Basicity comparison with nucleophilicity

The development of these various solvent parameters and scales has been accompanied by the realization that there are uncertainties in the physical property of the solvent that is correlated by a particular parameter in cases where systematic changes in solvent structure affect several solvent properties. Consider a reaction that shows no rate dependence on the basicity of hydroxylic solvents, and a second reaction that proceeds through a transition state in which there is a small transition state stabilization from a nucleophilic interaction with the hydroxyl group. The rate constants for the latter reaction will increase more sharply with changing solvent nucleophilicity than those for the former, and they should show a correlation with some solvent nucleophilicity parameter. This trend was observed in a comparison of the effects of solvent on the rate constants for solvolysis of 1-adamantyl and ferf-butyl halides, and is consistent with a greater stabilization of the transition state for reaction of the latter by interaction with nucleophilic solvents. ... 

In 30 (R3 = Aik), given that there is possibility of a competing formation of a-naphthylamines 153, the application of acetic acid catalysis, in some cases, leads to an adverse effect by increasing the yields of carbocycli-zation products 153. Such a marked influence of acid catalysis may be explained by the greater susceptibility of a substituted imino group in the ring-opened intermediate 150 toward exhibiting basic, but not nucleophilic, properties, in comparison with ammonium intermediate 136 (Scheme 8). This may lead to the formation of more nucleophilic enamines... 

A major weakness of semiempirical methods is that they must be assumed to be unreliable outside molecules of the kind used for their training set (the set of molecules used to parameterize them), until shown otherwise by comparison of their predictions with experiment or with high-level ab initio (or probably DFT) calculations. Although, as Dewar and Storch pointed out [125], the reliability of ab initio calculations, too, should be checked against experiment, the situation is somewhat different for these latter, at least at the higher levels studies of exotic species, in particular, are certainly more trustworthy when done ab initio than semiempirically. Semiempirical heats of formation are subject to errors of tens of kJ mol-1, and thus heats (enthalpies) of reaction and activation could be in error by scores of kJ mol-1. AMI and PM3 underestimate steric repulsions, overestimate basicity and underestimate nucleophilicity, and can give unreasonable charges and... 

In controlled/living systems reactions B and C can be avoided or converted into reversible ones, if ligands such as fluorides are not used, if the concentration of moisture is very low in comparison with initiator and if weakly basic/nucleophilic components (additives, counteranions) are used. Contribution of reaction A is reduced at low temperatures but can not be eliminated completely. 

Until recently, there had been no reports on the application of 2,7-disubstituted proton sponges like 44 and 65 in organic synthesis. Their advantage in comparison with compound 1 is their increased basicity coupled with their extremely low C-nucleophilicity. Lately, the 2,7-dimethoxy compound 44 was used for deprotonation of salts of other proton sponges, in particular of 88 and 89 (Section n.C.2)67. In another example, when the proton sponge 1 participated in the azo-coupling reaction, the addition of base 44 increased the degree of interaction (equation 30)56. The yield of the azo compound 269 was nearly quantitative, and the base 44 could be easily recovered. 

CH -groups introduced into p-positions of benzene rings attached to the phosphorus atoms in (III) make the reagents more basic and more nucleophilic (Tol)2P(0)CH=CHP(0)(Tol)2 (VI) is a somewhat better extractant of TPE and Eu in comparison with reagent (III). 

In comparison with the diazines, the inductive effects of the extra nitrogen(s) leads to an even greater susceptibility to nucleophilic attack and, as a result, all the parent systems and many derivatives react with water, in acidic or basic solution. Similarly, simple electrophilic substitutions do not occur some apparent electrophilic substitutions, such as the bromination of 1,3,5-triazine, probably take place via bromide nucleophilic addition to an N -Br triazinium salt. Attempted direct A-oxidation of simple tetrazines with the usual reagents generally results in ring cleavage, however it can be achieved satisfactorily with methyl(trifluoromethyl)dioxirane. ... 

