Carbocations or cyclopropyl gold carbenes in cyclizations of enynes

Article abstract of DOI:10.1002/asia.201000557

Theoretical and experimental studies on the gold(I)-catalyzed formal [4+2] cycloaddition of dienynes are consistent with a mechanism that proceeds by means of cyclopropyl gold(I) carbenes instead of simple carbocations. This work shows that homoallylic st

Full text of DOI:10.1002/asia.201000557

FULL PAPERS
DOI: 10.1002/asia.201000557
Carbocations or Cyclopropyl Gold Carbenes in Cyclizations of Enynes
Patricia Pꢀrez-Galꢁn,[a] Nolwenn J. A. Martin,[a] Araceli G. CampaÇa,[b]
Diego J. Cꢁrdenas,[b] and Antonio M. Echavarren*[a]
Dedicated to Professor Eiichi Nakamura on the occasion of his 60th birthday
Abstract: Theoretical and experimental studies on the gold(I)-catalyzed formal
[4+2] cycloaddition of dienynes are consistent with a mechanism that proceeds by
means of cyclopropyl gold(I) carbenes instead of simple carbocations. This work
shows that homoallylic stabilization is significant even for systems in which the ter-
tiary carbocation is stabilized by two methyl groups.
Keywords: carbenes · cyclization ·
density functional calculations
·
enynes · gold
Introduction
enynes.[11,12] Indeed, cyclopropyl metal carbenes were first
proposed as intermediates in PdII-catalyzed cyclizations of
1,6-enynes in the presence of 1,3-dienes.[13] Intermediates 2
are also involved in the skeletal rearrangement of 1,6-
enynes,[14] intramolecular cyclopropanations[15] of enynes,
and other reactions.[1,16–19] Different trapping experiments
support the involvement of intermediates 2 in these transfor-
mations.[5,20,21] The intermolecular reaction of alkynes with
alkenes catalyzed by gold(I) can also be interpreted as pro-
ceeding through related intermediates.[22]
According to DFT calculations, 1,6-enynes 1 react in the
presence of gold(I) catalysts through intermediates 2[1] that
are trapped by hetero- or carbonucleophiles to form addi-
tion products 3[2–10] (Scheme 1). A similar pathway was pro-
posed for related PtII- and PdII-catalyzed reactions of
Although structures such as 2 are depicted for conven-
ience in a simplified manner as cyclopropyl gold carbenes,[1]
DFT calculations reveal that these species have highly dis-
torted structures, which are intermediate between cyclo-
propyl gold carbenes and gold-stabilized homoallylic carbo-
cations.[4b,7d,9,14a,23] Indeed, we have demonstrated that if the
enyne is substituted at the alkene with strongly electron-re-
leasing R groups, skeletal rearrangements and addition reac-
tions catalyzed by gold(I) lose their usual stereospecificity.[9]
In these cases, the reactive intermediates are better de-
scribed as open carbocations 2’.
Scheme 1. Gold-catalyzed cyclization of 1,6-enynes by means of cyclo-
propyl gold(I) carbenes 2.
[a] P. Pꢀrez-Galꢁn, Dr. N. J. A. Martin, Prof. A. M. Echavarren
Institute of Chemical Research of Catalonia (ICIQ)
Av. Paꢂsos Catalans 16, 43007 Tarragona (Spain)
and
The nature of intermediates 2 has been discussed recently
and their carbocationic character 2’ has been stressed.[3g,24,25]
Actually, the mere involvement of 2 as intermediates in
gold(I)-catalyzed cyclizations of substituted enynes has been
questioned. Thus, for example, it was proposed that the re-
ported mechanism for the [4+2] cycloaddition of dienynes 4
to give hydrindanes 5 by means of intermediates 6 and 7[4]
could be explained more simply by a cyclization of carbocat-
ion 8[26] (Scheme 2). In favor of this interpretation, it was
argued that the more substituted cyclopropyl carbon atom
in intermediate 6 should be less reactive towards nucleophil-
ic attack.[26]
Departament de Quꢃmica Analꢃtica i Quꢃmica Orgꢄnica
Universitat Rovira i Virgili, C/ Marcel·li Domingo s/n
43007 Tarragona (Spain)
E-mail: aechavarren@iciq.es
[b] Dr. A. G. CampaÇa, Prof. D. J. Cꢁrdenas
Departamento de Quꢃmica Orgꢁnica
Universidad Autꢅnoma de Madrid
Cantoblanco, 28049 Madrid (Spain)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201000557.
482
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Chem. Asian J. 2011, 6, 482 – 486

