Gold(I)-catalyzed divergence in the preparation of bicyclic enol esters: From exclusively [3C+2C]-cycloaddition reactions to exclusive formation of vinylcyclopropanes

Article abstract of DOI:10.1002/chem.201202881

With the use of benzonitrile-stabilized Au^I catalyst [Au(IPr)(NCPh)]SbF₆ (Ic; IPr=1,3-bis(2,6-diisopropylphenyl)imidazol- 2-ylidene), a spectrum of reactivity is observed for propargyl ester 4 a with cyclic vinyl ethers, ranging from exclusively [3C+2C] cycloaddition reactions to exclusively cyclopropanation depending only on the structure of the substrate. Some initially formed cyclopropanation products rearrange into the corresponding formally [3C+2C] cycloaddition products after treatment with fresh Au ^I complex at 80 °C. Vinylcyclopropanes formed from dihydrofuran and dihydropyran resisted such rearrangement, even in the presence of fresh Au^I catalyst at elevated temperature. This study addresses an important mechanistic question concerning whether the five-membered-ring products were produced by a direct [3C+2C] cycloaddition reaction or by a sequential cyclopropanation/ring-expansion reaction. A dual pathway is proposed for the Au^I-catalyzed reactions between propargyl esters and cyclic vinyl ethers. The different behavior among vinyl cyclic ethers is attributed to the difference in the polarization of the π bond. Highly polarized bonds appear to undergo the cycloaddition reaction whereas less polar π-bonds produce cyclopropanes. Copyright

Full text of DOI:10.1002/chem.201202881

DOI: 10.1002/chem.201202881
Gold(I)-Catalyzed Divergence in the Preparation of Bicyclic Enol Esters:
From Exclusively [3C+2C]-Cycloaddition Reactions to Exclusive Formation
of Vinylcyclopropanes
Ryan C. Conyers and Benjamin W. Gung*[a]
Abstract: With the use of benzonitrile-
stabilized AuI catalyst [Au
(IPr)-
(NCPh)]SbF6 (Ic; IPr=1,3-bis(2,6-di-
isopropylphenyl)imidazol-2-ylidene), a
fresh AuI complex at 808C. Vinylcyclo-
propanes formed from dihydrofuran
and dihydropyran resisted such rear-
rangement, even in the presence of
fresh AuI catalyst at elevated tempera-
ture. This study addresses an important
[3C+2C] cycloaddition reaction or by a
sequential cyclopropanation/ring-ex-
AHCTUNGTRENNUNG
G
pansion reaction. A dual pathway is
proposed for the AuI-catalyzed reac-
tions between propargyl esters and
cyclic vinyl ethers. The different behav-
ior among vinyl cyclic ethers is attribut-
ed to the difference in the polarization
of the p bond. Highly polarized bonds
appear to undergo the cycloaddition
reaction whereas less polar p-bonds
produce cyclopropanes.
ACHTUNGTRENNUNG
spectrum of reactivity is observed for
propargyl ester 4a with cyclic vinyl
ethers, ranging from exclusively
[3C+2C] cycloaddition reactions to ex-
clusively cyclopropanation depending
only on the structure of the substrate.
Some initially formed cyclopropanation
products rearrange into the corre-
sponding formally [3C+2C] cycloaddi-
tion products after treatment with
mechanistic
question
concerning
whether the five-membered-ring prod-
ucts were produced by
a
direct
Keywords: cycloaddition
·
cyclo-
propanation · gold · homogeneous
catalysis · ring expansion
Introduction
Readily accessible propargylic esters are versatile substrates
for Au catalysis and can undergo either 1,2-acetoxyl migra-
tion[1] or 3,3-rearrangment reactions.[2] Two competing reac-
tive intermediates, gold vinyl carbenoids and Au-stabilized
allyl cations, have been invoked in reports in which propar-
gylic esters are employed as precursors in gold-catalyzed re-
actions. It is known that an allene function can be activated
to generate an Au-stabilized allyl cation, which may undergo
intramolecular [4C+3C]-cycloaddition reactions.[3] In 2010,
we reported AuI-catalyzed transannular [4C+3C]-cycloaddi-
tion reactions that take advantage of the allyl oxocarbenium
(2, Scheme 1) derived from a 3,3-rearrangement of propar-
gylic esters to provide the core structure of cortistatins (3).[4]
In addition, we reported a study of an intermolecular ver-
sion of the [4C+3C]-cycloaddition reaction,[5] in which we
discussed the mechanistic possibilities of a direct [4C+3C]
cycloaddition and an alternative two-step sequence of cyclo-
propanation followed by a Cope rearrangement.
Scheme 1. The AuI-catalyzed transannular [4C+3C]-cycloaddition reac-
tion.
more recent results in this area, which involve a benzoni-
trile-stabilized AuI catalyst. We have observed an interesting
shift in the outcome of the reaction from exclusive intermo-
lecular [3C+2C] cycloadditions to exclusive cyclopropana-
tion reactions depending on the structure of the vinyl ether
reagent.
Nevado and co-workers recently reported gold-catalyzed
cycloadditions of olefins with propargyl esters to form five-
or seven-membered rings (Scheme 2).[6a] Cyclopropane de-
rivatives were isolated and were suggested as the precursor
to the final five- and seven-membered-ring products. Subse-
quently, Fiksdahl et al. reported a number of gold-catalyzed
cyclopropanation reactions of vinyl esters and vinyl sulfona-
mides with different propargyl esters (Scheme 2).[6b] In con-
trast to the observations of the Nevado group, ring expan-
sion from cyclopropylvinyl esters was not successful and a
Recent reports[6] of AuI-catalyzed reactions between prop-
argyl esters and olefins have prompted us to report our
[a] R. C. Conyers, Prof. Dr. B. W. Gung
Department of Chemistry and Biochemistry
Miami University, Oxford, OH 45056 (USA)
Fax : (+1)513-529-5715
E-mail: gungbw@muohio.edu
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201202881.
654
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 654 – 664

FULL PAPER
plexes with N-heterocyclic carbene (NHC) ligands have
proven to be effective catalysts in a variety of reactions.[7]
Our screening for efficient AuI–NHC catalysts concentrated
on ligand structure and the use of an additional dynamic
ligand that further stabilizes the active cationic AuI cata-
lyst.[8] The dynamic ligand we employed was benzonitrile,
which made a significant difference to the reaction outcome.
As shown in Table 1, when propargylic esters 4 and vinyl
ether 5b were treated with [AuACTHNUGRTNEUNG(IMes)Cl] (Ia; IMes=1,3-
bis(2,4,6-trimethylphenyl)imidazol-2-ylidene) in the pres-
ence of AgSbF6, no desired product was isolated and the
cationic AuI catalyst slowly degraded (by formation of a
gold mirror).
In contrast, the employment of the same Au complex with
benzonitrile-containing dynamic ligand Ib provided the
[3C+2C]-cycloaddition products in 95% yield. The exclusive
formation of the [3C+2C]-cycloaddition product at room
temperature has not been reported previously by using a
AuI catalyst.[9] Near quantitative yields were obtained by
À
(NCPh)]+SbF6
employing the more stable catalyst [AuACTHNUTRGENNUG(IPr)ACHTUNGTRENNUGN
(Ic, Table 1, entries 3–5). Propargylic esters (4a–c) with dif-
ferent aryl groups (Ph, p-BrC6H4, or p-MeOC6H4) were ex-
amined briefly. The electron-withdrawing Br group (4b) hin-
dered the reaction whereas the electron-donating MeO
group (4c) allowed high yields that are comparable to those
for reactions with the parent phenyl group. A small amount
of the cyclopropanation product was also isolated when 4c
was employed (Table 1, entry 5).
Scheme 2. The AuI-catalyzed reactions of propargyl esters with olefins.
IPr=1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene; Tol=tolyl; M.S.=
molecular sieves.
direct [3+2]-cycloaddition pathway involving
a
Table 1. AuI-catalyzed [3+2]-cycloaddition reactions with various gold(I) complexes
and propargylic esters.[a]
gold-stabilized 1,3-dipole was proposed. An impor-
tant mechanistic question arose concerning wheth-
er the five-membered-ring products were produced
by a direct [3C+2C] cycloaddition or by an initial
cyclopropanation followed by a rearrangement.
Only two examples of such direct cycloaddition
products have been reported in the study by the
Fiksdahl group.[6b] We have now observed an entire
spectrum of reactivity from exclusive [3C+2C]-cy-
cloaddition reactions to 100% cyclopropanation
reactions depending only on the structure of the
substrate (Scheme 2). Consistent with previous re-
ports,[6] some cyclopropanation products rear-
ranged to the corresponding formally [3+2]-cyclo-
addition products in the presence of a fresh AuI
complex at 808C, although others resisted such re-
arrangement even when treated with a fresh AuI
catalyst at elevated temperature. Our results
Ar
[Au–L]
Time Yield
[h]
Ratio
ACHTUNGTRENNUNG(6+7)/8
[%][b]
1
2
3
4
5
Ph (4a)
Ph (4a)
Ph (4a)
4-Br-C6H4 (4b)
[Au
[Au(IMes)
[Au(IPr)
(NCPh)]+SbF6À (Ic)
Ic
N
18
2
2.5
24
1
0[c]
n/a
100:0
100:0
E
95 (55:45)
99 (61:39)
G
25[d] (60:40) 100:0
4-MeO-C6H4 (4c) Ic
99 (50:43)
93:7
[a] Reaction conditions: 4 (1 equiv), 5b (1.2 equiv), [Au–L] (5 mol%), 258C, 0.05m in
CH2Cl2. [b] Yield of the isolated product. Ratios in parentheses are for the two dia-
stereomers (anti/syn; (6:7)). [c] TLC indicated incomplete reaction and some forma-
tion of a gold mirror was observed. [d] Recovered 50% of 4b.
should shed some light on the divergence of the AuI-cata-
lyzed reactions and point to the possibility that both path-
ways are operating.
