Acid-induced processes

Several iodine-catalyzed organic transformations have been reported. Iodine-catalyzed reactions are acid-induced processes. Molecular iodine has received considerable attention because it is an inexpensive, nontoxic and readily available catalyst for various organic transformations under mild and convenient conditions. Michael additions of indoles with unsaturated ketones were achieved in the presence of catalytic amounts of iodine under both solvent-free conditions and in anhydrous EtOH (Scheme 19) [85,86]. l2-catalyzed Michael addition of indole and pyrrole to nitroolefins was also reported (Scheme 20) [87]. 

Acid-induced processes Although perfluoroaromatic nitrogen heterocyclic compounds are only weak bases, nucleophilic substitution can be induced by protonic or Lewis acids, as outlined in Figure 9.45, and interesting contrasts in orientation can sometimes be achieved because attack ortho to nitrogen is often preferred under these conditions. 

Hie o-(acyiamino)amtdroxamine (6.1) is a versatile intermediate and is cyclized to the )V-oxide by heating with ethanol and hydrochloric acid for 40 min (see also Section II.3). Annotation of the acylamino and oximino functions in the peri positions of the thioxanthene (6.2) is an efficient acid-induced process. 

The ASI workshop on "Selectivities in Lewis Acid Promoted Reactions" held in the Emmantina-Hotel in Athens-Glyfada, Greece, October 2-7, 1988 was held to bring some light into the darkness of Lewis acid induced processes. 

Another important acid-induced process in fluorinated diazine chemistry is fluorine-halogen exchange promoted by Lewis acids. In some cases the switching of regiochemistry also observed. Thus in a case of dimethyl-(3,5,6-trifluoro-pyrazin-2-yl)-amine 300 treatment with AICI3 leads to exchange of flnorine at 6-th position unlike with MeaNH substitution (compare with Fig. 14). Subsequently dialkylamino - AICI3 treatment leads to dichloropyrazine 303, which unavailable by selective manner from tetrachloropyrazine (Scheme 45) [164]. 

The rate of the Lewis-acid catalysed Diels-Alder reaction in water has been compared to that in other solvents. The results demonstrate that the expected beneficial effect of water on the Lewis-acid catalysed reaction is indeed present. However, the water-induced acceleration of the Lewis-add catalysed reaction is not as pronounced as the corresponding effect on the uncatalysed reaction. The two effects that underlie the beneficial influence of water on the uncatalysed Diels-Alder reaction, enforced hydrophobic interactions and enhanced hydrogen bonding of water to the carbonyl moiety of 1 in the activated complex, are likely to be diminished in the Lewis-acid catalysed process. Upon coordination of the Lewis-acid catalyst to the carbonyl group of the dienophile, the catalyst takes over from the hydrogen bonds an important part of the activating influence. Also the influence of enforced hydrophobic interactions is expected to be significantly reduced in the Lewis-acid catalysed Diels-Alder reaction. Obviously, the presence of the hydrophilic Lewis-acid diminished the nonpolar character of 1 in the initial state. 

The diastereoselectivity of these reactions was initially explained in terms of acyclic, an-tiperiplanar transition states23, although a synclinal process has since been proposed on the basis of the stereoselectivity of Lewis acid induced intramolecular allylsilane- aldehyde cycliza-tions56. 

Pitting Corrosion Pitting corrosion is a general term for a visible sign of concentration cell corrosion, where further induced-corrosion processes develop, as when chloride attack occurs (although pits also may occur with simple acid corrosion processes). 

Both thermal- and acid-induced equilibrations of 3,3-disubstituted thietane oxides were very slow (K 10-5 s-1)194. The results suggest that thietane oxides are similar to the various acyclic sulfoxides with respect to the rates of thermally induced pyramidal inversion at sulfur238, and that this inversion process, therefore, does not interfere significantly in the above exchange/racemization studies. 

Another interesting cationic domino process is the acid-induced ring opening of a-cyclopropyl ketones and subsequent endocyclic trapping of the formed carboca-... 

The mechanism of this transformation is a matter of debate, and may vary with the structure of the heteroanalogous carbonyl compound employed. Although a Diels-Alder-type process is conceivable [246], a Lewis acid-induced addition of the silyl enol ether moiety in 2-453 followed by a cyclizahon through a nucleophilic intramolecular attack of the amine and subsequent elimination of methanol is assumed in this case [247]. 

The process control of the post-exposure bake that is required for chemically amplified resist systems deserves special attention. Several considerations are apparent from the previous fundamental discussion. In addition for the need to understand the chemical reactions and kinetics of each step, it is important to account for the diffusion of the acid. Not only is the reaction rate of the acid-induced deprotection controlled by temperature but so is the diffusion distance and rate of diffusion of acid. An understanding of the chemistry and chemical kinetics leads one to predict that several process parameters associated with the PEB will need to be optimized if these materials are to be used in a submicron lithographic process. Specific important process parameters include ... 

The case of succinic acid cannot be discussed in terms of Coulombic interactions alone. Here, conformational changes induced by the binding process can contribute significantly to the correlation. Note also that g(l, 1) [or W(l, 1)] of succinic acid is not an average of the correlations in maleic and fiimaric acids. This could be partially due to the configurational changes in the succinic acid, induced by the binding process. We shall discuss below a simple two-state model for succinic acid, and a continuous model in the next subsection. 

Clay minerals behave like Bronsted acids, donating protons, or as Lewis acids (Sect. 6.3), accepting electron pairs. Catalytic reactions on clay surfaces involve surface Bronsted and Lewis acidity and the hydrolysis of organic molecules, which is affected by the type of clay and the clay-saturating cation involved in the reaction. Dissociation of water molecules coordinated to surface, clay-bound cations contributes to the formation active protons, which is expressed as a Bronsted acidity. This process is affected by the clay hydration status, the polarizing power of the surface bond, and structural cations on mineral colloids (Mortland 1970, 1986). On the other hand, ions such as A1 and Fe, which are exposed at the edge of mineral clay coUoids, induce the formation of Lewis acidity (McBride 1994). 

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