Reactive centres

Carrell, R.W., Evans, D.F., Stein, RE. Mobile reactive centre of serpins and the control of thrombosis. Nature 353 576-578, 1991. 

Stein, P.E., et al. Crystal stmcture of ovalbumin as a model for the reactive centre of serpins. Nature 347 99-102, 1990. 

The polymerisation reactions that occur by the chain mechanism are typically those involving unsaturated monomers. The characteristic reaction begins with the chemical generation of reactive centres on selected monomer... 

Although the most important chain reactions are those involving free radicals, they are not the only ones that are possible. The reactive centre at the growing end of a polymer molecule may alternatively be ionic in character or involve co-ordination to metal complexes. 

The term chemistry in interphases was first introduced in the field of reverse-phase chromatography [41], In 1995 Lindner et al. transferred the concept to the area of transition metal catalysis [42] and in a recent review the concept is explained in detail [43], The interphase is defined as a region within a system in which the stationary and a mobile component penetrate on a molecular level without the formation of a homogeneous mixture. In these regions the reactive centre on the stationary phase... 

In the presence of a proper second molecule bimolecular photochemical processes occur. Obviously such reactions can also occur in an intramolecular fashion in bifunctional molecules containing both reactive centres. These reactions are a) hydrogen abstraction by the excited molecule if the second molecule (or reactive centre) is a hydrogen donor RH (1.14) b) photodimerisation (1.15) c) photoaddition or photocycloaddition (1.16) d) electron transfer (1.17), if no bonding takes place between the reactants (or reactive centres). ... 

An isokinetic plot of EA versus in A for straight chain and isopentyl substrates is linear and all substrates, where branching of the alkyl chain is at least <5 to the reactive centre, nitrogen, behave similarly (Fig. 21, filled squares).43,181,182 The isopentyloxy and butyloxy substrates 25h and 25c have very similar transition state... 

Bimolecular reactions of aniline with /V-acyloxy-/V-alkoxyamides are model Sn2 processes in which reactivity is dictated by a transition state that resembles normal Sn2 processes at carbon. Electronic influences of substituents support a non-synchronous process which has strong charge separation at the transition state and which is subject to steric effects around the reactive centre, at the nucleophile but not on the leaving group. The sp3 character of nitrogen and disconnection between the amino group and the amide carbonyl renders these reactions analogous to the displacement of halides in a-haloketones. 

As pyramidal amides5,32 their Sn2 reactivity with neutral nucleophiles like /V-methylaniline parallels that of a-haloketones with amines, which, as described in an earlier section, are also strongly affected by steric effects on the a -carbon.183 SN2 reactions are in general strongly and adversely influenced by steric effects and branching / to the reactive centre and the same appears to be true for /V-acyloxy-/V-alkoxyamides 30b and 29a-e. Broadly speaking, their mutagenic activity is affected similarly. 

By correlating the gas-phase and solution data derived from proton transfer reaction, the deviations were accounted for by solvation effects on the reactive centre and/or the substituent. 

In the new structures of type (59) the reactive centre is the methanide site which behaves as a nucleophile. <86JCS(Pi)ii57, 92JCS(P1)1483> (Section 4.17.8). Tautomeric tetrazole-tetrazoline structures of general type (4 R = H) may show ambident reactivity with reactions occurring on the ring or the exocyclic X atom (Sections 4.17.6 and 4.17.7). 

The presence of the O reactive centres was confirmed by a sharp symmetric signal at g 2.003 following contact of Fe-MFI with N20. While interaction of N20 with oxide surfaces is widely used to generate surface O- centres, it is uncommon to observe such a signal at elevated temperatures, as in the Fe-MFI... 

Several generalizations emerge from an exhaustive study of these complexes the methoxide ion prefers to attack an a-position in the thiophene nucleus the resulting thiophene complexes are in general more stable than the corresponding Meisenheimer complexes in the benzene series and the stability is increased if the reactive centre already carries an alkoxy substituent. 

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