Stepwise elimination

The higher strain energy in thiirene dioxides (19) compared to thiirane dioxides (17) is obvious. Yet, the elimination of sulfur dioxide from the latter is significantly faster than one would expect for a thermally allowed concerted process. Consequently, either aromatic-type conjugative stabilization effects are operative in thiirene dioxides2,12 or the relative ease of S02 elimination reflects the relative thermodynamic stability of the (diradical )99 intermediates involved in the nonconcerted stepwise elimination process. 

Starting from 27, cyclo-Cig was prepared in the gas phase by laser flash heating and the neutral product, formed by stepwise elimination of three anthracene molecules in retro-Diels-Alder reactions, was detected by resonant two-photon-ionization time-of-flight mass spectrometry [23]. However, all attempts to prepare macroscopic quantities of the cyclocarbon by flash vacuum pyrolysis using solvent-assisted sublimation [50] only afforded anthracene and polymeric material. 

Attempts to prepare the corresponding tetrameric para-PAM from 62 were unsuccessful. Dehydrobromination in furan afforded a symanti mixture of cycloadducts 65, that were subsequently transformed to the known dibenzodiyne (66). Formation of 65 is likely to arise from stepwise elimination/cycloaddition rather than to involve the intermediacy of the highly strained tetrameric PAM. 

Although the concerted mechanism described in the preceding paragraph is available only to those azo compounds with appropriate orbital arrangements, the nonconcerted mechanism occurs at low enough temperatures to be synthetically useful. The elimination can also be carried out photochemically. These reactions presumably occur by stepwise elimination of nitrogen, and the ease of decomposition depends on the stability of the radical R ... 

Borazine Pyrolysis Studies. In a simplistic manner, one can visualize the formation of boron nitride via stepwise eliminations between rings, i.e.,... 

Wesdemiotis, C. Feng, R. McLafferty, F.W. Distonic Radical Ions. Stepwise Elimination of Acetaldehyde From Ionized Benzyl Ethyl Ether. J. Am. Chem. Soc. 1985, 707,715-716. 

Irradiation of bischlorobisdiazirine 22-R2 in an inert matrix like N2 [86] or Ar [87] causes stepwise elimination of two nitrogen molecules and leads to the formation of bischlorocarbene 22 (Scheme 4). Identification of 22 was achieved indirectly by trapping it with HCl (to form 23) and O2 (giving 25 and 26) and... 

When the number of noisy (noninformative) variables is too large, PLS models may also supply rather poor predictive performance. In order to overcome such a matter, a number of techniques for the elimination of noisy variables or the selection of useful predictors have been deployed, such as iterative stepwise elimination (ISE), iterative predictor weighting (IPW), uninformative variable elimination (UVE), and Martens uncertainty test (MUT) (Forina et ah, 2007). 

At very low pH (< 1), Fe exists as the purple, hexa-aquo ion, [Fe(H20)6] . Hydrolysis involves the stepwise elimination of protons from the six water moleeules that surround the central Fe eation to form mono- and binuclear speeies. These species then interact further to produce species of higher nuclearity (Sylva, 1972 Johnston and Lewis, 1986 Bottero et al. 1994 Rose et al. 1997 Schwertmann et al. 1999). The latter finally precipitate as a more or less crystalline product, the nature of which depends on the rate and conditions of the reaction. Examples of hydrolysis and polymerization reaetions are ... 

Similar stepwise elimination of Hg is observed from the polymer derived from (PhHjGe), i.e., (PhjGe Hgj), ... 

Lindane is absorbed from the gastrointestinal tract, the respiratory tract, and skin. The metabolism of lindane is complex and involves a number of pathways depending on which isomer of hexachlorocyclohexane (HCH) is involved (lindane is the gamma (y) isomer). It is nonetheless rapid. Lindane is metabolized in the liver by microsomal enzymes. The main pathways include stepwise elimination of chlorines to form tri- and tetrachloro-phenols and conjugation with sulfates or glucuron-ides and subsequent elimination. Other metabolic pathways involve the production of mercaptura-tes. These water-soluble products are eliminated in the urine. Lindane is bound by serum proteins in the blood. Storage is in adipose tissue and other... 

Stepwise elimination of a sulfur atom and nitrogen molecule from the adduct gives the Dewar pyrroles 120,121). The stability of the desulfurized products depends... 

Figure 8 Stepwise elimination of noninteracting peptides from a mixture of 32 peptides and identification of one tight-binding ligand for vancomycin. Interpretation of each electropherogram is described in the text. Specific experimental conditions are described elsewhere (86a). (Reproduced with permission of the copyright holder, publisher and Journal of Organic Chemistry.)...

The effect of cycloamylase on the hydrolysis of several diaryl methylphos-phonates in the pH range 5—11 appears to be due to an initial nucleophilic attack binding the phosphonyl group to the carbohydrate followed by a fast intramolecular nucleophilic attack by an adjacent hydroxy-group expelling a second mole of phenol. Under weakly acid condition intramolecular catalysis of the hydrolysis of (73) occurs, giving stepwise elimination of both... 

The addition or elimination of atoms leads to changes in number, position, and height of the peaks in an RDF descriptor. The elimination is restricted to the parts of a molecule where the skeletal structure remains intact — that is, where no skeletal bond is broken. The minimum is determined through stepwise elimination of all atoms in a definable number of iterations. The addition of atoms is restricted to the former positions of the hydrogen atoms, which are potential positions for new atoms. At first, carbon atoms are added, and the atom type is varied later on. The change of the bond length is considered by a recalculation of a 3D model. The same applies to the removal of atoms. The decision as to whether atoms are removed or added is determined by the number of peaks in the two RDF descriptors and the differences in the maximum distance. 

This scheme agrees with the reaction above, i.e., the addition of Et2GaH to 1-decene, and is preferred to the stepwise elimination of isobutylene ... 

Williams and his coworkers have shown that the hydrolysis and aminolysis of aryl toluene-a-sulphonates proceed via a stepwise elimination-addition (ElcB) mechanism (equation 101)137 139. 

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