Gold-catalyzed oxidative cyclizations on 1,4-enynes: Evidence for a γ-substituent effect on Wagner-Meerwein rearrangements

Article abstract of DOI:10.1002/anie.201210313

Gold-catalyzed oxidative cyclizations of 1,4-enynes were used to study the γ-effect on the Wagner-Meerwein rearrangement. Both experimental and theoretical work disclose that a gold substituent in the γ-position can direct a stereospecific 1,2-shift of the anti-β-substituent regardless of its intrinsic properties. Copyright

Full text of DOI:10.1002/anie.201210313

Angewandte
Chemie
DOI: 10.1002/anie.201210313
Rearrangement
Gold-Catalyzed Oxidative Cyclizations on 1,4-Enynes: Evidence for
a g-Substituent Effect on Wagner–Meerwein Rearrangements**
Satish Ghorpade, Ming-Der Su,* and Rai-Shung Liu*
The Wagner–Meerwein rearrangement refers to a 1,2-shift of
an alkyl, aryl, and alkenyl group to an adjacent carbocationic
center; formation of this carbon–carbon bond has found
widespread applications in many Lewis acid or Brønsted acid
mediated reactions [Eq. (1)].[1,2] For this well-known reaction,
the 1,2-shift of R1 versus R2 is determined primarily by the
relative stability of carbocation B (or B’), as well as the
intrinsic properties of the migrating group (Scheme 1).[1,2] No
instance of a g-substituent to stereospecifically direct a 1,2-
shift of the R group in b-position was reported to date; those
carbocations bearing a g-silyl group are no examples either.[3a]
Although the g-effect on this arrangement was claimed in an
early report,[3b] the 1,2-shift actually occurred within a ben-
zene skeleton comprising the Cb and Cg carbon atoms.
According to carbocation chemistry,[3–5] we envisage that
a metal substituent in the g-position might facilitate an anti
activation through hyperconjugation to induce a 1,2-R1-shift
(C!D) in an antiperiplanar conformation (C) [Eq. (2)].[3–5]
Alternatively, this metal might exert steric interaction to
induce a syn activation to enable a 1,2-R2-shift (C’!D’) in
a synperiplanar conformation C’ [Eq. (3)].[3,5] The realization
of such an unprecedented effect of a metal in g-position relies
on the availability of suitable carbocations applicable for
a study. Herein, we report our experimental and theoretical
work to support an anti-activation route [Eq. (2)] for M =
Au[6] (path C!D) even in a synperiplanar conformation. This
work represents an atypical Wagner–Meerwein rearrange-
ment, because the intrinsic properties of the migrating group
(R1, R2) are no longer decisive.
conrotation route. We employ these special 1,4-enynes,
because their bridged cyclopropanes can stabilize carbocation
F or F’ with a “bisected” conformation,[11] in which the ketone
group hinders the expansion of the cyclopropane ring.[12]
A
cyclopropyl group greatly enhances the electrophilicity of an
alkyne in the presence of a gold catalyst.[12] Our experimental
results disclose that only the cis-substituent R1 at the alkene is
transferable to give the observed cyclopentanone F selec-
tively. Notably, this stereospecificity of the migration is
unaffected when varying the R1 and R2 groups to methyl,
alkyl, and aryl groups, thereby truly reflecting this significant
g-effect. The object of this work is to clarify if an anti or syn
activation occurs in carbocations with a gold substituent in the
g-position, as in carbocations F or F’.
