350
Hydrosilanes easily undergo catalytic oxidation reactions (Eq. 14.5) (Marciniec
1974, 1975). These reactions are catalyzed by Cu
+
salts, which heve been formed by
reduction of Cu
2+
salts with hydrosilanes (Marciniec and Tran 1981a, b). H-silanes
are oxidized to silanols in the presence of O 2 and AgClO 4 , while in the absence of
oxygen the reduction of Ag
+
to Ag(0) occurs. Peracids (Eaborn 1960; Sommer et al.
1972) and dioxiranes (Adam et al. 1990; Lu et al. 1993) also oxidize hydrosilanes
to silanols. Using H 2 O 2 the tertiary silanes heve also been oxidized to silanols
(Limnios and Kokotos 2013).
R 3 Si-H
R 3 Si-OH
[O]
(14.5)
The hydrosilanes were quite often used in organic synthesis as reducing agents
(Weber 1996), e.g. ketones are readily reduced with PMHS in the presence of
bis(dibutylacetoxytin) oxide or Pd supported on C (Pd/C). PMHS forms insoluble
POSSs, which are removed from the reaction medium through filtration (Lipowitz
and Bowman 1973; Keinan and Greenspoon 1983). PMHS are safe and inexpensive
reagents, compared to LiAlH 4 and other metal hydrides. In the presence of ZnH 2 or
other metal hydrides, PMHS selectively reduces aldehydes, esters, ketones and lactones to alcohols (Mimoun 1999). In the presence of titanocene catalysts, activated
with n-BuLi, PMHS reduces carboxylic esters to alcohols (Barr et al. 1994).
Similarly, the reduction of carboxylic acids and esters occurs in the presence of
PMHS and Ti or Zr alkoxides (Breeden and Lawrence 1994; Reding and Buchwald
1995). The reduction of amines occurs enantioselectively in the presence of transition metal (Ni, Rh) complexes, containing chiral ligands (Bourhis and Frainnet
1975; Kohra et al. 1988). The nitro compounds are also reduced with PMHS to
amines, and nitryls to aldoimines (German Pat. 1,079,600, 1960), which under
hydrolysis conditions form aldehydes (Fry and Ott 1981).
By reduction of silver acetate with PMHS in toluene solution, silver nanoparticles (NPs) have been formed in the presence of trioctylamine catalyst (Chauhan and
Sardar 2004). Similarly, by reduction of palladium(II) acetate (Pd(OAc) 2 ) with
PMHS, stable powdered palladium-polysiloxane NPs have been prepared and isolated. The PMHS has also acted as a reducing agent and as a protective material for
the preparation and stabilization of catalytically active Pd-NPs. Pd nanoclusters
stabilized with polysiloxane in situ or ex situ have been used for chemoselective
hydrogenation of functional conjugated alkenes. Under moderate reaction conditions, high yields of reduction products have been achieved. Catalyst poisoning
experiments have unequivocally revealed the role of Pd nanoclusters encapsulated
in polysiloxane as the actual catalytic species (Chauhan et al. 2004).
Polysiloxane-encapsulated soluble Pd nanoclusters have also been used as catalyst for grafting alcohols onto PMHS. With primary, secondary, sterically bulky
tertiary and functional alcohols, selective alcoholysis of PMHS has been produced with good to excellent yields under moderate reaction conditions (Chauhan
et al. 2005). The esterification reactions of PMHS with carboxylic acids have also
been catalyzed by polysiloxane- stabilized Pd colloids generated in situ by reduction
J. J. Chruściel
Hydrosilanes easily undergo catalytic oxidation reactions (Eq. 14.5) (Marciniec
1974, 1975). These reactions are catalyzed by Cu
+
salts, which heve been formed by
reduction of Cu
2+
salts with hydrosilanes (Marciniec and Tran 1981a, b). H-silanes
are oxidized to silanols in the presence of O 2 and AgClO 4 , while in the absence of
oxygen the reduction of Ag
+
to Ag(0) occurs. Peracids (Eaborn 1960; Sommer et al.
1972) and dioxiranes (Adam et al. 1990; Lu et al. 1993) also oxidize hydrosilanes
to silanols. Using H 2 O 2 the tertiary silanes heve also been oxidized to silanols
(Limnios and Kokotos 2013).
R 3 Si-H
R 3 Si-OH
[O]
(14.5)
The hydrosilanes were quite often used in organic synthesis as reducing agents
(Weber 1996), e.g. ketones are readily reduced with PMHS in the presence of
bis(dibutylacetoxytin) oxide or Pd supported on C (Pd/C). PMHS forms insoluble
POSSs, which are removed from the reaction medium through filtration (Lipowitz
and Bowman 1973; Keinan and Greenspoon 1983). PMHS are safe and inexpensive
reagents, compared to LiAlH 4 and other metal hydrides. In the presence of ZnH 2 or
other metal hydrides, PMHS selectively reduces aldehydes, esters, ketones and lactones to alcohols (Mimoun 1999). In the presence of titanocene catalysts, activated
with n-BuLi, PMHS reduces carboxylic esters to alcohols (Barr et al. 1994).
Similarly, the reduction of carboxylic acids and esters occurs in the presence of
PMHS and Ti or Zr alkoxides (Breeden and Lawrence 1994; Reding and Buchwald
1995). The reduction of amines occurs enantioselectively in the presence of transition metal (Ni, Rh) complexes, containing chiral ligands (Bourhis and Frainnet
1975; Kohra et al. 1988). The nitro compounds are also reduced with PMHS to
amines, and nitryls to aldoimines (German Pat. 1,079,600, 1960), which under
hydrolysis conditions form aldehydes (Fry and Ott 1981).
By reduction of silver acetate with PMHS in toluene solution, silver nanoparticles (NPs) have been formed in the presence of trioctylamine catalyst (Chauhan and
Sardar 2004). Similarly, by reduction of palladium(II) acetate (Pd(OAc) 2 ) with
PMHS, stable powdered palladium-polysiloxane NPs have been prepared and isolated. The PMHS has also acted as a reducing agent and as a protective material for
the preparation and stabilization of catalytically active Pd-NPs. Pd nanoclusters
stabilized with polysiloxane in situ or ex situ have been used for chemoselective
hydrogenation of functional conjugated alkenes. Under moderate reaction conditions, high yields of reduction products have been achieved. Catalyst poisoning
experiments have unequivocally revealed the role of Pd nanoclusters encapsulated
in polysiloxane as the actual catalytic species (Chauhan et al. 2004).
Polysiloxane-encapsulated soluble Pd nanoclusters have also been used as catalyst for grafting alcohols onto PMHS. With primary, secondary, sterically bulky
tertiary and functional alcohols, selective alcoholysis of PMHS has been produced with good to excellent yields under moderate reaction conditions (Chauhan
et al. 2005). The esterification reactions of PMHS with carboxylic acids have also
been catalyzed by polysiloxane- stabilized Pd colloids generated in situ by reduction
J. J. Chruściel