Construction of extended π-conjugation systems utilizing novel multicarbene complexes of titanium

Article abstract of DOI:10.1055/s-2007-984512

Titanium-multicarbene complexes, namely organo-titanium species having a plurality of titanium-carbene complex substructures, were easily prepared by the reaction of aromatic nuclei possessing spatially separated thioacetal moieties with the titanocene(II

Full text of DOI:10.1055/s-2007-984512

LETTER
1715
Construction of Extended p-Conjugation Systems Utilizing Novel
Multicarbene Complexes of Titanium
C
onstructionof Ex
k
tended
p
-C
o
i
njugati
t
on
S
yst
o
ems shi Ogata, Shintaro Anno, Takeshi Kurata, Song Xu, Akira Tsubouchi, Takeshi Takeda*
Department of Applied Chemistry, Graduate School of Engineering, Tokyo University of Agriculture and Technology, Koganei,
Tokyo 184-8588, Japan
Fax +81(42)3887034; E-mail: takeda-t@cc.tuat.ac.jp
Received 10 April 2007
Dedicated to the memory of Professor Yoshihiko Ito
25 °C for one hour produced the bis- and tris(dimethyl-
phenylsilylmethyl)arenes 4a, 4b, and 4c in good yields,
suggesting the formation of the corresponding multicar-
bene complexes 2 (entries 2, 3, and 4, Table 1).
Abstract: Titanium–multicarbene complexes, namely organo-
titanium species having a plurality of titanium–carbene complex
substructures, were easily prepared by the reaction of aromatic
nuclei possessing spatially separated thioacetal moieties with the
titanocene(II) reagent Cp2Ti[P(OEt)3]2. Reaction of these multi-
carbene complexes with aromatic ketones gave various highly con-
jugated compounds in good yields.
In contrast, a similar treatment of 3a having two adjacent
thioacetal functions with dimethylphenylsilane gave no
carbenoid-insertion-type product and instead the bissul-
fide 5 was obtained in moderate yield (entry 1, Table 1).
Key words: carbene complexes, conjugation, olefination, thioace-
tals, titanium
The formation of 5 is attributable to preferential genera-
tion of the benzotitanacyclopentane 6 and its hydrolysis
during workup (Scheme 2). Indeed 6 was isolated as dark
green crystals in 42% yield by the reaction carried out
without aqueous workup.8
We have studied the preparation of various titanium–
carbene complexes by the desulfurizative titanation of
thioacetals with the low-valent titanium species
Cp2Ti[P(OEt)3]2 (1).1 These titanium–carbene complexes,
the typical Schrock-type metal carbenes, react with vari-
ous carbonyl compounds to give olefins.2 The major ad-
vantage of this olefination is that thioacetals are rather
stable under basic and acidic conditions, and hence a va-
riety of starting materials are readily available and can be
transformed into the corresponding carbene complexes.
Therefore, it is of utmost importance for the successful
generation of the multicarbene complexes 2 that thioace-
tal groups must be attached on a conformationally rigid
molecular platform and are separated from each other.
Synthetic utility of the multicarbene complexes 2 for the
construction of extended p-conjugation systems was de-
monstrated in the preparation of 8a–n by their reaction
with aromatic ketones 7a–d (Table 2).9 In addition to the
arene-cored carbene complexes, the multicarbene com-
plexes having a silicon and heterocycle core were readily
prepared and subjected to the reaction with benzophenone
derivatives to produce extended p-conjugation com-
pounds 8 in good to excellent yields.
Although preparations of the Fischer-type monomolecu-
lar multicarbene complex arrays of tungsten,3 rhodium,4
and chromium3a,5 have been reported, the Schrock-type
congener has not appeared yet. In this communication, we
describe the first generation of the Schrock-type multi-
carbene complexes 2 from multithioacetals 3 (Scheme 1)
and their reactions with aromatic ketones to construct ex-
tended p-conjugation systems, which are basic skeletons
of potential functional organic materials for electron
luminescent devices, optical data storages, photovoltaic
devices, and so on.6
Preparation of extended p-conjugation systems by multi-
ple formations of phenylenevinylene substructures in one
pot has been extensively investigated. Although various
methods such as the Wittig and Horner–Wadsworth–
Emmons reactions are employed for this purpose,10
ketones are generally poor substrates as compared with
aldehydes due to their steric hindrance.11 Efficiency com-
parison between the present and reported methods is
shown in Table 3. The reaction of the multicarbene com-
plex generated from 3c with benzophenone derivatives
affords the polyaromatics 8b and 8l in better yields than
the reaction using the bisphosphonate 9.11 Although ex-
tended p-conjugation systems can also be constructed by
the double Hiyama coupling of certain vinylsilanes with
diiodoarenes,12 the yields of coupling products largely
decrease when amino-substituted polyaromatics such as
8m and 8n are to be synthesized.
Insertion of transition-metal carbene complexes into M–H
bonds of group 14 metal hydrides R3MH is a well-estab-
lished process and employed as a prove for generation of
a variety of carbene complexes.7 We initially investigated
the reaction of organotitanium species generated by the
desulfurizative titanation of the multithioacetals 3 with
dimethylphenylsilane. Treatment of the multithioacetals
having a benzene or 1,3,5-triphenylbenezene core 3b, 3c,
and 3g with 1 in the presence of dimethylphenylsilane at
SYNLETT 2007, No. 11, pp 1715–1719
0
3
.0
7
.2
0
0
7
Advanced online publication: 25.06.2007
DOI: 10.1055/s-2007-984512; Art ID: U03207ST
© Georg Thieme Verlag Stuttgart · New York

