DMAP-accelerated rhodium(I) chloride catalyzed hydroboration of vinylarenes

Article abstract of DOI:10.1055/s-0028-1087415

Regioselective hydroboration of vinylarenes catalyzed by a Rh(I)-DPPB complex proceeded rapidly when DMAP was used as an additive. The catalyst loading could be reduced to 0.4 mol% Rh(I) to furnish the desired products in good to excellent yield with high

Full text of DOI:10.1055/s-0028-1087415

LETTER
3231
DMAP-Accelerated Rhodium(I) Chloride Catalyzed Hydroboration of
Vinylarenes
R
K
hodium(I) Chlori
o
de
C
atalyzed
h
Hydroborat
e
ionof
V
iny
i
larenes Endo,* Munenao Hirokami, Kazunari Takeuchi, Takanori Shibata*
Department of Chemistry and Biochemistry, School of Advanced Science and Engineering, Waseda University,
Shinjuku, Tokyo 169-8555, Japan
Fax +81(3)52868098; E-mail: kendo@aoni.waseda.jp; E-mail: tshibata@waseda.jp
Received 22 August 2008
4
). The enhancement of the reaction rate was observed
Abstract: Regioselective hydroboration of vinylarenes catalyzed
by a Rh(I)–DPPB complex proceeded rapidly when DMAP was
used as an additive. The catalyst loading could be reduced to 0.4
mol% Rh(I) to furnish the desired products in good to excellent
yield with high regioselectivity.
when using 4-tert-butylpyridine as an additive, although
the regioselectivity was moderate (entry 5). The dramatic
acceleration effect was observed when the reaction was
conducted using DMAP as an additive (entry 6). The re-
action in the presence of [RhCl(cod)] (5 mol% Rh),
DPPB (6 mol%), and DMAP (6 mol%) in 1,2-dichloroet-
Key words: rhodium, hydroborations, alkenes, boron, regioselec-
tivity
2
hane (DCE) furnished the product 2a in 98% NMR yield
5
exclusively. The reaction in the presence of 1,8-diazabi-
cyclo[5.4.0]undec-7-ene (DBU) as an additive also accel-
erated the reaction, but the regioselectivity lowered (entry
7). The reaction in the presence of excess DMAP (120
mol%) without DPPB ligand gave the products as a mix-
The hydroboration of unsaturated hydrocarbons is an es-
tablished protocol for the functionalization of carbon re-
sources. Especially the resulted organoboron compounds
1
are useful intermediates for the preparation of alcohols,
amines, and carboxylic acids. The cross-coupling reaction
of organoboron compounds is one of the fascinating syn-
thetic approaches to C–C bond formation. Typically, hy-
Table 1 Screening of Additives for Rh(I)-Catalyzed Hydroboration
I
droboration with pinacolborane (HBpin) using Rh X (X =
BF , Cl, acac, and OMe) requires a few hours to several
4
O
O
2
,3
O
O
days to complete. Recently, we reported the counteran-
B
I
HB
[RhCl(cod)]2 (5 mol% Rh)
DPPB (6 mol%)
ion-accelerated Rh OAc-catalyzed hydroboration of
Ph
vinylarenes using pinacolborane, where the reaction com-
pleted within 10 minutes to give the branched products re-
additive (x mol%)
2
a
(
1.2 equiv)
DCE, r.t., time
2
gioselectively. The present paper describes another
O
Ph
approach to the acceleration of hydroboration using 4-
N,N-(dimethylamino)pyridine (DMAP) as an additive to
the Rh(I) complex. This strategy reveals that DMAP–
Rh Cl complex realizes the rapid hydroboration of al-
kenes.
B
1
a
Ph
O
3
a
I
a
b
Entry Additive (mol%)
Time (min) Yield (%) 2a/3a
1
2
3
4
5
6
–
10
10
10
10
10
10
10
60
20
trace
trace
trace
trace
42
–
–
–
–
During the course of our study, we hypothesized that the
external additive might promote the Rh(I)-catalyzed hy-
droboration with HBpin. The screening of additives clari-
fied the specific nature of DMAP (Table 1). Under the
conventional conditions, the hydroboration of vinylarenes
with HBpin requires a relatively long reaction time. Actu-
ally, the reaction of styrene (1a) with HBpin gave only a
trace amount of desired product for 10 minutes in the pres-
Ph P (6.0)
3
py (6.0)
bpy (6.0)
4-tert-butylpyridine (6.0)
DMAP (6.0)
DBU (6.0)
79:21
98
>98:2
92:8
ence of [RhCl(cod)] (5.0 mol% Rh) and 1,4-bis(diphe-
2
4
7
80
nylphosphino)butane (DPPB, 6.0 mol%, entry 1). The
employment of triphenylphosphine as an additive did not
facilitate the reaction at all (entry 2); accordingly, the re-
action in the presence of pyridine (py) or 2,2¢-bipyridyl
8c
DMAP (120)
DMAP (0.5)
81
51:49
96:4
9d
98
(
bpy) as an additive gave the same results (entries 3 and
a
1
The yield was determined by the integration ratio of H NMR anal-
ysis with 1,1,2,2-tetrachloroethane.
