Gold(I)-catalyzed cyclodehydration enabled by the triisopropylsilyl group: A synthetically versatile methodology

Article abstract of DOI:10.1002/hlca.201200176

Introduction of a triisopropylsilyl group into allyl and allenyl carbinols greatly enhances the efficiency of gold(I)-catalyzed cyclodehydration, which can provide rapid access to a library of various compounds including 1H-indenes (Table 2 and Scheme 5), benzofulvenes (Table 3), indan-2-ones (Scheme 2), fulvenes (Table 4), cyclopentadienes (Table 4), 5H-dibenzo[a,c][7]annulenes (Scheme 6) and dibenzosuberones (Scheme 6). The developed method enables unprecedented product generality for several classes of cyclodehydration reactions, which is particularly notable for the preparation of 1H-indenes. The first synthesis of non-benzo-fused fulvenes via cyclodehydration of allenyl vinyl carbinols could be accomplished. The protocol is remarkable for mild conditions, operational convenience, and easy access to starting materials. Copyright

Full text of DOI:10.1002/hlca.201200176

Helvetica Chimica Acta – Vol. 95 (2012)
1773
Gold(I)-Catalyzed Cyclodehydration Enabled by the Triisopropylsilyl Group:
A Synthetically Versatile Methodology
by Dmitry L. Usanova)1), Marina Naodovica)2), Malte Brasholza)3), and Hisashi Yamamoto*a)b)
a) Department of Chemistry, The University of Chicago, 5735 South Ellis Avenue, Chicago, IL, 60637,
USA (e-mail: yamamoto@uchicago.edu)
b) Adjunct affiliation: Chemistry Department, King Abdulaziz University, Jeddah, 21589, Saudi Arabia
Dedicated to Prof. Dieter Seebach on the occasion of his 75th birthday
Introduction of a triisopropylsilyl group into allyl and allenyl carbinols greatly enhances the
efficiency of gold(I)-catalyzed cyclodehydration, which can provide rapid access to a library of various
compounds including 1H-indenes (Table 2 and Scheme 5), benzofulvenes (Table 3), indan-2-ones
(Scheme 2), fulvenes (Table 4), cyclopentadienes (Table 4), 5H-dibenzo[a,c][7]annulenes (Scheme 6)
and dibenzosuberones (Scheme 6). The developed method enables unprecedented product generality for
several classes of cyclodehydration reactions, which is particularly notable for the preparation of 1H-
indenes. The first synthesis of non-benzo-fused fulvenes via cyclodehydration of allenyl vinyl carbinols
could be accomplished. The protocol is remarkable for mild conditions, operational convenience, and
easy access to starting materials.
Introduction. – In the recent years, gold salts and complexes have been extensively
utilized as powerful and versatile catalysts for a large number of organic trans-
formations, being usually notable for high reactivity, chemoselectivity, and mild
reaction conditions [1]. Our recent communication [2] provided another contribution
to the field of gold catalysis describing the beneficial effect of the triisopropylsilyl
(TIPS) group on the efficiency of AuI-mediated cyclodehydration. In this report, we
would like to consider this methodology in a broader context and emphasize its
convenience and significant synthetic potential4).
1H-Indene Synthesis. – Cyclodehydration of easily accessible allyl alcohols is one of
the most straightforward approaches to the 1H-indene scaffold. However, before the
disclosure of our results [2], the majority of existing methods involved the use of strong
Lewis or Brønsted acids and required the presence of multiple aryl/alkyl stabilizing
groups in the substrates [3]. Thus, the synthesis of minimally functionalized 1H-indenes
1
)
)
)
)
Current Address: Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford
Street, Cambridge, MA 02138, USA.
Current Address: Department of Chemistry, University of Rochester, RC Box 270216, Rochester,
NY, 14627, USA.
Current Address: Institute of Organic Chemistry, University of Hamburg, Martin-Luther-King-
Platz 6, D-20146 Hamburg.
Data of compounds not given in the Exper. Part are available in the Supporting Information to [2].
2
3
4
ꢀ 2012 Verlag Helvetica Chimica Acta AG, Zꢁrich