is noteworthy since calculations (DFT at the B3LYP-6-31G
level) predict that 10a and its diastereomer 10a’ possess the
same energy (DE<0.1 kcalmolÀ1). Importantly, reaction of
selectively deuterated dienyne [D3]9a (80:20 E/Z ratio) with
catalyst A led to hydrindane [D3]10a with the labeled CD3
cis to the phenyl group. This result shows that a tertiary car-
bocation such as 12 is either not an intermediate in this re-
action or, if 12 is formed, cyclization is faster than bond ro-
tation that would give a 1:1 mixture of deuterated deriva-
tives.
To better define the mechanism of this process, we per-
formed DFT calculations on a model system that lacked the
substitution at the tether (Scheme 4). These calculations
show that the cycloaddition proceeds through intermediate
Scheme 2. Original mechanistic proposal for the intramolecular [4+2] cy-
cloaddition of dienynes through intermediate 6[4] and alternative proposal
by means of carbocation 8.[26]
The original mechanistic proposal for a Nazarov-type cyc-
lization through intermediates 6 and 7 was based on the
analogy with the mechanism of the [4+2] cycloaddition of
aryl-substituted enynes, which was supported by a theoreti-
cal study.[4b] Therefore, since our original interpretation for
the cyclization of dienynes has been questioned,[26] we decid-
ed to study in detail the mechanism of the gold(I)-catalyzed
cyclization of substrates of type 4 with the aim to shed light
on the nature of the intermediates involved in this and relat-
ed gold(I)-catalyzed processes.
Results and Discussion
We reported before that the cycloaddition of dienyne 9a
using gold(I) catalyst A gave hydrindane 10a stereoselec-
tively[4] (Scheme 3). Similarly, reaction of dienyne 9b with
an electron-withdrawing group on the aryl group in the pres-
ence of cationic gold(I) complex A gave exclusively 10b.
The total stereoselectivity observed in these cycloadditions
Scheme 4. Energy profile for the key step in the [4+2] cycloaddition of
enyne 9 with [AuPMe3]+ in CH2Cl2 calculated with the PCM model (D-
AHCTUNGTRENNUNG
(E+ZPE), kcalmolÀ1) and calculated distances (in red). Energy differen-
ces without solvent effects are given in parentheses.
13 by an outward rotation of the alkenylcarbene by means
of transition state TS13–14 to form allyl cation 14, which cor-
responds to intermediate 7 in Scheme 2. The alternative
inward rotation in intermediate 13 would form 15 through
TS13–15 in a less favorable process. Although the differences
in energy between TS13–14 and TS13–15 are small, these calcu-
lations predict formation of products with the configuration
determined experimentally.
According to DFT calculations, cationic gold intermedi-
ates 13 are not open carbocations and show instead a quite
regular cyclopropyl gold(I) carbene structure with cyclo-
Scheme 3. Gold-catalyzed cyclization of dienynes 9a,b and [D3]9a.
Chem. Asian J. 2011, 6, 482 – 486
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483

A. M. Echavarren et al.
FULL PAPERS
chloride when compared with phosphine ligands.[28,29] The
structures of 14i and 14l, with two methyl groups at the ter-
minus, show longer distances (1.76–1.86 ꢇ with B3LYP,
1.67–1.69 ꢇ with M05) that are nevertheless shorter than
those found for limiting structures with cyclopropyl or p-me-
thoxyphenyl substituents at the terminal carbon atom.[9]
When the gold(I)-catalyzed reaction of dienynes 9a–c was
carried out in methanol, products 16a–c of methoxycycliza-
tion[2] were isolated in 60–90% yield (Scheme 5). These re-
Table 1. Calculated distances [ꢇ] for intermediate 13.
À
Au C
Method
a
b
c
d
B3LYP
M06
2.076
2.083
1.439
1.430
1.547
1.537
1.643
1.611
1.483
1.457
propyl bonds (1.55Æ0.10 ꢇ, Table 1). According to that
shown in Scheme 4, the lengthening bond c (Table 1) in 13
leads to structures that resemble open carbocations, which
are, however, transition states (TS13–14 and TS13–15) and not
intermediates.
A more detailed analysis of intermediates 14 that bear
different R1–R3 substituents using the B3LYP and M05
functionals is shown in Table 2.[27] For the neutral structures
14a–f with L=ClÀ, regular cyclopropyl gold(I) carbenes
were found in all cases with distances c=1.55–1.64 ꢇ,
whereas this bond length was slightly longer in the case of
cationic intermediates 14g,h and 14j,k (1.57–1.66 ꢇ). These
differences are consistent with the higher donating ability of
Table 2. Calculated distances [ꢇ] for intermediates 24.[a]
Scheme 5. Gold-catalyzed methoxycyclization of dienynes 9a,b, [D3]9a,
and enyne [D3]15.
14
a
L
ClÀ
R1
H
R2
H
R3
H
c
1.555
sults show that trapping of the intermediates 13 by methanol
is faster than the cyclization process. Reaction of dienyne
[D3]9a (80:20 E/Z ratio) with catalyst A in methanol as sol-
vent led to [D3]16a as an 80:20 mixture of diastereomers,
which demonstrates again that the intermediate species 13 is
not a free carbocation that could undergo bond-rotation cyc-
lization to form an equal mixture of both diastereomers.
Furthermore, selectively labeled 1,6-enyne [D3]15 (84:16
E/Z ratio) reacted with catalyst A in the presence of metha-
nol to give [D3]17 with the same diastereomeric ratio. This
result demonstrates that bond rotation around bond c in in-
termediate 18 does not take place. Calculation for structures
14i and 18 with the M06 functional (Figure 1) show slightly
less distorted cyclopropyl gold(I) carbenes than those deter-
mined with B3LYP or M05 (Table 2). Importantly, even
though atoms-in-molecules (AIM) analysis[30] shows zero
electron density at the bond-critical point of bond c for
structures 14i and 14l,[27] the calculated bond angle between
bonds b and d (64.7–65.28) was much smaller than the tetra-
hedral angle of 109.58, in accord with a substantial homoal-
lylic stabilization of the cationic species.
AHCTUNGTRENNUNG
b
c
d
e
f
ClÀ
H
Me
Me
H
H
AHCTUNGTRENNUNG
ClÀ
H
Me
H
AHCTUNGTRENNUNG
ClÀ
CH=CH2
CH=CH2
CH=CH2
H
AHCTUNGTRENNUNG
ClÀ
Me
H
H
AHCTUNGTRENNUNG
ClÀ
Me
H
AHCTUNGTRENNUNG
g
h
i
PMe3
PMe3
PMe3
PMe3
PMe3
PMe3
H
AHCTUNGTRENNUNG
H
Me
Me
H
H
AHCTUNGTRENNUNG
H
Me
H
AHCTUNGTRENNUNG
j
CH=CH2
CH=CH2
CH=CH2
AHCTUNGTRENNUNG
k
l
Me
Me
H
AHCTUNGTRENNUNG
Me
AHCTUNGTRENNUNG
[a] B3LYP/6-31G(d) (C,H,P), LANL2DZ (Au). In parentheses: M05/6-
31G(d) (C,H,P), LANL2DZ (Au).
484
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Chem. Asian J. 2011, 6, 482 – 486

