Lithium perchlorate diethyl ether/methyl triflate catalyzed transfer of silicon from oxygen to carbon in silyl ketene (Thio) acetals

Article abstract of DOI:10.1246/cl.2003.226

Migration of a trimethylsilyl group from oxygen to the α-carbon (retro- [1,3]-Brook rearrangement) in silyl ketene acetals is catalyzed by LPDE/methyl triflate under mild conditions.

Full text of DOI:10.1246/cl.2003.226

226
Chemistry Letters Vol.32, No.3 (2003)
Lithium Perchlorate Diethyl Ether/Methyl Triflate Catalyzed Transfer of Silicon from
Oxygen to Carbon in Silyl Ketene (Thio) Acetals
Akbar Heydarià and Reza Alijanianzadeh
Chemistry Department, Tarbiat Modarres University, P. O. Box 14155-4838, Tehran, Iran
(Received October 22, 2002; CL-020902)
Migration of a trimethylsilyl group from oxygen to the a-
formation of a-trimethylsilyl esters,14 as examplified by Eq 1.
carbon (retro- [1,3]-Brook rearrangement) in silyl ketene acetals
is catalyzed by LPDE/methyl triflate under mild conditions.
R1
R2
O
OSiMe3
XR3
LPDE/MeOTf
r.t., 1h
Me3Si
R1
XR3
(1)
R2
The synthetic utility of a-silylated esters has been amply
demostrated.1 Although these systems have been generated in a
variety of ways, the majority of these preparations suffer from less
than readily available starting materials and/or a lack of general-
ity.2 The most general route to these interesting systems involves
reactions of trimethylsilyl triflates with esters to give the a-
trimethylsilyl esters in moderate to good yield with some
contamination from the O-silylated and bis-silylated products.3
All reports to date on the silylation of ester enolates have shown
that the predominat product is that of O-silylation. The two
exceptions to this general observation are tert-butyl acetates and
esters of cyclopropane carboxylic acid. No good example of C-
silylation of an enolates of an a-substituted ester has been
reported to date. The thermal migration of a trialkylsilyl group
from carbon to oxygen in a-silyl ketones by an intramolecular
concerted four-center mechanism has been well established.4 The
reverse process has been also observed in silylketene acetals,5 but
less is common because of competitive decomposition to ketenes
and trialkylsilyl ethers.6 In addition, thermal, high pressure7 and
Lewis acid catalyzed migration of the silyl group from O to the a-
C has been documented using mercury (II) iodide at 70 ꢀC,8
[Cp2Zr(Ot-Bu)THF][BPh4],9 organoaluminum compounds10
and lanthanoid trifluromethanesulfonates (trimethylsilyl ketene
acetals derived from ethyl isobutyrate, methyl phenylacetate and
methyl 3-phenylbutyrate were decomposed to the corresponding
esters quantitatively and no a-silylated products were
obtained).11 Nevertheles, in the presence of TiCl4 silyl ketene
acetals are oxidatively dimerized to succinates.12
In recent years, lithium perchlorate in diethyl ether (LPDE)
has gained importance as a versatile reaction medium for
effecting various organic transformations such as Diels–Alder
reactions;13a 1,3-Claisen rearrangements;13b Hetero Diels–Alder
reactions;13c substitution reactions of allylic acetates and allylic
alcohols;13d chelation controlled addition of allylstannanes to
aldehydes;13e [2+2] cycloadditions;13f selective carbonyl
protection;13g selective deoxygenation of allylic alcohols and
acetates;13h ene reactions;13i ionization of C7 methoxy group in
rapamycin;13j [2+3] addition of p-benzoquinone with alkenes;13k
selective rearrangement of epoxides;13l cationic [5+2] cycload-
dition reactions;13m tetrahydropyranylation of alcohols;13n
Baylis–Hilman reactions;13o a-aminoalkylation;13p a-amino-
cyanation;13q a-cyanohydroxylamination;13r and N-trimethylsi-
lyloxy-a-aminophosphonation.13s In an attempt to prepare a-
methylesters by reaction of O-silyl ketene acetals and methyl
trifluoromethanesulfonate in 5 MLPDE, we observed instead the
1
2
yield
2 (%)
R2
R3
R1
X
76
75
86
90
67
74
O
Et
Me
Me
i-Pr
H
H
H
a
b
c
d
e
f
Me
H
H
H
H
O
O
O
O
S
Me
Me
Ph
Bu
Et
H
Though the behavior of LiClO4 is not clear, it seems that
LiClO4 interacts with the methyl triflate to generate an active
anionic species, which accelerates the reaction. The reaction may
proceed via the trimethylsilyltriflate and it should be noted that
stoichiometric amounts of methyl triflate is needed to complete
the reaction.15 The reaction of trimethylsilyl ketene acetal 1b was
monitored by 13C NMR in LPDE (CDCl3 as internal standard).
Carbon absorbtions of the trimethylsilyl group and a-C atom of
1b appeared at À0:15 and 153 ppm respectively. Addition of
stochiometric amount of MeOTf in the reaction mixture gave new
signals at 179 and À4 ppm, after 15 min. These peaks correspond
to (C=O) and trimethylsilyl group of 2b. These results suggest
that the migration of the silyl group proceeds via initial generation
of lithium enolate(not detected by 13C NMR under conditions
used), and TMSOTf, followed by C-silylation of enolate. Such C-
silylation by trimethylsilyl triflate in the presence of trimethyla-
mine has already been reported.3
+
CF3SO3CH3
R1
CH3ClO4+ CF3SO2OLi
LiClO4
R1
R2
OLi
OSiMe3
CF3SO2OLi
- CF3SO3SiMe3
XR3
R2
XR3
OLi
R2 R1
R1
R2
CF3SO3SiMe3
O
Me3Si
XR3
XR3
In conclusion, we have described an LPDE/MeOTf mediated
retro [1,3]-Brook rearrangement, which provides silylated esters
directly from keteneacetals.
Research supported by the National Research Council of I. R.
Iran as a National Research project under the number 984.
Copyright ꢀ 2003 The Chemical Society of Japan

