Cobalt-catalyzed addition reaction of organoboronic acids with aldehydes: Highly enantioselective synthesis of diarylmethanols

Article abstract of DOI:10.1002/chem.201001160

Predicted outcomes: The addition reaction of organoboronic acids with aldehydes in the presence of K₂CO₃ catalyzed by CoI ₂/ACHTUNGRE(R,R)-BDPP gives chiral secondary alcohols in excellent yields with 90-99 % enantiomeric excess (see scheme; (R,R)-BDPP = (2R,4R)-(+)-2,4-bis(diphenyl-phosphino)pentane). This method provides an alternative to prepare an R and S enantiomeric pair by using the same chiral ligand and allows the stereochemical outcome of the reaction to be predicted.

Full text of DOI:10.1002/chem.201001160

COMMUNICATION
DOI: 10.1002/chem.201001160
Cobalt-Catalyzed Addition Reaction of Organoboronic Acids with
Aldehydes: Highly Enantioselective Synthesis of Diarylmethanols
Jaganathan Karthikeyan, Masilamani Jeganmohan, and Chien-Hong Cheng*[a]
Transition-metal-catalyzed addition of organometallic re-
agents to aldehydes is a key method for the synthesis of sub-
stituted secondary alcohols.[1] Various organometallic re-
agents, such as organomagnesium,[2] -zinc,[1,12e–i] -lithium,[3]
-silane,[4] -stannane[5] and -boron,[6] have been used in these
addition reactions. Among them, organoboron reagents
have gained much attention due to the advantages of air
and moisture stability, low toxicity, and availability. Rho-
dium,[1c,7] palladium,[8] platinum,[9a] and nickel[9b,c] complexes
efficiently catalyzed the addition reaction of organoboronic
acids to aldehydes. Recently, copper- and iron-catalyzed ad-
dition reaction of organoboronic acid with aldehydes were
also reported.[10] However, the scope of aldehydes in these
two addition reactions is rather limited. Only aromatic alde-
hydes with an electron-withdrawing substituent worked
well.[10]
Despite the fact that various metal-catalyzed addition re-
actions of organoboronic acids with aldehydes are available
in the literature,[6–10] only a few reports on asymmetric reac-
tions were discussed.[3a,b,11] In 2006, Zhou et al. reported a
rhodium-catalyzed enantioselective addition reaction of aro-
matic boronic acids with aromatic aldehydes.[11a] In the reac-
tion, enantiomeric excess (ee) values of 62–87% for chiral
biaryl methanols were observed. Recently, Miyaura and co-
workers reported a ruthenium-catalyzed enantioselective ad-
dition reaction of aromatic boronic acids with aromatic alde-
hydes. In the reaction, the expected chiral biarylmethanols
were observed in excellent enantiomeric excess.[11b] Howev-
er, in these reactions specially designed chiral ligands and
expensive ruthenium or rhodium catalysts were used. Chiral
secondary alcohols are key structural units present in vari-
ous biologically and pharmaceutically active com-
pounds.[12a–h] The development of new, mild, and convenient
methods using a low-cost catalyst for the synthesis of chiral
secondary alcohols remains highly attractive.[12]Recently, we
have reported a cobalt-catalyzed hydroarylation of alkynes
with organoboronic acids[13] and other cobalt-catalyzed reac-
tions.[14] Our continuing interest in developing new reactions
using less expensive cobalt complexes as catalysts prompted
us to investigate the addition of organoboronic acids with al-
dehydes and the enantioselective version of this reaction.
Herein, we wish to show that cobalt complexes very effi-
ciently catalyze this addition reaction to give diarylmetha-
nols in excellent yields and ee values.
Treatment of phenylboronic acid (1a) with 4-cyanobenzal-
dehyde (2a) in the presence of CoACTHNUTRGNENUG(acac)2 (5 mol%), 1,2-
bis(diphenylphosphino)ethane (dppe; 5 mol%) in THF/
CH3CN (1/1) at 808C for 12 h gave addition product 3aa in
96% isolated yield (Table 1, entry 1). In the present reac-
tion, no extra base was required and only 1.2 mmol of bor-
onic acid was used.[13] The use of binary solvent system
THF/CH3CN (1:1) appears to improve the yield of product
3aa. If the catalytic reaction was carried out in THF, prod-
uct 3aa was observed only in 75% yield along with benzene,
the protodeboronation product of 1a, in 18% yield. The cat-
alytic reaction also worked equally well using CoI2 or CoCl2
(5 mol%), dppe (5 mol%) as the catalyst, and THF as sol-
vent to afford 3aa in 96–97% yield, but base (K2CO3
(1.50 equiv)) was needed to activate the boronic acid.