The benzonitrile-stabilized AuI complex Ic and the parent
propargylic ester (4a) were chosen for our study into reac-
tions with various vinyl ethers (Table 2). All reactions were
performed in dichloromethane at 228C. We observed that
the vinyl ether structure plays a dominant role in the out-
come of the reactions under otherwise similar conditions for
all experiments.
Results
The influence of the AuI complex, the propargyl ester, and
As shown in Table 2, the acyclic vinyl ether 2-methoxy-
propene (5a), and the six-membered cyclic vinyl ether 5b
the vinyl ether structure on product distribution: AuI com-
Chem. Eur. J. 2013, 19, 654 – 664
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
655

B. W. Gung and R. C. Conyers
Table 2. AuI-catalyzed reactions of a propargylic ester with various vinyl ethers.[a]
that these cyclopropanation
products have very different
propensities to undergo AuI-
catalyzed rearrangement reac-
tions to produce the corre-
sponding formal [3C+2C]-cy-
cloaddition products. The evi-
dence from our study suggests
that the initial [3C+2C] prod-
ucts are indeed from such a
mechanism, that is, the direct
capture of the Au-stabilized
allyl cation by the vinyl ether.
Operating in parallel to the
[3C+2C] cycloaddition is the
cyclopropanation/rearrange-
Vinyl ether
Products
Time
[h]
Yield
[%][b]
Ratio
(6+7)/ACTHNGUETRNNU(G 8+9)
G
1
2
1.5
2.5
99 (90:10)[c]
99 (61:39)[c]
100:0
100:0
ment pathway. Some of the iso-
lated vinylcyclopropane prod-
ucts did rearrange into cyclo-
pentenes in the presence of
fresh AuI catalyst at higher tem-
peratures. However, the vinyl-
cylopropanes formed from di-
hydrofuran and dihydropyran
strongly resist the rearrange-
ment reaction even when
heated at 1108C in the presence
of fresh AuI catalyst (see
below).
3
4
16
16
88
79
33:67
55:45
5
6
7
16
16
2.5
99 (76:24)[c]
28:72
33:67
Product stereochemistry and
structure determination: Dia-
stereomers were isolated in
most of the reactions in Table 2.
For the [3C+2C]-cycloaddition
products, the relative stereo-
chemistry (syn or anti) with re-
spect to the methoxy and
phenyl groups was identified
based on the chemical shift of
the methoxy group (Table 3). In
the anti isomer, the methoxy
group has a normal chemical
shift of d=3.4 ppm whereas the
methoxy group in the syn
isomer displayed an upfield
79
89 (63:37)[c]
0:100
0:100
8
1.5
75 (52:48)[c]
[a] Reaction conditions are the same as in Table 1. Relative stereochemistry is shown. [b] Yield of the isolated
product. [c] Ratios in parentheses are diastereomers (anti:syn; (6:7)) in the case of [3+2] cycloadditions and
endo/exo ((8:9)) in the case of cyclopropanations.
provided exclusively the [3C+2C]-cycloaddition products, in
99% yield (Table 2, entries 1 and 2). The 5-, 7-, and eight-
membered cyclic vinyl ethers 5c, 5d, and 5e gave mixtures
of [3C+2C]-cycloaddition and cyclopropanation products in
very good combined yields (Table 2, entries 3–5). The six-
membered cyclic methoxycyclohexadiene 5 f also afforded a
mixture of these products, with the reaction selectively oc-
curring at the methoxy-substituted double bond (Table 2,
entry 6). When the oxygen atom is part of the ring, as in di-
hydropyran (5g) and dihydrofuran (5h), the cyclopropana-
tion products formed exclusively. We will demonstrate later
shift of d=2.9 ppm. The significant upfield shift of the me-
thoxy group is consistent with it being located in the region
of the phenyl ring current. This difference in methoxy chem-
ical shift holds true for other diastereomers of the [3C+2C]-
cycloaddition products.
As shown in Table 3, the methoxy group has a chemical
shift of d=3.4–3.5 ppm when it is not close to a phenyl ring,
such as in the cases of compounds 6, 8, and 9. In the three
syn isomers (7a–c), the chemical shift of the methoxy group
has moved to approximately d=2.9 ppm, indicating the
proximity of a phenyl ring.
656
www.chemeurj.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 654 – 664

Gold(I)-Catalyzed Preparation of Bicyclic Enol Esters
FULL PAPER
Table 3. Chemical shifts [ppm] for the MeO and vinyl protons in the
[3+2] cycloaddition (6 and 7) and cyclopropanation (8–10) diastereom-
ers.
Compound
series
6
7
8
9
10
MeO
vinyl
MeO
vinyl
MeO
vinyl
MeO
vinyl
MeO
vinyl
a
b
c
d
e
f
3.4, 5.9
3.5, 5.8
–
3.4, 5.9
3.5, 6.0
3.4, 5.8
2.9, 5.7
2.9, 5.7
2.8, 5.6
–
3.5, 6.1
3.4, 6.2
3.4, 6.1
–
3.2, 6.1
3.5, 6.2
À5.9
g
h
À6.0
À5.8
À6.0
À6.1
1
The protons of each diastereomer in the H NMR spectra
were assigned by a combination of two-dimensional NMR
methods, including COSY, HSQC, and HMBC spectroscopy.
The endo and exo diastereomers in the cyclopropanation re-
action products were identified by two-dimensional NOSE-
Y NMR spectroscopy. As shown in Figure 1, the vinyl
was confirmed by converting it into the bicyclic enone 12
through formation of first the methanolysis product 16 and
then through selective hydrogenation.
The dihydropyran and dyhydrofuran provided exclusively
the cyclopropanation products when treated with the same
AuI catalyst Ic and propargyl ester 4a. Upon treatment of
the cyclopropanation product 8g (Table 2, entry 7) with
K2CO3 in methanol overnight, the ring-opened product 18
was isolated in 69% yield [Eq. (2)]. The presumed inter-
mediate (17) was not observed under the conditions em-
ployed.
Figure 1. The stereochemistry of the endo- and exo-enol ester group rela-
tive to the ring was determined by two-dimensional NOSEY NMR spec-
troscopy.
proton of the enol ester group displays an NOE correlation
with the ring proton adjacent to the MeO group for the
endo diastereomer and a correlation with the cyclopropane
proton for the exo isomer.
Similarly, the cyclopropanation products formed from the
reaction of dihydrofuran (8h and 9h, Table 2, entry 8) were
treated under the same conditions as 8g to provide the ring-
opened product 19 (Scheme 3). The anti isomer was ob-
The structures of the cycloaddition products 6 and 7 were
confirmed by methanolysis of the enol ester functionality by
using K2CO3 in MeOH, as illustrated in Equation (1) by the
formation of ketone 12. Under these conditions, the me-
thoxy group was lost through b elimination to give the cor-
responding cyclopentenone. The other bicyclic enol esters
(6a–f) produced the corresponding ketones (11–16) under
the same conditions. Compound 11 and the bicyclic enones,
12 and 13, are known compounds,[10,11] whereas 14 and 15
have not been reported in the literature. It is worth noting
that the popular Pauson–Khand[12] reaction can be employed
Scheme 3. Synthesis of perhydroACHTUNGTRNEUNG
[2,3-b]furofuran (20) through the AuI-
catalyzed cyclopropanation of dihydrofuran.
to prepare the bicyclo
and the Rautenstrauch cyclization can produce cyclopente-
nones,[1c,13] although the bicyclo
[5.3.0]- and -[6.3.0]enones
(14 and 15) are more difficult to obtain through these meth-
ods. Thus, for compounds similar to enones 12 and 13, the
AuI-catalyzed cycloaddition followed by methanolysis pro-
vides an alternative preparation, whereas for structures simi-
lar to 14 and 15, the AuI-centered sequence provides an im-
proved synthetic route. The structure of the [3+2]-cycloaddi-
tion product 6 f, formed from methoxycyclohexadiene (5 f),
ACHTUNGTRENN[UNG 3.3.0]- and -[4.3.0]enones (12 and 13)
tained exclusively and the stereochemistry was determined
by the absence of coupling between the anomeric proton
and the proton on carbon number 2. This ketone was stereo-
selectively reduced with tri-tert-butoxy lithium aluminum
hydride at 08C, followed by an acid-induced ring closure, to
provide the bicyclic acetal 20. This bis(perhydrofuran) is a
common structural unit in a number of natural products.[14]
Similar structures to 20 have been synthesized through dif-
ferent routes.[14–15] The efficient synthesis of the perhydro-
AHCTUNGTRENNUNG
Chem. Eur. J. 2013, 19, 654 – 664
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
657

B. W. Gung and R. C. Conyers
A
Thus the rearrangement from vinylcyclopropane deriva-
tives to the cyclopentenes requires the presence of the AuI
catalyst and some heating. This is consistent with the recent
report by Nevado et al.[6a] However, a large difference in the
tendency for ring expansion/rearrangement is observed
among different vinylcyclopropanes.
