The regioselectivity of 1,3-disubstituted allylmetal species towards electrophiles: 1-(Trimethylsilyl)alk-2-enylpotassium compounds

Article abstract of DOI:10.1055/s-1998-2060

1-(Trimethylsilyl)alk-2-enylpotassium species are readily generated by deprotonation of (alk-2-enyl)trimethylsilanes with the superbasic mixture of butyllithium and potassium tert-butoxide. Like the parent compound 1- (trimethylsilyl)allylpotassium, they too react with a variety of electrophiles preferentially, if not exclusively, at the silyl-distant terminus of the allyl moiety, thus producing branched enesilanes. The opposite regioselectivity previously observed with iodomethane hence appears to be an exception.

Full text of DOI:10.1055/s-1998-2060

May 1998
SYNTHESIS
707
The Regioselectivity of 1,3-Disubstituted Allylmetal Species Towards Electrophiles:
1-(Trimethylsilyl)alk-2-enylpotassium Compounds
Manfred Schlosser,* Livia Franzini
Institut de Chimie organique de l’Université, Bâtiment de Chimie (BCh), CH-1015 Lausanne-Dorigny, Switzerland
Received 16 October 1997
Abstract: 1-(Trimethylsilyl)alk-2-enylpotassium species are
readily generated by deprotonation of (alk-2-enyl)trimethylsilanes
with the superbasic mixture of butyllithium and potassium tert-
butoxide. Like the parent compound 1-(trimethylsilyl)allylpotas-
sium, they too react with a variety of electrophiles preferentially,
if not exclusively, at the silyl-distant terminus of the allyl moiety,
thus producing branched enesilanes. The opposite regioselectivity
previously observed with iodomethane hence appears to be an
exception.
Key words: 1-(trimethylsilyl)alk-2-enylpotassium species, super-
bases, regioselectivity, electrophilic reaction
1-(Trimethylsilyl)allyllithium and 1-(triphenylsilyl)allyl-
lithium can be used as versatile building blocks for the
construction of organic molecules, as was demonstrated in
pioneering work by Corriu,1 Chan,2 and Matteson3 among
others. Electrophiles were found to attack mainly, if not
exclusively, at the unsubstituted terminus of the allyl moi-
ety. The (E) configuration of the resulting enesilanes sug-
gests that the bulky trimethylsilyl moiety occupies the exo
position in the organolithium intermediate in analogy to
the well-documented behavior of the potassium analog.4
The present work was undertaken in order to extend this
investigation and, in particular, to find out how the nature
of the electrophile affects the regioselectivity of the inter-
ception step. To this purpose, chlorotrimethylsilane, fluo-
rodimethoxyborane, and aldehydes were selected as the
principal reagents. The parent member of the series, 1-
(trimethylsilyl)allylpotassium, served as the first model
compound. It gave the enesilanes 1a (78%), 2a (50%), 3a0
(69%), 3a2 (45%) and 3a6 (65%) as the sole identifiable
products.
Later, Taddei, Mordini and co-workers5 reported on the
superbase-promoted deprotonation and subsequent sub-
stitution of homologous allylsilanes. When they consecu-
tively treated pure (Z) and (E) isomers of (but-2-
enyl)trimethylsilane or (hex-2-enyl)trimethylsilane with
butyllithium in the presence of potassium tert-butoxide6
and with iodomethane, they obtained exclusively the
branched allylsilanes having originated from the attach-
ment of the methyl group to the silicon-bearing carbon at-
om. However, when 1-iodobutane was used as the
electrophile, the regioselectivity was lost, with mixtures
of the allylsilanes and the isomeric (E)-enesilanes in ratios
ranging from 1:1 to 4:1 being formed. The stereochemis-
try of the precursor was completely retained in the allylsi-
lanes as long as metalation times were kept short (30 min
at –50 °C). On the other hand, if the intermediate was left
for 24 hours at –50°C before being trapped, torsional
equilibrium was established. As expected,7 the endo-alkyl
exo-trimethylsilyl conformer proved to be the thermody-
namically much favored species.
