Hindered pyridine

Another type of steric effect is the result of an entropy effect. The compound 2,6-di-fert-butylpyridine is a weaker base than either pyridine or 2,6-dimethylpyridine. The reason is that the conjugate acid (8) is less stable than the conjugate acids of nonsterically hindered pyridines. In all cases, the conjugate acids are hydrogen bonded to a water molecule, but in the case of 8 the bulky tert-butyl groups restrict rotations in the water molecule, lowering the entropy. 

Pyridine is added to neutralize small amounts of hydrogen iodide, which is often present in iodotrimethylsilane as a result of hydrolysis by contact with moisture. The amount of by-products, including cyclohexyl iodide, is reduced by the presence of pyridine. Hindered pyridine bases such as 2,6-di-terf-butyl-4-methylpyridine" have also been used for this purpose by the submitters. The pyridine bases do not appear to react with iodotrimethylsilane. 

Polymers with triflate groups react with alcohols to form alkoxysubstituted polysilanes. This reaction occurs readily in the presence of bases. The best results were obtained using triethylamine and hindered pyridine. In Fig. 3c the NMR spectrum of the reaction mixture containing the excess of triethylamine is shown, the methyl groups from the polymer chains absorb in the range typical for alkoxysilanes. Reaction in the presence of unsubstituted pyridine leads to the formation of insoluble polymer probably by attack at the p-C atom in the silylated pyridine. 

Thermodynamics of complex formation of silver with several ligands such amines,368 hindered pyridine bases,369 nitrogen donor solvents,370 and azoles371 have been carried out. Other studies include the secondary-ion mass spectra of nonvolatile silver complexes,372 the relationship between Lewis acid-base behavior in the gas phase and the aqueous solution,373 or the rates of hydride abstraction from amines via reactions with ground-state Ag+.374... 

Many synthetic applications of Rh-catalyzed hydrogenation of a-dehydroamino acid derivatives have recently been explored (Scheme 26.2). Takahashi has reported a one-pot sequential enantioseiective hydrogenation utilizing a BINAP-Rh and a BINAP-Ru catalyst to synthesize 4-amino-3-hydroxy-5-phenylpentanoic acids in over 95% ee. The process involves a first step in which the dehydroami-no acid unit is hydrogenated with the BINAP-Rh catalyst, followed by hydrogenation of the / -keto ester unit with the BINAP-Ru catalyst [87]. A hindered pyridine substituted a-dehydroamino acid derivative has been hydrogenated by a... 

Scheme 8.13. Disaccharide synthesis in the presence of a sterically hindered pyridine.

The more basic and less hindered pyridines undergo nucleophilic attack at an ethylene coordinated to platinum(II). Pyridine substitution reactions at platinum also occur, and in the presence of excess ethylene, alkene replacement is observed.72 ... 

Blazejewski, J.-C. Hofstraat, J. W. Lequesne, C. Wakselman, C. Wiersum, U. E. Formation of monomeric halogenoaryl acrylates in the presence of hindered pyridine bases. /. Fluorine Chem. 1998, 91, 175-177. 

Studies of proton transfers involving small ions with localized charge have shown that these reactions may proceed indeed with rate constants close to or even slightly larger than the collision rate constants predicted by the ADO theory (Mackay et al., 1976). However, rate-constant measurements of proton-transfer reactions between delocalized anions (Farneth and Brau-man, 1976) and sterically hindered pyridine bases (Jasinski and Brauman, 1980) and of SN2 displacement reactions (Olmstead and Brauman, 1977 Pellerite and Brauman, 1980 Pellerite and Brauman, 1983 Caldwell et al., 1984 for a review see Riveros et al., 1985) have shown that the rate constants can span the range from almost collision controlled values down to ones too slow to be observed. For these reactions the wide variation in rate constants has been explained on the basis of a double potential-well model which for a hypothetical SN2 substitution is schematically shown in Fig. 4. 

