Rhodium fluoroapatite catalyzed conjugate addition of arylboronic acids to α,β-unsaturated carbonyl compounds

Article abstract of DOI:10.1002/hlca.200890208

Rhodium fluoroapatite (RhFAP) is an efficient catalyst for conjugate addition of organoboron reagents to α,β-unsaturated carbonyl compounds. A variety of arylboronic acids and α,β-unsaturated carbonyl compounds were converted to the corresponding conjugat

Full text of DOI:10.1002/hlca.200890208

Helvetica Chimica Acta – Vol. 91 (2008)
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Rhodium Fluoroapatite Catalyzed Conjugate Addition of Arylboronic Acids
to a,b-Unsaturated Carbonyl Compounds
by Mannepalli Lakshmi Kantam*, Vura Bala Subrahmanyam, Kota Balaji Shiva Kumar, Gopaladasu T.
Venkanna, and Bojja Sreedhar
Inorganic & Physical Chemistry Division, Indian Institute of Chemical Technology, Hyderabad, 500067,
India (fax: þ 91-40-27160921; e-mail: mlakshmi@iict.res.in)
Rhodium fluoroapatite (RhFAP) is an efficient catalyst for conjugate addition of organoboron
reagents to a,b-unsaturated carbonyl compounds. A variety of arylboronic acids and a,b-unsaturated
carbonyl compounds were converted to the corresponding conjugate-addition products, demonstrating
the versatility of the reaction. The reaction is highly selective. RhFAP was recovered quantitatively by
simple filtration, and reused for four cycles.
Introduction. – Rhodium (Rh)-catalyzed conjugate addition of arylboronic acids to
a,b-unsaturated carbonyl compounds has received considerable attention in the field of
organic synthesis, ever since it was reported by Miyaura and co-workers in 1997 [1]. The
discovery of such an important reaction has triggered much research activity in the area
of C,C-bond formation catalyzed by Rh complexes [2]. Zou et al. reported the use of
phosphinerhodium catalyst for selective Heck-type coupling and conjugate addition by
synergistically tuning the supporting ligand, the boronic acid to olefin ratio, and other
reaction conditions [3]. Despite the advantages of homogeneous metal complex
catalysts, difficulties in recovering the expensive catalyst metals and ligands from the
reaction mixture severely limit their industrial applications. Heterogeneous catalysis is
particularly attractive as it allows production and ready separation of large quantities of
products with the use of a small amount of catalyst. Kaneda et al. reported the 1,4-
addition of organoboron reagents to a,b-unsaturated carbonyl compounds by using a
highly efficient heterogeneous catalyst, Rh-grafted hydrotalcite [4].
Apatites are metal basic phosphates for which the chemical formula is
M10(PO4)6(X)2 (M ¼ divalent metal, X ¼ monovalent anion), and the most typical
apatite is calcium hydroxyapatite: Ca10(PO4)6(OH)2 (CaHAP). Various kinds of
cations and anions can be readily introduced into their framework due to their large
ion-exchange ability, and such exchanged apatites are already in use in several organic
transformations [5 – 7]. Previously, we reported the preparation of recyclable hetero-
geneous Cu-exchanged fluoroapatite and Cu-exchanged (tert-butoxy)apatite catalysts
for N-arylation of imidazoles and other heterocycles with iodo-, bromo-, chloro-, and
fluoroarenes (electron-withdrawing groups) with good to excellent yields. We also
reported a PdFAP catalyst (fluoroapatite-supported Pd catalyst) for the Suzuki- and
Heck-coupling reaction [8]. Very recently, we described the preparation of a recyclable
heterogeneous rhodium fluoroapatite (RhFAP) catalyst by a simple ion-exchange
technique by treatment of fluoroapatite with RhCl3 in H2O, which successfully
ꢀ 2008 Verlag Helvetica Chimica Acta AG, Zürich

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Helvetica Chimica Acta – Vol. 91 (2008)
promoted cross coupling of Baylis – Hillman adducts with arylboronic acids to yield
trisubstituted olefins [9]. Thus, in continuation of our work with the RhFAP catalyst,
we herein report the conjugate addition of organoboron reagents to a,b-unsaturated
carbonyl compounds to yield conjugate-addition products (Scheme).
