Substituent effects upon rates of deamination and base strengths of substituted N-tritylamines
作者:Moisés Canle L.、Ibrahim Demirtas、Howard Maskill
DOI:10.1039/b104916j
日期:——
Substituted N-tritylamines undergo deamination in aqueous acidic solution containing organic cosolvents, and follow a pseudo first-order specific acid-catalysis rate law (kobs = k0 + kH[H3O+]); the products are the corresponding trityl alcohols in equilibrium with the trityl carbenium ions, and ammonium ions. The change from 4,4′-dimethoxy- to 4,4′,4″-trimethoxy-tritylamine increases reactivity by a factor of about 30 in both k0 and kH, but N-alkyl and N-aryl groups have much greater effects (ca. 106 for both k0 and kH for 4,4′-dimethoxytritylamine) which are largely independent of the nature of the N-alkyl and N-aryl groups. The anions of the catalytic strong acids (perchlorate, chloride,
bromide, and nitrate) have only small effects as also does the concentration of acetonitrile as cosolvent; ethanoic acid as cosolvent is mildly rate-enhancing. Substituents in the aniline residue of N-(4,4′-dimethoxytrityl)anilines have virtually no effect upon either k0 or kH (ρ = 0 for both). The results are interpreted by a mechanism involving a pre-equilibrium heterolysis of the (substituted) tritylammonium ion to give an ion–molecule pair which may undergo diffusional or acid-catalysed dissociation before the (substituted) trityl cation undergoes equilibrium nucleophilic capture by water. Base strengths of some substituted N-tritylamines have been measured; the substituted N-trityl group causes all amines (aliphatic and aromatic) to be comparably strongly basic (pKBH+ca. 9).
替代的N-三苯基胺在含有有机共溶剂的酸性水溶液中发生脱氨反应,并遵循伪一级的特定酸催化速率定律(kobs = k0 + kH[H3O+]); 产生的产物是相应的三苯基醇,与三苯基阳离子和铵离子处于平衡状态。从4,4′-二甲氧基到4,4′,4″-三甲氧基三苯基胺的变化使反应性增加了大约30倍,在k0和kH中都是如此,但N-烷基和N-芳基取代基对反应性的影响更大(对于4,4′-二甲氧基三苯基胺,k0和kH的影响约为106),且这些影响在很大程度上独立于N-烷基和N-芳基取代基的性质。催化强酸的阴离子(高氯酸盐、氯离子、溴离子和硝酸盐)对反应的影响很小,乙腈作为共溶剂的浓度也没有显著影响;乙酸作为共溶剂略微增强了反应速率。在N-(4,4′-二甲氧基三苯基)苯胺的苯胺残基上的取代基对k0或kH几乎没有影响(ρ = 0)。这些结果通过一种机制进行了解释,该机制涉及(替代的)三苯基铵离子的前平衡异裂,形成一个离子-分子对,该对可能在(替代的)三苯基阳离子通过水进行平衡亲核捕获之前,发生扩散性或酸催化解离。一些替代的N-三苯基胺的碱性已被测量;替代的N-三苯基基团使所有胺(包括脂肪族和芳香族)具有相似的强碱性(pKBH+约为9)。