We also observed a strong resistance to rearrangement by
some vinyl cyclopropane derivatives. This is consistent with
the report by Fiksdahl et al.[6b] In this study, the vinylcyclo-
propanes from the reaction of dihydrofuran and dihydropyr-
an (8g and 8h) exhibit resistance to ring expansion/rear-
rangement. No cyclopentene derivatives were isolated de-
spite the treatment of vinylcyclopropanes 8g and 8h with
fresh AuI catalyst Ic at elevated temperatures (1108C). For
8g, the treatment lead to the vinylcyclopropane with an exo
stereochemistry (9g) in 65% yield, which indicates a cyclo-
propane ring-opening and -closing process, but no rear-
rangement/ring expansion product was observed [Eq. (7)].
For 8h, only the starting material was recovered [Eq. (8)].
nation of dihydrofuran, followed by the sequence shown in
Scheme 3 demonstrates the synthetic utility of the reported
reactions.
Study of the AuI-catalyzed vinylcyclopropane/cyclopentene
rearrangement reactions: No change was observed when the
isolated cyclopropanation product 8e was heated to 1108C
in a sealed Schlenk tube for 2 h [Eq. (3)]. However, in the
presence of fresh AuI complex Ic at 808C in CH2Cl2, a mix-
ture of two diastereomers of the ring-expansion product (6e
and 7e) was isolated in 89% yield in favor of the anti dia-
stereomer in a 2:1 ratio [Eq. (4)].
Similarly, when the isolated cyclopropanation product 8d
was treated with 5 mol% of fresh AuI catalyst Ic at 808C for
2 h, a conversion to the [3+2]-cycloaddition products 6d
and 7d was observed in a 6:1 ratio in favor of the anti dia-
stereomer [Eq. (5)].
The most intriguing case of the rearrangement reaction
came from the treatment of vinylcyclopropane 8c with AuI.
Under carefully controlled conditions (with freshly distilled
(over CaH2) solvent and sealed under an inert atmosphere),
diene 21 could be isolated in excellent yield [Eq. (9)] when
compound 8c was allowed to stir in the presence of fresh
AuI complex Ic. If the solvent was not carefully dried or if
the reaction vessel was not completely sealed under an inert
atmosphere, varying amounts of enone 22 were isolated
along with diene 21. When dichloromethane, directly from a
commercial source, was used as the solvent without drying
over CaH2, a 50% yield of enone 22 was isolated after
30 min [Eq. (10)]. The fact that the vinyl cyclopropane 8c
does not follow the expected ring-expansion/rearrangement
sequence further supports the notion that the initially
Similar behavior was also observed for vinylcyclopropane
9 f [Eq. (6)]. Since the rearrangement of the vinylcyclopro-
panes (8d and 9 f) to the cyclopentenes (6d and 6 f) requires
heating to 808C in the presence of AuI catalyst Ic, the initial
formation of 6d and 6 f at room temperature must have oc-
curred through the [3C+2C] pathway by direct capture of
the cationic 1,3-dipole by the vinyl enol ether.
658
www.chemeurj.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 654 – 664

Gold(I)-Catalyzed Preparation of Bicyclic Enol Esters
FULL PAPER
sion under the same gold catalysis conditions to provide the
ring-expansion products with anti stereochemistry. In the
same year, a subsequent report from Fiksdahl et al. on simi-
lar reactions[6b] suggested that a direct cycloaddition was
more likely than the cyclopropanation/ring-expansion mech-
anism.
The current study into the reaction between cyclic vinyl
ethers and propargyl esters in the presence of AuI catalyst Ic
shows that the capture of the Au-stabilized allyl cation or
the gold carbenoid is dependent on the vinyl ether structure.
Our results are more consistent with a dual reaction path-
way involving both a direct [3C+2C] cycloaddition and a
cascade cyclopropanation/rearrangement reaction sequence.
The preference for one of the two pathways is dependent on
the structure of the vinyl ether substrate. A more polarized
p-bond appears to induce [3C+2C] cycloaddition, whereas a
less polarized p-bond preferentially undergoes cyclopropa-
nation reactions. Alkoxy vinyl ethers are known to be better
electron donors than vinyl acetates.[23] Accordingly, the
study reported by Fiksdahl et al. with vinyl acetates as reac-
tants resulted in mainly cyclopropanation reactions (Sche-
me 2).[6b] In contrast, Nevado et al. studied reactions of
mainly alkoxy vinyl ethers and thus obtained cycloaddition
products.[6a] Under our standard conditions (AuI catalyst Ic
(5%), CH2Cl2, room temperature, ꢀ2 h), the reactions of
vinyl ethers 5a and 5b gave exclusively [3C+2C]-cycloaddi-
tion products, vinyl ethers 5c–5 f yielded mixtures of
[3C+2C]-cycloaddition and cyclopropanation products, and
dihydropyran (5g) and dihydrofuran (5h) produced exclu-
sively the cyclopropanation products (see Table 2).
We were pleased to obtain the bicyclic enol esters under
extremely mild conditions with vinyl ethers 5a and 5b.
However, the appearance of the cyclopropanation products
in varying amounts when using vinyl ethers 5c–5h suggested
a dual reaction pathway in the AuI-catalyzed reactions. The
stereoselective formation of the anti-diastereomeric five-
membered rings provided supporting evidence for the pro-
posed AuI-catalyzed cascade cyclopropanation/rearrange-
ment reaction sequence.[6a] The aforementioned ring-expan-
sion/rearrangement reactions of vinylcyclopropanes 8d, 8e,
and 9 f also gave predominantly the anti diastereomers.
Therefore the anti-diastereomer 6b, from the reaction of 5b,
might be formed from the cascading cyclopropanation/rear-
rangement reaction sequence, whereas the syn-diastereomer
7b may arise from direct capture of the Au-stabilized allyl
cation intermediate (Scheme 4).
formed cyclopentene derivative 7c (Table 2, entry 3) was a
result of a direct [3C+2C]-cycloaddition reaction.
It should be pointed out that enone 22 is different from
the hydrolysis product of a direct [3C+2C]-cycloaddition re-
action. The direct cycloaddition product 7c (Table 1,
entry 3) yielded enone 13 upon hydrolysis of the vinyl ester.
Enone 22 differs from enone 13 in the location of the
phenyl group. Although the phenyl group is attached to the
carbon next to the carbonyl group in both compounds, it is
attached to the sp2 carbon atom in compound 13 and to the
sp3 carbon atom in compound 22. A discussion of possible
pathways leading to enone 22 will be presented in the next
section.
Discussion
The mechanism of gold-catalyzed reactions and the charac-
teristics of the gold–carbon bond have been elegantly exam-
ined by several leading groups.[16] The ancillary ligand at-
À
tached to the gold atom plays an important role on the Au
I
À
C bonding mode of Au complexes. The Au C bond in most
AuI complexes can exhibit either gold-stabilized-cation or
carbene-like properties. The first gold-catalyzed cyclopropa-
nation reaction of carbenoids derived from propargyl esters
was reported by the Toste group in 2005.[17] A subsequent
computational study found that the cyclopropanation reac-
tion occurs by a process that is close to the concerted proc-
ess proposed in the original report.[18] A gas-phase study in
combination with computation suggested that cyclopropana-
tion of electron-rich olefins by gold carbenes is a stepwise
process as a result of the stability of the cationic intermedi-
ates.[19] More recently, in a study of the gold(I)-catalyzed in-
termolecular cyclopropanation of alkenes with 1,6-enynes,
Echavarren et al. concluded that the cyclopropanation can
occur either in a concerted or a stepwise fashion depending
on the substituents present in the alkene.[20]
The divergent pathways, involving either a carbenoid or
an Au-stabilized allyl oxocarbenium, in the reactions of
propargylic esters have also been investigated by computa-
tional studies.[2b,16d] Furthermore, related 1,3-addition reac-
tions have recently been reported for a vinyl metal carbe-
noid by Doyle et al. with rhodium[21] and previously by
Toste et al. by using gold.[22] Nevado and co-workers recent-
ly reported an extensive list of gold-catalyzed cascade cyclo-
propanation/ring-expansion reactions of acyclic, mono-sub-
stituted olefins with a tris(2,4-di-tert-butylphenyl)phosphite–
gold complex (Scheme 2).[6a] The mechanistic proposal for
these reactions involves the initial formation of the cyclo-
propylvinyl esters, which undergo a subsequent ring expan-
In terms of synthetic utility, it was unfortunate that vinyl
ethers 5c–5 f yielded mixtures of [3C+2C]-cycloaddition and
cyclopropanation products at room temperature. However,
in terms of understanding the mechanism of the reaction,
the isolation of both products provided an opportunity to
examine the two pathways. The conversion of the cyclopro-
panation products into the cyclopentene derivatives requires
both fresh AuI catalyst and some heating. The fact that
some cyclopentene derivatives were formed at room temper-
ature was consistent with the proposal that the initially
formed cyclopentene derivatives (e.g., 7c) were a result of
Chem. Eur. J. 2013, 19, 654 – 664
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
659

B. W. Gung and R. C. Conyers
I
À
Au catalyst leads to an Au C
sigma bond in intermediate A.