1716
A. Ogata et al.
LETTER
2n Cp2Ti[P(OEt)3]2
SPh
SPh
1
TiCp2
n
– n Cp2Ti(SPh)2
n
3
2
SPh
SPh
SPh
SPh
SPh
SPh
SPh
SPh
SPh
PhS
PhS
PhS
PhS
SPh
3a
PhS
SPh
3b
3c
3d
SPh
PhS
SPh
PhS
H9C4
C4H9
PhS
PhS
SPh
SPh
PhS
PhS
SPh
SPh
3e
SPh
SPh
3f
PhS
PhS
O
PhS
PhS
SPh
SPh
PhS
PhS
3g
3h
SPh
SPh
S
Si
S
PhS
PhS
SPh
SPh
S
3j
3i
Scheme 1 Preparation of multicarbene complexes 2 from multithioacetals 3
Table 1 Reaction of Multithioacetals 3 with Dimethylphenylsilane in the Presence of Titanocene(II) 1
Entry
1
Thioacetal 3
1 (equiv)
Me2PhSiH (equiv) Product (yield, %)
3a
6
2.2
2.2
SPh
SPh
5 (41)
2
3b
6
SiPhMe2
Me2PhSi
4a (70)
3
4
3c
3g
6
9
2.2
3.3
SiPhMe2
Me2PhSi
4b (72)
SiPhMe2
Me2PhSi
SiPhMe2
4c (53)
Synlett 2007, No. 11, 1715–1719 © Thieme Stuttgart · New York

