PtCl2-catalyzed [6+2] cycloaddition of alkynes tethered to cycloheptatriene

Article abstract of DOI:10.1002/anie.200705482

(Chemical Presented) Summing up the situation: Eight-membered rings can be synthesized in an intramolecular formal [6+2] cycloaddition of alkynylcycloheptatrienes catalyzed by PtCl₂ (see scheme). The use of heteroatom-tethered substrates provide complex heterocyclic compounds, while the use of a higher temperature leads to an unusual formal [6+1] cycloaddition.

Full text of DOI:10.1002/anie.200705482

Communications
DOI: 10.1002/anie.200705482
Cyclization
PtCl2-Catalyzed [6+2] Cycloaddition of Alkynes Tethered to
Cycloheptatriene**
Alphonse Tenaglia* and Sylvain Gaillard
Transition-metal-catalyzed cycloisomerization reactions of
enynes have attracted considerable attention because of
their high synthetic potential for the construction of useful
carbo- and heterocyclic structural features.[1] Important
advances in this timely area of research have been achieved
using late-transition metals (for example, Ru, Pt, and Au) as
p-acid catalysts.[2] In this context, novel modes of connection
between alkenes and alkynes still remain an attractive field of
investigation, although the chemoselectivity of the reactions
often depends on the structural patterns of the substrates. The
cycloisomerization of polyene-ynes takes advantage of addi-
tional unsaturated bond(s) to trap transient reactive inter-
mediates, thus allowing the elaboration of complex poly-
cycles. For example, the gold-catalyzed cycloisomerization of
dienynes affords Diels–Alder adducts,[3] whereas cyclohexa-
dienyl alkynes generate tetracycles.[4,5] In the particular case
of aryl-tethered alkynes, the cycloisomerization with various
electrophilic metal catalysts afforded Friedel–Crafts-type
compounds, but these reactions generally required electron-
rich arenes.[6] As part of our recent studies on the cobalt-
catalyzed intermolecular [6+2] cycloaddition of cyclohepta-
triene (CHT) with alkynes,[7] we were intrigued by the
behavior of CHT tethered to alkynes in cycloisomerization
reactions involving p-acid catalysts. In these reactions,
nucleophilic attack of CHT on an electrophilic metal-coordi-
nated alkyne B is expected to generate a pentadienyl cation C
which may evolve through ring closure to produce [2+2],
[4+2], and/or [6+2] adducts (Scheme 1).
Scheme 1. [n+2] Cycloadditions through metal-catalyzed cycloisomeri-
zation of 1-(pent-4-ynyl)-1,3,5-cycloheptatriene.
Exposure of triene-yne 1a to catalytic amounts of PtCl2
(5 mol%) in toluene at room temperature resulted in a
complete and clean conversion into a single adduct 2a[8] in
94% yield (Table 1, entry 10). The cycloisomerization pro-
Table 1: Metal-catalyzed [6+2] cycloaddition of 1a into 2a.[a]
Entry
Catalyst
T [8C]
t [h]
Yield [%][b]
1
2
3
4
5
6
7
8
9
10
none
InCl3
80
110
80
80
80
65
25
80
25
25
24
24
24
24
24
8
5
24
5
–
–
RhCl3·nH2O
[RhCl(PPh3)3]
RuCl3·nH2O
[{RuCl2(CO)3}2]
AuCl3
Pt/C
PtI2
PtCl2
6[c]
8[c]
7[c]
77
90
7[c]
39
94
Herein, we report the platinum-catalyzed cycloisomeriza-
tion of 1-(pent-4-ynyl)-1,3,5-cycloheptatrienes, in which con-
nections are made between the carbon termini of the triene
and the acetylenic carbon atoms, to afford tricyclotrienes F,
similar to a formal intramolecular [6+2] cycloaddition.
5
[a] Conditions: 1 (0.36 mmol), catalyst (5 mol%), toluene (c=0.1m).