Scheme
Results and Discussion. – In an effort to develop a better catalytic system, various
reaction parameters were screened for the conjugate addition of (4-chlorophenyl)bor-
onic acid to butyl acrylate (¼ butyl prop-2-enoate) to yield butyl 4-chlorobenzenepro-
panoate, and the results are presented in Table 1. The RhFAP catalyst, combined with
cycloocta-1,5-diene (1,5-cod; 0.12 mmol), in MeOH at 658 exhibited the highest
catalytic activity leading to the conjugate-addition (CA) product (Table 1, Entry 4),
while the reaction conducted in the absence of 1,5-cod did not yield the CA product
(Table 1, Entry 3). No reaction took place with homogeneous RhCl3 catalyst in the
presence and absence of 1,5-cod under similar conditions (Table 1, Entries 1 and 2), and
there was no reaction with RhCl3 impregnated on FAP support (RhFAP-impregnated)
and Rh/SiO2 (Table 1, Entries 10 and 11). The solvent had a pronounced effect in these
reactions, both in terms of yield and selectivity of CA product. MeOH and EtOH were
good solvents with good selectivity, whereas H2O, toluene/H2O 3 :1, and THF/H2O 2 :1
provided moderate yields of CA product with poor selectivity (Table 1). The
concentration of boronic acid, olefin, Rh, and 1,5-cod were tuned to get the conjugate
addition product selectively, and these results are presented in Table 2. The CA product
was obtained with 99% selectivity when the ratio of (4-chlorophenyl)boronic acid to
olefin was 1.2 :1, and Rh to 1,5-cod was 1:12.
The controlled reaction conducted under identical conditions devoid of RhFAP and
1,5-cod gave no CA product, despite prolonged reaction times. RhFAP was recovered
quantitatively by simple centrifugation, and reused for four recycles with a minimal loss
of activity (Table 1, Entry 4). The X-ray photoelectron spectrum (XPS) of the used
RhFAP catalyst showed retention of the þ 3 oxidation state. To check the
heterogeneity of the catalyst, a reaction between (4-chlorophenyl)boronic acid and
butyl acrylate was terminated at 31% conversion (1 h), and the catalyst was separated
by simple filtration. The reaction was continued for additional 6 h, and the conversion
remained unchanged. These studies clearly demonstrate that no leaching of Rh had
taken place from the catalyst, and throughout the reaction, Rh was bounded to the
support.

Helvetica Chimica Acta – Vol. 91 (2008)
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Table 1. Screening of Reaction Parameters for the Synthesis of Butyl 4-Chlorobenzenepropanoatea)
Entry
Catalyst
Solvent
Yieldb) [%]
Ratio CA/HCc)
1d)e)
2f)e)
3d)
RhCl3
RhCl3
RhFAP
RhFAP
RhFAP
RhFAP
RhFAP
RhFAP
MeOH
MeOH
MeOH
MeOH
0
0
0
0
0
0
4f)
92, 90g)
99 : 1
92 : 8
99 : 1
73 : 27
82 : 18
96 : 4
0
5f)
H2O
68
12
64
79
91
0
6f)
1,4-dioxane
toluene/H2O 3 : 1
THF/H2O 2 : 1
EtOH
MeOH
MeOH
7f)
8f)
9f)
RhFAP
RhFAP (impregnated)
Rh/SiO2
10f)
11f)
0
0
a) X (1.2 mmol), Y (1 mmol), catalyst (100 mg), 1,5-cod (0.12 mmol), solvent (4 ml), reaction time
(6 h), N2, 658. b) Yield of isolated products. c) Ratio of CA (conjugative addition) and HC (Heck-type
coupling) product is based on GC analysis by comparison with authentic samples. d) Without any
f
additive. e) 2 mol-% of RhCl3 was used. ) 1,5-cod was used (0.12 mmol). g) Yield after fourth recycle.
Table 2. Screening of Molar Parameters for the Selective Formation of the Conjugate-Addition Product
X [mmol]
Y [mmol]
Rh [mmol]a)
1,5-Cod [mmol]
Ration CA/HCb)
0.5
0.5
0.6
1.0
1.2
1.0
1.0
1.0
1.5
1.0
0.005
0.005
0.01
0.01
0.01
0.06
0.12
0.12
0.12
0.12
89 : 11
88 : 12
90 : 10
98 : 2
99 : 1
a) Amount of Rh in the RhFAP catalyst. b) Ratio of CA (conjugate addition) and HC (Heck-type
coupling) product is based on GC by comparison with authentic samples.