Shown in Scheme 1 is our strategy to illustrate the g-
substituent effect; the key reaction involves a gold-catalyzed
oxidative cyclization of 1,4-enyne 3.[7,8] The initially formed a-
oxo gold carbene E[9] is expected to have its olefin p electrons
+
=
parallel to the positive Au C p orbital to achieve a through-
space interaction. We envisage that this spatial arrangement
will undergo a facile alkene/carbene coupling[10] to give 2-oxo-
cyclopent-1-yl cation F or F’, through either a disrotation or
[*] S. Ghorpade, Prof. Dr. R.-S. Liu
Department of Chemistry, National Tsing-Hua University
Hsinchu, 30043 (Taiwan, ROC)
E-mail: rsliu@mx.nthu.edu.tw
Prof. Dr. M.-D. Su
Department of Applied Chemistry, National Chiayi University
Chiayi, 60004 (Taiwan, ROC)
E-mail: midesu@mail.ncyu.edu.tw
[**] We thank National Science Council, Taiwan, for financial support of
this work.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201210313.
Scheme 1. Strategy to study the g-substituent effect.
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Table 2: 1,4-Enynes bearing two equivalent alkenyl substituents.[a]
Table 1 shows the oxidative cyclization of 1,4-enyne 1a by
using 8-methylquinoline oxide (3 equiv) as the oxidant in the
presence of different catalysts. The feasibility of this reaction
Table 1: Activity screening using various catalysts.
Entry
2
t
[h]
Entry
2
t
[h]
(yield [%])[b]
(yield [%])[b]
Entry
Catalyst[a]
Solvent
t [h]
Yield [%][b]
1a
2a
1
2
3
4
5
6
7
8
9
AuCl3
DCE
DCE
DCE
DCE
DCE
DCE
DCE
CH2Cl2
CH2Cl2
CH3CN
1
2
0.5
1
0.5
0.5
24
12
1
–
–
–
–
–
–
60
76
–
52
48
82
36
71
66
–
–
62
55
PPh3AuCl/AgNTf2
LAuCl/AgNTf2
IPrAuCl/AgNTf2
LAuCl/AgOTf
LAuCl/AgSbF6
AgNTf2
HOTf
LAuCl/AgNTf2
LAuCl/AgNTf2
1
2b (83)
6.5
6
7
2g (72)
12
2
3
2c (89)
7
7
2h (78)
0.5
10
1
–
[a] [Substrate]=0.027m, L=P(tBu)2(o-biphenyl). IPr=1,3-bis(diisopro-
pylphenyl)imidazol-2-ylidene. [b] Product yields are reported after pu-
rification from a silica gel column.
is reflected by AuCl3 (5 mol%) in dichloroethane (DCE,
258C), which gave the desired oxidative cyclization product
2a in 52% yield (Table 1, entry 1); this process involves a ring
expansion of a cyclopentylidene moiety while retaining
a cyclopropane ring. We tested further the reaction with
cationic gold catalysts; P(tBu)2(o-biphenyl)AuNTf2 gave
compound 2a in 82% yield, better than PPh3AuNTf2 (48%)
and IPrAuNTf2 (36%) (Table 1, entries 2–4). Other counter
anions, such as in LAuOTf and LAuSbF6 [L = P(tBu)2(o-
biphenyl)], gave decreased yields with 71% and 66%,
respectively (Table 1, entries 5–6). AgNTf2 and HOTf alone
failed to give the desired product 2a under similar conditions
(Table 1, entries 7–8). For LAuNTf2, the reactions proceeded
less efficiently in dichloromethane and CH3CN, giving cyclo-
pentenone 2a in 62% and 55% yield, respectively (Table 1,
entries 9–10).
2d (91)
8
9
2i (71)
2j (86)
1
3
4
5
2e (78)
2 f (75)
0.5
2.5
[a] [substrate]=0.027m, L=P(tBu)2(o-biphenyl). [b] Product yields are
reported after purification from a silica gel column.
