Isolation of a zwitterionic dienegold(III) Complex intermediate in the direct conversion of enyne-amines to cyclopentadienes

Article abstract of DOI:10.1002/chem.201000988

(Figure Presented) Golden ring: A route for the direct AuCl ₃-catalyzed conversion of an enyne-amine into cyclopentadiene is reported. The mechanism for the catalysis is proposed and supported by a resting-state zwitterionic gold(III)cyclopentadiene structure (see figure).

Full text of DOI:10.1002/chem.201000988

DOI: 10.1002/chem.201000988
Isolation of a Zwitterionic DienegoldACTHNUGTRNEUNG(III) Complex Intermediate in the
Direct Conversion of Enyne–Amines to Cyclopentadienes
Michele Melchionna,[a] Martin Nieger,[b] and Juho Helaja*[a]
Gold-mediated homogeneous catalysis has shown, recent-
ly, its power, uniqueness, and versatility especially in electro-
philic activation of alkynes for nucleophilic attack.[1] Cycloi-
somerization of enynes, and other alkynes carrying nucleo-
philic groups represents a large category in gold mediated
catalysis. Due to the fact that this chemistry is in its infancy,
only a few mechanistic studies have been reported.[1f,2]
The recent findings in gold and other late transition-metal
catalysis have provided effective alternatives to classical
Pauson–Khand cyclocarbonylation[3] and the Nazarov cycli-
zation[4] for the synthesis of cyclopentadienes (Cp). Toste
et al. have shown that the cationic AuI species Ph3PAuCl/
AgSbF6 catalyses the cycloisomerazation of 1,2,4-triene
(enallene; 2) to cyclopentadiene (Cp; 3; Scheme 1) with
high yields and turnover numbers.[5] In line with these find-
ings, Iwasawa et al. have shown that the same conversion
can be catalyzed by PtCl2 with equal performance.[6] An evi-
dent restriction in these methods is that the synthesis of the
prerequisite starting material (allenes) proceeds in many
cases with modest or low yields when traditional methods
are applied. Nevertheless, Cheꢀs group has recently intro-
duced a facile method to access (asymmetric) allenes, 2,
from aminoalkynes, 1, either by KAuCl4 or AgNO3 cataly-
sis.[7,8]
Inspired by these achievements we set out to investigate
the possibility of a direct conversion of enyne–amine 1 to
Cp 3, thus reducing a two-step sequence (synthesis of allene
followed by cycloisomerization) to a single catalytic event.
For the gold-catalyzed allene formation from propargyla-
mines, deuterium labeling experiments have proven that a
1,5-hydride migration from the amine moiety controls the
progress and direction of the reaction (Scheme 2).[8] For sys-
tems in which intramolecular migrations occur by other nu-
cleophiles than hydride, there are examples showing how
the migration mode, and therefore the direction of reaction
can be controlled, for example by the choice of cationic AuI
or neutral AuACTHNUGRTNEUNG(I/III) catalysts, as well as by slight modifica-
tions of the substrate. These two factors can both have a
Scheme 1. Transition-metal catalyzed stepwise conversions from enyne–
amine, 1 via enallene, 2 to Cp 3.
[a] Dr. M. Melchionna, Dr. J. Helaja
Department of Chemistry, University of Helsinki
A.I.Virtasen aukio 1, P.O. Box. 55, University of Helsinki
00014 Helsinki (Finland)
Fax : (+358)919150466
E-mail: juho.helaja@helsinki.fi
[b] Dr. M. Nieger
Laboratory of Inorganic Chemistry, Department of Chemistry
University of Helsinki, A.I.Virtasen aukio 1
P.O. Box. 55, 00014 Helsinki (Finland)
Scheme 2. Mechanistic pathways from enyne–amines by a) proven 1,5-hy-
dride migration to enallenes, or b) hypothesized 1,4-hydride migration to
Cps.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201000988.
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Chem. Eur. J. 2010, 16, 8262 – 8267

COMMUNICATION
major impact on the regioselectivity of the nucleophilic
attack and subsequently of the resulting cycloisomerization
products.[1f,9,10] Analogous to this, our strategy was based on
the assumption that a systematic variation of the catalyst
and substrate could eventually lead to a change in the hy-
dride migration mode, namely from a 1,5- to a 1,4-hydride
shift, thus inducing a direct conversion to a Cp in place of
an allene (Scheme 2).
Cyclohexene-functionalized propargylic amines, 4a–4 f,
were chosen as substrates to create ring-fused Cp (Table 1).
