2.6- di- tert-butylpyridine

The reaction of 2,6-di-tert-butylpyridine with SO3 at elevated temperatures gives (43) in addition to the 3-sulfonic acid. ... 

General Considerations. The following chemicals were commercially available and used as received 3,3,3-Triphenylpropionic acid (Acros), 1.0 M LiAlH4 in tetrahydrofuran (THF) (Aldrich), pyridinium dichromate (Acros), 2,6 di-tert-butylpyridine (Acros), dichlorodimethylsilane (Acros), tetraethyl orthosilicate (Aldrich), 3-aminopropyltrimethoxy silane (Aldrich), hexamethyldisilazane (Aldrich), tetrakis (diethylamino) titanium (Aldrich), trimethyl silyl chloride (Aldrich), terephthaloyl chloride (Acros), anhydrous toluene (Acros), and n-butyllithium in hexanes (Aldrich). Anhydrous ether, anhydrous THF, anhydrous dichloromethane, and anhydrous hexanes were obtained from a packed bed solvent purification system utilizing columns of copper oxide catalyst and alumina (ether, hexanes) or dual alumina columns (tetrahydrofuran, dichloromethane) (9). Tetramethylcyclopentadiene (Aldrich) was distilled over sodium metal prior to use. p-Aminophenyltrimethoxysilane (Gelest) was purified by recrystallization from methanol. Anhydrous methanol (Acros) was... 

Reaction of Chlorodimethyl(2,3,4,5-tetramethyl-2,4-cyclopentadien-l-yl)silane with Silica Materials. Chlorodimethyl-(2,3,4,5-tetramethyl-2,4-cyclopentadien-l-yl)silane (excess) was added to a mixture of amine-functionalized silica with hexanes in a drybox. 2,6-Di-tert-butylpyridine (excess) was added as a proton sponge. The mixture was allowed to react while stirring for 24 h. The solid was filtered and washed with hexanes and THF in the drybox. The solid was then contacted with another aliquot of chlorodimethyl-(2,3,4,5-tetramethyl-2,4-cyclopentadien-l-yl)silane and the procedure was repeated. 

As evidenced by experiments carried out on partially poisoned catalysts with 2,6-di-tert-butylpyridine, a significant decrease in the catalytic activity (of about 65-70 %) occurs because of a partial neutralization of the external acid sites. This means that the alkylation takes place predominantly on the external surface. 

Addition of a bulky pyridine base Some workers have added an equivalent of 2,6-di-tert-butylpyridine or 2,6-di-tert-butyl-4-methylpyridine to the glycosylation mixture. While the reasons for this addition have not been discussed, one can note that according to a speculative equation for the glycosylation reaction ... 

Hindered nonnucleophilic bases are typically added to sulfoxide glycosylations to buffer the acidic by-products. Classically, the 2,6-di-tert-butylpyridines have been employed for this purpose [86], but the more highly crystalline and easily handled 2,4,6-tri-fert-butylpyrimidine is finding increasing favor in this regard [356]. 

In the case of protonated pyrroles, the p Ta value lies in the range between 4 and —4, whereas the pATa value of acetonitrile is about —10. Therefore, the oligomerization of pyrrole in pure acetonitrile may already stop at the level of a-intermediates of hi- or more likely of tetrapyrrole. Acetonitrile is a weaker base than the a-intermediates. Consequently, a stronger base must be used to initiate the elimination of protons. Water fulfills this condition. Pyrrole can be polymerized in acetonitrile in the presence of 1% water [6, 37]. A similar effect results from the application of a sterically hindered base such as 2,6-di-tert-butylpyridine [38]. However, the concentration should be kept low because, at high concentrations proton, abstraction from the monomeric radical cation may occur, thus forming a neutral radical [28d]. The base effect can be also observed in the case of thiophenes. 

For example, the rate of oligomerization and polymerization increases when 2,6-di-tert-butylpyridine is added to a solution of bithiophene. However, in the case of monomeric thiophene, the high oxidation potential of the starting species between (>1.6 V versus Ag/AgCl) prevents any formation of conducting polymeric material. 

The Soft Nef-Reaction was possible using the sterically shielded 2,6-di-tert-butylpyridine (49). But when the concave pyridine 3c was used, different products (nitro compounds 45 or carbonyl compounds 47) were found for... 

Steric shielding of the heteroatom in pyridine results in non-nucleophilic bases such as 2,6-di-tert-butylpyridine and its 4-methyl congener (the latter is easily... 

SCHEME 12.9 Danishefsky s second-generation synthesis of Globo-H. DMF, /V,/V-dimcth-ylformamide DTBP, 2,6-di-tert-butylpyridine IDCP, iodonium di-.vym-collidinc perchlorate LHMDS, lithium bis(trimethylsilyl)amide. 

For example, addition of 2 equivalents of a nonnucleophilic base, 2,6-di-tert-butylpyridine, to the reaction system almost totally suppressed the reaction (Table 9). This proposes that acid catalyzed enolization is important for the a-Umpolung. In support of mechanism 1 the yields of the a-methoxylated products increased with decreasing oxidation strength of the oxidant (Table 10), since endergonic oxidation of the ketone 70 is slowed down (Fig. 2). On the other hand, in the presence of stronger aminium salts the ketone tautomer is oxidized which however does not lead to deprotonation in the benzylic position... 

