Rhodium(I)-catalyzed decarbonylative spirocyclization through C=C bond cleavage of benzocyclobutenones: An efficient approach to functionalized spirocycles

Article abstract of DOI:10.1002/anie.201310149

The rhodium-catalyzed formation of all-carbon spirocenters involves a decarbonylative coupling of trisubstituted cyclic olefins and benzocyclobutenones through C=C activation. The metal-ligand combination [{Rh(CO)₂Cl}₂]/P(C₆F₅)₃ catalyzed this transformation most efficiently. A range of diverse spirocycles were synthesized in good to excellent yields and many sensitive functional groups were tolerated. A mechanistic study supports a hydrogen-transfer process that occurs through a β-H elimination/decarbonylation pathway. All-carbon spirocenters were formed by a decarbonylative coupling of trisubstituted cyclic olefins and benzocyclobutenones through C=C activation. The metal-ligand combination of [{Rh(CO)₂Cl}₂] and P(C₆F ₅)₃ catalyzed this transformation most efficiently; a range of spirocyclic rings was thus synthesized in good to excellent yields. Copyright

Full text of DOI:10.1002/anie.201310149

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Chemie
DOI: 10.1002/anie.201310149
À
C C Activation
À
Rhodium(I)-Catalyzed Decarbonylative Spirocyclization through C C
Bond Cleavage of Benzocyclobutenones: An Efficient Approach to
Functionalized Spirocycles**
Tao Xu, Nikolas A. Savage, and Guangbin Dong*
Dedicated to Professor Chul-Ho Jun on the occasion of his 60th birthday
Abstract: The rhodium-catalyzed formation of all-carbon
spirocenters involves a decarbonylative coupling of trisubsti-
cyclization through an intramolecular coupling between
olefins and benzocyclobutenones (Scheme 1), in which the
reaction course can be controlled by the choice of the ligand
on the metal catalyst.
À
tuted cyclic olefins and benzocyclobutenones through C C
activation. The metal–ligand combination [{Rh(CO)2Cl}2]/
P(C6F5)3 catalyzed this transformation most efficiently. A
range of diverse spirocycles were synthesized in good to
excellent yields and many sensitive functional groups were
tolerated. A mechanistic study supports a hydrogen-transfer
process that occurs through a b-H elimination/decarbonylation
pathway.
À
À
C
atalytic carbon carbon bond (C C) activation/functional-
ization provides a unique platform to develop novel trans-
formations that are challenging using conventional
approaches.[1] In particular, C C activations that involve
À
a decarbonylation process are of significant synthetic value
because they allow the preparation of compounds that lack
a carbonyl moiety from a more readily available ketone
precursor.[2] Given the ubiquity of carbonyl compounds, the
À
À
decarbonylative C C activation followed by a C C forming
process would potentially offer a distinct synthetic strategy
using carbonyl groups as a “traceless handle”.
À
Although highly attractive, the decarbonylative C C
activation strategy has been largely limited to its reaction
scope. Previous efforts primarily involved a direct CO
extrusion to give the corresponding hydrocarbon products
[Eq. (1)].[3,4] Only a few examples are known to engage the
addition of simple olefins [Eq. (2)].[5] To broaden the scope
Scheme 1. RhI-catalyzed decarbonylative coupling between olefins and
À
benzocyclobutenones through C C activation.
À
and applicability of the decarbonylative C C activation/
functionalization strategy, new classes of synthetically useful
transformations are highly sought. Herein we describe our
development of a rhodium-catalyzed decarbonylative spiro-
[*] Dr. T. Xu, N. A. Savage, Prof. Dr. G. Dong
Department of Chemistry, University of Texas at Austin
100 east 24th street, Austin, TX 78712 (USA)
E-mail: gbdong@cm.utexas.edu
Recently, we developed a Rh-catalyzed intramolecular
carboacylation between benzocyclobutenones and olefins.[6]
Homepage: http://gbdong.cm.utexas.edu/
[**] We thank UT Austin and CPRIT for a startup fund, and NIGMS
(1R01GM109054-01) and the Welch Foundation (F 1781) for
research grants. G.D. is a Searle Scholar. We thank Professors
Sessler, Siegel, and Anslyn for loaning chemicals. Dr. Lynch is
acknowledged for X-ray crystallography. We also thank Johnson
Matthey for a generous donation of Rh salts. Dr. Alpay Dermenci is
thanked for proof reading of this manuscript.