For substrate 4a, Che and co-workers previously reported
that pure allene was formed with 83% yield based on 77%
existence of a distinct reaction pathway for the formation of
Cp, rather than originating from an allene intermediate.
This was somehow expected because if the Cp was to be
produced in a second step from the initially formed allene,
mixtures of 5 and 6 would be found regardless of what prop-
argylamine is employed.
To probe the reaction mechanism the isoquinoline protons
were deuterium labeled at the bridging amino benzylic posi-
tion (Scheme 3). These experiments revealed that only a
Table 1. Reaction of enynamines 4a–4 f with gold complexes.[a]
Substrate R1
Catalyst Conversion Yield of
Ratio 5/6[c]
(10%)
[%]
5+6 [%][b]
4a
4b
KAuCl4 77[d]
KAuCl4 67
83[d]
100:0
34
76
ꢀ99:1
4c
KAuCl4 69
77:23
Scheme 3. Proposed AuCl3 catalytic cycle from enyne–amine 7 to enal-
lene 8 or Cp 9.
4d
4e
KAuCl4
0
–
–
KAuCl4 56
91
40:60
1,5-deuteride transfer had taken place in contrast with our
initial hypothesis. Apparently, the large isoquinoline moiety
did not change the mode of hydride migration, but favored
the Cp formation over the allene possibly due to electronic
effects (i.e., the 3,4-dihydroisoquinolinium cation being a
poorer leaving group than other iminium leaving groups).
The latter hypothesis was further investigated by carrying
out the catalysis on substrate 7, in which the bulkiness of
the cyclohexene moiety was also increased by replacing it
with the more space demanding 2,2-dimethylcyclohexene
moiety (Table 2). To our delight, conversion of 7 to Cp 9 by
KAuCl4 occurred with an increased Cp selectivity with only
traces of allene 8. Lowering the catalyst loading from 10 to
5% slightly increased the conversion (35!40%), but de-
creased the Cp formation selectivity. Meanwhile, a higher
loading (15%) boosted the selectively towards Cp, but low-
ered the yield. Hashmi et al. have shown that N,O-ligated
AuIII complexes (e.g., picolinate and pyridine) are beneficial
precatalysts for chemical transformations of complex sub-
strates.[11] However, in our case, it is noteworthy that the use
of different gold catalysts resulted in relatively minor
changes in terms of conversion (highest conversion taking
place with (pyridine)AuCl3; Table 2, entry 5), whereas the
[a] All reactions carried out at 408C. [b] Yield of the isolated products
(flash column) based on conversion. [c] Determined by 1H NMR spec-
troscopy.[7] [d] Reported for the S enantiomer at the benzylic carbon.
conversion using the KAuCl4 catalyst.[7] In our study the
prolinol moiety was replaced by piperidine (4b), azepane
(4c), indoline (4d), and 1,2,3,4-tetrahydroisoquinoline (4 f)
to assess whether a difference in steric and electronic prop-
erties of the amine would have an effect on the hydride
transfer process. When catalysis was performed on 4b only
allene product was formed, while 4d remained totally un-
reacted. The reaction with 4c produced, somewhat promis-
ingly, some Cp, 5, with the allene, 6, still being the dominat-
ing product. A far more encouraging result was obtained
with substrate 4e, with which a 4:6 mixture of 5 and 6 was
isolated.
1
Monitoring the catalysis of 4e by H NMR spectroscopy
indicated that 5 and 6 were formed steadily in the course of
the reaction. Moreover, the addition of extra catalyst into
the reaction mixture did not lead to any further consump-
tion of the formed allene. Together these facts suggest the
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J. Helaja et al.
Table 2. Reaction of 7 with various catalysts.[a]
terestingly the vinylic proton of the cyclohexenyl ring could
not be traced in the spectra and full analysis of the 2D
NMR spectroscopy couplings through bond and space was
consistent with a gold metallacycle type structure. Further-
more, the assignment of the carbon signals at d=170 and
160 ppm to carbon atoms adjacent to gold implies that the
electronic nature of the gold species is either neutral or
anionic, since strongly upfield-shifted dc values have been
reported for cationic Au-coordinated olefinic carbons.[12]
When the reaction was continued by warming either the re-
action mixture or the crystallized complex 7c at 408C, for-
mation of Cp 9 was observed.