The observed regioselectivity is noteworthy in two re-
spects. It applies not only to bulky, but also to small elec-
trophiles (such as fluorodimethoxyborane and monomeric
formaldehyde). Furthermore, benzaldehyde reacts unidi-
rectionally with 1-(trimethylsilyl)allylpotassium whereas
its lithium counterpart has been reported to produce a
83:17 mixture of enesilane- and allylsilane-type regioiso-
mers.8

708
Short Papers
SYNTHESIS
1H NMR: d = 6.02 (1 H, dt, J = 18.2, 7.7 Hz), 5.44 (1 H, dt, J = 18.2,
1.3 Hz), 1.63 (2 H, dd, J = 7.9, 1.2 Hz), 0.04 (9 H, s), 0.00 (9 H, s).
The intriguing question now was how these electrophiles
would behave towards medium- and long-chain 1-(tri-
methylsilyl)alk-2-enylpotassium species. Chlorotrimeth-
ylsilane reacted with 1-(trimethylsilyl)hex-2-enyl-
potassium and 1-(trimethylsilyl)oct-2-enylpotassium again
with excellent g-selectivity affording the bissilanes 1c
(75%) and 1d (78%), while 1-(trimethylsilyl)pent-2-
enylpotassium, after borylation and oxidation, selectively
(E)-Hex-1-ene-1,3-diylbis(trimethylsilane) (1c):
The same procedure as used for 1a was applied to (Z)-(hex-2-
enyl)trimethylsilane11 (3.9 g, 25 mmol); bp 95–97°C/25 Torr; nD20
1.4430; yield: 4.3 g (75%).
1H NMR: d = 5.83 (1 H, dd, J = 18.8, 8.7 Hz), 5.40 (1 H, dd, J =
18.7, 1.2 Hz), 1.6 (1 H, m), 1.4 (3 H, m), 1.2 (1 H, m), 0.87 (3 H, t,
gave the allyl alcohol 2b (50%). When 1-(trimethylsi- J = 6.8 Hz), 0.04 (9 H, s), –0.04 (9 H, s).
Anal.: calcd for C12H28Si2 (228.52) C 63.07, H 12.35; found C
63.54, H 12.19.
lyl)pent-2-enylpotassium, 1-(trimethylsilyl)hex-2-enyl-
potassium and 1-(trimethylsily])dodec-2-enylpotassium
were allowed to combine with benzaldehyde or formalde-
hyde, mixtures of regioisomers were obtained: 3b6 (61 %)
and 4b6 (small amounts; not characterized), 3c6 (68%) and
4c6 (10%; isolated as (1E,3Z)-1-phenylhepta-1,3-diene
after chromatography on silica gel9) and, respectively, 3e0
(46%) and 4e0 (31% ; isolated as (Z)-trideca-1,3-diene af-
ter chromatography on silica gel9). In other words, enesi-
lanes were no longer the exclusive, though still the
predominant, products. Thus, one has to consider the op-
posite regioselectivity of alkylation, in particular in the re-
action with iodomethane,5 as an exception.
Oct-1-ene-1,3-diylbis(trimethylsilane) (1d):
The same procedure as used for 1a was applied to (Z)-(oct-2-enyl)tri-
methylsilane (4.6 g, 25 mmol); bp 65–70°C/0.7 Torr; nD20 1.4335;
yield: 5.0 g (78%).
1H NMR: d = 5.82 (1 H, dd, J = 18.7, 8.8 Hz), 5.39 (1 H, dd, J =
l8.7, 1.1 Hz), 1.5 (1 H, m), 1.4 (2 H, m), 1.3 (6 H, m), 0.88 (3 H, t,
J = 6.8 Hz), 0.04 (9 H, s), –0.05 (9 H, s).
Anal.: calcd for C14H32Si2 (256.58) C 65.54, H 12.57; found C
65.51, H 12.42.
