Platinum and ruthenium chloride-catalyzed cycloisomerization of 1-alkyl-2-ethynylbenzenes: Interception of π-activated alkynes with a benzylic C-H bond

Article abstract of DOI:10.1021/jo901045g

(Chemical Equation Presented) Air-stable and commercially available alkynophilicmetal salts, such as PtCl₂, PtCl₄, and [RuCl₂(CO)₃]₂, catalyze the cycloisomerization of 1-alkyl-2-ethynylbenzenes to produce substituted indenes even at an ambient temperature. Electrophilically activated alkynes can be intercepted by simple benzylic C-Hbonds at primary, secondary, and tertiary carbon centers. Mechanistic studies, such as labeling studies and kinetic isotope and substituent effects, indicate that the cycloisomerization proceeds through the formation of a vinylidene intermediate and turnover-limiting 1,5-shift of benzylic hydrogen.

Full text of DOI:10.1021/jo901045g

pubs.acs.org/joc
Platinum and Ruthenium Chloride-Catalyzed Cycloisomerization of
1-Alkyl-2-ethynylbenzenes: Interception of π-Activated Alkynes with a
Benzylic C-H Bond
Mamoru Tobisu,‡ Hiromi Nakai,† and Naoto Chatani*,†
†Department of Applied Chemistry, Faculty of Engineering and ‡Frontier Research Base for Global Young
Researchers, Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871, Japan
chatani@chem.eng.osaka-u.ac.jp
Received May 24, 2009
Air-stable and commercially available alkynophilic metal salts, such as PtCl2, PtCl4, and [RuCl2(CO)3]2,
catalyze the cycloisomerization of 1-alkyl-2-ethynylbenzenes to produce substituted indenes even at
an ambient temperature. Electrophilically activated alkynes can be intercepted by simple benzylic
C-H bonds at primary, secondary, and tertiary carbon centers. Mechanistic studies, such as labeling
studies and kinetic isotope and substituent effects, indicate that the cycloisomerization proceeds
through the formation of a vinylidene intermediate and turnover-limiting 1,5-shift of benzylic
hydrogen.
Introduction
especially gold and platinum, is a rapidly evolving area of
research.1 To date, this alkyne activation strategy has been
applied to a diverse range of transformations, the mechanism
of which can be well-rationalized, in most cases, on the basis
of two limited resonance structures resulting from π-alkyne
complex I (Scheme 1). One form is a vinyl cation species II,
which is susceptible to nucleophilic attack by lone-pair
electrons of heteroatoms and π-electrons of arenes.1,2 The
other form is a carbene-carbenoid species III, which can be
intercepted by unsaturated bonds via cyclopropanation.1,3
Apart from these typical reaction modes via II and III,
involvement of a vinylidene intermediate IV has also been
suggested in several Pt- and Au-catalyzed reactions,4 as is
frequently proposed for other metals.5 Regardless of the
intermediate involved, the electrophilically activated alkyne
moiety is trapped, in most cases, by electron-rich function-
alities, including heteroatoms, stabilized carbanions, and
unsaturated bonds. A few notable exceptions exist, however,
The development of new catalytic reactions triggered by
π-activation of alkynes using electrophilic metal salts,
€
(1) For selected recent reviews: (a) Furstner, A.; Davies, P. W. Angew.
Chem., Int. Ed. 2007, 46, 3410. (b) Gorin, D. J.; Toste, F. D. Nature 2007, 446,
395. (c) Hashmi, A. S. K. Chem. Rev. 2007, 107, 3180. (d) Arcadi, A. Chem.
ꢀ
ꢀ~
Rev. 2008, 108, 3266. (e) Jimenez-Nunez, E.; Echavarren, A. M. Chem. Rev.
2008, 108, 3326. (f) Li, Z.; Brouwer, C.; He, C. Chem. Rev. 2008, 108, 3239.
(g) Gorin, D. J.; Sherry, B. D.; Toste, F. D. Chem. Rev. 2008, 108, 3351.
(h) Patil, N. T.; Yamamoto, Y. Chem. Rev. 2008, 108, 3395. (i) Shen, H. C.
Tetrahedron 2008, 64, 3885. (j) Skouta, R.; Li, C.-J. Tetrahedron 2008, 64,
4917. (k) Kirsch, S. F. Synthesis 2008, 3183. (l) Lee, S. I.; Chatani, N. Chem.
Commun. 2009, 371.
