Platinum-catalyzed intramolecular [4C+3C] cycloaddition between dienes and allenes

Article abstract of DOI:10.1002/anie.200704566

(Chemical Equation Presented) Crossing the seven C's: The three carbon atoms of several allenyl fragments can be incorporated into seven-membered carbocycles by means of a Pt-catalyzed intramolecular [4C+3C] cycloaddition with dienes (see scheme). The tra

Full text of DOI:10.1002/anie.200704566

Angewandte
Chemie
DOI: 10.1002/anie.200704566
Metal-Catalyzed Cycloadditions
Platinum-Catalyzed Intramolecular [4C+3C] Cycloaddition between
Dienes and Allenes**
Beatriz Trillo, Fernando López,* MoisØs Gulías, Luis Castedo, and JosØ L. Mascareæas*
Polycyclic structures containing seven-membered carbocycles
are important synthetic targets because they constitute the
basic structural framework of a wide variety of biologically
relevant products.[1] Among the different strategies for
assembling cycloheptanoic systems, cycloaddition routes are
particularly attractive because of their inherent potential for
achieving a rapid increase in skeletal complexity.[2] One of the
most convenient and best established cycloaddition
approaches to carbocyclic seven-membered rings is the
[4C+3C] annelation of dienes and allyl cation derivatives,
principally oxy- and amino-allyl cations.[3–7] These cationic
species are typically generated from halo, alkoxy, or sulfone
carbonyl derivatives,[4] although their preparation from
[6]
alkoxyallylic sulfoxides,[5] allenamides,
or methylene-
Scheme 1. Mechanistic hypothesis for a diene–allene [4C+3C] cyclo-
addition. [M]=Pt or Au complex.
aziridines[7] have also been described.
Despite notable advances in the implementation of these
methodologies, the requirement of relatively elaborate and
sometimes unstable allyl cation precursors, the need for
stoichiometric activators,[8] and the frequent restriction of the
success of the cycloadditions to furan and/or cyclopentadiene
components, currently represent important limitations.
Recent reports on the use of Pt and Au complexes to
induce reactions of allenes through allyl cation type inter-
mediates,[9,10] led us to envisage the possibility of using allenes
as three-carbon components in metal-promoted [4C+3C]
cycloadditions with dienes.[11] Our mechanistic working
hypothesis (Scheme 1) consists of an initial activation of the
allene (A) to form an allylic cation–metal complex (B), which
may react with the diene in a standard 4p(4C)-2p(3C) mode.
The resulting metal–carbene intermediate C could then
undergo a 1,2-hydrogen shift to provide the desired seven-
membered carbocycles (2 and/or 3) with regeneration of the
metal catalyst.
Herein, we demonstrate the viability of the approach by
reporting several examples of a platinum-catalyzed intra-
molecular [4C+3C] cycloaddition between dienes and allenes.
The strategy provides a novel and particularly efficient way to
construct bicyclic systems that contain a seven-membered
carbocycle.
Initial studies to test the viability of the process were
carried out on the furan-allene derivative 1a (Table 1).[12]
Treatment of 1a with 10 mol% of the catalyst generated
[13,9b]
from [Ph3PAuCl] and AgSbF6
in CH2Cl2 provided, after
24 h at room temperature, the [4C+3C] adduct 2a in 30%
yield (65% conversion, Table 1, entry 1). Poorer yields of 2a
were obtained on increasing the temperature to 408C or to
758C (using toluene instead of CH2Cl2; see the Supporting
Information). Heating under reflux in toluene led to the
formation of the [4+2] adducts 4 (3.5:1 exo/endo mixture)
together with a small proportion of 2a (4:2a 6.2:1, 25%
combined yield; Table 1, entry 2). Control experiments con-
firmed that the adducts 4 can be obtained by simple
thermolysis of 1a, which suggests that the metal is not
involved in their formation.[14] The use of other gold-based
catalysts such as AuCl3 or AuCl[15] led to poor conversions at
moderate temperatures, whereas heating at 1108C provided
the Diels–Alder cycloadducts as the major products (Table 1,
entries 3–5). We then turned our attention to platinum
complexes, which have been recently employed in catalytic
processes involving allene activation.[9a,e] The use of PtCl4 and
PtCl2 provided mixtures of 4 and 2a, but with higher
proportions of 2a than in the above experiments (Table 1,
entries 6 and 7). As in previous cases, performing the
reactions at lower temperatures led to poor conversions.