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was beyond the scope of dehydrative cyclization. On the contrary, we discovered that
introduction of a triisopropylsilyl auxiliary group alleviates the necessity of polysub-
stitution and enables excellent yields in the 1H-indene synthesis catalyzed by a milder
system consisting of chloro(triphenylphosphine)gold(I) ([AgCl(PPh3)]) and silver
hexafluoroantimonate(V) (AgSbF6). The efficiency of the catalysis appeared to be
strongly dependent on steric factors: bulkier silyl moieties provided better yields
(Table 1). The nature of the catalyst and the silyl auxiliary demonstrated synergistic
effects on the reaction outcome: classical Lewis acids, e.g., boron trifluoride, could also
be employed with silylated substrates albeit with significantly lower yields; an identical
trend with respect to the size of the silyl moiety was observed (Table 1).
Table 1. Influence of the Catalyst and the Silyl Group on the 1H-Indene Synthesis
Ra)
Yield [%]
BF3 · Et2O
[AuCl(PPh3)]
H
< 1
6
51
67
< 1
5
56
92
TMS
TES
TIPS
a) Abbreviations: TMS ¼ trimethylsilyl, TES ¼ triethylsilyl, TIPS ¼ triisopropylsilyl.
On the contrary to the [AuCl(PPh3)]/AgSbF6 system, other gold catalysts appeared
to be of low preparative importance, with the notable exception of gold(III) chloride
which demonstrated efficient catalysis albeit with somewhat lower yields. The use of
the [AuCl(PPh3)]/AgBF4 combination furnished a similar outcome; however, the
catalytic system involving AgSbF6 appeared to be more stable. The use of gold(I)
choride resulted in virtually no turnover due to the rapid decomposition of the catalyst.
AuBr3 provided far inferior results compared to the corresponding chloride. A gold(I)
system with a more electron-rich phosphine ligand ([1,1’-biphenyl]-2-yl)di(tert-
butyl)phosphine) was shown to be catalytically inactive even with the most reactive
substrates. NHC-Derived systems (NHC ¼ N-heterocyclic carbene ligand) did not
render the reaction sufficiently clean.
As can be seen from Table 2, various types of products 2 can be obtained in
excellent yields from allyl alcohols 1 with the [AuCl(PPh3)]/AgSbF6 system. This
involves the simple benzaldehyde-derived 1H-indenes 2a – 2g as well as the products
obtained from naphthalenecarboxaldehydes (see 2h and 2i) and ketones (see 2j – 2n).
Substitution at position 1 of the 1H-indene product was possible by the use of
terminally alkylated substrate 1f. The naphthalene-2-carboxaldehyde-derived sub-
strate 1i demonstrated exclusive formation of angular product 2i. It is particularly
noteworthy that the isomeric couples 2h/2i and 2f/2j could be isolated without any
complications associated with the C¼C bond scrambling, which would take place under

Helvetica Chimica Acta – Vol. 95 (2012)
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Table 2. Substrate Screening for AuI-Catalyzed Cyclodehydrationa). TIPS ¼ iPr3Si.
Yield [%]b)
90
Yield [%]b)
92
1a
1b
1c
1d
1e
1f
1g
2a
2b
2c
2d
2e
2f
2g
92
96
72
92
73
96
97
92
70
91
99
80
1h
93
2h
99

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Helvetica Chimica Acta – Vol. 95 (2012)
Table 2 (cont.)
Yield [%]b)
Yield [%]b)
87
1i
99
2i
c
1j
1k
1l
75
97
77
)
2j
2k
2l
82d)
99
94
1m
1n
99
99
2m
2n
90d)
no reaction
–
1o
99
61
2o
–e)
–
1p
1q
2p
2q
–e)
–
72f)
35g)
1r
76f)
2r
–e)
–

Helvetica Chimica Acta – Vol. 95 (2012)
1777
Table 2 (cont.)
Yield [%]b)
Yield [%]b)
–
h
1s
99
)
2s
2t
–e)
1t
95
84
1u
1v
94
69
2u
2v
–e)
–
91
a)Reaction conditions: i) 1. BuLi, THF, ꢀ 788, 1 h; 2. 0.8 equiv of aldehyde or ketone, THF, ꢀ 788, 1 h. ii)
10 mol-% [AuCl(PPh3)], 10 mol-% AgSbF6, CH2Cl2, r.t., 10 min to 2 h. b) Yield of isolated 1 or 2 after
c
CC. ) Prepared via transmetallation of the organolithium compound with 2 LiCl · CeCl3 for 1 h at ꢀ 788,
followed by the addition at ꢀ 788 for 16 h. d) Carried out at 08. e) A complex mixture of products was
f
obtained. ) A bromide/aldehyde ratio of 1 : 1 was used. g) Reaction conditions: 10 mol-% [AuCl(P(o-
tol)3)], 10 mol-% AgSbF6, CH2Cl2, ꢀ 208, 9 h. h) A bromide/aldehyde ratio of 2.5 : 1 was used.
Brønsted acid catalysis. meta-Substituted benzaldehyde derivatives expectedly fur-
nished mixtures of isomers; meta-bromo substrate 1d demonstrated greater regiose-
lectivity (3.3 :1) compared to its meta-methyl counterpart 1c (1.7:1), which apparently
is due to the combination of steric and electronic factors. Interestingly, di-ortho-
alkylated substrate 1g could also undergo annulation resulting in rearranged product
2g; this can easily be explained by a 1,2-methyl shift in the intermediate cationic species
(Scheme 1) [4].
Scheme 1. Rationale for the Rearrangement on the Way to 2g and the Loss of the TIPS Group
Concomitant with the Formation of 2j [4]

