Construction of Hexahydrophenanthrenes By Rhodium(I)-Catalyzed Cycloisomerization of Benzylallene-Substituted Internal Alkynes through C?H Activation

Article abstract of DOI:10.1002/anie.201605640

The treatment of benzylallene-substituted internal alkynes with [RhCl(CO)₂]₂effects a novel cycloisomerization by C(sp²)?H bond activation to produce hexahydrophenanthrene derivatives. The reaction likely proceeds through consecutive formation of a rhodabicyclo[4.3.0] intermediate, σ-bond metathesis between the C(sp²)?H bond on the benzene ring and the C(sp²)?Rh^IIIbond, and isomerization between three σ-, π-, and σ-allylrhodium(III) species, which was proposed based on experiments with deuterated substrates.

Full text of DOI:10.1002/anie.201605640

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201605640
German Edition: DOI: 10.1002/ange.201605640
Cycloisomerizations
Construction of Hexahydrophenanthrenes By Rhodium(I)-Catalyzed
Cycloisomerization of Benzylallene-Substituted Internal Alkynes
ꢀ
through C H Activation
Yasuaki Kawaguchi, Shigeo Yasuda, and Chisato Mukai*
Abstract: The treatment of benzylallene-substituted internal
other hand, changing the RhI catalyst to RhCl(dppp)2 effected
3
ꢀ
alkynes with [RhCl(CO)2]2 effects a novel cycloisomerization
a unique C(sp ) H bond activation to produce the novel
2
by C(sp ) H bond activation to produce hexahydrophenan-
spiro[2.4]heptane derivatives 3.[3] Some sort of s-bond meta-
ꢀ
III
ꢀ
ꢀ
threne derivatives. The reaction likely proceeds through
thesis would occur between the g-C H bond and the C Rh
bond in intermediate 4’ (a conformational isomer of 4), which
would be followed by reductive elimination to give 3.[5] Sato,
consecutive formation of a rhodabicyclo[4.3.0] intermediate,
2
ꢀ
s-bond metathesis between the C(sp ) H bond on the benzene
2
III
ꢀ
ring and the C(sp ) Rh bond, and isomerization between
three s-, p-, and s-allylrhodium(III) species, which was
proposed based on experiments with deuterated substrates.
Oonishi, and co-worker reported a similar cycloisomerization
3
ꢀ
reaction of tert-butylallene-substituted alkynes by C(sp ) H
bond activation.[9d] Thus these results allow for the assump-
tion that the allene-alkyne unit functions as a highly reactive
p component towards the RhI catalyst[9] to form a rhodabicy-
clic intermediate (e.g., 4 or 4’), which would subsequently
C
yclization reactions that make use of transition-metal-
[1]
[2]
ꢀ
ꢀ
catalyzed C H and/or C C bond activation are powerful
step- and atom-economic methods for the straightforward
construction of complex polycyclic skeletons that are inac-
cessible by other conventional methods. These activation
processes often require directing groups in the substrates,
which bind the transition-metal catalyst at a position close to
the reactive site. Relief of the high strain energy would be an
ꢀ
ꢀ
activate a C C and/or C H bond near the Rh species.
While investigating the RhI-catalyzed cyclization of
allene-alkynes, we observed a novel cycloisomerization for
the benzylallene-substituted terminal alkynes 5 (R1 = H, R2 =
2
ꢀ
alkyl) that proceeds through C(sp ) H bond activation on the
benzene ring to produce 6.[10] This reaction was assumed to
ꢀ
alternative driving force to facilitate the cleavage of C C
proceed via the unique vinylidene carbene Rh intermediate
ꢀ
bonds in cycloalkanes. We recently disclosed that the RhCl-
(PPh3)3 catalyzed intramolecular cycloisomerization of
alkynes with a pendant allenylcyclopentane moiety 1 effi-
ciently provides bicyclo[7.4.0]tridecatrienes 2 (Scheme 1).[3,4]
We presumed that this reaction occurred by the initial
formation of rhodabicyclo[4.3.0] intermediate 4,[3,4b,5] fol-
lowed by b-carbon elimination and reductive elimination. The
plausible key intermediate 4 could accelerate the unprece-
dented cleavage of the unactivated cyclopentane ring by
releasing its low strain energy (6.3 kcalmolꢀ1).[3,6–8] On the
6’. Herein, we describe another type of C H activation of
benzylallene-alkyne species
5
(R1 ¼ H, R2 = alkyl); the
replacement of the terminal alkyne with an internal one
dramatically changed the ring-closing mode to furnish the
hexahydrophenanthrene skeleton 7 in high yields (Scheme 2).