Final confirmation for the structure of this intermediate
was obtained from single crystal analysis (Figure 2).[13] The
gold atom is featured as AuIII, lying in a square planar ge-
ometry, with two covalent bonds to carbon atoms, and two
chloride ligands still coordinated, therefore formally indicat-
Catalyst (10%) Conversion [%][b] Yield of 8+9 [%][c] Ratio 8/9[b]
1
2
3
4
5
6
7
8
9
KAuCl4
KAuCl4 (5%)
KAuCl4 (15%) 34
PicolinateAuCl2 49
35
40
54 (15)
89 (36)
34 (12)
29 (14)
56 (28)
40 (16)
39 (14)
0
ꢀ1:99
19:81
0:100
33:67
12:88
50:50
90:10
–
PyridineAuCl3
AuCl
50
41
35
–
AuCl[d]
K2PtCl4
AgNO3
12
56 (7)
100:0
[a] All reactions carried out in MeCN at 408C for 18 h except where indi-
cated. [b] Determined by 1H NMR spectroscopy. [c] Yield of the isolated
product based on conversion, absolute yields in the parenthesis. [d] Re-
action carried out at RT.
ꢁ
ing an excess of negative charge on the metal. The C21 C20
ꢁ
and C1 C2 bond lengths are typical of C=C double bonds,
ꢁ
whereas a distance of 1.465 ꢂ for C20 C1 is characteristic
for a conjugated single bond, thus supporting the view of a
metallacyclopentadiene for the C5 ring defined by the gold
ꢁ
Cp/allene ratio was more significantly affected. Most inter-
estingly, use of AuCl resulted in the conversion of 7 to a
mixture of 8 and 9 in a notably faster rate at 408C, whereas
lowering the temperature to room temperature led to
almost selective formation of
and the coordinated ligand. The N4 C5 distance is that of a
C=N double bond, as expected once the hydride has been
transferred. Since the bonding in the imine moiety denotes
a positive charge on the nitrogen atom, a zwitterionic nature
the allene. All of the goldACTHNUTRGNE(UNG III)
catalysts were inactive at room
temperature. No conversion
was observed with K2PtCl4,
whereas poor conversion to the
allene only was obtained with
AgNO3.
The progress of the reaction
of 7 with 15% of KAuCl4 was
monitored by 1H NMR spec-
troscopy at 408C. The reaction
was found to proceed in a step-
wise manner; after an induction
time of about 15 min, an inter-
mediate was observed, which
persisted for about another
15 min at 408C before slowly
starting to convert into the Cp.
The experiment was repeated
with higher catalyst loadings
(Figure 1) to trace possible in-
termediates. When the temper-
ature was lowered to 108C,
after 30 min the progress of the
reaction stopped and the ob-
served intermediate structure
could be characterized at this
temperature by 2D NMR spec-
troscopy experiments (COSY,
Figure 1. 1H NMR spectroscopy monitoring of gold catalysis on enyne 4 (0–35 min time frame) with a stoichio-
metric amount of KAuCl4 in CD3CN at 408C. The characteristic signals identified are traced to 4 or its salt
form and 7c. From this mixture 7c was crystallized out and fully characterized by using 2D NMR spectroscopy
NOESY, HSQC, HMBC). In- techniques and X-ray.
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Chem. Eur. J. 2010, 16, 8262 – 8267

Isolation of a Zwitterionic DienegoldACTHUNRTGNEUNG(III) Complex
COMMUNICATION
triple bond to the gold followed by 1,5-hydride migration.
The subsequent step is crucial for the direction of the reac-
tion: a combination of the bulkiness of the cyclohexene and
ꢁ
amine moieties drives the catalysis towards vinylic C H ac-
tivation and thereby AuCl3 migration and Cp formation,
whereas with less hindered substrates enallene 8 is formed.
Presumably, steric crowding pushes the methyl groups away
from both the gold and isoquinoline proximities bringing
the vinylic proton into close contact with the anionic AuCl3
(7b). It turns out that electronic properties of the amine
also give an important contribution to the selectivity of the
process, since it produces extra stabilization of the inter-
mediate 7b by delocalizing the positive charge onto the 3,4-
dihydroisoquinolinium moiety.
To gain a theoretical insight into the proposed mechanism
we studied computationally selected Au-complexed isoatom-
ic intermediates (7a, 7b, and 7d) and hydride migration
transition-state (TS) structures (1,4- and 1,5-shifts) with
DFT theory (Figure 3).[17,18] In the energy-minimized starting
geometry structure, 7a, AuCl3 is coordinated to the terminal
enyne carbon against the expected bisected coordination to
both triple bond carbons. This structure is also geometrically
rather close to the TS of the 1,5-hydride migration, in which
the free-energy barrier height is only 2.2 kcalmolꢁ1. In con-
trast to this, the TS for the 1,4-hydride migration is
19 kcalmolꢁ1, with this high energy level making it unlikely
to occur. The overall inspection of the relative intermediate
energies of 7b and 7d supports the proposed mechanism,
that is, both the 1,5-hydride migration and the 1,4-AuCl3 mi-
gration are energetically favorable steps. The details relating
to the ring-closing steps to form 7c and 9 will be investigat-
ed in future studies.