Under similar reaction conditions, a variety of substituted
aromatic aldehydes, heterocyclic aldehydes, and aliphatic al-
dehydes were examined with phenylboronic acid (1a)
(Table 1). Thus, benzaldehydes with electron-withdrawing
groups, such as 4-NO2 (2b), 4-CHO (2c), 4-CO2Me (2d),
and 4-CF3 (2e) provided diarylmethanols 3ab–3ae in excel-
lent yields (89–97%; Table 1, entries 2–5). For these sub-
strates, only the CHO group participated in the reaction, the
other functional groups remained essentially intact. The re-
action of dialdehyde 2c with 1a also proceeds selectively at
one of the CHO groups. Halo-substituted benzaldehyde de-
rivatives are compatible with the present catalytic reac-
[a] J. Karthikeyan, Dr. M. Jeganmohan, Prof. Dr. C.-H. Cheng
Department of Chemistry, National Tsing Hua University
Hsinchu, 30043 (Taiwan)
Fax : (+886)35724698
E-mail: chcheng@mx.nthu.edu.tw
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201001160.
Chem. Eur. J. 2010, 16, 8989 – 8992
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
8989

Table 1. Results of the addition reaction of arylboronic acid with various
aldehydes.[a]
effectively with 2d to furnish substituted diarylmethanols
3bd–3hd in 93, 93, 84, 92, 97, 96, and 75% yield, respective-
ly (Table 1, entries 20–26). These results clearly indicate that
the present addition reaction shows excellent tolerance to-
wards Br, F, CHO, Me, and OMe functional groups. The cat-
alytic reaction also worked very well with alkenylboronic
acid. Thus, (E)-1-phenylvinyl boronic acid (1i) reacted with
2d to afford allylic alcohol 3id in 78% yield (Table 1,
entry 27).
The great importance of chiral secondary alcohols in or-
ganic synthesis prompted us to explore the enantioselective
addition of organoboronic acids with aldehydes. Phenylbor-
onic acid (1a) and 2d were used as the model substrates in
this study. Cobalt catalyst CoI2 (5 mol%), a bidentate chiral
ligand (5 mol%), and K2CO3 (1.5 equiv) in THF were used
in the reaction. Various bidentate chiral ligands, including
(R)-Prophos, (R)-Tol-BINAP, (S)-BINAP, (S,S)-Chiraphos,
(S)-BINOL, (R,R)-Ph-BPE, (R)-Quinap, (R)-MOP, (S,S)-
DIPAMP, (R)-Monophos, (R,R)-BDPP, and (S,S)-DIOP,
were examined (for the structure of chiral ligands and de-
tailed optimization studies, see the Supporting Information).
Among them, (R,R)-BDPP is most effective, affording (S)-
diarylmethanol 3ad in 98% yield with an ee value of 94%
(Table 2, entry 3).[11] Other ligands provided 3ad in 44–86%
yields with an ee value of 10–67%. Under the reaction con-
ditions, (S,S)-BDPP provided the other enantiomer (R)-dia-
rylmethanol 3ad in 95% yield with an ee of 93% (Table 2,
Entry Substrates
Product Yield[b] [%]
1
2
3
4
5
6
7
8
1a: R1 =H; 2a: R=4-CN-C6H4
3aa
3ab
3ac
3ad
3ae
3af
3ag
3ah
3ai
96
97
93
98
89
92
82
80
96
84
73
75
75
67
57
76
69
78
79
93
93
84
92
97
96
75
78
1a: R1 =H; 2b: R=4-NO2-C6H4
1a: R1 =H; 2c: R=4-CHO-C6H4
1a: R1 =H; 2d: R=4-CO2Me-C6H4
1a: R1 =H; 2e: R=4-CF3-C6H4
1a: R1 =H; 2 f: R=4-F-C6H4
1a: R1 =H; 2g: R=3-F-C6H4
1a: R1 =H; 2h: R=2-F-C6H4
1a: R1 =H; 2i: R=4-Cl-C6H4
1a: R1 =H; 2j: R=4-Br-C6H4
1a: R1 =H; 2k: R=Ph
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
3aj
3ak
3al
1a: R1 =H; 2l: R=1-napthyl
1a: R1 =H; 2m: R=2-napthyl
1a: R1 =H; 2n: R=4-Me-C6H4
1a: R1 =H; 2o: R=4-OMe-C6H4
1a: R1 =H; 2p: R=4-pyridinyl
1a: R1 =H; 2q: R=2-furyl
3am
3an
3ao
3ap
3aq
3ar
3as
3bd
3cd
1a: R1 =H; 2r: R=2-thienyl
1a: R1 =H; 2s: R=cyclohexyl
1b: R1 =4-Br; 2d: R=4-CO2Me-C6H4
1c: R1 =4-F; 2d: R=4-CO2Me-C6H4
1d: R1 =4-CHO; 2d: R=4-CO2Me-C6H4 3dd
1e: R1 =4-Me; 2d: R=4-CO2Me-C6H4
3ed
1 f: R1 =4-OMe; 2d: R=4-CO2Me-C6H4 3 fd
1g: R1 =2-OMe; 2d: R=4-CO2Me-C6H4 3gd
1h: R1 =4-vinyl; 2d: R=4-CO2Me-C6H4 3hd
entry 4). Another cobalt catalyst, CoACHTNUTRGNENUG(acac)2/ACHTUNGTREN(NGUN R,R)-BDPP, in
THF without base is also effective, giving chiral (S)-3ad in
92% yield and 89% ee.