To understand the scope and limitations of RhFAP-catalyzed conjugate-addition
reactions, various structurally different arylboronic acids possessing a wide range of
functional groups were treated with butyl acrylate to give the corresponding conjugate-
addition products, and the results are summarized in Table 3. Phenylboronic acids with
a substituent at ortho-position (Table 3, Entries 6 and 7) afforded lower yields
compared with the phenylboronic acids having a substituent at para-position (Table 3,
Entries 4 and 8), which might be due to an ortho steric effect. Heteroaromatic boronic

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Table 3. Conjugate Addition of Organoboron Reagents X to Butyl Acrylate (Y) Catalyzed by RhFAPa)
Entry
Ar
Time [h]
Yieldb) [%]
1
2
3
4
5
Ph
7
6
6
86
4-ClC6H4
4-BrC6H4
4-MeC6H4
4-MeOC6H4
2-MeC6H4
2-FC6H4
92, 90c)
90
8
93
82
78
79
12
12
8
6
7
8
4-FC6H4
8
89
9
1-naphthyl
3-CHOC6H4
3-NO2C6H4
PhCH¼CH
furan-3-yl
3-thienyl
8
8
8
10
8
81
82
84
76
84
81
10
11
12
13
14
8
a) X (1.2 mmol), Y (1 mmol), catalyst (100 mg), cycloocta-1,5-diene (0.12 mmol), MeOH (4 ml), N
658.b) Yield of isolated CA product. c) Yield of CA after the 4th recycle.
,
2
acids such as furan-3-ylboronic acid and 3-thienylboronic acid (Table 1, Entries 13 and
14) gave good yields of the corresponding conjugate-addition products. Aliphatic
boronic acids such as vinylboronic acids gave lower yields of the desired product after a
long reaction time (Table 3, Entry 12). To expand the scope of the method, a variety of
a,b-unsaturated carbonyl compounds were successfully treated with (4-chlorophenyl)-
boronic acid, and the results are summarized in Table 4. The cyclic enones such as
cyclohex-2-en-1-one and cyclopent-2-en-1-one reacted slowly with good yields
compared to other a,b-unsaturated carbonyl compounds. Selectivity of all the products
was determined by both GC and 1H-NMR, the selectivity of all the CA products being
> 96%.
Conclusions. – We developed a simple and efficient method for the conjugate
addition of organoboron reagents to a,b-unsaturated carbonyl compounds to yield
conjugate-addition products by using the RhFAP catalyst. A variety of arylboronic
acids and a,b-unsaturated carbonyl compounds were converted to the corresponding
conjugate-addition products, demonstrating the versatility of the reaction. The catalyst
can be readily recovered and reused. The process is highly selective towards conjugate-
addition products. We believe that this methodology will find widespread use in organic
synthesis for the preparation of conjugate-addition products.
We wish to thank CSIR and UGC, New Delhi, for financial support.

Helvetica Chimica Acta – Vol. 91 (2008)
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Table 4. Conjugate Addition of 4-(Chlorophenyl)boronic Acid to a,b-Unsaturated Carbonyl Compounds
Catalyzed by RhFAPa)
Entry
1
Olefin
Time [h]
8
Yieldb) [%]
86
2
3
8
8
84
85
4
5
6
8
9
9
89
90
88
a) Boronic acid (1.2 mmol), olefin (1 mmol), catalyst (100 mg), MeOH (4 ml), cycloocta-1,5-diene
(0.12 mmol), N2, 658. b) Yield of isolated product.
Experimental Part
General. All reactants were commercially available and used without purification. NMR Spectra:
Avance-300 and Gemini-200 spectrometer; d in ppm rel. to Me4Si as internal standard, J in Hz. GC
Analyses: Shimadzu GC 2010, with ZB-5 capillary column.