In summary, a direct AuCl3 catalytic route from an
enyne–amine to a Cp is reported and a related mechanistic
pathway proposed, which is supported by the isolation of
single crystals of one of the intermediates, as well as DFT
calculations. We are currently applying these reactivity find-
ings to other related AuIII-mediated catalysis. In a wider per-
spective the proposed novel gold metallacyclopentadiene in-
termediate, as characterized by the crystal structure, can be
expected to inspire further reactivity-based method develop-
ment of AuIII-mediated catalysis.
Figure 2. Molecular structure of 7c (displacement parameters are drawn
ꢁ
ꢁ
at 50% probability level). Key bond lengths (ꢂ): Au1 C2 2.027(3), Au1
ꢁ
ꢁ
ꢁ
C21 2.043(4), Au1 Cl1 2.3664(10), Au1 Cl2 2.3988(12), C20 C21
[13]
ꢁ
ꢁ
ꢁ
1.344(6), C1 C20 1.465(6), C1 C2 1.343(5), N4 C5 1.283(5).
for the neutral complex has to be invoked. The extraordina-
ry stability of 7c arises presumably from the fact that the
cationic isoquinoline imine moiety (3,4-dihydroisoquinolini-
um) is a relatively poor leaving group, therefore providing
the intermolecular counterion for the anionic gold moiety.
Overall the characterized AuIII metallacyclodiene struc-
ture is exceptional in itself,[14] but in particular because it is
proposed as a catalytic intermediate. A number of neutral
and cationic AuI complexes have been reported as proposed
catalytic intermediates.[15] Among these, only two have been
structurally characterized as vinylic intermediates, both fea-
turing cationic linear AuI species. The AuCl3 characteristic
ꢁ
in C H activation of aromatic compounds has been proven
ꢁ
ꢁ
by a trapped Ar AuCl2
N
ogous heterocyclic cycloaurated complexes.[16] Among them
Cinellu et al. have reported the first anionic gold(III) oxo
AHCTUNGTRENNUNG
intermediate.[16f] However, to the best of our knowledge, the
present structure represents the first example of a truly zwit-
terionic AuIII trapped intermediate in the gold-catalyzed ac-
tivation of a p-system in which the Au moiety possesses
anionic character.
Experimental Section
See the Supporting Information for experimental details.
The experimental data suggests that the zwitterionic struc-
ture 7c is at least a resting structure along the catalytic path-
way from enyne–amine 7 to Cp 9. We propose that the com-
plex is a key resting intermediate deriving from HCl elimi-
nation, which functions as a shuttle between two structures
(7b and 7d) in which the AuCl3 group migrates from C2 to
C21 (Scheme 3). In the proposed mechanistic pathway the
allene 8 and Cp 9 formations are derived from the same cat-
Acknowledgements
We are grateful to Dr. Petri Heinonen and Dr. Jorma Matikainen for the
HRMS measurements. This work was partially supported by Academy of
Finland (JH no. 118586 and 113317). The National Centre for Scientific
Computing (CSC) is acknowledged for computational resources.
ꢂ
alytic cycle; the cycle starts with coordination of the C C
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J. Helaja et al.
Figure 3. Structures and Gibbs free energies [kcalmolꢁ1] of AuCl3 complexed intermediates and transition states related to Scheme 3 (B3LYP 6-31G-
ACHTUNGTRENNUNG
(d,p)/LANL2DZ).[17]
ꢁ
[9] Example of subtle substrate-dependent regioselectivity in propargy-
lamide cycloisomerizations: N-propargyl carboxamides, having ter-
minal alkynyl groups undergo 5-exo-dig cyclizations and N-alkynyl
carbamates experience the 5-endo-dig cyclization, whereas different
Keywords: alkynes · C H activation · cyclopentadienes ·
gold · zwitterions
ꢂ
N-propargyl carboxamides having internal, alkyl-substituted C C
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Isolation of a Zwitterionic DienegoldACTHUNRTGNEUNG(III) Complex
COMMUNICATION
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Received: April 16, 2010
Published online: June 29, 2010
Chem. Eur. J. 2010, 16, 8262 – 8267
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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