Radical reactions transition-metal catalysts

Addition. Chlorine adds to vinyl chloride to form 1,1,2-trichloroethane [79-00-5] (44—46). Chlorination can proceed by either an ionic or a radical path. In the Hquid phase and in the dark, 1,1,2-trichloroethane forms by an ionic path when a transition-metal catalyst such as ferric chloride [7705-08-0], FeCl, is used. The same product forms in radical reactions up to 250°C. Photochernically initiated chlorination also produces... 

The previous sections show that certain ionic liquids, namely the chloroalumi-nate(III) ionic liquids, are capable of acting both as catalyst and as solvent for the polymerization of certain olefins, although in a somewhat uncontrolled manner, and that other ionic liquids, namely the non-chloroaluminate(III) ionic liquids, are capable of acting as solvents for free radical polymerization processes. In attempts to carry out polymerization reactions in a more controlled manner, several studies have used dissolved transition metal catalysts in ambient-temperature ionic liquids and have investigated the compatibility of the catalyst towards a range of polymerization systems. 

A direct addition of cydoethers to terminal alkynes has been discovered by Zhang and Li (Scheme 6.136) [271]. The best results were obtained when the reactions were run without additional solvent and in the absence of additives such as transition metal catalysts, Lewis acids, or radical initiators. Typically, the cycloether was used in large excess (200 molar equivalents) as solvent under sealed-vessel conditions. At a reaction temperature of 200 °C, moderate to good yields of the vinyl cycloether products (as mixtures of as and trans isomers) were obtained. The reaction is proposed to follow a radical pathway. 

Not all C-H activation chemistry is mediated by transition metal catalysts. Many of the research groups involved in transition metal catalysis for C-H activation have opted for alternative means of catalysis. The activation of methane and ethane in water by the hexaoxo-/i-peroxodisulfate(2—) ion (S2O82) was studied and proceeds by hydrogen abstraction via an oxo radical. Methane gave rise to acetic acid in the absence of external carbon monoxide, suggesting a reaction of a methyl radical with CO formed in situ. Moreover, the addition of (external) CO to the reaction mixture led to an increase in yield of the acid product (Equation (ll)).20... 

The addition of organodichalcogenides to alkynes does not necessarily require the aid of transition metal catalysts. Indeed, the reaction proceeds via different mechanisms under various conditions (radical, In, CsOH,183 SnCl4,184 and phase-transfer catalysts185). Treatment of a mixture of (PhS)2 and terminal alkynes with GaCl3 affords (E)-products (E Z=>20 1).186 The reaction is assumed to involve a thiirenium ion as the intermediate (Scheme 38). 

In contrast to the free-radical polymerizations, there have been relatively few studies on transition metal catalysed polymerization reactions in water. This is largely due to the fact that the early transition metal catalysts used commercially for the polymerization of olefins tend to be very water-sensitive. However, with the development of late transition metal catalysts for olefin polymerizations, water is beginning to be exploited as a medium for this type of polymerization reaction. For example, cationic Pd(II)-bisphosphine complexes have been found to be active catalysts for olefin-CO copolymerization [21]. Solubility of the catalyst in water is achieved by using a sulfonated phosphine ligand (Figure 10.5) as described in Chapter 5. 

A series of internal alkynes has been investigated, revealing that the presence of an alkyl or aryl group does not appear to change the course of the reaction. Internal alkynes, however, do not undergo germylformylation in this system. In the case of tin, two reports exist of the stannylformyla-tion of unsaturated carbon substrates, but both proceed by a free radical mechanism initiated by AIBN and do not require a transition metal catalyst.128... 

Radical intermediates and transition metal-catalyzed reactions are in principle ideally suited to be linked together. A prerequisite to perform successful radical reactions is that the concentration of radicals has to be kept low to promote the desired reaction and to avoid competing homocoupling and disproportionation, which occur often diffusion-controlled. Including radical intermediates in the regime of a transition metal catalyzed process is thus ideal to keep their concentrations low, since their maximum concentration cannot exceed that of the metal catalyst. On the other hand, radicals are much more reactive than closed-shell organotransition metal intermediates. Thus, the involvement of radicals in transition metal catalysis often leads to a strong acceleration of the reactions compared to a process where only closed-shell intermediates are involved [101]. 

Transition metal-catalyzed radical based processes often occur under mild conditions and are quite fast. The fine-tuning of the electronic properties of the metal complex catalysts to maximize the facility of radical generation and to modulate the lifetime of the radicals for follow-up reactions is still in its infancy and has not been studied systematically. This is, however, important to allow the design of radical processes at the slower end of the reactivity scale, as the current methodology is mostly limited to very fast radical processes in the framework of transition metal catalysis. The kinetics of almost all processes described in this review is not known. Only detailed investigations in this field will allow the rational design of radical-based transition metal-catalyzed processes. 

In the Bu3SnH-promoted radical reactions to aliphatic alkynes, using initiators such as AIBN, Et3B, and ultrasound104 furnishes /3-adducts as a mixture of (E)- and (Z)-isomers. Lewis acid catalysts give /3-(Z) isomers,96 whereas transition metal catalysts furnish the predominant formation of (3-(E) isomers.105 The a-stannylation of simple aliphatic alkynes, however, is particularly difficult because of the absence of anchor substituents such as ethers. In the general hydrostannations of aliphatic alkynes, a-adducts are obtained only as minor adducts in the Pd-catalyzed reaction (Equation (35)). 

Polymerisation reactions in ionic liquids have so far focused on processes that do not involve a transition metal catalyst. Examples include acid-catalysed/34"361 free-radical,[37 50] electrochemical19,51"551 and laser1561 induced polymerisation reactions and a review is available on the topic.1571... 

The term hydrosilation (or hydrosilylation) refers to the addition of a molecule containing a Si-H bond across the multiple bond of a substrate, usually an alkene, alkyne, or carbonyl compound (equation 1). The reaction can be promoted by UV-light, radiation (y- and X rays), radical initiators, Lewis acids, nucleophiles, or, most importantly, transition metal catalysts. Hydrosilation is related to the important processes of hydrogenation (see Hydrogenation) and hydroboration (see Hydroboration), all of which belong to the general reaction class of hydroelementation. 

The compehtion of one-electron pathways is sometimes detectable in the epoxidations catalyzed by transition metal catalysts [67]. However, in the epoxidahon of unhindered olefins on TS-1, the typical radical products are below the detection limits. Their presence could no longer be neglected when the rate of epoxidation is so slow as to become comparable to that of homolytic side reactions, for example with bulky olefins (see also Section 18.11). It is possible that, within these limits only, the epoxide is produced in part through the addition of a radical peroxy intermediate to the double bond [68, 69]. Even so, a homolytic pathway has again been proposed as a generally vahd epoxidation mechanism [7]. 

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