Enantioselective gold(I)-catalyzed intramolecular (4+3) cycloadditions of allenedienes

Article abstract of DOI:10.1002/anie.201105815

Allene-tethered dienes (1) undergo an intramolecular and highly enantioselective (4+3) cycloaddition when treated with suitable chiral phosphoramidite/gold(I) catalysts (see scheme; Ar=9-anthracenyl). The reactions provide synthetically relevant [5.3.0] and [5.4.0] fused bicyclic systems 2 with good yields, complete diastereocontrol, and excellent enantioselectivities.

Full text of DOI:10.1002/anie.201105815

Communications
DOI: 10.1002/anie.201105815
Enantioselective Cycloaddition
Enantioselective Gold(I)-Catalyzed Intramolecular (4+3)
Cycloadditions of Allenedienes**
Isaac Alonso, Hꢀlio Faustino, Fernando Lꢁpez,* and Josꢀ L. MascareÇas*
The (4+3) cycloaddition of conjugated
dienes and allylic cations represents a
highly valuable strategy for the prep-
aration of seven-membered carbocy-
cles.[1] Indeed, this type of annulation
has been successfully used as a key
step in the synthesis of several com-
plex natural products and advanced
intermediates.[2,3]
However,
very
important challenges, such as the
development of catalytic versions
that work with readily available pre-
cursors,[4] and principally, the imple-
mentation of enantioselective var-
iants, remain to be satisfactorily devel-
oped. Indeed, we are aware of only
two isolated reports on catalytic enan-
tioselective (4+3) cycloadditions of
allylic cations, and both deal with the
Scheme 1. Pt- and Au-catalyzed cycloadditions of allenedienes 1.[8,10a]
intermolecular
annulation
of
Additionally, we have found that when using suitable chiral
furans.[5–7]
phosphoramidite/gold(I) catalysts (e.g. (R,R,R)-Au3–Au5)
the cycloadditions proceed in an enantioselective manner to
provide optically active bicyclic compounds 3.[10a,11] Mecha-
nistic studies suggest that both types of products, 2 and 3, arise
from the common carbenic species II,[12] itself coming from
the (4+3) cycloaddition of 1 via allylic cation intermediate I
(Scheme 1). Species II might then evolve either by ring
contraction (1,2-alkyl migration, route b) or by a standard 1,2-
H shift (route a).[13] Although the phosphite type of ligands
seem to favor the ring contraction process over the 1,2-H
shift, theoretical data suggest that the activation barriers for
both processes are not so different.[10a,b] Therefore, we
reasoned that chiral phosphoramidite/gold catalysts might
be also capable of inducing (4+3) annulations, provided that
the ring-contraction route (route b) could be slightly deacti-
vated. Herein, we demonstrate the viability of this approach
by reporting a highly enantioselective intramolecular (4+3)
cycloaddition of allenedienes 1. The reactions are promoted
by the chiral phosphoramidite/gold(I) catalyst (R,R,R)-Au5/
AgSbF6 and provide a straightforward route to optically
active, synthetically relevant bicyclo[5.3.0]decadiene and
bicyclo[5.4.0]undecadiene skeletons. To the best of our
knowledge the transformation represents the first example
of an intramolecular, highly enantioselective (4C + 3C) cyclo-
addition.
We have recently reported a new type of (4+3) cyclo-
addition strategy based on the platinum- or gold-catalyzed
intramolecular reaction of allene-tethered dienes such as 1
(Scheme 1). The reaction is particularly efficient when
catalyzed by the cationic gold(I) complex Au1/AgSbF6,
which features
a
s-donating N-heterocyclic carbene
ligand.[8,9] In the course of these studies, we also discovered
that allenedienes 1, when dialkylated at the distal position of
the allene (R, R’ = alkyl), preferentially provide (4+2) cyclo-
adducts of type 3, as long as the gold catalyst incorporates a p-
acidic ligand such as a phosphite or a phosphoramidite.[10,8b]
[*] I. Alonso, H. Faustino, Prof. Dr. J. L. MascareÇas
Departamento de Quꢀmica Orgꢁnica
Centro Singular de Investigaciꢂn en Quꢀmica Biolꢂgica y Materiales
Moleculares y Unidad Asociada al CSIC
Universidad de Santiago de Compostela
15782, Santiago de Compostela (Spain)
E-mail: joseluis.mascarenas@usc.es
Dr. F. Lꢂpez
Instituto de Quꢀmica Orgꢁnica General (CSIC)
Juan de la Cierva 3, 28006, Madrid (Spain)
E-mail: fernando.lopez@iqog.csic.es
[**] This work was supported by the Spanish MEC (SAF2007-61015,
SAF2010-20822-C02, Consolider Ingenio 2010 CSD2007-00006), the
ERDF and the Xunta de Galicia INCITE09 209 084PR, GRC2010/12.
