Silylcupration of 1,3-dienes followed by an electrophilic trapping reaction

Article abstract of DOI:10.1021/ol015910f

(matrix presented) Silylcupration reaction of 1,3-dienes with a cyanocuprate reagent, PhMe₂SiCuCNLi, followed by an electrophilic trapping has been reported for the first time. The use of allylic phosphates as electrophiles resulted in a highly

Full text of DOI:10.1021/ol015910f

ORGANIC
LETTERS
2001
Vol. 3, No. 12
1861-1864
Silylcupration of 1,3-Dienes Followed by
an Electrophilic Trapping Reaction
Vilnis Liepins and Jan-E. Ba1ckvall*
Department of Organic Chemistry, Arrhenius Laboratory, Stockholm UniVersity,
SE-10691 Stockholm, Sweden
jeb@organ.su.se.
Received March 30, 2001
ABSTRACT
Silylcupration reaction of 1,3-dienes with a cyanocuprate reagent, PhMe2SiCuCNLi, followed by an electrophilic trapping has been reported for
the first time. The use of allylic phosphates as electrophiles resulted in a highly regioselective reaction with overall 1,4-addition of the silyl
and allyl moieties across the diene.
Silylcupration reaction of acetylenes and allenes has been
studied by Fleming and co-workers.1,2 Scarce literature data,
however, is available on silyl-metalations of alkenes. Silyl-
manganation of acetylenes and 1,3-dienes3 and catalytic
silicon-silicon bond addition to double bonds4 and 1,3-
dienes5 involving silylpalladation have been reported. How-
ever, to the best of our knowledge, there are no publications
in the literature on silylcupration of dienes. Recently, we
reported on a silylcupration reaction of substituted and
unsubstituted styrene.6 These results encouraged us to study
the silylcupration reaction of 1,3-dienes.
Most of the silylcupration reactions have been done with
the silylcuprate reagent (PhMe2Si)2CuLi‚LiCN or more bulky
silylcuprate reagents.2d Recently, cyanocuprate reagent PhMe2-
SiCuCNLi7 has been used by Pulido8 and us9 in allene
silylcupration reactions. The advantage of the silylcopper
reagent PhMe2SiCuCNLi over the disilyl cuprate reagent
(PhMe2Si)2CuLi‚LiCN is that it makes use of all the silyl
on copper. With the latter reagent only one of the two groups
is transferred and the remaining silyl copper reagent may
lead to side products upon reaction with the electrophile
employed.
Here we wish to report for the first time on a silylcupration
reaction of 1,3-dienes. The copper intermediate obtained can
be efficiently trapped by different electrophiles, including
allylic phosphates,10 which are readily accessible allylic
substrates for metal-catalyzed reactions.6,9,11
(1) (a) Fleming, I.; Roessler, F. J. Chem. Soc., Chem. Commun. 1980,
276. (b) Fleming, I.; Newton, T. W.; Roessler, F. J. Chem. Soc., Perkin
Trans. 1 1981, 2527. (c) Fleming, I.; Martinez de Marigorta, E. J. Chem.
Soc., Perkin Trans. 1 1999, 889. (d) Barbero, A.; Pulido, F. J. Recent Res.
DeV. Synth. Org. Chem. 1999, 2, 1.
(2) (a) Fleming, I.; Pulido, F. J. J. Chem. Soc., Chem. Commun. 1986,
1010. (b) Cuadrado, P.; Gonzalez, A. M.; Pulido, F. J.; Fleming, I.
Tetrahedron Lett. 1988, 29, 1825. (c) Cuadrado, P.; Gonzalez-Nogal, A.
M.; Pulido, F. J.; Fleming, I.; Rowley, M. Tetrahedron 1989, 45, 413. (d)
Barbero, A.; Cuadrado, P.; Gonzalez, A. M.; Pulido, F. J.; Fleming, I. J.
Chem. Soc., Perkin Trans. 1 1991, 2811. (e) Fleming, I.; Landais, Y.;
Raithby, P. R. J. Chem. Soc., Perkin Trans. 1 1991, 715.
(3) (a) Fugami, K.; Nakatsukasa, S.; Oshima, K.; Utimoto, K.; Nozaki,
H. Chem. Lett. 1986, 6, 869. (b) Fugami, K.; Oshima, K.; Utimoto, K.;
Nozaki, H. Tetrahedron Lett. 1986, 27, 2161. (c) Fugami, K.; Hibino, J.;
Nakatsukasa, S.; Matsubara, S.; Oshima, K.; Utimoto, K.; Nozaki, H.
Tetrahedron 1988, 44, 4277.
(4) Hayashi, T.; Kobayashi, T.; Kawamoto, A. M.; Yamashita, H.;
Tanaka, M. Organometallics 1990, 9, 280.
(5) (a) Tamao, K.; Okazaki, S.; Kumada, M. J. Organomet. Chem. 1978,
146, 87. (b) Matsumoto, H.; Shono, K.; Wada, A.; Matsubara, I.; Watanabe,
H.; Nagai, Y. J. Organomet. Chem. 1980, 199, 185. (c) Sakurai, H.; Eriyama,
Y.; Kamiyama, Y.; Nakadaira, Y. J. Organomet. Chem. 1984, 264, 229.
(d) Carlson, C. W.; West, R. Organometallics 1983, 2, 1801.
(6) Liepins, V.; Ba¨ckvall, J.-E. Chem. Commun. 2001, 265.
(7) (a) Fleming, I. In Organocopper Reagents. A Practical Approach;
Taylor, R. J. K., Ed.; Oxford University Press: New York, 1994; Chapter
12, p 257. (b) Fleming, I.; Roberts, R. S.; Smith, S. C. J. Chem. Soc., Perkin
Trans. 1 1998, 1209.
(8) (a) Blanco, F. J.; Cuadrado, P.; Gonzalez, A. M.; Pulido, F. J.;
Fleming, I. Tetrahedron Lett. 1994, 35, 8881. (b) Barbero, A.; Garcia, C.;
Pulido, F. J. Tetrahedron Lett. 1999, 40, 6649. (c) Barbero, A.; Garcia, C.;
Pulido, F. J. Tetrahedron 2000, 56, 2739.
(9) Liepins, V.; Karlstro¨m, A. S. E.; Ba¨ckvall, J. E. Org. Lett. 2000, 2,
1237.
10.1021/ol015910f CCC: $20.00 © 2001 American Chemical Society
Published on Web 05/17/2001