The regioselectivity was determined by H NMR analysis.
The reaction was conducted in the absence of DPPB.
b
1
c
SYNLETT 2008, No. 20, pp 3231–3233
1
6
.1
2
.2
0
0
8
d
Conditions: [RhCl(cod)] (0.4 mol% Rh), DPPB (0.5 mol%),
Advanced online publication: 26.11.2008
DOI: 10.1055/s-0028-1087415; Art ID: U08808ST
2
DMAP (0.5 mol%), DCE (6 M), r.t., 20 min.
©
Georg Thieme Verlag Stuttgart · New York

3
232
K. Endo et al.
LETTER
ture of regioisomers 2a and 3a in 81% NMR yield with Table 2 Rh(I)–DMAP-Catalyzed Hydroboration of Vinylarenesa
5
1:49 ratio (entry 8). These results imply the cooperative
Entry
1
Vinylarene
Product (%)b,c
influence of DPPB and DMAP for the present catalysis. It
is notable that the catalyst loading could be reduced to 0.4
mol% Rh(I) to give the products 2a and 3a in 98% NMR
yield with 96:4 ratio for 20 minutes (entry 9). In general,
the low catalyst loading often provides unfavorable rhod-
ium intermediates due to impurities, which participates
the hydride reduction and/or oxidative borylation, and the
2a, 98 (87)
1a
Cl
2
3
4
5
2b, 97 (77)
2c, 98 (83)
2d, 98 (83)
2e, 98 (92)
6
yield of desired products decreased.
1
b
The present catalysis facilitated the synthesis of various
products regioselectively (Table 2). The reaction of sty-
7
rene (1a) gave the product 2a in 98% NMR and 87% iso-
lated yield with excellent regioselectivity (entry 1). The
halogen-substituted vinylarene could participate the reac-
tion. The reactions of 3-chlorostyrene (1b) and 4-chlo-
rostyrene (1c) afforded the products 2b in 97% NMR and
Cl
1c
F
1
d
7
7% isolated yield and 2c in 98% NMR and 83% isolated
yield, respectively (entries 2 and 3). 4-Fluorostyrene (1d)
gave the corresponding product 2d in 98% NMR and 83%
isolated yield (entry 4). The electron-donating group did
not retard the reaction. The reaction of 4-methylstyrene
Me
1
e
(
1e) provided the product 2e in 98% NMR and 92% iso-
6
7
8
2f, 96 (81)
2g, 93 (81)
2h, 95 (80)
lated yield (entry 5). The reaction of 4-vinylanisole (1f)
gave the comparable result (entry 6). The reactions of 2-
vinylnaphthalene (1g) and 6-methoxy-2-vinylnaphthalene
MeO
1
f
(
1h) require longer reaction time to afford the products in
8
high yield regioselectively (entries 7 and 8). The reaction
of 4-vinylbiphenyl (1i) proceeded in 10 minutes to give
the product 2i in 98% NMR and 78% isolated yield (entry
1
g
9
).
MeO
A postulated mechanism is shown in Scheme 1. Although
the influence of DMAP is unclear, it could be expected to
work as a ligand on rhodium center. The reaction starts
from the oxidative addition of HBpin to complex A to
generate complex B. Given the coordination of DMAP on
Rh(III) center is the key to the present catalysis, the coor-
1
h
9
2i, 98 (78)
Ph
1i
a
The reactions using HBpin (1.2 equiv) were carried out in the pres-
dination of the olefinic moiety of vinylarene on Rh(III) ence of [RhCl(cod)] (5 mol% Rh), DPPB (6.0 mol%), and DMAP
2
(
6.0 mol%) in DCE (1 M) at r.t. for 10 min, except entries 7 and 8; en-
would be inhibited; the precedents explain that the Rh(I)-
catalyzed hydroboration reaction would proceed via the
coordination and insertion pathway. Thus, the cationic
complex C might generate, followed by the insertion of
try 7 required 20 min (c = 5 M) and entry 8 required 30 min (c = 2 M).
b
1
The H NMR yields with 1,1,2,2-tetrachloroethane were described.
