Cycloaddition of Fischer aminocarbene complexes: An efficient annulation of fulvenes to indene derivatives

Article abstract of DOI:10.1016/j.jorganchem.2005.07.017

Alkenyl(amino)carbene chromium(0) complexes 1 undergo regioselective [6 + 3] cycloaddition to pentafulvenes 2, affording substituted dihydroindenes 5a-d and indene 6. In the case of 6,6-dimethylpentafulvene 2a dihydroindene chromium complexes 4a-c were al

Full text of DOI:10.1016/j.jorganchem.2005.07.017

Journal of Organometallic Chemistry 690 (2005) 5696–5700
www.elsevier.com/locate/jorganchem
[6 + 3] Cycloaddition of Fischer aminocarbene complexes:
An efficient annulation of fulvenes to indene derivatives
*
´
´
´
´
Jose Barluenga , Silvia Martınez, Angel L. Suarez-Sobrino, Miguel Tomas
Instituto Universitario de Qu´ımica Organometa´lica ‘‘Enrique Moles’’, Unidad Asociada al CSIC, Universidad de Oviedo,
Julia´n Claver´ıa 8, 33071-Oviedo, Spain
Received 2 June 2005; accepted 5 July 2005
Available online 15 August 2005
Abstract
Alkenyl(amino)carbene chromium(0) complexes 1 undergo regioselective [6 + 3] cycloaddition to pentafulvenes 2, affording
substituted dihydroindenes 5a–d and indene 6. In the case of 6,6-dimethylpentafulvene 2a dihydroindene chromium complexes
4a–c were also isolated. Isopropylfulvene 2b similarly produces 5d or, under oxidative reaction conditions, the indene 6. The reaction
involves: (i) 1,2-nucleophilic addition of fulvene to the carbene carbon, (ii) [1,2]-Cr(CO)5 migration, (iii) either metal–indene coor-
dination or reductive metal elimination/aromatization.
Ó 2005 Elsevier B.V. All rights reserved.
Keywords: [6 + 3] Annulation; Amino carbene complexes; Chromium; Pentafulvenes; Indenes
1. Introduction
Recently, we found that alkenyl(alkoxy)carbenes
react with pentafulvenes, a particular type of polar
system due to a unique C@C bond conjugation pattern,
at 80 °C in MeCN to produce efficiently indene deriva-
tives [4]. This process was regarded as the first [6 + 3]
cyclization of Fischer carbene complexes [5]. Interest-
ingly, chromium alkenyl(amino)carbene complexes were
found to be reactive towards activated fulvenes, for
instance 6-acetoxyfulvene, allowing a facile access to
substituted indenes (Fig. 1). This feature is of relevance
in the Fischer carbene complexes area as the metal–
carbon bond is much less reactive for aminocarbenes
than for alkoxycarbenes [6]. Thus, the major role of
the aminocarbene functionality lies generally on the acti-
vation of the Ca–H bond and of conjugated unsaturated
moieties, a fact that has served to achieve some funda-
mental asymmetric syntheses [7].
Since their discovery by Fischer and Maasbo¨l [1],
heteroatom stabilized carbene complexes have demon-
strated to be useful organometallic reagents for carbo-
and heterocyclization reactions [2]. The flexibility as well
as the varied reaction pathways that they show towards
different reagents and/or reaction conditions makes
them one of the most relevant organometallic species
in organic synthesis mediated by transition metal com-
plexes. In recent years, we have particularly focused on
the potential of alkenyl(alkoxy)carbenes which act as a
three-carbon synthon towards different substrates. In
this way, a number of two-component and multicompo-
nent cyclization reactions have been devised for synthe-
sis of molecules with either normal and high structural
complexity [3].
Herein, we report an easy access to indene and dihy-
droindene derivatives via [6 + 3] cyclization reaction
between pentacarbonyl[(alkenyl)(amino)carbene]chro-
mium(0) complexes 1 and 6-alkyl- and 6,6-dialkylpenta-
fulvenes 2 (Fig. 2).
*
Corresponding author. Tel./fax: +34 98 510 34 50.