Over the last
a few years,
stable Au C
(sp3) bonds have
À
been reported both in AuI[24]
and in AuIII complexes.[25] In
the presence of protic solvents,
the gold catalyst is typically re-
generated by proto-deaura-
tion.[26] In this case, ring clo-
sure and deauration produce
the new five-membered ring in
6d (Scheme 6). The bicyclo-
Scheme 4. Possible dual pathways leading to syn and anti diastereomers.
AHCTUNGTRENNUNG
direct capture of the Au-stabilized allyl cation involving AuI
catalyst Ic. The syn stereochemistry of [3C+2C]-cycloaddi-
tion product 7c further supports this reaction pathway
(Scheme 5).
the bicycloAHCTUNGTRENNNUG
conformations than 8c or 8h to allow for the alignment of
the involved orbitals and the formation of the new five-
membered ring. In contrast, the bicycloACTHNUTRGNEUNG[3.1.0]hexane deriva-
tive 8c and the dihydropyran and dihydrofuran derivatives
(8g and 8h) are more rigid and the alignment of the orbitals
favors the re-closure of the three-membered ring. Subse-
quently, different pathways may occur depending on the
substrate structure. The two most common events were
1) ring closure to form a five-membered ring and deauration
(Scheme 6, top), 2) ring closure to form a three-membered
ring and deauration to return to the starting material
(Scheme 6, bottom), or 3) ring closure to give a vinylcyclo-
propane with exo configuration [Eq. (7)].
A third pathway in which a hydride shift leads to the for-
mation of a diene ester was also observed [Eq. (9)] . As de-
scribed earlier the AuI-catalyzed conversion of vinylcyclo-
propane 8c leads to diene 21 and enone 22. Initially, we con-
sidered the possibility that enone 22 was formed through a
rearrangement of vinylcyclopropane 8c with the opening of
the cyclopropane ring from the unexpected side, followed
by hydrolysis of the enol ester (Scheme 7). However, this
pathway would require opening of the cyclopropane ring
from the side leading to an unstabilized secondary carboca-
tion. This was deemed unlikely because a previous study has
shown that the opening of a cyclopropane ring requires a
substituent that can stabilize the developing positive
charge.[27] A methoxy group is capable of stabilizing the de-
Scheme 5. Proposed dual pathways leading to the respective products 7c
and 8c.
As described earlier, the outcome of the AuI-catalyzed
ring-expansion/rearrangement reaction depends on the
structure of the vinylcyclopropane. However, the difference
in structure for the compounds studied herein is limited to
changes in the ring that is fused to the cyclopropane ring in
the product (Table 2).
We attribute the difference in the behavior of the vinylcy-
clopropanes to a combination of stereo- and electronic prop-
erties of the fused ring on the
cyclopropane. In accordance
with the mechanism proposed
by Nevado et al.,[6a] the initial
coordination of the enol ester
double bond of 8d or 8h to the
cationic AuI center should lead
to the opening of the cyclopro-
pane ring and the formation of
an oxycarbenium ion (A;
Scheme 6). This is
a highly
strained intermediate due to
the cis configuration of the
double bond. The activation of
the enolate double bond by the
Scheme 6. Proposed AuI-catalyzed transformation of vinylcyclopropanes.
660
www.chemeurj.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 654 – 664

Gold(I)-Catalyzed Preparation of Bicyclic Enol Esters
FULL PAPER
transposition through a five-membered ring has previously
been documented.[24,25] Methoxy-group migration occurs
concurrently with the formation a new carbon–carbon bond
(IIb) and deauration to produce the unstable acetal IIc. The
presence of a trace amount of water can initiate the hydroly-
sis and methanolysis of the unstable acetal IIc to give enone
22. Acetals with structures similar to IIc have previously
been shown to spontaneously equilibrate to the correspond-
ing ketones and esters.[28]
Scheme 7. A possible, but unlikely, pathway to 22.
veloping positive charge and
the ring opening should occur
on the side of the ring with the
methoxy group attached. Fur-
thermore, the hydrolysis of the
enol ester requires the solvent
to be either H2O or MeOH
and this reaction was per-
formed in anhydrous CH2Cl2.
Therefore, this pathway seems
unlikely.
To examine the mechanism
for the formation of enone 22,
two additional experiments
Scheme 8. Proposed mechanism for the formation of diene 21 and enone 22.
were performed. When diene
21 was treated with fresh AuI
catalyst and heated to 808C for 30 min [Eq. (11)], enone 22
was isolated in 62% yield. This experiment shows that diene
21 is an intermediate on the pathway leading to enone 22
Conclusion
It has been shown that the outcome of the AuI-catalyzed re-
action of propargyl esters with a cyclic vinyl ether is sensi-
tive to substrate structure. This is consistent with several
recent studies that showed the impact of alkene substituents
on the mechanism of cyclopropanation. Similarly, the AuI-
catalyzed ring-expansion/rearrangement reactions of the vi-
nylcyclopropanes also produce several different outcomes
depending on the structure of the bicyclic enol esters. Based
on documented mechanisms and model structures of AuI
complexes,[14,18–20] a dual reaction pathway is proposed to ex-
plain the observed divergence in the AuI-catalyzed reactions.
The preference for the [3C+2C]-cycloaddition pathway may
depend on the degree of polarization of the vinyl ether
double bond. A more polarized p-bond appears to induce
the [3C+2C] cycloaddition, whereas a less polarized p-bond
preferentially undergoes cyclopropanation reactions. How-
ever, the difference in the polarization of the p-bond does
not have to be large for a change from one pathway to the
other to occur. Our current effort is directed toward a
better understanding of the AuI-catalyzed ring-expansion/re-
arrangement reactions and our results will be reported in
due course.
from the vinyl cyclopropane derivative 8c.
A second experiment was performed in [D8]toluene in a
NMR tube. Vinylcyclopropane 8c was stirred with 5% AuI
catalyst Ic in [D8]toluene at room temperature. The result-
ing mixture was filtered through a cotton plug into a high
quality NMR tube. Proton NMR spectra were taken at in-
tervals of 10 min. After each spectrum, the temperature of
the NMR probe was raised by 108C. Diene 21 started to
form at approximately 658C with concurrent disappearance
of vinylcyclopropane 8c. Enone 22 started to form at ap-
proximately 858C with simultaneous disappearance of diene
21. Methyl toluate and a small amount of methanol were
also observed in the final NMR spectrum. Based on this evi-
dence, we propose that the formation of the diene occurs
through a sequence of events involving first the {Au}+-pro-
moted opening of the cyclopropane ring, and then a hydride
shift from the allylic carbon atom to the oxycarbenium
carbon atom (Scheme 8). Once the diene has formed, it is
further activated by the AuI catalyst to form an oxygen-
bridged acetal IIa. Similar AuI-promoted methoxy-group
Experimental Section
General conditions: Unless otherwise stated, all reactions were carried
out under an inert nitrogen atmosphere with anhydrous solvents. Tetrahy-
drofuran (THF) was distilled over sodium benzophenone, and dichloro-
methane (CH2Cl2) and benzene were distilled over calcium hydride. Re-
Chem. Eur. J. 2013, 19, 654 – 664
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
661

B. W. Gung and R. C. Conyers
agents were purchased and used without further purification unless other-
wise stated. Yields refer to chromatographically and spectroscopically
(1H NMR) homogeneous materials, unless otherwise stated. Reactions
were monitored by thin-layer chromatography (TLC) carried out on
Merck silica gel plates (60F-254; 0.25 mm) by using UV light as the visu-
alizing agent and an acidic mixture of anisaldehyde, phosphomolybdic
acid, ceric ammonium molybdate, or basic aqueous potassium permanga-
nate (KMnO4) and heat as developing agents. Merck silica gel (60, parti-
cle size 0.043–0.063 mm) was used for flash column chromatography.
NMR spectra were recorded on Bruker Av-500 and Av-300 instruments
and calibrated by using residual undeuterated solvent as an internal ref-
erence (CHCl3: d=7.26 (1H), 77.0 ppm (13C)). The following abbrevia-
tions (or combinations thereof) were used to explain the multiplicities:
s=singlet, d=doublet, t=triplet, q=quartet, m=multiplet, br=broad.
High-resolution mass spectra (HRMS) were recorded at Ohio State Uni-
versity.
8.5, 11, 12 Hz), 2.15 (1H, dd, J=8, 12 Hz), 2.43 (3H, s), 3.45 (3H, s), 6.14
(1H, s), 7.11 (1H, t, J=7.5 Hz), 7.19 (2H, t, J=7.5 Hz), 7.28 (2H, d, J=
8 Hz), 7.38 (2H, d, J=8 Hz), 8.03 ppm (2H, d, J=8 Hz); 13C NMR
(125 MHz, CDCl3): d=21.2, 21.7, 26.7, 28.8, 30.0, 30.13, 30.17, 56.3, 75.7,
116.3, 126.7, 127.0, 128.3, 128.36, 128.39, 129.4, 130.2, 134.7, 144.3, 146.3,
164.3 ppm; HRMS: m/z calcd for C23H24O3 +Na: 371.1623; found:
371.1616.