We tested this oxidative reaction on additional 1,4-enynes
1b–1j to examine the substrate scope; the reactions were
performed with LAuNTf2 (5 mol%) and 8-methylquinoline
oxide (3 equiv) in DCE (258C). As shown in Table 2,
entries 1–3, the reactions are extensible to internal alkyne
substrates 1b–1d bearing electron-deficient phenyl substitu-
ents (R2 = 4-XC6H4, X = F, CN, and NO2); the resulting
cyclopentenone products 2b–2d were obtained in 83–91%
yield. The molecular structure of compound 2d was charac-
terized by X-ray diffraction.[13] We also prepared 1,4-enyne 1e
bearing a cyclohexylidene moiety; its Au-catalyzed reaction
gave the desired cyclopentenone 2e including a fused seven-
membered ring (Table 2, entry 4). Such oxidative ring expan-
sions were also operable for internal alkynes 1 f (R2 = Me), 1g
(R2 = Ph), and 1h (R2 = 4-MeCOC6H4) to give expected
products 2 f–2h in (72–78%) yield (Table 2, entries 5–7). The
reaction is further extensible to the synthesis of eight-
membered carbocycle 2i (71% yield) by using cycloheptyli-
dene derivative 1i. We tested the reaction on a propylidene
substrate 1j that afforded the vicinal dimethyl cyclopente-
none 2j in 86% yield.
We prepared both E- and Z-configured 1,4-enynes 3a–f to
assess the migration ability of the alkenyl cis- and trans-
substituents (Table 3). With E-configured olefins, the reac-
tions proceeded more rapidly than with their corresponding
Z-isomers, because the former easily attain conformation E
(see Scheme 1, size: R2 > R1). For 1,4-enyne E-3a, we
observed a 1,2-methyl migration for its resulting product 4a
(Table 3, entry 1), but for its Z-isomer Z-3a, a 1,2-phenyl shift
occurred to deliver a distinct regioisomer 4a’ (entry 2). These
observations reveal that the cis-position is the preferable
migration site. As shown in Table 3, entries 3–6, the 1,2-shifts
of the cis-substituents are again observed for 1,4-enynes E-3b,
E-3c, Z-3b, and Z-3c bearing different electron-rich or
2
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Table 3: Oxidative cyclization of 1,4-enynes in E- and Z-forms.[a]
respectively
(Table 3,
entries 11,12). The oxidative cycli-
zation of disubstituted olefin E-3g
is compatible with our expectation
to give only species 4g in 76% yield
(Table 3, entry 13), whereas its Z-
isomer Z-3g gave a mixture of
cyclopentenone species 4g (34%)
and 4g’ (43%), which were separa-
ble by using silica column chroma-
tography (entry 14). We believe
that a portion of species 4g in
entry 14 was produced from an
intrinsic deprotonation in the pro-
Entry
3
4
t
[h]
Entry
3
4
t
[h]
(yield [%])[b]
(yield [%])[b]
1
2
E-3a[a]
Z-3a
4a (78)
4a’ (81)
0.5
2
10
11
12
Z-3e
E-3 f
Z-3 f
4e’ (71)
4 f (71)
4 f’ (73)
5
7
posed intermediate
F
(or F’;
Scheme 1, R2 = H). This stereose-
lective migration pattern also
occurs with internal alkyne sub-
strates E-3h and Z-3h, producing
desired products 4h and 4h’,
respectively. The fact that only cis-
substituents are transferable for
various aryl, methyl, and alkyl sub-
stituents strongly indicates the pro-
nounced g-effect of Au to direct
a 1,2-shift with stereospecificity.