The isolated yields were described in parentheses.
c
The regioselectivity is >98:2 except entries 7, 8, and 9; entry 7 is
olefin into Rh–H bond occurs to give complex D. The re- 96:4, entry 8 is 96:4, and entry 9 is 97:3.
ductive elimination affords the product 2 and regenerates
complex A. The tentative reaction using the chiral pyri-
9
dine analogue, (R)-PPY*, provided the slight asymmetric
Bpin
HBpin
Bpin
P
P
Cl
induction (Scheme 2), thus the insertion of olefinic moi-
ety to Rh–H bond might proceed with the coordination of
DMAP on Rh(III) center. The further investigation to
clarify the specific nature of DMAP is an ongoing study.10
Rh
Ar
DMAP
2
A
Cl
Rh
Cl
P
P
Bpin
P
P
In conclusion, we have demonstrated the dramatic influ-
ence of the second ligand, DMAP, for Rh(I)–DPPB com-
plex-catalyzed hydroboration of vinylarenes using
pinacolborane. The present catalysis furnishes the rapid
hydroboration within 20 minutes regioselectively even in
the reduced catalyst loading to 0.4 mol% Rh. The novel
Rh
H
DMAP
DMAP
Ar
D
+
B
DMAP
–Cl
P
P
Bpin
Ar
Rh
H
1
I
DMAP–DPPB–Rh Cl complex represents an extremely
Ar
C
high catalytic performance to date. These insights can
contribute to the discussion to design Rh(I) complex and Scheme 1 Postulated mechanism of Rh(I)–DMAP catalysis
Synlett 2008, No. 20, 3231–3233 © Thieme Stuttgart · New York

LETTER
Rhodium(I) Chloride Catalyzed Hydroboration of Vinylarenes
3233
1
994, 116, 4062. (g) Fernandez, E.; Maeda, K.; Hooper,
HBpin
[RhX(cod)]2 (5 mol% Rh)
DPPB (6 mol%)
M. W.; Brown, J. M. Chem. Eur. J. 2000, 6, 1840.
O
O
(h) Demay, S.; Volant, F.; Knochel, P. Angew. Chem. Int.
Ed. 2001, 40, 1235. (i) Kwong, F. Y.; Yang, Q.; Mak,
T. C. W.; Chan, A. S. C.; Chan, K. S. J. Org. Chem. 2002,
67, 2769. (j) Crudden, C. M.; Edwards, D. Eur. J. Org.
Chem. 2003, 4695.
(
1.2 equiv)
+
(
R)-PPY* (6 mol%)
B
*
DCE, r.t., time
Ph
1
a
2a
+
(
2) Endo, K.; Hirokami, M.; Shibata, T. Organometallics 2008,
3a
27, 5390.
N
X = Cl, 40 min:
a 19%, 10% ee
a 36%
(3) (a) Westcott, S. A.; Blom, H. P.; Marder, T. B.; Baker, R. T.
J. Am. Chem. Soc. 1992, 114, 8863. (b) Doucet, H.;
2
3
N
Fernandez, E.; Layzell, T. P.; Brown, J. M. Chem. Eur. J.
Fe
X = OAc, 10 min:
a 80%, 13% ee
a 18%
1
999, 5, 1320. (c) Chen, A.; Ren, L.; Crudden, C. M. J. Org.
Me
Me
Me
Me
2
3
Chem. 1999, 64, 9704. (d) Murata, M.; Kawakita, K.;
Asana, T.; Watanabe, S.; Masuda, Y. Bull. Chem. Soc. Jpn.
Me
2
002, 75, 825. (e) Crudden, C. M.; Hleba, Y. B.; Chen, A. C.
(
R)-PPY*
J. Am. Chem. Soc. 2004, 126, 9200. (f) Segarra, A. M.;
Daura-Oller, E.; Claver, C.; Poblet, J. M.; Bo, C.; Fernández,
E. Chem. Eur. J. 2004, 10, 6456. (g) Moteki, S. A.; Wu, D.;
Chandra, K. L.; Reddy, D. S.; Takacs, J. M. Org. Lett. 2006,
Scheme 2 Chiral DMAP analogue for asymmetric induction
8
, 3097. (h) Carroll, A.-M.; O’Sullivan, T. P.; Guiry, P. J.
clarify the exact mechanism of Rh(I)-catalyzed hydrobo-
ration, which is not yet settled due to the intriguing de-
bates to date. Further investigations to reveal the special
features of the present Rh complex are currently under-
way in our laboratory, for hydroboration as well as for
other catalytic reactions.
Adv. Synth. Catal. 2005, 347, 609. (i) Edwards, D. R.;
Hleba, Y. B.; Lata, C. J.; Calhoun, L. A.; Crudden, C. M.
Angew. Chem. Int. Ed. 2007, 46, 7799.
(
(
4) The preliminary examination for hydroboration typically
proceeds regioselectively in the presence of DPPB as a
ligand. Other ligands often diminished the regioselectivity;
see refs. 1c, 2, and 3e.