H.F. acknowledges Fundażo para a CiÞncia e a Tecnologia-Portugal
for a PhD Grant SFRH/BD/60214/2009.
Previous studies in the group suggested that reducing the
number of substituents at the allene terminus of 1 (from two
to one) has a drastic negative effect on the formation of
cyclohexene adducts 3, while cycloheptenyl products 2 and 2ꢀ
can still be satisfactorily obtained.[14] Therefore, we initially
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201105815.
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Angew. Chem. Int. Ed. 2011, 50, 11496 –11500

checked the performance of monosubstituted allenediene 1a
(R = Me, R’ = H, X = C(CO2Me)2) in the presence of several
chiral phosphoramidite/gold catalysts (Table 1). Gratifyingly,
ratio and a good 74% combined yield (Table 1, entry 3).
Importantly, analysis of the enantioselectivity of this reaction
revealed that 2b was obtained in 87% ee,[16] thus confirming
the potential of this phosphoramidite/gold catalyst to induce
high levels of asymmetry in these (4+3) cycloadditions.
As is shown in Table 1 (Table 1, entries 3–6), precatalyst
(R,R,R)-Au5, which bears 9-anthracenyl groups at the 3 and 3’
positions of the binaphthol unit, provided the best selectivity
in favor of the (4+3) adducts, as well as the highest ee values.
A related phenanthryl-derived complex (R,R,R)-Au6 also
provided the (4+3) adducts 2b and 2b’ with a good 85% ee,
however, the reaction also gave a considerable amount of the
(2+2) cycloadduct 4b (Table 1, entry 4).[17] Other precatalysts,
such as the phenyl-derived complex (R,R,R)-Au4 and the 3,3’
nonsubstituted catalyst (R,R,R)-Au3, led to much lower
ee values (Table 1, entries 5 and 6). The counterion seems to
have little effect on the enantioselectivity (Table 1, entries 7–
10). Thus, an equimolar combination of AgSbF6 and (R,R,R)-
Au5 turned out to be the optimum catalyst. Importantly,
reduction of the catalyst loading to 5 mol%, and even
2 mol%, did not affect the enantioselectivity, although the
transformation requires longer reaction times and leads to
slightly lower yields (Table 1, entries 11 and 12).[18]
Once an optimum catalytic system had been established,
we evaluated the versatility and scope of the process, typically
using 5 mol% of (R,R,R)-Au5/AgSbF6. As shown in the
Table 2, allenediene 1c, with an electron-donating ortho-
methoxy substituent on the aryl group of the allene, also
participated in the cycloaddition, thus providing the (4+3)
cycloadducts 2c/2c’ in 91% yield and 88% ee (Table 2,
entry 2). In contrast, electron-withdrawing substituents on
the aromatic ring, such as a para-trifluoromethyl group, were
not tolerated, thus leading to complete recovery of the
starting material (Table 2, entry 3). The presence of the
Table 1: Preliminary screening on cycloadditions of 1a–b.[a,b]
Entry
1
1
AgX
Au*
Au5
t [h]
Products
(ratio)[c]
Yield
[%][d]
ee (2)
[%][e]
[f]
1a
AgSbF6
4
2a/2a’/4a
(3:2:1)
4a[g]
2b/2b’
(8:1)
55
–
2
3
1a
1b
AgSbF6
AgSbF6
Au2
Au5
0.5
8
40[h]
74
–
87
4
5
6
7
8
9
1b
1b
1b
1b
1b
1b
AgSbF6
AgSbF6
AgSbF6
AgBF4
Au6[i]
Au3
Au4
Au5
Au5
Au5
5
17
5
2b/2b’/4b
(1:1:1)
2b/2b’
(2:1)
2b/2b’
(4:1)
69[j]
70
75
46
64
10
85
13
40
81
84
84
48
36
35
2b/2b’
(1:0)
AgNTf2
AgOTf
2b/2b’
(13:1)
2b/2b’
(15:1)
–
2b/2b’
(9:1)
2b/2b’
(9:1)
[k]
10
1b
1b
AgOTs
AgSbF6
Au5
Au5
–
11
–
–
87
11[l]
64
12[m]
1b
AgSbF6
Au5
14
56
85
Table 2: Enantioselective (4+3) cycloadditions of allenedienes 1, cata-
lyzed by (R,R,R)-Au5/AgSbF6.[a,b]
[a] Allenediene 1 (1 equiv) was added to a mixture of AgX (10 mol%) and
(R,R,R)-Au* (10 mol%), in CH2Cl2 (0.1m) at À158C and the mixture was
slowly warmed to RT. [b] Conversions are greater than 99%, as
1
determined by H NMR spectroscopy, unless otherwise noted.