E-mail address: barluenga@uniovi.es (J. Barluenga).
0022-328X/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2005.07.017

J. Barluenga et al. / Journal of Organometallic Chemistry 690 (2005) 5696–5700
5697
In the case of the reaction of 6-isopropylfulvene 2b
(R2 = i-Pr; R3 = H) with the carbene complex 1b
(R1 = 2-furyl), under the same reaction conditions, the
expected violet tricarbonyl(dihydroindene)chromium
complex was not formed, but the metal-free product
5d was instead isolated. Column chromatography purifi-
cation afforded pure 5d (65% yield) as an inseparable 5:1
mixture of diastereoisomers (Scheme 2). Moreover,
when the reaction crude containing 5d was taken up in
hexanes/EtOAc (5:1 v/v) and subjected to light-air treat-
ment the oxidative aromatization of the dihydroindene
system took place and the trisubstituted indene 6 (72%
yield) was formed as a 4:1 mixture of tautomers (Table
1, entries 7, 8) [9].
OAc
R1
R1
Cr(CO)5
R2N
NR2
Fig. 1. [6 + 3] Cyclization of Fischer alkenyl aminocarbenes and
6-acetoxyfulvene.
R2 R3
NR2
(CO)5Cr
R1
1
2
Compound
Compound
NR2
R1
R2
R3
A suitable mechanism that would account for this
[6 + 3] carbocyclization reaction is based on the well rec-
ognized capability of the M(CO)5 fragment of metallate
species to undergo 1,2-migration thus inducing unusual
cyclization processes (Scheme 3) [10]. The initial 1,2-
nucleophilic addition of fulvene to the electrophilic car-
bene carbon would lead to the zwitterionic intermediate
I. Then a [1,2]-shift of the chromium pentacarbonyl
fragment would take place and facilitate a subsequent
cyclization to the intermediate II, which might generate
the enamine intermediate III via consecutive metal elim-
ination and g2-olefin coordination. Finally, the latter
would undergo either isomerization to the more stable
complexes 4a–c or metal-decoordination and tautomer-
ization to indene derivative 5d. A relevant finding is that
the metal–carbon bond of amino carbene complexes is
able to undergo 1,2-addition, a fact that is well known
for alkoxy carbenes but rather rare for the much less
electrophilic amino carbene analogs. In fact, the penta-
fulvene system can be regarded as the first carbon nucle-
ophile reagent that adds to a Fischer aminocarbene
complex in 1,2-manner [11].
Further interest of the reaction herein described can
be envisaged as follows: (i) the unusual strategy of
constructing the indene skeleton in the sense that most
procedures are based by far on the annulation of the
cyclopentene unit into the benzo ring, while the present
work reports an easy benzannulation of the fulvene
system, thus permitting to incorporate functionality into
the aromatic domain [12], (ii) biological properties, e.g.,
as dopamine receptors ligands, have been reported for
some amino substituted 2,3-dihydroindenes [13], (iii)
since compounds 5 still maintain the reactive fulvene
NMe2
NMe2
Ph
1a
1b
1c
Me
Me
H
2a
2b
2-Fu
i-Pr
N(CH2)4 CO2Et
Fig. 2. Carbene complexes 1 and fulvenes 2 used.
2. Results and discussion
Thus, a solution of aminocarbene 1a (R1 = Ph) and
6,6-dimethylfulvene 2a in hexane was heated at 130 °C
in a sealed tube. After stirring for 5 h a violet precipitate
was formed. Flash chromatography purification allowed
the isolation of the tricarbonyl complex 4a in 86% yield
(Scheme 1; Table 1, entry 1), wherein the fulvene ligand
is best represented by means of its zwiterionic structure
[8]. The adduct 4a was formed as a sole diastereoisomer
whose stereochemistry – anti relationship between R1
and Cr(CO)3 – was tentatively assigned on the basis of
minimum steric repulsion between the R1 substituent
and the tricarbonylchromium fragment. Further demet-
allation of 4a could be readily achieved at room temper-
ature by stirring with silica gel under air to yield
quantitatively the dihydroaminoindene 5a (Table 1, entry
2). In the same way, carbene complex 1b (R1 = 2-furyl)
and 6,6-dimethylfulvene 2a afforded complex 4b (80%
yield) and the metal-free dihydroindene 5b (quantitative
yield from 4b) (Table 1, entries 3, 4). Very unusual amino-
carbene complexes with an electron-acceptor functional-
ity, like complex 1c (R1 = CO2Et), allowed to obtain the
functionalized dihydroindene derivatives 4c (90% yield)
and 5c (quantitative yield) under slightly milder reaction
conditions (Table 1, entries 5, 6).
