, 95% ee, entry 12).
Changing the ester moiety of the diazoester to more sterically
bulky tert-butyl further improved the enantioselectivity to an
exciting 99% (entry 13). Notably, 0.8 mol% of catalyst (Rh(I)/
Figure 1. X-ray crystal structures of (R)-3m and (R)-5c.
Encouraged by success of the asymmetric B−H insertion of
α-diazoesters, we turned our attention to the more challenging
α-diazoketone substrates to explore the potential of generating
chiral α-boryl ketones. Unlike α-diazoesters, the use of α-
diazoketones as carbene precursors for the X−H insertion
reaction is far less studied.14 The asymmetric B−H insertion of
α-diazoketone has not yet been reported. Under the same
conditions, a structurally diverse set of α-diazoketones were
subject to reaction with amine−borane adduct 2c (Table 3).
B
DOI: 10.1021/jacs.5b00892
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Journal of the American Chemical Society
Communication
Table 3. Substrate Scope of Asymmetric B−H Insertion
Scheme 1. Proposed B−H Insertion Mechanism
a d
,
Reactions of α-Diazoketones
b
c
entry
4
Ar
R
5
yield (%)
ee (%)
1
4a
4b
4c
4d
4e
4f
Ph
Ph
Ph
Ph
Ph
Ph
5a
5b
5c
5d
5e
5f
76
67
69
64
65
73
75
68
90
48
71
70
73
73
54
96
96
96
96
95
96
97
96
98
95
96
98
94
95
93
2
2-naphthyl
3
3-BrC6H4
4
4-ClC6H4
5
4-MeC6H4
6
Ph
4-FC6H4
4-MeC6H4
4-ClC6H4
4-MeOC6H4
2-MeC6H4
3-FC6H4
4-MeC6H4
Et
7
4g
4h
4i
Ph
5g
5h
5i
8
Ph
9
Ph
10
11
12
13
14
15
4j
Ph
5j
4k
4l
Ph
5k
5l
3-BrC6H4
Ph
4m
4n
4o
5m
5n
5o
Ph
Bu
Ph
i-Pr
a
The reaction was performed with 4 (0.1 mmol), 2c (0.15 mmol),
[Rh(C2H4)2Cl]2 (1.5 mol%), and L6 (3.3 mol%) in 2 mL of CH2Cl2
b
c
at room temperature. Isolated yield. Determined by chiral HPLC.
To highlight the versatility of this method, the insertion
product 3f was readily converted into the protected β-hydroxy
pinacol borate ester in good yield with 98% ee (Scheme 2).
d
The absolute configuration was determined by the X-ray crystallo-
graphic analysis of 5c; see the Supporting Information for details.
Scheme 2. Product Transformations
Remarkably, all reactions of α-diazoketones 4a−l bearing two
aryl groups adjacent to diazo and carbonyl occurred in a highly
enantioselective fashion (95−98% ee), providing the corre-
sponding insertion products 5 in moderate to good yields
(entries 1−12). It appeared that substitution on the either aryl
ring did not affect the stereoselectivity of the reaction.
Substrates 4m−o that possess an alkyl group such as ethyl,
butyl, and isopropyl at the ketone moiety also afforded the
desired organoborane products 5m−o with good yields and
enantioselectivities (93−95% ee, entries 13−15). Thus, the
scope of this new reaction is substantially expanded. The same
reaction stereochemistry was confirmed by X-ray analysis of the
product 5c (Figure 1).
A plausible reaction mechanism is illustrated in Scheme 1.
The bis(rhodium/diene) complex initially dissociates to give an
active monorhodium catalyst I, which reacts with diazo
compound to generate Rh(I)−carbene intermediate II having
a square-planar coordination. Insertion of this highly reactive
carbenoid species into the B−H bond of the amine−borane
adduct probably via a concerted transition state1a,c,8 then
affords the corresponding organoborane product and regener-
ates the catalyst. To explain the observed stereochemical
outcome, an empirical stereocontrol model is also proposed.
We assume that the rhodium−carbene complex with unsym-
metrically substituted diene ligand L6 may take the
conformation with RhC on the site trans to the more
electron-deficient double bond,15 where the carbenoid moiety
is orthogonal to the coordination plane. To minimize the steric
repulsion, the bulky ester orients away from the phenyl group
of the ligand, and accordingly the two phenyl rings are likely to
form a π−π stacking interaction (TS-1 and TS-2). Thus, the
B−H insertion takes place preferentially from the unblocked
carbene face at the site adjacent to Cl to provide the R product.
Furthermore, the reaction of chiral α-boryl ketones 5a and 5c
with DIBAL-H followed by treatment with pinacol gave the
valuable β-boryl alcohols 9a and 9b having two stereogenic
centers. It should be noted that both excellent diastereo-
selectivity and enantioselectivity were observed. The cis-
conformation was determined by the X-ray analysis of
intermediate 8.