Scheme 2. RhI-catalyzed cycloisomerization of benzylallene-alkynes by
ꢀ
C H bond activation.
Our initial study employed 5a, which has a dimethyl
group at the benzylic position so that unfavorable b-hydride
elimination of the possible rhodacyclic intermediate A is
avoided (Table 1). A solution of 5a in toluene was heated to
reflux in the presence of 5 mol% [RhCl(CO)2]2 (an effective
catalyst for the cycloisomerization of 5 when R1 = H)[10] for
0.2 h to produce hexahydrophenanthrene derivative 7a in
77% yield.[11] The bicyclo[4.3.0]nonadienone derivative 8a
was isolated in 19% yield as a byproduct (entry 1, method I).
Compound 8a was regarded as the product of a Pauson–
Khand-type reaction (PKTR), which is formed by CO
Scheme 1. Our previous study: RhI-catalyzed intramolecular cyclization
[3]
ꢀ
ꢀ
of allenylcyclopentane-alkynes by C C and C H bond activation.
[*] Y. Kawaguchi, Dr. S. Yasuda, Prof. Dr. C. Mukai
Division of Pharmaceutical Sciences
Graduate School of Medical Sciences
Kanazawa University
insertion into
A
followed by reductive elimination.[12]
Kakuma-machi, Kanazawa 920-1192 (Japan)
E-mail: mukai@p.kanazawa-u.ac.jp
Decreasing the amount of [RhCl(CO)2]2 to 2.5 mol% dimin-
ished the production of 8a (10%) and 7a (56%) as well
(entry 2). Other catalysts, such as RhCl(PPh3)3, RhCl(CO)-
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201605640.
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Table 1: Optimization of the reaction conditions for the RhI-catalyzed
cycloisomerization of benzylallene-alkyne 5a.[a]
5d and 5e, with n-butyl and ethyl substituents at the alkyne
terminus, provided 7d and 7e in good yields (86 and 83%;
entries 3 and 4). On the other hand, the less hindered methyl-
substituted alkyne derivative 5 f afforded the desired com-
pound 7 f in rather low yield (40%),[14] with the formation of
considerable amounts of unidentified side products (entry 5).
The reaction of 5g, which does not benefit from geminal
disubstitution,[15] smoothly proceeded to exclusively furnish
7g in 91% yield (entry 6). The nitrogen (5h) and oxygen (5i)
congeners also exclusively afforded 7h and 7i (89 and 85%;
entries 7 and 8). Overall, a bulky tert-butyl group at the
alkyne terminus seemed to provide better results than linear
alkyl chains. Thus we next examined some silylacetylene
derivatives. Treatment of the TIPS derivatives 5j and 5k with
[RhCl(CO)2]2 in xylene or toluene at reflux provided the
desired vinylsilane derivatives 7j (71%) and 7k (70%) along
with 8j (18%) and 8k (18%), respectively (entries 9 and 10).
The reactions of the TBS analogues 5l and 5m gave similar
results to afford 7l and 7m (67 and 61%; entries 11 and 12).