R1
R1
R1
Hexane
SiO2
Cr(CO)5
100-130oC
CH2Cl2
R2N
Cr(CO)3
NR2
4a-c
Scheme 1. Cyclization reaction of aminocarbenes 1 with 6,6-dimethylfulvene 2a.
NR2
(80-90% yield)
(quantitat.)
1a-c
2a
5a-c

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J. Barluenga et al. / Journal of Organometallic Chemistry 690 (2005) 5696–5700
Table 1
Chromium pentacarbonyl complexes obtained by cyclization reaction of aminocarbenes 1 and fulvenes 2, and its subsequent demetallation
#
NR2
R1
R2
R3
Product
T (°C)
Yield (%)a
1
2
3
4
5
6
7
8
NMe2
Ph
Me
Me
4a
5a
4b
5b
4c
5c
5dc
6
130
20
130
20
100
20
130
130
86
100b
80
NMe2
2-Fu
Me
Me
i-Pr
Me
Me
H
100b
90
N(CH2)4
NMe2
CO2Et
2-Fu
100b
65
72d
a
Isolated yields after flash chromatography.
Yields from compounds 4.
Obtained as a 5:1 mixture of diastereomers.
From crude 5d by air-light oxidation.
b
c
d
2-Fu
2-Fu
i) Hexane, 130oC
Hexane
1b + 2b
130oC
ii) light, air, rt
hexanes, EtOAc
NMe2
NMe2
(65% yield)
(72% yield)
5d
6
Scheme 2. [6 + 3] Cyclization of aminocarbene 1c and 6-isopropylfulvene 2b.
R3
R2 R3
R1
R2
R1
R1
[1,2]-M
NR2
[M]
[M]
M=Cr(CO)5
[M] NR2
NR2
I
II
R2 R3
R1
4a-c
5d
-[M]
[M]
NR2
III
Scheme 3. Proposed mechanism for the [6 + 3] cyclization of carbenes 1 and fulvenes 2.
unit they seem to be excellent candidates to produce
extended polycyclic systems, a hypothesis that was dem-
onstrated feasible in the case of alkenyl(alkoxy)carbenes
[4].
aminocarbenes require a higher reaction temperature
than the corresponding alkenyl alkoxycarbenes to be
reactive, the excellent yields and the interest of the
structure of the new products contribute to expand
the scope of the reactivity of Fischer carbene complexes
with unsaturated system and their use in cyclization
processes.
3. Conclusion
In summary, we have described herein the first cycli-
zation of alkenylaminocarbene complexes following the
1,2-addition/[1,2]-M(CO)5 shift mechanistic model. This
process takes place with pentafulvenes to give amino
indenes in a formal [6 + 3] cyclization. Although
4. Experimental
All reactions involving air sensitive compounds
were carried out under a N2 atmosphere (99.99%). All

J. Barluenga et al. / Journal of Organometallic Chemistry 690 (2005) 5696–5700
5699
1
glassware was oven-dried (120 °C), evacuated and
purged with nitrogen. All common reagents and sol-
vents were obtained from commercial suppliers and
used without any further purification unless otherwise
indicated. Fischer carbene complexes [1,6b,14] and ful-
venes [15] were prepared following described proce-
dures. Solvents were dried by standard methods and
distilled prior to use. Flash column chromatography
was carried out on silica gel 60, 230–240 mesh. NMR
experiments were recorded on Bruker AC-200, AC-300
or DPX-300 spectrometers. 1H NMR spectra were
recorded in CDCl3 at 300.08 MHz at 20 °C with tetra-
methylsilane (d = 0.0) as the internal standard. 13C
NMR spectra were recorded in CDCl3 at 75.46 MHz
Tricarbonylchromium complex (4b): yield = 80%; H
NMR (CD2Cl2): d = 1.0 (s, 3H); 1.1 (s, 3H), 2.9 (m,
2H); 3.3 (s, 3H); 3.4 (s, 3H); 3.6 (m, 1H); 4.9 (m, 1H);
5.2 (m, 2H); 6.2 (m, 1H); 6.4 (m, 1H); 7.4 (m, 1H);
13C NMR (CD2Cl2): d = 25.8 (q); 26.9 (q); 33.5 (t);
34.3 (s); 44.0 (q); 44.5 (q); 46.3 (d); 79.9 (s); 84.5 (d);
85.0 (d); 89.9 (d); 109.11 (d); 110.8 (d); 122.1 (s); 142.4
(d); 154.3 (s); 173.6 (s); 240.6 (s, 3C); HRMS m/z calcd.