Table 4 depicts our control
3
4
E-3b
Ar=4-ClC6H4
Z-3c
4b (72)
4c (79)
0.5
0.5
10
Ar=4-MeOC6H
5
6
Z-3b
Ar=4-ClC6H4
Z-3c
4b’ (77)
4c’ (83)
2.5
1.5
13
E-3g
Ar=4-MeOC6H4
4g (76)
0.5
Ar=4-MeOC6H4
experiments to confirm the inter-
mediacy of oxo gold carbenes E. We
prepared diazo-containing E-con-
figured olefin E-5, and its treatment
with gold catalyst (5 mol%) gave
only cyclopentenone 4a in 64%
7
E-3d
4d (62)
1
14
Z-3g
Ar=4-OMeC6H4
4g’ (43) + 4g (34) 1.5
yield with
a methyl migration
(Table 4, entry 1). For its Z-isomer
Z-5, we obtained the other regio-
isomer 4a’ in 68% yield with a 1,2-
phenyl migration (Table 4, entry 2);
compound 4a’ represents the prod-
uct from a typical Wagner–Meer-
wein rearrangement. Again, only
cis-substituents are found to
migrate. Entries 3–6 (Table 4) illus-
trate the ligand effects of gold
catalysts on the Wagner–Meerwein
rearrangement. A large electron-
8
9
Z-3d
E-3e
4d’ (68)
4e (79)
4
1
15
16
E-3h
4h (67)
8
Z- 3h
4h’ (64)
15
[a] [Substrate]=0.027m, 5 mol%, LAuNTf2 L=P(tBu)2(o-biphenyl); oxidant (3 equiv), DCE, 258C.
[b] Product yields are reported after purification by using a silica gel column.
electron-deficient benzenes, giving cyclopentenone deriva-
tives 4b and 4c (72% and 79%) through a 1,2-methyl
migration and their regioisomers 4b’ and 4c’ (77% and 83%)
through a 1,2-phenyl shift. We tested the ring expansions on
substrates E-3d and Z-3d, which conformed to the same
migration pattern to give desired products 4d and 4d’
selectively. Such a site selectivity works well with E- and Z-
configured olefins 3e bearing ethyl and phenyl groups; their
corresponding products 4e and 4e’ were obtained in satisfac-
tory yields (79–71%, Table 3, entries 9,10). This stereospeci-
ficity of the migration is applicable to 1,4-enynes E-3 f and Z-
3 f bearing two distinct methyl and benzyl groups, delivering
desired products 4 f and 4 f’ in 71% and 73% yields,
deficient phosphite ligand as in AuP(OPh)3NTf2 (5 mol%)
gave the Wagner–Meerwein product 4a’ as the major species
(44%) when using E-configured olefin E-5 (entry 3); this
phosphite ligand gave 4a’ exclusively with Z-configured
olefin Z-5 (entry 4). In contrast, the small ligand in Au-
(PMe3)NTf2 gave the methyl migration product 4a (45%)
preferably with E-5 (entry 5). Accordingly, the g-effect is
more prominent for an electron-rich phosphine ligand
according to the observed trend: P(tBu)2(o-biphenyl) >
PMe3 > P(OPh)3.
We tested also the migratory cyclizations of diazo species
Z-5 or E-5 with other metal carbene intermediates (Table 5).
[Rh2(OAc)4] (0.05 mol%) also showed a 1,2-shift of the
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Table 4: Control experiments and ligand effects.
consider the solvent effects (in dichloroethane); the results
are provided in Figure 1 and Figure S1 in the Supporting
Information, respectively; the computational results on the
two solvent models gives well agreeable conclusions. Figure 1
Entry
Diazo[a]
Ligand(L)
t [h]
Yield [%][b]
4a
4a’
1
2
3
4
5
6
E-5
Z-5
E-5
Z-5
E-5
Z-5
P(tBu)2(o-biphenyl)
P(tBu)2(o-biphenyl)
P(OPh)3
P(OPh)3
PMe3
0.5
1.0
0.5
2
0.5
2
64
–
29
–
45
–
–
68
44
80
27
78
PMe3
[a] [substrate]=0.027m. [b] Product yields are reported after purification
by using a silica gel column.
Table 5: Reaction with other metal catalysts.
Figure 1. Energy profiles for four possible 1,2-migrations. The black
line corresponds to the observed path. DFT calculations were per-
formed by using the B3LYP/LAN2DZ method in the gaseous phase
(values in []), and the Onsager method was used to consider solvent
effects in dichloromethane (values without brackets). Energies are
given in kcal molꢀ1. PR3 =P(tBu)2Ph.