This “cut and sew” transformation begins with the oxidative
I
À
addition of Rh into the benzocyclobutenone C1 C2 bond,
followed by migratory insertion into the olefin to give a 7-
membered metallacycle (C), and subsequent reductive elim-
ination to afford fused-ring systems (D). However, if a b-H
elimination/decarbonylation process can be promoted instead
À
of direct C C reductive elimination, spirocycles containing
all-carbon quaternary centers would be rapidly formed when
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201310149.
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Table 1: Selected optimization conditions.
cyclic trisubstituted olefins were employed as the coupling
partner (Scheme 1). Given that spirocycles are important
structural motifs often found in bioactive natural products,
whereas efficient synthesis of functionalized spirocycles has
heretofore been challenging,[7] this decarbonylative C C
À
activation strategy would provide a complementary approach
to the previous spirocyclization methods.[8] However, the
challenge here is how one can promote the b-H elimination
and decarbonylation instead of direct reductive elimination?
Previously, we discovered that bidentate phosphine
ligands with a large bite angle promote the direct reductive
elimination; however, a Lewis acid is needed to enhance the
electrophilicity of the substrate to permit coupling with
polysubstituted olefins [Eq. (3)].[6a] We hypothesized that the
use of monodentate p-acidic ligands (inspired by a recent
work of Tang[9]) would benefit the formation of the spirocycle
in two different aspects: 1) the faster ligand exchange
(compared to bidentate ligands) would facilitate the forma-
tion of open coordination sites on Rh, which in turn could
favor both b-H elimination and CO deinsertion; 2) the more
electron-deficient catalyst would coordinate more strongly
with the trisubstituted olefins, thus enhancing subsequent
migratory insertion.[10]
Entry
Precatalyst
Ligand/Additive
Yield [%][a]
3a/4a
2a
1
[{Rh(CO)2Cl}2]
[Rh(CO)2(acac)]
[{Rh(CO)2Cl}2]
[{Rh(CO)2Cl}2]
[{Rh(CO)2Cl}2]
[{Rh(CO)2Cl}2]
[{Rh(CO)2Cl}2]
[{Rh(CO)2Cl}2]
[{Rh(CO)2Cl}2]
[{Rh(CO)2Cl}2]
[{Rh(CO)2Cl}2]
none
none
AgSbF6 (10 mol%)
PPh3
P(OCH2CF3)3
P[OCH(CF3)2]3
P(2-furyl)3
P[3,5-(CF3)2C6H3]3
P(C6F5)3
3/<1%
0
0
0
0
0
7/ꢀ4
14/14
30/7
72[f]/ꢀ4
66/ꢀ5
2
0
0
0
0
0
22
39
60
22[f]
15
2[b]
3[c]
4
5
6
7
8
9[d]
10[d,e]
11[d,g]
P(C6F5)3
P(C6F5)3
[a] Yields were determined by 1H NMR spectroscopy using mesitylene as
the internal standard. [b] 10 mol% [Rh(CO)2(acac)] was used. [c] 3-OH-
benzocyclobutenone was isolated in 53% yield. [d] The reaction time was
36 h. [e] 10 mol% P(C6F5)3 of the ligand was used. [f] Yields of isolated
products. [g] 5 mol% P(C6F5)3 was used. acac=acetylacetonates.
coordination sites for b-H elimination.[10] It is interesting to
note that under these reaction conditions the C3 olefin isomer
(after one “chain walk”[11]) of the spirocyclic product was
selectively afforded (for a mechanistic study, see below).[12,13]
Further lowering the ligand/metal ratio to 0.5:1 gave a similar
result (entry 11, Table 1).
