A new synthetic route to allylsilanes: The reaction of silyllithium reagents with aromatic carbonyl compounds and aluminium tris(2,6-diphenylphenoxide) (ATPH)

Article abstract of DOI:10.1039/a702684f

Conjugate 1,6-addition of silyllithium reagents to aromatic carbonyl substrates in the presence of the carbonyl protector aluminium tris(2,6-diphenylphenoxide) (ATPH) gives allylsilanes.

Full text of DOI:10.1039/a702684f

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A new synthetic route to allylsilanes: the reaction of silyllithium reagents with
aromatic carbonyl compounds and aluminium tris(2,6-diphenylphenoxide)
(ATPH)
Susumu Saito,a Kazuto Shimada,a Hisashi Yamamoto,*†a Eduardo Mart´ınez de Marigortab and Ian Fleming*b
a Graduate School of Engineering, Nagoya University, CREST, Japan Science and Technology Corporation (JST), Furo-Cho,
Chikusa, Nagoya 464-01, Japan
b Department of Chemistry, Lensfield Road, Cambridge, UK CB2 1EW
Conjugate 1,6-addition of silyllithium reagents to aromatic
carbonyl substrates in the presence of the carbonyl protector
aluminium tris(2,6-diphenylphenoxide) (ATPH) gives
allylsilanes.
low-temperature with deuterated acid to give the mono-
deuterated products 6 (Scheme 2).
When the generation of dienolate 5 was followed by
treatment with an excess of methyl trifluoromethanesulfonate
(10 equiv.) at 278 °C for 1 h, the a-methylated product 7 was
formed in 43% yield after the usual low-temperature acidic
work-up. Methylation occurred predominantly at the a-carbon,
in contrast to the exclusive g-protonation with hydrochloric acid
(Scheme 2).
Allylsilanes are versatile synthetic intermediates,1 and several
methods are available to prepare these reagents.2 We recently
described the conjugate addition of several alkyllithiums to
aromatic carbonyl compounds complexed with aluminium
tris(2,6-diphenylphenoxide) (ATPH),3 in which the Lewis acid
effectively blocks carbonyl carbons from nucleophilic attack.
This result led us to explore the possibility of using this strategy
for the synthesis of allylsilanes using silyllithium reagents4,5 in
place of the alkyllithium reagents. We report here that the
1,6-addition of silyllithium reagents to the aromatic carbonyl
compound–ATPH complex, and subsequent enolate capture
with an appropriate electrophilic reagent, does provide a route
to functionalised allylsilanes (Scheme 1).
Treatment of the precomplexed salt 1a of benzaldehyde and
ATPH with phenyldimethylsilyllithium (PhMe2SiLi)5 at
278 °C produced, after stirring for 30 min and quenching with
concentrated hydrochloric acid at 278 °C, selective 1,6-addi-
tion to give allylsilane 2a in 59% yield.‡ When a similar
procedure was applied to aromatic ketones 1b–d, 1,6-addition
products 2b–d were accompanied by small amounts of the
1,4-addition products 3b–d§ and the aromatised silylated
products 4b–d (Scheme 1). These results suggest that the
g-carbon of the carbonyl substrate was selectively protonated by
way of dienolate intermediates, which was confirmed by
quenching the intermediates 5 in the acetophenone series at
Similarly, phenyldimethylsilyllithium underwent conjugate
addition to methyl benzoate in the presence of ATPH with high
1,6-selectivity (1,6:1,4 = 87:13), but relatively low re-
gioselectivity on protonation to give a mixture (52:35:13) of
the regioisomeric pentadienyl silanes 9 (Scheme 3), including a
stereoisomeric mixture (cis:trans = 1:3.1) in the case of the
a-protonated product 9b. The cis:trans ratio was determined by
1H NMR spectroscopy and assigned by a NOESY measure-
ment. In contrast, methylation of the dienolate intermediates 8
gave cis-10 regio- and stereo-selectively, with the methyl group
entering anti to the resident silyl group. This finding is similar
to the alkylation of cyclic enolates having a silyl group adjacent
to the nucleophilic centre, which are highly selective for attack
by the incoming electrophile anti to the silyl group.6 The
relative stereochemistry of cis-10 was assigned by a NOESY
measurement by converting the mixture 10 into the alcohols 11
by reduction with diisobutylaluminium hydride (DIBAL-H).
1,6-Addition also took place, in somewhat higher yield, with
1-naphthaldehyde 12a and with 1-acetonaphthone 12b to give
the adducts 13 (Scheme 4). In an attempt to prepare the hydrate
of an aromatic ring, we also carried out the reaction with the
latter substrate using the more easily functionalised 2-me-
thylbut-2-enyldiphenylsilyl group7 to give the adduct 14.
Functionalisation of the silyl group was carried out using
protodesilylation at the exocyclic allylsilane unit to give the
ATPH
O
O
O
i, ii
R
R
R
+
D
COMe
PhMe2Si
SiMe2Ph
3a —
b 6%
c 13%
d 13%
γ
γ
O–
1a R = H
2a 59% + 1a >10%
b 38% + 1b 43%
c 27% + 1c
PhMe2Si
PhMe2Si
b R = Me
c R = Pri
d R = But
i
and
+
d 45% + 1d 38%
D
COMe
O–
Ph
O
R
SiMe2Ph
SiMe2Ph
5
6
O
Al
+
3
COMe
Me
PhMe2Si
Ph
4a —
ii
ATPH
b ca. 2%
c ca. 2%
d ca. 2%
PhMe2Si
7 43%
Scheme 1 Reagents: i, PhMe2SiLi; ii, HCl, H2O
Scheme 2 Reagents: i, DCl, D2O; ii, MeOSO2CF3
Chem. Commun., 1997
1299

