Hydrogen bonds basic principles

Residues at N-caps. The rank order of preference at the N-caps of bamase is Asp,Thr,Ser > Asn,Gly > Glu,Gln > His > Ala > Val (Table 17.2).35 36 The basic principles behind these preferences are similar to those at the C-cap. (1) Hydrogen-bond acceptors are particularly effective at stabilizing the NH groups... 

The addition of a single-bonded reagent across a multiple bond is one of the fundamental reactions of organic radicals. The basic principles of this reaction were first advanced by Kharasch in pioneering studies on the mechanism of the peroxide-initiated anti-Maikovnikov addition of hydrogen bromide to alkenes.1 In the atom transfer method, the generation and removal of radicals are coupled and occur in the key atom transfer step. Compared to other methods, the atom transfer method provides unique options for synthetic reactions. But there are also important limitations. Recently, there has been a renewed interest in the application of the characteristics of atom transfer reactions in synthesis and new developments have been reviewed.5,161... 

The basic principle of the stereochemistry of the hydrogenation of the C=C bond is syn (cis) addition. Established already in the early 1930s, it means that both hydrogen atoms add from the same face of the double bond. One of the highest selectivities was observed in the hydrogenation of isomeric 2,3-diphenyl-2-butenes144 the mesa compound is formed from the cis isomer in 98% yield (equation 8), whereas the trans isomer gives the racemic compound in 99% yield (equation 9). 

We have already discussed (p. 106) that T+ and T ionic forms can give the ester and amine products only. Thus, in acidic and neutral media which favor the formation of T+ and T1, imidate salts should always give the ester and amine products. In basic medium, which favors the formation of T , there is the possibility for the formation of both types of products, i, e., ester and amine or amide and alcohol. The cleavage of the C—N bond in the T" tetrahedral intermediate will take place only if the nitrogen electron pair can form a hydrogen bond with a solvent molecule. Thus, experimental evidence in favor of the principle of stereoelectronic control can be obtained with imidate salts, only when the hydrolysis is carried out under basic conditions. 

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