Introduced substitution

Thus the reactions of cyclic or acyclic enamines with acrylic esters or acrylonitrile can be directed to the exclusive formation of monoalkylated ketones (3,294-301). The corresponding enolate anion alkylations lead preferentially to di- or higher-alkylation products. However, by proper choice of reaction conditions, enamines can also be used for the preferential formation of higher alkylation products, if these are desired. Such reactions are valuable in the a substitution of aldehydes, which undergo self-condensation in base-catalyzed reactions (117,118). Monoalkylation products are favored in nonhydroxylic solvents such as benzene or dioxane, whereas dialkylation products can be obtained in hydroxylic solvents such as methanol. The difference in products can be ascribed to the differing fates of an initially formed zwitterionic intermediate. Collapse to a cyclobutane takes place in a nonprotonic solvent, whereas protonation on the newly introduced substitutent and deprotonation of the imonium salt, in alcohol, leads to a new enamine available for further substitution. 

Pyrrolo[l,2-fl]thieno[2,3-c]pyrazine (186) brominated mainly in the fused pyrrole ring. Only the third bromine atom introduced substituted the thiophene /3-position (86JHC17). 

Besides to introduce substitutents onto the ra ophenyl ring or the P-pyrrolic positions, modification of the molecular structures by fusing a small aromatic ring to the porphyrin ring has also been an attractive way to extend the absorption spectra and tune the electrochemical properties of porphyrins in the prospect of improving the sensitization behavior. Imahori has investigated the application of a fused five-membered porphyrin (84) as sensitizer in DSSC. Compared with the reference porphyrin, the fused porphyrin shows broaden and red-shifted absorption spectra... 

Introduction their functional groups. In this chapter, we consider the properties and reactions of alkyl halides. We use alkyl halides to introduce substitution and elimination, two of the most important types of reactions in organic chemistry. Stereochemistry (Chapter 5) will play a major role in our study of these reactions. Many other reactions show similarities to substitution and elimination, and the techniques introduced in this chapter will be used throughout our study of organic reactions. 

In 2001, Jain and co-workers (104) used Anderson and Kochi s discovery to introduce substitution on a histidine ring. Although yields are moderate and the scope is limited, this selective introduction of an alkyl group to a heterocyclic system has potential to be optimized for synthetic applications. Later, Jain et al. (105) used this method to build various quinoline derivatives that may have anti-Tuberculosis activity. In 2001, Frost and co-workers (106) used a silver-mediated... 

In addition to changing the P-substitution pattern of porphycene, Vogel and coworkers have introduced substitution at the w 50-positions of the macrocycle. Specifically, the 9,10,19,20-tetraalkyl porphycenes 3.37-3.39 were prepared via the reductive carbonyl coupling of the corresponding 5,5 -diacyl-2,2 -bipyrroles... 

Continuous reduction of risk by introducing substitutes and setting limitations of exposure... 

Since absorbances can only be measured in a certain dynamic range, the dilute solutions used in fluorescence measurements cannot be taken. Therefore the assumption is necessary that the photochemical reaction is governed by the same mechanism for 10 and 10 M solutions. Furthermore the higher concentration forbids an approximation according to eq. (5.111). However, in Section 3.3.11 a principle was introduced substituting the normal time scale by a transformation into a pseudo time (f) In consequence the three equations of eq. (5.147) have to be substituted by... 

A completely different approach for the doping of a-Si H has been followed by Beyer and Fischer (1977). They showed that n-type doping is also obtained by interstitially incorporated lithium brought into the films by in-diffusion as well as by ion implantation. The possibility of introducing substitutional dopants in a-Si H by ion implantation has been proved by Muller et al. (1977). Subsequently, these authors demonstrated the doping effect of most elements of the groups I, III, and V of the periodic table by ion implantation experiments (Kalbitzer et al., 1980). 

The absence of cis-di-exo-dihalogen derivatives is attributed by Wilt to steric reasons. On the conversion of trans-dichloride 288, however, Cristol has shown this explanation to be unsatisfactory. The nucleophihc attack at the position nonvicinal to the already introduced substitutent can be accounted for, according to Cristol, either by the high stability of cation 291 in comparison with cation 290 or by a higjier value of the positive charge on in ion 289. 

Hartwig and co-workers once again exploited the high-throughput approach and FRET screening to uncover new palladium catalysts for the arylation of a-cyanoacetates. The approach was very similar to those described for the aryl amination and Heck coupling reactions previously described (refer to Sections 1.13.2.4 and 1.13.2.6). The authors uncovered effective catalysts for this transformation, introducing substituted ethyl cyanoacetates as new and versatile synthons. 

In an analogous way the equilibrium condition can be found when the gel and the solution have no common ion, or when one of the ions has a valency differing from unity. Further, the theory of strong electrolytes may be introduced, substituting ionic activities for ionic concentrations. ... 

One of the general methods to introduce substitutions at C-9 involves amidation of 9-amino-NeuAc, which is obtained by reduction of 9-azido-NeuAc/ The routine use of 9-azido-NeuAc as a key intermediate in the synthesis of 9-substituted sialic acids is limited by lengthy procedures for its preparation. Current chemooizymatic or chemical syndiesis methods for preparation of 9-azido-NeuAc involve multi-step, low yield procedures starting from eidier N-acetyl-mannosamine or NeuAc. We developed an efficient, high yield, two step thesis of 9-azido-NeuAc as depicted in Scheme 1 and briefly described below. Detailed procedures can be found in the supplemental methods to Han et at htto //www.nature.coni/nchembio ouraal/vl/n2/extre nchembio713-S7.pdf. 

Ion implantation of graphene is a possible route to modifying the material s electrical properties by introducing substitutional impurities... 

In 1978, Heck and coworkers [la, 9] introduced substituted fiiarylphosphines associated with Pd(OAc)2. Among them, the tri-o-tolylphosphine, P(o-Tol)3, was found to be more efficient than PPhs in reactions involving aryl bromides (experimental conditions of Scheme 1.7 at 75 °C). In 1983, Spencer improved the Mizoroki-Heck reactions catalysed by Pd(OAc)2 associated with P(o-Tol)3 upon using the polar solvent DMF and NaOAc as... 

Multilayers of substituted and unsubstituted phthalocyanines have been investigated by several authors using the LB technique and have been characterized by electron microscopy, UV-vis spectra and electron paramagnetic resonance [193-198]. Since unsubstituted phthalocyanines are insoluble in common organic solvents they are made soluble by introducing substitutents such as t-butyl [195], cumyl phenoxy... 

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