1i: (E)-styryl; 2d: R=4-CO2Me-C6H4
3id
[a] Unless otherwise mentioned, all of the reactions were carried out by
using arylboronic acid 1 (1.20 mmol), aldehydes 2 (1.00 mmol), Co(acac)2
(5 mol%), dppe (5 mol%), and THF/CH3CN (1:1) at 808C for 12 h.
In the presence of CoI2 (5 mol%)/ACTHNUTRGNEUNG(R,R)-BDPP (5 mol%)
AHCTUNGTRENNUNG
and K2CO3 (1.5 equiv) in THF, the reaction of various sub-
stituted aldehydes with phenylboronic acid (1a) were then
examined (Table 2). Electron-withdrawing groups, 4-CN-
(2a), 4-NO2- (2b), 4-CO2Me- (2d), and 4-CF3-substituted
(2e) benzaldehydes afforded chiral (S)-diarylmethanols 3aa,
3ab, 3ad, and 3ae in excellent 97, 95, 98, and 97% yield
with 92, 93, 94 and 92% ee, respectively (Table 2, entries 1–
3, 5). As expected, if CoI2/ACTHNUTRGNE(UNG S,S)-BDPP was employed as the
catalyst, the reaction of 1a with 2d afforded the correspond-
ing (R)-3ad in 93% ee (Table 2, entry 4). Similarly, by using
[b] Isolated yields.
tion.[15] For example, 4-F (2 f), 3-F (2g), 2-F (2h), 4-Cl (2i),
and 4-Br (2j) also reacted efficiently with 1a to give the cor-
responding diarylmethanols 3af–3aj in good to excellent
yields (Table 1, entries 6–10). Similarly, benzaldehyde (2k),
1-napthaldehyde (2l) and 2-napthaldehyde (2m) underwent
addition reaction with 1a to afford products 3ak–3am in
good yields (Table 1, entries 11–13). Benzaldehydes contain-
ing electron-donating groups, such as 4-Me (2n) and 4-OMe
(2o) also gave addition products 3an and 3ao albeit in mod-
erate yields (Table 1, entries 14 and 15). Heterocyclic alde-
hydes, including 4-formylpyridine (2p), 2-formylfuran (2q),
and 2-formylthiophene (2r) also reacted efficiently to give
addition products 3ap–3ar in 76, 69, and 78% yields, respec-
tively (Table 1, entries 16–18). Aliphatic aldehyde, cyclohex-
anecarbaldehyde (2s), also effectively participated in the ad-
dition reaction affording product 3as in 79% yield (Table 1,
entry 19).
