Mg-promoted regioselective carbon-silylation of α-phosphorylacrylate derivatives

Article abstract of DOI:10.1246/cl.2002.228

Treatment of aromatic α-phosphorylacrylate derivatives 1a-h with Mg turnings in the presence of trimethylsilyl chloride brought about facile and regioselective C-silylation to give the corresponding β-silylated phosphonates 2a-h in excellent yields. The r

Full text of DOI:10.1246/cl.2002.228

2
28
Chemistry Letters 2002
Mg-Promoted Regioselective Carbon-Silylation of ꢀ-Phosphorylacrylate Derivatives
Ã
Makoto Kyoda, Takamichi Yokoyama, Takeshi Kuwahara, Hirofumi Maekawa, and Ikuzo Nishiguchi
Department of Chemistry, Nagaoka University of Technology, 1603-1 Kamitomioka-cho, Nagaoka, Niigata 940-2188
(Received November 5, 2001; CL-011102)
Treatment of aromatic ꢀ-phosphorylacrylate derivatives 1a-
diastereoisomers, as shown in Table 1. The diastereoisomer
ratios of the products 2a-h were approximately 1 to 2, but their
structures have not been determined as yet. Large excess amount
of TMSCl must be required for smooth reaction because it was
assumed that TMSCl did not only work as an electrophilic agent
but also played a critical role of continuous activation of Mg metal
surface as a weak Lewis acid. The similar silylation of ethyl ꢀ-
phosphoryl-ꢁ-thienylacrylates 1h having a heteroaromatic ring
also successfully took place with a similar diastereoisomer ratio
of 2h. The yields of the products 2a-h were little influenced by the
nature of substituents on the benzene rings of the substrates 1a-h.
h with Mg turnings in the presence of trimethylsilyl chloride
brought about facile and regioselective C-silylation to give the
corresponding ꢁ-silylated phosphonates 2a-h in excellent yields.
The reaction took place smoothly for activated olefins possessing
appropriate reduction potentials and may be initiated by electron
transfer from Mg to 1a-h.
Regioselective introduction of a silyl group to a desired
position of organic compounds may be one of challenging
subjects in organic synthesis because of increasing interests in
1
{3
Table 1. Mg-Promoted ꢁ-silylation of aromatic ꢀ-phosphory-
lacrylate derivatives 1a-h
specific and valuable functions of organosilicon compounds. It
4
has been reported by us and Calas’s group that electrochemical
and Mg-promoted reduction5 of aromatic ꢀ,ꢁ-unsaturated
esters such as ethyl cinnamate in the presence of trimethylsilyl
chloride(TMSCl) affords ꢁ-trimethylsilyl-ꢁ-arylpropionates in
moderate to good yields.7 A regioselective carbon-silicon bond
formation proceeds at the ꢁ-position of the ester groups through
electron transfer from a cathode or Mg metal to the substrates. On
the other hand, phosphorus-containing organic compounds are of
much use as a variety of speciality chemicals such as
agrochemicals, pharmaceutical drugs, flame retardants, metal-
extraction agents and electronics materials.
;6
;8
In this study, we wish to report facile and efficient
regioselective carbon-silylation of aromatic ꢀ-phosphorylacry-
late derivatives 1a-h to give the corresponding ꢁ-silylated
phosphonates 2a-h in excellent yields. Synthesis of these specific
compounds bearing both of a phosphonate group and a silyl group
in a same molecule may be interesting in material science as well
as in organic chemistry.
Scheme 1.
It may be noteworthy that Mg-promoted reduction of diethyl
1
3
The starting ꢀ-phosphorylacrylates 1a-h were prepared
according to the reported literatures.9 The typical procedure
of thepresent Mg-promoted reductive silylation isas follows: Into
a solution of N,N-dimethylformamide (DMF) containing TMSCl
2-phenylethenylphosphonate (3) under the similar conditions
led to formation of the hydrodimerization product 3C (19%
{11
1
4
yield) as the main product accompanying with formation of a
small amount of the ꢁ-silylated product 3A (10% yield) and much
tarry material. On the other hand, the reaction of diethyl
benzylidenmalonate (4) and ethyl benzylideneacetoacetate (5)
resulted in considerable decrease in the yield of the corresponding
ꢁ-silylated products 4A and 5A (70–67% yield) , and consider-
able amounts (23–24 yield) of simply hydrogenated products 4B
and 5B were obtained as the by-products. Also, from the reaction
of ethyl benzylidenecyanoacetate (6), only some of a simply
hydrogenated products 6B (23% yield) and a small amount of the
(
15equiv) and Mg turnings (6 equiv) for Grignard reaction
without any pre-treatment was added a DMF solution of ꢀ-
ꢀ
phosphorylacrylates 1a-h dropwise at À10 C under nitrogen
atmosphere and the solution was continued to stir for 9 h at room
ꢀ
temperature (15–20 C). After the usual work-up of the reaction
mixture, the products were isolated by flash column chromato-
graphy on silica gel. The corresponding ꢁ-silylated phosphonates
1
2
2
a-h were obtained in 84–97% yields as mixtures of the
Copyright Ó 2002 The Chemical Society of Japan