LETTER
Construction of Extended p-Conjugation Systems
1717
Table 2 Reaction of Multicarbene Complexes 2 with Benzophenone Derivatives 7a
Cp2
Ti
SPh
SPh
1) 2n 1
R
O
n
– n Cp2Ti=O
O
n
n
2)
n
3
R
R
R
8
R
R
7
Entry
3
7 (R)
1 (equiv)
Temp (°C)
Time (h)
Product 8
Yield (%)
1
2
3b
3c
3d
3f
7a (H)
8
6
25
1
8a
8b
8c
8d
8e
8f
64
66
90
49
64
80
56
64
80
58
57
61
62
72
7a (H)
25
1
3
7a (H)
8
reflux
3
4
7a (H)
12
12
8
25
1
5
3g
3i
7a (H)
reflux
3
6b
7b
8
7a (H)
0, then reflux
0, then reflux
reflux
0.3, 3
3j
7a (H)
8
0.3, 3
8g
8h
8i
3c
3d
3e
3h
3c
3b
3c
7b (BuO)
7b (BuO)
7b (BuO)
7b (BuO)
7c (Ph2N)
7d (Me2N)
7d (Me2N)
8
3
9
8
reflux
3
10
11b
12
13
14
8
reflux
3
8j
8
0, then reflux
reflux
0.3, 3
8k
8l
8
3
3
3
8
reflux
8m
8n
8
reflux
a All reactions were performed with a similar procedure as described in the text, unless otherwise noted.
b The thioacetal 3 and the ketone 7 were added to a THF solution of 1 in one portion.
8a
8b
8c
8d
8e
O
O
S
S
Si
O
S
O
O
O
8g
8f
8h
8i
O
O
N
O
O
N
N
N
O
O
8j
N
N
N
8m
O
O
N
O
N
N
N
O
O
N
8l
8n
8k
Synlett 2007, No. 11, 1715–1719 © Thieme Stuttgart · New York

1718
A. Ogata et al.
LETTER
SPh
TiCp2
SPh
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1 (2 equiv)
H2O
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5
3a
– Cp2Ti(SPh)2
6
Scheme 2 Formation of titanacycle 6 from bisthioacetal 3a
Table 3 Efficiency Comparison between the Present and Reported
Reactions
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Entry
Product Yield (%)
Present reaction Horner–Wadsworth–
Emmons reaction using 9
1
2
8b
8l
66a
42,b 61c
37,11a 2711b
1711c
a Carried out using 1 (6 equiv) and 7a (2 equiv) at 25 °C for 1 h.
b Carried out using 1 (6 equiv), 3c (1.1 equiv), and 7c (2 equiv) under
reflux for 3 h. The yield is based on the amount of 7c used.
c Carried out using 1 (8 equiv) and 7 (4 equiv) under reflux for 3 h.
O
O
P(OEt)2
(EtO)2P
9
In conclusion, we have established the facile method for
the preparation of titanium multicarbene complexes uti-
lizing multithioacetals as starting materials. The mandato-
ry requirement for the preparation of such unprecedented
organotitanium species is that the thioacetal functional-
ities in a molecule must be arranged so as to be separated
from each other, otherwise the titanacycle is formed. A
variety of extended p-conjugation systems were con-
structed in one pot by the use of these multicarbene com-
plexes.
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Acknowledgment
This work was supported by Grant-in-Aid for Scientific Research
(No. 18350018) and Grant-in-Aid for Scientific Research on Priori-
ty Areas ‘Advanced Molecular Transformations of Carbon Re-
sources’ (No. 18037017) from the Ministry of Education, Culture,
Sports, Science and Technology, Japan. This research was carried
out under the 21st Century COE program of ‘Future Nanomaterials’
at Tokyo University of Agriculture and Technology.
References and Notes
(8) (a) The titanacycle 6 was isolated by column
chromatography over alumina gel (hexane–EtOAc, 98:2)
under N2, mp 85–87 °C. 1H NMR (300 MHz, CDCl3): d =
4.88 (s, 2 H), 5.88 (s, 5 H), 6.44 (s, 5 H), 7.03–7.28 (m, 14
H). 13C NMR (75 MHz, CDCl3): d = 58.7, 114.6, 115.8,
123.0, 124.1, 125.0, 125.6, 128.7, 142.4, 146.1. IR (KBr):
n = 3057, 2914, 1579, 1476, 1439, 1085, 1022, 824, 739, 690
cm–1.
(1) (a) Takeda, T. Bull. Chem. Soc. Jpn. 2005, 78, 195.
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Takeda, T., Ed.; Wiley-VCH: Weinheim, 2004, 151.
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H. A.; Lappert, M. F.; Skelton, B. W. J. Chem. Soc., Dalton
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Russ. Chem. Bull. 2005, 54, 807.
Lappert and co-workers reported the preparation of 2-
titanaindane8b and meso-1,3-bis(trimethylsilyl)-2-
titanaindane8c complexes. The NMR signals of the Cp rings
Synlett 2007, No. 11, 1715–1719 © Thieme Stuttgart · New York

