A practical synthesis of difunctional organosilane reagents and their application to the Diels-Alder reaction

Full text of DOI:10.1021/jo990082d

1780
J . Org. Chem. 1999, 64, 1780-1781
Sch em e 1. Electr on -With d r a w in g Gr ou p on th e
Dien op h ile Low er s th e Rea ction Tem p er a tu r e a n d
Im p r oves th e Selectivity of th e Rea ction
A P r a ctica l Syn th esis of Difu n ction a l
Or ga n osila n e Rea gen ts a n d Th eir Ap p lica tion
to th e Diels-Ald er Rea ction
Scott McN. Sieburth* and J erome Lang
Department of Chemistry, State University of New York,
Stony Brook, New York 11794-3400
Received J anuary 15, 1999
Difunctional organosilanes, with a leaving group and a
reactive organic functionality, have become basic tools for
synthetic chemistry.1 Prominent examples include (bromo-
methyl)chlorosilanes2 and chloro(vinyl)silanes.3,4 Unfortu-
nately, only a small number of these reagents are com-
mercially available, and synthesis of even simple examples
can be experimentally demanding.5 A case in point are the
versatile â-(chlorodialkylsilyl) acrylates 5 (X ) Cl). The
unsaturated ester provides enhanced reactivity and a high
level of stereoselectivity in cycloadditions,6-8 while the silyl
ether (e.g., 1) provides the regioselectivity and entropic
advantages of intramolecularity (Scheme 1). For example,
ester 1b cyclizes at a moderate temperature and yields a
single product,3 whereas 1a requires rather high tempera-
tures and is low in stereoselectivity.3,4
Sch em e 2. P r ep a r a tion of Silyl Acr yla tes 9 a n d 12
The two published procedures for the synthesis of 5 (X )
Cl, R ) Me, i-Pr) each require four steps, the use of -100
°C reaction temperatures, and the isolation of moisture-
sensitive intermediates.3,9 We report here synthetic se-
quences that make compounds such as 5 and 1 readily
available and do not require the isolation of moisture
sensitive intermediates. While both earlier procedures em-
ployed â-iodo acrylate 4,10 the protocols described here utilize
commercially available silanes 6 or 7.
in the presence of the ester. Silyl triflates O-silylate carbo-
nyls13 and therefore both functional groups in the same
molecule might be expected to yield, at best, a polymeric
complex. Nevertheless, generation of silyl triflates by prot-
olytic cleavage of phenylsilanes with triflic acid is a reliable
reaction,14 and we therefore targeted triphenylsilane 9 for
study (Scheme 2).
Ethyl ester 9 has not been previously prepared; however,
two methods provide useful quantities of this acrylate.
Takeuchi’s ethoxycarbonylation, originally described for
trimethylsilylacetylene,15 also works well with triphenylsi-
lylacetylene 8 to give exclusively 9 as a crystalline solid in
93% isolated yield. The utility of this process, however, is
somewhat attenuated by the requirement for high-pressure
reaction vessels. The alternative procedure of Seki and
Murai,16 cobalt-catalyzed oxidative hydrosilylation of ethyl
acrylate, gave the same crystalline product with complete
regio- and stereoselectivity and an acceptable yield (75%).
Importantly, this latter procedure can be used to prepare
tens of grams of 9. Under the same conditions, dimethylphe-
nylsilane 11 yields acrylate 12.
Compound 5 as a triflate (X ) OTf) was anticipated to be
significantly more reactive than the corresponding chlorosi-
lane11,12 and, therefore, quite useful if it could be generated
* To whom correspondence should be addressed. Tel: (516) 632-7851.
Fax: (516) 632-8882. E-mail: ssieburth@notes.cc.sunysb.edu.
(1) For reviews, see: Bols, M.; Skrydstrup, T. Chem. Rev. 1995, 95, 1253-
1277. Fensterbank, L.; Malacria, M.; Sieburth, S. McN. Synthesis 1997,
813-854.
(2) Stork, G.; Kahn, M. J . Am. Chem. Soc. 1985, 107, 500-501.
(3) Stork, G.; Chan, T. Y.; Breault, G. A. J . Am. Chem. Soc. 1992, 114,
7578-7579.