1-Methoxy-10-phenylbicyclo
[5.3.0]dec-9-enyl
4-methylbenzoate
(6d):
1H NMR (500 MHz, CDCl3): d=0.64–0.73 (1H, m), 0.94–1.05 (1H, m),
1.26–1.35 (2H, m), 1.45–1.50 (2H, m), 1.59–1.65 (2H, m), 1.65–1.71 (1H,
m), 1.80–1.90 (2H, m), 2.35 (3H, s), 2.76 (1H, d, J=11 Hz), 3.41 (3H, s),
4.19 (1H, s), 5.92 (1H, s), 7.14 (2H, d, J=8 Hz), 7.21 (2H, d, J=7 Hz),
7.26–7.29 (3H, m), 7.74 ppm (2H, d, J=8.5 Hz); 13C NMR (125 MHz,
CDCl3): d=21.6, 22.4, 30.7, 31.1, 33.2, 33.4, 49.4, 55.5, 58.5, 88.6, 119.2,
126.8, 128.3, 190.1, 129.9, 138.7, 144.0, 146.7, 164.2 ppm; LCMS: m/z
calcd for C25H28O3 +Na: 399.2; found: 399.3.
General procedure for gold-catalyzed cycloaddition reactions: Gold cata-
lyst Ic (0.009 mmol) in freshly distilled CH2Cl2 (1.0 mL) was added to a
round-bottomed flask equipped with a stirring bar under an atmosphere
of nitrogen. The propargyl ester (0.18 mmol) and vinyl ether
(0.22 mmol), were both dissolved in freshly distilled CH2Cl2 (1.0 mL),
were added dropwise to the reaction, stirred at RT, and monitored by
TLC and/or 1H NMR spectroscopy until completion. The reaction mix-
ture was diluted with Et2O, filtered through a pad of Celite, the solvent
removed under reduced pressure, and the product purified by column
chromatography (1–2% EtOAc/hexanes).
1-(1-MethoxybicycloACTHUNTGRENNG[U 5.1.0]oct-8-yl)-2-phenylvinyl 4-methylbenzoate (8d):
1H NMR (500 MHz, CDCl3): d=0.95–1.02 (1H, m), 1.12 (1H, dd, J=1.5,
13 Hz), 1.21 (1H, d, J=13 Hz), 1.40–1.50 (1H, m), 1.52–1.54 (1H, m),
1.62 (1H, d, J=7 Hz), 1.64–1.70 (2H, m), 1.84 (1H, d, J=10.5 Hz), 2.29–
2.36 (1H, m), 2.44 (3H, s), 2.63–2.65 (1H, m), 3.43 (3H, s), 6.17 (1H, s),
7.15 (1H, d, J=7 Hz), 7.23 (2H, t, J=7.5 Hz), 7.32 (2H, d, J=7.5 Hz),
7.41 (2H, d, J=7.5 Hz), 8.06 ppm (2H, d, J=8 Hz); 13C NMR (125 MHz,
CDCl3): d=21.7, 25.6, 28.7, 30.1, 31.5, 31.6, 32.0, 38.1, 54.2, 71.5, 115.5,
126.7, 127.0, 128.3, 129.4, 129.8, 130.2, 134.8, 144.3, 144.6, 164.4 ppm;
HRMS: m/z calcd for C25H28O3 +Na: 399.1936; found: 399.1938.
4-Methoxy-4-methyl-5-phenylcyclopent-1-enyl 4-methylbenzoate (6a):
1H NMR (500 MHz, CDCl3): d=0.90 (3H, s), 2.34 (3H, s), 2.43 (1H, dd,
J=2, 17 Hz), 2.73 (1H, dd, J=2.5, 16.5 Hz), 3.38 (3H, s), 4.21 (1H, s),
5.91 (1H, s), 7.14 (2H, d, J=8 Hz), 7.19–7.22 (3H, m), 7.27–7.30 (2H,
m), 7.77 ppm (2H, d, J=8.5 Hz); 13C NMR (125 MHz, CDCl3): d=21.6,
22.8, 40.4, 50.3, 58.8, 84.5, 112.6, 126.8, 128.3, 128.4, 128.9, 129.05, 129.09,
129.12, 129.15, 129.88, 129.92, 137.9, 144.0, 149.2, 164.2 ppm; HRMS: m/z
calcd for C21H22O3 +Na: 345.1467; found: 345.1461.
1-Methoxy-11-phenylbicycloACTHNUTRGNE[NUG 6.3.0]undec-10-enyl 4-methylbenzoate (6e):
1H NMR (500 MHz, CDCl3): d=0.93 (1H, t, J=13.5 Hz), 1.10 (1H, d,
J=9 Hz), 1.36–1.44 (2H, m), 1.61–1.68 (5H, m), 1.78 (1H, d, J=
14.5 Hz), 1.87–1.90 (2H, m), 2.35 (3H, s), 2.68 (1H, d, J=8.5 Hz), 3.47
(3H, s), 4.16 (1H, s), 5.96 (1H, s), 7.14 (2H, d, J=8 Hz), 7.21–7.28 (5H,
m), 7.77 ppm (2H, d, J=7 Hz); 13C NMR (125 MHz, CDCl3): d=21.6,
23.9, 25.4, 26.4, 28.0, 30.6, 32.4, 50.0, 52.8, 57.4, 87.3, 120.7, 126.8, 126.9,
128.1, 129.0, 129.9, 138.6, 143.9, 147.0, 164.3 ppm; LCMS: m/z calcd for
C26H30O3 +Na: 413.2; found: 413.3.
4-Methoxy-4-methyl-5-phenylcyclopent-1-enyl 4-methylbenzoate (7a):
1H NMR (500 MHz, CDCl3): d=1.61 (3H, s), 2.36 (3H, s), 2.45 (1H, dd,
J=3, 15.5 Hz), 2.79 (1H, d, J=15.5 Hz), 2.90 (3H, s), 3.87 (1H, s), 5.74
(1H, s), 7.16 (2H, d, J=8 Hz), 7.25–7.28 (5H, m), 7.76 ppm (2H, d, J=
8 Hz).
1-(1-MethoxybicycloACTHUNTGRENNG[U 6.1.0]non-9-yl)-2-phenylvinyl 4-methylbenzoate (8e):
1H NMR (500 MHz, CDCl3): d=0.93–0.95 (1H, m), 1.07 (1H, td, J=3.5,
15.5 Hz), 1.23 (1H, d, J=6.5 Hz), 1.33–1.37 (1H, m), 1.37–1.43 (1H, m),
1.43–1.45 (1H, m), 1.45–1.47 (1H, m), 1.47–1.50 (1H, m), 1.57–1.63 (1H,
m), 1.63–1.64 (1H, m), 1.64–1.70 (2H, m), 2.17 (1H, dd, J=3.5, 14.5 Hz),
2.44 (3H, s), 2.50 (1H, d, J=15.5 Hz), 3.42 (3H, s), 6.13 (1H, s), 7.10
(1H, t, J=7.5 Hz), 7.19 (2H, t, J=7.5 Hz), 7.29 (2H, d, J=8 Hz), 7.38
(2H, d, J=7.5 Hz), 8.03 ppm (2H, d, J=8 Hz); 13C NMR (125 MHz,
CDCl3): d=21.7, 25.1, 26.17, 26.22, 27.2, 28.4, 29.0, 31.8, 33.1, 53.9, 69.1,
114.9, 126.6, 127.1, 128.26, 128.31, 129.4, 130.2, 134.8, 144.3, 146.8,
164.4 ppm; HRMS: m/z calcd for C26H30O3 +Na: 413.2093; found:
413.2087.
1-Methoxy-9-phenylbicyclo
[4.3.0]non-8-enyl
4-methylbenzoate
(6b):
1H NMR (500 MHz, CDCl3): d=1.05–1.12 (2H, m), 1.31–1.37 (2H, m),
1.45–1.50 (1H, m), 1.53–1.56 (2H, m), 1.70–1.71 (1H, m), 2.35 (3H, s),
3.03 (1H, s), 3.45 (3H, s), 4.51 (1H, s), 5.81 (1H, s), 7.15–7.27 (7H, m),
7.78 ppm (2H, d, J=8 Hz); 13C NMR (125 MHz, CDCl3): d=19.8, 21.7,
26.6, 28.1, 43.6, 50.2, 55.8, 86.0, 118.4, 126.5, 126.7, 127.9, 128.30, 128.33,
129.09, 129.12, 129.3, 129.5, 129.9, 130.2, 136.7, 144.0, 148.3, 164.2 ppm;
HRMS: m/z calcd for C24H26O3 +Na: 385.1780; found: 385.1770.
1-Methoxy-9-phenylbicyclo
[4.3.0]non-8-enyl
4-methylbenzoate
(7b):
1-(1-MethoxybicycloACTHNGUTERNNU[G 6.1.0]non-9-yl)-2-phenylvinyl 4-methylbenzoate (9e):
1H NMR (500 MHz, CDCl3): d=1.45–1.50 (2H, m), 1.50–1.55 (2H, m),
1.69–1.75 (2H, m), 1.96 (1H, dt, J=3.5, 12 Hz), 2.13 (1H, d, J=13.5 Hz),
2.36 (3H, s), 2.89 (3H, s), 3.15 (1H, s), 3.77 (1H, s), 5.68 (1H, s), 7.16
(2H, d, J=8 Hz), 7.21–7.32 (5H, m), 7.79 ppm (2H, d, J=8 Hz);
13C NMR (125 MHz, CDCl3): d=21.5, 21.7, 22.0, 25.9, 30.72, 44.9, 51.2,
59.7, 83.7, 117.9, 126.7, 126.8, 128.1, 129.1, 129.4, 129.9, 136.7, 144.1,
151.7, 164.6 ppm; LCMS: m/z calcd for C24H26O3 +Na: 385.2; found:
385.2.