The silane functional group can be easily elaborated in
various ways.[16] We hence attempted transformations of the
silane moieties in 7j and 7k (Scheme 3). Treatment of 7j with
Entry
Method
Time [h]
Product and yield [%][b]
1
I
I
II
II
0.2
2
0.2
0.2
7a: 77
7a: 56
7a: 82
7a: 91
8a: 19
8a: 10
8a: 13
8a: 3
–
2[c]
3
5a: 34
–
–
4[d]
[a] Method I: A solution of 5a in toluene was heated to reflux in the
presence of the RhI catalyst for 0.2–2 h. Method II: A solution of 5a in
toluene or xylene was added to a toluene or xylene solution of 5 mol%
[RhCl(CO)2]2 at reflux; the reaction mixture was heated at reflux for an
additional 0.2 h. [b] Yield of isolated product. [c] With 2.5 mol% [RhCl-
(CO)2]2. [d] Xylene was used as the solvent.
(PPh3)2, RhCl(dppp)2, and [RhCl(CO)dppp]2, furnished poor
results. To obtain better results, the procedure was slightly
modified: A solution of 5a in toluene was added to a toluene
solution of 5 mol% [RhCl(CO)2]2 at reflux, and the reaction
mixture was heated to reflux for an additional 0.2 h to afford
7a in 82% yield together with 8a in 13% yield (entry 3,
method II).[13] Finally, the best result (highest yield of 7a
(91%) and lowest yield of 8a (3%)) was obtained when
method II was used in combination with a higher reaction
temperature (xylene at reflux; entry 4).
The optimized reaction conditions (Table 1, entry 4) were
then applied to other substrates 5 (Table 2). Treatment of the
ethyl- (5b) and n-butyl-substituted (5c) allene derivatives
with [RhCl(CO)2]2 in xylene at reflux gave the tricyclic
products 7b and 7c (96 and 95%; entries 1 and 2). Substrates
Scheme 3. Transformations of 7j and 7k.
1.5 equiv of trifluoroacetic acid (TFA) effected the isomer-
ization of the vinylsilane group into an allylsilane group to
afford 9 in 82% yield. Exposure of 7j and 7k to an excess of
TFA led to desilylation, which was accompanied by migration
of the exo double bond to give 10j (85%) and 10k (92%).[17]
We then turned our attention to the RhI-catalyzed cyclo-
isomerization of 11, which has an electron-withdrawing group
(EWG) at the alkyne terminus, to determine the effect of the
electronic properties of the substituent at the alkyne terminus
(Table 3). Treatment of phosphonate derivative 11a with
[RhCl(CO)2]2 produced 12a in 87% yield alongside a negli-
gible amount of the PKTR adduct 13a (entry 1). Neither an
ester nor a ketone functional group inhibited this cyclo-
isomerization. Indeed, substrates with an ester moiety, 11b
and 11c, exclusively furnished 12b and 12c (80 and 91%;
entries 2 and 3). The acetyl derivative 11d afforded 12d
(79%) and 13d (9%; entry 4). Chloro-substituted 11e could
be converted into 12e in acceptable yield (67%; entry 5). The
reaction of 11 f, which features an electron-withdrawing para-
nitrophenyl group, furnished 12 f[14] in 79% yield (entry 6).