1
[c] Determined by H NMR spectroscopy of the crude mixtures.
[d] Combined yield of 2 and 2’ upon isolation unless otherwise noted.
[e] Determined by HPLC. [f] The ee value was not determined. [g] Result
taken from reference [10a]: 4a was observed together with other
unknown products. [h] Yield of 4a as determined by GC analysis. [i] Au6:
Ar=9-phenanthryl. [j] Combined yield of 2b, 2b’, and 4b. [k] 0%
conversion; 1b was recovered after 24 h. [l] Used 5 mol% of (R,R,R)-
Au5/AgSbF6 . [m] Used 2 mol% of (R,R,R)-Au5/AgSbF6.
Entry
1
R
X
2/2’[c] Yield [%][d] ee (2) [%]
1
2
3
4
5
6
7
1b C6H5
C(CO2Me)2 9 :1
64
87
88[e]
–
1c 2-OMeC6H4 C(CO2Me)2 3.5:1 91
1d 4-F3CC6H4
1e C6H5
1 f 2-MeC6H4
1g 3-MeC6H4
1h 2-OMeC6H4 NTs
[f]
C(CO2Me)2
NTs
NTs
–
–
1:0
1:0
1:0
1:0
74
68
75
80
95
95
95
98
treatment of this substrate with (R,R,R)-Au5/AgSbF6
(10 mol%), provided the (4+3) cycloadducts 2a and 2a’
(3:2 ratio), together with a lower quantity of the (2+2) adduct
4a, in an 55% combined yield (Table 1, entry 1).[15] Interest-
ingly, the racemic phosphite/gold catalyst Au2/AgSbF6
affords a more complex mixture of products than the above
phosphoramidite catalyst (Table 1, entry 2).[10a] Pleasingly, the
cycloaddition of the related allenediene 1b, which bears a
phenyl group at the allene terminus, was completely selective,
thus providing the (4+3) cycloadducts 2b and 2b’ in a 8:1
NTs
[a] Allenediene 1 (1 equiv) was added to a mixture of AgSbF6 (5 mol%)
and (R,R,R)-Au5 (5 mol%), in CH2Cl2 (0.1m) at À158C and the mixture
slowly warmed to RT. [b] Conversions are greater than 99%, as
determined by H NMR spectroscopy, unless otherwise noted.
[c] Determined by H NMR spectroscopy of the crude mixtures.
1
1
[d] Combined yield of 2 and 2’ upon isolation. [e] Used 10 mol% of
catalyst. [f] 1d was recovered after 24 h at RT.
Angew. Chem. Int. Ed. 2011, 50, 11496 –11500
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Communications
geminal diester in the connecting chain of 1 is not essential.
