A tandem allylsilane–vinylsilane difunctionalization by silylcupration
of allene followed by reaction with a,b-unsaturated nitriles
Asunción Barbero, Yolanda Blanco and Francisco J. Pulido*
Departamento de Química Orgánica, Universidad de Valladolid, 47011 Valladolid, Spain.
E-mail: pulido@qo.uva.es
Received (in Cambridge, UK) 17th April 2001, Accepted 10th July 2001
First published as an Advance Article on the web 9th August 2001
Silylcupration of allene using phenyldimethylsilylcopper 1
followed by BF3-mediated reaction with a,b-unsaturated
nitriles at 240 °C affords allylsilane–vinylsilane-containing
ketones resulting from consecutive addition (1,2 and 1,4) of
the intermediate allyl- and vinylcopper species formed in the
silylcupration of allene.
Me2CuCNLi2 slowly produced a poor yield of the methyl
ketone resulting from 1,2 addition.
Silylcupration of allene (2 eq., following our usual protocol1)
followed by addition of BF3·Et2O (2 equiv.) and subsequent
slow addition of nitriles 6–9 (1 eq., THF, 240 to 0 °C) afforded,
after hydrolysis at 0 °C, the ketones 10–13, which seem to be
produced by addition of the vinylsilane–allylcopper species 3 to
the nitrile group and conjugate addition of the allylsilane–
vinylcopper species 2 to the b position (Table 1).7 The reaction
(one pot) is clean and high yielding. Monoaddition products
were not found. Remarkably, when this reaction is carried out
with one equivalent each of allene and 1, BF3 and nitrile, the
same diaddition products were formed, and almost half of the
nitrile was wasted, which suggests that the second stage of the
overall addition occurs quickly. It should be also noted that
silylcupration of allene and further reaction with 8, 9 (nitrile–
organocopper; 1+2) at 0 °C, in the absence of BF3, gives
exclusively the 1,2-adducts 14 and 15 (Table 1).
We recently reported1–3 that addition of phenyldimethyl-
silylcopper (PhMe2SiCu·LiCN, 1) to propa-1,2-diene leads to a
silylated copper intermediate, which can be imprecisely formu-
lated as a mixture of the allylsilane–vinylcopper species 2 and
the vinylsilane–allylcopper species 3. The reversibility of the
reaction is one of its features. Quenching of the intermediate
copper species with different electrophiles results in formation
of compounds of type 4 or 5 (Scheme 1).
The reaction is temperature dependent and the presence of
species 2 and 3 can be demonstrated by protonation of the
reaction mixture at different temperatures. Thus, at 240 °C (or
lower temperature) only formation of 4 (E = H) was observed,
whereas 5 (E = H) was the major product obtained near to
0 °C.
The order in which the two steps (1,2 and 1,4) of addition take
place and the mechanistic pathway are not certain. Weiberth and
Hall8 have proposed the intervention of Cu(III) intermediates in
the copper-activated reaction of benzonitriles with Grignard
reagents. Although we defer any definitive statement about the
mechanism, it seems feasible—in view of the absence of
1,4-monoaddition products—that the reaction proceeds by
initial addition of species 3 to the nitrile group followed by BF3-
catalyzed conjugate addition of species 2 to the intermediate
a,b-unsaturated ketimine (Scheme 2). The chemoselectivity
observed and the preference for the attack of 3 to the nitrile and
of 2 to the conjugate position can be related to the different
hardness of the nucleophilic species.
In general, for most of the reported work1 the reactive species
at low temperature is 2. Thus, at 240 °C, a,b-unsaturated
ketones or aldehydes react with 2 giving exclusively 1,4-addi-
tion. However, at temperatures around 240 °C, simple ketones
are unreactive toward species 2, but they are readily attacked by
3 as the temperature increases to 0 °C, giving hydroxymethyls
bearing the vinylsilane moiety.1 In this case, conversion of
species 2 into species 3 seems to occur rapidly as the
temperature increases, which probably accounts for the ob-
served result.
We now report an interesting and unusual tandem diaddition-
hydrolysis process by successive addition (1,2 and 1,4) of
species 3 and 2 to a,b-unsaturated nitriles. Protonation of the
intermediate adduct, gives products that have both allylsilane
and vinylsilane functionality.
Reaction of a,b-unsaturated nitriles with organocopper
compounds has not been widely explored. Yamamoto4 reported
that the reaction of a,b-unsaturated nitriles with RCu·BF3 was
not satisfactory; yields were low, and mixtures of mono-
alkylated and dialkylated products were obtained. Alexakis
et al.5 showed that introduction of an additive like TMSCl in the
reaction of Me2CuLi with some a,b-unsaturated nitriles gave
the dialkylated ketone, whereas Lipshutz et al.6 found that
A completely different pattern of reaction is observed when
the bulky tert-butyldiphenysilyl group is used instead of
PhMe2Si. Silylcupration of propa-1,2-diene with tert-BuPh2Si-
Cu·LiCN9 followed by addition of BF3 and reaction with 6 and
8—under the same conditions used before—affords selectively
the 1,4-adducts 16 and 17 as the only products (Table 1). We
have noted before1 that species of type 2, but carrying the bulky
tert-butyldiphenysilyl group, are regiochemically stable at any
temperature between 240 °C and 0 °C.
The one-pot tandem diaddition-hydrolysis process reported
here could be of importance in synthetic work since the
allylsilane and vinylsilane moieties are versatile synthons for
organic synthesis. Thus, the diadducts undergo selective
intramolecular allylsilane terminated cyclization,1 while the
vinylsilane unit remains unchanged, as shown in Scheme 3 with
the conversion of 10–12 into 18–20.
In summary, silylated organocopper reagents, like those used
in this work, react well with a,b-unsaturated nitriles, giving
Scheme 1
Scheme 2
1606
Chem. Commun., 2001, 1606–1607
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b103387p
Barbero