Chemistry Letters Vol.32, No.3 (2003)
227
References and Notes
(1992); J. Ipaktschi, A. Heydari, and H.-O. Kalinowski,
Chem. Ber., 127, 905 (1994). f)W. Srisiri, A. B. Padias, and H.
K. Hall, Jr., J. Org. Chem., 58, 4185 (1993). g) V. Geetha
Saraswathy and S. Sankaraman, J. Org. Chem., 59, 4665
(1994). h) D. J. Wustrow, W. J. Smith, III, and L. D. Wise,
Tetrahedron Lett., 35, 61 (1994). i) W. J. Kinat, J. Chem. Res.,
Synop., 1994, 486. j) J. Luengo, A. K. Beck, and D. A. Holt,
Tetrahedron Lett., 36, 7823 (1995). k) J. Asano, Y. Ryu, K.
Chiba, and M. Tada, J. Chem. Res., Synop., 1995, 124. l) R.
Sudha, K. Maloia Narashimhan, V. Geetha Saraswathy, and
S. Sankararaman, J. Org. Chem., 61, 1877 (1996). m) J. L.
Collins, P. A. Grieco, and J. K. Walker, Tetrahedron Lett., 38,
1321 (1997). n) B. S. Babu and K. K. Balasubramanian,
Tetrahedron Lett., 39, 9287 (1998). o) M. Kawamura and S.
Kobayashi, Tetrahedron Lett., 40, 1539 (1999). p) A. Heydari
and J. Ipaktschi, Chem. Ber., 127, 1761 (1994). q) A. Heydari,
P. Fatemi, and A.-A. Alizadeh, Tetrahedron Lett., 39, 3049
(1998). r) A. Heydari, A. Karimian, and J. Ipaktschi,
Tetrahedron Lett., 39, 6729 (1998). s) A. Heydari, H.
Larijani, J. Emami, and B. Karami, Tetrahedron Lett., 41,
2471 (2000). t) A. Heydari, M. Zarei, R. Alijanianzadeh, and
H. Tavakol, Tetrahedron Lett., 42, 3629 (2001).
1
E. Nakamura, T. Murofushi, M. Shimizu, and I. Kuwajima, J.
Am. Chem. Soc., 98, 2346 (1976); S. L. Hartzell, D. F.
Sullivan, and M. W. Rathke, Tetrahedron Lett., 15, 1403
(1974); K. Shimoji, H. Taguchi, K. Oshima, H. Yamamoto,
and H. Nozaki, J. Am. Chem. Soc., 96, 1620 (1974); S. L.
Hartzell and M. W. Rathke, Tetrahedron Lett., 42, 2038
(1977).
2
3
4
R. J. Fessenden and J. S. Fessenden, J. Org. Chem., 32, 3535
(1967); A. G. Brook, J. M. Duff, and D. G. Anderson, J. Am.
Chem. Soc., 92, 7567 (1970).
C-Silylation by trimethylsilyl triflate in the presence of
trimethylamine has been reported. H. Emde and G. Simchen,
Synthesis, 1977, 867.
A. G. Brook, Acc. Chem. Res., 7, 77 (1974); A. G. Brook and
A. R. Bassindale, ‘‘Molecular Rearrangements of Organosi-
licon Compounds,’’ Eassy 9 in ‘‘Rearrangements in Grund
and Excited States,’’ ed. by P. De Mayo, Academic Press,
New York (1980).
5
6
7
Y. Yamamoto, K. Maryuyama, and K. Matsumoto, Organo-
metallics, 3, 1583 (1984); S. Raucher and D. C. Schindele,
Synth. Commun., 17, 637 (1987).
Y.-N. Kuo, F. Chen, and C. Ainsworth, J. Am. Chem. Soc., 93,
4504 (1971); C. Ainsworth, F. Chen, and Y.-N. Kuo, J.
Organomet. Chem., 46, 59 (1972).
I. F. Lutsendo, Y. I. Baukov, G. S. Burlachenko, and B. N.
