Nucleophilic processes

The nucleophilic route can be improved, by using diphenyl sulfone as a high-boiling solvent. In this process, hydroquinone is transformed into its dipotassium salt by heating with an equivalent amount of potassium carbonate or potassium bicarbonate, with simultaneous removal of the water at 150-200°C, followed by the addition of the second monomer, namely, 4,4 -difluorobenzophenone. The polymerization reaction is carried out at 320-350°C to obtain a polymer of an IV in the range of 0.8 to 1.4 dl g with a melting point of 335-350°C. The polymers by this process are claimed to be very useful for wire coating.  

It is difficult to prepare composite polymer materials with liquid crystalline polyesters, because the liquid crystalline molecules aggregate and do not mix with the other component. Therefore, block copolymers are prepared. 

The polymer can be synthesized from 4,4 -difluorobenzophenone bis-phenol A and 4 -hydroxy phenyl-4-hydroxybenzoate. NMP and toluene are used as a solvent and for azeotropic water removal. The condensation is conducted at 155°C for 8 h using a Dean-Stark trap. In the final stage of condensation, the toluene is drained and the temperature is increased to 190°C. 

Liquid crystalline poly(aryl ether ketone) polyesters are then prepared by copolymerization of the ester group containing poly(aryl ether ketone) with a liquid crystalline polyester. The crystalline polyester is synthesized from phenylhydroquinone ferf-butylhydroquinone, 2-chloroterephthaloyl chloride and isophthaloyl chloride.  

In a variant of the process, instead of the ketone monomer, a ketimine monomer is used to prepare a poly(ether ketimine).The ketimine polymer can be dissolved in NMP. Tbe ketimine groups in the polymer can then be hydrolyzed by means of a dilute aqueous acid solution. Hydrolysis gives back the ketone polymer that precipitates out as a fine powder. Particles of the size of 0.5-5 pm are produced by this procedure. 

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The reactions of these nucleophilic processes are usually S 2 rather than S l. The reaction rate is methyl > ethyl > isopropyl, as with the alkyl hahdes. As the species to be alkylated becomes more nucleophilic, alkylation becomes faster, eg, a sulfur-containing anion alkylates more quickly than a phenohc anion. 

Nucleophilic processes that generate chloroindoles are largely confined to the displacements of oxy functions and Sandmeyer reactions of diazo-nium salts [81 H( 15)547]. A low yield of 2-chloroindole was obtained by a reaction sequence that involved treatment of oxindole with phosphoryl chloride, and then treatment of the Vilsmeier salt with sodium bicarbonate [66JOC2627 86H(24)2879]. It is, however, much better to prepare this compound from 2-lithioindole (92JOC2495). With phosphoryl chloride and dimethylformamide ethyl l-hydroxyindole-2-carboxylate failed to give the expected 3-formyl derivative. Instead there was a 50% yield of the 3-chloro derivative (84CPB3678). Diazonium salts have been used as precursors in... 

Among the nucleophilic processes available for introduction of bromine to quinolines are reactions of the diazonium salts (87JHC181) and syntheses based on hydroxyquinolines or quinolones (91M935) (Scheme 36). The former processes are especially useful for making 5-, 6-, 7-, and 8-bromo derivatives. Halogen-halogen exchange reactions have also been reported, but they are not common. When perfluoroquinoline was heated... 

However, if we consider the alternative nucleophilic displacement, it is known that nucleophilic processes are accelerated by ionic liquids, but more pertinent is the fact that the Sn2 displacement of iodide from alkyl iodide (Mel) by Rh(CO)2l2 is slightly accelerated by ionic liquids (7). Unfortunately, ionic liquids would also be expected to accelerate the nucleophilic displacement of iodide from ethyl iodide by propionic acid to form ethyl propionate (Reaction 8). In fact, as an Sn2 Type II displacement (the interaction of two neutral species), the ester formation from propionic acid and ethyl iodide would be expected to be significantly increased compared to the reaction of Rh(CO)2l2 with EtI. Therefore, by operating in iodide containing ionic liquids, we had set up a situation in which we suppressed the normally predominant hydride mechanism, slightly accelerated the alternative nucleophilic mechanism, but dramatically increased the ethyl propionate by-product forming pathway. 

While the mechanism in the absence of Eti or HI is still a matter of conjecture, it is unlikely that a hydride mechanism was operable since, whereas we could possibly envision an imidazolium salt donating a hydrogen via carbene formation, there is no corresponding viable source of hydride when using pyridinium and phosphonium salts which are also effective solvents for the process. Therefore, by process of elimination, it was more likely that the process was operating via a nucleophilic process. 

Based on our success with the carbonylahon of ethylene and the evidence supporhng the nucleophilic process, there was reason to believe we might also be able to conduct Rh catalyzed carbonylahon of methanol in the absence of Mel. As displayed in Table 37.3, the Rh catalyzed carbonylahon of methanol was achievable in the absence of Mel with a variety of iotuc liquids. More importantly, at most, only traces of Mel could be detected in the medium. The lack of any Mel was significant since we were concerned that we could generate significant levels of Mel in situ via the nucleophilic displacement of the methyl group by iodide (Eqn. 23.)... 

Sjoberg, P., and P. Politzer. 1990. The Use of the Electrostatic Potential at the Molecular Surface to Interpret and Predict Nucleophilic Processes. J. Phys. Chem. 94, 3959. 