LETTER
of the latter titanacycle occur as two singlets at d = 4.46 and
Construction of Extended p-Conjugation Systems
1719
136.4, 141.6, 158.5, 158.8. IR (KBr): n = 2956, 2932, 2871,
1604, 1570, 1509, 1467, 1390, 1284, 1247, 1175, 1146,
1111, 1070, 1027, 1010, 973, 834, 813, 617 cm–1. Anal.
Calcd for C50H58O4: C, 83.06; H, 8.09. Found: C, 82.82; H,
8.12.
5.23 ppm. The spectrum of 6 shows the protons of two Cp
rings as two singlets at d = 5.88 and 6.44 ppm, suggesting
that 6 has the meso configuration. (b) Bristow, G. S.;
Lappert, M. F.; Martin, T. R.; Atwood, J. L.; Hunter, W. F.
J. Chem. Soc., Dalton Trans. 1984, 399. (c) Lappert, M. F.;
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Dalton Trans. 1984, 893.
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(9) General Procedure
Cp2TiCl2 (398 mg, 1.6 mmol), magnesium turnings (43 mg,
1.76 mmol), and finely powdered 4 Å MS (128 mg) were
placed in a flask and dried by heating with a heat gun in
vacuo (2–3 mmHg). After cooling, THF (2.4 mL) and
P(OEt)3 (0.55 mL, 3.2 mmol) were added successively with
stirring under argon. During the addition, the reaction
mixture was cooled in a water bath so that the temperature
was maintained between 20 °C and 30 °C. After stirring for
3 h at 25 °C, a THF (1.0 mL) solution of the thioacetal 3c
(108 mg, 0.2 mmol) was added. Then, a THF (4.0 mL)
solution of 7b (261 mg, 0.8 mmol) was added dropwise over
10 min and the reaction mixture was stirred for 3 h under
reflux. The reaction was quenched by addition of 1 M NaOH
and the insoluble materials were filtered off through Celite®
and washed with CHCl3. The layers were separated, and the
aqueous layer was extracted with CHCl3. After the combined
organic extracts were dried with Na2SO4 and concentrated,
the remaining triethyl phosphate, formed by the oxidation of
triethyl phosphite, was removed by azeotropic distillation
with MeOH. Purification of the residue by PTLC on silica
gel (hexane–CHCl3, 96:4) gave 8h as yellow crystals (102
mg, 64%), mp 130–132 °C. 1H NMR (300 MHz, CDCl3):
d = 0.96 (t, J = 7.4 Hz, 6 H), 0.99 (t, J = 7.4 Hz, 6 H), 1.41–
1.59 (m, 8 H), 1.67–1.85 (m, 8 H), 3.94 (t, J = 6.2 Hz, 4 H),
3.96 (t, J = 6.1 Hz, 4 H), 6.70 (s, 2 H), 6.75–6.89 (m, 12 H),
7.07 (d, J = 8.4 Hz, 4 H), 7.20 (d, J = 8.4 Hz, 4 H). 13C NMR
(75 MHz, CDCl3): d = 13.8, 13.9, 19.2, 19.3, 31.3, 31.4,
67.6, 67.7, 114.1, 114.4, 125.8, 128.8, 129.0, 131.6, 136.0,
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Lett. 2004, 6, 4093. (b) Itami, K.; Ohashi, Y.; Yoshida, J. J.
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Chem. Soc. Jpn. 2006, 79, 811. (d) Itami, K.; Yoshida, J.
Chem. Eur. J. 2006, 12, 3966.
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