Department of Organic Chemistry, Southern Federal University, Zorge str. 7, 344090 Rostov-on-Don, Russian Federation.
Org Biomol Chem. 2014 Apr 21;12(15):2360-9. doi: 10.1039/c3ob41986j.
A possibility of non-conventional two-step protonation of 1,8-bis(dimethylamino)naphthalene (proton sponge) is discussed. Unlike the generally accepted mechanism, involving relatively slow direct penetration of a proton into the cleft between the peri-NMe2 groups, it consists of the rapid addition of a proton to the out-inverted NMe2 group with the subsequent slower rotational transfer of the proton into the inter-nitrogen space to produce a stable chelated cation. The following approaches were employed during the work: (1) competitive hydrogen bond formation in a specially designed alcohol in which the OH group might chelate either the proton sponge 1-NMe2 group or another basic center (N,N-dimethylaniline) of known basicity; (2) measuring the basicity of naphtho[1,8-b,c]diazabicyclo[3.3.3]undecane considered to be a close analogue of the proton sponge capable exclusively of out-protonation; (3) X-ray, spectral and DFT studies of structure, energy and stereodynamics of compounds obtained, including their conformers. For the first time, the pKa value of an organic base with a perfectly flat nitrogen atom is reported. The final conclusion is made that both pathways of proton sponge protonation, traditional and non-conventional, contribute in parallel with a still undefined ratio. The estimated out-basicity of the proton sponge is at least 5.5 orders of magnitude lower than the directly measured in-basicity.
讨论了 1,8-双（二甲氨基）萘（质子海绵）非常规两步质子化的可能性。与普遍接受的机制不同，该机制涉及质子相对缓慢地直接穿透peri-NMe2 基团之间的裂缝，而是由质子快速加成到外-反式 NMe2 基团，随后质子较慢地旋转转移到氮间空间，以产生稳定的螯合阳离子。在这项工作中采用了以下方法：（1）在专门设计的醇中竞争氢键形成，其中 OH 基团可能螯合质子海绵 1-NMe2 基团或另一个已知碱性的碱性中心（N,N-二甲基苯胺）；（2）测量被认为是质子海绵的紧密类似物的萘并[1,8-b,c]二氮杂双环[3.3.3]十一烷的碱性，其只能进行外向质子化；（3）对获得的化合物的结构、能量和立体动力学进行 X 射线、光谱和 DFT 研究，包括它们的构象。首次报道了具有完美平面氮原子的有机碱的 pKa 值。最终得出的结论是，质子海绵的质子化传统和非传统途径都以尚未定义的比例平行贡献。质子海绵的估计外向碱性至少比直接测量的内向碱性低 5.5 个数量级。
