26
J. Oliver–Meseguer and A. Leyva–Pérez
in high yield and selectivity with ppm amounts of Pt catalyst or with other metal
catalysts including Rh and Ir for the β–(E) product (the most thermodynamically
stable) and Ru and Ir for the β–(Z)–vinylsilane. In order to get the α-vinylsilane, Pt 3
clusters were made to catalyze the Markovnikov hydrosilylation of terminal alkynes
to afford a wide variety of new α-vinylsilanes in good isolated yields using low
amounts of catalyst [59]. These clusters can be formed in situ with < 100 ppm of
simple Pt compounds or with externally added Pt “Chini” clusters [41].
In order to prove that the Pt clusters were the active catalysts and not the precursors, UV–Vis absorption spectroscopy measurements were performed during reaction at 110 °C, in which the α-isomer is formed. These measurements showed the
appearance of new bands at approximately 300 nm, and the fluorescence spectrum
confirmed the expected emission band for Pt 3–4 (ca. 360 nm according to the Jellium
model) when the α product was predominant. In contrast, only the plasmonic band
of Pt nanoparticles was observed under β-favored reaction conditions, that is, reaction temperatures <70° C, without any absorption or emission band corresponding
to sub-nanometer clusters. These results supported the formation of Pt 3–4 clusters
during the hydrosilylation reaction at 110 °C with 0.005 mol% of Kardstedt’s catalyst, without nanoparticle formation, and indicate that Pt clusters are the species
needed for the α isomer formation. These results were confirmed also by ElectroSpray–Ionization Quadrupole Time-Of-Flight Mass Spectrometry (ESI–QTOF) and
zeta potential measurements. Moreover, the Chini clusters [8] [NEt 4 ] 2 [Pt 3 (CO) 6 ] 3 ,
Na 2 [Pt 3 (CO) 6 ] 5 and Na 2 [Pt 3 (CO) 6 ] 10 showed the production of α-vinylsilanes under
classical (anti-Markovnikov) reaction conditions. The absence of induction time at
the beginning in the reaction and the presence of the Pt 3 UV–VIS signals during
the reaction confirmed that the Pt 3 clusters are the active catalysts for obtaining the
α-isomer.
Considering the reactivity trends of the Pt 3 clusters, we can discuss the origin
of the inverse selectivity toward the hydrosilylation with respect to the traditional
Pt compounds. The Chalk–Harrod and a modified Chalk–Harrod mechanism are
presented in Fig. 1.25, which is the most accepted mechanism for hydrosilylation
of alkynes and alkenes catalyzed by Pt [42, 64]. Once the active catalyst is formed,
the mechanism consists of four steps: (1) alkyne coordination to the Pt; (2) silane
oxidative addition; (3) migratory insertion in Chalk–Harrod and silylplatination for
modified Chalk–Harrod and (4) reductive elimination. Based on this mechanism, the
key step that determines the regioselectivity of the reaction is the migratory insertion,
which proceeds through hydroplatination for the former and silylplatination for the
latter mechanism.
1.3.2.4 Andrussow Reaction (Cyanide Synthesis, C–N)
Hydrocyanic acid HCN is a very versatile molecule produced in multi-ton amounts
using the Andrussow process. This process consists of an endothermic reaction
between methane (CH 4 ) or carbon monoxide (CO) with ammonia (NH 3 ) at temperatures higher than 500 °C using Pt–Rh gauze catalysts in flow conditions to obtain
J. Oliver–Meseguer and A. Leyva–Pérez
in high yield and selectivity with ppm amounts of Pt catalyst or with other metal
catalysts including Rh and Ir for the β–(E) product (the most thermodynamically
stable) and Ru and Ir for the β–(Z)–vinylsilane. In order to get the α-vinylsilane, Pt 3
clusters were made to catalyze the Markovnikov hydrosilylation of terminal alkynes
to afford a wide variety of new α-vinylsilanes in good isolated yields using low
amounts of catalyst [59]. These clusters can be formed in situ with < 100 ppm of
simple Pt compounds or with externally added Pt “Chini” clusters [41].
In order to prove that the Pt clusters were the active catalysts and not the precursors, UV–Vis absorption spectroscopy measurements were performed during reaction at 110 °C, in which the α-isomer is formed. These measurements showed the
appearance of new bands at approximately 300 nm, and the fluorescence spectrum
confirmed the expected emission band for Pt 3–4 (ca. 360 nm according to the Jellium
model) when the α product was predominant. In contrast, only the plasmonic band
of Pt nanoparticles was observed under β-favored reaction conditions, that is, reaction temperatures <70° C, without any absorption or emission band corresponding
to sub-nanometer clusters. These results supported the formation of Pt 3–4 clusters
during the hydrosilylation reaction at 110 °C with 0.005 mol% of Kardstedt’s catalyst, without nanoparticle formation, and indicate that Pt clusters are the species
needed for the α isomer formation. These results were confirmed also by ElectroSpray–Ionization Quadrupole Time-Of-Flight Mass Spectrometry (ESI–QTOF) and
zeta potential measurements. Moreover, the Chini clusters [8] [NEt 4 ] 2 [Pt 3 (CO) 6 ] 3 ,
Na 2 [Pt 3 (CO) 6 ] 5 and Na 2 [Pt 3 (CO) 6 ] 10 showed the production of α-vinylsilanes under
classical (anti-Markovnikov) reaction conditions. The absence of induction time at
the beginning in the reaction and the presence of the Pt 3 UV–VIS signals during
the reaction confirmed that the Pt 3 clusters are the active catalysts for obtaining the
α-isomer.
Considering the reactivity trends of the Pt 3 clusters, we can discuss the origin
of the inverse selectivity toward the hydrosilylation with respect to the traditional
Pt compounds. The Chalk–Harrod and a modified Chalk–Harrod mechanism are
presented in Fig. 1.25, which is the most accepted mechanism for hydrosilylation
of alkynes and alkenes catalyzed by Pt [42, 64]. Once the active catalyst is formed,
the mechanism consists of four steps: (1) alkyne coordination to the Pt; (2) silane
oxidative addition; (3) migratory insertion in Chalk–Harrod and silylplatination for
modified Chalk–Harrod and (4) reductive elimination. Based on this mechanism, the
key step that determines the regioselectivity of the reaction is the migratory insertion,
which proceeds through hydroplatination for the former and silylplatination for the
latter mechanism.
1.3.2.4 Andrussow Reaction (Cyanide Synthesis, C–N)
Hydrocyanic acid HCN is a very versatile molecule produced in multi-ton amounts
using the Andrussow process. This process consists of an endothermic reaction
between methane (CH 4 ) or carbon monoxide (CO) with ammonia (NH 3 ) at temperatures higher than 500 °C using Pt–Rh gauze catalysts in flow conditions to obtain