(4) Sieburth, S. McN.; Fensterbank, L. J . Org. Chem. 1992, 57, 5279-
5281.
(5) For an example, see: Stork, G.; Keitz, P. F. Tetrahedron Lett. 1989,
30, 6981-6984.
(6) For the intramolecular cycloadditions of Si-tethered â-silylacrylates,
see refs 3 (Diels-Alder) and 9 (nitronate [3 + 2] cycloaddition).
(7) For cycloadditions of â-silylacrylate derivatives not tethered through
silicon, see the following. Diels-Alder reactions: Hermeling, D.; Scha¨fer,
H. J . Angew. Chem., Int. Ed. Engl. 1984, 23, 233-235. Hermeling, D.;
Scha¨fer, H. J . Chem. Ber. 1988, 121, 1151-1158. Wilson, S. R.; Di Grandi,
M. J . J . Org. Chem. 1991, 56, 4766-4772. Nitrile-oxides: Lukevics, E.;
Dirnens, V.; Kemme, A.; Popelis, J . J . Organomet. Chem. 1996, 521, 235-
244.
Protodesilylation of organic groups by triflic acid has been
extensively studied by Bassindale17 and Uhlig,14 and on the
basis of this work, a phenyl group is expected to be cleaved
from silicon before a simple vinyl group. The ester reinforces
this tendency, as triflic acid would initially protonate the
carbonyl (13), protecting the acrylate from further electro-
philic attack (14) and leading to triflate 16 via 15 (Scheme
3).
(8) For examples of cyclizations utilizing â-silylacrylate esters, see the
following. Aziridination: Lukevics, E.; Dirnens, V. V.; Goldberg, Y. S.;
Liepinsh, E. E. J . Organomet. Chem. 1986, 316, 249-254. Michael addition/
Claisen condensation: Oliver, J . E.; Waters, R. M.; Lusby, W. R. Tetrahedron
1990, 46, 1125-1130. Cyclopropanation: Hanessian, S.; Cantin, L.-D.; Roy,
S.; Andreotti, D.; Gomtsyan, A. Tetrahedron Lett. 1997, 38, 1103-1106.
(9) Denmark, S. E.; Hurd, A. R.; Sacha, H. J . J . Org. Chem. 1997, 62,
1668-1674.
(13) For an example of a characterized trimethylsilyl triflate-ketone
complex see: Emde, H.; Go¨tz, A.; Hofmann, K.; Simchen, G. Liebigs Ann.
Chem. 1981, 1643-1657.
(10) Biougne, J .; The´ron, F. C. R. Seances Acad. Sci., Ser. C 1971, 272,
858-860.
(14) Uhlig, W. J . Organomet. Chem. 1993, 452, 29-32. Uhlig, W. Chem.
Ber. 1996, 129, 733-739.
(11) Trimethylsilyl triflate is more than 108-fold more reactive than
chlorotrimethylsilane. See: Hergott, H. H.; Simchen, G. Liebigs Ann. Chem.
1980, 1718-1721.
(15) Takeuchi, R.; Sugiura, M. J . Chem. Soc., Perkin Trans. 1 1993,
1031-1037.
(12) Reviews: Emde, H.; Domsch, D.; Feger, H.; Frick, U.; Go¨tz, A.;
Hergott, H. H.; Hofmann, K.; Kober, W.; Kra¨geloh, K.; Oesterle, T.; Steppan,
W.; West, W.; Simchen, G. Synthesis 1982, 1-26. Uhlig, W. Chem. Ber. 1996,
129, 733-739. See also: Uhlig, W. J . Organomet. Chem. 1993, 452, 29-32.
(16) Takeshita, K.; Seki, Y.; Kawamoto, K.; Murai, S.; Sonoda, N. J . Org.
Chem. 1987, 52, 4864-4868.
(17) Bassindale, A. R.; Stout, T. J . Organomet. Chem. 1984, 271, C1-
C3.
