Regioselective alkylation and olefination of allylic carbanion stabilized by two different heteroatoms: Phosphorus and silicon

Article abstract of DOI:10.1016/S0040-4039(01)00170-8

Treatment of allylphosphonate with LiHMDS, followed by successive addition of chlorotrimethylsilane and carbonyl reagent, afforded dienylsilanes in good yields. 3-Substituted dienylsilanes were obtained by alkylation and olefination of allylic carbanion of 3-trimethylsilylallylphosphonates.

Full text of DOI:10.1016/S0040-4039(01)00170-8

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 2345–2347
Regioselective alkylation and olefination of allylic carbanion
stabilized by two different heteroatoms: phosphorus and silicon
Bum Sung Lee, Jun Mo Gil and Dong Young Oh*
Department of Chemistry, Korea Advanced Institute of Science and Technology, Taejon 305-701, South Korea
Received 21 December 2000; accepted 26 January 2001
Abstract—Treatment of allylphosphonate with LiHMDS, followed by successive addition of chlorotrimethylsilane and carbonyl
reagent, afforded dienylsilanes in good yields. 3-Substituted dienylsilanes were obtained by alkylation and olefination of allylic
carbanion of 3-trimethylsilylallylphosphonates. © 2001 Elsevier Science Ltd. All rights reserved.
The reactions of allyl anions, stabilized by two different
heteroatoms at the a- and g-positions, have been one of
the well-known methods of introducing various func-
tional groups at the allylic position over the years.1 Allylic
systems bearing two different heteroatoms—sulfur with
oxygen, silicon, or halogen,2a oxygen with nitrogen or
silicon,2b and nitrogen with sulfur or silicon2c—have been
studied particularly. The regioselectivity of an allylic
carbanion is determined by numerous factors, including
steric effects, electron density, ionizing cosolvents and
various species of electrophiles.3 Presently, our interest
focused on functionalized allylic phosphonates that could
be transformed into versatile compounds.4 In the course
of our study, a lithiated 3-trimethylsilylallylphosphonate,
i.e. an allylic system of phosphorus and silicon, was
expected to be a useful precursor for various transforma-
tions, but its tendency toward olefination and alkylation
has been rarely studied. We herein report a facile
synthetic route to substituted dienylsilanes, which are
valuable precursors for Diels–Alder reactions,5 using
regioselective alkylation and olefination of lithiated 3-
trimethylsilylallylphosphonate.
The lithiated 3-trimethylsilylallylphosphonate (I) was
generated by treatment of phosphonate (1) with 2 equiv.
of LiHMDS in THF at −78°C, followed by addition of
chlorotrimethylsilane (Scheme 1). The trimethylsilyl
group exclusively took the g-position of allylphospho-
nate.6 Protonolysis of intermediate (I) at −78°C or room
temperature afforded 2a exclusively. The a-carbanion
stabilizing power of the diethylphosphonyl group seemed
to be superior to that of trimethylsilyl group, because the
position of double bond indicates that the a-carbon of
diethylphosphonyl group has a higher electron density.
Also, all efforts to isolate the intermediate, 3-trimethyl-
silyl-1-propenylphosphonate, failed. Exposure of I to
carbonyl reagents afforded dienylsilanes (3) by Horner–
Wadsworth–Emmons olefination7 in good yields. The
Peterson olefination8 product, i.e. the g-addition product,
was not detected by 1H NMR analysis of the crude
product. The a-selectivity of lithiated allylic systems
containing diethylphosphonyl and trimethylsilyl groups
could be explained by the higher electron density, and
both by a-selectivity9 of allylic phosphonate and g-
selectivity10 of allylic trialkylsilane in reaction with
O
R1
O
β
R1COR2
SiMe3
2 eq. LiHMDS
(CH3)3SiCl
(EtO)2P
(EtO)2P
SiMe3
R2
-
I
γ
α
1
3
H3O+ -78oC or rt.
O
(EtO)2P
SiMe3
2a
Scheme 1.
* Corresponding author.
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)00170-8

