Intramolecular reactions theoretical calculations

Inter- and intramolecular hydrogen bondings are also important for the formation of epoxytwinol A (71), which would be produced by the quite rare formal [4 -I- 4] cycloaddition reaction. Theoretical calculations suggest that preassociation... 

A series of theoretical studies of the SCV(C)P have been reported [38,40,70-74], which give valuable information on the kinetics, the molecular weights, the MWD, and the DB of the polymers obtained. Table 2 summarizes the calculated MWD and DB of hyperbranched polymers obtained by SCVP and SCVCP under various conditions. All calculations were conducted, assuming an ideal case, no cyclization (i.e., intramolecular reaction of the vinyl group with an active center), no excluded volume effects (i.e., rate constants are independent of the location of the active center or vinyl group in the macromolecule), and no side reactions (e.g., transfer or termination). 

Thus Marvel and Levesque found that from 79 to 85 percent of the oxygen was removed in this process, to be compared with the theoretically calculated figure of 81.6 percent (fraction unreacted equal to 1/c) for intramolecular reaction of this type in a head-to-tail polymer. A head-to-head, tail-to-tail arrangement, consisting of 1,4-diketone structures, should be expected to yield furan rings... 

Although this type of reaction is symmetry forbidden in an unadsorbed molecule, theoretical calculations showed that in a molecule adsorbed on transition metals, such a shift is allowed [3-5], Later, other theoretical calculations suggested another type of 1,3-hydrogen shift, one in which the allylic cxo-hydrogen is abstracted by the surface fi-om an adsorbed alkene (either 1,2-diadsorbed or n-complexed) and the resulting 7i-allyl species moves over the abstracted hydrogen in such a way that it adds to the former vinylic position and causes, in effect, a stepwise intramolecular 1,3-hydrogen shift (bottom shift) [6],... 

Cozzi and co-workers (243,263) studied the influence of the double-bond configuration on the stereochemical course of the intramolecular cycloaddition of chiral alkenes, where the stereocenter is located outside the isoxazoline ring (Table 6.15). On the basis of experimental results as well as theoretical calculations, two models were proposed for the reaction with (Z)- and ( )-aIkenes, in accord with the model proposed for a-X-substituted alkenes (see Section 6.2.3.1). 

There is continued expansion in the use of metals as catalysts in substitution reactions. Copper iodide in the presence of /V./V -dimcthylcthylcncdiamine has been shown to be effective in the intramolecular substitution of aryl bromides carrying an o-l,3-dicarbonyl substituent reaction may involve either an oxygen centre or a carbon centre of the dicarbonyl moiety.26 The reaction of aryl halides with sodium trifluoroacetate in the presence of copper iodide may lead to the formation of the tri-fluoromethylated derivatives, possibly via CF3CuI as an intermediate.27 There have been theoretical calculations, PM3 and ab initio, on complexes formed from copper... 

In a series of theoretical calculations of addition to protonated formaldehyde it was also revealed that the intrinsic properties of the attacking nucleophile (X) are reflected in the bond energy of the X-CH2-OH+ intermediate (X = NH3 >H20 >HF >H2), and in the barrier for the subsequent 1,3-intramolecular proton transfer leading to water elimination [129]. In comparing different substituents X and Y, it was also found that for the same X,but different CH2Y+ ions the reactivity order is Y = NH2 reactions with formamide, it has been found that Y = OH [Pg.16]

A comparison between the overall rate coefficients, calculated in different ways, and those recommended by Atkinson et al [89] is provided in Table 12.1 and Figure 12.3. All the theoretical calculations were performed within the CTST approach. As these results show, taking into account the reactant complexes and the intramolecular interactions in the transition structures in the modeling of alcohols + OH radical reactions in gas phase, improve the agreement of the theoretical calculations with the experimental data. 

Next, Ch. 11 by Lochbrunner, Schriever and Riedle deals with excited electronic state intramolecular tautomerization proton transfers in nonpolar, rather than polar, solvents. But there is a connection to the previous chapter the ultrafast optical experiments discussed here emphasize evidence that the proton is not the reaction coordinate. The proton transfer is controlled by low vibrational modes of the photo-acids, rather than by the proton motion itself, an interpretation supported by separate vibrational spectroscopic studies and theoretical calculations The key role of modes reducing the donor-acceptor distance for proton transfer is highlighted, and for the featured molecule of this chapter, the proton adiabatically follows the low frequency modes, and no tunneling or barrier for the proton occurs. (See also Ch. 15 by Elsaesser for direct ultrafast vibrational studies on these issues). 

An SN2-type substitution at the sp -hybridized nitrogen atom of a C=N bond has recently been reported for the acid-catalyzed intramolecular reaction of oxime 14 (Scheme 11). The results have been rationalized by theoretical calculations. 

A large number of papers related to this subject has been published since then, which can be classified into three types of mechanistic approaches (i) studies of initial product distribution, (ii) hydrogen deuterium exchange studies and (iii) theoretical calculations. In this review we will limit ourselves essentially to H/D isotope exchange studies between the acidic catalyst and small alkanes. In most cases, it is possible to observe inter-molecular H/D exchange in the absence of side reactions such as intramolecular atom scrambling, isomerisation or cracking. 

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