Benzylic deprotonation is often an inefficient process. It may be more important than it would appear from the end products, however, since radical cation deprotonation followed by reduction of the radical and reprotonation may regenerate the starting material. This mechanism has been proposed to explain the inefficiency of some PET alkylations [68]. In suitable models such a process has been revealed, e.g. deuterium incorporation at the bis-benzylic position in 2-(4-methoxyphenyl)-2-phenylethyl methyl ether and cis-trans isomerization in 2-methoxy-l-(4-methoxyphenyl)indane (but not in the corresponding 3-methoxyphenyl derivatives) [204], as well as deconjugation of 1-phenylalkenes to 3-phenylalkenes in the presence of 1,4-dieyanobenzene, biphenyl (as a secondary donor) and a hindered pyridine as the base [205]. Deprotonation of N,N-dimethylaniline has likewise been observed (Scheme 38) [206-207],... 

In addition to protonic acids, Lewis acids are the most common initiators of carbocationic polymerizations. Two mechanisms are possible. Direct initiation is rare and usually slow. The more prevalent mechanism is by cocatalysis in binary systems, with the Lewis acid acting as a coinitiator or catalyst rather than as initiator. Cationating or protonating species are the true initiators, which are therefore the species incorporated at the polymer s end group. The most common initiator is adventitious water in insufficiently dried systems. Thus, mechanistic studies should be performed under stringently dry conditions or in the presence of proton traps such as hindered pyridines. In addition to water, the protonating reagent may be an alcohol, carboxylic acid, amine, or amide [Eq. (28)]. 

Slow exchange may lead to extremely broad polydispersities and often to polymodal MWDs. It is recommended to study the evolution of molecular weights with conversion and especially the proportion and position of various peaks in the MWD by size-exclusion chromatography (SEC). Use of scavengers is helpful in the identification of the origin of peaks in SEC (MWD) traces. For example, salts with common ions suppress free ions and reduce the intensity of peaks formed by free ions. Hindered pyridines trap protons and reduce peaks resulting from protonic initiation, especially in systems with adventitious moisture. Apparently, stability of complex anions MtX +, and MtX OH can be different and slow exchange may lead to polymodal MWD. 

Hindered pyridines can act in three different ways [150,293-295]. The first is to trap protonic impurities and prevent adventitious initiation by water. This improves the control of molecular weights. The second role is similar to that of salts with common anions. The pyridinium salts formed in the system are accompanied by complex anions which may scavenge free ions in a similar manner as tetrabutylammonium salts. Hindered pyridines may also act as nucleophiles (or donors) and interact with some Lewis acids. These interactions will be directed toward the aromatic ring rather than the nitrogen atom which is protected by bulky tert-butyl groups in ortho position [293]. 

In a paper emphasizing the preparative value, Schulz and Kluge described the a-hydroxylation of ketones in good yields by using 2 equivalents of triarylarainium salts in moist acetonitrile [172]. In contrast to the oxidative functionalization of 68, 70 and 72 the reaction with cyclohexanone (64) and methyl isopropyl ketone (80) was run in the presence of a hindered pyridine base. Thus, mechanistically, it cannot rigorously be stated whether ends or the enolates are oxidized (cf Sect. 3.2). 

Polymerisations with the most active salts were extremely fast. Moisture was found to be detrimental to these processes and traces of alcohols and amines inhibited them completely. Tests with hindered pyridines proved that the active sites on the salt surface... 

Large downfield shifts of the N-oxide NMR signal for hindered pyridine-N-oxides (7,8) and quinoline-N-oxides (9,11) systems in which torsion angle twist is not thought to be likely led to a broader study of rigid planer systems[39,40]. 

Little A-alkylation occurs during hydrogenation of pyridine in the presence of polyols (27, b). Substituted pyridines undergo little N-alkylation in the presence of a branched chain alcohol. An example of this is seen in the hydrogenation of 2-picoline in isopropyl alcohol at 230-240°. Only 7.3% of l-isopropyl-2-pipecoline was obtained (28, a). With more sterically hindered pyridines chain length of the alcohol is a factor (28, b). In the hydrogenation of 2,6-lutidine in various alcohols A -methylation occurred to the extent of 56.6% at 250°C. A dramatic drop occurred when ethyl alcohol was used giving only 19% of the 1-ethyl derivative. Less than 10% A-alkylation took place with n-propyl and w-butyl alcohols. 

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