Heteroatom elimination

In most carbon and graphite processes, the initial polymerization reactions occur in the Hquid state. The subsequent stages of crystal growth, heteroatom elimination, and molecular ordering occur in the soHd phase. The result is the development of a three-dimensional graphite stmcture. 

The conductivity of many metal modified systems is reportedly enhanced due to various factors such as charge transfer between metal ions and the electron-rich heteroatoms, elimination of impurities, and changes in the transport number of cations and anions due to environmental changes in the solid electrolytes. Even interesting cases have been reported where a polymer film can reach the electronically conducting metallic level by cis-trans isomerization. 

Heteroatom Elimination. Sulfur elimination from hydrorefined filtrate varied from 42 wt % at 650°F to 98 wt % under the most severe conditions at 750°F. The data presented in Figure 4 shows a very rapid initial decrease in sulfur content probably related to an initially rapid catalytic conversion of sulfur compounds particularly susceptible to catalytic reaction but thermally stable. Such compounds could be di-... 

The heteroatom elimination from total feed is plotted vs. temperature in Figure 11. The data show an increase in sulfur, oxygen, and nitrogen removal as the temperature increases. The scatter in the oxygen data is considerably greater than for either nitrogen or sulfur. 

Table X. Heteroatom Elimination from Total Product for Ni—W...

A palladium(II)-catalyzed three component coupling reaction was established by Lu, who performed the intermolecular carbopalladation involving propargyl alcohols and alkenes, and this was followed consecutively by allylic chloride insertion to the C-Pd bond and its quenching by p-heteroatom elimination in the presence of an excess of chloride ions. An example is shown below <03TL467>. 

There are several limitations on these methods with regard to the heteroatom involved. Reaction of the tin heterocycle with NBS results in cleavage of the aromatic-tin bonds to give o,o -dibromobibenzyl (81). Reaction of the tin heterocycle with DDQ resulted only in products of heteroatom elimination (81). Attempts to generate a silicon heterocycle with a functional group on the Si atom have failed to produce the unsaturated system (XVII). Dehydrobromination reactions of the silane afford highly colored reaction products but no isolable identifiable monomer. It is probable that the dehydrobromination reaction occurs not only at the ethylene bridge but also by transannular elimination. 

In the reactions shown above, preferential jS-heteroatom elimination occurs rather than jS-H elimination. Lu reported that the presence of a halide ligand effectively blocks jS-H elimination. Thus the different products were obtained depending on the amounts of added LiBr in the reaction of 3-heptynoic acid (393) with acrolein. In the presence of 200 mol% LiBr in AcOH, protonolysis occurs to give 395 and Pd(II), while the unsaturated aldehyde 394 was obtained by a... 

Fig. 4.30) with various substitution pattern at Cl and C2 were investigated. Experiments with alkyl substituents at C2, in place of the sulfur heteroatoms, eliminate the viability of a chelation-controlled selectivity and reveal the significance of geminal substitution [geminal substitution influenced the selectivity in other oxocarbenium ion systems [108]]. Perturbation of the Cl substituent of the oxocarbenium ion intermediate has little effect on reaction stereoselectivity, and analysis of this observation lends additional support for stereoelectronically preferred inside attack of the nucleophile. The results demonstrate that selective formation of the 1,4-c/s product 84 does not require a chelated transition stracture, reinforcing the utility of the inside attack model to analyze the reactivity of complex five-membered ring oxocarbenium ion intermediates. 

The key point in divalent Pd-catalyzed cyclization of allylic alkynoates is the method of quenching of the carbon-paUadium bond formed after cyclization. In order to develop a divalent Pd-catalyzed reaction, a divalent palladium species must be regenerated in the quenching of the carbon-paUadium bond. Lu and co-workers developed different quenching methods for the carbon-palladium bonds, for example, /3-heteroatom elimination, copper halide mediated oxidative cleavage, and carbonylation and protonolysis of... 

The results of the four types of Pd(II)-catalyzed reactions are sununarized in Table 1. In addition, the stereochemistry of the /3-heteroatom elimination and the oxidative cleavage reaction were also studied. The results show that the /3-heteroatom elimination proceeds in an anti manner, most probably through an E2-like mechanism with a reactivity order Cl > OMe > OAc > OH While the oxidation cleavage in this reaction... 

Carbozincation of oxabicyclic and azabicyclic alkenes can be controlled to proceed without p-heteroatom elimination using a derivative of dppbz as ligand (Scheme 4-273). The intermediate zinc organyl can be quenched with acid or treated with an electrophile. The latter gives the corresponding cis-disubstituted heterobicycloalkane. ... 

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