(2) Selected reviews on catalytic hydroarylation of alkynes: (a) Nevado,
C.; Echavarren, A. M. Synthesis 2005, 167. (b) Kitamura, T. Eur. J. Org.
Chem. 2009, 1111. (c) Chatani, N.; Inoue, H.; Ikeda, T.; Murai, S. J. Org.
Chem. 2000, 65, 4913. (d) Inoue, H.; Chatani, N.; Murai, S. J. Org. Chem.
2002, 67, 1414.
(3) Interception of metal carbenoid species via double cyclopropanation:
(a) Chatani, N.; Kataoka, K.; Murai, S.; Furukawa, N.; Seki, Y. J. Am.
ꢀ
Chem. Soc. 1998, 120, 9104. (b) Mendez, M.; Munoz, M. P.; Nevado, C.;
Cardenas, D. J.; Echavarren, A. M. J. Am. Chem. Soc. 2001, 123, 10511.
~
ꢀ
ꢁ
(c) Mainetti, E.; Mouries, V.; Fensterbank, L.; Malacria, M.; Marco-Contelles,
J. Angew. Chem., Int. Ed. 2002, 41, 2132. (d) Nieto-Oberhuber, C.; Munoz,
~
€
(4) (a) Mamane, V.; Hannen, P.; Furstner, A. Chem.;Eur. J. 2004, 10,
ꢀ
ꢀ
M. P.; Bunuel, E.; Nevado, C.; Cardenas, D. J.; Echavarren, A. M. Angew.
Chem., Int. Ed. 2004, 43, 2402. (e) Peppers, B. P.; Diver, S. T. J. Am. Chem. Soc.
4556. (b) Soriano, E.; Marco-Contelles, J. Organometallics 2006, 25, 4542.
(c) Seregin, I. V.; Gevorgyan, V. J. Am. Chem. Soc. 2006, 128, 12050.
(d) Bajracharya, G. B.; Pahadi, N. K.; Gridnev, I. D.; Yamamoto, Y. J.
Org. Chem. 2006, 71, 6204. (e) Seregin, I. V.; W., S. A.; Gevorgyan, V.
Tetrahedron 2008, 64, 6876.
~
2004, 126, 9524. (f) Nieto-Oberhuber, C.; Munoz, M. P.; Lopez, S.;
Jimenez-Nunez, E.; Nevado, C.; Herrero-Gomez, E.; Raducan, M.;
ꢀ
ꢀ
ꢀ~
ꢀ
Echavarren, A. M. Chem.;Eur. J. 2006, 11, 1677. (g) Marco-Contelles, J.;
ꢁ
Arroyo, N.; Anjum, S.; Mainetti, E.; Marion, N.; Cariou, K.; Lemiere, G.;
Mouries, V.; Fensterbank, L.; Malacria, M. Eur. J. Org. Chem. 2006, 4618.
(5) For recent reviews on catalytic reactions involving vinylidene inter-
mediates, see: (a) Bruneau, C.; Dixneuf, P. H. Angew. Chem., Int. Ed. 2006,
ꢁ
ꢀ
ꢀ
ꢀ
(h) Lopez, S.; Herrero-Gomez, E.; Perez-Galan, P.; Nieto-Oberhuber, C.;
Echavarren, A. M. Angew. Chem., Int. Ed. 2006, 45, 6029. (i) Kim, S. M.; Park,
J. H.; Choi, S. Y.; Chung, Y. K. Angew. Chem., Int. Ed. 2007, 46, 6172.
45, 2176. (b) Verela, J.; Saa, C. Chem.;Eur. J. 2006, 12, 6450. (c) Ohe, K.
Bull. Korean Chem. Soc. 2007, 28, 2153. (d) Trost, B. M.; McClory, A. Chem.
Asian J. 2008, 3, 164. (e) Liu, R.-S. Synlett 2008, 801.
DOI: 10.1021/jo901045g
r
Published on Web 06/18/2009
J. Org. Chem. 2009, 74, 5471–5475 5471
2009 American Chemical Society

Tobisu et al.