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Alcohols derived from both alkyl aryl (see 1j) and aryl aryl ketones (see 1k – 1m)
could be employed with equal success. Indeed, a complete regioselectivity of
annulation was observed for the benzophenone-derived substrate 1l with a deactivated
aryl moiety. Besides 1H-indenes, a tetracyclic system 2m could be constructed from
dibenzosuberone (¼ 5H-dibenzo[a,d]cyclohepten-5-one). An analogous transforma-
tion could not be accomplished with 1n, which is a precursor to an antiaromatic cation;
the starting material was isolated back unaltered. Notably, for substrates 1j and 1m,
partial loss of the silyl moiety was detected when the reactions were run at room
temperature; this might be associated with longer lifetimes of the more stable
intermediates, which increases the chances of the silyl moiety to be attacked by some
nucleophilic species (Scheme 1). Fortunately, this issue could be easily overcome by
carrying out the catalysis at 08. The attempts to prepare 6- and 7-membered rings were
not successful: complex mixtures of products were obtained on exposure of substrates
1o and 1p to the gold(I) catalytic system. However, the construction of 7-membered
rings via the developed methodology was achieved on a different type of substrates –
vide infra.
We then turned our attention to the more sensitive heterocyclic substrates 1q – 1s.
The standard reaction conditions resulted in rapid decomposition of the thiophene
derivative 1q; however, conducting the catalysis at ꢀ 208 enabled isolation of dimeric
compound 2q as a result of aromatic substitution in the highly reactive bicyclic
dehydration product. No preparative outcome could be achieved for the furan
counterpart 1r. We decided to tune the reactivity of heterocyclic substrates by the
introduction of benzo fusion, which enabled the isolation of tricyclic product 2t from 1t
but was not helpful for the furan analogue 1u. The reactivity of the highly activated 1H-
indole derivative 1s could also be successfully tuned; the introduction of an N-tosyl
moiety resulted in clean cyclodehydration of 1v leading to compound 2v. Despite the
inapplicability of the developed methodology to a number of reactive heterocyclic
systems, the possibility to isolate the sensitive products 2t and 2v in high yields is quite
remarkable, as these compounds would not be compatible with conventional Lewis or
Brønsted acids previously utilized for cyclodehydration.
Catalyst loadings lower than 10 mol-% could also be used for efficient cyclo-
dehydration as exemplified for substrate 1h: the use of 5 mol-% of [AuCl(PPh3)]/
AgSbF6 still furnished the 1H-indene product 2h in excellent yield (99%). Further
decrease of the gold content was possible (1 mol-%); however, a lower yield was
achieved (82%), presumably due to catalyst decomposition. The use of 10 mol-% was
chosen for the substrate-screening experiments (Table 2) due to the generally higher
yields compared to lower catalyst loadings.
It is important to emphasize the sharp contrast between our methodology and the
previous approaches involving polysubstituted compounds. As the presence of a sole
silyl auxiliary group is sufficient for clean cyclodehydration, silyl 1H-indenes with
unsubstituted positions 1 and 3 could be easily prepared. Moreover, unlike alkyl or aryl
stabilizing groups, the triisopropylsilyl moiety is labile and could be replaced by other
groups. As shown in Scheme 2, the synthetic versatility of the TIPS group in 1H-indenes
2 can provide access to a number of different types of compounds. Halodesilylation [5]
enabled preparation of synthetically valuable 2-iodo- and 2-bromo-1H-indenes 3 and 4,
which in turn could be converted to the arylated 1H-indenes 5 via Suzuki coupling. The

Helvetica Chimica Acta – Vol. 95 (2012)
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Scheme 2. Synthetic Utility of Silyl 1H-Indenes 2
i) Epoxidation/Meinwald rearrangement: 1. m-chloroperbenzoic acid, CH2Cl2; 2. 0.1 mol-%
(CF3SO2)2NH. ii) N-Bromosuccinimide, (CF3)2CHOH, CH2Cl2. iii) Bu4NF, THF. iv) N-Iodosuccini-
mide, (CF3)2CHOH, CH2Cl2. v) Suzuki coupling: PhB (OH)2, [Pd(OAc)2], P(o-tol)3, Ba(OH)2.
epoxidation/Meinwald rearrangement sequence is noteworthy as a convenient route to
a-silyl-substituted indan-2-ones 6. Whereas the overwhelming majority of studies on
the Meinwald rearrangement involves the use of Lewis acids [6], we could carry out this
transformation without introduction of any metal species; it is particularly notable that
as low as 0.1 mol-% of triflimide ((CF3SO2)2NH) was sufficient for a rapid one-pot
isomerization. The possibility to run this process under so mild conditions apparently
stems from the electronic influence of the silyl moiety, as the developing positive charge

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Helvetica Chimica Acta – Vol. 95 (2012)
can be stabilized by hyperconjugation (b-Si effect) [7]. The silyl indanones 6 could also
be easily desilylated to furnish unsubstituted indan-2-ones 7. Thus, the presence of the
silyl auxiliary not only enabled the 1H-indene synthesis but also significantly increased
the synthetic value of the resulting 1H-indene products.
The general nature of the developed methodology is additionally emphasized by the
convenient accessibility of the starting materials. The common precursor of compounds
1 could be easily synthesized in three steps in 82% overall yield (Scheme 3, R ¼ H) [8].
This approach employed inexpensive chemicals and was operationally straightforward;
the intermediate reactions proceeded cleanly so that only one purification step was
required for the whole sequence. Substituted analogues could also be prepared likewise
by using various Grignard reagents in the second step [8b] (Scheme 3), which provided
ample functionalization opportunities at position 1 of the silyl 1H-indene products
(exemplified herein by the methylated compound 2f).
Scheme 3. Preparation of the Precursors for Silylated Allyl Alcohols 1. TIPS ¼ iPr3Si; LDA ¼ lithium
diisopropylamide.
Silylated allyl alcohols 1 were prepared via standard organolithium chemistry at
ꢀ 788; it is noteworthy that undesired elimination leading to (triisopropylsilyl)acety-
lene proceeded under these conditions, which could result in the formation of ca. 10%
of the corresponding propargylic alcohols. Fortunately, this problem was solved by
using an aldehyde/bromovinylsilane ratio of 1:1.25 as the undesired alkynyllithium
species were far less reactive at ꢀ 788 compared to the vinyl counterparts. Substrate 1j
derived from highly enolizable acetophenone was prepared via transmetallation of the
organolithium species with the 2 LiCl · CeCl3 complex [9].
Preparation of Benzofulvenes. – Along with the cyclodehydration of allyl alcohols
(vide supra) we considered an analogous transformation with allenyl carbinols leading
to benzofulvenes. Only few protocols involving direct construction of the benzofulvene
scaffold from acyclic/monocyclic compounds have been reported; similar to the 1H-
indene synthesis, all the reported methods required the presence of multiple stabilizing
aryl groups in the substrates [10]. To our delight, the alcohols 8 substituted by a sole
silyl group could be smoothly converted to the corresponding benzofulvenes 9
(Scheme 4), this finding represents the first example of the cyclodehydrative approach
to minimally functionalized benzofulvenes, which dramatically expands the scope of
accessible products compared to the existing methodologies. Interestingly, in contrast
to the 1H-indene case, benzofulvene synthesis was successfully accomplished even for
trimethylsilyl-substituted allenyl carbinols; however, lower yields were obtained. The
effect of the triisopropylsilyl group was also superior to other bulky silyl substituents
(Scheme 4).