[b] Yields of isolated products after column chromatography. [c] Deter-
mined by 1H NMR spectroscopic analysis.
[*] Dr. A. Tenaglia, Dr. S. Gaillard
Laboratoire de Synth›se AsymØtrique
UMR 6180 CNRS
ceeded with excellent chemoselectivity, whereas heating at
808C in toluene in the absence of catalyst resulted in the
recovery of unchanged 1a (Table 1, entry 1). To the best of
our knowledge, intramolecular [6+2] cycloaddition reactions
between alkynes and CHT have been only been achieved by
using tricarbonyl(h6-7-exo-alkynyl-1,3,5-cycloheptatriene)-
chromium(0) complexes at high temperatures (140–
1708C).[9] In comparison, the present cycloisomerization is
performed catalytically and under mild conditions (room
temperature), and affords higher yields without preliminary
coordination of the triene to the metal. A catalyst loading as
little as 1 mol% can be used, but the reaction time increases
UniversitØ d’Aix-Marseille P. CØzanne
FacultØ des Sciences et Techniques de St JØrôme, Case A62
Avenue Escadrille Normandie Niemen
13397 Marseille Cedex 20 (France)
Fax: (+33)4-9128-2742
E-mail: alphonse.tenaglia@univ-cezanne.fr
[**] We would like to thank the CNRS for financial support, Dr. I.
De Riggi (ECM, Marseille) for help with the NMR studies, and Dr.
M. Giorgi (Spectropole, UniversitØ P. Cezanne, Marseille) for the X-
ray analyses. S.G. gratefully acknowledges the MNERT for a doctoral
fellowship.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or fromthe author.
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significantly (16 h instead of 5 h) and with a slight decrease in
yield (86% instead of 94%). Interestingly, under these
conditions, the reaction can be scaled-up, thus allowing for
the efficient preparation of 2a on a gram scale.
species, afforded the expected adduct 2l together with
significant decomposition of the starting materials.[10,12,15]
Gratifyingly, the reaction of 1l at 1008C in a CO atmos-
phere[16] resulted in complete conversion and the isolation of
2l in 53% yield (Table 2, entry 11).
The cycloisomerization of 1a to afford 2a was examined
with other metal salts and complexes (Table 1).
Substrates 3a–e with an internal alkyne moiety proved to
be reluctant to cycloisomerize even if the reactions were
conducted at 1108C (Scheme 2). Alkyl- (3a,b) or phenyl-
[{RuCl2(CO)3}2][5a,10,11] (Table 1, entry 6) and AuCl3
[10,11a,12]
(Table 1, entry 7) proved satisfactory, whereas rhodium com-
plexes were poor catalysts (Table 1, entries 3 and 4). The
higher efficiency of PtCl2, compared to PtI2, is in agreement
with its higher electrophilicity (Table 1, entries 9 and 10).[13]
The reaction can be conducted in a wide range of solvents—
THF (85%), acetone (91%), and toluene (94%) were the
most effective—but performing the reaction in MeOH,
MeCN, or DMF left 1a unchanged.
The reaction scope with respect to the type of tether and
its length was also investigated (Table 2). The formal
Scheme 2. Platinum-catalyzed cycloisomerization of trienynes 3a–e.
Table 2: PtCl2-catalyzed [6+2] cycloaddition of alkynes tethered to cyclo-
heptatriene.
substituted (3c) substrates were recovered unchanged while
electron-deficient alkynes (3d,e) afforded the [6+2] cyclo-
adducts 4d,e, albeit in fair to low yields. Skeletal reorgan-
ization[11b] or metathesis[17] products, such as those observed in
cycloisomerizations of 1,6-enyoates (or enynones), were not
detected. The heterocyclic adduct 5 (39%), which was formed
from 3e as a single diastereomer, suggested the interception
of the putative cationic intermediate C (Scheme 1) by the
pendant acetyl group.
The cycloisomerization of trienynes 6a–g bearing a
heteroatom in the tether was also examined (Table 3).
