Rhodium(i)-catalysed skeletal reorganisation of benzofused spiro[3.3]heptanes: Via consecutive carbon-carbon bond cleavage

Article abstract of DOI:10.1039/c6ob01344a

Skeletal reorganisation of benzofused spiro[3.3]heptanes has been achieved using rhodium(i) catalysts. The reaction of benzofused 2-(2-pyridylmethylene)spiro[3.3]heptanes proceeds via sequential C-C bond oxidative addition and β-carbon elimination. On the

Full text of DOI:10.1039/c6ob01344a

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Rhodium(I)-catalysed skeletal reorganisation of
benzofused spiro[3.3]heptanes via consecutive
carbon–carbon bond cleavage†‡
Cite this: DOI: 10.1039/c6ob01344a
Takanori Matsuda,* Itaru Yuihara and Kazuki Kondo
Skeletal reorganisation of benzofused spiro[3.3]heptanes has been achieved using rhodium(I) catalysts.
The reaction of benzofused 2-(2-pyridylmethylene)spiro[3.3]heptanes proceeds via sequential C–C bond
oxidative addition and β-carbon elimination. On the other hand, benzofused spiro[3.3]heptan-2-ols
undergo two consecutive β-carbon elimination processes. In both cases, substituted naphthalenes are
obtained.
Received 31st May 2016,
Accepted 24th June 2016
DOI: 10.1039/c6ob01344a
www.rsc.org/obc
discovering and developing C–C bond cleavage reactions of
Introduction
strained carbocycles9 led to the conception of catalytic ring
Transition-metal-catalysed C–C bond cleavage reactions have opening of benzofused spiro[3.3]heptane derivatives involving
seen increasingly active development, showcasing the feasi- two C–C bond cleavages (Fig. 1(b)). Herein, we report the first
bility of unique transformations that are difficult to achieve by experimental study of the rhodium(I)-catalysed skeletal re-
conventional methods.1 In particular, ring opening of benzo- organisation of benzofused 2-(2-pyridylmethylene)spiro[3.3]-
cyclobutenones2 and benzocyclobutenols3 has recently attracted heptanes and spiro[3.3]heptan-2-ols. In both cases, the reactions
much attention as it enables facile access to benzofused cyclic afford substituted naphthalenes via two C–C bond cleavages.
structures via coupling and rearrangement reactions.4,5 Several
research groups have performed computational studies on the
site selectivity of the catalytic ring opening of benzocyclobu-
Results and discussion
tenes possessing oxygen functionalities.2b,c,3f,i However, more
experiments on benzocyclobutenes without heteroatom substi- The benzofused 2-(2-pyridylmethylene)spiro[3.3]heptanes
tuents as well as the factors governing the selectivity of the used in this study were prepared via the Wittig olefination or
bond cleavage are required (Fig. 1(a)).
McMurry coupling of the corresponding ketones, which were
1
Two consecutive C–C bond cleavages of spiro[3.3]heptan-2- obtained from benzocyclobutenones in three steps.10 Heating
ones and spiropentanes, catalysed by rhodium(I) have been 1a at 150 °C in p-xylene in the presence of 5 mol% RhCl
reported,6,7 whereas the reaction of benzofused spirocycles has (PPh3)3, which was the best catalyst identified in our previous
not yet been investigated. We recently reported rhodium(I)-cat- study,8 led to the formation of 1-methyl-3-(2-pyridylmethyl)
alysed skeletal reorganisation of (2-pyridylmethylene)cyclo- naphthalene (2a) in 90% yield via skeletal reorganisation invol-
butanes to form indane skeletons.8 Our continuing interest in ving two consecutive C–C bond cleavages (Scheme 1). From
our recent studies and the structure of the product, the skel-
etal reorganisation of 1a would proceed as follows. The pyri-
dine nitrogen directs the first C–C bond cleavage by oxidative
addition of the C–C bond to rhodium(I) to afford spirocyclic
rhodacycle A, which contains a (cyclobutylmethyl)rhodium(III)
moiety. The second C–C bond cleavage by β-carbon elimin-
ation from this intermediate can proceed via two pathways.
Fig. 1 C–C bond cleavage of benzocyclobutene and benzospiro[3.3]
heptane.