JOCArticle
SCHEME 1. π-Activation of Alkynes by Electrophilic Metal Salts
Results and Discussion
We initially examined the cycloisomerization of ethynyl-
benzene derivative 1, since prior to this study, no effective
catalysts were identified for a cycloisomerization of this type
of simple substrate (Table 1).8 To our delight, the use of
commercially available platinum salts, PtCl2 and PtCl4,
furnished the desired indene 2 at 60 °C in good yield (entries
1 and 2). Notably, with a PtCl4 catalyst, the reaction
proceeded at ambient temperature, although it was accom-
panied by an undesired decomposition of 1 (entry 3). Inter-
estingly, PtBr2, which was reportedly inactive for
cycloisomerization in the absence of olefinic moiety,4d
worked well under the present conditions (entry 4).9 Whereas
the use of other alkynophilic metal salts, including gold
complexes, led to a complicated mixture (entries 5, 6, and
9), [RuCl2(CO)3]2 exhibited a catalytic activity comparable
to that for platinum salts (entry 7).10 It should be noted that
isomerization of the double bond in 2 to a more substituted
isomer did not occur under these conditions.
With several effective catalysts in hand, we next exami-
ned the scope of this intriguing cycloisomerization
reaction (Table 2). The benzylic C-H bond of a methyl
group can participate in this reaction to afford indene in
modest yield (entry 3). Introduction of sterically demanding
polyaromatic groups at the homobenzylic position success-
fully furnished the corresponding indene derivatives, which
are potentially useful for the synthesis of extended indenyl
systems (entries 4-9). The cycloisomerization took place
smoothly in the presence of relatively acidic protons, such as
those in diphenylmethyl (entries 4 and 5) and fluorenyl
(entries 6 and 7) groups. Most notably, tertiary C-H bonds
could be annulated efficiently with platinum and ruthenium
catalysts (entries 10-17, 21-23, and 27-29). Moreover,
cycloisomerization proceeded even at 30 °C, when PtCl4
was used as a catalyst (entries 12, 15, 20, 23, 26, and 29).
This tertiary C-H bond functionalization can successfully be
applied to the synthesis of spirocyclic indenes (entries 13-17).
These reactions demonstrate the potential power of this
π-activation approach, whereby a tertiary C-H bond is
directly transformed into an all-carbon quaternary center in
a catalytic manner at room temperature. Ethynylbenzenes
bearing the benzylic C-H bonds adjacent to heteroatoms,
such as oxygen (entries 18-29) and nitrogen (entries 30 and
31), also serve as good substrates, furnishing heteroatom-
substituted indenes. In the case of substrates bearing secondary
benzylic C-H bonds, the formation of enol ethers or enamines
via double-bond isomerization was not observed.11 In addi-
tion, we detected no byproducts formed through carbon-
heteroatom bond fission, which has been known to occur in
the reactions of alkynophilic metal salts with similar
substrates.12
TABLE 1. Cycloisomerization of 1 by Various Alkynophilic Metalsa
entry
catalyst
time (h)b
GC yield (%)
1
PtCl2
PtCl4
PtCl4
PtBr2
AuCl3
22
1
12
15
5
22
24
29
22
>99
87
51
72
0d
2
3c
4
5
6e
7
AuCl[P(4-CF3C6H4)3]/AgSbF6
[RuCl2(CO)3]2
[IrCl(CO)3]n
Rh2(O2CCF3)4
0d
86
44
0d
8
9
a Reaction conditions: 1 (0.1 mmol), catalyst (0.01 mmol) in toluene
(0.5 mL) at 60 °C. b The approximate time required for complete consump-
tion of 1. c Run at 30 °C. d Complicated mixture. e 1,2-Dichloroethane
was used as a solvent.
in which an inert C(sp3)-H bond participates in the react-
ion involving activated alkynes. Yamamoto reported the
Pt-catalyzed cycloisomerization reaction of 1-ethynyl-2-
(1-alkoxybut-3-enyl)benzenes through a vinylidene intermedi-
ate, as in IV.4d In the reaction, a benzylic C-H bond adjacent
to both allyl and methoxy groups is added across a terminal
alkyne moiety to afford indenes. However, a benzylic C-H
bond of simple alkyl groups is inapplicable, and the reac-
tion requires heating at an elevated temperature (120 °C).
