Catalyzed Reactions of Alkenes

Various reactions of alkenes are surveyed separately in several chapters. Pd(II)-promoted oxidative reactions of alkenes are treated in Chapter 2.2 and Pd(0)-catalyzed reactions of alkenes with organic halides are discussed in Chapter 3.2. Other Pd(0)-catalyzed reactions of alkenes are discussed in this chapter. 

The oxidative carbonylation of alkenes, which is promoted by Pd(II), is described in Chapter 2.2.7. The Pd(0)-catalyzed carbonylation reactions, which are mechanistically different from the oxidative carbonylation, are treated in this section. 

Several Pd complexes have been claimed to be highly active catalysts. The linear ester 4 was obtained as the main product along with the branched ester 3 as the minor product by the carbonylation of 1-octene using [Pd(MeCN)2(PPh3)2](BF4)2 

Palladium Reagents and Catalysts—New Perspectives for the 21st Century J. Tsuji 2004 John TOey Sons, Ltd ISBNs 0-470-85032-9 HB) 0-470-85033-7 (PB) 

The formation of branched esters suggests the potential for asymmetric carbonylation. Only a few successful asymmetric carbonylations are known. Zhou and co-workers reported the highly successful asymmetric carbonylation of styrene. They obtained the branched ester 6 with 99.3 % ee and high regioselectivity of the branched ester using the Alper catalyst, PdCl2-CuCl2 in the presence of the chiral monophosphine DDPPI 9 in methyl ethyl ketone [6]. 

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Palladium-catalyzed reactions of alkenes containing nitrogen nucleophiles have proven to be a powerful methodology for C—N bond formation leading to pyrroles. 

The oxo synthesis (hydroformylation), which was discovered by Otto Roelen in 1938 at Ruhrchemie AG, is one of the most important metal-complex-catalyzed reactions of alkenes. Today, the worldwide capacity for oxo products is about 6 million tons/year. 

Caiboxylate ions are also effective nucleophiles in palladium-catalyzed reactions of alkenes and several classes of lactones including 7-pyrones (equation 11)17 and isocoumarins (equation 12)18 have been made in this manner. These early studies used stoichiometric amounts of palladium salts, since efficient redox systems had not yet been developed. However, with more modem techniques catalysis in these systems should be relatively straightforward. The more recent catalytic cyclization-caibonylation process in equation (13) is indicative of this.19... 

An alternate route to formation of alkyl monolayers is via Lewis acid catalyzed reactions of alkenes with the hydrogen terminated surface. In this approach, a catalyst such as ethyl aluminum dichloride is used to mediate the hydrosilylation reaction of an alkene (or alkyne), resulting in the same type of product as in the case of the photochemical or thermal reactions. This type of reaction is well known based on molecular organosilane chemistry and has also been used successfully to alkylate porous silicon [31]. Although this route has been shown to work on H/Si(lll), the resulting monolayers are found to have lower coverages than those achieved using the photochemical or thermal approach [29], Another concern with this approach is the possibility of trace metal residues from the catalyst that could adversely affect the electronic properties of these surfaces (even when present at levels below the detection limit of most common surface analysis techniques). 

Acid-Catalyzed Synthesis. The acid-catalyzed reaction of alkenes with hydrogen sulfide to prepare thiols can be accomplished using a strong acid (sulfuric or phosphoric acid) catalyst. Thiols can also be prepared continuously over a variety of solid acid catalysts, such as zeolites, sulfonic acid-containing resin catalysts, or aluminas (22). The continuous process is utilized commercially to manufacture the more important thiols (23,24). The acid-catalyzed reaction is commonly classed as a Markownikoff addition. Examples of two important industrial processes are 2-methyl-2-propanethiol and 2-propanethiol, given in equations 1 and 2, respectively. 

Coordinated alkenes and alkynes undergo two kinds of reactions substitution reactions and nucleophilic additions to the coordinated ligand. While a relahvely few examples of nucleophihc attack exist for stable Pd -alkene complexes, a far larger number of examples can be inferred from the palladium-catalyzed reactions of alkenes. 

Vicinal hydroxy carbamates are prepared by the osmium-catalyzed reaction of alkenes with A-chloro-A-metallocarbamates generated in situ from A-chloro-A-sodiocarbamates (easily prepared from the carbamate, tert-butyl hypochlorite, and sodium hydroxide in methanol) by reaction with silver nitrate77 or mercury(II) salts78 in acetonitrile, The greatest reactivity was, however, displayed by the A-chloro-A-sodiocarbamate/mercuryUI) nitrate/tetraethylam-monium acetate (relative ratio 1.5 0.75 1) system, unfortunately low yields of hydroxycarba-mates from trisubstituted alkenes were obtained, due to competitive formation of organomer-... 