Helvetica Chimica Acta – Vol. 95 (2012)
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Scheme 4. Screening of the Silyl Auxiliary in AuI-Mediated Benzofulvene Synthesis
A range of benzofulvenes were obtained with excellent yields regardless of the
electronic properties of the substituents at the aromatic ring (Table 3). Preparation of
the starting materials could be accomplished in a very straightforward manner from
aldehydes and methyl(triisopropylsilyl)acetylene via organotitanium species. An AuI-
based catalytic system appeared to be even more specific for allenyl carbinols: various
Brønsted and soft Lewis acids (including AuCl3) did not catalyze the reaction.
Table 3. Substrate Screening for AuI-Mediated Benzofulvene Synthesisa). TIPS ¼ iPr3Si.
R
Yield [%]b)
Yield [%]b)
8a
8b
8c
8d
8e
H
Me
Br
MeO
NO2
82
77
84
78
81
9a
9b
9c
9d
9e
94
95
90
90
89
a) Reaction conditions: i) 1. tBuLi, THF, ꢀ 788, 1 h; 2. Ti(OiPr)4, 08, 30 min; 3. aldehyde, ꢀ 788, 2 – 12 h.
ii) 5 mol-% [AuCl(PPh3)], 5 mol-% AgBF4, CH2Cl2, r.t., 30 min. b) Yield of isolated 8 or 9 after CC.
We were very interested in the synthetic potential of benzofulvene products 9,
which could potentially be used for the construction of a variety of diverse silylated 1H-
indenes 10 through the sequential interactions with appropriate nucleophile/electro-
phile combinations (Scheme 5) [11]. Various organolithium reagents could add to the
conjugated system of 9 furnishing carbanions which were then trapped by benzalde-
hyde, resulting in a rapid construction of product complexity. Thus, the highly
functionalized 1H-indenes 10 could be prepared via a highly regioselective one-pot
procedure in good yield, which further expanded the product scope available through
the cyclodehydration of allyl alcohols.
Thus, the combination of the gold(I)-based protocols reported herein enabled the
preparation of 1H-indenes with unprecedented structural freedom hitherto never
accessible via the cyclodehydration pathway. Positions 1, 2, and 3 of the five-membered
fragment as well as the aromatic part could be variously functionalized, yet largely
unsubstituted products were easily obtained from silylated allyl alcohols. The use of
allenyl carbinols allowed the convenient preparation of a diverse library of otherwise

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Helvetica Chimica Acta – Vol. 95 (2012)
Scheme 5. A Three-Step Synthetic Approach to Diverse Libraries of Compounds from Aldehydes.
TIPS ¼ iPr3Si; LDA ¼ lithium diisopropylamide
poorly accessible 1- and 1,3-substituted 2-silyl 1H-indenes through benzofulvene
intermediates. In combination with operational simplicity, robustness and accessibility
of the starting materials, the generality of the developed methodology makes it superior
to any of the previously designed cyclodehydration methods used for the 1H-indene
synthesis.
Preparation of Cyclopentadienes and Fulvenes. – To our delight, the stabilizing
effect of the silyl auxiliary appeared to be sufficient for the construction of non-benzo-
fused five-membered rings by the [AuCl(PPh3)]/AgSbF6-promoted cyclodehydration;
thus, sensitive cyclopentadiene and fulvene products could also be successfully isolated
(Table 4). The importance of the a-substituent R1 in the aldehyde precursors to the

Helvetica Chimica Acta – Vol. 95 (2012)
1783
Table 4. AuI-Mediated Cyclopentadiene and Fulvene Synthesisa). TIPS ¼ iPr3Si.
Yield [%]b)
Yield [%]b)
–
11a
11b
11c
98
89
94
12a
12b
12c
–
73c)
91c)
11d
11e
90
54
12d
12e
89d)
21c)
11f
11g
11h
11i
89
12f
12g
12h
12i
96d)
69d)
–
84
90e)
95
–f)
–f)
–