1H NMR (500 MHz, CDCl3): d=0.87–0.90 (2H, m), 1.32–1.45 (4H, m),
1.55–1.65 (5H, m), 1.98 (1H, d, J=10.5 Hz), 2.31–2.38 (2H, m), 2.45
(3H, s), 3.20 (3H, s), 6.05 (1H, s), 7.16 (1H, d, J=7 Hz), 7.21–7.30 (2H,
m), 7.31 (2H, d, J=8 Hz), 7.40 (2H, d, J=7.5 Hz), 8.01 ppm (2H, d, J=
8 Hz); 13C NMR (125 MHz, CDCl3): d=21.7, 24.0, 24.5, 25.2, 26.1, 28.6,
29.0, 29.7, 31.4, 31.9, 53.5, 66.7, 119.8, 120.5, 126.9, 127.2, 128.4, 128.5,
129.5, 130.1, 144.5, 145.6, 164.4 ppm; LCMS: m/z calcd for C26H30O3 +
Na: 413.2; found: 413.3.
1-Methoxy-8-phenylbicyclo
[3.3.0]oct-7-enyl
4-methylbenzoate
(7c):
1-Methoxy-9-phenylbicyclo
[4.3.0]nona-3,7-diene-8-yl
4-methylbenzoate
1H NMR (500 MHz, CDCl3): d=1.58–1.61 (2H, m), 1.77–1.82 (2H, m),
1.86–1.90 (1H, m), 1.99–2.04 (1H, m), 2.23–2.26 (1H, m), 2.35 (3H, s),
2.75 (3H, s), 3.25 (1H, d, J=8 Hz), 4.04 (1H, s), 5.57 (1H, s), 7.15 (2H,
d, J=8 Hz), 7.21 (1H, d, J=7 Hz), 7.27–7.32 (4H, m), 7.73 ppm (2H, d,
J=8.5 Hz); 13C NMR (125 MHz, CDCl3): d=21.7, 24.4, 30.0, 36.9, 51.8,
52.4, 59.2, 93.7, 117.6, 126.7, 127.9, 128.0, 129.08, 129.12, 129.5, 129.6,
129.86, 129.9, 138.2, 144.1, 149.6, 164.5 ppm; LCMS: m/z calcd for
C23H24O3 +Na: 371.2; found: 371.2.
(6 f): 1H NMR (500 MHz, CDCl3): d=1.80 (2H, s), 1.94–1.99 (1H, m),
2.35 (3H, s), 2.44–2.47 (1H, m), 3.05–3.08 (1H, m), 3.41 (3H, s), 4.50
(1H, s), 5.53–5.56 (1H, m), 5.82–5.86 (2H, m), 7.14 (2H, d, J=8 Hz),
7.19–7.27 (5H, m), 7.74 ppm (2H, d, J=8 Hz); 13C NMR (125 MHz,
CDCl3): d=21.6, 28.7, 29.7, 30.9, 46.1, 50.2, 58.5, 88.4, 118.3, 126.7, 127.4,
127.5, 128.0, 128.3, 129.1, 129.9, 130.0, 138.1, 144.1, 144.2, 146.6, 148.1,
164.1 ppm; LCMS: m/z calcd for C24H24O3 +Na: 383.2; found: 383.2.
1-(1-MethoxybicycloACTHNUGRTENUNG[4.1.0]hept-3-en-7-yl)-2-phenylvinyl 4-methylbenzoate
1-(1-Methoxybicyclo
N
(9 f): 1H NMR (500 MHz, CDCl3): d=1.67 (1H, t, J=5.5 Hz), 1.89 (1H,
d, J=6.5 Hz), 2.40 (1H, d, J=4 Hz), 2.44 (3H, s), 2.51–2.54 (1H, m),
2.55 (1H, d, J=17.5 Hz), 2.76–2.80 (1H, m), 3.46 (3H, s), 5.51 (1H, d,
662
www.chemeurj.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 654 – 664

Gold(I)-Catalyzed Preparation of Bicyclic Enol Esters
FULL PAPER
J=9.5 Hz), 5.52 (1H, d, J=3 Hz), 6.17 (1H, s), 7.11 (1H, t, J=7.5 Hz),
7.20 (2H, t, J=7.5 Hz), 7.29 (2H, d, J=8 Hz), 7.39 (2H, d, J=7.5 Hz),
8.03 ppm (2H, d, J=8 Hz); 13C NMR (125 MHz, CDCl3): d=21.7, 24.2,
25.0, 27.4, 29.5, 54.4, 65.7, 116.1, 123.2, 123.7, 126.7, 127.1, 128.3, 129.3,
130.2, 134.8, 144.3, 146.6, 164.3 ppm; HRMS: m/z calcd for C24H24O3 +
Na: 383.1623; found: 383.1611.
132.6, 141.2, 181.1, 207.0 ppm; HRMS: m/z calcd for C17H20O+Na:
263.1412; found: 263.1398.
[4.3.0]nona-1,7-diene-3-one (16): 1H NMR (500 MHz,
2-PhenylbicycloACTHNUTRGNEUNG
CDCl3): d=1.59–1.63 (1H, m), 2.18–2.24 (2H, m), 2.47 (2H, d, J=2 Hz),
2.81 (1H, dd, J=7, 18.5 Hz), 2.92–2.97 (1H, m), 6.37–6.40 (1H, m), 6.78
(1H, d, J=10 Hz), 7.23 (1H, t, J=7 Hz), 7.36–7.42 ppm (4H, m);
13C NMR (125 MHz, CDCl3): d=27.1, 28.9, 29.7, 36.8, 42.1, 50.5, 123.1,
127.7, 128.3, 128.7, 129.2, 129.5, 131.4, 134.8, 140.9, 166.4, 206.2 ppm;
LCMS: m/z calcd for C15H14O+Na: 233.1; found: 233.1.
1-(2-OxabicycloACHTUNGTRENNUNG[4.1.0]hept-7-yl)-2-phenylvinyl 4-methylbenzoate (8g):
1H NMR (300 MHz, CDCl3): d=1.45–1.51 (3H, m), 1.91 (2H, dd, J=2.1,
6.9 Hz), 2.04–2.10 (1H, m), 2.44 (3H, s), 3.27–3.35 (1H, m), 3.60 (1H, d,
J=10.8 Hz), 3.77 (1H, dd, J=2.1, 7.5 Hz), 5.99 (1H, s), 7.10–7.21 (3H,
m), 7.25–7.35 (4H, m), 8.00 ppm (2H, d, J=8.4 Hz); 13C NMR (75 MHz,
CDCl3): d=18.7, 19.1, 22.1, 28.2, 58.3, 64.4, 114.9, 126.7, 126.9, 128.2,
128.4, 129.3, 130.2, 134.3, 144.4, 148.1, 164.1 ppm; LCMS: m/z calcd for
C22H22O3 +Na: 357.2; found: 357.2.
Hydrogenation of ketone 16: Pd/C (5 mg) and ketone 16 (0.03 mmol) dis-
solved in freshly distilled benzene was added to a round-bottomed flask
equipped with a stirring bar under an atmosphere of hydrogen, and the
resulting mixture was stirred for 45 min. The reaction mixture was diluted
with Et2O and filtered through Celite. The solvent was removed under
reduced pressure and the product was purified by column chromatogra-
phy (5–10% EtOAc/hexanes) to give 12 in 83% yield as a colorless oil.
1-(2-Methoxytetrahydropyran-3-yl)-3-phenylpropan-2-one (18): 1H NMR
(500 MHz, CDCl3): d=1.17–1.19 (1H, m), 1.49–1.52 (2H, m), 1.87–1.89
(1H, m), 2.08–2.09 (1H, m), 2.35 (1H, dd, J=7.5, 17 Hz), 2.67 (1H, dd,
J=6.5, 16.5 Hz), 3.37 (3H, s), 3.38–3.45 (1H, m), 3.70 (2H, s), 3.91–3.93
(1H, m), 4.07 (1H, d, J=6.5 Hz), 7.19 (2H, d, J=7.5 Hz), 7.26–7.28 (1H,
m), 7.33 ppm (2H, d, J=7 Hz); 13C NMR (125 MHz, CDCl3): d=23.8,
26.6, 36.0, 43.6, 50.2, 55.7, 64.0, 104.4, 127.0, 128.66, 128.69, 129.4, 129.5,
134.2, 206.9 ppm; LCMS: m/z calcd for C15H20O3 +Na: 271.1; found:
271.1.
1-(2-Methoxytetrahydrofuran-3-yl)-3-phenylpropan-2-one (19): 1H NMR
(500 MHz, CDCl3): d=1.34–1.41 (1H, m), 2.17–2.21 (1H, m), 2.19 (1H,
dd, J=6.5, 12.5 Hz), 2.56 (2H, s), 3.29 (3H, s), 3.67 (2H, s), 3.85 (2H, t,
J=7.5 Hz), 4.60 (1H, s), 7.18 (2H, d, J=7 Hz), 7.24 (1H, t, J=7.5 Hz),
7.31 ppm (2H, d, J=8 Hz); 13C NMR (125 MHz, CDCl3): d=29.8, 40.4,
44.5, 50.3, 54.7, 66.3, 108.8, 127.1, 128.8, 129.4, 133.9, 206.6 ppm; HRMS:
m/z calcd for C14H18O3 +Na: 257.1153; found: 257.1149.