[*] B. Trillo, Dr. F. López, Dr. M. Gulías, Prof. Dr. L. Castedo,
Prof. Dr. J. L. Mascareæas
Departamento de Química Orgµnica
Universidad de Santiago de Compostela
Facultad de Química
15782, Santiago de Compostela (Spain)
Fax: (+34)981-595-012
E-mail: qojoselm@usc.es
Homepage: http://qoweb.usc.es/jlmgroup/index.html
[**] This work was supported by the Spanish MEC(SAF2007-61015),
Consolider Ingenio 2010 (CSD2007-00006), and the Xunta de
Galicia (GRC2006/132). B.T. and M.G. thank the Spanish M.E.C.
and Xunta de Galicia for predoctoral fellowships. F.L. thanks the
Xunta de Galicia (PGIDIT06PXIB209126PR) for research support
and the Spanish MECfor a Ramon y Cajal contract. Lucía Saya is
gratefully acknowledged for technical support. We also thank
Johnson–Matthey for the generous gift of metal complexes.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Table 1: Optimization of the [4C+3C] cycloaddition of 1a.[a]
Table 2: Platinum-catalyzed [4C+3C] cycloaddition of acyclic dienes to
allenes.[a]
Entry Substrate[12]
Product
T [8C] t [h] Yield[b]
1
110
110
5
5
62
Entry
[M]
[Ph3PAuCl]/AgSbF6
[Ph3PAuCl]/AgSbF6
AuCl3
AuCl3
AuCl
PtCl4
PtCl2
Products (ratio)[b] T [8C] Yield [%][c]
1
2a
23
110
40
110
110
110
110
110
30[d]
25
95[c]
2
3
4
5
6
7
8
4:2a (6.2:1)
2a
2
26[e]
57
4:2a (6.1:1)
4:2a (19:1)
4:2a (4.4:1)
4:2a (3.4:1)
2a
89[f]
57
3
4
1c
2c:3c
65
12
2
87[d]
92
73
PtCl2/CO (1 atm)
45[f]
110
[a] Conditions: 1a (0.2m), 10 mol% [M]. Reactions at 23 or 408Cwere
carried out in CH2Cl2, otherwise toluene was employed. Conversions are
>99% unless otherwise noted. [b] Ratios determined by 1H NMR
spectroscopy of the crude mixtures. [c] Determined by 1H NMR spec-
troscopy against an internal standard (1,2,3-trimethoxybenzene), unless
otherwise stated. [d] 65% conversion. [e] 66% conversion. [f] Yield of
isolated product.
