Alkene enamides

The difference between this catalytic system and Wilkinson s catalyst lies in the sequence of the oxidative addition and the alkene complexation. As mentioned above, for the cationic catalysts the intermediate alkene (enamide) complex has been spectroscopically observed. Subsequently oxidative addition of H2 and insertion of the alkene occurs, followed by reductive elimination of the hydrogenation product. 

Asymmetry in metal-alkene coordination plays a critical role in asymmetric catalysis, with implications far beyond the scope of the present treatment. An instructive example is provided by catalytic asymmetric hydrogenation of enamides,... 

When assessing catalytic results reported for new ligands, one must bear in mind that their quality and relevance differ widely. For most new ligands only experiments with selected model test substrates carried out under standard conditions are available, and very few have already been applied to industrially relevant problems. The test substrates for alkenes used most frequently are Aceta-mido Cinnamic Acid (ACA) or its methyl ester (MAC), Methyl Acetamido Acrylate (MAA), ITaconic Acid or DiMethyl ITaconate (ITA, DMIT) and selected aryl enamides (Fig. 25.3). 

When the diphosphine is chiral, binding of a prochiral alkene creates diastereomeric catalyst-alkene adducts. (Diastereomers result because binding of a prochiral alkene to a metal center generates a stereogenic center at the site of unsaturation.) Through a powerful combination of3lP and l3C NMR methods, Brown and Chaloner first demonstrated the presence of two diastereomeric catalyst-enamide adducts with bidentate coordination of the substrate to the metal (Figure 1) [19]. 

Shortly after the discovery of the first asymmetric phosphoric acid-catalyzed transformation of enecarbamates, Zhou et al. expanded the scope of the Friedel-Crafts alkylation of indoles 29 with electron-rich alkenes to enamides 127 (Scheme 51) [74],... 

A significant decrease in reactivity was observed when the IV-aryl zinc enamide generated from 4-methylaniline was used. It was suggested that this 2-methyl effect on reactivity may arise from a steric effect contributing to the conformational stabilization of the transition state. These recent results have thus greatly expanded the scope of the so-called olefinic aldol condensation as the experimental procedure is quite simple and applicable to a variety of monosubstituted or aqa-disubstituted alkenes. 

F-Teda BF4 is effective for the selective addition of fluorine to steroids in good yield, re-gioselectively and, in many cases, stereoselectively at the 6- and 16-positions, under very mild reaction conditions (Table 7).92 Further, 6 will also efficiently fluorinate silyl and alkyl enolates, enamides, carbanions, a-alkenes and actived aromatic compounds (Table 8). As an extension of this method F-Teda BF+ has been used for the electrophilic fluorination of (fluorovinyl)tin compounds affording terminal fluoroalkenes (see Table 9).88... 

As part of studies pertaining to the total synthesis of the complex hexacyclic alkaloid gelsemine, Overman and Abelman applied the Heck reaction to the cycli-zation of /V-(2-halophenyl)enamides.54 They found that the addition of silver salt suppressed alkene isomerization, a frequent problem in Heck reactions, and dramatically accelerated the reaction (Scheme 10.32). 

In addition to the well exploited radical additions to enamides, thiovinyl ethers have been shown to be suitable intramolecular acceptors for aryl radicals. By placing a thiophenyl substituent on terminal alkene of 33, the cyclization to 34 occurred exclusively in the 6-exo mode and was not complicated by internal hydrogen abstraction from the allylic positions of the alkene (Scheme 13) [83, 84]. 

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