Rhodium(i)-catalysed intermolecular alkyne insertion into (2-pyridylmethylene)cyclobutenes

Article abstract of DOI:10.1039/c6ob00734a

Cyclobutenes with 2-pyridylmethylene groups at the 3 position underwent an intermolecular alkyne insertion reaction in the presence of a rhodium(i) catalyst at 170 °C to afford substituted benzenes. Among the different 2-pyridylmethylene groups examined, 3-methyl-2-pyridyl derivatives showed superior activity and readily coupled with various alkynes, including sterically demanding, heteroaromatic and terminal alkynes.

Full text of DOI:10.1039/c6ob00734a

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DOI: 10.1039/C6OB00734A
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Rhodium(I)-catalysed intermolecular alkyne insertion into (2-
pyridylmethylene)cyclobutenes†‡
Received 00th January 20xx,
Accepted 00th January 20xx
Takanori Matsuda* and Takeshi Matsumoto
DOI: 10.1039/x0xx00000x
www.rsc.org/
R
OH
R
OH
Cyclobutenes with 2-pyridylmethylene groups at the 3 position
underwent an intermolecular alkyne insertion reaction in the
presence of a rhodium(I) catalyst at 170 °C to afford substituted
benzenes. Among the different 2-pyridylmethylene groups
examined, 3-methyl-2-pyridyl derivatives showed superior activity
and readily coupled with various alkynes, including sterically
demanding, heteroaromatic and terminal alkynes.
1) β-carbon elimination
R
R
(a)
(b)
2) alkyne insertion
R
R
N
N
1) directed C–C bond cleavage
2) skeletal rearrangement
Me
R
Ph
R
The catalytic cleavage of the carbon–carbon bond and its
utilisation in organic synthesis has become one of the most
intensely researched fields in recent years.1 In particular, the
cleavage of carbon–carbon bonds of small ring compounds and
the subsequent migratory insertion of unsaturated molecules,
N
1) directed C–C bond cleavage
2) alkyne insertion
N
R
(c)
R
R
R
such as alkynes and carbon monoxide, provide unique access Scheme 1 Our rhodium-catalysed reactions of cyclobutanes and cyclobutenes.
to ring expansion reactions. Insertion reactions of four-
membered ring ketones2 and alcohols3 are well established;
3-(2-Pyridylmethylene)-1-phenylcyclobutene (1a) was
however, there are only limited examples of successful
insertions into cyclobutanes that lack heteroatom
substituents.4
We recently reported the alkyne insertion reaction of
cyclobutenols (Scheme 1, (a))3b and the skeletal
rearrangement reaction of (2-pyridylmethylene)cyclobutanes
(Scheme 1, (b)),5 both of which are catalysed by rhodium(I). In
this study, we focus on (2-pyridylmethylene)cyclobutenes. We
selected as the initial substrate for this study, and it was
prepared by the Wittig reaction of 3-phenylcyclobut-2-enone
with (2-pyridylmethylene)phosphorane.6 The reaction gave an
E/Z mixture of 1a’s stereoisomers, which were separated by
column chromatography. Mixing (E)-1a and diphenylacetylene
(2a) in the presence of Wilkinson’s catalyst at 170 °C in
mesitylene led to the formation of 1,2,5-triphenyl-3-(2-
pyridylmethyl)benzene 3aa (77%) (Scheme 2). The use of the
corresponding (Z)-isomer also gave 3aa, but with a slight
decrease in yield (57%). Based on this result, (2-
pyridylmethylene)cyclobutenes 1 were used as stereoisomeric
mixtures for further study.
have developed
a
rhodium(I)-catalysed intermolecular
cyclobutene–alkyne coupling reaction, which proceeded via an
alkyne insertion into the C(3)–C(4) bond of cyclobutene to
form a 3-(2-pyridylmethylene)-1,4-cyclohexadiene (Scheme 1,
(c)). The initial coupling product then undergoes double bond
isomerisation to create a benzene ring skeleton.
