Stereoselective formation of (Z)-γ-substituted allylsilanes by the titanocene(II)-promoted reaction of thioacetals with trialkyl(allyl)silanes

Article abstract of DOI:10.1039/a707177i

The reaction of organotitanium species prepared by the desulfurizative titanation of thioacetals with (C₅H₅)₂Ti-[P(OEt)₃]₂ in the presence of trialkyl(allyl)silanes gives γ-substituted allylsilanes with Z stereoselectivity.

Full text of DOI:10.1039/a707177i

View Article Online / Journal Homepage / Table of Contents for this issue
Stereoselective formation of (Z)-g-substituted allylsilanes by the
titanocene(ii)-promoted reaction of thioacetals with trialkyl(allyl)silanes
Tooru Fujiwara, Mayumi Takamori and Takeshi Takeda*
Department of Applied Chemistry, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184, Japan
The reaction of organotitanium species prepared by the
desulfurizative titanation of thioacetals with (C Ti-
P(OEt) in the presence of trialkyl(allyl)silanes gives
g-substituted allylsilanes with Z stereoselectivity.
In a similar manner, the reactions of several thioacetals 2 with
H
5 5
)
2
trialkyl(allyl)silanes 3 were performed, and the mixtures of the
corresponding substituted allylsilanes 4 and the homoallyl-
silanes 5 were produced in a ratio of 7.1 :1 to 1.9:1 depending
on the alkyl substituents on silicon atom of 3 and the structure
of thioacetal 2. For example, the reaction using
allyltriisopropylsilane gave substituted allylsilanes preferen-
tially. On the other hand, in the case of the reaction using
allyltrimethylsilane or allyldimethyl(phenyl)silane, a substan-
tial amount of 5 was produced. What is striking is that
[
3 2
]
Since allylsilanes are useful synthetic intermediates in organic
synthesis, a number of procedures for their preparation have
been reported. Among them, the methods utilizing an olefina-
tion process are attractive because they enable us to prepare
regioisomeric pure allylsilanes. Two types of olefination have
been investigated; (i) Wittig olefination of carbonyl compounds
1
using Ph
3
PNCHCH
2
SiMe
3
, and (ii) metal alkylidene catalysed
Table 1 Reaction of thioacetals 2 with trialkyl(allyl)silanes 3
Products (% yield; E:Z)
a
olefin cross metathesis using trialkyl(allyl)silane. Crowe et al.
showed that the cross metathesis of allyltrimethylsilane with
terminal olefins using Schrock’s molybdenum catalyst
i
Entry
2
3
4
5b
PhMe
2
CCHNMoNN(2,6-Pr C
6
H
3
)[OCMe(CF
3
)
2
]
2
gave (E)-g-
2
substituted allylsilanes predominantly. On the other hand, the
stereochemistry of Grubb’s ruthenium {Cl [c-C
P]
1
2
3
4
5
6
7
8
9
2a
2a
2b
2b
2b
2c
2c
2d
2d
3a
3b
3a
3b
3c
3b
3c
3a
3b
4a (64; 13:87)
4b (64; 12:88)
4c (55; 16:84)
4d (85; 23:77)
4e (45; 14:86)
4f (78; 11:89)
4g (48; 10:90)
4h (44; 14:86)
4i (44; 14:86)
5a (13)
5b (9)
2
6 11 3
H ) -
2
RuNCHPh} catalysed ring opening metathesis of strained
5c (29)
5d (12)
5e (23)
5f (16)
5g (20)
5h (20)
5i (18)
cyclic olefins with allyltrimethylsilane is reported to be
unpredictable.3
Recently we reported the titanocene(ii)-promoted reactions
of thioacetals with organic compounds having a multiple bond
4
5
6
such as carbonyls, alkenes and alkynes. It was suggested that
these reactions proceeded through the carbene complexes of
titanium 1 formed by the desulfurization of thioacetals with low
valent titanium species. Since the intermediate of olefin
metathesis is assumed to be a metal alkylidene species, we
expected that substituted allylsilanes would be formed by the
titanocene(ii)-promoted reaction of thioacetals 2 with allyl-
silanes 3 through the metathesis process.
a
b
Carried out with 5 equiv. of 3.
determined.
The stereochemistry was not
6
2
1
3
R1
Ti(C5H5)2
(C5H5)2Ti
R1
2
3
1
SiR 2R
+
5 5 2
CH2 Ti(C H )
4
8
H
As would be expected, 1-triisopropylsilyl-5-phenylpent-
7
2
-ene 4b was produced in 50% yield along with a small amount
of the homoallylsilane 5b (7%) when the thioacetal 2a was
treated with the titanocene(ii) species (C Ti[P(OEt) 6 in
the presence of a two-fold excess allyltriisopropylsilane 3b at
room temperature for 48 h (Scheme 1). The starting material
disappeared within a short period of time (3 h), and 4b was
obtained in a comparable yield when the reaction was carried
out in refluxing THF. The yield of 4b was increased by
increasing the molar ratio of 3b (Table 1, entry 2).†
5 5
H )
2
3
]
2
R1
2
3
SiR 2R
5
–Ti(C5H5)2
Ti(C5H5)2
H
9
Scheme 2
2
3
PhS
3 2
(C H ) Ti[P(OEt) ] 6
SiR 2R
5
5 2
2
3
SiR 2R
+
+
2
3
R1
R1
SiR 2R
SiR 2R3
2
R1
SPh
R1
(Z)-4
1
3a R = R3 = Me
2
2
a R = Bn
(E)-4
5
1
2
3
i
b R = C6H13
b R = R = Pr
1
i
c R = Me, R3 = Ph
2
c R = Pr
1
d R = PhO(CH2)3
Scheme 1
Chem. Commun., 1998
51

