Rhodium(I)-catalyzed ene-allene-allene [2+2+2] cycloadditions: Stereoselective synthesis of complex trans-fused carbocycles

Article abstract of DOI:10.1002/anie.201100272

A complex situation: The title reaction was utilized for the construction of a variety of trans-fused hydrindanes and decalins in a highly convergent manner (see scheme; binap=2,2′-bis(diphenylphosphanyl)-1,1′- binaphthyl, Tf=trifluoromethanesulfonate), w

Full text of DOI:10.1002/anie.201100272

Communications
DOI: 10.1002/anie.201100272
Synthetic Methods
Rhodium(I)-Catalyzed Ene–Allene–Allene [2+2+2] Cycloadditions:
Stereoselective Synthesis of Complex trans-Fused Carbocycles**
Andrew T. Brusoe and Erik J. Alexanian*
The development of new reactions that increase molecular
complexity is a paramount goal of modern chemical syn-
thesis.[1] Processes that enable the construction of multiple
bonds and/or stereogenic centers in a single synthetic
operation offer decisive advantages in developing step-
economical[2] or greener[3] syntheses of complex synthetic
targets. Three-component transition-metal-catalyzed cyclo-
additions of the general form [m + n + o] have demonstrated
the capability to rapidly generate complex molecules, as no
less than three new s bonds and a new ring system are formed
from easily accessed p components.[4]
The metal-catalyzed [2+2+2] cycloaddition process is
among the most useful of this group of transformations, and is
applicable to the preparation of a variety of synthetically
valuable carbo-[5] and heterocyclic[6] six-membered rings.
Scheme 1. Prototypical transition-metal-catalyzed [2+2+2] cycloaddi-
tions.
Although all [2+2+2] cycloadditions forge three s bonds in
a single step, these reactions generate varying levels of
stereochemical complexity, as dictated by the nature of the
p systems involved.[7,8] In this regard, the alkyne p compo-
nents commonly utilized in [2+2+2] cycloadditions limit the
potential of these reactions to generate stereochemical
complexity, as each alkyne reduces the maximum number of
stereocenters created by two. For example, the alkyne
cyclotrimerization reaction delivers benzenoid systems that
possess no stereocenters, whereas an ideal [2+2+2] cyclo-
addition for increasing molecular complexity would use only
alkenes and could theoretically provide access to cyclohex-
anes containing six contiguous stereogenic centers
(Scheme 1). Herein, we report efforts towards this goal
through the development of an alkyne-free rhodium(I)-
catalyzed [2+2+2] cycloaddition by using simple alkenes
and allenes as the p components. These reactions deliver
synthetically valuable carbocycles and construct up to four
contiguous stereogenic centers, including quaternary stereo-
centers, in a single synthetic step.
1 as our model substrate. We initially employed a catalyst
system comprised of rhodium(I) with bidentate phosphine
ligands because of the demonstrated ability of these systems
to facilitate [m+n+o] cycloaddition processes.[4] Upon heat-
ing to 1008C in toluene for 2 h in the presence of 2.5 mol%
[{Rh(C2H4)2Cl}2], 5 mol% AgOTf, and 6 mol% H8-binap, the
[2+2+2] cycloaddition between substrate 1 and 2.0 equiva-
lents of allenoate 2 delivered trans-hydrindane 3 in 79% yield,
isolated as a single regioisomer and diastereomer (Table 1,
entry 1).[9] We examined several other catalytic systems
involving alternative rhodium(I) sources and bidentate phos-
phine ligands, each of which was less effective than our
standard reaction conditions (Table 1, entries 2–6). In the
absence of silver(I) salts the reaction was much less efficient
(Table 1, entry 7), and AgOTf was superior to AgBF4
(Table 1, entry 8). Performing the cycloaddition at a lower
reaction temperature (Table 1, entry 9), or with polar solvents
(Table 1, entries 10 and 11) proved suboptimal. Either a
decrease (Table 1, entry 12) or an increase (Table 1, entry 13)
in the amount of ethyl allenoate 2 added also lowered the
reaction yields.
The structure of product 3 was determined by 2D NMR
spectroscopy and subsequently confirmed by X-ray crystal-
lography (Figure 1).[10] This cycloaddition generates two
carbocyclic rings, three s bonds, and four contiguous stereo-
genic centers. Furthermore, the trans-hydrindane framework,
which is accessed in this highly convergent manner, con-
stitutes the core of many classes of bioactive natural products
and small molecules,[11] yet still presents a formidable
synthetic challenge.[12]
We began our studies of [2+2+2] cycloadditions of alkene
and allene p systems using the readily synthesized ene–allene
[*] A. T. Brusoe, Prof. E. J. Alexanian
Department of Chemistry
The University of North Carolina at Chapel Hill
Chapel Hill, NC 27599 (USA)
Fax: (+1)919-962-2388
E-mail: eja@email.unc.edu
[**] This work was supported by generous start-up funds provided by
UNC Chapel Hill. We also gratefully acknowledge the American
Chemical Society Petroleum Research Fund for the partial support
of this research.
Supporting information (including experimental procedures) for
this article is available on the WWW under http://dx.doi.org/10.
1002/anie.201100272.
Encouraged by this initial result, we sought to explore the
generality of this process (Table 2). The rhodium(I)-catalyzed
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Angew. Chem. Int. Ed. 2011, 50, 6596 –6600