Table 1. Silylcupration of 1,3-Dienes Followed by Trapping with “Reactive” Electrophiles
a Isolated yields. b >97% Z-isomer for 5b, and E-isomer in the case of 5d. Stereochemistry established by NOESY.
1
Silylcupration of 1,3-dienes with 1 proceeds smoothly at
temperatures ranging from -78 to 0 °C.12 The resulting
copper intermediates 2 and 3 were allowed to react with
various electrophiles to give products 4 and 5. The ratio
between products 4 and 5 depends very much on the
electrophile employed. For example with more reactive
electrophiles the regioisomer 4 predominates, and protonation
of the isoprene adduct afforded 4a and 5a in a ratio of 85:
15 (entry 1, Table 1). Acetyl chloride quenching of the
isoprene adduct gave 4b and 5b in a ratio of 63:37. The use
of dienes 7 and 8 in the silylcupration followed by quenching
with water gave 4c and 5c in a ratio of 60:40 and 4d and 5d
in a ratio of 70:30, respectively.
Z-isomer, and 5d the E-isomer. In both cases H NMR
indicated that the selectivity was greater than 97%.
The electrophilic trapping reaction of allylic cuprates 2
and 3 with reactive electrophiles such as a proton (H2O) or
CH3COCl is expected to proceed through direct electrophilic
attack on the allyl moiety to form the product. As with other
allylmetal species, reaction of 2 and 3 with these more
reactive electrophiles can occur in two ways: it may proceed
in an SE2 and/or an SE2′ fashion leading to R- and/or
γ-cleavage, respectively. Thus, the ratio 4:5 will depend on
the eletrophile employed. We believe that there is only slow
equilibration between 2 and 3. This is supported by the
observation that when isoprene was reacted with silylcopper
reagent 1 at different temperatures and quenched with water,
the ratio between 4a and 5a was constant (Table 1, entries
1-3).
The stereochemistry of products 5b and 5d was determined
by NOESY experiments. Product 5b was found to be the
(10) (a) Yamamoto, H.; Nomura, N.; Yanagisawa, A. Tetrahedron 1994,
50, 6017. (b) Knochel, P.; Jubert, C.; Tucker, E. C.; Nowotny, S. J. Org.
Chem. 1995, 60, 2762.
(11) Allylic phosphates have previously been used as electrophiles in a
copper-catalysed cross-coupling reaction with Grignard reagents: (a)
Yanagisawa, A.; Noritake, Y.; Nomura, N.; Yamamoto, H. Synlett 1991,
251. (b) Yanagisawa, A.; Nomura, N.; Yamamoto, H. Synlett 1993, 689.
(c) Yanagisawa, A.; Nomura, N.; Yamamoto, H. Tetrahedron 1994, 50,
6017.
A direct 1,2-silylcupration of one of the double bonds of
the diene in a concerted addition would give adduct 2
(Scheme 1). To explain formation of adduct 3 one would
Scheme 1
(12) Typical Procedure. A total of 0.65 mmol of PhMe2SiLi (∼1 M
solution in THF) was added to a stirred suspension of CuCN (1 equiv) in
dry THF (0.65 mL) at 0 °C and stirred at this temperature for 30 min.
Then the mixture was cooled to -40 °C, isoprene (1.2 equiv) was added
dropwise, and the reaction mixture was stirred for 1 h at this temperature.
The temperature was lowered to -78 °C, and allylic phosphate 11 (1.2
equiv) in 0.6 mL of THF was added slowly over 30 min. After the reaction
mixture had been stirred for 1 h at -78 °C, 4 mL of saturated aqueous
NH4Cl was added, and the aqueous phase was extracted with pentane (5 ×
3 mL). Column chromatography on silica with pentane as eluent afforded
compound 5f as a colorless oil in 71% yield. Rf ) 0.43 (hexane). 1H NMR
(CDCl3 400 MHz): δ 7.54 (m, 2H), 7.38 (m, 3H), 5.20 (t, J ) 8.5 Hz,
1H), 4.71 (s, 1H), 4.69 (s, 1H), 2.10 (m, 2H), 2.00 (m, 2H), 1.75 (s, 3H),
1.72 (m, 3H), 1.68 (d, J ) 8.5 Hz, 2H), 0.31 (s, 6H).13C NMR (CDCl3
75.4 MHz): δ 146.4, 139.3, 133.8, 133.6, 129.0, 127.8, 120.0, 109.6, 36.0,
30.2, 23.5, 22.7, 17.6, -3.0.
1862
Org. Lett., Vol. 3, No. 12, 2001

