Addition/ring-opening reaction of organoboronic acids to cyclobutanones catalyzed by rhodium(I)/P(t-Bu)3 complex

Article abstract of DOI:10.1246/bcsj.78.1528

An addition/ring-opening reaction of aryl- and alkenylboronic acids to cyclobutanones took place in 1,4-dioxane at 100 °C in the presence of a rhodium(I) catalyst bearing tri-t-butylphosphine, affording ring-opened ketones. Mechanistically, the reaction proceeded through the addition of an organorhodium species to the carbonyl group of a cyclobutanone and a subsequent ring-opening of the resulting rhodium cyclobutanolate through β-carbon elimination. A deuterium-labeling experiment revealed that an alkylrhodium species generated by the β-carbon elimination underwent successive β-hydride elimination/re-addition processes to form the η³-oxaallylrhodium intermediate, which was readily protonated to afford the product.

Full text of DOI:10.1246/bcsj.78.1528

1528 Bull. Chem. Soc. Jpn., 78, 1528–1533 (2005)
Ó 2005 The Chemical Society of Japan
Addition/Ring-Opening Reaction of Organoboronic Acids
to Cyclobutanones Catalyzed by Rhodium(I)/P(t-Bu)3 Complex
ꢀ
Takanori Matsuda, Masaomi Makino, and Masahiro Murakami
Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering,
Kyoto University, Katsura, Kyoto 615-8510
Received February 3, 2005; E-mail: murakami@sbchem.kyoto-u.ac.jp
ꢁAn addition/ring-opening reaction of aryl- and alkenylboronic acids to cyclobutanones took place in 1,4-dioxane at
100 C in the presence of a rhodium(I) catalyst bearing tri-t-butylphosphine, affording ring-opened ketones. Mechanis-
tically, the reaction proceeded through the addition of an organorhodium species to the carbonyl group of a cyclobuta-
none and a subsequent ring-opening of the resulting rhodium cyclobutanolate through ꢀ-carbon elimination. A deuteri-
um-labeling experiment revealed that an alkylrhodium species generated by the ꢀ-carbon elimination underwent succes-
sive ꢀ-hydride elimination/re-addition processes to form the ꢁ3-oxaallylrhodium intermediate, which was readily pro-
tonated to afford the product.
Carbon–carbon bond forming reactions with organoboron
compounds have gained considerable interest in synthetic or-
ganic chemistry because of the ease of accessibility, substan-
tial stabilities, and low environmental impact of organoboron
compounds compared with other conventional organometal-
lics, like organolithium and organotin reagents.1 Especially,
the palladium-catalyzed cross-coupling reaction of aryl- and
alkenylboron reagents2 has found wide applications in industri-
al processes as well as in laboratory syntheses. Recently, new
rhodium-catalyzed addition reactions of arylboron compounds
to unsaturated functionalities have been developed.3 An aryl-
rhodium species, generated from arylboron compounds via
transmetalation, adds to ꢂ,ꢀ-unsaturated carbonyl com-
pounds,4 aldehydes,5 imines,6 alkynes,7 and alkenes.8
On the other hand, carbon–carbon bond cleavage reactions
by means of transition metals have also attracted much atten-
tion. They achieve an otherwise inaccessible organic transfor-
mation.9 We have explored the catalytic carbon–carbon bond
cleavage reactions of cyclobutanones, wherein a rhodium(I)
complex inserts between the carbonyl carbon and the ꢂ-car-
bon.10 In search of a rhodium complex with higher catalytic
activity, we envisaged that an arylrhodium(I) species, which
is more electron-rich than a rhodium(I) halide, could be more
easily oxidized to a rhodium(III) species.11 Hence, its insertion
into the carbon–carbon single bond would be more facilitated.
Thus, we investigated the rhodium-catalyzed reaction of cyclo-
butanones with arylboronic acids. We report here a full ac-
count of the rhodium-catalyzed addition/ring-opening reaction
of cyclobutanones with organoboronic acids.12 The related
reaction with a benzocyclobutenone is also described.
and tri-t-butylphosphine (Rh:P = 1:2) (Table 1, Entry 1).
