Reactive paradigm

Shaik, S., Filatov, M., Schroder, D., Schwarz, H., 1998, Electronic Structure Makes a Difference Cytochrome P-450 Mediated Hydroxylations of Hydrocarbons as a Two-State Reactivity Paradigm , Chem. Eur. J., 4, 193. 

Since the two-spin state forms can lead to different products, the products obtained will be a mixture that reflects the initial fractionation of the reaction between the two-spin states. The fractionation in turn is a reflection of the interplay and the probability of cross-over between the two-spin states (8). Thus, the two-state reactivity paradigm resolves the dilemma of whether a radical recombination or a direct insertion mechanism governs cytochrome P450-catalyzed hydroxylation actually they are both involved and the degree to which either is expressed depends upon the specific substrate hydroxylated and the specific enzyme. 

S. S. Shaik, A. C. Reddy, A. Ioffe, J. P. Dinnocenzo, D. Danovich, J. K. Cho, J. Am. Chem. Soc. 117, 3205 (1995). Reactivity Paradigms. Transition State Structures, Mechanisms of Barrier Formation, and Stereospecificity of Nucleophilic Substitutions on a-Cation Radicals. 

Shaik S, Filatov M, Schroder D, Schwarz H. Electronic structure makes a difference cytochrome P-450 mediated hydroxylations of hydrocarbons as a two-state reactivity paradigm. Chem Eur J 1998 4(2) 193-199. 

Treit D, Berridge KC (1990) A comparison of benzodiazepine, serotonin, and dopamine agents in the taste-reactivity paradigm. Pharmacol Biochem Behav 27 451 456. 

The model lends itself to articulation of new ideas and new applications. Some potential applications are mentioned above, e.g. the concept of twin-states [5,11,102] and its applications to photochemistry, etc., the concept of catalysis by spin crossover [35] and its application to bond activation processes, the application to problems of electron delocalization [29], and so on. Other articulations of the diagram serve to solve chemical puzzles, such as the recent applications [53] to dissociation of alkoxyradicals, to the invention of a new mechanism, Sri,j2 and its wide range of applications [88 -90,113], to transition metal catalyzed reactions [57-59], to the concept of entangled mechanisms [11], and so on. The future acceptance of the VB diagram model as a general reactivity paradigm depends on such articulations. 

It s worth emphasizing again that for Si and other elements below period 2, the S 2 displacement is not a major reactivity paradigm. Instead, nucleophilic displacement typically occurs via the two-step Sisf2-Si pathway. The first step of the SN2-Si path is an A reaction, which typically leads to a hypervalent intermediate. Hypervalent molecules and ions are very common for Si and all its heavier congeners. 

Avoided Crossing States and Valence Bond Mixing Fundamental Reactivity Paradigms. 

Soc., 117, 3205 (1995). Reactivity Paradigms. Transition State Structures, Mechanisms of Barrier Formation, and Stereospecificity of Nucleophilic Substitutions on a-Cation Radicals. 

For decades, the accelerating effect of ultrasonic irradiation has been a useful reactivity paradigm most physical and chemical effects arise from cavitations without an alteration of the rotational or vibrational states of molecules. In contrast to classical chemistry, in sonochem-istry it is not necessary to go to higher temperatures in order to accelerate the chemical process. To drive the chemical transformations the released kinetic energy from the cavitational collapse is sufficient [177]. Such an effect was also observed in this esterification reaction, where at room temperature (Table 6.10, entry 5) both the reaction rate and the selectivity in the main product were enhanced in comparison to the values obtained at 80°C (Table 6.10, entry 4) the reaction rate increased 43 times when compared with thermal activation and around 6 times when compared with microwaves. Even more importantly the selectivity to DAG and TAG after 30 min was at almost the same level as that obtained by thermal heating at 100°C for 22 h. 

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