have to assume isomerization of 2 to 3. Recently, we
proposed6 that silylcupration of styrene proceeds via a
mechanism involving nucleophilic attack by the silylcopper
reagent 1 at the terminal carbon of the double bond in
analogy with a conjugate addition. A similar mechanism has
been proposed for the carbocupration of acetylenes.13 It is
likely that the analogous mechanism operates for silyl-
cupration of 1,3-dienes, which would involve copper(III)-
intermediate 9.14 Intermediate 9, formed by addition of
reagent 1 to the terminal carbon of the diene, would undergo
reductive elimination to give a mixture between the two
copper intermediates 2 and 3 (Scheme 2).
Table 2. Silylcupration of 1,3-Dienes Followed by Trapping
with “Less Reactive” Electrophilesa
Scheme 2
Reaction of cuprate adducts 2 and 3 with less reactive
electrophiles such as allylic phosphates, allylic phosphinates,
and MeI gave only the regioisomer of type 5 where the
electrophile has attacked the terminal carbon (Table 2). Thus,
when allylic phosphates 10, 11, 12, and 13 were used to
trap the silylcupration adduct from isoprene (6), products
5e, 5f, 5g, and 5h, respectively, were obtained in high
regioselectivity (>98%, entries 1-4). Also phosphinate 14
and MeI gave the same regioselectivity with the isoprene
silylcupration adduct to give 5i and 5j, respectively (entries
5 and 6). Some other dienes, 7 and 8, were used in the
silylcupration-cleavage reaction employing the “less reactive”
electrophiles. Also with these dienes only one regioisomer
was formed (entries 7-10, Table 2).
The high regioselectivity obtained with less reactive
electrophiles (Table 2) can be explained by a copper(III)-
intermediate, which is formed after oxidative addition of the
electrophile, e.g., allylic phosphate, to 2 and 3 (Scheme 3).
The two copper(III)-intermediates 15 and 16 produced would
be in equilibrium with one another. Since the copper atom
in the formal oxidation state +3 is very electrophilic, it tends
to form π-allyl complexes of type 17 with the available allyl
fragments in the molecule. Thus, the equilibration proceeds
through a 16e complex 17. This process would result in a
thermodynamically more stable copper(III)-intermediate 16,
which on reductive elimination would give the 1,4-addition
product 5e.
a Reactions were carried out according to the experimental procedure
given.12 b Isomers 5e, 5f, 5g, 5i were of >97% Z-stereochemistry; 5m and
5n were E-isomers. The stereochemistry was established by NOESY.
c Isolated yields. d The E and Z isomers could not be separated (ratio
approximately 1:1 in all cases).
phosphate can do so. This can be seen from the different
regiochemistry of the allylic substitution (Table 2, entries 3
and 4). In entry 3 (Table 2) only the γ-substitution product
Not only can the diene allyl fragment equilibrate via a
π-allyl complex, but also the allyl fragment of the allylic
(13) Nakamura, E.; Mori, S. Angew. Chem., Int. Ed. 2000, 39, 3750 and
references therein.
(15) (a) Posner, G. H. Org. React. 1975, 22, 253. (b) Lipshutz, B. H.;
Sengupta, S. Org. React. 1992, 41, 135. (c) Ba¨ckvall, J. E.; Sellen, M.;
Grant, B. J. Am. Chem. Soc. 1990, 112, 6615. (d) Magid, R. M. Tetrahedron
1980, 36, 1901. (e) Goering, H. L.; Kantner, S. S. J. Org. Chem. 1984, 49,
422. (f) Underiner, T.; Goering, H. L. J. Org. Chem. 1991, 56, 2563 and
preceding papers in this series. (g) Ba¨ckvall, J. E.; Persson, E. S. M.;
Bombrun, A. J. Org. Chem. 1994, 59, 4126. (h) Persson, E. S. M.; Ba¨ckvall,
J. E. Acta Chem. Scand. 1995, 49, 899. (i) Persson, E. S. M.; van Klaveren,
M.; Grove, D. M.; Ba¨ckvall, J. E.; van Koten, G. Chem. Eur. J. 1995, 1,
351. (j) Karlstro¨m, A. S. E.; Ba¨ckvall, J. E. Chem. Eur. J. 2001, 7, 1981.
(14) (a) Woodward, S. Chem. Soc. ReV. 2000, 29, 393. (b) Kingsbury,
C. L.; Smith, R. A. J. J. Org. Chem. 1997, 62, 4629. (c) Uerdingen, M.;
Krause, N. Tetrahedron 2000, 56, 2799. (d) Dorigo, A, E.; Wanner, J.;
Schleyer, P. von R. Angew. Chem., Int. Ed. Engl. 1995, 34, 476. (e)
Nakamura, E.; Yamanaka, M. J. Am. Chem. Soc. 1999, 121, 8941. (f)
Nakamura, E.; Mori, S.; Morokuma, K. J. Am. Chem. Soc. 1998, 120, 8273.
(g) Nakamura, E.; Yamanaka, M.; Mori, S. J. Am. Chem. Soc. 2000, 122,
1826.
Org. Lett., Vol. 3, No. 12, 2001
1863