for C20H21CrNO4 391.0876. Found: 391.0872. Anal.
Calc. for C20H21CrNO4: C, 61.38; H, 5.41; N, 3.58.
Found: C, 61.25; H, 5.47; N, 3.55%.
1
Tricarbonylchromium complex (4c): yield = 90%; H
NMR (CD2Cl2): d = 1.1 (t, J = 7.0 Hz, 3H); 1.3 (m,
6H); 1.9–2.2 (m, 4H); 2.8–3.0 (m, 2H); 3.2 (m, 1H);
3.6–3.8 (m, 4H); 4.1–4.2 (m, 2H); 4.9 (m, 1H); 5.1 (m,
2H); 13C NMR (CD2Cl2): d = 14.9 (c, 2C); 25.1 (t);
26.5 (q); 27.5 (q); 32.5 (t); 33.6 (s); 49.8 (d); 53.4 (t);
54.2 (t); 61.8 (t); 79.5 (s); 84.2 (d); 84.6 (d); 89.6 (d);
122.2 (s); 169.4 (s); 172.7 (s); 240.7 (s, 3C); HRMS m/z
calcd. for C21H25CrNO5 423.1133. Found: 423.1135.
Anal. Calc. for C21H25CrNO5: C, 59.57; H, 5.95; N,
3.31. Found: C, 59.48; H, 6.00; N, 3.27%.
1
at 20 °C. H NMR splitting pattern abbreviations are:
s, singlet; d, doublet; t, triplet; q, quartet; sp, septuplet;
m, multiplet. 13C NMR multiplicities were determined
by DEPT, abbreviations are: q, CH3; t, CH2; d, CH;
s, quaternary carbons. NOESY experiments were car-
ried out on a Bruker AMX-400 spectrometer. Standard
pulse sequences were employed for the DEPT experi-
ments. High resolution mass spectra (HRMS) were
obtained with a Finnigan Mat95 Mass Spectrometer
and electron impact techni-ques (70 eV) were employed.
Elemental analyses were carried out with a Perkin–
Elmer 240 B microanalyzer.
6,7-dihydro-7,7-dimethyl-4-dimethylamino-6-phenyl-
1
5H-indene (5a): H NMR: d = 1.1 (s, 3H); 1.2 (s, 3H);
2.7 (dd, J = 15.6 and 3.3 Hz, 1H); 3.0–3.3 (m, 2H);
3.35 (s, 6H); 6.1 (dd, J = 2.6 and 1.8 Hz, 1H); 6.4 (dd,
J = 4.9 and 2.6 Hz, 1H); 6.5 (dd, J = 4.9 and 1.8 Hz,
1H); 7.2–7.4 (m, 5H). 13C NMR: d = 24.7 (q); 27.6 (c);
33.8 (t); 35.2 (s); 43.0 (q, 2C); 53.0 (d); 112.4 (d); 113.3
(s); 114.1 (d); 122.2 (d); 126.6 (d); 127.8 (d, 2C); 129.3
(d, 2C); 141.5 (s); 144.1 (s); 156.9 (s). HRMS m/z calcd.
for C19H23N 265.1830. Found: 265.1835. Anal. Calc. for
C19H23N: C, 85.99; H, 8.74; N, 5.28. Found: C, 85.80;
H, 8.81; N, 5.19%.
