A substance that contains platinum and has a catalytic function is called a platinum catalyst. Platinum is a silvery-white shiny metal with good ductility, thermal conductivity and electrical conductivity. Platinum is chemically inert and stable in air and humid environments. When heated below 450 ℃, a platinum dioxide film is formed on the surface. Platinum reacts with sulfur, phosphorus, and halogens at high temperatures. The oxidation state of platinum is +2, +3, +4, +5, +6. The valence of platinum in a common platinum catalyst (PtCl2, PtCl4) is usually +2 or +4. Platinum readily forms a catalytically active coordination compound with a ligand. For example, [Pt(NH3)2]Cl2 and [PtCl2(CH2=CH2)]2 are catalytically active platinum coordination compounds. Platinum is rarely present in the earth's crust, so platinum catalysts are often expensive.
Platinum catalysts are widely used in the field of organic synthesis because of their various types, good catalytic activity and environmental friendliness.
- Hydrosilylation reaction: The hydrosilylation reaction refers to the addition reaction between an organic or inorganic Si-H bond and unsaturated multiple bonds, which can form a Si-C bond to synthesize an organosilicon compound having a graft functional group on a carbon atom. Hydrosilylation is one of the important methods to produce silicone products. Hydrosilylation catalysts currently used in industrial applications are primarily homogeneous platinum catalysts, namely Speier catalysts and Karstedt catalysts. Supported platinum catalysts are also occasionally used to catalyze hydrosilylation reactions. For example, the preparation of a supported platinum catalyst using H2PtCl6 as a platinum source can catalyze the hydrosilylation reaction of an olefin with trimethoxysilane.
Figure 1. Speier catalysts and Karstedt catalysts
- Asymmetric hydrogenation: Asymmetric catalytic hydrogenation is one of the important methods for the preparation of chiral compounds. Platinum catalysts have many advantages, such as good atom economy, environmental friendliness, ease of industrialization and recyclability of the catalyst, and are therefore often used as catalysts for asymmetric catalytic hydrogenation. Typically, an unmodified platinum catalyst catalyzes an asymmetric hydrogenation reaction to obtain a racemization product, and a chiral modifier-modified platinum catalyst catalyzes an asymmetric hydrogenation reaction to obtain a chiral compound. For example, in a toluene solution, the chiral modifiers cinchonidine, quinine and 9-phenoxyoctylamine modified Pt/Al2O3 catalyst can catalyze the asymmetric hydrogenation of the α-ketoester substrate to give α-hydroxy ester product. Platinum catalysts are commonly used to catalyze the asymmetric hydrogenation of organic compounds such as alpha-ketoesters, beta-ketoesters and alpha, beta-unsaturated carboxylic acids.
Figure 2. Platinum catalyst catalyzed asymmetric hydrogenation
- Cycloaddition reaction: The cycloaddition reaction is one of the important methods for synthesizing monocyclic and polycyclic compounds. As a transition metal element, metal platinum has a special Lewis acid property in addition to the properties of a general transition metal, and thus a platinum catalyst can be used to catalyze a cycloaddition reaction. The cycloaddition reaction catalyzed by platinum catalysts has the advantages of high atomic utilization rate, mild reaction condition, high stereospecificity and high yield. In addition to catalyzing the common [4+2] cycloaddition reaction, the platinum catalyst can also catalyze the [3+2] cycloaddition reaction, the [2+2+2] cycloaddition reaction, the [3+2+2] ring addition reaction, and [4+2+2] cycloaddition reaction. For example, PtCl2 can catalyze an intramolecular [3+2] cycloaddition reaction of a 1,2,4-triene compound having a propadiene structure to form a polysubstituted 1,3-cyclopentadiene compound in high yield. [PtCl2(CH2=CH2)]2 can catalyze the [4+3] cycloaddition reaction of a 2-substituted alkynylamine derivative with 1,3-butadiene-2-trialkylsilyl ether.
Figure 3. Platinum catalyst catalyzed cycloaddition reaction
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