Fragmentation reactions, solvent effects

An analogous solvent effect was observed upon treatment of the chiral a-alkoxy aldehyde 11 with 2-lithio-4-methylfuran in the presence of zinc bromide. This highly diastereoselective addition reaction was the key step in a synthesis of the enantiomcrically pure C-10-C-20 fragment of the immunosuppressant KK 506139. 

The effects of solvent density on the observed rates can also be interpreted through the onset of cage effects. As the solvent loading increases, the system density increases from a gas-like to a liquid-like value. In dense fluids, an addition reaction such as A + B AB is considered to follow, in series, the formation of an ecountered pair AB (6 ). Similarly, the fragmentation reaction AB - ... 

To express the collective solvent reaction coordinate as in equation (6), it is necessary to define the specific diabatic potential surface for the reactant and product state. This, however, is not a simple task, and there is no unique way of defining such diabatic states. What is needed is a method that allows the preservation of the formal charges of the fragments of reactant and product resonance states. At the same time, solvent effects can be incorporated into electronic structure calculations in molecular dynamics and Monte Carlo simulations. Recently, we developed a block-localized wave function (BLW) method for studying resonance stabilization, hyperconjugation effects, and interaction energy decomposition of organic molecules.20-23 The BLW method can be formulated to specify the effective VB states.14... 

Dipolar activated complexes differ considerably in charge separation or charge distribution from the initial reactants. Dipolar transition-state reactions with large solvent effects can be found amongst ionization, displacement, elimination, and fragmentation reactions such as ... 

Similar solvent effeets ean be found in fragmentation reaetions [109], Sueh reactions may proceed heterolytically or homolytically depending on the solvent used as reaction medium. Thus, the observed solvent effect for the thermolysis of 4-CH3-C6H4-N=N-S-C6H4-4-C(CH3)3 indicates that this benzenediazoaryl sulfane decomposes homolytically in apolar non-HBD solvents according to Eq. (5-148a) [393],... 

A recent modification [la] uses potassium tert-butoxide in anhydrous dimethyl sulphoxide, when reaction is rapid even at room temperature. The enhanced rate is attributed to the high reactivity of anions in this solvent (c/. p. 45). The reaction is usually formulated as a prototropic rearrangement of the hydrazone to give a diimide (i), followed by fragmentation of the diimide anion (2) to give a nitrogen molecule and the carbanion (3) [2], Rapid protonation then affords the reduced product. A recent study [3] oi solvent effects on the reduction... 

The methodology for studying M-Ng complexes in the gas phase is essentially the same as the TRIR method for liquified noble gases a pump pulse photolyzes a metal carbonyl ion and the fragment is detected with the aid of a continuous IR laser. In these experiments helium is utilized as the standard buffer gas. A xenon complex may be detected by alteration in the spectrum and kinetics on addition of xenon. Since the spectra are free of solvent effects, the effect of coordination should be more easily discerned than in the liquid phase. This method has been used to study M(CO)sXe (M = Cr, Mo, W) and W(CO)sKr. Metal-xenon bond energies of ca. 35 kJmol are deduced from the kinetics of reaction with CO. The variation between metals in comparable to the error bars, about 4 kJ mol . The W-Kr bond energy is estimated to be less than 25 kJmol . ... 

The arguments can be extended to more than two substituent variations and the next level of complexity is where a substituent on the fragment E of Scheme 1 would be varied. Such variation is exemplified in displacement reactions at centres adjacent to substituted aromatic rings. Solvent effects can also be included in equations similar to (1) but these are not considered here. 

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