Reaction, coupling

Phosphodiesteric/peptidic/glycosidic bond formation (coupling reaction) is a nucleophilic substitution reaction of a hydroxy/amino/hydroxy group at a phosphoesteric/carboxyl/ acetal group, resulting in dehydration. The coupling reaction can be conducted in two procedures activation-substitution (via stable activated intermediate) and direct condensation 

1 Activation-substitution. To promote coupling of biomonomers via a nucleophilic substitution, which has to be performed under mild conditions, the electrophilic (esteric/carboxylic/acetal) site is activated to increase its electrophilicity. This is achieved by an introduction of electron-withdrawing moieties (decrease the electron density at the electrophilic site), thereby favoring the subsequent nucleophilic attack. Some of the common activated structures are illustrated in Table 8.6. 

2 Direct condensation. The direct condensation of biomonomers between the two coupling centers can be initiated by the use of coupling reagents such as carbodiimides  

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An interesting coupling reaction with the diazonium salt derived from anthranilic acid leads to an excellent method for the preparation of dlphenic acid. The reaction occurs with cuprous salts in ammoniacal solution ... 

Carbonyl Coupling Reactions (McMurry Reaction) (Smith Ch. 13.7.F)... 

Transition Metal Catralyzed Cross-Coupling Reactions... 

Syntheses of alkenes with three or four bulky substituents cannot be achieved with an ylide or by a direct coupling reaction. Sterical hindrance of substituents presumably does not allow the direct contact of polar or radical carbon synthons in the transition state. A generally applicable principle formulated by A. Eschenmoser indicates a possible solution to this problem //an intermolecular reaction is complex or slow, it is advisable to change the educt in such a way. that the critical bond formation can occur intramolecularly (A. Eschenmoser, 1970). 

As stated above, intermolecular coupling reactions between carbon atoms are of limited use. In the classical Wurtz reaction two identical primary alkyl iodide molecules are reduced by sodium. /i-Hectane for example, has been made by this method in 60%... 

Palladium-catalyzed coupling reactions of organic halides with olefins or dienes (R. F. Heck, 1979) are broad in scope and simple to carry out. Anhydrous conditions or any special technique are not required and most functional groups are tolerated. 

The use of reducing metals nowadays is mainly restricted to acyloin and pinacol coupling reactions (see p. 53f.) and Birch reductions of arenes (A.A. Akhrcm, 1972 see p. 103f.) and activated C—C multiple bonds (see p. 103f.). 

Reaction that can be carried out by the oxidative coupling of radicals may also be initiated by irradiation with UV light. This procedure is especially useful if the educt contains oleflnic double bonds since they are vulnerable to the oxidants used in the usual phenol coupling reactions. Photochemically excited benzene derivatives may even attack ester carbon atoms which is generally not observed with phenol radicals (I. Ninoraiya, 1973 N.C. Yang, 1966). 

Success of the reactions depends considerably on the substrates and reaction Conditions. Rate enhancement in the coupling reaction was observed under high pressure (10 kbar)[l 1[. The oxidative addition of aryl halides to Pd(0) is a highly disfavored step when powerful electron donors such as OH and NHt reside on aromatic rings. Iodides react smoothly even in the absence of a... 

Monosubstitution of acetylene itself is not easy. Therefore, trimethylsilyl-acetylene (297)[ 202-206] is used as a protected acetylene. The coupling reaction of trimethylsilylacetylene (297) proceeds most efficiently in piperidine as a solvent[207]. After the coupling, the silyl group is removed by treatment with fluoride anion. Hexabromobenzene undergoes complete hexasubstitution with trimethylsilylacetylene to form hexaethynylbenzene (298) after desilylation in total yield of 28% for the six reactions[208,209]. The product was converted into tris(benzocyclobutadieno)benzene (299). Similarly, hexabutadiynylben-zene was prepared[210j. 

The coupling reaction has widespread use in the construction of enediyne systems present in naturally occurring anticancer antibiotics[234]. The Pd-Cul catalyzed coupling reaction of the alkenyl bromides 323 and 326 with the... 

The Li compound 588 formed by the ort/io-lithiation of A. A -dimethylaniline reacts with vinyl bromide to give the styrene derivative 589(433]. The 2-phe-nylindole 591 is formed by the coupling of l-methyl-2-indolylmagnesium formed in situ from the indolyllithium 590 and MgBr2, with iodobenzene using dppb[434]. 2-Furyl- and 2-thienyllithium in the presence of MgBr2 react with alkenyl halides[435]. The arylallenes 592 and 1,2,4-alkatrienes are prepared by the coupling reaction of the allenyllithium with aryl or alkenyl halides[436]. 

Regiocontrolled q- or / -alkenylation and arylation of cyclic enones are possible without protection of the ketone by applying the coupling reaction of the Q- or /3-halo enones 607 and 608 with aryl and alkenylzinc reagents[468,469]. 

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