Liu reported that a similar transformation can be accom-
plished with simple allylic6a and benzylic6b C-H bonds by
using [TpRu(PPh3)(CH3CN)2]+ which is a typical vinylidene
generator. However, even with this elaborate catalyst, the
benzylic C-H bonds in secondary alkyl groups cannot be
functionalized. Quite recently, Urabe reported a rhodium-
catalyzed cycloisomerization reaction, in which benzylic
C-H bonds of benzyl ethers react with a vinyl cation inter-
mediate, as in II, to form dihydropyrans.7 However, the
substrates require highly electron-deficient sulfonyl alkynes
and benzylic C-H bonds adjacent to an oxygen atom. Herein,
we report that commonly used alkynophilic metal salts, such as
PtCl2 and [RuCl2(CO)3]2, serve as the general catalyst for the
cycloisomerization of 1-alkyl-2-ethynylbenzenes under rela-
tively mild reaction conditions (30-80 °C), leading to benzylic
C-H bond functionalization of simple alkyl groups. It is
demonstrated, for the first time, that simple tertiary C-H
bonds can participate in this type of catalytic cycloisomeriza-
tion reaction to form all-carbon quaternary centers. The
mechanistic aspect of this intriguing cycloisomerization is
thoroughly discussed.
To gain insight into the reaction mechanism, we next
investigated the reaction of deuterated alkyne 3 in the
(8) With a similar substrate, 38% yield after 100 h of stirring at 105 °C
was obtained with a Ru catalyst. See ref 6b.
(9) The reaction did not proceed under conditions similar to that
reported by Yamamoto (10 mol % PtBr2 in CH3CN at 60 °C). See ref 4d.
(10) Alkynophilic Lewis acids, such as GaCl3 and InCl3, afforded no
desired product.
(11) With a TpRu catalyst, isomerization to silyl enol ether occurred
predominantly. See ref 6b.
(12) Dube, P.; Toste, F. D. J. Am. Chem. Soc. 2006, 128, 12062.
(6) (a) Datta, S.; Odedra, A.; Liu, R.-S. J. Am. Chem. Soc. 2005, 127,
11606. (b) Odedra, A.; Datta, S.; Liu, R.-S. J. Org. Chem. 2007, 72, 3289.
(7) Shikanai, D.; Murase, H.; Hata, T.; Urabe, H. J. Am. Chem. Soc.
2009, 131, 3166.
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TABLE 2. Catalytic Cycloisomerization of 1-Alkyl-2-ethynylbenzenesa
water.13 Furthermore, the cycloisomerization of bromoalk-
yne 5 furnished 3-bromoindene 6 as a sole product (eq 2),
while other internal alkynes afforded no indenes under these
conditions. These results clearly indicate the intermediacy of
a vinylidene complex.5 The catalytic cycloisomerization of
the substrate labeled at the benzylic position, as in 7,
afforded indene 8, in which the cleaved deuterium atom
was incorporated at the C2 position (eq 3).
The possible mechanisms that can rationalize both the
hydrogen shift events observed above are provided in
Scheme 2. The vinylidene complex IV0 is formed via
π-activation of an alkyne moiety under the influence of
alkynophilic metal salts. The 1,5-hydrogen shift through a
conjugated system in IV0 then forms metal-carbenoid V. The
intermediate V can be viewed as a metallohexatriene, thus
electrocyclization of V would form the metallacycle VI.
Subsequent reductive elimination of VI leads to the final
product. As an alternate pathway, the direct insertion of
metal vinylidene IV0 into a benzylic C-H bond nearby would
also afford the indene product.4d
The present catalytic cycloisomerization involved the
migration of two hydrogen atoms, one at the alkyne terminus
and the other at the benzylic position. To obtain a more
detailed description of these hydrogen shifts, additional
labeling experiments were conducted. A crossover experi-
ment using a pair of substrates, 9 and 10, afforded indenes 11
and 12, and no exchange of the deuterium label was observed
by 1H NMR analysis, indicating that the 1,2-shift of the
alkynyl hydrogen proceeded through an intramolecular path-
way (eq 4). Similar crossover experiments using a substrate
a Reaction conditions: substrate (0.5 mmol), catalyst (0.05 mmol) in
toluene (2.5 mL). b A: [RuCl2(CO)3]2; B: PtCl2; C: PtCl4. c Oil bath
temperature. d Isolated yields unless otherwise noted. e GC yields.
f 9-Fluorenyl. g 9-Methyl-9-fluorenyl. h Run in THF as a solvent.
i 9-Carbazolyl.
presence of ruthenium and platinum salts used in this study
(eq 1). As a result, indene 4, in which a deuterium atom is
selectively incorporated at the 3-position, was obtained
exclusively with all catalysts, although, in the case of a
ruthenium catalyst, the deuterium content was decreased
to 64% probably due to a H/D exchange with the residual
(13) (a) Yeh, K.-L.; Liu, B.; Lo, C.-Y.; Huang, H.-L.; Liu, R.-S. J. Am.