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Helvetica Chimica Acta – Vol. 95 (2012)
Table 4 (cont.)
Yield [%]b)
Yield [%]b)
87
11j
71
70
12j
11k
12k
83
a) Reaction conditions: i) 1. BuLi, THF, ꢀ 788, 1 h; 2. 0.8 equiv of aldehyde, THF, ꢀ 788, 1 h. ii) 10 mol-
% [AuCl(PPh3)], 10 mol-% AgSbF6, CH2Cl2, r.t., 30 min. iii) 1. tBuLi, THF, ꢀ 788 1 h; 2. Ti(OiPr)4, 08,
30 min; 3. aldehyde, ꢀ 788, 2 – 12 h. iv) 5 mol-% [AuCl(PPh3)], 5 mol-% AgBF4, CH2Cl2, r.t., 30 min.
c
b) Yield of isolated 11 or 12 after CC. ) > 90% of the shown isomer. d) A mixture of C¼C bond isomers
f
was obtained. e) The substrate with a diastereoisomer ratio of 1 : 0.3 was used. ) A complex mixture of
products was detected.
divinyl carbinols was realized: under various conditions of gold(I) catalysis, a (E)-
cinnamaldehyde derivative 11a furnished a complex mixture of products. However, an
a-methyl group in substrate 11b was already sufficient to render the reaction clean; a
bulkier alkyl group provided further increase of the yield (! product 12c). An aryl
substituent was necessary for efficient catalysis in either a- or b-position; inferior
performance was observed with dialkyl analogues 11g – 11i. Interestingly, whereas the
derivatives of a-alkyl-b-aryl-substituted aldehydes, i.e., 11b, 11c, and 11e, furnished
virtually single isomers of cyclopentadiene 12, the use of a-aryl-b-alkyl-substituted
counterparts, i.e., 11d and 11f, resulted in all possible arrangements of the C¼C bonds;
excellent overall yields of 12 were still obtained. The use of methylated substrate 11e
resulted in a sluggish reaction; the corresponding tetrasubstituted cyclopentadiene 12e
was isolated in poor yield. It is notable that the few existing reports on cyclopentadiene
synthesis from divinyl carbinols did involve the use of multiple aryl/alkyl stabilizing
groups [12]; preparation of trisubstituted cyclopentadienes was thus beyond the
product scope of cyclodehydration before the disclosure of our results.
Disubstituted fulvenes 12j and 12k were also prepared in good yields (Table 4); on
the contrary to cyclopentadienes, neither a-substitution nor the presence of an aryl
group in the parent aldehydes was crucial for the efficient reaction outcome. To the best
of our knowledge, this is the first example of a fulvene synthesis via cyclodehydration of
allenyl vinyl carbinols.
Construction of Seven-Membered Rings. – Our studies showed that the developed
methodology was not limited to the construction of five-membered rings. Terphenyl-
carboxaldehyde-derived allyl alcohols 13a – 13c with the ortho-positions blocked by
aryl groups underwent a different type of transformation: the intermediate cationic
species appeared to attack one of the ortho-substituents to furnish 5H-dibenzo[a,c]-

Helvetica Chimica Acta – Vol. 95 (2012)
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[7]annulene derivatives 14a – 14c with excellent yields (Scheme 6)5). The resulting
6,7,6-tricyclic system is particularly interesting in light of its presence in allocolchici-
noids, the class of compounds of great importance in cancer chemotherapy6).
Interestingly, an identical transformation could also be carried out with 13d, leading
Scheme 6. Construction of Seven-Membered Rings via AuI-Catalyzed Cyclodehydration. TIPS ¼ iPr3Si;
mCPBA ¼ 3-ClC6H4CO3H.
5
)
The 13C-NMR spectrum of 14c exhibited three peaks in the area of d(C) 110 – 120, which indicates
the broken symmetry of one of the aryl groups leading to three structural types of C-atoms adjacent
to the atoms bearing MeO groups. In the IR spectrum of 15a, the C¼O band was observed at
1681 cmꢀ1 which is in agreement with a 7-membered cyclic ketone structure (C ¼ O frequencies of
indanones 6 are located at 1720 – 1740 cmꢀ1). Along with NOESY studies (see formula below),
these features provide ample evidence that in the course of the reaction, an ortho-aryl group is
attacked leading to the formation of the 5H-dibenzo[a,c][7]annulene scaffold. The axial chirality of
1
compounds 14a – 14c results in the presence of an AB system of benzylic H-atoms in the H-NMR
spectra, as two d separated by 0.28 – 0.29 ppm with J ¼ 12.6 – 12.8 Hz.
6
)
The members of this family, e.g., allocolchicine, N-acetylcolchicinol methyl ether, N-acetylcolchi-
cinol, and its dihydrogen phosphate (ZD 6126) have received much attention in the context of
antitumor activity. For the most recent synthetic works and further details, see [13].