Ph
N
5 mol% RhCl(PPh3)3
N
+
Ph
Ph
mesitylene, 170 °C, 7 h
Ph
Ph
Ph
1a
2a (1.2 equiv)
3aa
77% from (E)-1a
57% from (Z)-1a
Scheme 2 Intermolecular insertion of 2a into (E)- and (Z)-1a in the presence of
RhCl(PPh3)3.
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A proposed mechanism for the rhodium(I)-catalysed
intermolecular alkyne insertion into cyclobutene is depicted in
Scheme 3. The cyclobutenes, (E)- and (Z)-1a, are in equilibrium
under the stipulated reaction conditions (mesitylene, 170 °C).7
The rhodium(I) metal, guided by the pyridine nitrogen (as in A),
would initially insert into the C(3)–C(4) bond of (E)-1a to
produce the five-membered rhodacycle B.8 Next, the alkyne 2a
coordinates to the rhodium metal (as in C) and its migratory
insertion into the Rh–C(sp3) bond results in a ring expansion to
form the seven-membered rhodacycle D. Finally, a reductive
elimination yields the (2-pyridylmethylene)cyclohexadiene E,
which aromatises by double bond isomerisation to 3aa under
the reaction conditions used.9
Table 1 Insertion of diphenylacetylene (2a) into cyclobutenes 1
View Article Online
DOI: 10.1039/C6OB00734A
R
R
Ph
Ph
5 mol% RhCl(PPh3)3
mesitylene, 170 °C
N
N
+
Ph
Ph
Ph
Ph
1
3
2a (1.2 equiv)
Entry
1
Deviation from the
Time
(h)
6
3
Yield
(%)a
77
standard conditions
1
2
3ba
3ca
1b
1c
N
Me
Me
5
4
83
93
N
Me
Me
3
3da
1d
N
N
N
Ph
N
N
4
5
3
5
3ea
3fa
86
27
Rh
Ph
Ph
Ph
Ph
1e
Rh
N
N
Ph
Ph
Ph
Ph
Ph
Ph
Ph
1f
D
E
3
C
N
Ph
Rh
6
11
3ga
NRb
1g
1d
1d
1d
1d
1d
1d
1d
Ph
7
8
9
10
11
12
13
p-xylene, 150 °C
PhCl, 130 °C
5
16
6
8
8
3da
3da
3da
3da
3da
3da
3da
84
45
38
53
56
N
2
N
N
N
toluene, 110 °C
Rh
Rh
c
[RhCl(cod)]2
RhCl(CO)(PPh3)2
2.5 mol% RhCl(PPh3)3
without Rh catalyst
Ph
Ph
(E)-1a
Ph
(Z)-1a
Ph
6
5
73
B
A
NRb
Scheme 3 Proposed mechanism of the rhodium-catalysed reaction between 1a
and 2a.
a Isolated yield. b No reaction. c 2.5 mol% of dimer (5 mol% Rh) was used.
The rhodium(I)-catalysed alkyne insertion of 1 was then
screened under various reaction parameters, and the results
are summarised in Table 1. Firstly, the effect of pyridine ring
substituents was evaluated. Introducing methyl substituents
on the pyridine ring of 1 increased the yield of the insertion
products 3 (entries 1–3) and the highest yield was achieved
with 3-methyl-2-pyridyl 1d (entry 3). While 2-quinolyl 1e gave
a satisfactory result (entry 4), the reaction was far less efficient
with 2-pyrazyl 1f (entry 5). More importantly, compound 1g,
which lacks a nitrogen atom, failed to react, hence implying
that the reaction is initiated by pyridine-directed C–C bond
cleavage rather than cyclobutene ring opening10 and
subsequent oxidative cyclisation with the rhodium(I)
complex.11 The effect of reaction temperature was obvious,
with the yields decreasing with the reaction temperature
(entries 7–9).12 Among the rhodium catalysts tested,
RhCl(PPh3)3 exhibited the highest catalytic activity (entries 10
and 11), and a 73% yield of 3da was obtained with a catalyst
loading of 2.5 mol% RhCl(PPh3)3 (entry 12). Unsurprisingly, no
reaction was observed in the absence of the rhodium catalyst
(entry 13).