However, cleavage occurs site-selectively at the distal C–C bond
to give the seven-membered rhodacycle B. The interaction
between rhodium and benzene would be insufficient to cleave
Department of Applied Chemistry, Tokyo University of Science, 1-3 Kagurazaka,
Shinjuku-ku, Tokyo 162-8601, Japan. E-mail: mtd@rs.tus.ac.jp
the proximal C–C bond, probably because of the restrained
†Dedicated to Prof. Masahiro Murakami on the occasion of his 60th birthday.
flexibility of the spirocyclic intermediate. Instead, the more
accessible C(sp3) in the four-membered ring interacts with
‡Electronic supplementary information (ESI) available: Experimental procedures
and characterisation data for new compounds. See DOI: 10.1039/c6ob01344a
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Scheme 2 Proximal C–C bond cleavage in reaction of 1j.
Scheme 1 Rhodium(I)-catalysed reaction of benzofused 2-(2-pyridyl-
methylene)spiro[3.3]heptane 1a.
When 1j having a methyl substituent at the spiro[3.3]
heptane skeleton11 was used in the reaction, a mixture of two
isomeric naphthalenes (2j : 3j = 57 : 43) was obtained in 23%
combined yield after 8 h, indicating that the substituent influ-
ences the site selectivity as well as reaction efficiency
(Scheme 2).12 In this case, the expected second distal C–C
bond cleavage would be sterically obstructed by the methyl
group, allowing partial cleavage of the proximal C–C bond.
We anticipated that benzofused spiro[3.3]heptan-2-ols
would also be amenable to an analogous skeletal reorganis-
ation, because four-membered tert-alcohols have often been
employed in rhodium(I)-catalysed C–C bond cleavage reac-
tions.13 The reaction of benzofused spiro[3.3]heptan-2-ol 4a 14
at 150 °C in p-xylene, in the presence of a catalytic amount of
[Rh(OH)(cod)]2 (10 mol% Rh; cod = cycloocta-1,5-diene), gave
two products: naphthalene 5a and α,β-unsaturated ketone 6a
in 54% and 29% yields, respectively (Scheme 3). Both products
were formed via two consecutive β-carbon elimination pro-
cesses; after the first C–C bond cleavage of rhodium(I) cyclobu-
rhodium to induce distal C–C bond cleavage. Subsequent
reductive elimination yields tetralin C, which aromatises to
naphthalene 2a under the reaction conditions.
The scope of the rhodium(I)-catalysed skeletal reorganis-
ation was explored, and the results are shown in Chart 1 and
Scheme 2. The reaction of 3-methyl-2-pyridyl derivative 1b
afforded 2b in 86% yield. Mono- and dialkoxynaphthalenes
2c–f were obtained in 51–82% yields by the reaction of the
corresponding substrates 1c–f possessing alkoxy groups on the
benzene ring. Naphthalene products 2g and 2h were obtained
in high yields when using substrates 1g and 1h possessing
substituents at the vinylic position. Naphthalene-fused spiro-
heptane 1i also participated in the reaction to afford 2,4-di-
substituted phenanthrene 2i.
Chart 1 Rhodium(I)-catalysed skeletal reorganisation of 1b–i. Reaction
conditions: 1 (0.081–0.150 mmol), RhCl(PPh3)3 (4.0–7.5 µmol, 5 mol%
Rh), p-xylene (1.0–1.5 mL), 150 °C, 0.5–1.0 h.
Scheme 3 Rhodium(I)-catalysed reaction of benzofused spiro[3.3]
heptan-2-ol 4a.
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Table 1 Skeletal reorganisation–dehydration of 4b–g a
C–C bond cleavages. In the reaction of 2-pyridylmethylene-sub-
stituted derivatives 1, the first C–C bond cleavage proceeded
via oxidative addition and the second β-carbon elimination
occurred via preferential distal bond cleavage. In contrast, the
reaction of hydroxyl-substituted derivatives 4 involved two
β-carbon elimination processes, and the second cleavage
occurred predominantly at the proximal bond. Particularly in
6 (% yield)b this case, selective proximal C–C bond cleavage during the
Entry
4
R1
R2
R3
5 (% yield)b
second β-carbon elimination was feasible by substitution at
the appropriate positions of substrate 4.