Although the simple phenyl derivative 11g also gave 12g[14]
(entry 7), the yield (62%) was somewhat lower than for 12 f
(entry 7 vs. 6). Phenyl derivative 11h, with a methyl-substi-
tuted allene, gave 12h[18] in 96% yield (entry 8). It thus
became clear that various EWGs were tolerated at the alkyne
terminus in this cyclization. In combination with the results
shown in Tables 2 and 3, we concluded that the ring-closing
Table 2: [RhCl(CO)2]2 catalyzed cycloisomerization of benzylallene-
alkynes 5.[a]
Entry
5
R1
R2
X
Time [h] Product and yield [%][b]
1
2
3
4
5b tBu Et
5c tBu nBu C(CO2Me)2 0.2
5d nBu Me C(CO2Me)2 0.2
C(CO2Me)2 0.2
7b: 96
7c: 95
7d: 86
7e: 83
7 f: 40[d]
7g: 91
7h: 89
7i: 85
8b: 4
8c: 5
8d: 6
8e: 4
–
–
5e Et
Me C(CO2Me)2 0.2
5[c] 5 f Me Me C(CO2Me)2 0.2
[e]
6
7
8
5g tBu Me CH2
5h tBu Me NTs
5i tBu Me
0.2
0.2
0.2
–
–
O
9[f] 5j TIPS Me C(CO2Me)2 0.7
7j: 71
7k: 70
7l: 67
8j: 18
8k: 18
8l: 19
8m: 18
10[c] 5k TIPS nBu C(CO2Me)2
1
11[c] 5l TBS Me C(CO2Me)2 0.2
12[c] 5m TBS nBu C(CO2Me)2 0.5
7m: 61
[a] The reaction conditions are the same as those in Table 1, entry 3 or 4.
[b] Yield of isolated product. [c] Toluene as the solvent. [d] Yield
1
determined by H NMR analysis. [e] Unidentified byproducts were
obtained. [f] With 10 mol% [RhCl(CO)2]2. TBS=tert-butyldimethylsilyl,
TIPS=triisopropylsilyl, Ts=para-toluenesulfonyl.
2
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Table 3: [RhCl(CO)2]2 catalyzed cycloisomerization of other benzylallene-
alkyne substrates 11.[a]
regarding the reaction mechanism (see below), namely that
geminal disubstitution of the benzylic position of the starting
materials is mandatory and that substituents at the benzylic
position that are bulkier than a methyl group retard the
cycloisomerization step.
To obtain more information on the mechanism, we
performed two experiments with [D5]-5d and [D1]-5d
(Scheme 5). Treatment of [D5]-5d with [RhCl(CO)2]2 pro-
Entry
11
R1
R2
Product and yield [%][b]
1
11a
11b
11c
11d
11e
11 f
11g
11h
P(O)(OEt)2
CO2Me
CO2Me
Ac
Me
nBu
Me
Me
Me
nBu
nBu
Me
12a: 87
12b: 80
12c: 91
12d: 79
12e: 67
12 f: 79[d]
12g: 62[d]
12h: 96
13a: 3
–
–
13d: 9
–
–
13g: 4
13h: 3
2[c]
3[c]
4[c]
5
Cl
6
7
8
p-NO2C6H4
Ph
Ph
[a] The reaction conditions are the same as those in Table 2. [b] Yield of
isolated product. [c] Toluene as the solvent. [d] Yield determined by
1H NMR analysis.
Scheme 5. [RhCl(CO)2]2 catalyzed cycloisomerization of [D5]-5d and
[D1]-5d.
reaction occurs irrespective of the electronic properties of the
substituent at the alkyne terminus while the substituent has to
be bulky for successful construction of the hexahydrophenan-
threne framework.
Treatment of substrate 14, with one carbon atom shorter
chain, with [RhCl(CO)2]2 also led to the construction of
a tricyclic framework to afford 15 in 82% yield along with 16
in 17% yield [Eq. (1)].[19]
duced [D5]-7d in 87% yield. One deuterium atom was
exclusively incorporated at the terminal carbon atom of the
exocyclic olefin moiety. For monodeuterated substrate
[D1]-5d, the deuterium atom at the allenic position was
completely incorporated into the olefinic position of the six-
membered ring in [D1]-7d.
The experimental results in Schemes 4 and 5 provide fairly
informative insight into the reaction mechanism (Scheme 6).
Upon exposure to the standard conditions, substrate 17a,
which features a diethyl group, gave 18 in rather low yield
(45% NMR yield)[14] along with the formation of consider-
able amounts of unidentified side products (Scheme 4).
Scheme 6. Plausible mechanism for the formation of 7 (or 12) and 8
(or 13).