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at 278 °C under argon, and the resulting orange solution of the ATPH–
benzaldehyde complex was stirred at this temperature for 10 min. A solution
of phenyldimethylsilyllithium (0.35 mol dm23 in THF, 2.86 cm3, 1.0 mmol)
was added at 278 °C, and the reaction mixture was stirred at this
temperature for 30 min. The reaction was quenched with concentrated
hydrochloric acid (12 mol dm23; 1.5 cm3) at 278 °C, stirred at room
temperature for 15 min, diluted with water (4.5 cm3) and extracted with
diethyl ether (3 3 15 cm3). The organic layer was dried (Na2SO4) and
concentrated. The residue was purified by column chromatography on silica
gel (CH2Cl2–hexane to give the 1,6-adduct 2a (59% isolated). Selected data
for 2a: Rf = 0.35 (CH2Cl2–hexane, 1:2); nmax (film)/cm21 1682, 1250,
1115. dH (CDCl3, 300 MHz) 9.22 (1 H, s, CHO), 7.55–7.30 (5 H, m, SiCPh),
6.44 (1 H, t, J 5.6, SiCHCH2CH), 6.28 (1 H, d, J 9.6, SiCHCHCH), 5.92 (1
H, dd, J 5.2 and 9.6, SiCHCH), 2.77 (1 H, ddd, J 3.9, 10.8 and 18.9,
CHAHB), 2.48 (1 H, ddd, J 5.7, 5.7 and 18.9, CHAHB), 2.07 (1 H, m, SiCH),
0.32 (6 H, s, SiMe2); dC(CDCl3) 190.7, 145.7, 138.2, 136.6, 133.8, 131.1,
129.3, 127.8, 116.6, 24.5, 23.9, 24.8 and 25.0 (Found: C, 74.2; H, 7.6.
C15H18OSi requires C, 74.3; H, 7.5%).
OMe
O–
OMe
O–
CO2Me
i
+
PhMe2Si
SiMe2Ph
8
ii
CO2Me
CO2Me
+
CO2Me
+
PhMe2Si
PhMe2Si
SiMe2Ph
9a
9c
9b
cis : trans 1: 3.1
(9a+9b+9c 70%, 9a:9b:9c 52:35:13)
CO2Me
Me
CO2Me
Me
iii
8
+
§ Selected data for 3b: Rf = 0.28 (CH2Cl2–hexane, 4:1); nmax(film)/cm21
1671(CNO), 1612 (conjugated CNC) and 1239 (C–Si); dH(CDCl3, 250
MHz) 7.50–7.26 (5 H, m, SiPh), 6.63 (1 H, ddd, J 1.25, 2.5 and 5.7,
CHNCCO), 5.75 (1 H, ddd, J 2.9, 5.2 and 9.6, CHNCHCHSi), 5.44 (1 H, br
dd, J 4.6 and 9.6, CHNCHCHSi), 3.23 (1 H, dt, J 3.6 and 5.7, CHSi), 2.73
(1 H, dddd, J 4.5, 4.7, 4.7 and 23, CHAHB, equatorial), 2.39 (1 H, dddd, J
22.9, 8.8, 2.6 and 2.6, CHAHB, axial), 2.24 (3 H, s, MeCO), 0.24 (6 H, s,
SiMe2); dC(CDCl3) 198.2, 140.8, 135.4, 134.5, 134.2, 129.4, 128.2, 127.8,
119.7, 30.03, 29.5, 25.6, 23.2 and 24.2 (Found: M+, 256.1280. C16H20OSi
requires M, 256.1283).
PhMe2Si
PhMe2Si
cis-10
trans-10
51% >95 : 5
iv
H
HO
H
OH
Me
Me
+
Me
Me
Si
PhMe2Si
Ph
cis-11
trans-11
References
Scheme 3 Reagents and conditions: i, ATPH, toluene, PhMe2SiLi, 278 °C,
30 min; ii, conc. HCl, 278 °C to room temp.; iii, MeOSO2CF3 (10 equiv.),
278 °C, 1 h; iv, DIBAL-H, 0 °C
1 E. W. Colvin, Silicon in Organic Synthesis, Butterworths, London, 1981,
ch. 9; W. P. Weber, Silicon Reagents for Organic Synthesis, Springer-
Verlag, 1983, ch. 11; I. Fleming, J. Dunogue`s and R. Smithers, Org.
React. (N.Y.), 1989, 37, 57.
2 T. K. Sarkar, Synthesis, 1990, 969 and 1101; I. Fleming, in Compre-
hensive Organic Synthesis, ed. B. M. Trost and I. Fleming, Pergamon,
Oxford, 1991, vol. 2, ed. C. H. Heathcock, ch. 2.2, pp. 563–593.
Allylsilanes derived by silylation of allyl-metal species: M. C. Henry and
J. G. Nortes, J. Am. Chem. Soc., 1960, 82, 555; V. F. Mironov and
V. V. Nepomnina, Izv. Akad. Nauk SSSR, Ser. Khim., 1960, 1419 (Chem.
Abstr., 1961, 55, 358); J.-P. Pillot, J. Dunogue`s and R. Calas,
