Acetylene complexes electrophilicity

Related displacements with acetylenes and electrophilic olefins have been reported to give complexes formulated as (olefin or... 

Related displacements with acetylenes and electrophilic olefins have been reported to give complexes formulated as [Pd P(CeH5)3 2 (olefin or acetylene)]. Also, oxidative additions of alkyl and aryl halides have been shown to occur giving palladium(II) complexes, Pd(P(C6H5)3 2(R)Cl. ... 

The A -unsubstituted complex 16 reacts with various electrophilic acetylenes to yield adducts 25 as mixtures of Z- and -stereoisomers the adducts afford the corresponding TV-substituted 1,2-diazepines 26 on decomplexation.88... 

The reactions of electrophilic alkynes, such as DMAD (dimethyl acetylene-dicarboxylate), with metal per- and poly-chalcogenido complexes have been exploited for the synthesis of homoleptic and heteroleptic 1,2-dithiolene,... 

Besides the above electrophiles, the acetylene—titanium complexes react regioselectively with other acetylenes providing the corresponding titanacyclopentadienes. An example of a homo-coupling reaction is shown in Eq. 9.11 [30], which also displays some synthetic applications [30,31]. Especially noteworthy is the highly regioselective cross-coupling reaction of unsymmetrical internal and terminal acetylenes, which is illustrated in Eq. 9.12... 

If the alkenes and acetylenes that are subjected to the reaction mediated by 1 have a leaving group at an appropriate position, as already described in Eq. 9.16, the resulting titanacycles undergo an elimination (path A) as shown in Eq. 9.58 [36], As the resulting vinyltitaniums can be trapped by electrophiles such as aldehydes, this reaction can be viewed as an alternative to stoichiometric metallo-ene reactions via allylic lithium, magnesium, or zinc complexes (path B). Preparations of optically active N-heterocycles [103], which enabled the synthesis of (—)-a-kainic acid (Eq. 9.59) [104,105], of cross-conjugated trienes useful for the diene-transmissive Diels—Alder reaction [106], and of exocyclic bis(allene)s and cyclobutene derivatives [107] have all been reported based on this method. 

From the investigation of all these data it is clear that the aromaticity of phosphinine is nearly equal to that of benzene. Their chemical reactivity, however, is different. Most important is the effect of the in-plane phosphorus lone pair, which (i) is able to form a complex and (ii) can be attacked by electrophiles to form A -phosphinines. Thus, electrophilic substitution reaction on the ring carbon is impossible. In Diels—Alder reactions, phosphinines behave as dienes, providing similar products to benzene but under less forcing condition (the reaction with bis(trifluoromethyl) acetylene takes place at 100 °C with 3, while for benzene 200 °C is required). 

Pyrrole and indole rings can also be constructed by intramolecular addition of nitrogen to a multiple bond activated by metal ion complexation. Thus, 1-aminomethyl-l-alkynyl carbinols (obtained by reduction of cyanohydrins of acetylenic ketones) are cyclized to pyrroles by palladium(II) salts. In this reaction the palladium(II)-complexed alkyne functions as the electrophile with aromatization involving elimination of palladium(II) and water (Scheme 42) (81TL4277). 

Nonempirical quantum-chemical calculations of acetylide molecules support the ready displacement of alkali metal cations to the bridge position (87IZV2777 88IZV1335, 88IZV1339). This naturally leads to the conclusion that the polarization and deformation of the ir-electronic shell of acetylene must depend on the atomic number of the cation attached to the acetylene anion. However, the acetylene activation in the reaction with ketoximes via acetylides suggests nucleophile attack at a carbanionlike complex, which is of course a week point of the hypothesis. Nevertheless, the electrophilic assistance from the alkali metal cation (Na+) to the... 

From the qualitative point of view, the structure of the ethylene/ketene complex is similar to the geometry of the TS of the same system in cycloaddition reaction58. In 22, R (= 3.46 A) is the distance between the centre of mass of the ethylene and the carbon of the carbonyl group of ketene this carbon is the most electrophilic centre of the ketene. In the case of the complex between acetylene and ketene, the same distance between the centre of mass of acetylene and the carbon of ketene was evaluated (by the same method) at 3.60 A. 

To the extent that the enolate resulting from conjugate addition at the (3-carbon can be stabilized, the rate of this reaction pathway is enhanced. For example, (3-Michael additions are observed for MVK, acrolein, and acetylenic electrophiles even without the presence of a Lewis acid. Furthermore, MVK reacts with the 2,5-dimethylpyrrole complex (22) to form a considerable amount of (3-alkylation product, whereas only cycloaddition is observed for methyl acrylate. The use of a Lewis acid or protic solvent further enhances the reactivity at the (3-position relative to cycloaddition. While methyl acrylate forms a cycloadduct with the 2,5-dimethylpyrrole complex (22) in the absence of external Lewis acids, the addition of TBSOTf to the reaction mixture results in exclusive conjugate addition (Tables 3 and 4). 

Additions of aromatic C-H bond to olefins and acetylenes result in the formation of aryl-alkyl and aryl-alkenyl bonds. This type of addition reaction is not applicable to aryl-aryl bond formation. Catellani and Chiusoli [52] reported the first example of this type of arylation in 1985. To date, several arylation reactions of aromatic rings have been developed. In almost all cases, C-H bond cleavage proceeds through electrophilic substitution with transition-metal complexes [53]. 

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