Thus, the reaction of N-tosyl derivative 1e provided the (4+3)
cycloadduct 2e in 74% yield, with complete regioselectivity
and with an excellent 95% ee (Table 2, entry 4). Also, the
ortho-methyl and meta-methyl derivatives 1 f and 1g provided
the corresponding adducts 2 f and 2g with good yields and
95% ee (Table 2, entries 5 and 6). The cycloaddition of ortho-
methoxy-substituted derivative 1h provided even better
enantioselectivity, and adduct 2h was isolated in 80% yield
and 98% ee (Table 2, entry 7).[19]
Figure 1. Solid-state molecular structure of (3aR, 8aR)-2i.[20]
Interestingly, while disubstitution at the allene terminus
favors the formation of (4+2) cycloadducts 3,[10a] introduction
of a third substituent at the internal position instead of the
terminal position of the allene, results in the generation of
cycloheptenyl products 2. Therefore, treatment of the 1,3-
dimethyl-substituted allenediene 1i under the standard con-
ditions ((R,R,R)-Au5/AgSbF6) yielded the (4+3) adduct 2i
with complete regioselectivity, 78% yield and an excellent
95% ee (Table 3, entry 1). The structure of the product, which
features a quaternary bridgehead carbon, was unambiguously
determined by NMR spectroscopy and X-ray crystallography,
which also established the absolute configuration
(Figure 1).[20]
(4+3) adduct 2k (98% ee) and an interesting bicyclic product,
5k, resulting from a new type of formal (4+2) annulation
(Table 3, entry 3). The structure and stereochemical config-
uration of 2k and 5k could be determined by NMR
spectroscopy and X-ray crystallography (Figure 2).[22] Impor-
Figure 2. Solid-state molecular structure of 5k.[22]
Table 3: Enantioselective (4+3) cycloadditions of other allenedienes
1.[a,b]
tantly, both adducts were obtained with almost the same
enantioselectivity (Æ 2% ee), thus suggesting that they could
arise from a common intermediate. Indeed, the formation of
5k can be rationalized in terms of 1,2 migration of the
bridgehead tertiary carbon atom on a 5,7-cycloheptyl gold
carbene intermediate such as II (Scheme 1). This migration
entails a ring expansion of the five-membered ring and a
concomitant contraction of the seven-membered carbocy-
cle.[23]
The cycloaddition of allenedienes bearing alkyl substitu-
ents at the diene, such as 1l, can also be achieved with high ee.
However, in addition to the desired (4+3) adduct 2l
(93% ee), the reaction also gave the 6,6-bicyclic product 5l,
and a second side product 6l, which must arise from the
rupture of the allenediene and a subsequent intramolecular
hydroamination reaction (Table 3, entry 4).[24]
Finally, the cycloaddition of allenedienes 1m and 1n,
which feature a longer connecting chain between the allene
and the diene, proceeded efficiently, thus providing the
corresponding cycloadducts with good yields and excellent
ee values (Table 3, entries 5 and 6). Moreover, the structure
2m could be resolved by X-ray crystallography, thus confirm-
ing the absolute stereochemistry of the major enantiomer
(Figure 3).[25] Overall, these results demonstrate that the
method constitutes an efficient asymmetric approach to
enantiopure 5,7- and 6,7-fused bicyclic systems with a
quaternary stereocenter at the ring fusion.
Entry
1
n
R1
R2
2/5
Yield [%][c]
ee (2) [%]
1
2
3
4
5
6
1i
1j
1k
1l
1m
1n
1
1
1
1
2
2
Me
iPr
C6H5
Me
Me
iPr
H
H
H
Me
H
1:0
1:0
78
76
95
95
4:6
82[d]
80[g]
95
98[e]
93
1:1[f]
1:0
98
95
H
1:0[h]
95[i]
[a] Allenediene 1 (1 equiv) was added to a mixture of AgSbF6 (5 mol%)
and (R,R,R)-Au5 (5 mol%), in CH2Cl2 (0.1m) at À158C and slowly
warmed to RT. [b] Conversions are greater than 99%, as determined by
1H NMR spectroscopy, unless otherwise noted. [c] Yields are of isolated
2, unless otherwise noted. [d] Combined yield of 2k and 5k. [e] The
ee value of 2k; the ee value of 5k is 96%; [f] Ratio of 2l/6l/5l=1:2:1.
[g] Combined yield of 2l, 5l, and 6l. [h] A small amount (<10%) of 3n
was also obtained together with 2n.[21] [i] Combined yield of 2n and 3n.
Allenediene 1j, which also contains a methyl substituent
at the internal allenic position, provided the corresponding
(4+3) product 2j with good yield and 95% ee (Table 3,
entry 2). Curiously, allenediene 1k, with a phenyl group at the
allene terminus, reacted to give a 4:6 mixture of the expected
In summary, we have described the first examples of a
catalytic and highly enantioselective intramolecular (4C+3C)
cycloaddition reaction. This method leads to synthetically
appealing
bicyclo[5.3.0]decadiene
and
bicyclo-
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Angew. Chem. Int. Ed. 2011, 50, 11496 –11500

[6] Other asymmetric approaches are based on chiral auxiliary
strategies. For a review, see: a) M. Harmata, Adv. Synth. Catal.
2006, 348, 2297 – 2306; see also reference [1].
[7] To the best of our knowledge, only three other types of highly
enantioselective and catalytic formal (4C + 3C) cycloadditions
are known: Annulations of vinyl carbenoids with dienes: a) B. D.