Conjugate-Addition Product: General Procedure. RhFAP (100 mg, i.e., 0.01 mmol of Rh) was added
to a mixture of arylboronic acid (1.2 mmol), olefin (1.0 mmol), and 1,5-cod (0.12 mmol) in MeOH (4 ml)
under N2 at 658, and the mixture was stirred for 6 – 12 h. On completion of the reaction (TLC
monitoring), the mixture was centrifuged, and the centrifugate was concentrated under reduced pressure
to give the crude product. The latter was purified by column chromatography (silica gel) to afford the
corresponding conjugate-addition product. The recovered catalyst was washed with MeOH, dried, and
reused for the next cycle. The conjugate-addition products were characterized by 1H-NMR spectroscopy
and MS.
Butyl Benzenepropanoate (Table 3, Entry 1) [10a]: 1H-NMR (300 MHz, CDCl3): 7.28 – 7.20 (m, 2 H);
7.18 – 7.05 (m, 3 H); 4.03 (t, J ¼ 6.8, 2 H); 2.92 (t, J ¼ 7.9, 2 H); 2.57 (t, J ¼ 7.9, 2 H); 1.62 – 1.50 (m, 2 H);
1.38 – 1.28 (m, 2 H); 0.92 (t, J ¼ 7.2, 3 H). ESI-MS: 229 ([M þ Na]þ).
Butyl 4-Chlorobenzenepropanoate (Table 3, Entry 2) [10a]: 1H-NMR (300 MHz, CDCl3): 7.22 (d, J ¼
8.3, 2 H); 7.11 (d, J ¼ 8.3, 2 H); 4.03 (t, J ¼ 6.8, 2 H); 2.90 (t, J ¼ 7.5, 2 H); 2.56 (t, J ¼ 7.5, 2 H); 1.62 – 1.50
.
(m, 2 H); 1.40 – 1.25 (m, 2 H); 0.92 (t, J ¼ 7.2, 3 H). EI-MS: 240 (100, Mþ ), 184 (77), 167 (36), 138 (86),
125 (85), 103 (69), 89 (32), 77 (55), 57 (62), 41 (69).
1
Butyl 4-Bromobenzenepropanoate (Table 3, Entry 3) [10b]: H-NMR (300 MHz, CDCl3): 7.20 (d,
J ¼ 8.2, 2 H); 7.08 (d, J ¼ 8.2, 2 H); 4.03 (t, J ¼ 6.8, 2 H); 2.89 (t, J ¼ 7.5, 2 H); 2.54 (t, J ¼ 7.5, 2 H); 1.64 –
1.48 (m, 2 H); 1.38 – 1.23 (m, 2 H); 0.93 (t, J ¼ 7.15, 3 H). ESI-MS: 308 ([M þ Na]þ).
Butyl 4-Methylbenzenepropanoate (Table 3, Entry 4) [10a]: 1H-NMR (200 MHz, CDCl3): 7.12 – 6.97
(m, 4 H); 4.06 (t, J ¼ 6.52, 2 H); 2.89 (t, J ¼ 7.56, 2 H); 2.57 (t, J ¼ 7.56, 2 H); 2.32 (s, 3 H); 1.67 – 1.50 (m,
2 H); 1.40 – 1.25 (m, 2 H); 0.92 (t, J ¼ 7.2, 3 H). ESI-MS: 243 ([M þ Na]þ).

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Helvetica Chimica Acta – Vol. 91 (2008)
1
Butyl 4-Methoxybenzenepropanoate (Table 3, Entry 5) [10a]: H-NMR (300 MHz, CDCl3): 7.06 (d,
J ¼ 8.3, 2 H); 6.75 (d, J ¼ 8.3, 2 H); 4.03 (t, J ¼ 6.8, 2 H); 3.75 (s, 3 H); 2.86 (t, J ¼ 7.9, 2 H); 2.54 (t, J ¼ 7.9,
.
2 H); 1.65 – 1.50 (m, 2 H); 1.40 – 1.27 (m, 2 H); 0.92 (t, J ¼ 7.2, 3 H). EI-MS: 236 (21, Mþ ), 134 (67), 121
(100), 103 (24), 91 (34), 77 (44), 41 (56).
