Coupling reagent mode

In addition to suitable protection of other coreactive functionalities, activation of the carboxy group prior to reaction with the amino group is required for a controlled formation of the amide bond between two a-amino acid or peptide components (Scheme 1). For this purpose activated species can be generated separately, isolated, and stored for subsequent use. Alternatively, the activation can be carried out in situ with specific coupling reagents. Independently of the mode of activation, an electron-withdrawing group X must be incorporated at the acyl carbon in a transient mode in the in situ activation or in the formation of reactive intermediates. 

Fig. 2. (A) Fourier filtered ex-situ STM image, taken in constant-height mode, of a thermally oxidized HOPG electrode modified with K2[Fe(Cl J)5 [4-(aminomethyl)pyridine]] using DCC as a coupling reagent. Tip bias = -50 mV, tunneling current =1.0 nA, scan rate = 30.5 Hz. (B) Structure and proposed binding mode of the covalently immobilized iron complex.

Some sulfonates of strongly acidic AT-hydroxy compounds, such as HOBt-substituted derivatives and HODhbt, are excellent coupling reagents for amide bond formation in solution [138] but of limited applicability in solid-phase mode. The main drawback of this method is the formation of the corresponding sulfonylamide [138]. The arenesulfonyl-1,2,4-triazoles,... 

A major improvement in DCC mediated coupling was brought about by the use of the reagent in symmetrical-anhydride-mode . This means that one equivalent of the coupling reagent is added to the solution containing two... 

In the present work it has been shown that on-line coupling of flowthrough fractionation in RCC with ICP-EAS detection enables not only the fast and efficient fractionation of trace elements (TE) in environmental solids to be achieved but allows real-time studies on the leaching process be made. A novel five-step sequential extraction scheme was tested in on-line mode. The optimal conditions for the fractionation were chosen. Investigating elution curves provides important information on the efficiency of the reagents used, the leaching time needed for the separation of each fraction, and the potential mobility of HM forms. 

The oxidative dimerization of the anion of methyl phenyl sulfone (from a Grignard reagent) in ethereal solution in the presence of cupric chloride in 5% yield has been reported47. Despite the reported48 poor stability of the a-sulfonyl C-centered radicals, Julia and coworkers49 provoked the dimerization (in 13 to 56% yields) of the lithiated carbanion of alkyl phenyl sulfones using cupric salts as oxidants. The best results are obtained with cupric triflates in THF-isobutyronitrile medium (56% yield for R = H). For allyl phenyl sulfones the coupling in the 3-3 mode is predominant. 

In the Grignard reaction, which is very important in the manufacture of a variety of fine chemicals, the continuous reactor for production of the reagent consists of a column of magnesium particles through which a solution of the organohalogen compound in an ether-class solvent is passed. The continuous mode of operation reduces side reactions, particularly Wurtz-type coupling, which make many reactions impractical. 

Electron transfer from the alkene leads to a radical cation that can undergo coupling (Scheme la). The radical cation can also react with the nucleophilic heteroatom of a reagent to afford addition or substitution products (Scheme lb). Adducts can be likewise obtained by oxidation of the nucleophile to a radical that undergoes radical addition. Reactions between alkenes and nucleophiles can be realized too with chemical oxidants that are regenerated at the anode (mediators) (see Chapter 15). Finally, cycloadditions between alkenes can be initiated by a catalytic anodic electron transfer. These principal reaction modes are subsequently illustrated by selected conversions. 

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