Low basicity

Fig. 1. Frontier electron levels for polymethines containing the simplest end groups with low basicity (a, high basicity (P, 63°).

The nitrogen of aHphatic and aromatic amines is alkylated rapidly by alkyl sulfates yielding the usual mixtures. Most tertiary amines and nitrogen heterocycles are converted to quaternary ammonium salts, unless the nitrogen is of very low basicity, eg, ia tn phenylamine. The position of dimethyl sulfate-produced methylation of several heterocycles with more than one heteroatom has been examined (22). Acyl cyanamides can be methylated (23). Metal cyanates are converted to methyl isocyanate or ethyl isocyanate ia high yields by heating the mixtures (24,25). 

Work on the development of the hiU time lean bum engine continues with efforts to push back the lean bum limit. A problem recognized by the developers is that although low basic engine NO emissions are possible, it is not yet possible to meet the NO emissions required by California and the... 

IV-Oxidation of 3,6-dichloropyridazine is more difficult because of its low basicity, and the hydrolysis product 6-chloropyridazin-3(2//)-one is also formed together with the corresponding IV-oxide. 

The low basicity of pyrrole is a consequence of the loss of aromaticity which accompanies protonation on the ring nitrogen or on carbon 2 or carbon 3 of the ring. The thermodynamically most stable cation is the 2H-pyrrolium ion, and the p/sTa for protonation at C-2 has been recorded as -3.8 the corresponding pK values for protonation at C-3 and at nitrogen are -5.9 and ca. -10 (Scheme 7). 

Azoles can form stable compounds in which metallic and metalloid atoms are linked to nitrogen. For example, pyrazoles and imidazoles Af-substituted by B, Si, P and Hg groups are made in this way. Imidazoles with a free NH group can be Af-trimethylsilylated and Af-cyanated (with cyanogen bromide). Imidazoles of low basicity can be Af-nitrated. 

Isoxazolium salts can be prepared by reaction with alkyl iodides or sulfates, although the low basicity of isoxazoles and their sensitivity to nucleophilic attack may necessitate special care. Isoxazolium salts containing bulky Af-substituents can be prepared by the reaction of isoxazoles with alcohols in the presence of perchloric acid. For example, the reaction of 3,5-dimethylisoxazole (53) with some alcohols in the presence of 70% perchloric acid gave isoxazolium salts, (54a) in 29%, (54b) in 57% and (54c) in 82% yield 79AHC(25)147, 68JOC2397). Attempts to quaternize 3,5-dimethyl-4-nitroisoxazole failed 71JCS(B)2365). 

Cationic polymerization in hot melts has been applied to epoxidized polymers [38,39]. No hot melts based on vinyl ether or other cation-sensitive functionalized polymers have been described in the literature. With cationic systems, it is important that the other ingredients in the adhesive be of low basicity to avoid scavenging the initiating acid generated by the photoinitiator. 

Diffusion-limited rate control at high basicity may set in. This is more eommonly seen in a true Br nsted plot. If the rate-determining step is a proton transfer, and if this is diffusion controlled, then variation in base strength will not affect the rate of reaction. Thus, 3 may be zero at high basicity, whereas at low basicity a dependence on pK may be seen. ° Yang and Jencks ° show an example in the nucleophilic attack of aniline on methyl formate catalyzed by oxygen bases. 

Acid chlorides, which have a very good leaving group (low basicity and nucleo-philicity), may react in this manner. 

Spectroscopic evidence indicates that protonation of 2-fluoro-and 2-chloro-quinoline is not appreciable in O.OlJf aqueous hydrochloric acid. Protonation becomes evident in more strongly acidic solution in the case of the chloro compound without any accompanying decomposition, but the fluoro compound hydrolyzes to carbostyril under the latter conditions. The hydrolysis is acid-catalyzed, but it is doubtful whether protonation on the heterocyclic nitrogen is responsible, owing to its low basicity (presumably below that for the chloro compound). An alternative explanation in this case would be hydrogen bond formation with fluorine, Ar—F. .. H-O+H2. 

Goi. As noted previously, an a-chlorine atom renders a ring-nitrogen atom very weakly basic. Cyanuric chloride (5) is a very weak base both because s-triazines are of low basicity and because each of the ring-nitrogen atoms is alpha to two chlorine atoms. Hence, this compound should be insensitive to acid catalysis or acid autocatalysis and this has been observed for the displacement of the first chlorine atom with alcohols in alcohol-acetone solution and with water (see, however. Section II,D,2,6). 

Azide is widely useful as a surrogate for ammonia in nucleophilic substitution reactions, due to its high nucleophilicity, low basicity, and stability towards a variety of conditions for subsequent transformations. In particular, the azidolysis of... 

In contrast to bilateral triple-ion formation, unilateral triple-ion formation may also occur in solvents of high permittivity, when ion-pair association is increased by noncoulombic specific ion-ion interactions in solvents of low basicity such as PC or AN. Exclusive formation of anionic tripleions [A-C+A-] ", is observed in these solvents when large organic molecular anions A interact with small cations such as Li + or H+. For example, in contrast to lithium acetate in DMSO [97], where ion association is moderate, ion association as well as unilateral triple-ion formation is observed in the solvent PC [105] due to the much lower basicity of this solvent, (see Table 2)... 

The symmetrical bis(ylidyl)phosphenium chlorides 103, obtained from the reaction of trimethylsilyl ylides 102 with PCI3 are the first phosphenium salts which do not need counterions of low basicity such as AICI4 to be isolated (Scheme 30) [119]. The explanation of their stability lies in the delocalisation of the phosphenium charge in the two phosphonium parts. The reactivity study of these species is reported and for example the phosphenium 103 (R=Ph) adds ortho quinones to the central phosphorus to give the corresponding dioxaphospholenium salts 104 via a [4-1-1] cycloaddition. 

Finally, acid-catalyzed mechanisms are generally fast but must overcome the relatively low basicity of the tetrahedral intermediate (with electron-withdrawing substituents necessarily present, the basicity is low). 

The relatively high aromaticity of the parent 1,2,5-thiadiazole renders it good thermal stability (stable up to 220 °C) despite this, 3,4-diphenyl-l,2,5-thiadiazole 8 suffers slow photochemical degradation to give benzonitrile and sulfur. The low basicity of 1,2,5-thiadiazole indicates a relatively high electron density in the Jt-orbital and corresponding low electron density of the nitrogen lone pairs. Addition reactions such as Walkylation do not occur readily. A-Oxidation is... 

Actually, because of the abnormally low basicity of DXL the second stage of reaction scheme (8), i.e., the cationation of the DCA to give the ionic formate (VIII), will be slower with DXL than with its oligomers or any other DCA [17]. 

Amines of low basicity require a higher acid concentration in order to avoid the formation of diazoamino compounds, which do not couple ... 

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