Butyl 2-Methylbenzenepropanoate (Table 3, Entry 6) [10c]: 1H-NMR (300 MHz, CDCl3): 7.18 – 6.96
(m, 4 H); 4.06 (t, J ¼ 6.8, 2 H); 2.91 (t, J ¼ 7.9, 2 H); 2.54 (t, J ¼ 7.9, 2 H); 2.33 (s, 3 H); 1.65 – 1.50 (m, 2 H);
1.40 – 1.27 (m, 2 H); 0.92 (t, J ¼ 7.2, 3 H). ESI-MS: 243 ([M þ Na]þ).
1
Butyl 2-Fluorobenzenepropanoate (Table 3, Entry 7): H-NMR (200 MHz, CDCl3): 7.30 – 7.10 (m,
2 H); 7.10 – 6.92 (m, 2 H); 4.06 (t, J ¼ 6.9, 2 H); 2.97 (t, J ¼ 7.63, 2 H); 2.60 (t, J ¼ 7.63, 2 H); 1.69 – 1.46 (m,
2 H); 1.46 – 1.22 (m, 2 H); 0.93 (t, J ¼ 7.2, 3 H). ESI-MS: 247 ([M þ Na]þ).
1
Butyl 4-Fluorobenzenepropanoate (Table 3, Entry 8): H-NMR (300 MHz, CDCl3): 7.13 (d, J ¼ 8.1,
2 H); 6.90 (d, J ¼ 8.1, 2 H); 4.03 (t, J ¼ 6.8, 2 H); 2.90 (t, J ¼ 7.9, 2 H); 2.55 (t, J ¼ 7.9, 2 H); 1.65 – 1.44 (m,
2 H); 1.42 – 1.23 (m, 2 H); 0.92 (t, J ¼ 6.9, 3 H). ESI-MS: 247 ([ M þ Na]þ).
1
Butyl Naphthalene-1-propanoate (Table 3, Entry 9): H-NMR (200 MHz, CDCl3): 7.99 (d, J ¼ 8.08,
1 H); 7.88 – 7.73 (m, 2 H); 7.67 (d, J ¼ 8.08, 1 H); 7.55 – 7.37 (m, 3 H); 4.06 (t, J ¼ 6.6, 2 H); 3.38 (t, J ¼ 7.7,
2 H); 2.71 (t, J ¼ 7.7, 2 H); 1.65 – 1.50 (m, 2 H); 1.41 – 1.23 (m, 2 H); 0.92 (t, J ¼ 7.3, 3 H). EI-MS: 256 (56,
.
Mþ ), 200 (19), 183 (15), 154 (92), 141 (100), 129 (28), 115 (45), 41 (39).
1
Butyl 3-Formylbenzenepropanoate (Table 3, Entry 10): H-NMR (300 MHz, CDCl3): 9.98 (s, 1 H);
7.95 (s, 1 H); 7.71 (s, 1 H); 7.50 – 7.41 (m, 2 H); 4.03 (t, J ¼ 6.8, 2 H); 3.02 (t, J ¼ 7.55, 2 H); 2.64 (t, J ¼ 7.55,
2 H); 1.63 – 1.50 (m, 2 H); 1.40 – 1.25 (m, 2 H); 0.92 (t, J ¼ 7.2, 3 H). ESI-MS: 257 ([M þ Na]þ).
1
Butyl 3-Nitrobenzenepropanoate (Table 3, Entry 11): H-NMR (200 MHz, CDCl3): 8.14 – 8.01 (m,
2 H); 7.60 – 7.38 (m, 2 H); 4.03 (t, J ¼ 6.87, 2 H); 3.06 (t, J ¼ 7.56, 2 H); 2.65 (t, J ¼ 7.56, 2 H); 1.67 – 1.47 (m,
2 H); 1.40 – 1.23 (m, 2 H); 0.92 (t, J ¼ 7.2, 3 H). ESI-MS: 252 ([M þ H]þ).
1
Butyl 5-Phenylpent-4-enoate (Table 3, Entry 12): H-NMR (200 MHz, CDCl3): 7.30 – 7.20 (m, 5 H);
6.42 (d, J ¼ 14.7, 1 H); 6.23 – 6.12 (m, 1 H); 4.05 (t, J ¼ 6.8, 2 H); 2.48 – 2.54 (m, 4 H); 1.62 – 1.51 (m, 2 H);
1.39 – 1.29 (m, 2 H); 0.93 (t, J ¼ 7.2, 3 H). ESI-MS: 255 ([M þ Na).