5
6
7
1d
1d
1d
3d
3d
3d
110
23
23
12
18
2
90[e]
95
98[f]
8
–
110
110
12
–
9
2
81
On the assumption that the [4C+3C] cycloaddition
process might be favored by increasing the electrophility of
the PtII center, we carried out the reaction with PtCl2 under an
atmosphere of CO.[16] Interestingly, complete disappearance
of 1a was observed after just one hour, and 2a was the only
cycloadduct detected, although it was obtained in a moderate
yield of 45% (Table 1, entry 8).[17]
10
11
110
110
3
62
72
Envisaging that the presence of the furan unit could be
detrimental for the desired [4+3] annulation by favoring the
competitive Diels–Alder reaction as well as metal-induced
secondary processes, we decided to test the behavior of
substrate 1b, which features a simple, acyclic diene. Remark-
ably, 1b underwent the desired cycloaddition when heated
under reflux in toluene in the presence of PtCl2 (10 mol%),
and provided 2b in 62% yield (Table 2, entry 1). [18,19] It is
worth highlighting the complete diastereoselectivity of the
process, which provided exclusively the trans-fused 5,7-system
2b. The reaction is also completely regioselective: the
formation of regioisomers of type 3 was not detected. The
use of an atmosphere of CO did not bring about any
significant improvement in the yield of 2a, it just led to
slightly faster reaction rates. Therefore, further experiments
addressing the scope of the transformation were carried out
using the operationally safer standard inert atmosphere. It can
be seen from the results in Table 2that substrate 1c, which
includes an allene with a terminal phenyl substituent instead
of the methyl group, underwent a very efficient [4+3]
cycloaddition (Table 2, entry 2). In this case, the reaction
provided both regioisomers 2c and 3c (in a 6:4 ratio) in 95%
combined yield. Decreasing the reaction temperature to 658C
led to a slight increase in the regioselectivity (Table 2,
entry 3). Further lowering of the temperature gave incom-
plete conversions.
12
12
13
110
110
12
24
73
[g]
–
–
[a] E=CO2Et; conditions: Toluene 1108C, 1 (0.1–0.2m), 10 mol% PtCl2
unless otherwise stated. Conversion >99% (determined by 1H NMR
spectroscopy). [b] Yield of isolated product. [c] Combined yield of the
1
mixture of 2c and 3c (6:4). Ratio of products determined by H NMR
spectroscopy in the crude reaction mixtures. [d] Ratio 2c/3c=7:3.
[e] 2 mol% of PtCl2. [f] Reaction carried out under CO (1 atm). [g] The
starting material was recovered unaltered. The use of CO (1 atm) gave
the same results.
the cycloadduct 3d in excellent yield (92%) after only 2h at
1108C (Table 2, entry 4).[20] It is noteworthy that the catalyst
loading could be reduced to 2mol% without affecting the
efficiency of the process (Table 2, entry 5). The cycloaddition
of 1d could also be carried out at room temperature to give
the desired cycloadduct as the only detectable product and in
equally excellent yield (Table 2, entries 6 and 7). The superior
performance of 1c and 1d than 1b in the cycloaddition
Trisubstituted allenes are excellent partners for the cyclo-
addition. Thus, reaction of 1d with PtCl2 (10 mol%) provided
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reaction might be related to the higher stability of the
hypothetical allylic cation generated upon activation of the
allene with PtCl2 (B, Scheme 1). Consistent with this hypoth-
esis, substrate 1e, which contains a monosubstituted allene,
failed to undergo the [4+3] cycloaddition when heated under
standard conditions. The reaction gave rise to a mixture of
oligomers together with a small amount of a [4+2] adduct.[21]
We next analyzed the influence of the diene substitution
on the reaction. The introduction of a methyl substituent at
the terminal position of the diene is well tolerated, as evident
from the efficient conversion of 1 f into the [4C+3C] adduct
3 f with complete stereoselectivity (Table 2, entry 9). Reac-
tion of the oxygen analogue 1g, which lacks the geminal
diester moiety of the tether, also proceeded smoothly to
afford 3g in 62% yield (Table 2, entry 10). Finally, allene-
dienes 1h and 1i, which are substituted at internal positions of
the diene fragment, provided the corresponding cycloadducts
3h and 3i in satisfactory yields (Table 2, entries 11 and 12).
Allenediene 1j, which has a methyl substituent at the
most internal position of the diene, failed to undergo the
cycloaddition reaction when heated in the presence of PtCl2
or PtCl2/CO (1 atm); the starting material was recovered after
24 h under reflux (Table 2, entry 13). The failure of this
reaction could be related to the high energetic cost associated
with the S-cis conformation of this diene, a conformation that
is required if the cycloaddition involves a concerted [4p + 2p]
process.