10.1021/jo990082d CCC: $18.00 © 1999 American Chemical Society
Published on Web 02/25/1999

Communications
J . Org. Chem., Vol. 64, No. 6, 1999 1781
Sch em e 3. P r oton a tion P r otects th e Acr yla te
Sch em e 5. Secon d a r y Alcoh ols Also Rea ct Well w ith
16
Sch em e 4. On e-F la sk Con ver sion of 9 a n d 12 to Silyl
Eth er s F ollow ed by Cycloa d d ition a n d Oxid a tion
Sch em e 6. Acr yla te 27 Is Sta ble to Tr iflic Acid
Tamao-Fleming oxidation, gave diol 22 as a single isomer.
Use of an unadorned vinyl silane lead to a mixture of the
isomeric products 23 and 24 (Scheme 5).4
A potentially simpler and “greener”23 approach to di-
methylsilyltriflate 5 (X ) OTf, R ) CH3) is the protolytic
cleavage of a methyl group instead of phenyl group. Alkyls
are the most difficult of groups to remove from silicon with
acid,14 yet tetramethylsilane evolves methane rapidly and
quantitatively on treatment with triflic acid, forming tri-
methylsilyl triflate 26 (Scheme 6).24 Although vinyl groups
are protolytically cleaved from silanes much more easily
than methyl groups,14,17 initial protonation of the carbonyl
of 27 was expected to protect the acrylate. Surprisingly, this
protonation protects all of the groups on silicon; with a large
excess of triflic acid, silane 27 was stable at 50 °C for several
hours without noticeable decomposition (NMR).
The general method described here should allow a variety
of functionalized silyl acrylate derivatives to be rapidly and
easily prepared without the need for isolation of reactive
intermediates. The crystalline and easily synthesized 9 is
particularly convenient. With the increasingly common use
of organosilane reagents, we anticipate that this chemistry
will find broad application.
This process was evaluated by proton NMR. Addition of
1 equiv of triflic acid to acrylate 9 in deuteriochloroform at
ambient temperature led to minor changes in the proton
NMR spectrum attributed to simple protonation of the
carbonyl (13). Following addition of a second equivalent of
acid, substantial changes in the chemical shift of the alkene
proton signals were observed, accompanied by the appear-
ance of a prominent singlet at 7.3 ppm consistent with the
formation of benzene and triflate 16. This transformation
was very clean and complete within minutes at room
temperature.18
Isolation of 16 is unnecessary. After addition of triflic acid
at 0 °C, treatment with pyridine19 and sorbyl alcohol directly
forms silyl ether 17 (Scheme 4). The use of phenyl groups
on silicon, rather than methyl groups, leads to substantial
hydrolytic stability for this silyl ether, and it can be purified
by silica gel chromatography in good overall yield. Warming
this triene leads to Diels-Alder adduct 18 as a single isomer,
and Tamao-Fleming oxidation20-22 then yields the known
diol 19.3 Substitution of dimethylphenylsilane 12 for tri-
phenylsilyl 9 led to similar results, except that the hydro-
lytically more labile dimethylsilyl ether intermediate (1b)
was not purified, but cyclized directly to give 20.3
Ack n ow led gm en t is made to the donors of the Petro-
leum Research Fund, administered by the American Chemi-
cal Society, and to the NSF (CHE9712772) for support of
this research. NMR spectrometers used in this research
were purchased with funds from the NSF (CHE9413510).
The authors thank Ian Fleming for helpful discussions.
Su p p or tin g In for m a tion Ava ila ble: Characterization data
and procedures for the synthesis of 9, 12, 17-19, and 22, and
proton NMR spectra for 9, 12, 13, 16-19, and 22.
Secondary alcohol 21 reacted smoothly but more slowly
with triflate 16 and, after heating to 80 °C overnight and
J O990082D
(18) See the Supporting Information.
(19) Olah, G. A.; Klumpp, D. A. Synthesis 1997, 744-746.
(20) J ones, G. R.; Landais, Y. Tetrahedron 1996, 52, 7599-7662.
(21) Fleming, I. ChemtractssOrg. Chem. 1996, 9, 1-64.
(22) Chan, T.-Y. Ph.D. Thesis, Columbia University, 1993.
(23) Designing Safer Chemicals: Green Chemistry for Pollution Preven-
tion; DeVito, S. C., Garrett, R. L., Ed.; American Chemical Society:
Washington, DC, 1996; Vol. 640.
(24) Demuth, M.; Mikhail, G. Synthesis 1982, 827.