Besides the desired [6+2] cycloadducts 7, dihydropyranes or
tetrahydropyridines 8 were formed with concomitant 1,2-
hydrogen or 1,2-alkyl migration.[18] Additionally, new adducts
Entry Trienyne
n
R
Adduct Yield [%][a]
1
1b
1c
1d
1e
1 f
1g
1h
1i
1
1
1
1
1
1
1
1
1
1
2
R1 =R2 =SO2Ph
R1 =R2 =CN
2b
2c
2d
2e
66
79
92
85
2[b]
3
R1 =R2 =CH2OAc
R1 =R2 =CH2OTs
4
5
6
7
R1, R2 =CH2OCMe2OCH2 2 f
82
72
R1, R2 =CH2OC(O)OCH2 2g
R1 =H, R2 =CO2Me
R1 =H, R2 =CH2OAc
R1 =H, R2 =CH2OBn
R1 =H, R2 =CH2Br
R1 =R2 =CO2Me
2h
2i
2j
2k
2l
99 (4:1)[c,d]
83 (4:1)[c]
86 (2.8:1)[c]
75 (3.3:1)[c]
53
8
9
1j
10
1k
1l
11[e]
Table 3: Platinum-catalyzed cycloisomerization of (cyclohepta-1,3,5-
trien-1-yl)methyl, propargyl ethers 6a–f or amine 6g.
[a] Yields of isolated compounds after column chromatography. [b] 658C,
12 h. [c] Diastereomeric ratio (d.r.) determined by 1H NMR spectroscop-
ic analysis of the crude reaction mixture. [d] For the major diastereomer,
the sustituent R2 is anti to the methano bridge; see the Supporting
Information. [e] 1008C, 18 h in a CO atmosphere. RT=roomtemper-
ature. Ts=toluene-4-sulfonyl, Bn=benzyl.
[6+2] cycloaddition under the initial conditions (5 mol%
PtCl2, c = 0.1m, toluene, RT) proceeded efficiently with
triene-ynes 1b–l carrying acetal, ether, ester, sulfonate,
cyclic carbonate, or halide substituents. Even 1c with nitrile
groups, which are known for their ability to coordinate with
metal salts, afforded 2c,[14] but the reaction required higher
temperature (658C; Table 2, entry 2). In all cases, no other
Entry
Substrate
Conditions
Yields [%] (7/8/9)[a]
1
1
2
3
4
5
6
7
8
6a
6b
6c
6c
6d
6e
6 f
6g
808C, 10 h
808C, 12 h
258C, 7 days
808C, 10 h
808C, 14 h
808C, 12 h
808C, 30 h
908C, 10 h
48/34/–
adduct could be detected by H NMRspectroscopic analysis
41/46 (8b/9b=1:1.7)[b]
41/41/–
of the crude reaction mixture. Monosusbtitued substrates 1h–
k were converted into the corresponding [6+2] cycloadducts
2h–k as a mixture of diastereomers (d.r. 2.8:1–4:1; Table 2,
entries 7–10). Lengthening the tether by one carbon unit, such
as in 1l, did not result in the [6+2] cycloaddition occurring
under the usual conditions. Carrying out the reactions at
higher temperatures (80–1008C), or in the presence of silver
salts (AgOTf, AgPF6, or AgSbF6) to generate cationic Pt2+
–/40/–[c]
34/–/19
78 (d.r. 2.81:1)/–/–[d]
68/–/–
45/10/31
[a] Yields of isolated products. [b] Not separated. [c] Adduct 10 (51%)
was also formed. [d] Adduct 11 (20%) was also formed.
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such as 9 (which feature a cyclobutane ring), 10, or 11 were
observed from the geminal-substituted precursors 6c and 6d,
respectively (Table 3, entries 3 and 5). The formation of 9
from 8 (see above) might involve the coordination of Pt2+
species to the remote double bond of 8 followed by an
electronic redistribution involving cleavage of the cyclopro-
pane ring.