Schwartz, J. R. Denton, Y. Lian, H. M. L. Davies, C. M. Wil-
liams, J. Am. Chem. Soc. 2009, 131, 8329 – 8332, and references
therein; formal intermolecular (4+3) annulation of cyclopenta-
diene and a,b-unsaturated aldehydes: b) X. Dai, H. M. L.
Davies, Adv. Synth. Catal. 2006, 348, 2449 – 2456; Pd-catalyzed
(4+3) cycloaddition of g-methylidene-d-valerolactones and 1,1-
dicyanocyclopropanes: c) R. Shintani, M. Murakami, T. Tsuji, H.
Tanno, T. Hayashi, Org. Lett. 2009, 11, 5642 – 5645; for isolated
examples of a modestly enantioselective Pd-catalyzed (4 + 3)
cycloaddition of alkylidene cyclopropanes, see: d) M. Gulꢁas, J.
Durꢂn, F. Lꢃpez, L. Castedo, J. L. MascareÇas, J. Am. Chem. Soc.
2007, 129, 11026 – 11027.
[8] a) B. Trillo, F. Lꢃpez, M. Gulꢁas, L. Castedo, J. L. MascareÇas,
Angew. Chem. 2008, 120, 965 – 968; Angew. Chem. Int. Ed. 2008,
47, 951 – 954; b) B. Trillo, F. Lꢃpez, S. Montserrat, G. Ujaque, L.
Castedo, A. Lledꢃs, J. L. MascareÇas, Chem. Eur. J. 2009, 15,
3336 – 3339; c) S. Montserrat, G. Ujaque, F. Lꢃpez, J. L. Mascar-
eÇas, A. Lledꢃs, Top. Curr. Chem. 2011, 302, 225 – 248.
[9] For related work of other groups, see also: a) P. Mauleꢃn, R. M.
Zeldin, A. Z. Gonzꢂlez, F. D. Toste, J. Am. Chem. Soc. 2009, 131,
6348 – 6349; b) D. Benitez, E. Tkatchouk, A. Z. Gonzalez, W. A.
Goddard, F. D. Toste, Org. Lett. 2009, 11, 4798 – 4801; c) B. W.
Gung, D. T. Craft, Tetrahedron Lett. 2009, 50, 2685 – 2687;
d) B. W. Gung, D. T. Craft, L. N. Bailey, K. Kirschbaum, Chem.
Eur. J. 2010, 16, 639 – 644; for a review on Au-catalyzed
cycloadditions, see: e) F. Lꢃpez, J. L. MascareÇas, Beilstein J.
Org. Chem. 2011, 7, 1075 – 1094.
[10] a) I. Alonso, B. Trillo, F. Lꢃpez, S. Montserrat, G. Ujaque, L.
Castedo, A. Lledꢃs, J. L. MascareÇas, J. Am. Chem. Soc. 2009,
131, 13020 – 13030; b) S. Montserrat, I. Alonso, F. Lꢃpez, J. L.
MascareÇas, A. Lledꢃs, G. Ujaque, Dalton Trans. 2011, DOI:
10.1039/C1DT11061F; for related work of other groups, see
references [9a,b] and: c) M. Alcarazo, T. Stork, A. Anoop, W.
Thiel, A. Fꢄrstner, Angew. Chem. 2010, 122, 2596 – 2600; Angew.
Chem. Int. Ed. 2010, 49, 2542 – 2546.
[11] For more recent related work with other phosphoramidite/gold
catalysts, see: a) A. Z. Gonzꢂlez, F. D. Toste, Org. Lett. 2010, 12,
200 – 203; b) H. Teller, S. Flugge, R. Goddard, A. Fꢄrstner,
Angew. Chem. 2010, 122, 1993 – 1997; Angew. Chem. Int. Ed.
2010, 49, 1949 – 1953.
[12] This gold(I) carbene intermediate II can also be understood as a
gold(I)-stabilized carbocation. The degree of carbenic character
depends on the nature of the ligands on gold: a) D. Benitez,
N. D. Shapiro, E. Tkatchouk, Y. M. Wang, W. A. Goddard, F. D.
Toste, Nat. Chem. 2009, 1, 482 – 486; b) G. Seidel, R. Mynott, A.
Fꢄrstner, Angew. Chem. 2009, 121, 2548 – 2551; Angew. Chem.