Next, various symmetrical alkynes 2 were subjected to the
alkyne
insertion
reaction
with
(2-
pyridylmethylene)cyclobutene
1a
and (3-methyl-2-
pyridylmethylene)cyclobutene 1d (Table 2). In all cases
examined, 3-methyl-2-pyridyl 1d produced the six-membered
ring products 3 in higher yields than 2-pyridyl 1a. Cyclobutenes
1a and 1d reacted with the diphenylacetylene derivatives 2b–
2f to afford the 1,2,5-triaryl-3-(2-pyridylmethyl)benzenes 3ab–
af in 42–62% and 3db–3df in 61–86% yields, respectively
(entries 1–5). The insertion of sterically demanding alkynes (2g
and 2h) to 1d was also possible (entries 6 and 7). We also
established that heteroaromatic and aliphatic alkynes (2i and
2j) reacted with 1a and 1d to afford the corresponding
coupling products (entries 8 and 9).
Table 2 Insertion of various symmetrical internal alkynes 2 into 1a and 1da
R1
R1
R2
5 mol% RhCl(PPh3)3
mesitylene, 170 °C
N
N
+
R2
R2
R2
Ph
Ph
1a (R1 = H)
2 (1.2 equiv)
3
1d (R1 = Me)
2 | J. Name., 2012, 00, 1-3
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R1 = H
3 (%Yieldb)
3ab (57)
3ac (42)
3ad (47)
3ae (62)
3af (59)
3ag (–c)
3ah (–c)
3ai (46)
3aj (57)
R1 = Me
3 (%Yieldb)
3db (74)
3dc (61)
3dd (83)
3de (86)
3df (84)
3dg (61)
3dh (75)
3di (87)
3dj (82)
Entry
1 (R1, R2)
3 (%Yieldb)
View Article Online
Entry
2 (R2)
DOI: 10.1039/C6OB00734A
1
2
3
4
5
6
7
1h (H, 4-MeC6H4)
1i (Me, 4-MeC6H4)
1j (H, 4-MeOC6H4)
1k (Me, 4-MeOC6H4)
1l (H, 4-ClC6H4)
3ha (69)
1
2
3
4
5
6
7
8
9
2b (4-MeC6H4)
2c (4-MeOC6H4)
2d (4-ClC6H4)
2e (4-CF3C6H4)
2f (3,5-Me2C6H3)
2g (2-MeC6H4)
2h (1-naphthyl)
2i (2-thienyl)
2j (Bu)
3ia (87)
3ja (58)
3ka (85)
3la (52)
3ma (86)
3na (69)
1m (Me, 4-ClC6H4)
1n (H, 1-naphthyl)
a Reaction conditions: 1, 2a (1.2 equiv), RhCl(PPh3)3 (5 mol%), mesitylene, 170 °C,
5–24 h. b Isolated yield.
a Reaction conditions: 1 (0.10 mmol), 2a (0.12 mmol, 1.2 equiv), RhCl(PPh3)3 (5.0
µmol, 5 mol%), mesitylene (0.50 mL, 0.2 M), 170 °C for 4–16 h. b Isolated yield. c
Not examined.
The reaction between cyclobutenes 1o and 1p (i.e.
compounds with tetrasubstituted alkene moieties) and 2a
afforded 3oa and 3pa, both of which have substituents at the
benzylic position (Scheme 4).
Results of the reaction with unsymmetrical alkynes are
shown in Table 3. To our surprise, the reaction of 1d and
phenylacetylene (2k) was efficient, and gave the 1,4-diphenyl
derivative 3dk in 80% yield, with no other possible isomer
being found in the crude reaction mixture (entry 1).13 However,
the reaction with the aliphatic terminal alkyne 2l gave 3dl in
53% yield as a 3.2:1 isomeric mixture of products (entry 2).
Mixtures of isomers were also obtained from the reaction of
unsymmetrical internal alkynes 2m and 2n (entries 3 and 4).