reaction. All attempts to silylcuprate 1,3-hexadiene, 1,3-
cyclohexadiene or 4-methyl-1,3-pentadiene failed and did not
give any reaction product. We think that this is due to the
electron-releasing effect of the alkyl groups that hinder the
nucleophilic attack of the silylcuprate reagent on the terminal
carbon atom of the 1,3-diene.
Scheme 3
Some allylic phosphates are unstable and difficult to
prepare.16 We have demonstrated that allylic phosphinates17
from an allylic alcohol that failed to give the corresponding
phosphate can be used as a more stable allyl-donor alternative
to allylic phosphates (entry 5, Table 2).
In conclusion, the silylcupration of 1,3-dienes followed
by electrophilic trapping is reported for the first time. With
the use of allylic electrophiles the highly regioselective
overall 1,4-carbosilylation of the conjugated diene constitutes
a novel approach toward silylated 1,5-diene systems. The
reaction provides an extension to the widely studied silyl-
cupration reaction of other unsaturated compounds.
Acknowledgment. Financial support from the Swedish
Natural Science Research Council and The Swedish Founda-
tion for Strategic Research is gratefully acknowledged. We
thank Prof. A. Alexakis for fruitful discussions.
5g is formed, whereas in entry 4 only the R-substitution
product 5h is obtained. According to the generally accepted
mechanism for copper-catalyzed allylic substitution,13,15 the
allylic cuprate (2 or 3) would attack the γ-position of the
allylic substrate to give a new σ-allyl on copper. The latter
allyl can then undergo equilibration on copper so that only
the terminal allyl is formed in both cases (5g and 5h).
However, only 1,3-dienes with nonsubstituted terminal
double bonds turned out to be reactive in the silylcupration
OL015910F
(16) (a) Miller, J. A.; Wood, H. C. S. Angew. Chem. 1964, 76, 301. (b)
Yasui, K.; Fugami, K.; Tanaka, S.; Tamaru, Y. J. Org. Chem. 1995, 60,
1365.
(17) McCallum, J. S.; Liebeskind, L. S. Synthesis 1993, 819.
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Org. Lett., Vol. 3, No. 12, 2001