Crich D, Smith M, Yao QJ, Picione J. 2,4,6-Tri-tert-butylpyrimidine (TTBP) A cost effective, readily available alternative to the hindered base 2,6-Di-tert-butylpyridine and its 4-substituted derivatives in glycosylation and other reactions. Synthesis 2001 323—326. 

Roth and coworkers obtained CIDNP evidence for a singly bonded intermediate in the radical cation Diels-Alder dimerization of spiro[2,4]heptadiene [107]. Gassman and Singleton pointed out that acid-catalyzed (4 - - 2) addition reactions can compete with the radical cation chain process when a hindered base (2,6-di-tert-butylpyridine) is not included. For 2,4-dimethyl-l,3-pentadiene (89 of Scheme 19)... 

No other adducts are formed, and the endo/exo diastereomeric ratio is essentially the same for all of these methods. Further, the existence of an acid catalyzed mechanism for cycloaddition can be explicitly excluded by using an excess of a hindered amine base (2,6-di-tert-butylpyridine, DTBP) in the aminium salt induced reaction and by examining the results of an authentic acid catalyzed reaction (using, for example, triflic acid). In the former case, the same endo and exo adducts are formed in virtually the same relative amounts, but in the latter case neither of these adducts is formed. It is worth noting that acid catalyzed reactions have indeed sometimes been observed under typical aminium salt conditions [70], but these have never been observed, nor would they be expected, under PET conditions. Finally, in the instance where cation radicals are generated by the aminium salt method, the intervention of substrate cation radicals can usually be verified by the addition of the reduced form of the catalyst, i.e., the neutral triarylamine, to the reaction mixture. 

Abbreviations Ac acetyl Bn benzyl BSP 1-benzenesulfinyl piperidine BTIB bis(trifluoroacetoxy)iodobenzene DAST (diethylamino)sulfur trifluoride DDQ 2,3-dichloro-5,6-dicyano-/)-benzoquinone DMDO dimethyldioxirane DMTSF dimethyl(methylthio)sulfonium tetrafluoroborate DMTST dimethyl(methylthio)sulfonium triflate DTBMP 2,6-Ai-tert-butyl-4-methylpyridine DTBP 2,6-di-tert-butylpyridine DTBPl 2,6-di-tert-butylpyridinium iodide FDCPT l-fluoro-2,6-dichloropyridinium triflate FTMPT l-fluoro-2,4,6-trimethylpyridinium triflate IDCP iodonium dicollidine perchlorate IDCT idonium dicollidine triflate LPTS 2,6-lutidinium p-toluenesulfonate LTMP lithium tetramethylpiperidide Me methyl MPBT S-(4-methoxyphenyl) benzenethiosulflnate NBS A-bromosuccinimide NIS A-iodosuccinimide NlSac A-iodosaccharin PPTS pyridinium p-toluenesulfonate TBPA tris(4-bromophenyl)ammoniumyl hexachloroantimonate Tf trifluoromethanesulfonyl TMTSB methyl-bis(methylthio)sulfonium hexachloroantimonate TMU tetramethylurea Tr trityl TTBP 2,4,6-tri-tert-butylpyrimidine. 

The pyran indicated, 2,3-dihydro-2,2-dimethyl-6-phenyl-4H-pyran-4-one and benzoylacetylene in chloroform containing 2.5 moles of 2,6-di-tert-butylpyridine unden/vent additton in the presence of excess triisopropylsilyl triflate by stirring... 

The importance of chemical structure in determining ease of sulfonation is further illustrated in the pyridine series. These compounds as a group are very, difficult to sulfonate, ordinarily requiring a mercury catalyst at over 250°C using oleum. 2,6-Di-tert-butylpyridine has been found, on the other hand, to sulfonate easily even at — 10°C, using SO3 dissolved in SOj. In this exceptional case, steric hindrance prevents formation of the difficultly sulfonatable addition compound with SO3. 

The resultant delaminated materials, when are prepared in a silica-alumina form, show strong Bronsted acidity of the zeolitic type, which are much stronger than those of MCM-41 and accessible to large molecules, as indicated by the adsorption of 2,6-di-tert-butylpyridine. 

Type and distribution of aluminium and iron in the framework of zeolite MCM-22 were investigated using adsorption of ds-acetonitrile, pyridine and 2,6-di-tert-butylpyridine followed by FTIR spectroscopy, sodium ion-exchange and UV-Vis spectroscopy of Co " ions located in cationic positions. Detailed analysis of aluminium and iron distribution among single ions, ion pairs, Bronsted and Lewis sites and internal and external surface is provided. 

Similarly, Karge et al. [398] employed pyridine and 2,6-di-tert-butylpyridine to differentiate between internal and external acid sites of zeolite crystallites. In this respect, another possibility is the use of lutidine [139] or quinoline. The latter probe was employed by Corma et al. [693] for the determination of external Bronsted and Lewis acid sites of H, Na-Y and Al, Na-Y zeolites (cf. also [694,695]). For a characterization of the external Bronsted and Lewis acidity of ZSM-5 samples, Keskinen et al. [696] utilized as sufficiently bulky bases trimethylsi-lyldiethylamine and, like Karge et al. [398], 2,6-di-ferf-butylpyridine. For the discrimination of external from internal acid sites of shape-selective H-ZSM-5 catalysts,Take et al. [135] utilized pyridine andabulkytrialkylamine (e.g.,Et3N, n-Pr3N and n-Bu3N) as a pair of probes, with the former indicating the total amount of acid sites. For quantitative evaluation they determined the extinction... 

---