Trapping with Fluorodimethoxyborane Followed by Oxidation:
(E)-3-(Trimethylsilyl)prop-2-en-1-ol13 (2a):
Allyltrimethylsilanel11 (4.0 mL, 2.8 g, 25 mmol) in THF (25 mL) was
treated with BuLi and t-BuOK as described for 1a. At –75°C, fluo-
rodimethoxyborane–diethyl ether complexl4 (6.7 mL, 6.0 g,
36 mmol) was added to the red mixture followed, when the white
suspension formed had reached 25°C, by 30% aq H2O2 (5.7 mL,
56 mmol). After 12 h of vigorous stirring, sat. aq NH4Cl (20 mL)
was added. Repetitive extraction with Et2O (3 ´ 30 mL), washing of
the combined organic layers with brine (2 ´ 20 mL) and distillation
afforded a colorless liquid; bp 135–140°C; nD20 1.4412; yield: 1.6 g
(50%).
1H NMR: d = 6.17 (1 H, dt, J = 18.9, 4.4 Hz), 5.91 (1 H, dt, J = 18.9,
1.6 Hz), 4.16 (2 H, dd, J = 4.5, 1.5 Hz), 1.9 (1 H, s, broad), 0.07 (9
H, s).
(E)-1-(Trimethylsilyl)pent-1-en-3-ol15 (2b):
The same procedure as used for 2a was applied to (Z)-(pent-2-
enyl)trimethylsilane16 (3.5 g, 25 mmol); a colorless liquid was col-
lected upon distillation; bp 80–85 °C/25 Torr; nD20 1.4483 ; yield: 2.0 g
(50%).
1H NMR: d = 6.02 (1 H, dd, J = 19.0, 5.3 Hz), 5.84 (1 H, dd, J =
19.0, 1.1 Hz), 4.02 (1 H, sym. m), 1.55 (3 H, quint, J = 6.9 Hz), 0.91
(3 H, t, J = 7.0 Hz), 0.07 (9 H, s).
Regarding standard laboratory practice and formalities, see related
articles.10,11 1H NMR spectra were recorded of samples dissolved in
CDCl3 at 250 MHz (or at 400 MHz, if marked by an asterisk). The
preparation of (Z)-(hex-2-enyl)-, (Z)-(hept-2-enyl)-, (Z)-(oct-2-enyl)-,
(Z)-(non-2-enyl)-, (Z)-(dec-2-enyl)-, (Z)-(undec-2-enyl)-, and (Z)-
(dodec-2-enyl)trimethylsilane using a similar procedure has been
described in another context.11
Trapping with Aldehydes:
4-(Trimethylsilyl)but-3-en-1-ol15 (3a0):
Allyltrimethylsilane11 (4.0 mL, 2.8 g, 25 mmol) in THF (25 mL) was
treated with BuLi and t-BuOK as described for 1a. At –75 °C, 0.8 M
monomeric formaldehyde18 in THF (30 mmol) was added. The red
mixture slowly decolorized. After 1 h at 25 °C, the solvent was evap-
orated under reduced pressure and the residue neutralized with sat.
aq NH4Cl (20 mL). The organic material was extracted with Et2O (3
´ 30 mL), washed with brine (2 ´ 20 mL) and distilled; bp 50–55 °C/
2 Torr; nD20 1.4587; yield: 2.5 g (69%).
Trapping with Chlorotrimethylsilane:
(E)-Prop-1-ene-1,3-diylbis(trimethylsilane)12 (1a):
At –75°C, precooled THF (25 mL), allyltrimethylsilane11 (4.0 mL,
2.8 g, 25 mmol) and t-BuOK (3.1 g, 28 mmol) were consecutively
added to BuLi (28 mmol) from which the commercial solvent (hex-
ane) had before been stripped off. As soon as the alcoholate had dis-
solved under gentle shaking, the mixture was allowed to stand for 1 h
in a –50°C cold bath. Upon treatment with chlorotrimethylsilane
(3.5 mL, 3.0 g, 28 mmol), its bright red color disappeared immedi-
ately. The white suspension was concentrated and the residue dis-
tilled. A colorless liquid was collected; bp 48–50°C/10 Torr; nD20
1.4332; yield: 3.6 g (78%).