6-(2-furyl)-6,7-dihydro-7,7-dimethyl-4-dimethyl-
amino-5H-indene (5b): 1H NMR: d = 1.1 (s, 3H); 1.3 (s,
3H); 2.8 (dd, J = 17.1 and 4.3 Hz, 1H); 3.0–3.1 (dd,
J = 17.1 and 11.7 Hz, 1H); 3.25 (m, 1H); 3.3 (s, 6H);
6.1 (m, 2H), 6.3–7.5 (m, 3H); 7.4 (m, 1H). 13C NMR:
d = 25.3 (q); 27.4 (q); 32.3 (t); 35.2 (s); 43.0 (q, 2C);
46.4 (d); 106.8 (d); 109.9 (d); 112.5 (d); 113.2 (s); 114.2
(d); 122.1 (d); 140.8 (d); 143.5 (s); 156.0 (s); 156.1 (s).
HRMS m/z calcd. for C17H21NO 255.1623. Found:
255.1619. Anal. Calc. for C17H21NO: C, 79.96; H,
8.29; N, 5.49. Found: C, 80.02; H, 8.31; N, 5.44%.
Ethyl 5,6-dihydro-4,4-dimethyl-7-(1-pyrrolidinyl)-4H-
indene-5-carboxylate (5c): 1H NMR: d = 1.2 (s, 3H);
1.3 (t, J = 7.0 Hz, 3H); 1.4 (s, 3H); 2.0 (m, 4H); 2.7
(dd, J = 17.0 and 3.9 Hz, 1H); 3.0 (dd, J = 12.0 and
3.9 Hz, 1H); 3.1 (dd, J = 17.0 and 12.0 Hz, 1H); 3.7
(m, 4H); 4.2 (m, 2H); 6.1 (m, 1H); 6.4 (m, 2H). 13C
NMR: d = 14.3 (q); 25.7 (q); 27.7 (t); 30.9 (t); 34.4 (s);
50.0 (t); 52.0 (d); 60.3 (t); 112.0 (d); 112.6 (s); 113.4
(d); 121.5 (d); 141.7 (s); 152.9 (s); 173.4 (s); HRMS
m/z calcd. for C18H25NO2 287.1885. Found: 287.1879.
4.1. General procedure for the reaction between 1a–c and
2a
A mixture of carbene complex 1a–c (1 mmol) and 6,6-
dimethylfulvene 2a (1.5 mmol) in hexane (15 mL) was
heated at 100–130 °C (as indicated in Table 1) in a
sealed tube. After stirring for 5 h a violet solid precipi-
tated from the solution. The solvent was removed under
vacuum and the resulting chromium complex was puri-
fied by column chromatography (CH2Cl2/hexanes, 3:1)
affording compounds 4a–c as violet solids.
The chromium tricarbonyl complexes 4 were subse-
quently dissolved in CH2Cl2 (10 mL) and stirred with
silica gel (0.5 g) for 4 h at r.t. After filtration, the solvent
was removed under vacuum and the crude mixture was
purified by flash chromatography (hexanes/EtOAc 1:1),
yielding dihydroindenes 5a–c quantitatively.
1
Tricarbonylchromium complex (4a): yield = 86%; H
NMR (CD2Cl2): d = 1.0 (s, 3H); 1.1 (s, 3H), 2.9 (m,
2H); 3.3 (s, 3H); 3.5 (s, 3H); 3.6 (m, 1H); 5.0 (m, 1H);
5.2 (m, 2H); 7.1–7.4 (m, 5H); 13C NMR (CD2Cl2):
d = 26.1 (q); 26.4 (q); 34.4 (s); 35.2 (t); 44.8 (q); 46.6
(q); 50.2 (d); 80.2 (s); 84.5 (d); 84.9 (d); 90.0 (d); 124.6
(s); 128.0 (d); 128.8 (d, 2C); 130.5 (d, 2C); 140.1 (s);
175.2 (s); 240.8 (s, 3C); HRMS m/z calcd. for C22H23-
CrNO3 401.1081. Found: 401.1083. Anal. Calc. for
C22H23CrNO3: C, 65.83; H, 5.78; N, 3.49. Found: C,
65.78; H, 5.69; N, 3.40%.