1
Butyl Furan-3-propanoate (Table 3, Entry 13): H-NMR (300 MHz, CDCl3): 7.30 (s, 1 H); 7.20 (s,
1 H); 6.21 (s, 1 H); 4.04 (t, J ¼ 6.4, 2 H); 2.74 (t, J ¼ 7.55, 2 H); 2.52 (t, J ¼ 7.2, 2 H); 1.71 – 1.51 (m, 2 H);
.
1.51 – 1.30 (m, 2 H); 0.93 (t, J ¼ 7.2, 3 H). EI-MS: 196 (36, Mþ ), 167 (41), 138 (29), 111 (24), 96 (31), 55
(65), 41 (100).
Butyl Thiophene-3-propanoate (Table 3, Entry 14) [10a]: 1H-NMR (300 MHz, CDCl3): 7.23 – 7.17 (m,
1 H); 6.95 – 6.86 (m, 2 H); 4.05 (t, J ¼ 6.8, 2 H); 2.95 (t, J ¼ 7.5, 2 H); 2.59 (t, J ¼ 7.5, 2 H); 1.65 – 1.51 (m,
.
2 H); 1.41 – 1.30 (m, 2 H); 0.93 (t, J ¼ 7.2, 3 H). EI-MS: 212 (67, Mþ ), 156 (31), 139 (29), 113 (100), 97
(29), 41 (60).
Ethyl 4-Chlorobenzenepropanoate (Table 4, Entry 1) [11a]: 1H-NMR (300 MHz, CDCl3): 7.22 – 7.02
(m, 4 H); 4.2 (t, J ¼ 7.05, 2 H); 2.93 (t, J ¼ 7.9, 2 H); 2.56 (t, J ¼ 7.9, 2 H); 1.2 (t, J ¼ 7.2, 3 H). ESI-MS: 235
([M þ Na]þ).
1
Methyl 4-Chlorobenzenepropanoate (Table 4, Entry 2) [11b]: H-NMR (300 MHz, CDCl3): 7.22 (d,
J ¼ 8.3, 2 H); 7.10 (d, J ¼ 8.3, 2 H); 3.67 (s, 3 H); 2.93 (t, J ¼ 7.9, 2 H); 2.62 (t, J ¼ 7.9, 2 H). ESI-MS: 221
([M þ Na]þ).
tert-Butyl 4-Chlorobenzenepropanoate (Table 4, Entry 3): 1H-NMR (300 MHz, CDCl3): 7.22 (d, J ¼
8.31, 2 H); 7.10 (d, J ¼ 8.31, 2 H); 2.85 (t, J ¼ 7.55, 2 H); 2.48 (t, J ¼ 7.55, 2 H); 1.40 (s, 3 H). ESI-MS: 263
([M þ Na]þ).
4-(4-Chlorophenyl)butan-2-one (Table 4, Entry 4) [11c]: 1H-NMR (300 MHz, CDCl3): 7.26 – 7.05 (m,
4 H); 2.90 (t, J ¼ 6.8, 2 H); 2.75 (t, J ¼ 7.2, 2 H); 2.15 (s, 3 H). ESI-MS: 205 ([M þ Na]þ).
1
3-(4-Chlorophenyl)cyclohexanone (Table 4, Entry 5): H-NMR (200 MHz, CDCl3): 7.27 (d, J ¼ 8.6,
2 H); 7.12 (d, J ¼ 8.6, 2 H); 3.08 – 2.88 (m, 1 H); 2.61 – 2.23 (m, 4 H); 2.22 – 1.96 (m, 2 H); 1.92 – 1.67 (m,
.
2 H). EI-MS: 208 (26, Mþ ), 165 (24), 151 (56), 138 (49), 115 (58), 101 (58), 77 (41), 42 (100).
1
3-(4-Chlorophenyl)cyclopentanone (Table 4, Entry 6): H-NMR (300 MHz, CDCl3): 7.29 – 7.14 (m,
4 H); 3.42 – 3.36 (m, 1 H); 2.67 – 2.23 (m, 5 H); 2.02 – 1.96 (m, 1 H). ESI-MS: 217 ([M þ Na]þ).

Helvetica Chimica Acta – Vol. 91 (2008)
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Received March 31, 2008