Table 1: Influence of the reaction conditions on the [2+2+2] cyclo-
Table 2: Ene–allene–allene [2+2+2] cycloaddition.[a]
addition.
Entry Ene–allene substrate Allene
Product
Yield
[%][b]
1
59[c]
1
4
6
7
Entry Variation from standard conditions
Yield
[%][b]
1
2
3
4
none
79
76
61
2
61
[{Rh(coe)2Cl}2] instead of [{Rh(C2H4)2Cl}2], 2 h
[{Rh(nbd)Cl}2] instead of [{Rh(C2H4)2Cl}2], 7.5 h
[Rh(PPh3)3Cl]/AgOTf instead of [{Rh(C2H4)2Cl}2]/
H8-binap/AgOTf
<2
5
5
[RhCl(PPh3)3] instead of [{Rh(C2H4)2Cl}2]/H8-binap/AgOTf <2
6
7
BINAP instead of H8-binap, 3.5 h
No AgOTf
68
59
60
55
32
47
54
58
3
42
8
AgBF4 instead of AgOTf, 1 h
9
808C instead of 1008C
10
11
12
13
dioxane, 808C instead of PhCH3, 1008C
[{Rh(coe)2Cl}2]/DCE instead of [{Rh(C2H4)2Cl}2]/PhCH3
1.1 equiv allene instead of 2.0 equiv allene, 3 h
5.0 equiv allene instead of 2.0 equiv allene
5
[a] Reaction time was 24 h, or until full consumption of ene-allene
substrate was observed as indicated. [b] All yields are of isolated
products. coe=cis-cyclooctene, nbd=norbornadiene, OTf=trifluoro-
methanesulfonate.
76
4
9
1.2:1[d]
9:10
8
10
5
72
62
11
12
14
6
13
[a] Reaction conditions: allene (2 equiv), [{Rh(C2H4)2Cl}2] (2.5 mol%),
AgOTf (5 mol%), H8-binap (6 mol%), 1008C, PhMe, 2–24 h. [b] Yields of
the isolated product. [c] AgBF4 (5 mol%) was used instead of AgOTf.
[d] The diastereomeric ratio of 9:10 was determined by 1H NMR spectros-
copy of the crude reaction mixture.
Figure 1. ORTEP diagram of trans-hydrindane 3 (thermal ellipsoids set
at 50 % probability).
cycloaddition of substrate 1 with phenyl allene efficiently
provided the aryl-substituted trans-hydrindane product 4,
isolated as a single isomer (Table 2, entry 1), thus demon-
strating that the reaction is not limited to the addition of
electron-poor allenes. Reactions that utilized enones as the
alkene p component were also successful, as demonstrated by
the [2+2+2] cycloadditions of substrate 5 with ethyl allenoate
and phenyl allene to provide products 6 and 7, respectively
(Table 2, entries 2 and 3). An ene–allene substrate that
contains a Z-enoate p component, also underwent efficient
[2+2+2] cycloaddition, however a 1.2:1 mixture of trans-
hydrindane 9 and cis-fused product 10 was isolated (Table 1,
entry 4). We next explored the versatility of the ene–allene–
allene cycloaddition in the stereoselective construction of
carbocycles that contain quaternary stereocenters. The cyclo-
addition of substrates 11 and 13 furnished trans-hydrindanes
12 and 14 containing quaternary stereocenters at positions
both inside the ring system and at the ring junction,
respectively, both isolated as single diastereomers in good
yield.
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Communications
Table 3: Additional studies of ene–allene substrate scope in the [2+2+2] cyclo-
Further studies to survey the substrate scope of
addition.[a]
the ene–allene component of the [2+2+2] process
are shown in Table 3. The cycloaddition is not
limited to the synthesis of bicyclo[4.3.0] systems, as
bicyclo[4.4.0] trans decalins are also accessible
(Table 3, entry 1).[13] Substrates with tosylamide
linkages could also be used, as demonstrated by
the reaction of substrate 17 (Table 3, entry 2).
Importantly, while enoates and enones are excel-
lent alkene p components in the [2+2+2] process,
they are not required for the cycloaddition to
proceed. For example, styrenyl substrates 19 and
21 provided access to aryl-substituted trans-
hydrindanes 20 and 22, respectively (Table 3,
entries 3 and 4). The alkyl-substituted ene–allene
substrate 23 also afforded the [2+2+2] cyclo-
adduct, however the product 24 was isolated in low
yield (Table 3, entry 5).
Entry
Ene–allene substrate
Allene
Product
Yield [%][b]
48
1
15
16
2
3
45
63
17
18
The ene–allene substrates that contain malo-
nate-derived tethers are easily prepared by using
standard alkylation procedures. Furthermore,
simple substitution of the diester linkage for a
bis(sulfone) facilitates the removal of the tether
functionality after the [2+2+2] process. For exam-
ple, after the catalytic cycloaddition of the bis-
(sulfone) ene–allene 25, a mild reduction provides
the trans-hydrindane product 26 [Eq. (1)].[14,15]
The exocyclic 1,3-diene furnished by the
[2+2+2] cycloaddition facilitates a multitude of
further synthetic manipulations of the initial
reaction products.[16] Preparation of this useful
functionality commonly requires multistep proto-
cols.[17] We have found that the direct transforma-
tion of the initially formed 1,3-diene products is
possible in a one-pot process. For instance, upon
completion of the initial rhodium(I)-catalyzed
cycloaddition, a simple substitution of the Ar
atmosphere for H2 results in the 1,4-hydrogenation
19
20
4
5
58
27
21
23
22
24
[a] Reaction conditions: allene (2 equiv), [{Rh(C2H4)2Cl}2] (2.5 mol%), AgOTf
(5 mol%), H8-binap (6 mol%), 1008C, PhMe, 2–24 h. [b] Yield of the isolated product.
of the diene, thus providing cyclohexene 27 using sequential
rhodium(I) catalysis [Eq. (2)].[18] Elaboration of the initially
formed trans-hydrindane to an aromatic 6-6-5 tricycle is easily
achieved through a Diels–Alder/oxidation sequence. Subse-
quent to the [2+2+2] process, the direct addition of dime-
thylacetylenedicarboxylate (DMAD) to the reaction mixture
followed
by
2,3-dichloro-5,6-dicyano-1,4-benzoquinone
(DDQ) provides expedient access to product 28 in 82%
overall yield [Eq. (3)].
Preliminary studies have demonstrated the potential of
our current catalytic system in the development of an
enantioselective variant of the ene–allene–allene cycloaddi-
tion. The reaction of substrate 13 using 2.5 mol% of [{Rh-
(C2H4)2Cl}2], 5 mol% of AgOTf, and 6 mol% of (R)-H8-
binap delivered the [2+2+2] cycloadduct 14 in 62% yield as a
single diastereomer with an enantiomeric ratio of 87:13
[Eq. (4)].[19] The development of a general, highly enantiose-
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Angew. Chem. Int. Ed. 2011, 50, 6596 –6600