The opening of the tetrahydrofuran ring of 7c (or 7e)
triggered by adventitious HCl (generated from the PtCl2
catalyst) to give a stabilized tertiary allylic cation[23] may
explain the formation of adduct 10 (or 11).
In summary, we have developed a platinum-catalyzed
room-temperature intramolecular [6+2] cycloaddition of
alkynes tethered to cycloheptatriene to afford cyclopentane-
fused bicyclo[4.2.1]nona-2,4,7-trienes. Complex heterocyclic
compounds were obtained when this reaction was applied to
substrates containing a heteroatom (O, N) in the tether. At
observed depending on the structural patterns of the pre-
cursors 6 and/or the reaction conditions. For example, the
reaction of 6c at room temperature required seven days for
completion and afforded 7c (41%) and 8c (41%; Table 3,
entry 3). When the reaction was carried out at 808C for 10 h,
the “ring-opening” adduct 10 (51%) was formed at the
expense of 7c, while 8c (40%) was observed without a
significant change in yield (Table 3, entry 4).
Control experiments showed that exposure of 7c to the
same reaction conditions did not give the bicyclic structure 10,
but allowed the recovery of 7c with partial degradation.[19]
A
small variation in the ring size of the tertiary propargylic
ethers (6c,d) dramatically changed the selectivity of the
reaction. For example, under quite similar conditions, while
6c afforded 8c (40%) and 10 (51%), 6d was converted into
7d (34%) and 9d (19%; Table 3, entries 4 and 5). Since
formation of products 8 and 9 involved a 1,2-hydrogen (or
alkyl) shift, the question of a shared common intermediate or
the possibility of 8!9 interconversion arose. Indeed, subject-
ing 8g to PtCl2 (5 mol%) in toluene (858C, 16 h) led to 9g in
nearly quantitative yield.[20] The [6+2] cycloaddition is
favored with triene-ynes 6e and 6 f that bear at least one
phenyl substituent at the propargylic position (Table 3,
entries 6 and 7). In these cases, it is worth noting that 8 and
9 were not formed.
higher temperature,
a formal [6+1] cycloaddition was
observed at the expense of the intramolecular vinylcyclopro-
panation pathway.
Received: November 30, 2007
Published online: February 18, 2008
Keywords: cycloaddition · enynes · platinum· polycycles ·
.
strained molecules
A mechanistic scheme accounting for the structural
diversity of adducts arising from the cycloisomerization
reactions of 6 is proposed in Scheme 3.[21] The pentadienyl
cation I[22] derived from the established exo-cyclization mode
would undergo a kind of “concerted” electron redistribution
to release [6+2] adducts 7. If the alkyne carries an acyl
substituent, the carbonyl group would intercept the initially
formed cation to give the dihydropyran 5. The endo-cycliza-
tion mode would drive the reaction to product 8 through
zwitterion II and platinacarbene III intermediates. The III!8
transformation is assumed to occur through a 1,2-hydrogen or
alkyl shift before elimination of the metal species. The latter
case is evident from the ring-expansion adducts 8c and 9d
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[20] Subjecting a mixture of 8b and 9b (1:1.7; Table 3, entry 2) to the
same conditions resulted in total consumption of 8b but the
reaction suffered from substantial degradation of the starting
materials.
[21] We warmly thank one reviewer for helpful comments.
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[23] To test this assumption, we attempted to trap the stabilized
cation with nucleophilic species. Indeed, treatment of 7 f with
boron trifluoride and triethylsilane afforded 12 (51%), by
reductive ring cleavage, and oxatriquinane 13 (24%), by
isomerization through participation of the phenyl substituent.
See the Supporting Information..
[14] A single-crystal X-ray analysis of cycloadduct 2c secured the
tricyclic structure of the adduct. CCDC 655906 contains 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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[16] The strong p-acid character of this ligand, which increases the
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