The cyclobutanone 1a was consumed within 30 min, and a
mixture of the 1,2-addition product 4aa and the ring-opened
ketone 3aa was formed, with the former predominating. As
the reaction progressed, 4aa decreased, and after heating for
8 h, 3aa was formed as the exclusive product. This observation
indicated that cyclobutanol 4aa was initially produced, and
then underwent isomerization to the ketone 3aa under the re-
action conditions (vide infra). The reaction required three
equivalents of boronic acid and 5 mol % of rhodium catalyst
to reach completion. Reducing the amount of 2a decreased
the yield (Entries 2 and 3). With lower catalyst loadings
(1 or 3 mol %), the reaction was incomplete even after 45 h,
probably due to deterioration of the catalyst (Entries 4 and
5). No reaction occurred with the use of [Rh(acac)(cod)] and
[Rh(OH)(cod)]2 having cycloocta-1,5-diene (cod) instead of
ethylene ligands (Entries 6 and 7). An Rh:P ratio of 1:2 gave
the best yield of 3aa (Entries 8 and 9). Tricyclohexylphosphine
showed moderate activity, while triphenylphosphine failed to
promote the reaction (Entries 10 and 11). The reaction temper-
ature was also important to obtain 3aa in high yield. Lowering
the reaction temperature led to the predominant formation of
4aa, suggesting that conversion from 4aa to 3aa requires a
higher temperature (Entries 12–14). The addition of water to
the solvent (1,4-dioxane:water = 10:1) retarded the reaction
(Entry 15).
The results obtained with sterically more demanding o-
tolylboronic acid (2b) are given in Table 2. The reaction of
cyclobutanone 1a with 2b was sluggish (Table 2), and 1a
remained even after 48 h (Entry 1). To our delight, the addi-
tion of an inorganic base as an additive (1 equiv to 1a) facili-
tated the reaction. In particular, the use of Cs2CO3 led to the
quantitative conversion of 1a within 3 h.
Next, the reactivity of the isolated tertiary cyclobutanol was
examined. When 4aa and 4ab13 were heated under identical
conditions, the ring-opening reaction took place to afford the
Results and Discussion
A mixture of 3-phenylcyclobutanone (1a) and phenylboron-
ic acid (2a, 3.0 equiv to 1a) in 1,4-dioxane was heated at 100
ꢁC in the presence of a rhodium(I) complex (5 mol %) gener-
ated in situ from [Rh(acac)(C2H4)2] (acac = acetylacetonato)
Published on the web August 4, 2005; DOI 10.1246/bcsj.78.1528

T. Matsuda et al.
Bull. Chem. Soc. Jpn., 78, No. 8 (2005) 1529
Table 1. Reaction of 3-Phenylcyclobutanone (1a) with Phenylboronic Acid (2a) in the Presence of Rhodium(I)–
Phosphine CatalystsaÞ
Ph
Ph
Ph
Rh cat.
dioxane
+
+
PhB(OH)2
Ph
Ph
O
OH
4aa
O
1a
2a
3aa
Entry
1
2a/equiv
Rh complex/mol %
[Rh(acac)(C2H4)]2/5
Phosphine ligand/mol %
Temp
ꢁ
Time/h
3aa/%
4aa/%
3.0
P(t-Bu)3/10
100 C
0.5
2
8
15
52
97
80
44
—
ꢁ
ꢁ
2
3
2.0
1.2
[Rh(acac)(C2H4)]2/5
[Rh(acac)(C2H4)]2/5
P(t-Bu)3/10
P(t-Bu)3/10
100 C
100 C
8
8
82
67
—
—
ꢁ
ꢁ
4
5
3.0
3.0
[Rh(acac)(C2H4)]2/3
[Rh(acac)(C2H4)]2/1
P(t-Bu)3/6
P(t-Bu)3/2
100 C
100 C
45
45
12
2
55
35
ꢁ
ꢁ
6
7
3.0
3.0
[Rh(acac)(cod)]/5
[Rh(OH)(cod)]2/2.5
P(t-Bu)3/10
P(t-Bu)3/10
100 C
100 C
24
24
0
0
0
—
ꢁ
ꢁ
8
9
3.0
3.0
[Rh(acac)(C2H4)]2/5
[Rh(acac)(C2H4)]2/5
P(t-Bu)3/5
P(t-Bu)3/15
100 C
100 C
8
8
68
78
29
18
ꢁ
ꢁ
10
11
3.0
3.0
[Rh(acac)(C2H4)]2/5
[Rh(acac)(C2H4)]2/5
P(c-Hex)3/10
PPh3/10
100 C
100 C
24
24
65
5
14
—
ꢁ
ꢁ
12
13
14
3.0
3.0
3.0
[Rh(acac)(C2H4)]2/5
[Rh(acac)(C2H4)]2/5
[Rh(acac)(C2H4)]2/5
P(t-Bu)3/10
P(t-Bu)3/10
P(t-Bu)3/10
80 C
60 C
rt
8
8
8
38
11
3
53
89
55
ꢁ
15bÞ
3.0
[Rh(acac)(C2H4)]2/5
P(t-Bu)3/10
100 C
8
24
12
34
63
43
a) Unless otherwise noted, the reaction was carried out with 3-phenylcyclobutanone (1a, 0.50 mmol), phenylboronic
acid (2a, 1.50 mmol) in 1,4-dioxane (1.0 mL) in the presence of rhodium catalysts, and the yields were determined
by GC analysis. b) The reaction was carried out in 1,4-dioxane–H2O (10:1).