Next, the scope of this decarbonylative spirocyclization
was investigated (Table 2). First, cyclic olefins with different
ring sizes were examined. To our delight, 5-, 6-, 7-, 8-, and 12-
membered ring substrates all underwent this transformation
smoothly (entries 1–5, Table 2); a single olefin isomer was
observed except in the case of the 5-membered ring substrate
(entry 2, Table 2). Intriguingly, spirocyclization of the 12-
membered ring substrate (1e) proceeded to give a trans olefin
in 90% yield without further alkene isomerization (entry 5,
Table 2). The enhanced reactivity of substrate 1e is likely
attributed to a transannular interaction caused by the 12-
membered cycle.[14] The structures of spirocycles 3a, 3c, and
3d were unambiguously confirmed by X-ray crystallography.
Electron-deficient olefins, such as an enone, also reacted,
albeit with a lower conversion (entry 6, Table 2). It is note-
worthy that this spirocycle formation method is highly
chemoselective and a variety of sensitive functional groups,
including dienes, ketones, enamides, esters, benzyl and vinyl
ethers, and unprotected tertiary alcohols are tolerated
(entries 6–10, 15, and 16, Table 2), which is likely attributed
to the near neutral reaction conditions. For example, when
dihydrobenzopyran 1h was employed as the substrate, the
vinyl ether based spirocycle was isolated in 71% yield
(entry 8, Table 2).
To test our hypothesis, compound 1a, which previously
gave the tetracyclic “cut and sew” product 2a [Eq. (3)], was
employed as the model substrate for the formation of the
spirocycle. A number of RhI precatalysts and phosphine
ligands were examined. Indeed, when electron-rich or biden-
tate ligands, such as PCy3, dppb, and dppf, were employed, the
desired decarbonylative spirocyclization product was not
obtained; in contrast, use of the more electron-poor
[{Rh(CO)2Cl}2] alone produced the 2H-benzofuran-[4.5]-
spirocycle 3a in about 3% yield (entry 1, Table 1). Use of
an acac ligand on the Rh versus Cl was detrimental to the
catalyst reactivity, leading to slight decomposition of 1a
(entry 2, Table 1). The in situ generated cationic RhI led to the
dealkylation of 1a to give 3-OH-benzocyclobutenone
(entry 3, Table 1). Use of the more electron-rich PPh3 or
highly electron-deficient phosphites as the ligands completely
shut down the catalyst reactivity (entries 4–6, Table 1).
However, we found that employment of the p-acidic triaryl-
phosphine ligands significantly promoted the formation of the
desired spirocycle 3a (entries 7–11, Table 1), among which
the P(C6F5)3 ligand proved to be most efficient. Finally, simply
by lowering the ligand/metal ratio to 1:1, formation of the
undesired reductive-elimination product (2a) was signifi-
cantly inhibited; spirocycle 3a was isolated as the major
product in 72% yield (entry 10, Table 1). Presumably, when
less ligand is present, the metal tends to provide open
In addition, when the C8-methoxy-substituted benzo-
cyclobutenone 1j was utilized, the spirocycle containing
a benzyl ether moiety (3j) was isolated in 59% yield as
a single isomer (entry 10, Table 2). While the exact reason is
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Table 2: Substrate scope.[a]
[a] Each reaction was run on a 0.2 mmol scale in a sealed vial, using 5 mol% [{Rh(CO)2Cl}2] and 10 mol% P(C6F5)3, in THF, at 1308C, for 36 h.
1
[b] Yields of isolated products. [c] Ratios of olefin regioisomers were determined by H NMR spectroscopy or were based on the yields of isolated
products (see the Supporting Information for more details). [d] Numbers in parenthesis are yields based on recovered starting material (brsm).
[e] Compound 3k was characterized by a subsequent olefin hydrogenation (see the Supporting Information for details). [f] Isolated as inseparable
isomers.
still unclear, the presence of the methoxy group at C8 likely
Substrate 1n was readily synthesized from propargyl alcohol
and D2-paraformaldehyde in five steps with an overall yield of
44% (for details, see the Supporting Information). By
reacting 1n under the standard decarbonylative spirocycliza-
tion conditions, spirocycle 3n with more than 95% deuterium
incorporation at the methyl group was isolated in 79% yield.
