A rhodium(I)-catalysed formal intramolecular C-C/C-H bond metathesis

Article abstract of DOI:10.1039/c5cc01415h

Phenylcyclobutanes underwent skeletal reorganisation in the presence of Wilkinson's catalyst to afford indanes through a cascade process involving chelation-assisted C-C bond cleavage and intramolecular C-H bond cleavage.

Full text of DOI:10.1039/c5cc01415h

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DOI: 10.1039/C5CC01415H
COMMUNICATION
A rhodium(I)-catalysed formal intramolecular C–C/C–
H bond metathesis
Cite this: DOI: 10.1039/x0xx00000x
Takanori Matsuda* and Itaru Yuihara
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
(b) this work
R
R
Me
Phenylcyclobutanes underwent skeletal reorganisation in the
presence of Wilkinson’s catalyst to afford indanes through a
cascade process of chelation-assisted C–C bond cleavage and
intramolecular C–H bond cleavage.
X
N
Rh(I)
(2)
X
X = CH, N
N
Transition-metal-catalysed cleavage of unreactive bonds, such
as C–H and C–C bonds, offers conceptually novel and
strategically innovative methods for synthesizing organic
molecules. Catalytic functionalisations of C−H bonds have
emerged as useful tools in organic synthesis.1 Catalytic
cleavage of C–C bonds and its synthetic utilisation have also
gained considerable attention in recent years.2
(2-Pyridylmethylene)cyclobutanes 1 used in this study were
prepared by Wittig olefination or McMurry coupling of the
corresponding
cyclobutanones.
1-Methyl-1-phenyl-3-(2-
pyridylmethylene)cyclobutane (1a) was heated in p-xylene at
150 °C in the presence of 4 mol% of Wilkinson complex
(RhCl(PPh3)3) (Scheme 1). The starting material was converted
into a single product within 30 min. The product obtained after
We recently reported rhodium(I)-catalysed arylation of 2-
(3-arylcyclobutylidene)acetates with tetraarylborates (eqn (1)).3
The reaction includes one β-carbon elimination4 and two 1,4-
rhodium migration5 events in the catalytic cycle as the C–C
bond and C–H bond cleavage processes, respectively, to furnish
spirobiindanones. In our continuing effort to discover catalytic
molecular transformations involving multiple cleavages of
unreactive bonds, we envisaged that a new reaction might occur
with the structurally relevant (2-pyridylmethylene)cyclobutanes,
possibly triggered by the pyridine-directed cleavage of the
cyclobutane C–C bond. It was found that a rhodium(I) complex
purification
was
(Z)-1,1-dimethyl-3-(2-
pyridylmethylene)indane (2a, 89% yield).6 The reaction of
phenylcyclobutane 1a involved cleavage of the cyclobutane C–
C and aromatic C–H bonds, resulting in ring expansion to
indane 2a. This can be viewed as a formal intramolecular C–
C/C–H bond metathesis reaction. On the other hand, when the
reaction was performed in mesitylene at 170 °C for 4 h, a
different result was obtained: indane with E geometry 3a and
indene 4a were formed in 96% yield, at a ratio of 7:3.
Me
Me
Me
Me
Me
catalyses
a
new
skeletal
reorganisation
of
(2-
Me
Me
4 mol%
pyridylmethylene)cyclobutanes to ring-expanded products, as
reported herein (eqn (2)). In addition, the reaction was
successfully applied to the chelation-assisted rearrangement of
cyclobutanones, affording 1-indanones via imines formed from
cyclobutanones with 2-aminopyridines.
RhCl(PPh3)3
N
N
N
N
1a
2a
89%
3a
4a
p-xylene, 150 °C, 0.5 h
(a) previous work
R
R
Me
mesitylene, 170 °C, 4 h
68%
28%
Rh(I)
NaBPh4
Scheme 1 Reaction of 1a in the presence of RhCl(PPh3)3
(1)
OMe
O
Subjecting 2a to more severe reactions conditions (170 °C)
resulted in double bond isomerisation of the (Z)-isomer 2a to 3a
and 4a, indicating that 2a was the initial product and it
underwent isomerisation at higher temperature (eqn (3)). No
O
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isomerisation was observed in the absence of the rhodium (1i) groups participate in this reaction to afford indanes (2h and
catalyst under otherwise identical conditions.