In the polymerization of cyclic sulfides, cyclization becomes particularly important because of the enhanced basicity (and nucleophilicity) of the linear sulfides in comparison with their parent monomers43). The simplest cyclic oligomer, formed in the polymerization of thiiranes, is a dimer-1,4-dithiane or its derivatives. 1,4-Dithiane was first observed by Bell in the polymerization of ethylene sulfide 441 later Price isolated the styrene sulfide dimer 45) and recently Goethals obtained dimers of isobutylene and cyclohexene sulfides 46 . Structures of these dimers are ... 

When the platinum nucleophilicity scale was first proposed it was implied that one np, scale was applicable to all substrates and that plots of logk2 against npt°(Y) were linear, taking the form log 10 2 = S np, (Y) + C, where S is termed the nucleophilic discrimination factor of the substrate and C its intrinsic reactivity. Discussions of mechanism based on a comparison of nucleophilic discrimination factors are frequently encountered. Nucleophiles that do not retain their positions in the nucleophilicity scale, e.g. NO2", SeCN and SC(NH2)2, were termed biphilic by Cattahni since their behaviour could be explained by their n-acceptor properties. When the Pt reaction centre had a greater n-basicity than the standard complex (for example a smaller effective nuclear charge) the substrate was more reactive than predicted and vice versa. This concept had been deduced some years earlier by Bosnich from his work with octahedral Ru complexes... 

A comparison with data reported in Table 2 indicates that the basicity of F is much more affected by specific hydration than its nucleophilicity is, therefore confirming that anhydrous tetra-alkylammonium fluorides are not stable compounds with the single exception of Me4N F, which cannot undergo Hofmann-like elimination. 

The EPD solvents of high Lewis basicity include the most common dipolar non-HBD solvents (e.g., DMSO. acetonitrile, DMF, acetone, HMPA, etc.) and some open-chain polyethers (oligoethylene glycol dialkyl ethers 1 ( glymes ). They specifically solvate the cation but weakly interact with the anion and, for this reason, are found to enhance nucleophilicity and basicity of anion-promoted reactions in comparison with the reactivity realized in protic HBD media. ... 

The variant at which the molecule of urea is deformed is theoretically possible and gets such space arrangement at which -orbital of nitrogen atom are not blocked with -orbitals of C=0 group. In this case the basicity and nucleophility of amino group should increase sharply. It is known, for example, the cyclic lactam in which uncoupled electrons of nitrogen atom are not blocked with - electrons of carbonyl group [4]. As the result of it is the increase in basicity parameter at five orders in comparison with usual (not cyclic) lactams. However, it is hardly possible to deform a molecule of urea and to deduce -orbitals from the interface with n-orbitals in conditions of coordination catalysis. 

Another method for sequence analysis is the Sanger N-terminal analysis, based on the use of 2,4-dinitrofluorobenzene (DNFB). When a polypeptide is treated with DNFB in mildly basic solution, a nucleophilic aromatic substitution reaction (SnAt, Section 21.1 lA) takes place involving the free amino group of the N-terminal residue. Subsequent hydrolysis of the polypeptide gives a mixture of amino acids in which the N-terminal amino acid is labeled with a 2,4-dinitrophenyl group. After separating this amino acid from the mixture, it can be identified by comparison with known standards. 

There have been several studies reporting equilibrium and rate measurements for reactions of TNB with thiolate ions. Values of the equilibrium constants measure the thermodynamic affinity of the nucleophile for the carbon center, a quantity usually known as the carbon basicity. This is distinct from nucleophilicity, which reflects the rate constant for nucleophilic attack. The results show that in comparison with oxygen bases, the polarizable sulfur bases have higher carbon basicities and nucleophilicities than expected from the values of the parent thiols [84, 161,162]. 

A comparison of nucleophiles in which the nucleophilic atoms bear the same charge, and are in the same column of the periodic table, indicates that the nucleophilicity increases as one proceeds down the column. For example, the order of nucleophilicities for the halide anions is 1 > Br" > Cl > F in polar protic solvents. As discussed above, in these solvents, the anions are well solvated, and fluoride makes stronger bonds with the solvent because the anionic charge is more localized. This trend is opposite of that expected if there was a strict correlation between basicity and nucleophilicity (the order of basicity is F > Cl > Br > I ). Similarly, thiolate anions are consistently better nucleophiles than analogous alkox-ides, even though the alkoxides are significantly more basic. The increased nucleophilicity... 

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