Carbocations or Cyclopropyl Gold Carbenes
microwave. The resulting mixture was filtered through Celite and puri-
fied by flash chromatography (10% EtOAc in hexanes).
General Procedure for the Methoxycyclization of Enynes
The enyne (0.15 mmol) dissolved in dry MeOH (0.5 mL) was added to a
solution of cationic AuI complex A (0.002 mmol) in dry MeOH (0.5 mL).
The reaction was stirred at room temperature for 10 h. The resulting mix-
ture was filtered through Celite and purified by flash chromatography
(10% EtOAc in hexanes).
Computational Details
Calculations were performed with the Gaussian 03 series of programs.[35]
The geometries of all complexes were optimized at the DFT level using
the B3LYP functional[36] or the M05/M06 functionals[37] using the stan-
dard 6-31G(d) basis set for C, H, and P. The LANL2DZ basis set, which
includes the relativistic effective core potential (ECP) of Hay and Wadt
and employs a split-valence (double-z) basis set, was used for Au.[38] Har-
monic frequencies were calculated at the same level of theory to charac-
terize the stationary points and to determine the zero-point energies
(ZPE). The starting approximate geometries for the transition states
(TS) were graphically located. Intrinsic reaction coordinate (IRC) studies
were performed to confirm the relation of the transition states with the
corresponding minima. Solvent effects were considered by performing
single-point calculations in CH2Cl2 using the polarized continuum model
(PCM) on the optimized structures.
Figure 1. Calculated structure for intermediates 14i and 18 [M06 6-31G**
(C,H,P), LANL2DZ (Au) (Spartan 08)].
Conclusion
Our results show that the gold(I)-catalyzed cyclization of di-
enynes to form hydrindanes proceeds by means of cyclo-
À
propyl gold(I) carbenes as intermediates, which undergo C
C bond formation faster than rotation around the elongated
cyclopropane bond. Attack of methanol to these intermedi-
ates is also faster than possible rotation around that elongat-
ed bond.
Acknowledgements
We thank the MICINN (CTQ2010-16088/BQU, Consolider Ingenio 2010
grant no. CSD2006-0003, and FPI predoctoral fellowship to P.P.-G.), the
AGAUR (2009 SGR 47), and the ICIQ Foundation for support.
Although the depiction of the intermediates in gold(I)-
catalyzed cyclizations of 1,6-enynes as simple carbocations
has the merit of simplicity and mnemotechnic value, this is
nevertheless a simplistic representation. Our work shows
that homoallylic stabilization is significant even for systems
in which the cation is stabilized by two methyl groups.
This study may be relevant to other transformations of
enynes that proceed by annulation/trapping by nucleophiles
pendant to the alkyne or alkene terminus of the
enynes.[24,31,32] In general, it is important to distinguish pro-
cesses that occur by an overall anti addition of the electro-
phile (the metal-coordinated alkyne) and the nucleophile to
the alkene, such as alkoxycyclizations[2] and polycyclization
reactions,[32] from those that occur by an overall syn addi-
tion. In the first case, in analogy with the Stork–Eschenmos-
er model for squalene and oxidosqualene cyclizations,[33] re-
actions might be concerted, although this hypothesis has not
yet been substantiated theoretically. It is interesting to note,
however, that stepwise mechanisms might be operative even
for the reactions catalyzed by different cyclase enzymes.[34]
For gold(I)-catalyzed syn additions to alkenes, this work, in
line with previous theoretical and experimental results,[4b,16b]
supports a stepwise mechanism.
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Received: August 11, 2010
Published online: October 8, 2010
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