Entry
Diazo[a]
M (mol%)
t [h]
Yield [%][b]
4a
4a’
1
2
3
4
5
6
7
8
E-5
Z-5
E-5
Z-5
E-5
Z-5
E-5
E-5
[Rh2(OAc)4] (0.05)
[Rh2(OAc)4] (0.05)
Cu(OTf)2 (1)
Cu(OTf)2 (1)
AgSbF6 (5)
AgSbF6 (5)
LAgSbF6 (5)
LAgNTf2 (5)
0.5
0.5
2.5
3.0
1.0
2.0
8.0
12.0
56
–
–
–
–
–
16
21
–
65
77
79
68
71
60
47
presents the results of calculations to assess four possible 1,2-
shifts on the two carbocations. For carbocation F, a 1,2-phenyl
shift has a barrier (DH°(sol) =+ 0.62 kcalmolꢀ1) much smaller
than that of a methyl shift (DH°(sol) =+ 6.9 kcalmolꢀ1). On its
diastereomer F’, we obtained a small barrier for a methyl
migration (DH°(sol) =+ 3.6 kcalmolꢀ1), but a large activation
energy for a phenyl shift (DH°(sol) =+ 8.6 kcalmolꢀ1). These
differences (> 5.0 kcalmolꢀ1) are significant to distinguish the
syn and anti activation. These results clearly suggest that
a gold substituent in the g-position in the two cationic
intermediates activates a 1,2-shift of the b-anti substituent
through hyperconjugation. As a large amount of enthalpic
energy (> 30 kcalmolꢀ1) is released for these transpositions,
these 1,2-migrations should be irreversible. Accordingly, the
migrations with smaller barriers, that is, F!TS-1!G and F’!
TS-1’!G’ will become the dominant pathways. The geo-
metries of carbocations H,G and H’,G’ resemble those of p-
alkene complexes I,J and I’,J’, because their energy levels are
very close to each other.
[a] [Substrate]=0.027m. L=P(tBu)2(o-biphenyl). [b] Product yields are
reported after purification by using a silica gel column.
alkenyl cis-substituent to give cyclopentenone derivatives 4a
and 4a’, respectively, from E-5 and Z-5 diazo species (Table 5,
entries 1–2). Notably, the use of both Cu(OTf)2 (1 mol%) and
AgSbF6 (5 mol%) resulted in a typical Wagner–Meerwein
rearrangement to give species 4a’ (68–79%), when using
either E-5 or Z-5 olefins species (Table 5, entries 3–6). But
electron-rich phosphine-containing silver catalysts, LAgX
(X = SbF6, NTf2) gave the methyl migration product E-5 in
16% and 21% yields respectively (Table 5, entries 7–8).
The control experiments confirm the intermediacy of a-
oxo gold carbenes E in our working mechanism (Scheme 1),
but we are still uncertain about the role of two possible
carbocations F and F’ to produce the resulting cyclopente-
nones 4. We sought information from density functional
theory on the 1,2-shifts of two possible carbocations F and
F’;[14,15] these calculations were performed using the B3LYP/
We rule out the intermediacy of F’ in Scheme 1, because
its corresponding syn-activation (F’!4) is inconsistent with
our computational results. Our control experiments in Table 4
also support an anti-activation route, because the stereospe-
LANL2DZ and B97D/LANL2DZ methods, with AuL = P- cificity of the migration is more prominent when a small
(tBu)2PhAu in the gaseous phase. We also used the Onsager
electron-rich ligand, as in PMe3Au+, is used than when a large
electron-deficient ligand, as in P(OPh)3Au+, is used; ligand
model[16] and the PCM[17] (polarized continuum model) to
4
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Chem. 2011, 123, 5882 – 5885; Angew. Chem. Int. Ed. 2011, 50,
size is not a decisive factor. The postulated route E!F is
easily comprehensible, because this disrotation route allows
an efficient overlap between two interacting p orbitals in the
early stage of rotation [Eq. (4)], whereas the conrotation fails
to give an overlap until the late stage of rotation.