View Article Online
1
+
3
Footnotes and References
*
†
E-mail: takeda-t@cc.tuat.ac.jp
Typical experimental procedure: To a flask charged with finely powdered
2
3
R 2R Si
H
2
3
molecular sieves 4A (150 mg), magnesium turnings (3 mmol) and
titanocene dichloride (1.5 mmol) was added THF (3 ml), triethyl phosphite
(3 mmol) and trialkyl(allyl)silane (2.5 mmol) successively with stirring.
After 2 h, a THF (1 ml) solution of 2 (0.5 mmol) was added and the reaction
mixture was refluxed for 6 h. After being cooled to room temperature, the
reaction mixture was diluted with pentane (30 ml). The insoluble materials
were filtered off through Celite and the filtrate was concentrated.
Purification was accomplished by preparative TLC providing a mixture of
R 2R Si
H
H
H
Ti(C5H5)2
Ti(C5H5)2
R1
R1 trans-7
cis-7
4
and 5 as a clear, colourless oil; the excess trialkyl(allyl)silane was
removed by distillation prior to purification when necessary. The yields of
4 and 5 were determined by NMR analysis.
(
E)-4
(Z)-4
‡
The configurations of 4a–d were determined by comparison with
Scheme 3
authentic samples of the E- and Z-isomers. The Z-isomers (Z)-4 were
prepared by the stereoselective reduction of the corresponding prop-
2
-ynylsilanes with DIBAL-H reported by Rajagopalan and Zweifel,10 and
(
Z)-allylsilanes (Z)-4 were selectively produced by the present
E-isomers (E)-4 were obtained by their diphenyl disulfide mediated
photoisomerization [4a (E:Z
(E:Z 79:21), 4d (E:Z
reaction regardless of the substituents on the thioacetals or
allylsilanes.‡
11
=
74:26), 4b (E:Z
= 83:17), 4c
=
= 82:18)]. The configurations of other
The formation of organosilyl compounds 4 and 5 is explained
by the following reaction pathway (Scheme 2), which involves
the initial formation of a,b-disubstituted titanacyclobutane
allylsilanes were assigned on the basis of the coupling constant of their vinyl
protons.
1
2
D. Seyferth, K. R. Wursthorn and R. E. Mammarella, J. Org. Chem.,
977, 42, 3104; D. Seyferth, K. R. Wursthorn and T. F. O. Lim,
J. Organomet. Chem., 1979, 181, 293; I. Fleming and I. Paterson,
Synthesis, 1979, 446; J. G. Smith, S. E. Drozda, S. P. Petraglia,
N. R. Quinn, E. M. Rice, B. S. Taylor and M. Viswanathan, J. Org.
Chem., 1984, 49, 4112.
W. E. Crowe, D. R. Goldberg and Z. J. Zhang, Tetrahedron Lett., 1996,
37, 2117. The molybdenum-catalysed cross metathesis of terminal
olefins with allylstannanes has also been reported; J. Feng, M. Schuster
and S. Blechert, Synlett, 1997, 129.
7
intermediate 7. The metallacycle intermediate then cleaves to