Chemistry Letters 2002
229
Table 2. Reduction Potentials and Products of Aromatic Active
a
transfer from Mg metal to appropriately activated aromatic ꢀ-
phosphorylacrylates, which might be of much use in organic
synthesis and material science.
Olefins
This research was supported by a Grant-in-Aid for Scientific
Research on Priority Areas (No. 13029038) from Ministry of
Education, Science, Sports and Culture, Japan.
References and Notes
1
D. R. M. Walton, ‘‘Organosilicon Compounds, Dictionary of
Organometallic Compounds,’’ Chapman and Hall, London
(
1984), Vol. 2.
2
3
E. W. Colvin, ‘‘Silicon Reagents in Organic Synthesis,’’
Academic Press, London (1988).
‘‘Organosilicon and Bioorganosilicon Chemistry –Structure,
Bonding, Reactivity, and Synthetic Application,’’ ed. by H.
Sakurai, Ellis Horwood, Chichester (1985).
4
5
6
T. Ohno, H. Nakahiro, K. Sanemitsu, T. Hirashima, and I.
Nishiguchi, Tetrahedron Lett., 33, 5515 (1992).
J.-P. Picard, A. Ekouya, J. Dunogues, N. Duffaut, and R.
Calas, J. Organomet. Chem., 93, 51 (1975).
I. Nishiguchi, Y. Kita, Y. Ishino, H. Maekawa, Y. Yamazaki,
and T. Ohno, 43rd Symposium on Organometallic Chemistry
of Kinki Chemical Society, Osaka, 1996; Abstr., No. B212.
Calas et al. represented the used solvent, hexamethylpho-
sphortriamide ((Me2N)3PO) as the abbreviation HMPT in
their series of Mg-promoted reactions. However, the solvent,
7
(
Me2N)3PO, is represented as carcinogenic hexamethylpho-
ꢁ
-silylated product 6A (9% yield) were obtained accompanying
sphoric triamide (HMPA) and should be different from
hexamethylphosphorous triamide ((Me N) P) (HMPT)
which may be too reactive to use as a solvent.
with formation of much tarry material, as shown in Table 2.
Cyclic voltammetry of these aromatic ꢀ-phosphorylacrylates
2
3
8
1
a, 1e, 1h, 3 and the related compounds 4-6 may provide some
8
9
J.-P. Picard, J. Organometal. Chem., 34, 279 (1972).
S. Yamazaki, T. Takada, T. Imanishi, Y. Moriguchi, and S.
Yamabe, J. Org. Chem., 63, 5919 (1998).
significant informations for elucidation of remarkable differences
in their reaction behaviors, as shown in Table 2. There seems a
tendency in the present reactions that the reaction of a substrate
possessing more negative reduction potential gives a hydro-
dimerzation product (type C) while a simply hydrogenated
product (type B) is obtained to some extent from that of a substrate
possessing a less negative reduction potential. A substrate such as
10 C. N. Robinson and C. D. Slater, J. Org. Chem., 52, 2011
(1987).
11 M. T. Reetz, R. Peter, and M. V. Itzstein, Chem. Ber., 120, 121
(1987).
12 All the products 2a-h, 3A, 3C, 4A-C, 5A-B, 6A-B, and 7A-B
1
13
ꢀ
-phosphorylacrylates 1a-h, whose reduction potential are
were characterized by spectroscopic methods ( H- and C-
NMR, IR, Mass), and elementary analysis.
À1:70-À1:80 V, are most suitable for the present Mg-promoted
regioselective C-silylation, giving a type A product predomi-
nantly.
13 T. Hirao, T. Masunaga, N. Yamada, Y. Ohshiro, and T.
Agawa, Bull. Chem. Soc. Jpn., 55, 909 (1982).
The present Mg-promoted reductive silylation may proceed
through electron transfer from Mg metal to activated electron–
difficient olefins 1a-h to give the corresponding radical anions,
which are successively subjected to electrophlic attack of TMSCl,
second electron transfer and protonation. Different reaction
behaviors of a variety of activated olefins, depending upon their
reduction potentials, may be attributed to stability and reactivity
of the corresponding radical anions.
As a conclusion, efficient and facile regioselective formation
of C-Si bond was successfully accomplished through electron
14 Use of 0.5eq of TMSCl in the reaction of 3 resulted in some
increase in the yield of the hydrodimerization product 3C to
39%, and formation of a trace amount of the silylated product
3A.
15It may be one of plausible explanation that a less stable radical
anion from 3 is apt to dimerize at a proximity to Mg metal
surface while a more stable one form 4-6 has enough life-time
to diffuse into a bulk solution and to catch a proton rather than
reactive TMSCl.
15