Table 2. Effect of Base on the Rhodium-Catalyzed Reaction
of 1a with o-Tolylboronic Acid 2baÞ
Ph
R
Ph
cat. Rh(I)−P(t-Bu)3
dioxane, 100 °C
5 mol% [Rh(acac)(C2H4)2]
OH
10 mol% P(t-Bu)3
O
R
+
1a
B(OH)2
4aa: R = H
4ab: R = Me
without Cs2CO3, 16 h 3aa quant. (NMR)
1 equiv base
with Cs2CO3, 48 h
3ab 89%
dioxane, 100 °C
2b
Scheme 1.
Ph
Ph
+
alyst, suggesting that the ring-opening process was also cata-
lyzed by rhodium. These results are consistent with our pro-
posed mechanism for the reaction of 1 with 2 (vide infra).
In contrast to boronic acid 2c, the corresponding boronic es-
ter 20c failed to react with 1a in anhydrous 1,4-dioxane. The
reaction of 20c worked well, however, in a mixed solvent of
1,4-dioxane–water (1:1). It is likely that the reaction requires
protic hydrogens to regenerate the rhodium catalyst from the
produced rhodium compound. In the reaction with boronic
acid 2, protons of the boronic acid moiety served as the proton
source. These results led us to carry out a deuterium experi-
ment using 20c in order to obtain mechanistic information.
When the reaction of 20c was carried out in 1,4-dioxane–
D2O (1:1), deuterium was incorporated not at the expected
ꢃ-position of the produced ketone, but exclusively at the ꢂ-
position (Scheme 2).14
OH
O
3ab
4ab
Entry
Base
Time/h
3ab/%bÞ
4ab/%bÞ
1
2
3
4
none
NaHCO3
KF
48
24
24
3
24
3
51
54
81
75
94
90
99
15
12
—
11
—
10
—
KOH
5
Cs2CO3
24
a) A mixture of 1a (0.50 mmol), 2b (1.50 mmol),
[Rh(acac)(C2H4)2] (0.025 mmol), P(t-Bu)3 (0.050 mmol),
and base (0.50 mmol) was heated in 1,4-dioxane (1.0 mL) at
ꢁ
100 C. b) Determined by GC.
corresponding butyrophenone derivatives, 3aa and 3ab, re-
spectively, in high yield (Scheme 1). On the other hand, the
ring-opening failed to occur in the absence of the rhodium cat-
The formation of the intermediary cyclobutanol 4 suggested
that the insertion of rhodium into the carbonyl carbon and the
ꢂ-carbon giving a five-membered cyclic acylrhodium inter-

1530 Bull. Chem. Soc. Jpn., 78, No. 8 (2005)
Addition/Ring-Opening of Cyclobutanones
Ph
D
O
Ar
cat. Rh(I)−P(t-Bu)3
+
1a
B
Ar
OH
100 °C
O
4
O
O
O
3
2´c
Ar B(OH)2
1
3ac-d
dioxane
dioxane−D2O (1:1)
no reaction
83% (92% D)
2
[Rh] Ar
5
Scheme 2.