1
form a g-substituted allylsilane 4 and titanocene methylidene
complex 8. The homoallylsilane 5 is produced by elimination of
the b-hydride from 7, followed by elimination of the titanoce-
ne(ii) species from the homoallyltitanium 9. A similar process
was observed in the reaction of carbene complexes with
alkynes.6
As for the cleavage of a,b-disubstituted titanacyclobutane
leading to the formation of a carbene complex and an olefin, it
was shown that the configuration of the metallacycle derived
from Tebbe’s reagent, 3,3-dimethylcyclopropene and a nor-
3
4
M. F. Schneider, N. Lucas, J. Velder and S. Belchert, Angew. Chem., Int.
Ed. Engl., 1997, 36, 257.
Y. Horikawa, M. Watanabe, T. Fujiwara and T. Takeda, J. Am. Chem.
Soc., 1997, 119, 1127.
8
bornene diester was retained during the process. Since the cis-
a,b-disubstituted metallacycle is much less stable than the
5 Y. Horikawa, T. Nomura, M. Watanabe, T. Fujiwara and T. Takeda,
J. Org. Chem., 1997, 62, 3678.
corresponding trans-isomer,9 the preferential formation of
6
7
T. Takeda, H. Shimokawa, Y. Miyachi and T. Fujiwara, Chem.
Commun., 1997, 1055.
The regio- and stereo-selective formation of trans-a,b-disubstituted
titanacyclobutanes by the reaction of trimethylsilylmethylene complex
(
Z)-allylsilanes by the present reaction is explicable in terms of
much faster cleavage of cis-titanacyclobutane cis-7 than of
trans-7 (Scheme 3).
In conclusion, we have found that the reaction of titanium
carbene complexes with allylsilanes affords (Z)-allylsilanes
regio- and stereo-selectively. It should be noted that the present
study first revealed the stereochemistry of the cross metathesis
between an alkylidenetitanocene and an acyclic olefin. Further
study on the reactions of carbene complexes formed from
thioacetals with various olefins is now in progress.
This work was supported by a Grant-in-aid for Scientific
Research on Priority Area No. 09231213 from the Ministry of
Education, Science, Sports and Culture, Japan.
(
C
5
Me
R. A. Andersen and R. G. Bergman, J. Am. Chem. Soc., 1996, 118,
737.
5
)
2
3
TiNCHSiMe with terminal olefins was reported; J. L. Polse,
8
8 J. R. Stille, B. D. Santarsiero and R. H. Grubbs, J. Org. Chem., 1990, 55,
843.
9 D. A. Straus and R. H. Grubbs, J. Mol. Catal., 1985, 28, 9.
0 S. Rajagopalan and G. Zweifel, Synthesis, 1984, 113.
1 A. Thalman, K. Oertle and H. Gerlach, Org. Synth., 1990, Coll. Vol.
VII, 470.
1
1
Received in Cambridge, UK, 6th October 1997; 7/07177I
52
Chem. Commun., 1998