CoI2/ACTHNURTGNEU(GN R,R)-BDPP as the catalyst, 4-F- (2 f), 4-Cl- (2i), and
4-Br-substituted (2j) benzaldehydes provided (S)-diarylme-
thanols 3af, 3ai, and 3aj in excellent 95–97% yield with 99,
93, and 96% ee, respectively (Table 2, entries 6–8). Likewise,
1-naphth- (2l) and 2-naphthaldehyde (2m), 4-methyl (2n),
4-methoxybenzaldehyde (2o), and 2-formylthiophene (2r)
gave (S)-diarylmethanols 3al, 3am, 3an, 3ao, and 3ar, re-
spectively, in 77–90% yield with 86–93% ee (Table 2, en-
tries 9–13). A 2-naphthyl and 2-thienyl group on the alde-
hyde substrate appears to lower the ee value slightly com-
pared with other aryl groups used. In a similar manner, cy-
clohexanecarbaldehyde (2s) yielded (R)-3as in 84% yield
with 97% ee (Table 2, entry 14). It is interesting to note that
in the present chiral addition reaction with base, even elec-
tron-rich-substituted, heteroaromatic and aliphatic alde-
hydes provided the corresponding addition products in ex-
To further explore the scope of the addition reaction, vari-
ous substituted organoboronic acids were examined with
methyl 4-formyl benzoate (2d). 4-Bromo- (1b), 4-fluoro-
(1c), 4-formyl- (1d), 4-methyl- (1e), 4-methoxy- (1 f), 2-me-
thoxy- (1g) and 4-vinylphenylboronic (1h) acids all reacted
8990
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Highly Enantioselective Synthesis of Diarylmethanols
COMMUNICATION
Table 2. Results of the enantioselective addition reaction of various phe-
nylboronic acids with substituted aldehydes.[a]
known to be the key intermediates for biologically active
compounds (S)-cetirizine and (R)-neobenodine, respective-
ly.[12e,f]
In addition, other substituted phenylboronic acids, includ-
ing 4-bromo- (1b), 4-methyl- (1e), 4-methoxy- (1 f), and 2-
methoxyphenylboronic 1g acids, also reacted smoothly with
4-CO2Me-substituted benzaldehyde 2d to give diarylmetha-
nols 3bd, 3ed, 3 fd, and 3gd in excellent yields (92–99%)
and ee values (90–94%), respectively (Table 2, entries 15,
19–21).
It is interesting to note that for the present CoI2/ACHTUNGTRENNUNG(R,R)-
BDPP-catalyzed asymmetric addition of phenylboronic acid
to various substituted benzaldehydes, only the S products
were obtained (Table 2, entries 1–13). These results may be
explained by the proposed reaction model, A, in Scheme 1,
Entry Substrates
Yield (ee)
[%][b]
1
2
3
4
5
6
7
8
1a: R1 =H; 2a: R=4-CN-C6H4
(S)-3aa
(S)-3ab
(S)-3ad
97 (92)
95 (93)
98 (94)
1a: R1 =H; 2b: R=4-NO2-C6H4
1a: R1 =H; 2d: R=4-CO2Me-C6H4
1a: R1 =H; 2d: R=4-CO2Me-C6H4
1a: R1 =H; 2e: R=4-CF3-C6H4
1a: R1 =H; 2 f: R=4-F-C6H4
1a: R1 =H; 2i: R=4-Cl-C6H4
1a: R1 =H; 2j: R=4-Br-C6H4
1a: R1 =H; 2l: R=1-napthyl
1a: R1 =H; 2m: R=2-napthyl
1a: R1 =H; 2n: R=4-Me-C6H4
1a: R1 =H; 2o: R=4-OMe-C6H4
1a: R1 =H; 2r: R=2-thienyl
1a: R1 =H; 2s: R=cyclohexyl
1b: R1 =4-Br; 2d: R=4-CO2Me-C6H4
1b: R1 =4-Br; 2k: R=Ph
(R)-3ad 95[c] (93)
(S)-3ae
(S)-3af
(S)-3ai
(S)-3aj
(S)-3al
97 (92)
95 (99)
97 (93)
97 (96)
77 (89)
9
10
11
12
13
14
15
16
(S)-3am 89 (92)
(S)-3an
(S)-3ao
(S)-3ar
(R)-3as
90 (93)
85 (92)
82 (86)
84 (97)
(+)-3bd 99 (90)
(R)-3bk 84 (94)
(R)-3aj
(R)-3ek 91 (95)
(R)-3an
Scheme 1. Proposed addition of an aryl boronic acid to aldehyde cata-
lyzed by CoI2/ACTHNUGRTNEUNG(R,R)-BDPP. Note that enantiomers were obtained from A
and B.
17
18
1e: R1 =4-Me; 2k: R=Ph
1 f: R1 =4-OMe; 2k: R=Ph
(R)-3 fk
(R)-3ao
93 (94)
19
20
21
1e: R1 =4-Me; 2d: R=4-CO2Me-C6H4
(+)-3ed 95 (91)
although the exact structure of the cobalt intermediate is
not known. The chiral (R,R)-BDPP ligand appears to con-
trol efficiently the relative three-dimensional position of the
coordinated phenyl and substituted benzaldehyde. As a
result, the coordinated aryl group adds to the aldehyde
group from its si face leading to the formation of S product.