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[7] For gold-catalyzed oxidative cyclization of 1,5-enyne, see D.
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Angew. Chem. 2011, 123, 7043 – 7046; Angew. Chem. Int. Ed.
2011, 50, 6911 – 6914. The reported reactions herein are unre-
lated to Wagner– Meerwein rearrangements.
[8] For gold-catalyzed reactions on 1,4-enynes, see selected exam-
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132, 8550 – 8551; d) L. Ye, W. He, L. Zhang, Angew. Chem. 2011,
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To date, there is no example of the Wagner–Meerwein
rearrangement where a metal substituent in the g-position can
direct a 1,2-shift of the neighboring group with stereospeci-
ficity. To demonstrate this g-effect, we sought the solution
from the gold-catalyzed cyclization of 1,4-enynes 1; in this
cyclization only the cis-alkenyl substituent is transferable.
Our control experiments suggest the intermediacy of a-
carbonyl gold carbenes E’. We performed theoretical calcu-
lations to demonstrate a preferable anti activation for two
possible carbocations F and F’. Both experimental and
theoretical work disclose that a gold substituent in the g-
position can direct a 1,2-shift of the anti-b-substituent
regardless of its intrinsic properties. This discovery provides
insight into a new aspect of the Wagner–Meerwein rearrange-
ment.
Received: December 27, 2012
Revised: February 5, 2013
Published online: && &&, &&&&
Keywords: carbenes · cyclization · gold ·
[10] For related AuI- and AgI-catalyzed alkene/carbene couplings,
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Salathe, J. W. Bats, Adv. Synth. Catal. 2008, 350, 2059 – 2064.
Notably, there is no stereospecificity of a migration reported
herein.
.
homogeneous catalysis · Wagner–Meerwein rearrangement
[1] a) G. Wagner, J. Russ. Chem. Soc. 1899, 31, 690; b) H. Meerwein,
Justus Liebigs Ann. Chem. 1914, 405, 129 – 175.
[11] This bisected conformation allows an efficient interaction of
cyclopropyl s bonding electrons with their adjacent empty p
orbital. See reviews: a) G. A. Olah, V. Reddy, G. K. S. Prakash,
Chemistry of the Cyclopropyl Group, Part 2 (Eds.: Z. Rappo-
port), Wiley, Chichester, UK, 1995, pp. 813 – 859; b) G. A. Olah,
V. Reddy, G. K. S. Prakash, Chem. Rev. 1992, 92, 69 – 95.
[12] Cyclopropyl alkynes were often used in gold-catalyzed reactions
because of their higher reactivity compared to normal alkynes.
See: a) A. Fꢀrstner, C. Aꢁssa, J. Am. Chem. Soc. 2006, 128, 6306 –
6307; b) M. Shi, L.-P. Liu, J. Tang, J. Am. Chem. Soc. 2006, 128,
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H. Hung, R.-S. Liu, Angew. Chem. 2010, 122, 10087 – 10090;
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Liu, Chem. Commun. 2011, 47, 1339 – 1341; e) C.-Y. Yang, M.-S.
Lin, H.-H. Liao, R.-S. Liu, Chem. Eur. J. 2010, 16, 2696 – 2699;
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130, 3736 – 3737; g) S. Ye, Z.-X. Yu, Org. Lett. 2010, 12, 804 – 807.
[13] CCDC 916773 (2d) 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.
[2] Reviews: a) J. R. Hanson in Comprehensive Organic Synthesis,
Vol. 3 (Eds.: B. M. Trost, I. Fleming, G. Pattenden), Pergamon,
Oxford, 1991, chap. 3.1, pp. 705 – 719; b) B. Ricknorn in Com-
prehensive Organic Synthesis, Vol. 3 (Eds.: B. M. Trost, I.