lective variant of this cycloaddition will be a focus of future
studies.
generating three s bonds and two carbocyclic rings, as well as
up to four contiguous stereocenters, including quaternary
centers, in a single step. Further studies will continue to
develop [m + n + o]-type cycloaddition approaches to com-
plex carbocycle synthesis, and develop general enantioselec-
tive variants of these processes.
Received: January 12, 2011
Revised: April 4, 2011
Published online: May 27, 2011
Keywords: alkenes · allenes · carbocycles · cycloaddition ·
.
rhodium
A preliminary mechanistic hypothesis for the catalytic
ene–allene–allene [2+2+2] cycloaddition is shown in
Scheme 2 using substrate 1 and ethyl allenoate 2. Following
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[2+2+2] cycloaddition.
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cationic rhodium(I) species coordinates to the substrate ene–
allene and ethyl allenoate at the internal allenic p bonds.
Oxidative coupling then generates bis(methylene)-substi-
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metallacycle 30 may ultimately lead to the observed product,
further studies are necessary to determine whether the initial
oxidative-coupling step is highly stereoselective, or if equili-
bration between 30 and a trans-substituted metallacycle
occurs. Formation of the rhodacyclopentane is followed by a
stereoselective 1,2-insertion of the tethered alkene, thus
providing metallacycle 31 and establishing the trans ring
fusion of the bicyclic framework. Final carbon–carbon bond-
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regenerates the active catalyst.[21]
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addition has been developed that gives direct access to
important classes of stereochemically rich carbocycles, includ-
ing trans-fused hydrindanes and decalins, from simple p com-
ponents. This process increases molecular complexity by
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Communications
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