A straightforward procedure for the [2+2+2] cycloaddition of enediynes

Article abstract of DOI:10.1002/adsc.200800646

Enediynes undergo intramolecular [2+ 2+2] cycloaddition in the presence of cobalt(II) iodide (CoI₂), manganese and an N-heterocyclic carbene (IPr) generated in situ from the corresponding imidazolium salt and butyllithium (BuLi). Polycyclic cyc

Full text of DOI:10.1002/adsc.200800646

UPDATES
DOI: 10.1002/adsc.200800646
A Straightforward Procedure for the [2+2+2]Cycloaddition of
Enediynes
Anaꢀs Geny,a Sophie Gaudrel,a Franck Slowinski,a Muriel Amatore,a
Gaꢁlle Chouraqui,a Max Malacria,a Corinne Aubert,a,* and Vincent Gandona,*
a
Laboratoire de Chimie Organique UMR 7611, Institut de Chimie Molꢂculaire FR2769, Universitꢂ Pierre et Marie Curie,
Tour 44-54, 4 place Jussieu, 75252 Paris, France
Fax : (+33)-1-4427-7360; e-mail: corinne.aubert@upmc.fr or vincent.gandon@upmc.fr
Received: October 21, 2008; Published online: December 29, 2008
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.200800646.
Abstract: Enediynes undergo intramolecular [2+
2+2]cycloaddition in the presence of cobalt(II)
iodide (CoI2), manganese and an N-heterocyclic
carbene (IPr) generated in situ from the corre-
sponding imidazolium salt and butyllithium (BuLi).
Polycyclic cyclohexadienes are obtained selectively.
This new method represents an interesting alterna-
tive to those employing air-sensitive cyclopentadi-
stoichiometric amount of cobalt is required, for the
final product may trap the catalyst as a very stable h4-
(1,3-cyclohexadiene) complex that will be unable to
turn over.[9] However, the use of one equivalent of a
cobalt complex, especially simple salts such CoX2
(X=Cl, Br, I), is still competitive compared to rhodi-
um-, iridium-, or ruthenium-containing catalysts that
may require up to 20 mol% loads.[10] CoX2 and
A
CoACTHNGUTREN(UNG acac)3, usually associated with phosphines or
Moreover, the N-heterocyclic carbene can be used
catalytically, which is a significant improvement
compared to the corresponding phosphine-based
system which requires an excess of ligand.
amines, zinc, manganese, or Et2AlCl, and possibly
other additives, have been used with success to cata-
lyze [2+2+2]cyclotrimerizations of alkynes,[11] in-
cluding by us,[11a] and [2+2+2]cocyclizations of al-
kynes and nitriles.[12,13] Diels–Alder,[14] homo-Diels–
Alder reactions,[15] and Alder-ene reactions,[16] [6+
2],[17] reductive [3+2],[18] and [4+2+2]cycloaddi-
tions,[19] cyclizations of o-iodobenzoates with alde-
hydes,[20] aryl-sulfur bond formations,[21] Reformatsky
reactions,[22] cross-couplings and conjugate addi-
tions[23] were also performed under these conditions.
Therefore, common catalysts of type CpCoL2 could
be replaced by catalytic systems much easier to
Keywords: cobalt; cycloaddition; dienes; manga-
nese; N-heterocyclic carbenes; polycycles
Introduction
Cobalt complexes are powerful catalysts for the handle (and sometimes more efficient[11a]) to generate
chemo-, regio,- and diastereoselective formation of benzenes or pyridines by [2+2+2]approaches. Yet
1,3-cyclohexadienes by [2+2+2]cycloaddition of two the case of enediynes remained rather unexplored,
alkynes to one alkene.[1] In an intramolecular fashion, with only one example to the best of our knowl-
a,d,w-enediynes can be transformed into complex edge.[11a] In this update, we report a new cobalt-con-
structures,[2] allowing access to natural products such taining system that can easily transform enediynes
as steroids,[3] or illudol derivatives.[4] Numerous exam- into polycyclic fused 1,3-cyclohexadienes.
ples involving complexes of type CpCoL2 (L=C2H4,
CO) have been reported. However, both groups of
catalysts are air-sensitive and difficult to handle. Al- Results and Discussion
though CpCoACHTUNGTRENNUNG(C2H4)2 is very active and can be used
over at room or lower temperature,[5] it is quite diffi- We disclosed that the transformation of enediyne 1a
cult to prepare.[6] On the other hand, CpCo(CO)2 into cyclohexadiene 2a could be achieved in 30 min at
(commercially available) requires heat and/or visible room temperature using CoI2/PPh3/Mn (1/2/10) in di-
light to be active,[7] and these harsh conditions may chloromethane [Scheme 1, Eq. (1)].[11a] Using other
isomerize the desired product.[8] Most of the time, a cobalt halides (CoCl2, CoBr2), phosphines (e.g.,
Adv. Synth. Catal. 2009, 351, 271 – 275
ꢃ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
271