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Helvetica Chimica Acta – Vol. 95 (2012)
to product 14d with an exocyclic methylene group. Preparation of a dibenzosuberone
(¼ 5H-dibenzo[a,d]cyclohepten-6-one) scaffold, exemplified by derivative 15a, was
also achieved by means of the epoxidation/Meinwald rearrangement sequence
previously implemented in the indan-2-one synthesis (see above, Scheme 2).
Conclusions. – We discovered the beneficial effect of a triisopropylsilyl auxiliary
group on the efficiency of a gold(I)-catalyzed cyclodehydration of allyl and allenic
alcohols, which provides rapid access to a library of various compounds including 1H-
indenes, benzofulvenes, indan-2-ones, fulvenes, cyclopentadienes, 5H-dibenzo[a,-
c][7]annulenes, and dibenzosuberones. The protocol is mild, robust, and operationally
convenient; the starting materials can be easily prepared from inexpensive chemicals.
This approach provides unprecedented product generality for several product classes of
the cyclodehydration reaction, which is particularly notable for the 1H-indene
synthesis. The first example of the synthesis of non-benzo-fused fulvenes via cyclo-
dehydration is demonstrated. In light of the numerous advantages of the developed
methodology, we hope that extensive synthetic applications thereof can be expected in
future.
This work was made possible by the generous support of the National Science Foundation (CHE-
1049551). D. L. U. gratefully acknowledges the Martha Ann and Joseph A. Chenicek Graduate Research
Fund for a personal fellowship. We also thank Dr. Antoni Jurkiewicz for his assistance with NMR
experiments.
Experimental Part
General. IR Spectra: Nicolet 20-SXB FTIR spectrometer; thin films on NaCl plates; ˜n in cmꢀ1. 1H-
and 13C-NMR spectra: Bruker Avance-500 spectrometer; at 500 and 125 MHz, resp.; in CDCl3, at 298 K;
d in ppm rel. to Me4Si as internal standard, J in Hz. For additional experimental details, see [2].
AuI-Catalyzed Cyclodehydration: General Procedure. A dry test tube (Fisherbrand 16 ꢁ 100 mm)
sealed with a 14 mm sleeve stopper was charged with silver hexafluoroantimonate(V) (21 mg,
0.06 mmol) in the glovebox; it was taken out, quickly charged with [AuCl(PPh3)] (30 mg, 0.06 mmol),
evacuated, and filled with Ar. CH2Cl2 (4 ml) was added. The substrate (0.6 mmol) was added quickly in
CH2Cl2 (2 ml overall), and the mixture was left stirring for the indicated time (10 min to 2 h; TLC
AcOEt/hexanes 1:5) monitoring. The soln. was then passed through a short silica gel (SiO2) plug which
was thoroughly washed with CH2Cl2 (an alternative workup protocol involved quenching with sat. aq.
NaHCO3 soln. followed by extraction with hexanes). The soln. was concentrated and the residue purified
by column chromatography (CC) (SiO2, hexanes).
Allenic Alcohols 8, 11j, and 11k: General Procedure. To a soln. of triisopropyl(prop)-1-yn-1-yl)silane
(196 mg, 1.0 mmol) in THF, 1.7m tert-butyllithium in pentane (590 ml, 1.0 mmol) was added dropwise at 08
and stirred for 60 min. After 1 h, titanium(IV) tetrakis(isopropoxide) (296 ml, 284 mg, 1.0 mmol) was
added and the resulting mixture was stirred at 08 for 30 min. It was then cooled to ꢀ 788, and the
aldehyde (1.0 mmol) was added dropwise. The mixture was stirred at ꢀ 788 for 2 – 12 h. Upon completion
of the reaction (TLC monitoring), the crude mixture was quenched with H2O and extracted with Et2O,
the extract dried (Na2SO4) and concentrated, and the crude product purified by CC (SiO2, 0.35%
AcOEt/hexanes).
AuI-Mediated Fulvene/Benzofulvene Synthesis: General Procedure. A flame-dried test tube (16 ꢁ
100 mm) was charged with [AuCl(PPh3)] (12 mg, 0.025 mmol) and AgBF4 (5 mg, 0.025 mmol) in the
glovebox, and the resulting mixture was dissolved in CH2Cl2 (5 ml) and stirred under Ar. To that soln.,
the allenylsilane derivative was added dropwise, and the mixture was stirred at r.t. for 30 min, After
30 min, the crude mixture was filtered through a short alumina plug (basic alumina, Brockmann activity
I), the soln. concentrated, and the residue purified by CC (SiO2, hexanes/AcOEt).

