298
LETTER
Rhodium-catalyzed Addition of Aryl- and Alkenylsilanediols to Aldehydes
R
hodium-cataly
o
zed Addition
s
of Aryl-
h
and
A
lkenyl
i
silaned
n
iols ari Fujii, Tooru Koike, Atsunori Mori,* Kohtaro Osakada
Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Yokohama 226-8503, Japan
Fax +81(45)9245224; E-mail: amori@res.titech.ac.jp
Received 14 November 2001
Abstract: Arylation and alkenylation of aromatic aldehydes with
silanediols is shown to proceed by use of a catalytic amount of rhod-
ium complex. Treatment of ethyl(4-methoxyphenyl)silanediol with
benzaldehyde in the presence of 3 mol% of [Rh(OH)(cod)]2 affords
4-methoxyphenyl(phenyl)methanol in 59% yield after stirring at
70 °C for 24 hours. On the other hand, diarylketone was also ob-
tained at elevated temperature via -hydride elimination from inter-
mediary rhodium alkoxide.
Key words: silanediol, rhodium complex, arylation, alkenylation,
Equation
aldehyde
DME, or 1,4-dioxane was found to be excellent, while use
of water as co-solvent resulted in no reaction by contrast
to the related reaction with boronic acid.3a–c A protic sol-
vent, MeOH was also found to be uneffective. Aromatic
aldehydes bearing an electron-donating or -withdrawing
group 2b–e both afforded the corresponding secondary al-
cohols in better yields than benzaldehyde (2a). The reac-
tion of ethyl(4-methoxyphenyl)silanediol (1) with various
aromatic aldehydes also resulted in giving good yields. In
addition to arylsilanediols, the use of alkenylsilanediol 4
afforded the corresponding allylic alcohol in 79% yield.
On the other hand, the reaction with aliphatic aldehydes
(2h) was found to be unsuccessful.
Nucleophilic addition of aryl- and alkenyl-metal reagents
to carbonyl compounds has been a fundamental reaction
in organic synthesis. By contrast to the highly reactive or-
ganometallic nucleophilic reagents such as lithium or
magnesium, use of an organosilicon reagent has rarely
been studied so far.1–3 We have recently been studying the
synthetic application of silanols, a new class of organosil-
icon reagent bearing hydroxy groups on the silicon atom,
and revealed that the carbon-silicon bond of silanols suc-
cessfully affect several carbon-carbon bond-forming reac-
tions by the catalysis of a transition metal complexes.4
Indeed, we reported that organosilicon reagents bearing
hydroxy groups on the silicon atom affect Mizoroki–Heck
(MH) type reaction and conjugate addition to , -unsatur-
ated carbonyl compounds in the presence of a catalytic
amount of a rhodium complex.5 Hence, we envisaged the
addition of such aryl- and alkenylsilanediols to aldehydes
with the rhodium catalyst.
Oi and Inoue recently reported that aryldifluorosilanes
also reacted with aldehyde in the presence of KF as an ac-
tivator.2 By contrast, the present rhodium-catalyzed reac-
tion with silanediol proceeded without additive,
suggesting superior reactivity of silanediols to fluorosi-
lanes. The difference would be caused by the electron do-
nation of the oxygen atom to the d-orbital of silicon in
addition to the negative inductive effect of oxygen. Such
electronic effects would result in activating the carbon-sil-
icon bond of silanediol to facilitate transmetalation of the
organic group toward the rhodium catalyst. A related sub-
stituent effect of the hydroxy group, was also proposed re-
cently by Li in the rhodium-catalyzed reactions of
organostannanes.6
The reaction of ethyl(4-methoxyphenyl)silanediol (1)
with benzaldehyde (2a) was carried out in the presence of
3 mol% of [Rh(OH)(cod)]2 in THF at 70 °C, which condi-
tions were similar to those for the MH-type reaction of
, -unsaturated carbonyl compounds.5 The correspond-
ing secondary alcohol was obtained in 59% yield after
stirring for 24 hours as shown in the Equation.
The table summarizes the results on the rhodium-cata-
lyzed reactions of several aldehydes and silanediols. The
reaction with twice molar amounts of silanediol to alde-
hyde was found to be optimum. A cationic rhodium com-
plex, [Rh(cod)(CH3CN)2]BF4 similarly catalyzed the
addition reaction. However, addition of several phos-
phines as a ligand of the rhodium complex inhibited the
reaction. A polar and non-aqueous solvent such as THF,
It should be pointed out that a considerable amount of di-
arylketone was obtained along with the desired secondary
alcohol when the reaction was carried out at 100 °C in 1,4-
dioxane as shown in the Scheme. The findings suggest
that an addition-elimination reaction takes place as ob-
served in the Mizoroki–Heck-type reaction of , -unsat-
urated carbonyl compounds.7 Since no ketone was
obtained in the reaction at 70 °C in 1,4-dioxane at all, the
pathway to ketone would be the influence of a higher re-
action temperature. The formation of diarylketone was
particularly remarkable in the reaction with a silanediol
Synlett 2002, No. 2, 01 02 2002. Article Identifier:
1437-2096,E;2002,0,02,0298,0300,ftx,en;Y23201ST.pdf.
© Georg Thieme Verlag Stuttgart · New York
ISSN 0936-5214