Amines, tert hindered

TSH readily A( aminates stericaUy hindered p)mdines such as 2,6-di-tert-hutylpyridine, which can only he A( alkylated under forcing conditions. Ethyl pyridine-2-carhoxylate, 2-carhamido-pyridines,, methyl 2-(2-pyridyl)henzoate, and furo[3,2-c]-pyridines were likewise A(-aminated in high yields under mild conditions (eq 3). 

Full details on the phosphorylation of water and alcohols by 4-nitrophenyl dihydrogen phosphate and the NfC H ) - and N(CH3) -salts of its mono- and dianion have been published 146>. Phosphoryl group transfer from the monoanion and dianion is thought to proceed via the monomeric POf ion. Addition of the sterically unhindered amine quinuclidine to an acetonitrile solution containing the phosphate monoanion and tert-butanol produces t-butyl phosphate at a faster rate than does the addition of the more hindered diisopropylethylamine. This nucleophilic catalysis of the phosphorylation reaction is also explained by the intermediacy of the POf ion. 

Secondly, the interaction of hindered amines with hydroperoxides was examined. At room temperature, using different monofunctional model hydroperoxides, a direct hydroperoxide decomposition by TMP derivatives was not seen. On the other hand, a marked inhibitory effect of certain hindered amines on the formation of hydroperoxides in the induced photooxidation of hydrocarbons was observed. Additional spectroscopic and analytical evidence is given for complex formation between TMP derivatives and tert.-butyl hydroperoxide. From these results, a possible mechanism for the reaction between hindered amines and the oxidizing species was proposed. 

Complex formation constants could also be determined directly from UV spectrophotometric measurements. Addition of tert.-butyl hydroperoxide to a solution of nitroxide I in heptane at RT causes a shift of the characteristic absorption band of NO at 460 nm to lower wavelengths (Fig. 9). This displacement allows calculation of a complex equilibrium constant of 5 1 1/Mol. Addition of amine II to the same solution causes reverse shift of theC NO" absorption band. From this one can estimate a complex formation constant for amine II and +00H of 12 5 1/Mol (23 2 1/Mol was obtained for tert.-butyl hydroperoxide and 2,2,6,6-tetramethylpipe-ridine in ref. 64b). Further confirmation for an interaction between hindered amines and hydroperoxides is supplied by NMR measurements. Figure 10a shows part of the +00H spectrum in toluene-dg (concentration 0.2 Mol/1) with the signal for the hydroperoxy proton at 6.7 ppm. Addition of as little as 0.002 Mol/1 of tetra-methylpiperidine to the same solution results in a displacement and marked broadening of the band (Fig. 10b). 

The sterically hindered base 2,6-bis(tert-butyl)pyridine does not inhibit cyclization triaryl-amine retards this reaction photosensibilized one-electron oxidation of a diene leads to the same products, which are formed in the presence of ammoniumyl salt. As shown, in majority of cases, only the cation-radical chain mechanism of the diene-diene cyclization is feasible (Bauld et al. 1987). Meanwhile, cyclodimerizations of 2,4-dimethylpenta-l,3-diene (Gassman and Singleton 1984) and l,4-dimethylcyclohexa-l,3- or -1,4-diene (Davies et al. 1985) proceed through both mechanisms. 

The latter transformation requires the use of a small amount of an acid or its ammonium salt. By using [Cp2TiMe2] as the catalyst, primary anilines as well as steri-cally hindered tert-alkyl- and sec-alkylamines can be reacted.596 Hydroamination with sterically less hindered amines are very slow. This was explained by a mechanism in which equlibrium between the catalytically active [L1L2Ti=NR] imido complex and ist dimer for sterically hindered amines favors a fast reaction. Lantha-nade metallocenes catalyze the regiospecific addition of primary amines to alkenes, dienes, and alkynes.598 The rates, however, are several orders of magnitude lower than those of the corresponding intramolecular additions. 

The three-step procedure described here illustrates a convenient, general route to di-tert-alkylamines. Extensive purification or isolation of intermediates is not required. The reactions are easily monitored. Only in the final step is the exclusion of air and moisture necessary. It should be noted that tert-butyl-tert-octylamine is considerably more hindered than 2,2,6,6-tetramethylpiperidine. tert-Butyl-tert-octylamine is inert to methyl iodide, while 2,2,6,6-tetramethylpiperidine gives a white precipitate of the pentamethylammonium iodide within minutes upon treatment with methyl iodide at room temperature. The extreme hindrance of this amine has been exploited in the selective deprotonation of carbon adds and in other reactions.10... 

Howard, J.A. and E. Furimsky. 1973. Arrhenius parameters for reactions of tert-butylperoxy radicals with some hindered phenols and amines. Can.. Chem. 51 3738-3745. 

Examples of homopolymers are given. Poly(4-vinylphenol) was prepared as a prepolymer for the subsequent alkylation [55]. Poly[2-(4-vinylbenzyl)hydroqui-none] 65 is an example of the unhindered phenolic antioxidant for rubbers. Many homopolymers bear a hindered phenolic moiety. Homopolymer 66 was proposed for blending with BR and IR [56]. Other examples are poly[vinyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] [57] (67), poly(3,5-di-/ert-butyl-4-hydroxy-benzyl methacrylate) [58] (68) or poly[iV-3,5-di-tert-butyl-4-hydroxybenzyl) male-imide] [59] (69). Numerous polymeric antioxidants are functionalized with aromatic amine groups. Poly(4-anilinophenyi methacrylate) [53] (70) serves as an example. 

This class of compounds is kinetically stabilized by bulky tert-butyl groups (3-6) or amine groups (7,8). In 3, 7, and 8, HF elimination should be possible, but it is hindered by the very strong SiF bond energy. These mono(silyl)hydrazines are very stable molecules and show no tendency to undergo condensation at room temperature. 

Conditions for the efficient use of Fmoc-protected amino acid chlorides in stepwise peptide synthesis have been delineated. The acid chlorides are reasonably stable solids that are readily aminolyzed if a base is added to neutralize the hydrogen chloride liberated in the reaction. They form the corresponding oxazol-5(4//)-ones and undergo stereomutation if left in the presence of tertiary amines, however, amide formation without stereomutation can be obtained under appropriate conditions. The acid chlorides are advantageously applied to acylate weak nucleophiles and sterically hindered amino components, but their application may be limited by the lack of stability of tert-butyl-type side-chain protecting groups toward conditions of acid chloride preparation. 

The use of less basic and sterically hindered amines [e.g., collidine or 2,6-di-tert-butyl-4-(dimethylamino)pyridine ], the avoidance of preactivation, and the use of less polar solvent systems are additional appropriate strategies for the suppression of racemization. Except for fragment condensations in solution and couplings involving racemization-prone derivatives, the addition of HOXt to the reaction mixture has no pronounced effect on a uronium salt mediated racemization/epimerization, in contrast to the striking effects of HOAt on epimerization caused by PfPyU and haloamidinium salts such as CIP or TFFH (Section 3.8.1.2.1). 

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. 

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