2i) possessing tetrasubstituted alkene moieties (entries 7 and 8).
View Article Online
Cyclobutane 1j–l having 4-substituted phenyl groups at the 1
4 mol% RhCl(PPh3)3
+
DOI: 10.1039/C5CC01415H
position afforded the corresponding 5-substituted indanes 2j–l
2a
3a
4a
(3)
in excellent yields (entries 9–11).12 The reaction of 3-
bromophenyl and 2-naphthyl derivatives (1m and 1n) provided
single isomers (2m and 2n), which were derived from the
cleavage of the more accessible C–H bonds of the aromatic
rings (entries 12 and 13). A heteroaromatic substituent, a 2-
thienyl group, was tolerated under the reaction condition, but
the product was obtained as a mixture of Z- and E-isomers
(entry 14).
mesitylene, 170 °C
To gain mechanistic insights into the skeletal reorganisation,
deuterium-labelling experiment was conducted with
a
cyclobutane 1a-d5 having a C6D5 group (Scheme 2). In this
experiment, one deuterium atom was transferred to the indane
methyl group of 2a-d5.
D
Me
D
Me
D
D
D
Table 1 Rhodium(I)-catalysed skeletal reorganisation of 1a
4 mol% RhCl(PPh3)3
D
Entry
1
1
Time
(h)
1
2
Yieldb
(%)
N
D
p-xylene, 150 °C, 1 h
D
N
D
D
Me
Me
Me
74
Me
1a-d5
2a-d5 76%
N
Scheme 2 Deuterium-labelling experiment with 1a-d5
N
1b
We propose a possible mechanism for the rhodium(I)-
catalysed skeletal reorganisation of 1a in Scheme 3, albeit we
have no conclusive data to support it. First, the C–C activation
is assisted by the pyridine directing group7 to form five-
membered rhodacycle A. The resulting alkylrhodium moiety of
A then undergoes σ-complex-assisted metathesis with the
pendant arene C–H bond to generate six-membered rhodacycle
B.8,9 This intramolecular C–H activation step can be viewed as
Me
2b
Me
2c
24
35
90
37
Me
Me
N
Me
N
1c
Me
3c + 4c (70:30)
a
variant of 1,4-rhodium migration.10 Finally, reductive
elimination provides 2a and the active rhodium(I) catalyst.
Me
Me
Me
3
N
2a
Rh Cl
1a
N
‡
C' Rh
1d
C'
H
H
C
3d
C' Rh
Rh
C
Me
R
R
σ-complex-assisted
metathesis
H
C
H
N
Me
H
Me
N
Rh
Cl
Rh
N
N
Cl
4
5
1e (R = Ph)
1f (R = H)
2
2e
2f
80
NRd
69
B
A
6e
23
Me
Me
Scheme 3 Proposed mechanism for the skeletal reorganisation of 1a to 2a
Me
We next investigated the rhodium-catalysed skeletal
reorganisation of a variety of alkylidenecyclobutanes 1 (Table
1). First, the effect of the pyridine moieties was briefly
evaluated (entries 1–3). The reaction of [(3-methyl-2-
pyridyl)methylene]cyclobutane 1b afforded the corresponding
indane 2b as the sole product in good yield (entry 1). On the
other hand, the reaction efficiency was reduced with 6-methyl-
2-pyridyl and 2-quinolyl groups (1c and 1d, respectively); in
these cases, the isomerised products were obtained (entries 2
and 3). The reaction of 2-pyrazyl and 8-quinolyl derivatives
was unsuccessful (not shown in the table). Thus, the original 2-
pyridyl group was critical to the success of the reaction. Then,
the effects of substituents on the cyclobutane ring were
examined. While 1,1-diphenylcyclobutane 1e was successfully
converted into indane 2e in 80% yield (entry 4), no reaction
was observed with the monosubstituted 1f (entry 5).11 Substrate
1g, containing a spiro[3.3]heptane moiety, also underwent
skeletal reorganisation to furnish spiro[cyclobutane-1,2′-indane]
2g in 69% yield (entry 6). We also explored substitution at the
vinylic position and established that methyl (1h) and phenyl
N
N
1g
2g
Me
Me
R
Me
N
R
N
7e
8e
1h (R = Me)
1i (R = Ph)
24
18
2h
2i
46
58
Me
Me
Me
N
R
R
N
9
10
11
1j (R = Me)
1k (R = Cl)
1l (R = CF3)
0.7
0.8
1.3
2j
2k
2l
84
87
82
2 | J. Name., 2012, 00, 1-3
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12e
9.5
7.5
24
45
63
54
Scheme 5 Skeletal reorganisation of 1q
Me
Me
Me
Br
Br
N
View Article Online
Considering the structural similarity between the 2-
pyridylmethylene and 2-pyridylimino gDroOuI:p1s0,.10w3e9/Ca5nCtiCc0ip14a1t5eHd
that ketimines derived from cyclobutanones and 2-
aminopyridines are also amenable to the skeletal reorganisation.