Ar
Ar
O
O
[Rh]
[Rh]
[Rh]
6
8
Table 3. Rhodium-Catalyzed Addition/Ring-Opening Re-
action of Cyclobutanones 1 with Phenylboronic Acid (2a)aÞ
5 mol% [Rh(acac)(C2H4)2]
R
R
10 mol% P(t-Bu)3
Ph
+
2a
H
[Rh]
Ar
H
1 equiv Cs2CO3
dioxane, 100 °C
O
H
(3 equiv)
O
Ar
[Rh]
1
3
O
O
Entry Cyclobutanone 1
Product 3
Yield/%bÞ
95
Ar
7
1
1a
3aa
O
2-naphthyl
Scheme 3.
O
Ph
2
78
79
75
O
1b
5 mol% [Rh(acac)(C2H4)2]
3ba
p-(t-Bu)C6H4
10 mol% P(t-Bu)3
Ph
Ph
Ph
Ph
Ph
+
2a
O
1 equiv Cs2CO3
dioxane, 100 °C
OH
t-Bu
O
Ph
1h
4ha 42%
3
4
O
1c
Scheme 4.
3ca
n-C8H17
tion. Scheme 3 summarizes our mechanistic hypothesis. The
catalytic cycle involves (i) the addition of arylrhodium species
5 to 1, (ii) ring-opening of rhodium cyclobutanolate 6 by ꢀ-
carbon elimination to alkylrhodium intermediate 7,16 (iii) rep-
etition of a ꢀ-hydride elimination/re-addition process17 twice
leading to ꢁ3-oxaallylrhodium 8, and (iv) protonolysis and
transmetalation with boronic acid 2, generating butyrophenone
3 and arylrhodium species 5.
n-C8H17
O
Ph
O
1d
3da
Ph
Ph
Ph
Ph
+
Ph
O
O
O
5
52
Other examples of the rhodium-catalyzed addition/ring-
opening reaction of a series of cyclobutanones under the opti-
mized reaction conditions using Cs2CO3 are listed in Table 3.
The reaction of 3-aryl or 3-alkylcyclobutanones 1a–d afforded
the corresponding products 3aa–3da in good yield (Entries 1–
4). In the reaction of 2-phenylcyclobutanone (1e), ꢀ-carbon
elimination took place either at the C(1)–C(4) or C(1)–C(2)
bond to yield two products, 3ea and 30ea, with the former pre-
dominating (Entry 5). Regioselective ꢀ-carbon elimination at
the less-substituted carbon was observed with indene-derived
cyclobutanone 1f to give product 3fa as a mixture of cis-
and trans-isomers (Entry 6). This stereoisomerism is consis-
tent with the proposed mechanism involving an ꢁ3-oxaallyl-
rhodium species. No reaction occurred with 2,2-disubstituted
cyclobutanone 1g due to steric reasons (Entry 7).
1e
1f
3ea + 3′ea (77:23)c),d)
O
Ph
O
6
7
76
0
3fa (cis:trans = 28:27)c)
Ph Ph
O
1g
a) All reaction were carried out with cyclobutanone 1 (0.50
mmol), phenylboronic acid (2a, 1.50 mmol), [Rh(acac)-
(C2H4)2] (5 mol %), P(t-Bu)3 (10 mol %), and Cs2CO3 (0.50
mmol) in 1,4-dioxane (1.0 mL) at 100 ꢁC for 24–72 h. b) Iso-
lated yield. c) Obtained as a mixture of isomers. The ratios
were determined by 1H NMR. d) Cyclobutanol was also isolat-
ed in 10% yield.
3,3-Disubstituted cyclobutanone 1h gave cyclobutanol 4ha
in 42% yield upon a treatment with 2a (Scheme 4).18 ꢀ-Carbon
elimination from 6 might be sterically hampered by disubstitu-
tion at the 3-position.