1H NMR*: d = 6.00 (1 H, dt, J = 18.6, 6.3 Hz), 5.78 (1 H, dt, J =
18.6, 1.4 Hz), 3.69 (2 H, t, J = 6.3 Hz), 2.39 (2 H, qd, J = 6.4,
1.4 Hz), 1.5 (1 H, s, broad), 0.06 (9 H, s).
(E)-6-(Trimethylsilyl)hex-5-en-3-ol17 (3a2):
This product was obtained in the same manner as described for 3a0
using propionaldehyde (2.0 mL, 1.6 g, 28 mmol) as the reagent; bp
55–60°C/0.05 Torr; nD20 1.4492; yield: 1.9 g (45%).
1H NMR: d = 6.02 (1 H, ddd, J = 18.6, 7.0, 6.6 Hz), 5.76 (1 H, d, J =

May 1998
SYNTHESIS
709
18.6 Hz), 3.57 (1 H, quint-like m, J ~ 8 Hz), 2.34 (1 H, dddd, J = Anal.: calcd for C16H34OSi (270.53) C 71.04, H 12.67; found C
14.0, 6.0, 4.3, 1.5 Hz), 2.18 (1 H, dtd, J = 14.0, 7.5, 1.0 Hz), 1.72 (1 71.28, H 12.93.
H, s, broad), 1.48 (2 H, quint d, J = 7.1, 2.0 Hz), 0.94 (3 H, t, J = 7.5
Hz), 0.05 (9 H, s).
This work was financially supported by the Schweizerische National-
fonds zur Förderung der wissenschaftlichen Forschung, Bern (grants
(E)-1-Phenyl-4-(trimethylsilyl)but-3-en-1-ol19 (3a6):
This product was obtained in the same manner as described for 3a0 20-29¢838-90 and 20-36¢385-92).
using benzaldehyde (2.8 mL, 3.0 g, 28 mmol); bp 75–80°C/0.2 Torr;
nD20 1.4582; yield: 3.5 g (65%).
1H NMR: d = 7.36 (4 H, d, J = 4.5 Hz), 7.3 (1 H, m), 6.04 (1 H, dt,
J = 18.6, 6.5 Hz), 5.81 (1 H, dt, J = 18.6, 1.3 Hz), 4.76 (1 H, dd, J =
7.9, 5.3 Hz), 2.56 (2 H, dd, J = 8.1, 5.4 Hz), 2.1 (1 H, s, broad), 0.07
(9 H, s).
(1) Corriu, R. J. P.; Masse, J. P. J. Organomet. Chem. 1973, 57, C5.
Corriu, R. J. P.; Guerin, C.; M’Boula, J. Tetrahedron Lett. 1981,
22, 2985.
(2) Koumaglo, K.; Chan, T. H. Tetrahedron Lett. 1984, 25, 717.
(3) Tsai, D. J. S.; Matteson, D. S. Tetrahedron Lett. 1981, 22, 2751.
(4) Schlosser, M.; Hartmann, J. J. Am. Chem. Soc. 1976, 98, 4674.
(5) Mordini, A.; Palio, G.; Ricci, A.; Taddei, M. Tetrahedron Lett.
1988, 29, 4991.
(6) Schlosser, M. J. Organomet. Chem. 1967, 8, 9.
Schlosser, M.; Strunk, S. Tetrahedron Lett. 1984, 25, 741.
Schlosser, M. Pure Appl. Chem. 1988, 60, 1627.
(7) Schlosser, M.; Desponds, O.; Lehmann, R.; Moret, E.; Rauch-
schwalbe, G. Tetrahedron 1993, 49, 10175.