Ph
Ph
R
R
N
5 mol% RhCl(PPh3)3
mesitylene, 170 °C
N
+
Ph
Ph
Ph
Ph
1o (R = Me) 2a (1.2 equiv)
1p (R = Ph)
3oa 75%
3pa 63%
Scheme 4 Reaction of 1o and 1p with 2a.
Table 3 Reaction of 1d with unsymmetrical alkynes 2k–na
In the case of 1q, another cyclobutene with
a
Me
Me
tetrasubstituted alkene moiety, two isomeric pentasubstituted
benzenes 3qa and 3ʹqa were obtained in 49% combined yield
via the insertion reaction (Scheme 5).14 The major product 3qa
was formed by a normal alkyne insertion into the C(3)–C(4)
bond of 1q. Conversely, the formation of 3ʹqa was likely a
result of double bond isomerisation of 1q to 1ʹq under the
reaction conditions used. The insertion of 2a into the original
C(2)–C(3) bond of 1ʹq led to the isomer 3ʹqa.
R1
5 mol% RhCl(PPh3)3
mesitylene, 170 °C
N
N
+
R1
R2
R2
Ph
Ph
1d
2
3
Entry
2 (R1, R2), Equiv
2k (Ph, H), 3.0
2l (Oct, H), 1.5
2m (Ph, Me), 1.2
3 (%Yieldb)
3dk (80)
3dl (53)
3dm (50)
3dn (78)
Ratioc
>20:1
3.2:1
2.2:1
5.8:1
1
2
3
4
Me
Me
Me
2n (isopropenyl, Et), 3.0
5 mol%
RhCl(PPh3)3
Ph
Ph
N
a Reaction conditions: 1d (0.10 mmol), 2 (0.12–0.30 mmol), RhCl(PPh3)3 (5.0 µmol,
5 mol%), mesitylene (0.50 mL, 0.2 M), 170 °C for 6–7 h. Isolated yield.
N
N
Ph
+
+
b
c
Pr
Ph
Ph
mesitylene
170 °C
Pr
Determined by 1H NMR.
Ph
Ph
Ph
Ph
1q
Pr
2a (1.2 equiv)
3qa 36%
3'qa 13%
Cyclobutenes with substituted phenyl and 1-naphthyl
groups at the 1 position of cyclobutene (1h–n) reacted with 2a
to produce 1,2,5-triaryl-3-(2-pyridylmethyl)benzenes 3ha–3na
(Table 4).
Me
N
Pr
Table 4 Reaction of 1h–n with 2aa
Ph
1'q
R1
R1
Ph
Ph
Scheme 5 Reaction of 1q with 2a.
5 mol% RhCl(PPh3)3
mesitylene, 170 °C
N
N
+
Ph
Ph
In this cyclobutene–alkyne coupling reaction, the directing
pyridylmethylene groups are retained in the products as
pyridylmethyl groups. Thus, we studied the derivatisation of
functionality that is present in the products. The copper-
catalysed aerobic oxidation of the methylene group belonging
to 3da and 3dj afforded the ketones 4a and 4b, respectively.15
The resulting 3-methylpicolinoyl group of 4b was removed
R2
R2
1
3
2a (1.2 equiv)
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Chem. Soc., 2012, 134, 20005; (o) P.-H. Chen, T. Xu and G.
upon treatment with t-BuOK to furnish 1,2,4-trisubstituted
benzene 5b.16
Dong, Angew. Chem., Int. Ed., 2014, 53, 1674;V(iepw)AErt.iclPeaOrnkliener
DOI: 10.1039/C6OB00734A
and N. Cramer, Organometallics, 2014, 33, 780; (q) L.
Souillart, E. Parker and N. Cramer, Angew. Chem., Int. Ed.,
2014, 53, 3001; (r) H. M. Ko and G. Dong, Nat. Chem., 2014, 6,
739; (s) L. Souillart and N. Cramer, Angew. Chem., Int. Ed.,
2014, 53, 9640; (t) T. Xu and G. Dong, Angew. Chem., Int. Ed.,
2014, 53, 10733; (u) F. Juliá-Hernández, A. Ziadi, A.
