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C. Ferreri et al. / Journal of Organometallic Chemistry 554 (1998) 135–137
Table 1
Table 2
Et3SiH/PdCl2 mediated conversion of alcohols to halidesa
Et3SiH/PdCl2 mediated conversion of alcohols to alkanes
ROHb
Halogenating
agentb
Time (h)/T,
(°C)
RX yieldc
(%)
ROH
Methoda Time (h)/T (°C)
RH yieldb, (%)
PhCH2OH
2-Octanol
1-Decanol
A
B
A
B
A
B
3/25
16/25
4/25
1/25
24/25
1/60
98
98
98
98
90
80
PhCH2OH
CH3I
0.3/25
1/25
5/60
1/25
3/25
24/60
0.6/25
1.5/25
28/60
24/25
23/60
48/60
24/25
23/60
72/60
2/60
98
98
CH2Br2
CCl3CCl3
CH3I
CH2Br2
CCl3CCl3
CH3I
CH2Br2
CCl3CCl3
CH3I
CH2Br2
CCl3CCl3
CH3I
CH2Br2
CCl3CCl3
CH3I
CH2Br2
CCl3CCl3
94d
83d
91
PhCHꢀCHCH2OH
(CH3)3COH
c-C6H11OH
1-Decanol
86
98
98
80
a CH3I and CH2Br2 were used in methods A and B, respectively.
b Determined by GC analysis on the basis of product formation in the
crude reaction mixture using an internal standard.
98
89d
80
Et SiH
3
RX+PdRPdX Et3SiX+Pd+RH
(2)
90
81
13
Upon alcohol addition, silyl ethers are initially formed
prior to the appearance of the halides4. Trialkylsilyl
bromides and iodides are known to effect the transforma-
tion of alcohols to bromides and iodides [14,15], whereas
trimethylsilyl chloride needs some activation [16–18]. On
the other hand, trialkylsilyl iodides are efficient for the
conversion of silyl ethers to iodides whereas the halo-
genating ability of bromo trimethylsilane is quite poor
[15]. Therefore, the presence of metallic Pd in the reaction
mixture may also play a role in the second part of this
process.
1,4-Butanediol
96d
90d
80d,e
23/60
24/60
a PdCl2 (2 mol%).
b ROH:(hal.agent):Et3SiH=1:1:1.4.
c Determined by GC analysis on the basis of product formation in the
crude reaction mixture using an internal standard.
d 2.8 equiv. of Et3SiH.
e Products refer to a 1:2:5 ratio of 1-triethylsilyloxy-4-chlorobutane:4-
chloro-1-butanol:tetrahydrofurane.
The transformation of alcohol to the corresponding
alkane was also achieved. Following the above protocol,
as soon as the alcohol was transformed into its halide
(followed by GC), another portion of Et3SiH was added.
This methodology provided the transformation of alco-
hols to alkanes in an one-pot reaction with very good
yields. The results of the reduction process are given in
Table 2 [12]. It is quite clear that the reduction of
intermediate halides took place with Et3SiH mediated by
metallic Pd in a fashion similar to that illustrated in Eq.
(2).
ondary iodides and bromides in good yields with the
exception of 1-decanol and cyclohexanol, which gave
poor yields of the corresponding chlorides and 1,4-bu-
tanediol which could be chlorinated only at one position,
the other alcoholic function being mainly transformed
into its corresponding silylether. Additional experiments
showed that the reaction proceeds with inversion of
configuration. For example, optically pure (+)-2-oc-
tanol was converted to (−)-2-octyl iodide with an optical
purity of 87.3% [13].
A mechanistic picture of these reactions can be deduced
by following the reaction course. The identification by
GC of small amounts of Et3SiCl in the reaction with CH3I
and CH2Br2 as well as the formation of metallic Pd and
the H2 evolution suggest the following initial step:
These preliminary results account for the efficiency and
flexibility of the palladium catalyzed transformation of
alcohols in the presence of organosilicon hydrides. It
should also be noted that the absence of solvent in these
reactions is beneficial from an environmental point of
view [19].
2Et3SiH+PdCl22Et3SiCl+Pd+H2
(1)
GC analysis of the mixtures prior to the alcohol addition
revealed the in situ formation of the triethylsilyl halide
in quantitative yield. The propagation steps for these
reactions (Eq. (2)) may involve oxidative addition of the
halogenating agent (RX) to Pd followed by reduction of
the palladium complex by triethylsilane in analogy with
the initiation step3.
References
[1] W.P. Weber, Silicon Reagents for Organic Synthesis, Springer-
Verlag, Berlin, 1983.
[2] I. Ojima, in: S. Patai, Z. Rappoport (Eds.), The Chemistry of
Organosilicon Compounds, Wiley, Chichester, 1989, Chapter 15,
pp. 1479–1526.
3 An alternative mechanism [10], in which the silane first adds
oxidatively to the Pd followed by exchange between the silyl and alkyl
groups, cannot be ruled out at present.
4 Quantitative formation of silyl ether was obtained in the presence
of a base such as pyridine or triethylamine.