This experiment strongly supports our hypothesis that this
transformation includes a b-H elimination, decarbonylation,
À
promotes a faster C H reductive elimination rather than
olefin migration.[10] Interestingly, spirocycle 3b was also
isolated in 18% yield.[15] Furthermore, we discovered that,
besides forming benzofuran-based products, other classes of
spirocycles can also be efficiently synthesized: first, this
transformation works well with substrates that lack an ether
linkage (entry 11, Table 2); second, the cyclization that gives
a 6-membered ring proceeded equally well to give benzopyr-
ane-based spirocycle 3l in 79% yield (entry 12, Table 2). Note
that, except for substrates 1a, 1i, and 1m,[16] the (“cut and
sew”) product of a direct reductive elimination was not
observed for other substrates depicted in Table 2. In addition,
substrates with different substituents at the C4 and C5
positions of the benzocyclobutenone also reacted smoothly
in the spirocyclization (entries 13–16, Table 2).[17] It is encour-
aging to note that substrates bearing substituents with differ-
ent steric and electronic properties, including methyl, 1-butyl-
1-hydroxypentyl, and methyl ester groups, all provided good
yields of the desired spirocycles.[17b]
À
and then C H reductive elimination pathway (see above,
Scheme 1).
Another mechanistic problem regards the selective olefin
chain walk with most substrates. We postulated that two
reaction pathways are possible for the olefin migration:
1) alkene isomerization after the decarbonylative spirocycli-
zation (that is, the C3 isomer comes from the C2 isomer), or
To provide mechanistic insights for this transformation,
a deuterium-labeling experiment [Eq. (4)] was designed.
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2) alkene isomerization during the decarbonylative spirocyc-
lization (that is, the C3 and C2 isomers are formed independ-
ently). Compounds 3i and 3i’ were chosen as the model
compounds because they can be readily separated through
column chromatography. When pure 3i and 3i’ were sub-
jected to the standard reaction conditions (shown in Table 2),
no isomerization for either product was observed after 36 h
[Eq. (5)], which indicates the [{Rh(CO)2Cl}2]/P(C6F5)3 system
cannot isomerize the alkene in the products.
the 1,2-disubstituted olefins proceeded smoothly [Eq. (9)].[19]
It is interesting to note that the olefin geometry does not
significantly affect the reactivity.
In conclusion, we have developed a Rh-catalyzed decar-
bonylative spirocyclization of olefins and benzocyclobute-
À
nones through C C activation, which provides a complemen-
tary but distinct way to generate all-carbon spirocenters. This
reaction exhibits several key features. First, it operates at near
neutral reaction conditions, and therefore tolerates many
acid- or base-sensitive functional groups, which represents
a major advantage over classical spirocycle syntheses. Second,
it has a broad substrate scope (both electron-rich and
electron-poor olefins undergo the reaction), thus providing
a range of structurally diversified spirocycles, which indicates
great potential for application in the synthesis of complex
molecules. Furthermore, to the best of our knowledge, this
A cross-over experiment was also carried out to examine
whether the intermediates generated in the catalytic cycle, for
example, a RhIII H species, can isomerize the alkenes.
À
Reaction of compounds 1c and 3i under the standard reaction
conditions provided 3c in 80% yield, but no isomerization of
3i was found [Eq. (6)]. When the reactions of 1i and 1b were
monitored separately over time, the ratio between the C2 and
C3 isomers remained largely unchanged [Eqs. (7) and (8)],
which is consistent with our earlier observation for substrate
1a.[13] In combination with the deuterium-labeling experiment
[Eq. (4)], these studies suggest that the olefin chain walk
likely occurs during (instead of after) the process of the
spirocycle formation, thus the C2 and C3 isomers are
expected to form independently (not shown in Scheme 1).
À
represents the only report that combines C C activation,
olefin insertion, b-H elimination, and decarbonylation in
a one-reaction sequence. The unique mode of reactivity
described here may provide broad implications for designing
new tandem transformations. Further expansion of the
substrate/reaction scope and the development of an enantio-
selective version of this reaction[20] are ongoing.
Received: November 22, 2013
Published online: January 20, 2014
À
Keywords: C C activation · decarbonylation ·
homogeneous catalysis · rhodium · spirocycles
.