There have been reports of a novel synthesis of 2,4,5-trialkyl-imidazoles by the rhodium-catalyzed reactions of alkenes with carbon monoxide and ammonia. Yields are 50-60%. Benzylamine and derivatives react with carbon tetrachloride in the presence of a catalytic amount of metal carbonyls to yield 2,4,5-triarylimidazoles and -imidazolines. The suggested reaction mechanism implicates an initially formed radical species which coordinates with the metal carbonyl. 

R. D. Sanner, R. G. Austin, M. S. Wrighton, W. D. Honnick, C. U. Pittman, Jr., Photoactivation of Polymer-Anchored Catalysts. Iron Carbonyl Catalyzed Reactions of Alkenes, Chapter 2 in Interfacial Photoprocesses Energy Conversion and Synthesis, M. S. Wrighton, Ed., pp. 13-26, Advances in Chemistry Series 184, ACS Publishers, 1980. 

Insertion of alkenes into Ru—H bonds is step in many Ru-catalyzed reactions of alkenes such as homogeneous hydrogenation. Stoichiometric formation of isolable cr-alkyl complexes is also observed. The reaction of ethylene with Ru hydrides such as RuHCl(PPh3)3 or RuH(OCOCF3)(PPh3)3 is an equilibrium process ... 

When reversible addition and elimination reactions are carried out under similar conditions, they follow the same mechanistic path, but in opposite directions. The principle of microscopic reversibility states that the mechanism of a reversible reaction is the same in the forward and reverse directions. The intermediates and transition structures involved in the addition process are the same as in the elimination reaction. Under these circumstances, mechanistic conclusions about the addition reaction are applicable to the elimination reaction and vice versa. The reversible acid-catalyzed reaction of alkenes with water is a good example. Two intermediates are involved a carbocation and a protonated alcohol. The direction of the reaction is controlled by the conditions, which can be adjusted to favor either side of the equilibrium. Addition is favored in aqueous solution, whereas elimination can be driven forward by distilling the alkene from the reaction solution. The reaction energy diagram is show in Figure 5.1. 

Competition between carbonyl and hydrocarbon ligands, though difficult to predict, is another key issue in catalytic reactions. For example, the products of the Pd-catalyzed reaction of alkenes with carbon monoxide and alcohol with the aid of an oxidant depended on whether a base was present or not, where the base played a key role to determine the site of the initial nucleophilic attack of the alcohol as shown in Scheme 8.3 [7]. Thus, with some bases added, the first step of the catalysis was thought to be formation of Pd-COOR intermediate by the attack... 

Surprisingly, the catalyzed reaction of alkenes with propiolate esters results in [2 + 2] cycloadducts, as well as ene adducts, in a manner that is related to the substitution on the alkene. No cycloadducts are formed from alkenes with two substituents on one end of the double bond. ... 

These results provide the fundamental characteristics of the Pd(II)-catalyzed reactions of alkenes with amines or its analogs. Thus, aliphatic amines coordinate strongly to electrophilic Pd(II). Therefore, alkenes coordinated to the metal are readily replaced by these amines, resulting in (amine)2PdCl2 complexes from which the Schiff base is formed... 

Although palladacyclopropanes and palladacyclopropenes can be postulated as intermediates when palladium in the oxidation state 0 or +2 interacts with an alkene or an alkyne, there is very little evidence that Pd-catalyzed reactions of alkenes and alkynes actually... 

The catalytic carboxylation of unsaturated hydrocarbons with CO2 to a,P-unsaturated carboxylates (i.e., acrylates) has been a topic of academic as well as industrial research for three decades, since seminal work of Hoberg and Yamamoto in the early 1980s. Although Hoberg et al. reported the catalytic nickel-catalyzed reaction of alkenes and iso-... 

Nguyen V, Nishino H, Kajikawa S, Kurosawa, K. Mn(III)-catalyzed reactions of alkenes and alkynes with thiols. An approach toward substituted 2,3-dihydro-l,4-oxathiins and simple route to (E)-vinyl sulfides. Tetrahedron 1998 54 11445-11460. 

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