Halide promoters, effects

The reaction is catalyzed by a group VIII metal species, particularly that of rhodium or palladium. The initial metal species may be any variety of complexes (e.g., PdCl2 Pd acetate, etc.). A source of halide is necessary iodide is especially effective. The most convenient source is methyl iodide, since it is likely a reaction intermediate. In addition, an organic promoter must be included for catalytic activity. These promoters are generally tertiary phosphines or amines. Also, chromium complexes were found to have an important promotional effect. 

Reaction pressure has a dramatic effect on the rate of product formation in the halide-promoted ruthenium system. As shown in Fig. 18, the dependence of the glycol-forming reaction on total H2/CO pressure is approximately fourth-order, whereas that for the methanol-producing reaction is... 

In view of the presence of similar labile chlorine atoms in PVC and the promoting effect of such halides in the preparation of cw-1,4-polybutadiene, the polymerization of butadiene was carried out in the presence of PVC using the Et2AlCl-Co compound catalyst system. The reaction proceeded readily, and the product was identified as a graft copolymer of cis-1,4-polybutadiene on PVC (7). 

A frequent theme in alcohol carbonylations by transition metals is the use of a halide or pseudo-halide promoters or cocatalysts. Despite major problems of corrosion associated with its use, iodide is almost always found to be most effective in this capacity. This is because the halide serves several purposes, for each of which iodide is ideally suited. One of the most important roles of these anion promoters can be that of facilitating the formation of metal-carbon bonds via the formation of intermediate alkyl halides. Under typical catalytic conditions for a variety of systems, at least some portion of the added halide is converted to the corresponding strong halo-acid. In fact, conditions are generally set so that this event is maximized. 

A third effect of halide promoters is to enhance the relative nucleophilici-ties of the metal center. This generally occurs by halide displacement of a coordinated neutral ligand or by bridge-splitting reactions to generate anionic metal species. In cases where this type of halide promotion has been independently demonstrated, the effect of the increase in electron density on the reactivity is very large. 

It is noted that the coupling of aryl halides, especially iodides, with a number of active methylene and methine compounds are promoted effectively by a stoichiometric amount of copper(I) halides [8, 37, 38]. The reaction using cy-anoacetate esters and 1,3-diketones can catalytically proceed [39-41]. 

Exceptionally clean cyclization can be accomplished by utilizing conjugated enones as precursors for the Barbier reaction (equation 43). High diastereoselectivity is achieved in these reactions, and under the mild conditions required for cyclization the TMS ether protecting group remains intact. It is also interesting that a neopentyl halide is effective in the cyclization. This result would appear to exclude an 5N2-type displacement of an organic halide by a samarium ketyl as a possible mechanism for the Smh-promoted intramolecular Barbier reaction. 

Other studies, again conducted by Geoffrey s group, also led to the discovery of interesting promotion effects of halides. It was found that the addition of [PPN][X] (PPN " = (PPh3)2N+, X = Cl, Br, I) strongly accelerates the reaction between Ru3(CO)i2 and nitrosobenzene, to afford an imido cluster in which the... 

We next analyzed the role of the chloride anion. We have already mentioned that chloride has been reported to accelerate the reaction of nitrosoarenes with Ru3(CO)i2 and also to promote the formation of phenyl isocyanate from a trinu-clear imido-halide cluster. However, neither of these effects seems relevant to the effect observed in catalysis. In fact, apart from the problem of the cluster nature of the compounds involved, nitrosoarenes are always much more reactive than their nitro counterparts and the reaction of a free nitroso intermediate should be very fast relative to the other steps in the catalytic cycle, so that its further acceleration should not be influential. The promotion effect on the isocyanate-producing step, on the other hand, cannot be relevant to the catalytic cycle because it has been shown that the eorresponding imido clusters are not involved. So we must look for other effects. 

In addition, the Heck reaction of iodobenzene with butyl acrylate was studied in more detail [7]. In all experiments,butyl cinnamate was selectively produced. It was observed that the activity was only dependent on the pallaihum concentration in the hydrophihc Uquid,but neither on the total amount of complex applied nor on the quantity of the liquid used. In recycling ejqieriments, a short induction period occurred in the first runs. This was not observed in the following runs. Furthermore, repeated appUcation of the catalyst resulted in an increased reaction rate. Other investigators observed a decrease of activity during the recycling experiments [5]. The authors ejqilained their results with an accumulation of EtjNHI during consecutive runs. It was assumed that the accumulated ammonium salt had the same promotion effect as tetrabuty-lammonium halide additives in Heck reactions (Jeffery conditions) [8]. 

Formation of the initial carbocation depends on the catalyst used (62). Pure cyclohexane, for example, does not undergo isomerization upon contact with pure aluminum chloride, even at refluxing temperature. However, isomerization does occur in the presence of a trace of water. HCI-AICI3 and HBr-AlBra were also demonstrated by careful studies to exhibit isomerization activity only in the presence of traces of alkene or alkyl halide promoters. The added alkene serves as a source of the intermediate carbocations, thereby initiating the chain reaction via hydride transfer. The effect of water in carbocation formation is attributed to its ability to generate the strong acid H[Al(OH)Hlgs]. 

Recently, a promotion effect of copper halides or Fe(CO)s on the double carbonylation of phenyl iodide in the presence of diethylamine has been reported. The promoters were found effective in both CO insertion and attack of CO by diethylamine. An iodide bridged, copper-containing complex was proposed as the intermediate. 

Pd(PPh3)4. An analogous promoting effect of iodide anion on the oxidative addition of Mel to [Rh(CO)2l2] has been noted and the formation of [Rh(CO)2l3] proposed [115]. Based on this general reactivity, we propose that the effect of halide even in the Pd-phenanthroline and Ni-phosphine catalytic systems for the carbonylation of nitrobenzene discussed in this paragraph can be described by the following reaction scheme (Scheme 3) ... 

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