6c (16)
1
2
3
4
5
6
4b
4c
4d
4e
4f
2-Thienyl
Bu
Me
H
H
OMe
H
H
H
H
H
Me
Me
Et
5b (28)
5c (46)
5d (80)
5e (64)
5f (56)
5g (62)
6b (25)
6d (8)
—
—
Et
2-Thienyl
Me
Experimental section
4g
OMe
—
General procedure for rhodium(I)-catalysed rearrangement of 1
a Reaction conditions:
4
(0.100–0.150 mmol), [Rh(OH)(cod)]2
(5.0–7.5 µmol, 10 mol% Rh), p-xylene (1.0–1.5 mL), 150 °C, 1.0–2.5 h;
A Schlenk tube was charged with 1 (0.100 mmol) and RhCl
(PPh3)3 (5.0 µmol, 5 mol% Rh), and the tube was evacuated
and backfilled with nitrogen. p-Xylene (1.0 mL) was added via
a syringe through the septum, and the mixture was heated at
150 °C with stirring for the indicated period of time. The reac-
tion mixture was cooled to room temperature and then filtered
through a plug of Florisil® washing with hexane–AcOEt (1 : 1),
and the filtrate was concentrated. The residue was purified by
preparative TLC on silica gel to afford naphthalene 2.
then silica gel, 1.0–5.5 h. b Isolated yield.
tanolate D, the reaction followed two different pathways at
(cyclobutylmethyl)rhodium(I) intermediate E and eventually
led to the products. Proximal C–C bond cleavage by β-carbon
elimination was dominant in the reaction of hydroxy-substi-
tuted 4, unlike in the case of 2-pyridylmethylene derivatives 1,
wherein distal C–C bond cleavage is preferred. The resulting
arylrhodium(I) F adds intramolecularly to the ketonic carbonyl
group to give tetralinol G after protonation. Subsequent aroma-
tisation involving dehydration is facilitated upon the addition
of silica gel,15 resulting in the formation of naphthalene 5a.
On the other hand, distal C–C bond cleavage of (cyclobutyl-
methyl)rhodium(I) E generates benzylrhodium(I) H, which fur-
nishes ketone 6a through protonation and double bond
isomerisation.
Table 1 summarises the results obtained for other benzo-
fused spiro[3.3]heptanols 4b–g.14 Similar results were obtained
with spiroheptanols having 2-thienyl and butyl groups (4b and
4c); naphthalenes 5 and ketones 6 were obtained, with the
former being the predominating product (Table 1, entries 1
and 2). Installation of a methoxy group into the benzene ring
of substrate improved the yield of naphthalene 5d to 80%,
with an excellent 5d/6d ratio (10 : 1) (entry 3). This result could
be explained as follows: increasing the electron density of the
benzene ring strengthens the η2-coordination of benzene to
rhodium(I), thus making the proximal C–C bond cleavage
more probable so that arylrhodium(I) is generated. Methyl sub-
stitution into the spiro[3.3]heptane skeleton again impeded
distal C–C bond cleavage in the reaction of benzofused spiro
[3.3]heptanols 4e–g; the second C–C bond cleavage occurred
exclusively at the proximal bond, affording naphthalenes 5e–g
as the sole products.
General procedure for rhodium(I)-catalysed rearrangement of 4
A Schlenk tube was charged with 4 (0.100 mmol) and [Rh(OH)
(cod)]2 (5.0 µmol, 10 mol% Rh), and the tube was evacuated
and backfilled with nitrogen. p-Xylene (1.0 mL) was added via
a syringe through the septum, and the mixture was heated at
150 °C with stirring. Silica gel (50–100 mg) was added, and the
mixture was further heated at 150 °C. The reaction mixture was
cooled to room temperature and then filtered through a plug
of Florisil® washing with hexane–AcOEt (1 : 1), and the filtrate
was concentrated. The residue was purified by preparative TLC
on silica gel to afford naphthalene 5.
Acknowledgements
This work was supported by JSPS, Japan (Grant-in-Aid for
Scientific Research (C) No. 25410054 and 16K05783) and the
Sumitomo Foundation (No. 130325).
Notes and references
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which furnished substituted naphthalenes via two consecutive
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