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J. Barluenga et al. / Journal of Organometallic Chemistry 690 (2005) 5696–5700
Anal. Calc. for C18H25NO2: C, 75.22; H, 8.77; N, 4.87.
Found: C, 75.31; H, 8.69; N, 4.90%.
References
[1] E.O. Fischer, A. Maasbo¨l, Angew. Chem. 16 (1964) 645.
[2] Recent reviews: (a) W.D. Wulff, in: E.W. Abel, F.G.A. Stone, G.
Wilkinson (Eds.), Comprehensive Organometallic Chemistry II,
vol. 12, Pergamon, New York, 1995, p. 469;
4.2. Synthesis of dihydroindene 5d and indene 6
A solution of aminocarbene 1c (1 mmol) and 6-iso-
propylfulvene 2b (1.5 mmol) in hexane (15 mL) was
heated at 130 °C in a sealed tube for 5 h. Then the sol-
vent was removed in vacuo and the crude mixture was
purified by flash chromatography (hexanes/EtOAc,
5:1) furnishing the dihydroindene 5d in 65% yield.
Alternatively, the crude was dissolved in a mixture of
hexane/EtOAc (1:1) and air oxidized in an open flask
under sunlight or lamp light (6–8 h). The solution was
then filtered off over Celite and the filtrate concentrated.
The resulting crude was purified by flash chromatogra-
phy (hexanes/EtOAc, 1:1) and the indene 6 isolated in
72% yield.
´
(b) J. Barluenga, F.J. Fananas, Tetrahedron 56 (2000) 4597;
˜
(c) M.A. Sierra, Chem. Rev. 100 (2000) 3591;
(d) A. de Meijere, H. Schirmer, M. Duetsch, Angew. Chem., Int.
Ed. Engl. 39 (2000) 3964;
´
´
(e) J. Barluenga, J. Santamarıa, M. Tomas, Chem. Rev. 104
(2004) 2259;
(f) K.H. Do¨tz (Ed.), Topics Organomet. Chem., vol. 13, Springer-
Verlag, Berlin, Heidelberg, 2004;
(g) J.W. Herndon, Coord. Chem. Rev. 249 (2005) 999.
´
´
[3] (a) Recent review: J. Barluenga, M.A. Fernandez-Rodrıguez,
E. Aguilar, J. Organomet. Chem. 690 (2005) 539;
(b) J. Barluenga, M.A. Fernandez-Rodrıguez, F. Andina,
E. Aguilar, J. Am. Chem. Soc. 124 (2002) 10978.
´
´
´
´
´
[4] J. Barluenga, S. Martınez, A.L. Suarez-Sobrino, M. Tomas,
J. Am. Chem. Soc. 123 (2001) 11113.
[5] The [6 + 3] cycloaddition of fulvene ketene acetals with the oxallyl
cation is the only known example, see: B.-C. Hong, S.-S. Sun,
Y.-C. Tsai, J. Org. Chem. 62 (1997) 7717.
6-(2-furyl)-6,7-dihydro-7-isopropyl-4-dimethylamino-
1
5H-indene (5d): yield = 65%; major isomer: H NMR:
d = 0.78 (d, J = 6.7 Hz, 6H); 0.80 (s, 3H); 1.6–1.8 (m,
1H); 2.8–3.0 (m, 2H); 3.3 (s, 3H); 3.6 (m, 1H); 5.9 (m,
1H); 6.1 (m, 1H); 6.30 (m, 1H); 6.4–6.6 (m, 2H); 7.4
(m, 1H). 13C NMR: d = 21.2 (q); 22.4 (q); 28.7 (d);
32.8 (t); 39.3 (d); 43.1 (q, 2C); 44.7 (d); 105.3 (d);
110.2 (d); 114.3 (d); 115.7 (s); 116.4 (d); 121.7 (d);
135.0 (s); 140.6 (d); 155.8 (s); 157.3 (s). HRMS m/z
calcd. for C18H23NO 269.1780. Found: 269.1781. Anal.