Scheme 2. Cycloaddition/ring expansion of substrate 1k.
(Scheme 1), opens up an interesting alternative to construct-
ing 5-7-6-tricyclic systems, a type of skeleton present in a wide
variety of natural products.[22]
In conclusion, we have developed a novel [4C+3C]
cycloaddition process involving a platinum-catalyzed reaction
of allenes and dienes. The method represents the first use of
allenes as three-carbon components in any type of [4+3]
catalytic cycloaddition. The excellent atom economy and
stereoselectivity of the process, together with its operational
simplicity, allows this method to be ranked among the most
practical and rapid alternatives to construct cycloheptane-
containing polycycles. Further studies on the scope, including
an enantioselective version, and the mechanism of the process
are underway.
Received: October 3, 2007
Published online: December 14, 2007
Keywords: carbocycles · cycloaddition · homogeneous catalysis ·
.
platinum · polycycles
The stereochemical course of the reported cycloadditions,
which are completely diastereoselective, is consistent with a
concerted annulation process involving an exo-like or
extended transition state such as those depicted in
Figure 1.[3] This model explains the observed diastereoselec-
tivity for the cycloaddition of 1b, 1c, 1 f, and 1g.
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Figure 1. Proposed extended (exo-like) transition state (TS) for the
[4C+3C] cycloadditions of 1 f and 1c (X=C(CO2Et)2).
According to the working mechanism depicted in
Scheme 1, the metal-regeneration step requires a hydrogen
migration process; however, the required turnover could also
be hypothetically achieved by an alternative process, such as a
1,2-alkyl shift.[9a,10] Although we did not observe products
arising from a 1,2-methyl shift in the cycloaddition of
substrates bearing the dimethylallenyl moiety (1d and 1 f–
1i), we were intrigued to know the outcome in the cases of
allenes bearing a fused cycle.
Remarkably, treatment of allenediene 1k with PtCl2
(10 mol%) in refluxing toluene for 2h afforded, in a
completely diastereo- and regioselective fashion, the tricyclic
product 2k in 75% yield (Scheme 2). This transformation, in
addition to validating a cycloaddition mechanism involving a
1,2-shift on a platinum–carbene intermediate of type C
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[12] For the synthesis of precursors see the Supporting Information.
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[17] The rest of the mass balance corresponds to acyclic side
products.
[18] The Diels–Alder cycloadducts derived from 1b were not
observed under these conditions, probably because of the
higher energy barrier associated with acyclic, non-activated
dienes.
[19] Other platinum salts (PtCl4, PtBr2) led to considerably lower
conversions, whereas the use of [Ph3PAuCl]/AgSbF6 in CH2Cl2
led to mixtures of products even at lower temperatures. See the
Supporting Information for details.
[20] Besides extensive NMR experiments, definitive confirmation of
the structure of 3d was obtained by X-ray crystallography of an
immediate derivative, see the Supporting Information.
CCDC 661923 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via www.
ccdc.cam.ac.uk/data_request/cif.
[21] This [4+2] cycloaddition must be a PtCl2-catalyzed process as
thermolysis of 1e does not give the cycloadduct.
[22] An alternative mechanism for the cycloadditions of allenedienes
1 involving deprotonation of C and protonation of the resulting
vinyl platinum intermediate could be proposed. However,
performing the cycloaddition of 1c in toluene saturated with
D2O provided nondeuterated cycloadducts (2c:3c = 6:4, 82%
yield), thus making this pathway very unlikely.
[11] The use of allenes as 3C units in catalytic cycloadditions has been
very scarce and limited to [3+2] annulations of activated
derivatives, see a) Y. Xia, Y. Liang, Y. Chen, M. Wang, L. Jiao,
F. Huang, S. Liu, Y. Li, Z.-X. Yu, J. Am. Chem. Soc. 2007, 129,
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