(E)-2-Ethyl-1-phenyl-4-(trimethylsilyl)but-3-en-1-ol (3b6):
As described for 3a0, (Z)-(pent-2-enyl)trimethylsilane16 (3.5 g,
25 mmol) was consecutively treated with BuLi in the presence of t-
BuOK (both 28 mmol) and benzaldehyde (2.8 mL, 3.0 g, 28 mmol).
The mixture was neutralized with 2.3 M HCl in Et2O (ca. 25 mL),
absorbed on silica gel (25 mL) and transferred to a column filled with
more silica gel (0.15 L). Upon elution (Et2O/hexane 1:9) a yellow
liquid was collected consisting of two diastereoisomers in a 1:1 ratio,
which was purified by distillation; bp 85–90 °C/0.1 Torr; yield: 3.8 g
(61%).
(8) Yamamoto, Y.; Saito, Y.; Maruyama, K. Tetrahedron Lett.
1982, 23, 4597.
1H NMR*: d = 7.3 (5 H, m), 5.9 (0.5 ´ 2 H, m), 5.66 (0.5 ´ 1 H, dd,
J = 18.5, 7.5 Hz), 5.60 (0.5 ´ 1 H, d, J = 18.5 Hz), 4.59 (0.5 ´ 1 H, d,
J = 6.0 Hz), 4.41 (0.5 ´ 1 H, d, J = 7.7 Hz), 2.30 (0.5 ´ 1 H, sym. m),
2.2 (1 H, m), 1.64 (0.5 ´ 1 H, dqd, J = 12.0, 6.5, 3.3 Hz), 1.25 (1 H,
sym. m), 0.84 (0.5 ´ 3 H, t, J = 6.6 Hz), 0.72 (0.5 ´ 3 H, t, J =
6.6 Hz), 0.11 (0.5 ´ 9 H, s), 0.01 (0.5 ´ 9 H, s).
See also: Lau, P. W. K.; Chan, T. H. Tetrahedron Lett. 1978,
2383.
(9) Acid-catalyzed anti elimination of trimethylsilanol from b-hy-
droxysilanes:
Hudrlik, P. F.; Peterson, D. Tetrahedron Lett. 1974, 1133.
Hudrlik, P. F.; Peterson, D. J. Am. Chem. Soc. 1975, 97, 1464.
Hudrlik, P. F.; Peterson, D.; Rona, R. J. J. Org. Chem. 1975, 40,
2263.
Anal.: calcd for C15H24OSi (248.44) C 72.52, H 9.74; found C 72.01,
H 9.94.
(E)-1-Phenyl-2-propyl-4-(trimethylsilyl)but-3-en-1-ol (3c6) and (10) Schlosser, M.; Dahan, R.; Cottens, S. Helv. Chim. Acta 1984,
(Z)-1-Phenyl-2-(trimethylsilyl)hept-3-en-1-ol (4c6): 67, 284.
In the same way as for 3b6, reaction between (Z)-(hex-2- (11) Desponds, O.; Franzini, L.; Schlosser, M. Synthesis 1997, 150.
enyl)trimethylsilane11 (3.9 g, 25 mmol) and benzaldehyde (2.8 mL, (12) Dunoguès, J.; Calas, R.; Ardoin, N.; Biran, C. J. Organomet.
3.0 g, 28 mmol) was performed. According to the NMR spectrum,
Chem. 1971, 32, C31.
Tubul, A.; Santelli, M. Tetrahedron 1988, 44, 3975.
the crude mixture contained two regioisomers (3c6 and 4c6).20 It was
submitted to chromatographic purification as described for 3b6. (13) Stork, G.; Jung, M. E.; Calvin, E.; Noel, Y. J. Am. Chem. Soc.
(1E,3E)-1-Phenylhepta-1,3-diene21 (0.4 g; 10% ; from 4c6 by acid-
catalyzed elimination of trimethylsilanol) and 3c6 were isolated.