If the phenyl group on the boronic acid and aryl group of
the aldehyde exchange positions (model B), R products will
be obtained (Table 2, entries 16–18). It is interesting to note
that this method provides an alternative to prepare an R
and S enantiomeric pair by using the same chiral ligand.
Based on these reaction models, we expect that (+)-3bd,
(+)-3 fd, and (+)-3gd should have R configuration, whereas
(+)3ed should be an S product.
1 f: R1 =4-OMe; 2d: R=4-CO2Me-C6H4 (+)-3 fd 97 (94)
1g: R1 =2-OMe; 2d: R=4-CO2Me-C6H4 (+)-3gd 92 (90)
[a] Unless otherwise mentioned, all of the reactions were carried out by
using organoboronic acid 1 (1.50 mmol), aldehydes 2 (1.00 mmol), CoI2-
A
ACHTUNGTRENNUNG(R,R)-BDPP (5 mol%), K2CO3 (1.5 equiv), and THF (2.0 mL)
cellent yields in contrast to the results using CoACTHNUGTRENUNG(acac)2/dppe
because the catalyst gave only moderate to good yields.
The present asymmetric catalytic reaction can be extend-
ed successfully to other arylboronic acids. Under similar re-
action conditions, using CoI2/ACTHNUGRTNEUNG(R,R)-BDPP as the catalyst,
the reaction of 4-bromophenylboronic acid (1b) and benzal-
dehyde (2k) gave (R)-3bk in 84% yield with 94% ee
(Table 2, entry 16). It is noteworthy that (R)-3bk is the
enantiomer of (S)-3aj (Table 2, entry 8) prepared from phe-
nylboronic acid (1a) and 4-bromobenzaldehyde (2j) using
In conclusion, we have demonstrated, for the first time, a
cobalt-catalyzed racemic and enantioselective addition of or-
ganoboronic acids with aldehydes to give biologically useful,
substituted secondary alcohols in excellent yields and enan-
tiomeric excess. In the chiral reaction, highly stable, less ex-
pensive CoI2 catalyst, and commercially available chiral
ligand (R,R)-BDPP were used. A wide scope of organobor-
onic acids and aldehydes are compatible with this catalytic
reaction. Further detailed investigation on the mechanism
and the application of this methodology in natural product
synthesis are in progress.
the same chiral CoI2/ACTHNUGRTENUNG(R,R)-BDPP catalyst. In a similar
manner, product (R)-3ek (Table 2, entry 17) is enantiomer
of (S)-3an (Table 2, entry 11) and (R)-3 fk (Table 2,
entry 18) is the enantiomer of (S)-3ao (Table 2, entry 12).
Thus, the present catalytic asymmetric addition reaction pro-
vides a versatile method to prepare the two enantiomers by
using the same chiral catalyst. Moreover, in the present
asymmetric reaction, products (S)-3ai and (R)-3ek are
Chem. Eur. J. 2010, 16, 8989 – 8992
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
8991

C.-H. Cheng et al.
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Experimental Section
General procedure for the asymmetric arylation of aldehyde: A 10 mL
sealed tube containing CoI2 (0.050 mmol, 5.0 mol%), (R,R)-BDPP
(0.050 mmol, 5.0 mol%), K2CO3 (1.5 equiv), 2d (1.00 mmol) and phenyl-
boronic acid (1a) (1.50 mmol) was evacuated and purged with nitrogen
gas three times. Then freshly distilled THF (2.0 mL) was added to the
system and the reaction mixture was stirred at 808C for 12 h. The reac-
tion mixture was filtered through a short Celite and silica gel pad and
washed with 3:1 mixture of hexane and ethyl acetate several times. The
combined filtrate was concentrated and the residue was purified on a
silica gel column using hexane/ethyl acetate as eluent to afford the addi-
tion product 3ad.
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Acknowledgements
We thank the National Science Council of Republic of China (NSC 96-
2113M-007-020-MY3) for support of this research and we thank Prof.
Biing-Jiun Uang for support with chiral HPLC.
Keywords: alcohols · aldehydes · asymmetric synthesis ·
cobalt · organoboronic acid
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Received: April 30, 2010
Published online: July 2, 2010
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