1-(2-OxabicycloACHTUNGTRENNUNG[4.1.0]hept-7-yl)-2-phenylvinyl 4-methylbenzoate (9g):
1H NMR (500 MHz, CDCl3): d=1.47–1.50 (1H, m), 1.71–1.79 (1H, m),
1.83–1.87 (2H, m), 2.36 (3H, s), 2.92 (1H, d, J=5 Hz), 3.50–3.53 (1H,
m), 3.86–3.89 (1H, m), 4.01 (1H, t, J=4 Hz), 4.11 (1H, s), 5.91 (1H, s),
7.17 (2H, d, J=8 Hz), 7.19–7.22 (1H, m), 7.22–7.31 (4H, m), 7.82 ppm
(2H, d, J=8 Hz); 13C NMR (125 MHz, CDCl3): d=21.7, 22.2, 25.2, 38.8,
56.5, 65.0, 82.5, 116.3, 126.8, 126.9, 127.9, 128.6, 129.1, 130.0, 138.3, 144.1,
150.4, 164.1 ppm.
1-(2-OxabicycloACHTUNGTRENNUNG[3.1.0]hex-6-yl)-2-phenylvinyl 4-methylbenzoate (8h):
1H NMR (500 MHz, CDCl3): d=1.86–1.89 (1H, m), 2.02–2.06 (1H, m),
2.36 (3H, s), 3.58–3.61 (1H, m), 3.91–3.94 (2H, m), 4.09 (1H, s), 4.43
(1H, d, J=6 Hz), 5.79 (1H, s), 7.16 (2H, d, J=8 Hz), 7.19–7.26 (4H, m),
7.29–7.32 (2H, m), 7.78 ppm (2H, d, J=8 Hz); 13C NMR (125 MHz,
CDCl3): d=21.7, 31.5, 45.8, 58.3, 67.1, 87.6, 115.5, 126.7, 126.8, 127.5,
128.7, 129.1, 129.9, 140.4, 144.2, 150.2, 164.0 ppm; LCMS: m/z calcd for
C21H20O3 +Na: 343.1; found: 343.2.
1-(2-OxabicycloACHTUNGTRENNUNG[3.1.0]hex-6-yl)-2-phenylvinyl 4-methylbenzoate (9h):
1H NMR (500 MHz, CDCl3): d=1.94–1.97 (1H, m), 2.02–2.42 (3H, m),
3.52 (1H, q, J=9, 17.5 Hz), 4.04 (1H, td, J=2.5, 9 Hz), 4.14 (1H, d, J=
5.5 Hz), 5.95 (1H, s), 7.11 (1H, d, J=7.5 Hz), 7.18 (2H, t, J=8 Hz), 7.27
(2H, d, J=8 Hz), 7.34 (2H, d, J=8 Hz), 8.01 ppm (2H, d, J=8 Hz);
13C NMR (125 MHz, CDCl3): d=21.7, 23.1, 25.8, 28.0, 65.0, 67.2, 115.4,
126.7, 127.0, 128.2, 128.4, 129.4, 130.2, 134.1, 144.5, 146.8, 164.1 ppm;
HRMS: m/z calcd for C21H20O3 +Na: 343.1310; found: 343.1297.
2-Benzylhexa hydroACTHNUTRGNEU[GN 2,3-b]furofuran (20): Ketone 19 (95 mg, 0.41 mmol)
in freshly distilled THF (2 mL) was added to a round-bottomed flask
equipped with a stirring bar under an atmosphere of nitrogen, and the re-
sulting mixture was cooled to 08C. Lithium tri-tert-butoxyaluminum hy-
dride (0.81 mmol) was added dropwise, and the mixture was stirred for
5 h at RT. After TLC indicated that ketone 19 had disappeared, the reac-
tion was cooled to 08C and 4 n HCl (0.5 mL) was carefully added drop-
wise. The resulting mixture was stirred for 3.5 h at RT. Once the reaction
was complete by TLC, the mixture was diluted with Et2O (5 mL) and the
layers were separated. The aqueous layer was extracted with Et2O (3ꢁ
5 mL). The combined organic phases were washed with saturated aque-
ous NaHCO3 and brine, dried over MgSO4, filtered, and concentrated
under reduced pressure. The residue was purified by column chromatog-
raphy (10–20% EtOAc/hexanes) to give 20 (60 mg, 72%) as a colorless
1-(2-OxabicycloACHTUNGTRENNUNG[3.1.0]hex-6-yl)-2-phenylvinyl 4-methylbenzoate (10h):
1H NMR (500 MHz, CDCl3): d=1.61–1.71 (1H, m), 2.19–2.26 (1H, m),
2.47 (3H, s, overlap with diastereomer 9h), 2.63–2.70 (1H, m), 3.69–3.75
(1H, m), 3.88–4.00 (2H, m), 6.12 (1H, s), 7.12–7.39 (7H, m, overlap with
diastereomer 9h), 8.01–8.02 (2H, m, overlap with diastereomer 9h).
General procedure for ester hydrolysis: The ester (0.06 mmol) dissolved
in MeOH (3 mL) was added to a round-bottomed flask equipped with a
stirring bar under an atmosphere of nitrogen. K2CO3 (0.15 mmol) was
then added. The reaction mixture was stirred overnight at RT. TLC was
performed to confirm that the ester had disappeared. The reaction was
concentrated, and then diluted with EtOAc and quenched with saturated
aqueous NH4Cl. The aqueous layer was extracted three times with
EtOAc. The combined organic layers were washed with brine and dried
with MgSO4. The solvent was removed under reduced pressure and puri-
fied by column chromatography (2–5–10% EtOAc/hexanes).
1
oil. H NMR (500 MHz, CDCl3): d=1.27–1.29 (1H, m), 1.64 (1H, dd, J=
5, 12.5 Hz), 1.88–1.91 (1H, m), 2.10–2.12 (1H, m), 2.80 (2H, dd, J=7,
13.5 Hz), 3.03 (1H, dd, J=6, 13.5 Hz), 3.79–3.83 (1H, m), 3.92 (1H, t, J=
8 Hz), 4.01–4.04 (1H, m), 5.63 (1H, d, J=5.5 Hz), 7.20–7.29 ppm (5H,
m); 13C NMR (125 MHz, CDCl3): d=32.5, 36.4, 41.6, 42.9, 65.9, 79.7,
109.1, 126.3, 128.3, 129.3, 138.2 ppm; LCMS: m/z calcd for C13H16O2 +
Na: 227.1; found: 227.1.
General procedure for gold-catalyzed vinylcyclopropane/cyclopentene re-
arrangement reactions: Gold catalyst Ic (0.003 mmol) in freshly distilled
CH2Cl2 (0.5 mL) was added to a Schlenk tube equipped with a stirring
bar under an atmosphere of nitrogen. The vinylcyclopropane (0.06 mmol)
was dissolved in freshly distilled CH2Cl2 (0.5 mL), added to the reaction
mixture, and the resulting mixture was stirred at 808C and monitored by
TLC and/or 1H NMR spectroscopy until completion. The reaction mix-
ture was diluted with Et2O, filtered through Celite, the solvent removed
under reduced pressure, and residue purified by column chromatography
(1–2% EtOAc/hexane).
2-PhenylbicycloACHTUNGTRENNUNG
[5.3.0]dec-1-en-3-one (14): 1H NMR (500 MHz, CDCl3):
d=1.39–1.75 (5H, m), 1.83–1.92 (2H, m), 2.17 (1H, dd, J=2.5, 18.5 Hz),
2.72–2.78 (2H, m), 2.84 (1H, dd, J=6.5, 18.5 Hz), 3.01 (1H, s), 7.25–7.32
(3H, m), 7.39 ppm (2H, t, J=7.5 Hz); 13C NMR (125 MHz, CDCl3): d=
26.1, 28.9, 30.5, 31.9, 34.8, 42.9, 43.9, 127.5, 128.2, 129.1, 132.3, 139.6,
180.5, 206.7 ppm; LCMS: m/z calcd for C16H18O+Na: 249.1; found:
249.1.
2-PhenylbicycloACHTUNGTRENNUNG
[6.3.0]undec-1-en-3-one (15): 1H NMR (500 MHz,
CDCl3): d=1.32–1.41 (2H, m), 1.26–1.73 (7H, m), 2.20–2.29 (1H, m),
2.34 (1H, d, J=2.5 Hz), 2.41–2.46 (1H, m), 2.69 (1H, dd, J=6.5,
18.5 Hz), 2.88–2.94 (1H, m), 3.06 (1H, s), 7.23–7.30 (2H, m), 7.31 (1H, t,
J=6 Hz), 7.39 ppm (2H, t, J=8 Hz); 13C NMR (125 MHz, CDCl3): d=
24.1, 26.0, 26.2, 28.8, 29.2, 29.7, 40.9, 41.9, 107.6, 127.6, 128.4, 128.6, 128.9,
1-Methoxy-11-phenylbicycloACTHNUTRGNE[NUG 6.3.0]undec-10-enyl 4-methylbenzoate (7e):
1H NMR (500 MHz, CDCl3): d=1.48–1.65 (6H, m), 1.70–1.80 (2H, m),
1.84–1.88 (2H, m), 1.94–1.97 (1H, m), 2.34 (3H, s), 2.41 (1H, dd, J=7,
15.5 Hz), 2.47 (3H, s), 3.22 (1H, d, J=11 Hz), 4.14 (1H, s), 5.59 (1H, s),
Chem. Eur. J. 2013, 19, 654 – 664
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
663

B. W. Gung and R. C. Conyers
7.11 (2H, d, J=8 Hz), 7.20 (1H, d, J=7 Hz), 7.21–7.26 (2H, m), 7.31
(2H, d, J=7 Hz), 7.68 ppm (2H, J=8.5 Hz).