Thus, cyclobutanone 6a, 2-aminopyridines (7, 1.2 equiv), and
benzoic acid (0.5 equiv) were reacted in p-xylene at 150 °C in
the presence of the rhodium(I) catalyst (Table 2).14
Cyclobutanone ketimine G, formed in situ from the
condensation of 6a and 2-aminopyridine 7a, analogously
underwent skeletal reorganisation to afford indanone ketimine
H. Subsequent acid hydrolysis of H afforded 1-indanone 8a in
84% yield (entry 1).15 The reaction of 2-amino-3-picoline 7b
provided the product 8a in 79% yield. Other 3-aryl-3-
methylcyclobutanones 6b–f were also converted to indanones
8b–f under the same reaction conditions,16 and 3,3-
diphenylcyclobutanone 6g yielded 3-methyl-3-phenyl-1-
indanone 8g (entries 2–7). In the case of 3,3-
dibenzylcyclobutanone 6h, 1,5-rhodium migration occurred to
afford 1-tetralone 8h (entry 8).
N
1m
2m
Me
Me
Me
13
N
N
1n
1o
2n
14e
Me
Me
Me
S
S
N
N
2o + 3o (54:46)
Unless otherwise noted, cyclobutane 1 was heated in p-xylene (0.10 M) at
150 °C in the presence of 4 mol% RhCl(PPh3)3. Isolated yield. The
reaction was performed in mesitylene at 170 °C. d No reaction. e The reaction
was performed with 10 mol% RhCl(PPh3)3.
a
b
c
Table 2 Skeletal reorganisation of 6 to 8a
Me
1) 4 mol% RhCl(PPh3)3
1) 0.5 equiv PhCO2H
Me
Me
In the case of the sterically demanding 2-methylphenyl
derivative 1p, tetraline 5 was obtained in addition to the
expected products 3 and 4, indicating that 1,5-migration13 is
also possible with an appropriate substrate (Scheme 4).
R
O
+
H2N
1) p-xylene, 150 °C, 23 h
2) HCl aq.
N
O
R = H: 7a
R = Me: 7b
(1.2 equiv)
6a
8a
hydrolysis
condensation
H
Me
Me
Me
Me
4 mol% RhCl(PPh3)3
Me
Me
Rh
Cl
p-xylene, 150 °C, 19 h
N
N
N
R
skeletal reorganisation
N
H
1p
N
N
Me Me
R
Me
Me
Me
H
G
+
Entry
1
Cyclobutanone 6
Product 8
Yieldb
(%) with
7a;7b
N
N
5 36%
50%
(3p:4p = 54:46)
6a
8a
84;79
R3
Me
R3
Me
Scheme 4 Skeletal reorganisation of 1p
Me
O
R2
R1
R2
Indeed, dibenzylcyclobutane derivative 1q reacted under
identical conditions to provide tetraline 2q through
mechanism involving the formation of five-membered
rhodacycle E, σ-bond metathesis with the arene C–H bond to
form seven-membered rhodacycle F, and subsequent reductive
elimination (Scheme 5).