Analogous reactions were attempted with other less-strained
cyclic ketones. No reaction occurred with cycloalkanones with
five- to eight-membered rings under the present reaction con-
ditions. The increased reactivity of cyclobutanones is ascribed
to the strain of the four-membered ring;19 the carbonyl sp2 car-
mediate is unlikely. Rather, we assume a mechanism that in-
volves the 1,2-addition of arylrhodium to the carbonyl group15
of cyclobutanone 1, and a subsequent ring-opening of the
resulting rhodium cyclobutanolate through ꢀ-carbon elimina-

T. Matsuda et al.
Bull. Chem. Soc. Jpn., 78, No. 8 (2005) 1531
Table 4. Rhodium-Catalyzed Addition/Ring-Opening Re-
action of 3-Phenylcyclobutanone (1a) with Boronic
Acids 2aÞ
5 mol% [Rh(acac)(C2H4)2]
10 mol% P(t-Bu)3
B(OH)2
+
1a
Bu
1 equiv Cs2CO3
dioxane, 100 °C, 3 days
2j
R
5 mol% [Rh(acac)(C2H4)2]
10 mol% P(t-Bu)3
Ph
R
+
1a
B(OH)2
Ph
Ph
1 equiv Cs2CO3
Pr
dioxane, 100 °C
O
3
+
Bu
2 (3 equiv)
O
O
Entry Boronic acid 2 Time/h
Product 3
Yield/%bÞ
72
3aj 20%
3´aj 5%
1
2b
24
3ab
Scheme 5.
B(OH)2
2
24
3ac
82
Ph
O
O
2c
+
1a
B
Ph
dioxane
100 °C, 72 h
B(OH)2
OH
9
21%
3
24
24
72
77
O
2d
3ad
Ph
Ph
Rh(I)
O
O
B
100 °C
4cÞ
3ac
73
38
O
O
10 56%
2′c
Scheme 6.
Ph
B(OH)2
Ph
5
O
Rh cat.
+
+
2a
O
Ph
toluene
90 °C
2e
2f
O
3ae
Cl
Cl
Cl
O
Ph
OMe
11
12
13
MeO
B(OH)2
68%
(12:13 = 94:6)
6
7
8
9
24
120
72
42
39
44
47
O
Scheme 7.
3af
Ph
CF3
can participate in the addition/ring-opening with cyclobuta-
none 1a. Three isomeric tolylboronic acids 2b–d gave the cor-
responding butyrophenones 3ab–3ad in good yield (Entries 1–
3). Pinacol ester 20c also underwent the reaction in the pres-
ence of water (Entry 4). Sterically bulkier mesitylboronic acid
2e was less reactive, giving a 38% yield of 3ae (Entry 5). Only
moderate yields were obtained with both electron-donating and
-withdrawing aromatic substituents (Entries 6 and 7). Some
alkenylboronic acids, 2h and 2i, afforded the corresponding
ꢂ,ꢀ-unsaturated ketones, 3ah (44%) and 3ai (47%), respec-
tively (Entries 8 and 9).
In the reaction of 1a with (E)-hex-1-enylboronic acid (2j), a
small amount of by-product 30aj was isolated along with ordi-
nary product 3aj (Scheme 5). It is likely that the by-product
was formed via the isomerization of 2j to allylboronic acid,
the allylation of cyclobutanone 1a, the rhodium-catalyzed
ring-opening of the tertiary cyclobutanol, and isomerization
of the resulting ꢀ,ꢃ-unsaturated ketone to ꢂ,ꢀ-unsaturated
ketone 30aj under the reaction conditions. In fact, when allyl-
boronic ester 9 was reacted with 1a, the ꢂ,ꢀ-unsaturated
ketone 10 was produced through rhodium-catalyzed ring-
opening/isomerization (Scheme 6).
F3C
B(OH)2
O
2g
3ag
Ph
B(OH)2
Ph
Ph
O
2h
3ah
Ph
B(OH)2
t-Bu
t-Bu
72
O
2i
3ai
a) All reaction were carried out with 3-phenylcyclobutanone
(1a, 0.50 mmol), boronic acid 2 (1.50 mmol), Rh(acac)-
(C2H4)2 (5 mol %), P(t-Bu)3 (10 mol %), and Cs2CO3 (0.50
mmol) in 1,4-dioxane (1.0 mL) at 100 ꢁC. b) Isolated yield
by preparative TLC. c) The reaction was carried out in 1,4-
dioxane–H2O (1.0 mL:1.0 mL).
bon changes to an sp3 carbon upon adding the arylrhodium,
thereby diminishing the ring strain. The ꢀ-carbon elimination
step would also be facilitated by release of the ring strain.
