Vinyl halides structure

The palladium-catalyzed carbonylation of isomeric vinylic halides shows the reaction to be reasonably stereospecific and proceed with retention of the original halide structure. The degree of specificity, however, depends somewhat on the reaction conditions. Low reaction temperatures and/or excess triarylphosphine favor the stereospecific reaction (11). 

Substitution reactions with (E)- or (Z)-vinylic halides usually show predominant retention of structure in the olefin substitution as they do in the carboalkoxylation, but the specificity is quite dependent on reaction conditions. Low reaction temperature, excess organophosphine, and dilution with excess trialkylamine and/or olefin all appear to improve the specificity. Under favorable conditions, for example, (Z)-l-bromo-l-hexene and methyl acrylate give an 82% yield of the (E, Z) and only 10% of the (E, E) isomer of methyl 2,4-nonadienoate 29). 

While die above reactions represent only a small fraction of die reactions known for palladium, they form the basis of a powerful methodology for building carbon structures. Several variations have been developed which utilize certain types of reactants and give particular types of products. All diese variations, however, contain a common theme. In each case an electron-deficient reagent (e.g., a vinyl halide or aromatic triflate) reacts with an election-rich reagent (e.g., an alkene, an organoborane, or an organotin) witii the formation of a new carbon-carbon bond. In that sense diese reactions are related to die reactions between carbon nucleophiles and carbon electrophiles discussed previously in diis chapter. They are quite different, however, because diey proceed only in the presence of Pd(0). In fact they proceed only in die coordination sphere of Pd(0). The ability of Pd(0) to catalyze diese reactions is nearly unique We will now examine some of die more common processes. 

Control of the selectivity of the addition often is best achieved by choosing the proper reactants. The major influence is steric. The organic portion of the organopalladium intermediate normally adds to the less-hindered, double-bond carbon of the olefin. The secondary electronic effect is to cause the organic group to add to the more electron-deficient double-bond carbon. The structure of the vinylic halide also may influence the selectivity of the addition. Both vinyl bromide and 2-bromopropene (and other internal vinylic halides) add... 

Additions to functionalized three-carbon olefins have been studied extensively. We have used methyl acrylate as a standard olefin since it always reacts only at the terminal carbon and the a,/3-double bond in the product is always trans. The stereospecificity of its reactions with vinylic halides varies with structure. The simple 1-halo-l-alkenes with methyl acrylate under normal conditions give mixtures of E,Z- and E,E-dienoates. The reaction is more selective with the bromides than with the iodides and the stereoselectivity increases with increasing triphenylphosphine concentration. This occurs because the excess phosphine displaces the hydridopalladium halide group from the diene 7r-complex before readdition to form the ir-allylic species occurs (see Equation 6). The disubstituted vinylic bromides react stereospecifically with methyl acrylate (4). 

The irradiation of vinylic halides in solution is by now as well-established as a convenient and powerful method to generate vinyl cations as the photolysis of alkyl halides is to create alkyl cations. In the period under review the scope of the method has been considerably expanded, and so has the knowledge of the relationship between the structure of vinylic halides and their photoreactivity. A review has appeared in which results up to 1990 have been tabulated290. The bulk of the studies concerns the photoreactions of vinylic halides... 

Photolysis of 2-bromo-4,4-dimethyl-2-cyclohexenone only affords reduction, even in a nucleophilic medium343,344. Apparently, this substrate is structurally not suitable to form a vinyl cation. Formation of vinyl radical-derived products is also the main process for all vinylic halides, if their irradiation is performed in an apolar medium. Such photochemical reductive dehalogenation and especially dechlorination reactions have been extensively studied in the past, not in the least because of their importance as abiotic transformation of persistent polychlorinated environmental pollutants. Examples are the cyclodiene insecticides aldrin and dieldrin, which contain a vicinal dichloroethene chromophore. In recent... 

Examples of tetratopic reactions are the C—N bond dissociation in azo compounds, discussed in Section 7.2.2, C—X bond dissociation in alkyl halides, and the O—O bond dissociation in peroxides. Examples of pentatopic reactions are the dissociation of the C—X bond in vinyl halides, of the C=C bond in ketenes, and of the C—N bond in diazoalkanes. An example of a hexatopic bond dissociation is the fragmentation of an alkyl azide to a ni-trene and N,. A verification of the topicity rules at a semiempirical level was reported (Evleth and Kassab, 1978), and a detailed description of the electronic structure aspects of bond dissociations characterized by various topicity numbers, with references to the original literature, has appeared recently (Michl and BonaCi<5-Koutecky, 1990). 

Vinyl cations [1], the dicoordinated unsaturated analogs of divalent carbenium ions, were first detected by Grob and coworkers in the early 1960s in solvolysis reactions of a-aryl vinyl halides [2]. The direct NMR detection of vinyl cations in superacidic media was achieved in 1992 at temperatures below -100 °C [3]. We recently reported a convenient synthesis of unusually stable vinyl cations at room temperature [4, 5]. One reason for the unusual high thermodynamic stabilization of these vinyl cations is the presence of two 3-silyl substituents. [4]. We report here details of the X-ray structure of the vinyl cation 1 and discuss the structural and spectroscopic consequences of 3-SiC hyperconjugation [6]. 

The parallel between aryl and vinyl halides goes further both are unreactive toward nucleophilic substitution and, as we shall see, for basically the same reason. Moreover, this low reactivity is caused—partly, at least—by the same structural feature that is responsible for their anomalous influence on electrophilic attack partial double-bond character of the carbpn-halogen bond. 

The mechanism of the Sonogashira cross-coupling follows the expected oxidative addition-reductive elimination pathway. However, the structure of the catalytically active species and the precise role of the Cul catalyst is unknown. The reaction commences with the generation of a coordinatively unsaturated Pd species from a Pd " complex by reduction with the alkyne substrate or with an added phosphine ligand. The Pd " then undergoes oxidative addition with the aryl or vinyl halide followed by transmetallation by the copper(l)-acetylide. Reductive elimination affords the coupled product and the regeneration of the catalyst completes the catalytic cycle. 

The differences in selectivity between catalysts cannot be explained only in terms of the strength of reactant adsorption. A tentative explanation lies in the preference of platinum for concerted addition of protons to adsorbed alkenes with simultaneous electron transfer (25). The electronic structure of the surface intermediate of the concerted step appears to lead to halide cleavage. Palladium, on the other hand, can participate in insertion reactions (305) and promotes surface reaction between hydrogen atoms and adsorbed alkenes 4Sa. It is possible that palladium adsorbs vinyl halides on two different sites or at two different states, dependent on potential, one of which... 

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