Tetrahedron Lett., 1976, 1871. Allylsilanes derived from silylmethyl
anions: K. Ito, M. Fukui and Y. Kurachi, J. Chem. Soc., Chem. Commun.,
1977, 500; D. Seyferth, K. R. Wursthorn and R. E. Mammarella, J. Org.
Chem., 1977, 42, 3104. Allylsilanes derived by nucleophilic allylic
substitutions with silyl anions: I. Fleming and D. Marchi, Synthesis, 1981,
560; I. Fleming, D. Higgins, N. J. Lawrence and A. P. Thomas, J. Chem.
Soc., Perkin Trans. 1, 1992, 3331; 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. Allylsilanes from
b-silylcarbonyl compounds: I. Fleming, S. Gil, A. K. Sarkar and
T. Schmidlin, J. Chem. Soc., Perkin Trans. 1, 1992, 3351.
COR
COR
i, ii
+
12a >12%
14%
b
SiMe2Ph
12a R = H
b R = Me
13a 74%
b 64%
COMe
COMe
iv or iv, v
iii, ii
SiPh2
SiPh2
X
14 38%
15a X = F 71%
b X = OH 41%
3 K. Maruoka, M. Ito and H. Yamamoto, J. Am. Chem. Soc., 1995, 117,
9091; S. Saito and H. Yamamoto, Chem. Commun., in the press.
4 Preparation of silyl-lithium species: W. C. Still, J. Org. Chem., 1976, 41,
3063; H. Gilman and G. D. Lichtenwalter, J. Am. Chem. Soc., 1958, 80,
608; R. Balasubramanian and J. P. Oliver, J. Organomet. Chem., 1980,
197, C7; G. Gutekunst and A. G. Brook, J. Organomet. Chem., 1982, 225,
1.
Scheme 4 Reagents and conditions: i, ATPH, toluene, PhMe2SiLi, 278 °C;
ii, conc. HCl, 278 °C to room temp.; iii, ATPH, toluene, 2-methylbut-
2-enyldiphenylsilyllithium, 278 °C; iv, BF3·AcOH, CH2Cl2, 210 °C; v,
KF, H2O2, MeOH–THF, 0 °C
5 D. J. Ager, I. Fleming and S. K. Patel, J. Chem. Soc., Perkin Trans. 1,
1981, 2520; I. Fleming, T. W. Newton and F. Roessler, J. Chem. Soc.,
Perkin Trans. 1, 1981, 2527; I. Fleming, R. Roberts and S. C. Smith,
Tetrahedron Lett., 1996, 37, 9395.
6 H.-F. Chow and I. Fleming, J. Chem. Soc., Perkin Trans. 1, 1984, 1815;
I. Fleming, N. L. Reddy, K. Takaki and A. C. Ware, J. Chem. Soc., Chem.
Commun., 1987, 1472; I. Fleming and S. K. Ghosh, J. Chem. Soc., Chem.
Commun., 1994, 2285.
silyl fluoride 15a in 71% yield. Subsequent treatment under
Tamao’s conditions8 with potassium fluoride and hydrogen
peroxide in THF–MeOH produced hydroxy silane 15b in fair
yield, but not the naphthalene hydrate (Scheme 4). A method for
efficiently transforming the silyl group into a hydroxy group is
still under investigation.
7 I. Fleming and S. B. D. Winter, Tetrahedron Lett., 1993, 37, 7287; 1995,
36, 1733; I. Fleming and D. Lee, Tetrahedron Lett., 1996, 38, 6929.
8 K. Tamao, N. Ishida, T. Tanaka and M. Kumada, Organometallics, 1983,
2, 1694; K. Tamao and N. Ishida, J. Organomet. Chem., 1984, 269, C37;
K. Tamao, M. Kumada and K. Maeda, Tetrahedron Lett., 1984, 25, 321;
K. Tamao, N. Ishida, Y. Ito and M. Kumada, Org. Synth. (N.Y.), 1990, 69,
96.
We thank the Ministry of Culture, Science, Education and
Sports of Japan and the Basque Government for providing
financial support.
Footnotes
† E-mail: j45988a@nucc.cc.nagoya-u.ac.jp
‡ Typical experimental procedure: benzaldehyde (51 3 1023 cm3, 0.5
mmol) was added to a solution of ATPH (0.55 mmol) in toluene (4.0 cm3)
Received in Cambridge, UK, 21st April 1997; Com. 7/02684F
1300
Chem. Commun., 1997