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[3] Suginome and co-workers studied a 1,2-g-silyl shift activated by
a g-silyl group;[3a] the syn and anti activations relied on the
configurations of two silyl groups. In other words, there is no
stereospecificity induced by the g-silyl group. This example is
a
typical Wagner–Meerwein rearrangement, because both
diastereomeric carbocations show the same 1,2-silyl shift;
a) M. Suginome, A. Takama, Y. Ito, J. Am. Chem. Soc. 1998,
120, 1930 – 1931; b) S. M. Starling, S. C. Vonwiller, J. N. H. Reek,
J. Org. Chem. 1998, 63, 2262 – 2272.
[4] For selected example, see: a) P. F. Hudrlik, A. M. Hudrlik, G.
Nage-ndrappa, T. Yimenu, E. Zellers, E. Chin, J. Am. Chem. Soc.
1980, 102, 6894 – 6896; b) K. Miura, T. Hondo, H. Saito, H. Ito,
A. Hosomi, J. Org. Chem. 1997, 62, 8292 – 8293; c) T. Ooi, T.
Kiba, K. Maruoka, Chem. Lett. 1997, 519 – 520; d) K. Tanino, N.
Yoshitani, F. Moriyama, I. Kuwa-jima, J. Org. Chem. 1997, 62,
4206 – 4207; e) A. S. K. Hashmi, W. Yang, F. Rominger, Angew.
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Communications
[14] All computations were done using the Gaussian03 package:
M. J. Frisch, et al. Gaussian03, revision C. 02: Gaussian, Inc.,
Pittsburgh, PA, 2003.
[15] The B3LYP method, see: a) A. D. Becke, Phys. Rev. A 1988, 38,
3098 – 3100; b) A. D. Becke, J. Chem. Phys. 1993, 98, 5648 – 5652;
c) C. Lee, W. Yang, R. G. Parr, Phys. Rev. B 1988, 37, 785 – 789;
d) The B97D method. see: S. Grimme, J. Comput. Chem. 2006,
27, 1787 – 1799; e) The LANL2DZ basis sets, see: P. J. Hay, W. R.
Wadt, J. Chem. Phys. 1985, 82, 270 – 284.
[16] The Onsager model, see: a) M. W. Wong, M. J. Frisch, K. B.
Wbierg, J. Am. Chem. Soc. 1991, 113, 4776 – 4782; b) J. G.
Kirkwood, J. Chem. Phys. 1934, 2, 351 – 361; c) L. Onsager, J.
Am. Chem. Soc. 1936, 58, 1486 – 1493.
ˇ
[17] The PCM method, see: a) S. Miertus, E. Scrocco, J. Tomasi,
Chem. Phys. 1981, 55, 117 – 129; b) M. Cossi, V. Barone, R.
Canni, J. Tomasi, Chem. Phys. Lett. 1996, 255, 327 – 335; c) V.
Barone, M. Cossi, J. Phys. Chem. A 1998, 102, 1995 – 2001.
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Angewandte
Chemie
Communications
Rearrangement
S. Ghorpade, M.-D. Su,*
R.-S. Liu*
&&&& — &&&&
Gold-Catalyzed Oxidative Cyclizations on
1,4-Enynes: Evidence for a g-Substituent
Effect on Wagner–Meerwein
Gold-catalyzed oxidative cyclizations of
1,4-enynes were used to study the g-effect
on the Wagner–Meerwein rearrangement.
Both experimental and theoretical work
disclose that a gold substituent in the g-
Rearrangements
position can direct a stereospecific 1,2-
shift of the anti-b-substituent regardless
of its intrinsic properties.
Angew. Chem. Int. Ed. 2013, 52, 1 – 7
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These are not the final page numbers!