Anaꢀs Geny et al.
UPDATES
Scheme 1. Cyclization of enediynes using CoI2/Mn/PPh3.
dppe), and reducing agents (e.g., Zn), gave rise to CpCo(CO)2 and subsequent oxidative demetalla-
lower yields. Use of one equivalent of CoI2 was neces- tion[31] furnished the expected product 6a in 30%
sary to achieve complete conversion of the sub- yield, and the CoI2/Mn/PPh3 system in 41% yield (B),
strate.[24] Lowering the amount of PPh3 to one equiva- it was possible to reach 64% yield by replacing the
lent resulted in a lower yield of 17%. A large excess phosphine by an NHC (C). The standard carbene
of manganese was also necessary for the reaction to (IPr) was generated in situ from the corresponding
start rapidly. Under refluxing conditions the yield imidazolium chloride (commercially available) and
could be increased to 76% [Eq. (2)], however the ho- BuLi in THF at 08C.[32] This solution was then trans-
moannular 1,3-cyclohexadiene 2a was accompanied ferred to CoI2 at 08C and then to 5a and Mn at room
by the corresponding heteroannular 1,3-diene 3a and temperature before refluxing for 4 h.[33]
also a small amount of aromatization product 4a. The
isomerization of 2a into 3a was complete after a few
days in the NMR tube (CDCl3).[25] The same prob-
lems were encountered with other substitution pat-
terns and tether lengths (enediynes 1b–e).
Various cobalt catalysts were screened for effective-
ness in the above transformations, however Co
ACHTUNGTNER(NUNG acac)3,
Co2(CO)8 in the presence of dppe, or ClCo(PPh3)3 in
AHCTUNGTRENNUNG
different solvents were not active, the starting materi-
al remaining unchanged. It is now well established
that the replacement of phosphines, amines or cyclo-
pentadienyl ligands by N-heterocyclic carbenes
(NHC) can have a dramatic influence on both reactiv-
ity and selectivity.[26] Although NHC-Co complexes
are known,[27] the association of NHCs with cobalt for
synthetic purposes remains very scarce. Intramolecu-
lar cycloadditions of triynes catalyzed by NHC-CoCl2/
Zn have been reported,[28] as well as NHC-CoCl2-cata-
lyzed sequential cyclization/cross-coupling reactions
of 6-halo-1-hexene derivatives with Grignard re-
agents,[29] and also dehydrohalogenation reactions.[30]
In this context, an important breakthrough was made
while studying the cyclization of enediyne 5a under
various experimental conditions (Scheme 2). Whereas
the “classical” two-step conditions
A
involving Scheme 2. Cyclization of 5a under various conditions.
272
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ꢃ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2009, 351, 271 – 275