À
[1] For recent reviews on transition-metal-mediated C C activa-
tion, see: a) W. D. Jones, Nature 1993, 364, 676; b) M. Murakami,
Y. Ito, Top. Organomet. Chem. 1999, 3, 97; c) B. Rybtchinski, D.
Milstein, Angew. Chem. 1999, 111, 918; Angew. Chem. Int. Ed.
1999, 38, 870; d) C. Perthuisot, B. L. Edelbach, D. L. Zubris, N.
Simhai, C. N. Iverson, C. Mꢀller, T. Satoh, W. D. Jones, J. Catal.
Mol. A 2002, 189, 157; e) M. E. van der Boom, D. Milstein,
Chem. Rev. 2003, 103, 1759; f) C.-H. Jun, Chem. Soc. Rev. 2004,
33, 610; g) T. Satoh, M. Miura, Top. Organomet. Chem. 2005, 14,
1; h) C.-H. Jun, J. W. Park, Top. Organomet. Chem. 2007, 24, 117;
i) D. Necas, M. Kotora, Curr. Org. Chem. 2007, 11, 1566; j) R. H.
Crabtree, Chem. Rev. 1985, 85, 245; k) T. Kondo, T. A. Mitsudo,
Chem. Lett. 2005, 34, 1462; l) K. Ruhland, Eur. J. Org. Chem.
2012, 2683; m) A. Korotvicka, D. Necas, M. Kotora, Curr. Org.
Chem. 2012, 16, 1170; n) T. Seiser, T. Saget, D. N. Tran, N.
Cramer, Angew. Chem. 2011, 123, 7884; Angew. Chem. Int. Ed.
2011, 50, 7740; o) M. Murakami, T. Matsuda, Chem. Commun.
2011, 47, 1100.
Furthermore, we found this transformation is not limited
to cyclic trisubstituted olefins, as the linear disubstituted
olefins also underwent the decarbonylative cyclization to
provide the cyclization products. While coupling of linear
trisubstituted alkenes proved challenging,[18] reactions with
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À
[2] For a recent review of decarbonylative C C forming reactions,
Ozawa, A. Kubo, Y. Matsumoto, T. Hayashi, Organometallics
see: A. Dermenci, G. Dong, Sci. China Chem. 2013, 56, 685.
1993, 12, 4188; c) O. Loiseleur, P. Meier, A. Pfaltz, Angew.
Chem. 1996, 108, 218; Angew. Chem. Int. Ed. Engl. 1996, 35, 200.
[13] An attempt to capture the non-chain-walk intermediate (4a)
under the reaction conditions (shown in entry 10, Table 1) was
unfruitful. Even when the reaction was quenched at a low
conversion, 3a was observed as the dominant product.
[14] D. Krois, E. Langer, H. Lehner, Tetrahedron 1980, 36, 1345.
[15] The mechanism for the formation of 3b is unclear.
[16] For substrate 1i, the corresponding “cut and sew” product (2i)
was isolated in 14% yield. For substrate 1m, the corresponding
“cut and sew” product (2m) was isolated in 12% yield.
[17] a) The C6-substituted substrate was challenging to prepare.
b) The reaction of the C5-ester-substituted substrate 1s has been
attempted twice under the decarbonylative spirocyclization
conditions [Eq. (10)]. The desired spirocycle was obtained,
albeit in a lower yield with decomposition of the starting
material (the exact reason is unclear).
À
[3] For examples of catalytic CO extrusion followed by C C
formation, see: a) K. Kaneda, H. Azuma, M. Wayaku, S.
Teranishi, Chem. Lett. 1974, 215; b) M. Murakami, H. Amii, Y.
Ito, Nature 1994, 370, 540; c) M. Murakami, H. Amii, K. Shigeto,
Y. Ito, J. Am. Chem. Soc. 1996, 118, 8285; d) T. Matsuda, M.
Shigeno, M. Murakami, Chem. Lett. 2006, 35, 288; e) Z.-Q. Lei,
H. Li, Y. Li, X.-S. Zhang, K. Chen, X. Wang, J. Sun, Z.-J. Shi,
Angew. Chem. 2012, 124, 2744; Angew. Chem. Int. Ed. 2012, 51,
2690; f) A. Dermenci, R. E. Whittaker, G. Dong, Org. Lett. 2013,
15, 2242.