Oxidative cyclization of the distal allene double bond and the
alkyne in 5 (or 11) with RhI would initially occur to form
Scheme 4. [RhCl(CO)2]2 catalyzed cycloisomerization of 17.
bicyclic rhodacyclopentene intermediate A (R2 = Me). Then,
2
s-bond metathesis[21] between a C(sp ) H bond on the
ꢀ
2
III
ꢀ
Furthermore, both the butyl (17b) and cyclohexyl (17c)
congeners afforded intractable mixtures. On the other hand,
the reaction of the unsubstituted benzyl substrate 17d
produced triene 19 in 82% yield as a 1.8:1 mixture of the E
and Z isomers. The formation of 19 strongly supports the
intermediacy of rhodabicyclic species A (R2 = H), which then
undergoes b-hydride elimination.[9a,20] These experiments
indirectly provide two significant points of information
benzene ring and the C(sp ) Rh bond of A would lead to
s-allylrhodium intermediate B, which would be in equilibrium
with another s-allylrhodium intermediate D via p-allylrho-
dium intermediate C. Reductive elimination of RhIII would
finally give 7 (or 12). As mentioned above, A can alternatively
undergo a carbonylative [2+2+1] cycloaddition to afford 8 (or
13) by successive insertion of CO and reductive elimination.
We succeeded in the suppression of the PKTR process in the
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[7] For the allenylcyclobutane derivatives, the reaction would
proceed via a rhodabicyclo[4.3.0] intermediate similar to 4,
which could accelerate the cleavage of the unfunctionalized
reactions of 11 with an EWG at the alkyne terminus,
presumably by avoiding the CO insertion process owing to
the electronic effect of the EWG. With alkyl- and silyl-
substituted alkyne derivatives 5, bulky substituents at the
alkyne terminus tend to suppress CO insertion into A and
some other side reactions through steric hindrance. However,
substrate 17a, with a diethyl group at the benzylic position,
afforded the desired product 18 in a rather low yield
(Scheme 4). This might be due to a significant nonbonding
interaction between the diethyl group and the methyl group
on the allene moiety during the s-bond metathesis[21] leading
to B. For the dibutyl and cyclohexyl derivatives 17b and 17c,
the s-bond metathesis[21] of A can no longer proceed.
In summary, we have developed an efficient method for
the construction of the hexahydrophenanthrene skeleton by
taking advantage of the RhI-catalyzed cycloisomerization of
internal alkynes with a pendant benzylallene moiety. Prelimi-
nary studies with deuterated substrates and some additional
experiments provided some support for the proposed reaction
mechanism. Investigations of the scope and limitations of this
method as well as further mechanistic studies are currently in
progress.
ꢀ
cyclobutane C C bond by relief of the high ring-strain energy
(26.3 kcalmolꢀ1); see Refs. [4b,6].
[8] For the allenylcyclopropane derivatives, we assume that a six-
membered rhodacyclic intermediate is initially formed because
cyclopropane has a fairly high ring-strain energy (27.5 kcal
molꢀ1): see Ref. [4a] and: a) J. W. Knowlton, F. D. Rossini, J. Res.
Natl. Bur. Stand. 1949, 43, 113 – 115; a similar rhodacyclic
intermediate could be considered for the reaction of allenylcy-
clopropane-alkenes; see: b) K. Sugikubo, F. Omachi, Y. Miya-
naga, F. Inagaki, C. Matsumoto, C. Mukai, Angew. Chem. Int.
Ed. 2013, 52, 11369 – 11372; Angew. Chem. 2013, 125, 11579 –
11582.
[9] For selected examples of RhI-catalyzed cycloisomerizations or
cycloadditions of allene-alkyne substrates, see: a) K. M. Brum-
mond, H. Chen, P. Sill, L. You, J. Am. Chem. Soc. 2002, 124,
15186 – 15187; b) X. Jiang, S. Ma, J. Am. Chem. Soc. 2007, 129,
11600 – 11607; c) Y. Oonishi, A. Hosotani, Y. Sato, J. Am. Chem.