Chem. Soc. 2002, 124, 6510. (b) Odedra, A.; Wu, C.-J.; Pratap, T. B.; Huang,
C.-W.; Ran, Y.-F.; Liu, R.-S. J. Am. Chem. Soc. 2005, 127, 3406. (c) Kim, H.;
Lee, C. J. Am. Chem. Soc. 2005, 127, 10180. (d) Cheong, P. H.-Y.;
Morganelli, P.; Luzung, M. R.; Houk, K. N.; Toste, F. D. J. Am. Chem.
Soc. 2008, 130, 4517. (e) Fukamizu, K.; Miyake, Y.; Nishibayashi, Y. Angew.
Chem., Int. Ed. 2009, 48, 2534.
J. Org. Chem. Vol. 74, No. 15, 2009 5473

Tobisu et al.
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SCHEME 2. Possible Mechanisms
set of 7 and 9 revealed that the benzylic hydrogen shift was also
an intramolecular process (eq 5).
On the other hand, the kinetic isotope effect of the benzylic
hydrogen was examined by an intramolecular competition
reaction using the monodeuterated substrate 17 (eq 7). As a
result, kH/kD was determined to be 1.9 based on the ratio of
18/19, which was estimated by 1H NMR of the isolated
products.
The electronic effect of the benzylic C-H bond on the
reaction rate was also examined using aromatic alkynes
bearing electronically different aryloxy groups (Table 3).
Although the cycloisomerization of 20a-c proceeded
smoothly under the optimized conditions to furnish 1-aryl-
oxyindenes 21a-c in good yields, the difference in reactivity
between these substrates was clearly observed under condi-
tions where a reduced amount of catalyst (2 mol %) was
used. Under these conditions, the substrate bearing an
electron-donating group (20c) afforded the product at a
higher rate than 20b, whereas introduction of an electron-
withdrawing group (20a) significantly retarded the cyclo-
isomerization reaction. Based on the kinetic isotope effect
experiments (eqs 6 and 7), the observed electronic effect
presumably reflected the electronic sensitivity of the benzylic
hydrogen shift.
To obtain further insight into the benzylic hydrogen shift,
we next examined a cycloisomerization reaction of chiral
substrate 22. Under Pt-catalyzed conditions, a nearly race-
mic product was obtained (eq 8). If the benzylic hydrogen
shift proceeds in a concerted manner, which is expected in a
direct vinylidene insertion process, the configuration of the
benzylic carbon should be retained on the basis of an analogy
to metal-carbenoid insertion reactions into C-H bonds.14
The effect of these two hydrogen shifts on the over-
all reaction rate was next investigated. The intermol-
ecular competition reaction of labeled (14) and unlabeled
(13) alkynes resulted in the formation of indene containing
47% deuterium at low conversion (30%) (eq 6). The kinetic
isotope effect of the alkynic hydrogen was estimated to be
1.1, indicating that the 1,2-hydrogen shift was a relatively
facile process in this catalysis.
(14) (a) Taber, D. F.; Petty, E. H.; Raman, K. J. Am. Chem. Soc. 1985,
107, 196. (b) Nakamura, E.; Yoshikai, N.; Yamanaka, M. J. Am. Chem. Soc.
2002, 124, 7181.
5474 J. Org. Chem. Vol. 74, No. 15, 2009

Tobisu et al.
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SCHEME 3. Possible Mechanism for a Benzylic Hydrogen Shift
Conclusion
In summary, we have identified commonly used alkyno-
philic metal salts, such as PtCl2, PtCl4, and [RuCl2(CO)3]2, as
versatile catalysts for the cycloisomerization of 1-alkyl-2-
ethynylbenzenes to afford substituted indenes. The scope of
substrates have been significantly expanded compared to
related catalytic systems,4d,6b,7 especially noteworthy are the
applicability of tertiary C-H bonds and the achievement of a
room-temperature reaction. This reaction enables a direct
functionalization of the benzylic C-H bond in simple alkyl
groups with electrophilically activated alkynes.16 Labeling
studies and substituent effect and chirality transfer experi-
ments revealed that the reaction proceeds through the forma-
tionof avinylideneintermediate via a1,2-hydrogenshift of the
alkynic hydrogen and a 1,5-shift of benzylic hydrogen. Studies
aimed at expanding the scope of this π-activation-triggered
C-H bond functionalization are ongoing in our laboratory.