À
[4] For an example of olefin formation through C C activation/
decarbonylation/b-H elimination, see: M. Murakami, T. Itaha-
shi, Y. Ito, J. Am. Chem. Soc. 2002, 124, 13976, and ref. [3c].
[5] a) K. Teruyuki, N. Ayoko, O. Takumi, S. Nobuyoshi, W. Kenji, M.
Take-aki, J. Am. Chem. Soc. 2000, 122, 6319; b) T. Kondo, Y.
Taguchi, Y. Kaneko, M. Niimi, T.-A. Mitsudo, Angew. Chem.
2004, 116, 5483; Angew. Chem. Int. Ed. 2004, 43, 5369; c) Y.
Yamamoto, S. Kuwabara, H. Hayashi, H. Nishiyama, Adv. Synth.
Catal. 2006, 348, 2493.
[6] a) T. Xu, G. Dong, Angew. Chem. 2012, 124, 7685; Angew. Chem.
Int. Ed. 2012, 51, 7567; b) T. Xu, H. M. Ko, N. A. Savage, G.
Dong, J. Am. Chem. Soc. 2012, 134, 20005.
[7] A recent review on spirocycle synthesis by Kotha describes: “In
general, these (spirocycle formation) methods encounter prob-
lems associated with functional-group incompatibility at one or
more stages. For more details, see Ref. [8a].
[8] For recent reviews of the formation of spirocycles, see: a) S.
Kotha, A. C. Deb, K. Lahiri, E. Manivannan, Synthesis 2009, 165,
and references therein; b) M. Sannigrahi, Tetrahedron 1999, 55,
[18] A linear trisubstituted olefin substrate (1t) was also synthesized
and tested under the reaction conditions [Eq. (11)]. The desired
decarbonylative cyclization product 3t was obtained, albeit in
a low yield (low conversion), which is likely due to the steric
hindrance of the alkene group.
À
9007; for a seminal work on spirocycle formation initiated by C
H activation, see: c) E. M. Ferreira, B. M. Stoltz, J. Am. Chem.
Soc. 2003, 125, 9578; d) H. Zhang, E. M. Ferreira, B. M. Stoltz,
Angew. Chem. 2004, 116, 6270; Angew. Chem. Int. Ed. 2004, 43,
6144.
[9] For recent efforts on using electron-deficient ligands to control
the reactivity of RhI catalysts, see: a) seminal work: X.-Z. Shu, S.
Huang, D. Shu, I. A. Guzei, W. Tang, Angew. Chem. 2011, 123,
8303; Angew. Chem. Int. Ed. 2011, 50, 8153; b) a review: X.-Z.
Shu, D. Shu, C. M. Schienebeck, W. Tang, Chem. Soc. Rev. 2012,
41, 7698.
[10] a) J. F. Hartwig in Organotransition Metal Chemistry: from
Bonding to Catalysis, University Science Books, Mill Valley,
2009; b) J. F. Hartwig, Inorg. Chem. 2007, 46, 1936.
[11] For a recent review of transition-metal-catalyzed olefin isomer-
ization, see: K. Tanaka in Comprehensive Organometallic
Chemistry III, Vol. 10 (Eds.: R. H. Crabtree, D. M. P. Mingos, I.
Ojima), Elsevier, Oxford, UK, 2007, p. 71.
[12] Olefin migration has been previously observed in the Pd-
catalyzed formation of spirocycles. For examples, see: a) M.
Hatano, K. Mikami, J. Am. Chem. Soc. 2003, 125, 4704; b) F.
[19] The cyclization product from Equation (9) contains several
olefin isomers.
[20] Not surprisingly, when monophos was employed as the chiral
ligand for this transformation, the desired spirocyclization
product was not observed (see above, entries 5 and 6, Table 1).
Chiral monodentate electron-deficient triarylphosphines will be
developed and investigated for the asymmetric transformation in
due course.
[21] CCDC 962877 (3a), 962878 (3c), and 962879 (3d) contain the
supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
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