Int. Ed. 2009, 48, 2510 – 2513.
[13] It has been shown that disubstitution at the allene terminus and
p-acidic ligands on gold, the latter enhancing the zwitterionic
character of II, favor the ring contraction (1,2-alkyl shift) to give
the (4+2) adducts 3. Conversely, the use of PtCl2 or s-donating
ligands on gold favor the 1,2-H shift to give the (4+3) adducts 2
and/or 2’.[8–10]
[14] Indeed, bicyclic compounds such as 3 are only efficiently
obtained from allenedienes with two alkyl substitutents at the
allene terminus.[10, 11,9a,b]
[15] The formation of the (2+2) cycloadduct 4a can be explained in
terms of a cationic stepwise pathway as suggested in related Au-
catalyzed (2+2) cycloadditions of allene-tethered alkenes, see:
a) M. R. Luzung, P. Mauleꢃn, F. D. Toste, J. Am. Chem. Soc.
2007, 129, 12402 – 12403; for a related recent work, see: b) A. Z.
Figure 3. Solid-state molecular structure of (4aR, 9aS)-2m.[25]
[5.4.0]undecadiene skeletons with good yields, complete
diastereocontrol, and excellent enantioselectivities. The
atom economy and stereoselectivity of the process, together
with its operational simplicity, allows this method to be
ranked among the most practical alternatives to make
optically active 5,7-and 6,7-fused bicyclic systems.
Received: August 17, 2011
Published online: October 13, 2011
Keywords: allenes · cycloaddition · enantioselectivity · gold ·
.
homogeneous catalysis
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c) M. Harmata, J. A. Brackley III, C. L. Barnes, Tetrahedron
Lett. 2006, 47, 8151 – 8155; d) G. Priꢀ, N. Prevost, H. Twin, S. A.
Fernandes, J. F. Hayes, M. Shipman, Angew. Chem. 2004, 116,
6679 – 6681; Angew. Chem. Int. Ed. 2004, 43, 6517 – 6519.
[5] For the first enantioselective example based on organocatalysis,
see: a) M. Harmata, S. K. Ghosh, X. Hong, S. Wacharasindhu, P.
Kirchhoefer, J. Am. Chem. Soc. 2003, 125, 2058 – 2059; for an
alternative method based on the use of allenamides and furans,
and a chiral Lewis acid catalyst, see: b) J. Huang, R. P. Hsung, J.
Am. Chem. Soc. 2005, 127, 50 – 51.
Angew. Chem. Int. Ed. 2011, 50, 11496 –11500
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 11499

Communications
Gonzꢂlez, D. Benitez, E. Tkatchouk, W. A. Goddard, F. D.
[22] The absolute stereochemistry of 5k was unambiguously deter-
mined by X-ray crystallography. CCDC 836788 (5k) contains the
supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
[23] Mechanistic rational for the formation of 2k and 5k from the
same reactive intermediate II:
Toste, J. Am. Chem. Soc. 2011, 133, 5500 – 5507.
[16] Cycloadducts 2b and 2b’ are obtained with the same enantio-
selectivity, in consonance with the mechanistic pathway shown in
Scheme 1.
[17] The yields and selectivities provided by (R,R,R)-Au6/AgSbF6 in
the (4+2) cycloaddition of allenedienes disubstituted at the
allene distal position are typically similar to those provided by
(R,R,R)-Au5/AgSbF6 (unpublished results).
[18] Allenediene 1b slowly polymerizes even at low temperatures.
Therefore, long reactions times should be avoided.
[19] Cycloadduct 2h was characterized by X-ray crystallography;
CCDC 836785 (2h) contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via www.
ccdc.cam.ac.uk/data_request/cif.
[20] CCDC 836784 (2i) contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via www.
ccdc.cam.ac.uk/data_request/cif.
[21] 3n could not be separated from 2n by column chromatography
on silica gel. The identification of 3n as a side product was
confirmed by its independent preparation, which allowed us to
obtain suitable crystals for X-ray crystallography. CCDC 836787
(3n) contains the supplementary crystallographic data for this
paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.
uk/data_request/cif.
[24] See details in the Supporting Information. For a review on
intramolecular hydroamination reactions of allenes, see: R. A.
Widenhoefer, X. Han, Eur. J. Org. Chem. 2006, 4555 – 4563.
[25] CCDC 836786 (2m) contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
11500 www.angewandte.org
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 11496 –11500