Helvetica Chimica Acta – Vol. 95 (2012)
1787
a-{1-[Tris(1-methylethyl)silyl]propa-1,2-dien-1-yl}benzenemethanol (8a): IR: 3445, 2944, 2888, 2866,
1925, 1463, 1041, 699, 676. 1H-NMR: 7.40 (d, J ¼ 7, 2 H); 7.31 – 7.34 (t, J ¼ 7, 7.5, 2 H); 7.26 (s, 1 H); 5.16 (d,
J ¼ 6, 1 H); 4.65 (m, 2 H); 1.19 (m, 3 H); 1.08 (d, J ¼ 7.3, 18 H). 13C-NMR: 129.4; 128.2; 128.1; 127.7;
127.1; 127.0; 73.1; 73.0; 72.5; 72.4; 18.4.
4-Methyl-a-{1-[tris(1-methylethyl)silyl]propa-1,2-dien-1-yl}benzenemethanol (8b): IR: 3282, 3088,
2025, 1963, 1280, 1041, 867, 790. 1H-NMR: 7.28 (d, J ¼ 11, 2 H), 6.94 (d, J ¼ 11, 2 H); 5.12 (m, 1 H); 4.66
(m, 2 H); 2.87 (s, 3 H); 1.15 (m, 3 H); 1.09 (d, J ¼ 7, 9 H); 0.94 (d, J ¼ 7, 9 H ) . 13C-NMR: 132.7; 128.4;
113.5; 73.2; 71.8; 71.7; 55.1; 18.4; 11.6.
4-Bromo-a-{1-[tris(1-methylethyl)silyl]propa-1,2-dien-1-yl}benzenemethanol (8c): IR: 3372, 3040,
2866, 1958, 1684 1580, 1041, 814. 1H-NMR: 7.51 (d, J ¼ 10, 2 H); 7.26 (d, J ¼ 10, 2 H); 5.14 (m, 1 H); 4.62
(m, 2 H); 1.19 (m, 3 H); 1.09 (d, J ¼ 7.3, 9 H); 0.97 (d, J ¼ 7.3, 9 H). 13C-NMR: 131.2; 128.7; 73.3; 73.2;
71.8; 71.7; 18.5; 11.6.
4-Methoxy-a-{1-[tris(1-methylethyl)silyl]propa-1,2-dien-1-yl}benzenemethanol (8d): IR: 3582, 2786,
1
2431, 2277, 2094, 1968, 1276, 1021, 870. H-NMR: 7.29 (d, ¼ 11.5, 2 H); 6.86 (d, J ¼ 11.5, 2 H); 5.10 (m,
1 H); 4.66 (m, 2 H); 3.80 (s, 3 H); 1.14 (m, 3 H); 1.06 (d, J ¼ 7, 9 H), 0.94 (d, J ¼ 7, 9 H ) . 13C-NMR: 128.4;
113.5; 73.2; 71.8; 71.7; 55.1; 18.4; 11.6.
4-Nitro-a-{1-[tris(1-methylethyl)silyl]propa-1,2-dien-1-yl}benzenemethanol (8e): IR: 3679, 2515,
1
2054, 1880, 1527, 1351, 853. H-NMR: 7.59 (d, J ¼ 10, 2 H); 7.50 (d, J ¼ 10, 2 H); 5.24 (m, 1 H); 4.59
(m, 2 H); 1.19 (m, 3 H); 1.09 (d, J ¼ 5.0, 9 H); 0.97 (d, J ¼ 5.0, 9 H). 13C-NMR: 138.6; 124.2; 73.5; 73.2;
72.1; 71.7; 18.5; 11.6.
1-Methylene-2-[tris(1-methylethyl)silyl]-1H-indene (9a): IR: 3090, 3049, 1905, 1684. 1133. 1H-NMR:
7.61 (s, 1 H); 7.29 – 7.25 (m, 3 H); 7.21 – 7.19 (m, 1 H); 6.19 (s, 1 H); 5.82 (s, 1 H); 1.42 – 1.35 (m, 3 H);
1.15 – 1.09 (s, 18 H). 13C-NMR: 151.8; 127.8; 125.3; 120.2; 119.2; 115.2; 18.8; 11.8.
6-Methyl-1-methylene-2-[tris(1-methylethyl)silyl]-1H-indene (9b): IR: 3082, 1598, 1280, 1083.
1H-NMR: 7.44 (s, 1 H); 7.18 – 7.15 (m, 2 H); 6.15 (s, 1 H); 5.77 (s, 1 H); 2.40 (s, 3 H); 1.38 – 1.33 (m,
3 H); 1.11 – 1.09 (s, 18 H). 13C-NMR: 142.0; 138.3; 137.9; 136.8; 120.6; 124.7; 110.9; 50.2; 18.8; 11.9.
6-Bromo-1-methylene-2-[tris(1-methylethyl)silyl]-1H-indene (9c): IR: 3046, 1889, 1693, 1083.
1H-NMR: 7.40 (s, 1 H); 7.27 (m, 2 H); 6.19 (s, 1 H); 6.01 (s, 1 H); 1.37 – 1.33 (m, 3 H); 1.11 – 1.09 (s,
18 H). 13C-NMR: 151.8; 140.2; 139.8; 138.0; 136.7; 136.3; 129.5; 127.7; 17.8; 12.2.
6-Methoxy-1-methylene-2-[tris(1-methylethyl)silyl]-1H-indene (9d): IR: 3083, 3038, 1636, 1285, 1056.
1H-NMR: 7.25 – 7.12 (m, 3 H); 7.12 (s, 1 H); 6.80 (d, 1 H); 3.85 (s, 3 H); 1.38 – 1.32 (m, 3 H); 1.14 – 1.10 (d,
J ¼ 7.4, 18 H). 13C-NMR: 157.8; 139.4; 137.9; 134.6; 129.6; 114.7; 113.1; 55.6; 18.8; 11.9.
1-Methylene-6-nitro-2-[tris(1-methylethyl)silyl]-1H-indene (9e): IR: 3445, 2944, 2888, 2866, 1925,
1463, 1041, 699, 676. 1H-NMR: 7.38 (s, 1 H); 7.35 – 7.37 (m, 2 H); 6.20 (s, 1 H); 5.83 (s, 1 H); 1.37 – 1.34 (m,
3 H); 1.12 (s, 18 H). 13C-NMR: 153.0; 137.5; 137.4; 134.8; 117.6; 111.9; 111.1; 19.4, 10.7.
(1E)-1-Phenyl-4-[tris(1-methylethyl)silyl]hexa-1,4,4-trien-3-ol (11j): IR: 3287, 3094, 2866, 1985, 1663,
1
1052, 895. H-NMR: 7.39 – 7.24 (m, 5 H); 6.58 (d, J ¼ 16, 1 H); 6.29 (dd, J ¼ 8, 16, 1 H); 4.71 – 4.64 (m,
3 H); 1.24 (m, 3 H); 1.09 – 1.05 (m, 18 H). 13C-NMR: 136.7; 131.7; 130.2; 128.5; 127.6; 126.5; 72.9; 70.7;
70.6; 18.6; 11.6.
(5E)-3-[Tris(1-methylethyl)silyl]nona-1,2,5-trien-4-ol (11k): IR: 3445, 2944, 2888, 2866, 1925, 1463,
1041, 699, 676. 1H-NMR: 5.69 – 5.64 (m, 1 H); 5.57 – 5.55 (m, 1 H); 4.64 – 4.57 (dd, J ¼ 13, 24, 2 H); 4.48
(m, 1 H); 2.03 (dd, J ¼ 7, 21, 2 H); 1.41 (dd, J ¼ 8, 22, 2 H); 1.22 (m, 3 H); 1.09 – 1.06 (m, 18 H); 0.9 (m,
3 H). 13C-NMR: 132.2; 132.0; 72.6; 70.8; 70.7; 34.1; 22.1; 18.5; 13.7; 11.5.
5-Methylene-1-phenyl-4-[tris(1-methylethyl)silyl]cyclopenta-1,3-diene (12j): IR: 3083, 3019, 1754,
1671, 1177. 1H-NMR: 7.42 – 7.38 (m, 5 H); 6.34 (d, J ¼ 10.7, 1 H); 6.28 (s, 1 H); 5.94 (d, J ¼ 11, 1 H); 5.84 (s,
1 H); 1.40 – 1.36 (m, 3 H); 1.14 (s, 18 H). 13C-NMR: 144.8; 137.6; 137.2; 136.7; 135.4; 128.0; 127.7; 117.3;
21.4; 13.5.
5-Methylene-1-propyl-4-[tris(1-methylethyl)silyl]cyclopenta-1,3-diene (12k): IR: 3445, 2944, 2888,
2866, 1925, 1463, 1041, 699, 676. 1H-NMR: 6.27 (s, 1 H); 6.21 (d, J ¼ 12, 1 H); 5.87 (s, 1 H); 5.81 (d, J ¼
12.4, 1 H); 2.17 (d, J ¼ 8.2, 22, 2 H); 1.47 (d, J ¼ 8, 22, 2 H); 1.38 – 1.32 (m, 3 H); 1. 28 (m, 3 H); 1.11 (s,
18 H). 13C-NMR: 133.6; 132.9; 132.6; 132.5; 131.4; 131.3; 32.7; 24.9; 20.9; 15.3; 13.8.
a-{1-[Tris(1-methylethyl)silyl]propa-1,2-dien-1-yl}[1,1’:3’,1’’-terphenyl]-2’-methanol (13d): IR: 3576,
3551, 3440, 3078, 3056, 3026, 2942, 2864, 1926, 1812, 1599. 1H-NMR: 7.43 – 7.30 (m, 10 H); 7.28 (t, J ¼ 7.5,