Soc. 2011, 133, 10386 – 10389; d) Y. Oonishi, Y. Kitano, Y. Sato,
Angew. Chem. Int. Ed. 2012, 51, 7305 – 7308; Angew. Chem.
2012, 124, 7417 – 7420; e) Y. Oonishi, T. Yokoe, A. Hosotani, Y.
Sato, Angew. Chem. Int. Ed. 2014, 53, 1135 – 1139; Angew. Chem.
2014, 126, 1153 – 1157.
[10] Y. Kawaguchi, S. Yasuda, A. Kaneko, Y. Oura, C. Mukai, Angew.
Chem. Int. Ed. 2014, 53, 7608 – 7612; Angew. Chem. 2014, 126,
7738 – 7742.
[11] The stereochemistry of the exocyclic double bond in the cyclized
products 7 was determined to be as shown by NOE experiments
(see the Supporting Information for details).
Acknowledgements
This work was financially supported by JSPS KAKENHI
Grants (15H02490 and 15K18826).
[12] For selected examples of RhI-catalyzed carbonylative [2+2+1]
cycloadditions of allene-alkyne substrates, see: a) K. M. Brum-
mond, D. Chen, Org. Lett. 2008, 10, 705 – 708; b) F. Inagaki, S.
Kitagaki, C. Mukai, Synlett 2011, 594 – 614, and references
therein. For selected examples of RhI-catalyzed carbonylative
[2+2+1] cycloadditions of other allene substrates, see: c) F.
Inagaki, S. Narita, T. Hasegawa, S. Kitagaki, C. Mukai, Angew.
Chem. Int. Ed. 2009, 48, 2007 – 2011; Angew. Chem. 2009, 121,
2041 – 2045; d) T. Iwata, F. Inagaki, C. Mukai, Angew. Chem. Int.
Ed. 2013, 52, 11138 – 11142; Angew. Chem. 2013, 125, 11344 –
11348.
ꢀ
Keywords: alkynes · allenes · C H activation ·
cycloisomerization · rhodium
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[13] The reason for the improvement of the yield of 7a is uncertain at
this stage. Initiation of the reaction at the boiling point of
toluene or xylene might presumably help the rapid extrusion of
CO from the RhI catalyst, thereby improving the yield of 7a and
suppressing the formation of 8a.
[14] The yields of 7 f, 12 f, 12g, and 18 were determined by 1H NMR
analysis because these compounds could not be isolated in pure
form by column chromatography.
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[18] X-ray analysis of 12h unambiguously established its 5-alkylide-
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4
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ꢀ
ꢀ
between C H and C Rh bonds, see: a) L. Souillart, N.
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[19] We also attempted the construction of larger tricyclic frame-
works by the reaction of the one-carbon-homologated substrate
20. However, no reaction occurred under the standard con-
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[20] a) C. Mukai, F. Inagaki, T. Yoshida, K. Yoshitani, Y. Hara, S.
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Received: June 14, 2016
Published online: && &&, &&&&
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Cycloisomerizations
Hexahydrophenanthrene derivatives
were obtained from internal alkynes with
Y. Kawaguchi, S. Yasuda,
C. Mukai*
a pendant benzylallene moiety through
2
ꢀ
&&&& — &&&&
cycloisomerization by C(sp ) H bond
activation. The reaction likely proceeds by
formation of a rhodabicyclo[4.3.0] inter-
Construction of
Hexahydrophenanthrenes By
Rhodium(I)-Catalyzed
Cycloisomerization of Benzylallene-
Substituted Internal Alkynes through C
H Activation
mediate, s-bond metathesis between a C-
2
ꢀ
(sp ) H bond on the benzene ring and
2
III
ꢀ
the C(sp ) Rh bond, and isomerization
between three s-, p-, and s-allylrhodi-
um(III) species.
ꢀ
6
www.angewandte.org
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
These are not the final page numbers!