2346
B. S. Lee et al. / Tetrahedron Letters 42 (2001) 2345–2347
Table 1. Olefination of lithiated 3-trimethylsilylallylphosphonate
carbonyl
reagent
E/Zb
entry
product
yielda(%)
88
O
SiMe3
only E
3a
1
Ph
Ph
Ph
H
O
only E
_
3b
3c
2
3
86
84
SiMe3
H
O
SiMe3
SiMe3
O
4
3d
3e
86
83
82
70 / 30
Ph
O
5
6
SiMe3
67 / 33
58 / 42
O
3f
SiMe3
a
Yield of isolated product. b Geometries of 3, 4-olefins of trimethylsilyl group were
determined by NOE and 1H NMR analysis.
carbonyl compounds. Table 1 summarizes some results
of olefination with various carbonyl reagents, including
easily enolizable (entry 4) and sterically hindered
reagents (entry 6). Most olefins at the 1,2-position of
the trimethylsilane group were typically in a pro-
nounced E-geometry. The geometries of 3,4-olefins
were E/Z mixtures, except for entries 1 and 2. Only one
stereoisomer was obtained when aldehydes were added
as carbonyl reagents. The geometries were identified by
nates containing methyl (2b), ethyl (2c), and benzyl (2d)
substituents were obtained. Alkylation occurred exclu-
sively at the a-carbon of diethylphosphonyl groups.
Then, exposure of II to carbonyl reagents afforded
3-alkylated dienylsilanes (4) in good yields. It is note-
worthy that although benzylation of unsubstituted
allylphosphonate gave a,g-added (1:1) mixtures, only
one regioisomer was obtained when both silylation and
benzylation were carried out in one pot. It could be
assumed that the trimethylsilyl group, introduced
firstly, served as a blocking group for g-benzylation.
The substituents of methyl, ethyl, and benzyl at the
a-carbon of the diethylphosphonyl group had little
effect on orientation of olefination of the diethylphos-
phonyl–trimethylsilyl allylic anion, because a-selectivity
1
NOE experiments and H NMR analysis.
To investigate how regioselectivity of the allylic anion is
affected by substituents, alkylation and olefination were
carried out by LiHMDS in THF at −78°C, as shown in
Scheme 2. 1-Alkylated-3-trimethylsilylallylphospho-
β
H3O+
2 eq. LiHMDS
P
SiMe3
P
α
SiMe3
P
SiMe3
_
R3X
γ
R3
R3 = CH3
R3
2a
II
R1COR2
2b 90%
CH3CH2 2c 85%
Benzyl 2d 82%
R1
O
P =
P(OEt)2
SiMe3
R2
R3
4
carbonyl
reagent
yielda(%)
E/Zb
phosphonate
product
O
SiMe3
SiMe3
4a
4b
4c
78
76
69
Ph
only E
2b
Ph
H
H
O
53 / 47
2d
2b
Ph
SiMe3
_
O
a Yield of isolated product. b Geometries of 3, 4-olefins of trimethylsilylgroup were determined
by NOE and 1H NMR analysis.
Scheme 2.

B. S. Lee et al. / Tetrahedron Letters 42 (2001) 2345–2347
2347
of allylic anion (II) still occurred at the same carbon.
The high regioselectivity of alkylation and olefination
at the a-carbon of the diethylphosphonyl group was
likely to somewhat overwhelm steric hindrance, which
was generated by alkylation. Therefore, to synthesize
3-functionalized dienylsilane, 3-trimethylsilylallylphos-
phonate seems to be a more appropriate precursor than
1,3-bis(trimethylsilyl)propene,11 which has been practi-
cally used for synthesis of dienylsilanes, because alkyla-
tion and olefination could not occur at the same carbon
in a lithiated 1,3-bis(trimethylsilyl)propene anion stabi-
lized by two identical heteroatoms.
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In summary, we demonstrated the regioselectivity of an
allylic anion of two different heteroatoms containing
phosphorus and silicon, and an efficient route to 3-sub-
stituted dienylsilanes, using regioselectivity of the lithi-
ated 3-trimethylsilylallylphosphonate.
4. Lee, B. S.; Lee, S. Y.; Oh, D. Y. J. Org. Chem. 2000, 65,
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Acknowledgements
6. Yuan, C.; Yao, J.; Li, S. Phosphorus Sulfur Silicon 1990,
53, 21.
This work was supported by grants from the Korea
Advanced Instituted of Science & Technology and the
Department of Chemistry and Department of Chem-
istry & School of Molecular Science (BK21).
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