Nucleophilic processes that introduce chlorine include displacement of diazonium functions, but these are not well known in the imidazoles because of the instability of many simple aminoimidazoles. In one instance the lack of success may have been a function of the high stability of 5-ethoxycarbonyl-4-diazoimidazole. Other 1-substituted 4-diazonium salts showed expected reactivity, and 1-substituted 5-aminoimidazoles formed sufficiently reactive diazonium salts to give good yields of the 5-chloro compounds [80JCS(P1)2310]. Most of the thrust in this reaction strategy has focused on the preparation of fluoroimidazoles (see B,2,d). 

Other nucleophilic processes have included displacements of diazonium salts (66JMC733), and the reactions of potassium fluoride with 1-methoxy-2-phenyl-l,2,3-triazolium salts and their 4-methyl derivatives to give the... 

Chloro- and bromo-l,3,4-thiadiazoles are usually prepared by nucleophilic processes, e.g., Sandmeyer reactions of diazonium salts [56CB1534 68AHC(9)165 86CHE1148], and reactions of thiadiazolinones with phosphorus halides [68AHC(9)165]. The halogeno derivatives are important... 

Nitration of dibenzofuran at C-3 as opposed to other electrophilic substitutions such as acetylation at C-2 has been attributed to the intervention of a charge-transfer process [21]. The C-N bond formation step is mechanistically closer to the nucleophilic process, the aromatic moiety being the electron-deficient species. It is understandable that N02. attacks at a nuclear carbon which is meta to the oxygen donor. 

The hydrolyses of glycine methyl ester and glycine ethyl ester appear to be nucleophilic processes, since as usual in nucleophilic reactions, the methyl ester reacts approximately twice as fast as the ethyl ester. 

This is a case of allowed syn entry and departure in Table 6,j = 3 and the number of electrons is 6. Oxotropy, a nucleophilic process (Mackenzie, 1964), could be stereoselective, but we are not aware of any relevant examples. 

A His residue is often found at the active site of enzymes where it functions as a catalyst in acid-base and nucleophilic processes. Substitution of fluorine on His reduces the pKa by about 5 pH units, and this dramatic drop in basicity is reflected in altered biological properties of FHis-containing proteins. The presence of 2-FHis in cell cultures inhibits the stimulation of several enzymes, for example, the stimulation of pineal gland A-acetyltransferase activity, in cell culture and in vivo. This stimulation is accompanied by His and cyclohex-imide-sensitive incorporation of 2-FHis into cellular protein192,193. A direct comparison of His and 2-F-His in mouse L cells showed that the analogue is incorporated at about 17% the efficiency of the parent194.4-FHis showed none of the above biological activity. 

The effects contributed by alkyl groups to the relative rate constants, kreh for the reaction of ozone with cis- and trans-1,2-disubstituted ethylenes are adequately described by Taft s equation = k °reX -f pSo-, where So- is the sum of Taft s polar substituent constants. The positive p values (3.75 for trans- and 2.60 for cis-l,2-disubstituted ethylenes) indicate that for these olefins the rate-determining step is a nucleophilic process. The results are interpreted by assuming that the electrophilic attack of ozone on the carbon-carbon double bond can result either in a 1,3-dipolar cycloaddition (in which case the over-all process appears to be electrophilic) or in the reversible formation of a complex (for which the ring closure to give the 1,2,3-trioxolane is the nucleophilic rate-determining step). 

Perhaps the most interesting point which emerges from the results is that in ethylenes bearing electron-releasing alkyl substituents the ratedetermining step appears to be a nucleophilic process, as indicated by the positive p values. This does not contradict the assumption that the first step in the ozone—olefin reaction is an electrophilic attack of ozone on the carbon-carbon double bond. The present observations also agree with some of the results obtained recently by Pritzkow et al. (16) for alkyl mono-substituted ethylenes in ethanol solution at — 60 °C. 

Amidines 14 are cyclized to quinazolines 15 in lithium alkylamide or dialkylamide mediated reactions in which the construction of the quinazoline ring system and introduction of the amino substituent occur in the same reaction. Mechanistically, each fluorine of the trifluo-romethyl substituent of the amidine is displaced by a series of internal nucleophilic processes and the resulting quinazoline contains the amino function of the lithium reagent incorporated at the C4 position. The cyclization method is suitable for the synthesis of sterically congested quinazolin-4-amines and in syntheses of derivatives for which the corresponding chloroquinaz-olines are not readily available. ... 

Elsewhere in this chapter we have used oxygen nucleophilic processes to illustrate stereoselectivity (Sections II.B and II.C), coelectrophiles (Section I.A), etc. and shall not repeat these here. The literature of oxygen nucleophile attacks on alkynes includes trends in substituent effects but the data are often qualitative and usually scattered. Numerous examples indicate that the reactions of alkoxides with C2H2 are relatively slow and that most substituents facilitate the addition (Tables 13 and 14) ... 

Such an electron-transfer rather than nucleophilic process has been one of the most central propositions in reaction mechanism [152-160]. The second step in the formation of the final bis-adduct may also proceed by an electron transfer or alternatively by an Sn2 reaction (Scheme 1). An electron transfer from to RX... 

Although the reaction of electrophilic cyclopropanes with protonic acids cannot be considered as apparently strictly nucleophilic processes, this reaction can be classified under this section. For example hydrobromination of ethyl 1-acetylcyclopropane carboxylate (674) affords 5-bromo-2-pentanone (675) (equation 238) ... 

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