A Straightforward Procedure for the [2+2+2]Cycloaddition of Enediynes
Table 1. Cyclization of enediynes using CoI2/Mn/IPr.
UPDATES
Entry Enediyne X
R1
R2 Product Yield
[%][a]
1
2
3
4
5
6
7
8
5b
5c
5d
5e
5f
5g
5h
5i
NTs
O
O
O
O
O
O
O
H
H
Me
Ph
n-Bu
CO2Me H 6g
SiMe3
Br
H
H
H
H
H
6b
6c
6d
6e
6f
66
84
76
72
83
81
[b]
H
H
–
–
–
–
Scheme 3. Diastereoselective cyclizations of enediynes 7a–d.
[b]
9
5j
5k
5l
5m
5n
5o
5p
5q
O
O
O
H
H
Me 6j
Ph 6k
79
73
42
54
30
68
64
65
10
11
12
13
14
15
16
for steric reasons, impede the cyclization, substitution
at the terminal alkyne terminus was possible, as
shown by the transformation of methyl-, phenyl- and
butyl-substituted enediynes (entries 3–5). Interesting-
ly, the reaction proved to be also compatible with an
ester group at the alkyne (entries 6, 11 and 14). Sub-
stitution at the internal alkene carbon was also possi-
ble, as shown in entries 9–13, and 16. In particular, we
could achieve the cyclization of a disubstituted ene-
diyne (entry 11), albeit in a modest 42% yield. Over-
CO2Me Me 6l
H
H
CO2Me H 6o
H
H
C
C
C
C
C
N
Me 6m
Ph 6n
H
6p
Me 6q
[a]
[b]
Isolated yields.
No reaction.
It is worthy of note that CoI2 remains the reagent all, the yields range from 30 to 84%, most of them
of choice, even with IPr instead of PPh3. Indeed, being particularly good.
CoCl2, CoBr2, Co(acac)x (x=2 or 3) or FeCl3 failed to Substitution at the external alkene carbon was also
ACHTUNGTRENNUNG
mediate the cyclization. The association of Zn instead studied (Scheme 3). Methyl, ethyl, and phenyl groups
of Mn with any of these salts and IPr did not work gave the expected products 8a–d in moderate to good
either. In addition to the fact that the yield was ac- yields. The cyclization proved to be diastereospecific:
tually the best with the carbene-based system, several (E)-enediynes 7a–c led to products 8a–c in which H1
other major advantages compared to the phosphine- and H2 were found to be trans to each other (e.g.,
containing mixture could be found. (i) Whereas PPh3 11% nOe between H1 and Me in 8a, no effect be-
had to be used in excess, only a catalytic amount of tween H1 and H2), whereas the (Z)-enediyne 7d gave
carbene was required to promote the cyclization. The 8d exclusively, displaying a cis relationship between
optimized amount of carbene was found to be H1 and H2 (11% nOe).
6 mol%. Lower loads of IPr resulted in longer reac-
tion times (e.g., 24 h with 4 mol%, 4 days with
1 mol%), and extensive levels of aromatization. (ii) Conclusions
Of particular interest, when IPr was used instead of
PPh3, it was no longer required to use an excess of The association of NHCs with cobalt salts for media-
manganese, only one equivalent proved enough to ting organic transformations is still in its infancy. In
ensure complete conversion of the substrate.[34] (iii) this update, we describe a new cobalt-containing
Last but not least, neither isomerization of the diene system capable of mediatingthe [2+2+2]cycloaddi-
fragment nor aromatization were observed when tion of enediynes (CoI2/Mn/IPr). It is easier to imple-
6 mol% IPr were used instead of PPh3. The scope and ment than CpCo(CO)2 and seemingly more efficient.
limitations associated with these new reaction condi- The NHC can be used catalytically, and the chemose-
tions were investigated next. We found that the reac- lectivity of the reaction was improved compared to
tion tolerated the presence of heteroatoms in the the corresponding PPh3-based system. Efforts are un-
tethers (Table 1, entries 1–11). With the exception of derway to achieve chirality transfers from enantio-
SiMe3 (entry 7) and Br (entry 8), which, presumably pure NHCs to the reaction products.
Adv. Synth. Catal. 2009, 351, 271 – 275
ꢃ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
273

Anaꢀs Geny et al.
UPDATES
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Experimental Section
General Procedure for the Cyclization of Enediynes
A solution of 1,3-bis(2,6-diisopropylphenyl)-3H-imidazolium
carbene (0.06 mmol) in THF (3 mL) (prepared at 08C from
1,3-bis(2,6-diisopropylphenyl)-3H-imidazolium
chloride
(25 mg, 0.06 mmol) and BuLi (0.3 mL, 0.2M in hexanes,
0.06 mmol) in THF (3 mL) was added in the dark, at 08C to
a mixture of anhydrous CoI2 (313 mg, 1 mmol) in THF
(4 mL). After 15 min, this solution was added to a suspen-
sion of enediyne (1 mmol) and Mn (55 mg, 1 mmol) in THF
(3 mL) at room tmperature. The resulting mixture was
stirred at reflux in the dark for 4 h. After being cooled
down, the mixture was filtered through a pad of celite with
EtOAc and strd NH4Cl was added. The mixture was extract-
ed with EtOAc, washed with brine, dried over MgSO4. The
solvent was removed under vacuum. The crude product was
purified via flash column chromatography using gradient
mixtures of pentane and EtOAc to give the corresponding
cyclohexadiene.
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Supporting Information
slinger, C. Aubert, V. Gandon, M. Malacria, E. Paredes,
K. P. C. Vollhardt, Synlett 2008, 2056; b) V. Gandon, C.
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047084289X.rn00943.pub2.
Preparation of the starting enediynes and characterization
data of all new compounds is available in a Supporting In-
formation.
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This work was supported by CNRS, MRES, and IUF.
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[32] The carbene is now also commercially available and
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[33] We checked that CoI2/Mn/imidazolium chloride, CoI2/
Mn/BuLi, CoI2/NHC, or even the NHC alone did ac-
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[18] H.-T. Chang, T. T. Jayanth, C.-H. Cheng J. Am. Chem.
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Adv. Synth. Catal. 2009, 351, 271 – 275
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