5) The reaction with catecholborane (HBcat) for 10 min gave a
regioisomeric mixture of 1-phenylethanol and 2-
phenylethanol in 83% yield with 77:23 ratio after the
oxidation of the crude boronate compounds.
I
Rh Cl–DPPB–DMAP Catalyzed Hydroboration of Vinylar-
enes; Typical Procedure
(
6) (a) Westcott, S.; Marder, T. B. Organometallics 1993, 12,
To a mixture of [RhCl(cod)] (12.3 mg, 0.025 mmol, 5 mol% Rh),
2
975. (b) Brown, J. M.; Lloyd-Jones, G. C. J. Am. Chem. Soc.
DPPB (25.6 mg, 0.06 mmol, 6.0 mol%), and DMAP (7.4 mg, 0.06
mmol, 6.0 mol%) in DCE (1 mL) were added styrene 1a (104 mg,
1994, 116, 866.
(
7) The reaction of 1,2-disubstituted olefin, such as cis-stilbene,
gave lower yield along with the formation of reduction
product. The reaction of 1-hexene under the same reaction
conditions for 10 min as entry 6 in Table 1 gave the linear
product in 65% yield (unoptimized).
8) These reactions without DMAP additive gave the
corresponding products as a mixture of 2g and 3g in 29%
yield with 21:79 ratio and 2h and 3h in 8% yield with 13:87
ratio.
1
.0 mmol) and pinacolborane (153.6 mg, 1.2 mmol, 1.2 equiv) at r.t.
The reaction mixture was stirred at r.t. for 10 min and filtered
through a pad of silica gel with Et O (50 mL). The crude mixture
2
1
was concentrated to dryness. The H NMR analysis with 1,1,2,2-tet-
rachloroethane as an internal standard suggested that the yield is
(
9
8% and 2a/3a ratio is >98:2. The purification by column chroma-
tography (silica gel, 5% EtOAc in hexane) gave the product 2a in
7% yield (202 mg, 0.87 mmol) as colorless oil. All products 2a–i
8
are known compounds and analytical data matched the results in the
literature.2
,3
(9) (a) Fu, G. C. Acc. Chem. Res. 2004, 37, 542. (b) Fu, G. C.
Acc. Chem. Res. 2006, 39, 853.
(
10) The ESI-MS analyses of the mixture of [RhCl(cod)] , DPPB
2
Acknowledgment
(2 equiv), and DMAP (4 equiv) gave a single peak as a [Rh
+
+
DPPB + (DMAP) ] (m/z calcd: 773.2 (100%); found:
2
K.E. thanks the Teijin Pharma Award in Synthetic Organic Chemi-
stry, Japan.
7
73.3). Thus, the coordination of DMAP to Rh(I) center is
31
probable under the present catalysis. The P NMR analysis
in CDCl (phosphoric acid as external reference) of a
3
mixture of [RhCl(cod)] , DPPB (2 equiv), and DMAP (4
2
References and Notes
equiv) gave a single peak at d = 18.64 (d, J
= 99.2 Hz,
, DPPB (2 equiv), and
Rh–P
(
1) (a) Hayashi, T. In Comprehensive Asymmetric Catalysis,
Vol. 1; Jacobsen, E. N.; Pfaltz, A.; Yamamoto, H., Eds.;
Springer: New York, 1999, 349. (b) Hayashi, T.;
2 P). A mixture of [RhCl(cod)]
2
DMAP (2 equiv) gave four peaks at d = 20.85 (d, JRh–P
=
110.2 Hz), 20.69 (d, JRh–P = 110.2 Hz), 17.98 (d, JRh–P = 101
Hz), 17.82 (d, JRh–P = 101 Hz). In contrast, a mixture of
Matsumoto, Y.; Ito, Y. J. Am. Chem. Soc. 1989, 111, 3426.
(
c) Hayashi, T.; Matsumoto, Y.; Ito, Y. Tetrahedron:
[RhCl(cod)]
specified). These results offer the formation of monomeric
[Rh(dppb)(dmap) ]Cl as an intermediate. We assume that
2
and DPPB (2 equiv) gave many peaks (not
Asymmetry 1991, 2, 601. (d) Tucker, C. E.; Davidson, J.;
Knochel, P. J. Org. Chem. 1992, 57, 3482. (e) Brown,
J. M.; Hulmes, D. E.; Layzell, T. P. J. Chem. Soc., Chem.
Commun. 1993, 1673. (f) Togni, A.; Breutel, C.; Schnyder,
A.; Spindler, F.; Landert, H.; Tijani, A. J. Am. Chem. Soc.
2
the strong s-donation ability of DMAP is favorable, but the
details should be examined to determine the role of DMAP.
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