As summarized in Table 4, various organoboronic acids 2
When benzocyclobutenone 11 was subjected to identical
reaction conditions, two ring-opened products, 2-phenylaceto-
phenone 12 and 2-methylbenzophenone 13, were obtained in a
ratio of 94:6 (Scheme 7).

1532 Bull. Chem. Soc. Jpn., 78, No. 8 (2005)
Addition/Ring-Opening of Cyclobutanones
Ph
Ph
+
a
H
References
a) H. C. Brown, ‘‘Organic Syntheses via Boranes,’’ Wiley-
O
O
1
[Rh]
O
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Cl
Cl
Cl
11
[Rh] Ph
15
12
2
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[Rh]
Ph
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[Rh]
Ph
O
3
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14
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thermal
ring-opening
isomerization
Ph
Ph
c
OH
Cl
Cl
OH
18
17
Scheme 8.
A mechanistic explanation for the formation of 12 and 13
from 11 is illustrated in Scheme 8. The addition of arylrho-
dium to 11 gives the rhodium alcoholate 14. ꢀ-Carbon elimi-
nation with the aromatic ipso carbon produces phenylrhodium
15. Protonolysis of 15 furnishes 12 (path a). For the minor
product 13, there are two possible reaction pathways. Like
path a, ꢀ-carbon elimination with the benzylic carbon of 14 af-
fords 16, and then the protonolysis of 16 gives 13 (path b). On
the other hand, 14 or protonated tertiary alcohol 17 might un-
dergo a thermal ring-opening reaction in a concerted manner to
give the o-quinodimethane derivative 18 (path c).20 Prototropic
isomerization would also lead to 13. In fact, benzocyclobute-
nol 17, independently prepared, underwent thermal ring-open-
5
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ꢁ
ing at 90 C to give 13, suggesting that the pathway is also
conceivable.
8
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2002, 390.
Conclusion
We developed a rhodium(I)-catalyzed addition/ring-open-
ing reaction of aryl- and alkenylboronic acids to cyclobuta-
nones. The reaction gave ring-opened ketones through ꢀ-car-
bon elimination from a rhodium cyclobutanolate and succes-
sive ꢀ-hydride elimination/re-addition of the resulting alkyl-
rhodium species, leading to an ꢁ3-oxaallylrhodium. It was
demonstrated that ꢀ-carbon elimination from transition metal
cyclobutanolates occurs with rhodium(I) as well as with palla-
dium(II).16 The synthetic potential of the ring-opening protocol
of transition metal cycloalkanolates will be further exploited.
9
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This work was supported by a Grant-in-Aid for Young Sci-
entists (B) (No. 15750085) from the Ministry of Education,
Culture, Sports, Science and Technology, Japan. We thank
H. Ushitora for obtaining the HRMS data.
Supporting Information
Experimental procedures and characterization data for new
compounds. This material is available free of charge on the
Web at http://www.csj.jp/journals/bcsj/.
11 L. S. Hegedus, S. M. Lo, and D. E. Bloss, J. Am. Chem.
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T. Matsuda et al.
Bull. Chem. Soc. Jpn., 78, No. 8 (2005) 1533
12 A part of this work appeared as a preliminary communica-
tion: T. Matsuda, M. Makino, and M. Murakami, Org. Lett., 6,
1257 (2004).
13 Tertiary cyclobutanols 4aa and 4ab were synthesized inde-
pendently by the reaction of 1a with the corresponding Grignard
reagents and used as a diastereomeric mixture (4aa: cis:trans =
96:4; 4ab: cis:trans = 74:26).
17 For transition metal–hydride elimination followed by
re-addition with the opposite regiochemistry, see: a) K. Yoshida
and T. Hayashi, J. Am. Chem. Soc., 125, 2872 (2003). b) S. V.
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18 Ring-opened ketone was also isolated in 2%.
19 For synthetic applications of cyclobutane derivatives, see:
J. C. Namyslo and D. E. Kaufmann, Chem. Rev., 103, 1485
(2003).
14 The diastereomer ratio of 3ab-d was determined to be
1
64:36 by H NMR; however, the relative stereochemistries were
not determined.
15 Insertion of aldehyde into rhodium–aryl bond was ob-
served by Hartwig. C. Krug and J. F. Hartwig, J. Am. Chem.
Soc., 124, 1674 (2002).
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