According to GC (30 m DB-1, 190 °C; 30 m DB-Wax, 190°C), the
1974, 96, 3684.
Vedejs, E.; Engler, D. A.; Mullins, M. J. J. Org. Chem. 1977,
42, 3109.
latter product was composed of two diastereoisomers in a 1:1 ratio; (14) Rauchschwalbe, G.; Schlosser, M. Helv. Chim. Acta 1975, 58,
bp 105–110°C/0.5 Torr; yield: 4.5 g (68%).
1094.
1H NMR: d = 7.3 (5 H, m), 5.85 (0.5 ´ 2 H, d-like, sym. m), 5.6 (0.5
´ 2 H, m), 4.61 (0.5 ´ 1 H, d, J = 3.5 Hz), 4.40 (0.5 ´ 1 H, d, J =
4.5 Hz), 2.3 (1 H, m), 2.20 (0.5 ´ 1 H, s), 2.09 (0.5 ´ 1 H, s), 1.4 (1
H, m), 1.2 (2 H, m), 0.87 (0.5 ´ 3 H, t, J = 7.0 Hz), 0.80 (0.5 ´ 3 H, t,
J = 7.0 Hz), 0.10 (0.5 ´ 9 H, s).
To prepare fluorodimethoxyborane–diethyl ether complex
(1.00 mol), one has simply to mix BF3·Et2 (42 mL, 47 g,
0.33 mol), trimethyl borate (75 mL, 70 g, 0.67 mol) and Et2O
(70 mL, 50 g, 0.67 mol).
(15) Kishi, N.; Maeda, T.; Mikami, K.; Nakai, T. Tetrahedron 1992,
48, 4087.
(16) Lappert, M. F.; Nile, T. A.; Takahashi, S. J. Organomet. Chem.
1974, 72, 425.
Anal.: calcd for C16H26OSi (262.42) C 73.22, H 9.98; found C 73.09,
H 10.12.
(E)-2-Nonyl-4-(trimethylsilyl)but-3-en-1-ol (3e0) and 2-(Trimeth- (17) Hsiao, C. N.; Shechter, H. Tetrahedron Lett. 1983, 24, 2371.
ylsilyl)tridec-3-en-1-ol (4e0): (18) Schlosser, M.; Jenny, T.; Guggisberg, Y. Synlett 1990, 704.
In the same way as for 3b6, reaction between (Z)-(dodec-2- (19) Tsai, D. J. S.; Matteson, D. S. Organometallics 1983, 2, 236.
enyl)trimethylsilane11 (6.0 g, 25 mmol) and formaldehyde was per- (20) The regioisomer 4c6, contaminated with small amounts of 3c6,
formed. The crude mixture (consisting of 3e0 and 4e0) was submitted to
chromatographic purification, as described for 3b6, to give, besides (Z)-
trideca-1,3-diene22 (1.4 g; 31%; from 4e0 by acid-catalyzed elimination
of trimethylsilanol), the alcohol 3e0; nD20 1.4560; yield: 3.1 g (46%).
1H NMR: d = 5.8 (2 H, m), 3.53 (1 H, dd-like m, J ~ 11, 7 Hz), 3.41
(1 H, t-like m, J ~ 9.5 Hz), 2.23 (1 H, s-like m, broad), 1.4 (1 H, m),
1.26 (16 H, s), 0.88 (3 H, t, J = 7.0 Hz), 0.07 (9 H, s).
was separately prepared by the addition of dimethyl (Z)-hex-2-
enylboronate to benzaldehyde: Franzini, L. Ph. D. thesis, Uni-
versity of Lausanne, 1996, pp 29 and 88–89.
(21) Other stereoisomers: Inoue, Y.; Toyofuku, M.; Hashimoto, H.
Bull. Chem. Soc. Jpn. 1986, 59, 1279.
Tsukamoto, M.; Schlosser, M. Synlett 1990, 605.
(22) Ikeda, Y.; Yamamoto, H. Bull. Chem. Soc. Jpn. 1986, 59, 657.