[8] C. H. M. Amijs, V. Lopez-Carrillo, M. Raducan, P. Perez-Galan, C.
Ferrer, A. M. Echavarren, J. Org. Chem. 2008, 73, 7721–7730.
[9] H. Kusama, M. Ebisawa, H. Funami, N. Iwasawa, J. Am. Chem. Soc.
2009, 131, 16352–16353.
[10] A. Padwa, T. J. Blacklock, D. Getman, N. Hatanaka, R. Loza, J.
Org. Chem. 1978, 43, 1481–1492.
[11] a) T. Kobayashi, Y. Koga, K. Narasaka, J. Organomet. Chem. 2001,
624, 73–87; b) T. Sugihara, M. Yamada, H. Ban, M. Yamaguchi, C.
Kaneko, Angew. Chem. 1997, 109, 2884–2886; Angew. Chem. Int.
Ed. 1997, 36, 2801–2804.
[12] P. L. Pauson, I. U. Khand, Ann. N. Y. Acad. Sci. 1977, 295, 2–14.
[13] V. Rautenstrauch, J. Org. Chem. 1984, 49, 950–952.
[14] J. Vader, H. Sengers, A. De Groot, Tetrahedron 1989, 45, 2131–
2142.
1-(2-Methoxycyclopentylidenemethyl)-2-phenylvinyl
4-methylbenzoate
(21): 1H NMR (500 MHz, CDCl3): d=1.56–1.66 (3H, m), 1.77–1.80 (2H,
m), 2.34–2.38 (1H, m), 2.46 (3H, s), 3.36 (3H, s), 4.04 (1H, s), 6.27 (1H,
s), 6.30 (1H, s), 7.14 (1H, t, J=7 Hz), 7.20 (2H, t, J=7.5 Hz), 7.31 (2H,
d, J=8 Hz), 7.42 (2H, d, J=7.5 Hz), 8.07 ppm (2H, d, J=8 Hz);
13C NMR (125 MHz, CDCl3): d=21.8, 22.6, 28.5, 29.7, 31.2, 56.1, 85.7,
120.7, 120.9, 126.8, 127.4, 128.5, 128.6, 128.7, 129.5, 129.8, 130.3, 143.5,
144.5, 145.2, 146.1, 163.9 ppm; LCMS: m/z calcd for C23H24O3 +Na:
371.1; found: 371.2.
4-PhenylbicycloACHTUNGTRENNUNG
[3.3.0]dec-1-en-3-one (22): 1H NMR (500 MHz, CDCl3):
d=1.26–1.36 (1H, m), 2.00–2.06 (1H, m), 2.10–2.13 (1H, m), 2.26–2.28
(1H, m), 2.63–2.70 (2H, m), 3.00–3.05 (1H, m), 3.26 (1H, d, J=3.5 Hz),
5.99 (1H, s), 7.18 (2H, d, J=7 Hz), 7.25–7.27 (1H, m), 7.32 ppm (2H, t,
J=7 Hz); 13C NMR (125 MHz, CDCl3): d=25.6, 26.4, 29.7, 30.9, 123.8,
126.9, 128.3, 128.7, 138.9, 189.2, 210.2 ppm; LCMS: m/z calcd for
C14H14O+Na: 221.1; found: 221.1.
[15] D. A. Powell, T. Maki, G. C. Fu, J. Am. Chem. Soc. 2005, 127, 510–
511.
[16] a) A. Fꢃrstner, P. W. Davies, Angew. Chem. 2007, 119, 3478–3519;
Angew. Chem. Int. Ed. 2007, 46, 3410–3449; b) E. Jimenez-Nunez,
A. M. Echavarren, Chem. Rev. 2008, 108, 3326–3350; c) A. S. K.
Hashmi, Angew. Chem. 2008, 120, 6856–6858; Angew. Chem. Int.
Ed. 2008, 47, 6754–6756; d) D. Benitez, N. D. Shapiro, E. Tkatch-
ouk, Y. M. Wang, W. A. Goddard, F. D. Toste, Nat. Chem. 2009, 1,
482–486; e) A. M. Echavarren, Nat. Chem. 2009, 1, 431–433; f) G.
Seidel, R. Mynott, A. Fuerstner, Angew. Chem. 2009, 121, 2548–
2551; Angew. Chem. Int. Ed. 2009, 48, 2510–2513.
Acknowledgements
Partial financial support from Miami University through
a Shoupp
[17] M. J. Johansson, D. J. Gorin, S. T. Staben, F. D. Toste, J. Am. Chem.
Soc. 2005, 127, 18002–18003.
[18] E. Soriano, J. Marco-Contelles, Chem. Eur. J. 2008, 14, 6771–6779.
[19] L. Batiste, A. Fedorov, P. Chen, Chem. Commun. 2010, 46, 3899–
3901.
Award is gratefully acknowledged. BWG thanks the NSF TUES program
for a partial summer stipend. We also thank the Ohio Board of Regents
and Miami University for the funding of the NMR facility.
[20] P. Pꢄrez-Galꢅn, E. Herrero-Gomez, D. T. Hog, N. J. A. Martin, F.
Maseras, A. M. Echavarren, Chem. Sci. 2011, 2, 141–149.
[21] X. C. Wang, X. F. Xu, P. Y. Zavalij, M. P. Doyle, J. Am. Chem. Soc.
2011, 133, 16402–16405.
[22] N. D. Shapiro, Y. Shi, F. D. Toste, J. Am. Chem. Soc. 2009, 131,
11654.
[23] R. Rathore, J. K. Kochi, Tetrahedron Lett. 1994, 35, 8577–8580.
[24] a) R. L. LaLonde, W. E. Brenzovich, D. Benitez, E. Tkatchouk, K.
Kelley, W. A. Goddard, F. D. Toste, Chem. Sci. 2010, 1, 226–233;
b) A. A. Zuzek, S. C. Reynolds, D. S. Glueck, J. A. Golen, A. L.
Rheingold, Organometallics 2011, 30, 1812–1817; c) N. P. Mankad,
F. D. Toste, Chem. Sci. 2012, 3, 72–76.
[25] a) D. M. Fan, E. Melendez, J. D. Ranford, P. F. Lee, J. J. Vittal, J. Or-
ganomet. Chem. 2004, 689, 2969–2974; b) E. Langseth, C. H. Gor-
bitz, R. H. Heyn, M. Tilset, Organometallics. 2012, 31, 6567–6571.
[26] N. D. Shapiro, F. D. Toste, Synlett 2010, 675–691.
[27] D. Garayalde, E. Gomez-Bengoa, X. G. Huang, A. Goeke, C.
Nevado, J. Am. Chem. Soc. 2010, 132, 4720–4730.
[1] a) K. Miki, K. Ohe, S. Uemura, J. Org. Chem. 2003, 68, 8505–8513;
b) V. Mamane, T. Gress, H. Krause, A. Furstner, J. Am. Chem. Soc.
2004, 126, 8654–8655; c) X. D. Shi, D. J. Gorin, F. D. Toste, J. Am.
Chem. Soc. 2005, 127, 5802–5803.
[2] a) L. Zhang, J. Am. Chem. Soc. 2005, 127, 16804–16805; b) A.
Correa, N. Marion, L. Fensterbank, M. Malacria, S. P. Nolan, L.
Cavallo, Angew. Chem. 2008, 120, 730–733; Angew. Chem. Int. Ed.
2008, 47, 718–721; c) M. Yu, G. Z. Zhang, L. M. Zhang, Tetrahedron
2009, 65, 1846–1855; d) D. Garayalde, C. Nevado, ACS Catal. 2012,
2, 1462–1479.
[3] a) B. Trillo, F. Lopez, S. Montserrat, G. Ujaque, L. Castedo, A.
Lledos, J. L. Mascarenas, Chem. Eur. J. 2009, 15, 3336–3339; b) P.
Mauleꢂn, R. M. Zeldin, A. Z. Gonzalez, F. D. Toste, J. Am. Chem.
Soc. 2009, 131, 6348.
[4] a) B. W. Gung, D. T. Craft, L. N. Bailey, K. Kirschbaum, Chem. Eur.
J. 2010, 16, 639; b) B. W. Gung, D. T. Craft, Tetrahedron Lett. 2009,
50, 2685–2687.
[5] B. W. Gung, L. N. Bailey, J. Wonser, Tetrahedron Lett. 2010, 51,
2251.
[28] K. Bowden, J. M. Byrne, J. Chem. Soc. Perkin Trans. 2 1996, 1921–
1924.
[6] a) D. Garayalde, K. Kruger, C. Nevado, Angew. Chem. 2011, 123,
941–945; Angew. Chem. Int. Ed. 2011, 50, 911–915; b) C. A. Sperg-
er, J. E. Tungen, A. Fiksdahl, Eur. J. Org. Chem. 2011, 3719–3722.
[7] a) N. Marion, R. S. Ramon, S. P. Nolan, J. Am. Chem. Soc. 2009,
131, 448; b) N. Marion, P. Carlqvist, R. Gealageas, P. de Fremont, F.
Maseras, S. P. Nolan, Chem. Eur. J. 2007, 13, 6437–6451.
Received: August 10, 2012
Revised: September 15, 2012
Published online: November 13, 2012
664
www.chemeurj.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 654 – 664