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Helvetica Chimica Acta – Vol. 95 (2012)
1 H); 7.15 (d, J ¼ 7.5, 2 H); 5.38 – 5.33 (m, 1 H); 4.26 (dd, J ¼ 11.5, 3.7, 1 H); 4.18 (dd, J ¼ 11.5, 4.0, 1 H);
1.96 (d, J ¼ 11.4, 1 H); 1.02 (sept., J ¼ 7.5, 3 H); 0.78 (d, J ¼ 7.5, 9 H); 0.75 (d, J ¼ 7.5, 9 H). 13C-NMR:
210.3; 142.1; 141.4; 137.9; 132 – 129 (br.); 129.5; 128.1; 127.1; 125.9; 98.8; 72.4; 70.6; 18.3; 18.2; 11.5. TOF-
ES-MS (pos.): 477.2585 ([M þ Na]þ, C31H38NaOSiþ; calc. 477.2590).
5-Methylene-8-phenyl-6-[tris(1-methylethyl)silyl]-5H-dibenzo[a,c]cycloheptene (14d): IR: 3059,
3026, 2864, 2756, 2724, 1946, 1877, 1806, 1622. 1H-NMR: 7.67 – 7.61 (m, 2 H); 7.41 – 7.25 (m, 10 H); 6.58
(s, 1 H); 5.04 (s, 1 H); 4.97 (d, J ¼ 1.3, 1 H); 1.13 (sept., J ¼ 7.4, 3 H); 0.96 (d, J ¼ 7.4, 9 H); 0.90 (d, J ¼ 7.4,
9 H). 13C-NMR: 149.2; 146.5; 145.4; 141.8; 141.7; 139.6; 137.6; 136.9; 134.7; 129.8; 129.6; 129.0; 128.6;
128.0; 127.3; 127.1; 127.0; 126.9; 126.4; 112.4; 18.7; 18.7; 11.6. EI-MS (pos.): 436.25779 (Mþ, C31H36Siþ;
calc. 436.25863).
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Received April 5, 2012