R1
O
6b: R1 = Me, R2 = R3 = H
6c: R1 = Cl, R2 = R3 = H
6d: R2 = Me, R1 = R3 = H
6e: R2 = Br, R1 = R3 = H
8b: R1 = Me, R2 = R3 = H
8c: R1 = Cl, R2 = R3 = H
8d: R2 = Me, R1 = R3 = H
8e: R2 = Br, R1 = R3 = H
8f: R3 = Me, R1 = R2 = H
65;72
73;76
51;77
71;61
28;40
52;66
a
2
3
4
5
6
7
6f: R3 = Me, R1 = R2 = H
Ph
Me
Ph
O
Me
4 mol% RhCl(PPh3)3
O
6g
8g
p-xylene, 150 °C, 10 h
N
Me
8
41;77
N
O
1q
2q 57%
O
8h
6h
H
Ph
Ph
a
Reaction conditions: Cyclobutanone 6 (0.200 mmol), aminopyridine 7
(0.240 mmol), benzoic acid (0.100 mmol), and RhCl(PPh3)3 (0.010 mmol, 5
mol%) were heated in p-xylene (2.0 mL) for 11–24 h. b Isolated yield.
Rh
Rh
H
N
Cl
N
In summary, we elaborated a novel ring expansion reaction
of 1-aryl-3-(2-pyridylmethylene)cyclobutanes in the presence
E
F
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J. Name., 2012, 00, 1-3 | 3

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of Wilkinson’s
catalyst,
producing
3-(2-
and 1a 19%); with 2.5 mol% [RhCl(cod)]2 and 10 mol% PPh3 in
mesitylene at 170 °C (3a 57%).
pyridylmethylene)indanes. Overall, the reaction involves
intramolecular C–C/C–H bond metathesis, which is achieved
by successive pyridine-directed cleavage of the C–C bond of
cyclobutane and C–H bond cleavage through intramolecular σ-
bond metathesis. Apart from the one-carbon ring expansion,
two-carbon ring expansion triggered by 1,5-migration occurring
in the reaction of dibenzylcyclobutane derivatives was
exploited to obtain tetraline derivatives. This work
demonstrates that transition-metal-catalysed successive
cleavage of unreactive bonds has incredible potential for use in
unique and original bond manipulation of organic compounds.
We plan to explore this strategy further, in a future study.
This work was supported by JSPS, Japan (Grant-in-Aid for
Scientific Research (C) No. 25410054) and the Sumitomo
Foundation.
7
For pyridine-directed C–C bond cleavage reactions,Vsieewe:Ar(tiac)le JO.nlWine.
DOI: 10.1039/C5CC01415H
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Notes and references
Department of Applied Chemistry, Tokyo University of Science, 1–3
Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan. E-mail:
mtd@rs.tus.ac.jp; Fax: +81 3 5261 4631
†
Electronic Supplementary Information (ESI) available: Experimental
8
9
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1
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11 No
reaction
observed
with
1,1-dipropyl-3-(2-
pyridylmethylene)cyclobutane.
12 The reaction time was crucial to obtain these products as the Z-
isomers; prolonged reaction times often resulted in the formation of
isomeric mixtures of products 2–4.
13 For examples of 1,5-rhodium migration, see: (a) M. Tobisu, J.
Hasegawa, Y. Kita, H. Kinuta and N. Chatani, Chem. Commun., 2012,
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3
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6
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Lett., 2006, 8, 3379; (b) T. Seiser, G. Cathomen and N. Cramer,
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Results with other conditions: In diglyme at 170 °C (2a 82%); with 4
mol% [Rh(cod)2]SbF6 and 8 mol% PPh3 (2a 48%, 3a 20%, 4a 5%,
16 No tetralone products could be found in the reaction of 6f, indicating
that the benzylic C–H bond is not susceptible towards activation
under this reaction conditions.
4 | J. Name., 2012, 00, 1-3
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