In summary, we have achieved the first rhodium(I)−carbene-
involved asymmetric insertion into B−H bonds for facile
synthesis of a wide range of chiral α-carbonyl-containing
organoboranes, which are versatile and potentially interesting
intermediates in organic synthesis. The newly developed C1-
symmetric chiral bicyclo[2.2.2]octadiene ligand has demon-
strated its remarkable efficiency in this Rh(I)-catalyzed C−B
bond-forming reaction, enabling the achievement of exception-
C
DOI: 10.1021/jacs.5b00892
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Journal of the American Chemical Society
Communication
(5) (a) Nishimura, T.; Maeda, Y.; Hayashi, T. Angew. Chem., Int. Ed.
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Chem. Commun. 2007, 4717.
ally high enantioselectivities (93−99% ee). Moreover, the use
of easily accessible and inexpensive amine−borane adduct as
boron source makes the present protocol particularly attractive
and practical. Studies to fully understand the reaction
mechanism and further exploit the Rh(I)−carbene chemistry
in asymmetric catalysis are in progress.
ASSOCIATED CONTENT
* Supporting Information
■
S
(7) (a) Zheng, G.; Jones, M., Jr. J. Am. Chem. Soc. 1983, 105, 6487.
(b) Bedel, C.; Foucaud, A. Tetrahedron Lett. 1993, 34, 311.
(c) Imamoto, T.; Yamanoi, Y. Chem. Lett. 1996, 705. (d) Monnier,
L.; Delcros, J.-G.; Carboni, B. Tetrahedron 2000, 56, 6039.
Experimental procedures and characterization data, including
CIF data for 3m, 5c, and 8b. This material is available free of
charge via the Internet at http://pubs.acs.org.
(e) Ramírez-Lo
́
pez, P.; Sierra, M. A.; Go
́
mez-Gallego, M.;
AUTHOR INFORMATION
Corresponding Author
*xumh@simm.ac.cn
Notes
■
Mancheno, M. J.; Gornitzka, H. Organometallics 2003, 22, 5092.
̃
(8) (a) Li, X.; Curran, D. P. J. Am. Chem. Soc. 2013, 135, 12076.
(b) Cheng, Q.-Q.; Zhu, S.-F.; Zhang, Y.-Z.; Xie, X.-L.; Zhou, Q.-L. J.
Am. Chem. Soc. 2013, 135, 14094.
(9) For a review, see: (a) Li, Y.; Xu, M.-H. Chem. Commun. 2014, 50,
3771. For representative examples, see: (b) Wang, Z.-Q.; Feng, C.-G.;
Xu, M.-H.; Lin, G.-Q. J. Am. Chem. Soc. 2007, 129, 5336. (c) Zhu, T.-
S.; Jin, S.-S.; Xu, M.-H. Angew. Chem., Int. Ed. 2012, 51, 780.
(d) Wang, H.; Jiang, T.; Xu, M.-H. J. Am. Chem. Soc. 2013, 135, 971.
(e) Yu, Y.-N.; Xu, M.-H. Org. Chem. Front. 2014, 1, 738.
(10) For reviews, see: (a) Staubitz, A.; Robertson, A. P. M.; Sloan, M.
E.; Manners, I. Chem. Rev. 2010, 110, 4023. (b) Carboni, B.; Monnier,
L. Tetrahedron 1999, 55, 1197.
(11) (a) Tokunaga, N.; Otomaru, Y.; Okamoto, K.; Ueyama, K.;
Shintani, R.; Hayashi, T. J. Am. Chem. Soc. 2004, 126, 13584.
(b) Otomaru, Y.; Okamoto, K.; Ueyama, K.; Shintani, R.; Hayashi, T.
J. Org. Chem. 2005, 70, 2503.
(12) A series of unprecedented C1-symmetric dienes were
synthesized by taking advantage of the creative work of Abele on
scalable synthesis of chiral dienes: Abele, S.; Inauen, R.; Spielvogel, D.;
Moessner, C. J. Org. Chem. 2012, 77, 4765.
(13) Compared to the phosphine−borane adducts, the amine−
borane adducts are highly stable, easy to access, and rather inexpensive.
In the case of 2c, it can be readily obtained by mixing cheap 1-
methylpyrrolidine with BH3 in THF at room temperature for 0.5 h.
(14) For a recent breakthrough in N−H insertion of α-diazoketone,
see: Xu, B.; Zhu, S.-F.; Zuo, X.-D.; Zhang, Z.-C.; Zhou, Q.-L. Angew.
Chem., Int. Ed. 2014, 53, 3913.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support was provided by the National Natural Science
Foundation of China (21325209, 21472205) and the Shanghai
Municipal Committee of Science and Technology
(14XD1404400). We thank Prof. Wenhao Hu for helpful
discussion.
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D
DOI: 10.1021/jacs.5b00892
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX