With its impressive features, gold has led to completely new reaction types in recent years, which in turn have strongly influenced both organic catalysis and material science. Other fields where a significant amount of new results has been obtained include nanotechnology, self assembly/supramolecular systems and biochemical/medicinal chemistry. As a result, gold is one of the hottest topics in catalysis at the moment, with an increasing amount of research being carried out in this field. While focusing on homogeneous catalysis, this monograph also covers the main applications in heterogeneous catalysis. Following a look at the gold-catalyzed addition of heteroatom nucleophiles to alkynes, it goes on to discuss gold-catalyzed additions to allenes and alkenes, gold-catalyzed benzannulations, cycloisomerization and rearrangement reactions, as well as oxidation and reduction reactions. The whole is finished off with a section on gold-catalyzed aldol and related reactions and the application of gold-catalyzed reactions to natural product synthesis. Of interest to synthetic chemists and inorganic chemists, as well as organic chemists working in homogeneous catalysis, physical and technical chemists.
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In recent years, there has been a revolution in the chemistry of gold. Until recently, gold had been considered to be less reactive than its neighbors in the periodic table (Figure 1.1) owing to it being the most noble of metals, having the highest standard electrode potential. Indeed, until very recently, there had been the perception that gold would be unreactive in catalytic applications. As gold does not chemisorb O2 as a bulk metal, it was considered that gold could not be effective in redox applications. As this was the perceived wisdom, then it is unsurprising that the exceptional catalytic activity of gold lay undiscovered for so long. For it is only when gold is prepared in nanoparticulate form or as soluble complexes that its high activity is observed.
Figure 1.1 Examples of commercial catalysts using elements near gold in the periodic table.
The traditional view of gold as an unreactive immutable metal has inspired great works of art and literature. In the late nineteenth century Kipling wrote about the relative roles of the coinage metals (gold, silver and copper) and iron in society:
āGold is for the mistress ā silver for the maid ā
Copper for the craftsman, cunning at his trade.
āGood!ā said the Baron, sitting in his hall,
āBut iron ā cold iron ā is ruler of them allā.ā
from Cold Iron
Rudyard Kipling
At that time, gold was used mainly in coinage and in artwork, whereas iron had underpinned the industrial revolution and was at the heart of the manufacturing industry. It is no wonder that iron was the prized commercial metal. This traditional view has now been overturned and currently there is an amazingly rich chemistry of catalysis based on gold nanoparticles and complexes. It is over 25 years since the prediction was made that gold would be the best catalyst for the hydrochlorination of acetylene. Since then, there has been a phenomenal growth in gold-based reactions of acetylenes and a very rich field of homogeneous catalysis based on gold has been established. To date, gold is still the best catalyst for acetylene hydrochlorination, in addition to being the catalyst of choice for the low-temperature oxidation of carbon monoxide. In this chapter, we introduce the reactions of gold and acetylenes and set out in greater detail the discovery and chemistry of gold as a catalyst for acetylene hydrochlorination.
1.2 Reactions of Alkynes Using Gold Chloride as Catalyst
The discovery that cationic gold supported on carbon is a highly effective catalyst for hydrochlorination of acetylene to vinyl chloride led over the years to a surge of applications of gold salts involving alkyne-based substrates [1, 2]. These include hydrofluorination [3], hydration [4, 5], hydroxylation [6], and hydrocarboxylation [7] reactions, and also hydroamination [8] and reactions over alkynes containing proximate oxygen nucleophiles [9]. Some of these reactions will be briefly described in this section; however, it should be noted that one of the first uses of Au3+ salts for catalytic purposes can be traced back to Thomas and co-workers in 1976 [10] in the oxidation of phenylacetylene (1) in aqueous methanol in presence of HAuCl4, which gave the ketone 2 as the major product, followed by the ether 3 and the chlorinated vinyl adduct 4 (Scheme 1.1).
Scheme 1.1 Reaction of phenylacetylene (1) with tetrachloroauric acid in aqueous methanol. The ketone 2 is the major product, which is obtained by Markovnikov addition. Minor amounts of methyl vinyl ethers (3) and vinyl chlorides (4) are obtained.
However, despite a turnover number of
6 in this reaction, the role of gold was merely assigned as a reagent and not as a catalyst. It was almost 10 years later, in 1985 [11], that the role of cationic gold was recognized in full as it was the best catalyst for the hydrochlorination reaction of acetylene (5) to vinyl chloride monomer (VCM) (6) and can be considered as a milestone for revealing cationic gold as an effective catalyst (Scheme 1.2).
Scheme 1.2 Hydrochlorination of acetylene (5) to vinyl chloride monomer (6). The reaction occurs at 180 Ā°C and is catalyzed by an Au/C catalyst on which the active sites are Au3+ centers.
Soon afterwards, in 1987, the intramolecular addition of amines to triple bonds (7) to yield cyclic adducts (8) with NaAuCl4 as catalyst was reported [12] (Scheme 1.3).
Scheme 1.3 Intramolecular addition of amine 7 to cyclic adduct 8, using sodium tetrachloroaurate as the catalyst.
Addition of alcohols to alkynes has also been successfully achieved [13], as shown by the cyclization of (Z)-3-ethylallyl alcohols (9), which can cyclize to furans (10 and 11) (Scheme 1.4).
Scheme 1.4 Cyclization of (Z)-3-ethylallyl alcohols (9) to furans, with R5 = hydrogen to yield 10 or R5 = alkyl to yield 11.
Moreover, if two alcohol functional groups are present, such as in 12, it is even possible to obtain bicyclic ketals such as 13 by means of AuCl or AuCl3 (Scheme 1.5) [14].
Scheme 1.5 Intramolecular reaction from the alcohol 12 to the bicyclic ketal 13. The reaction can be catalyzed by AuCl or AuCl3.
The interest in the latter reaction also relies on the circumstance that Au3+ is active towards carbonācarbon triple bonds, but not towards carbonācarbon double bonds. Analogies can be found also in the hydrochlorination reaction of acetylene, where Au/C-based catalysts are unreactive towards ethylene [15], making them suitable for selective chemical synthesis.
1.3 The Correlation of EĀ° with the Activity of Gold for the Hydrochlorination of Acetylene
1.3.1 The Initial Correlation
In this section, we demonstrate how the activity of gold can be correlated with the standard electrode potential (EĀ°) of the metal used to carry out the hydrochlorination reaction and how this correlation can predict gold to be the ...
Table of contents
Cover
Further Reading
Title Page
Copyright
List of Contributers
Chapter 1: Hydrochlorination of Acetylene Catalyzed by Gold