TABLE 3. Electronic Effect in the Pt-Catalyzed Cycloisomerization of 20a
isolated yield of 21(%)
Experimental Section
X
2 h
6 h
under optimized conditions
PtCl2-Catalyzed Cycloisomerization of tert-Butyl(2-ethynyl-
benzyloxy)dimethylsilane (Table 2, Entry 25). To a flame-dried,
10-mL, two-necked flask equipped with a reflux condenser were
added tert-butyl(2-ethynylbenzyloxy)dimethylsilane (123 mg,
0.5 mmol), PtCl2 (13 mg, 0.05 mmol), and toluene (2.5 mL)
under nitrogen atmosphere. After the mixture was stirred at
60 °C for 22 h, it was cooled to room temperature. The volatiles
were removed in vacuo, and the residue was purified by column
chromatography on silica gel (hexane/EtOAc = 20:1) to give
(1H-inden-1-yloxy)(tert-butyl)dimethylsilane as a colorless oil
(116 mg, 93%): Rf 0.20 (hexane/EtOAc = 20/1); 1H NMR
(CDCl3) δ 0.08 (s, 3H), 0.17 (s, 3H), 0.95 (s, 9H), 5.25 (s, 1H),
6.35 (dd, J=5.7, 1.8 Hz, 1H), 6.71 (d, J=5.7 Hz, 1H), 7.17-7.26
(m, 3H), 7.42 (d, J=6.8 Hz, 1H); 13C NMR (CDCl3) δ -4.2,
18.3, 25.9, 78.0, 121.3, 123.3, 125.8, 128.0, 131.8, 138.1, 142.1,
146.08; IR (neat) 3068 s, 2929 s, 2856 s, 1558 m, 1469 s, 1389 m,
1360 s, 1333 s, 1255 s, 1171 s, 1120 s, 1092 s, 1009 s; MS m/z
(relative intensity) 246 (M+, 5), 190 (19), 189 (40), 116 (12), 115
(100), 75 (21); exact mass calcd for C15H22OSi 246.1440, found
246.1435.
CF3
H
OMe
trace
7
19
trace
25
44
77 (10 mol % of PtCl2, 80 °C, 24 h)
82 (10 mol % of PtCl2, 60 °C, 3.5 h)
66 (10 mol % of PtCl2, 60 °C, 2 h)
a Reaction conditions: substrate 20 (0.5 mmol), catalyst (0.01 mmol)
in toluene (2.5 mL) at 60 °C.
The mechanism that can accommodate all the experimental
observations is depicted inScheme3. Thebenzylichydrogen is
transferred as a hydride to the most electrophilic vinylidene R-
carbon, resulting in the formation of benzyl cation intermedi-
ate VII. Since the resonance structure of benzyl cation inter-
mediate VII is carbene intermediate V, which will lead to the
indene product as shown in Scheme 2. Although the observed
kinetic isotope effect was relatively small (eq 7), all of the
experimental data can be rationalized with certainty by
assuming a 1,5-hydride shift (IV0 f VII) as the turnover-
limiting step of this catalytic cycloisomerization. Thus, the
introduction of electron-donating groups to stabilize the
benzylic cation VII should accelerate the overall reaction,
which was indeed observed experimentally (Table 3). The
lifetime of the benzylic cation VII is likely to be long enough to
allow therotation aroundthe C(aryl)-C(benzylic) axis, which
leads to complete racemization of the benzylic stereocenter.15
Acknowledgment. This work was carried out, in part, by
the Program of Promotion of Environmental Improvement
to Enhance Young Researchers’ Independence, the Special
Coordination Funds for Promoting Science and Technology,
Ministry of Education, Culture, Sports, Science and Tech-
nology (MEXT), Japan. We also thank the Instrumental
Analysis Center, Faculty of Engineering, Osaka University,
for assistance with the MS, HRMS, and elemental analyses.
Supporting Information Available: Detailed experimental
procedures and characterization of products. This material is
available free of charge via the Internet at http://pubs.acs.org.
(15) Chirality transfer is accomplished in some gold-catalyzed cyclo-
isomerizations, in which carbocationic intermediates are involved.
Nakamura, I.; Sato, T.; Terada, M.; Yamamoto, Y. Org. Lett. 2008, 10,
2649. See also ref 12.
(16) On the submission of this manuscript, a similar transformation using
internal alkynes has been reported: Yang, S.; Li, Z.; Jian, X.; He, C. Angew.
Chem., Int. Ed. 2009, 48, 3999.
J. Org. Chem. Vol. 74, No. 15, 2009 5475