Nishimura and R. Martin, Angew. Chem., Int. Ed., 2015, 54,
9537; (v) X. Zhou and G. Dong, J. Am. Chem. Soc. 2015, 137,
13715.
Me
Me
O
N
N
H
(a)
(b)
R
R
R
R
Bu
Bu
Ph
Ph
Ph
3da (R = Ph)
3dj (R = Bu)
4a (R = Ph) 95%
4b (R = Bu) 66%
5b 52%
3
4
(a) N. Ishida, S. Sawano, Y. Masuda and M. Murakami, J. Am.
Chem. Soc., 2012, 134, 17502; (b) T. Matsuda and N. Miura,
Org. Biomol. Chem., 2013, 11, 3424; (c) Y. Xia, Z. Liu, Z. Liu, R.
Ge, F. Ye, M. Hossain, Y. Zhang and J. Wang, J. Am. Chem.
Soc., 2014, 136, 3013; (d) N. Ishida, N. Ishikawa, S. Sawano, Y.
Masuda and M. Murakami, Chem. Commun., 2015, 51, 1882;
(e) J. Yu, H. Yan and C. Zhu, Angew. Chem., Int. Ed., 2016, 55,
1143.
(a) B. L. Edelbach, R. J. Lachicotte and W. D. Jones,
Organometallics, 1999, 18, 4040; (b) C. N. Iverson and W. D.
Jones, Organometallics, 2001, 20, 5745; (c) T. Shibata, G.
Nishizawa and K. Endo, Synlett, 2008, 765; (d) A. Korotvicka,
D. Frejka, Z. Hampejsová, I. Císařová and M. Kotora,
Synthesis, 2016, 48, 987.
Scheme 6 Derivatisation of 3: (a) 10 mol% CuI, AcOH, DMSO, 100 °C, O2; (b) 3.1
equiv t-BuOK, 1.2 equiv H2O, 1,4-dioxane, 100 °C.
In conclusion, we have developed a rhodium(I)-catalysed
reaction that affords tetra- and pentasubstituted benzenes,
involving an alkyne insertion into the C–C bond of cyclobutene.
Sterically demanding, heteroaromatic and terminal alkynes are
all
effectively
coupled
to
3-(2-pyridylmethylene)-1-
phenylcyclobutenes. The 2-pyridylmethyl groups of the
products can be removed using established procedures.
5
6
7
T. Matsuda and I. Yuihara, Chem. Commun., 2015, 51, 7393.
See the Supporting Information for the preparation of 1.
Heating (E)- and (Z)-1a at 170 °C in mesitylene for 24 h in the
absence of RhCl(PPh3)3 induced the E/Z isomerisation,
resulting in the formation of 2:1 and 1:3 mixtures of the
isomers, respectively.
Acknowledgements
This work was supported by JSPS, Japan (Grant-in-Aid for
Scientific Research (C) No. 25410054) and the Sumitomo
Foundation (No. 130325).
8
For related pyridine-directed C–C bond cleavage that forms
five-membered rhodacycles, see: (a) C.-H. Jun and H. Lee, J.
Am. Chem. Soc., 1999, 121, 880; (b) C.-H. Jun, H. Lee and S.-G.
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Zhang and J. Wang, Angew. Chem., Int. Ed., 2012, 51, 12334.
See also refs 2r and 5.
Notes and references
1
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9
Any attempts to isolate cyclohexadienes E failed.
10 M. Murakami and T. Matsuda, Comprehensive Organic
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12 No significant solvent effect was observed between these
solvents in entries 7–9.
13 For a computational study on the regioselectivity of insertion
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Soc., 2012, 134, 11012.
2
14 Attempts to obtain hexasubstituted benzenes via this
reaction were unsuccessful.
15 J. De Houwer, K. Abbaspour Tehrani and B. U. W. Maes,
Angew. Chem., Int. Ed., 2012, 51, 2745.
16 G. A. Swan, J. Chem. Soc., 1948, 1408.
4 | J. Name., 2012, 00, 1-3
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