Khasapov, J. Organomet. Chem., 5, 20 (1966); Y. Makioka,
Y. Taniguchi, K. Takaki, and Y. Fujiwara, Chem. Lett., 1994,
645.
14 A typical experimental procedure, To a mixture of ketene
acetal (2 mmol) in 5 MLPDE (4 ml) was added methyl triflate
(2.2 mmol) at room temperature. The mixture was stirred for
1 h and the product was extracted with CH2Cl2. The organic
phase was collected, dried (Na2SO4) and evaporated to afford
the crude product. The product was purified by preparative
gas chromatography OV-101, 150 ꢀC isotherm. 1H NMR, 13
C
8
9
I. B. Dicker, 189th National meeting of the American
Chemical Society, Miami Beach, Florida, April 28-May 3
1985, Abstr., ORGN 238; W. J. Brittain and I. B. Dicker,
Polym. Int., 30, 101 (1993); Chem. Abstr., 118, 22709v
(1993).
NMR, IR and MS spectra were entirely consistent with the
assigned structures. Selected data as follow: 2a (R1 = R2 =
Me, R3 = Et, X = O): 1H NMR (500 MHz, CDCl3) ꢀ 3.5 (q,
2H), 1.2 (t, 3H), 1.15 (s, 3H), 1.1 (s, 3H), 0.1 (s, 9H); 13C NM R
(125 MHz, CDCl3) ꢀ 180.5 (C=O), 61.99 (OCH2), 53.41 (C),
18.5 (CH3), 14.3 (CH3), 13.6 (CH3), À0:7 (SiMe3); 2C (R1 =
H, R2 = iso-propyl, R3 = Me, X = O): 1H NMR (400 MHz,
CDCl3) ꢀ 3.5 (s, 3H), 2.1 (m, 1H), 1.7 (d, 1H), 0.9 (d, 3H), 0.8
(d, 3H), 0.1 (s, 9H); 13C NMR (100 MHz, CDCl3) ꢀ 175
(C=O), 50.6 (OCH3), 46.3 (CH), 28.2 (CH), 23.5 (CH3), 22.7
(CH3), À0:7 (SiMe3); 2d (R1 = R2 = H, R3 = Ph, X = O): 1H
NMR (400 MHz, CDCl3) ꢀ7.3–7.0 (m, 5H), 2.1 (s, 2H), 0.1 (s,
9H); 13C NMR (100 MHz, CDCl3) ꢀ 171.3 (C=O), 151 (C),
129.3 (CH), 125.5 (CH), 121.7 (CH), 27 (CH2), À1:3
(SiMe3).
Y. Hong, D. J. Norris, and S. Collins, J. Org. Chem., 58, 3591
(1993).
10 K. Maruoka, H. Banno, and H. Yamamoto, Synlett, 1991, 253.
11 Y. Makioka, Y. Taniguchi, K. Takati, and Y. Fujiwara, Chem.
Lett., 1994, 645.
12 S. Inaba and I. Ojima, Tetrahedron Lett., 18, 2009 (1977).
13 a) P. A. Grieco, J. J. Nunes, and M. D. Gaul, J. Am. Chem.
Soc., 112, 4595 (1990). b) P. A. Grieco, J. D. Clark, and C. T.
Jagoe, J. Am. Chem. Soc., 113, 5489 (1991). c) P. A. Grieco, J.
D. Clark, and C. T. Jagoe, J. Am. Chem. Soc., 113, 5488
(1991); K. K. Balasubramanian and N. Palani, Tetrahedron
Lett., 36, 9527 (1995). d) W. H. Pearson and J. M.
Scvheryantz, J. Org. Chem., 57, 2986 (1992). e) K. J. Henry,
P. A. Grieco, and C. T. Jagoe, Tetrahedron Lett., 33, 1817
15 The solution of silyl ketene acetals and methyl triflate in
diethyl ether remains unchange after 3 h at room temperature.
In addition the solution of silyl ketene acetals in LPDE
remains unchange after 3 h at room temperature.