Calc. for C18H23NO: C, 80.26; H, 8.61; N, 5.20. Found:
C, 80.33; H, 8.54; N, 5.99%.
[6] (a) For cascade reactions initiated by alkyne insertion into de
M@C bond of aminocarbenes, see: H. Rudler, A. Parlier, S.
Benzennine-Lafollee, J. Vaissermann, Eur. J. Org. Chem. (1999)
2825, and references therein;
(b) For the benzannulation reaction of alkenyl(amino)carbene
´
´
complexes, see: J. Barluenga, L.A. Lopez, S. Martınez, M.
´
Tomas, J. Org. Chem. 63 (1998) 7588, and references cited
therein;
(c) For the reactivity of iminocarbene complexes, see: P.J.
Campos, D. Sampedro, M.A. Rodr´ıguez, J. Org. Chem. 68
(2003) 4674, and references cited therein.
[7] W.D. Wulff, Organometallics 17 (1998) 3116.
[8] B. Lubke, U. Behrens, J. Organomet. Chem. 149 (1978) 327.
[9] The structure of the chromium complexes 4 as well as products 5
and 6 was confirmed by NMR, elemental analysis and high
resolution mass spectroscopy.
7-Dimethylamino-4-isopropyl-5-(2-furyl)-1H-indene-
1
(6): yield = 72%; Mayor tautomer: H NMR: d = 1.3
(s, J = 7.1 Hz, 6H); 2.8 (s, 6H); 3.38 (sp, J = 7.14 Hz,
1H); 3.44 (m, 2H); 6.3 (m, 1H); 6.5 (m, 1H); 6.6 (m,
1H); 6.9 (s, 1H); 7.2 (m, 1H); 7.5 (m, 1H). 13C NMR:
d = 23.0 (q, 2C); 30.0 (d); 38.4 (t); 43.4 (q, 2C); 107.9
(d); 110.9 (d); 116.0 (d); 129.5 (s); 131.7 (d); 133.3 (d);
134.0 (s); 136.3 (s); 141.6 (d); 144.3 (s); 146.8 (s); 155.3
(s); HRMS m/z calcd. for C18H21NO 267.1623. Found:
267.1628. Anal. Calc. for C18H21NO: C, 80.86; H, 7.92;
N, 5.24. Found: C, 80.95; H, 7.98; N, 5.20%.
´
´
[10] J. Barluenga, M. Tomas, A. Ballesteros, J. Santamarıa, R.J.
´
´
Carbajo, F. Lopez-Ortiz, S. Garcıa-Granda, P. Pertierra, Chem.
Eur. J. 2 (1996) 88.
[11] For the 1,2-addition of hydride to aminocarbene complexes, see:
´
´
M. Gomez-Gallego, M.J. Mancheno, P. Ramırez, C. Pinar,
˜
˜
M.A. Sierra, Tetrahedron 56 (2000) 4893.
[12] (a) The synthesis of the indene skeleton by cyclization on the
cyclopentene unit is little known, and yields are very low: C.
Wang, H. Kohn, Org. Lett. 2 (2000) 1773;
(b) Other strategies imply the preparation of complex starting
materials: J.W. Grissom, D. Huang, Angew. Chem., Int. Ed. 34
(1995) 2037.
Acknowledgment
[13] F. Claudi, G.M. Cingolani, A.D. Stefano, G. Giorgioni, F.
Amenta, P. Barilli, F. Ferrari, D. Giulliani, J. Med. Chem. 39
(1996) 4238.
[14] U. Klabunde, E.O. Fischer, J. Am. Chem. Soc. 89 (1967) 7141.
[15] K.J. Stone, R.D. Little, J. Org. Chem. 49 (1984) 1849.
This research was partially supported by the Spain
and Principado de Asturias Governments (BQU-2001-
3853 and GE-EXP01-11).