Callahan Ronald, Prip Ron, Shariff Navroz, Sklyut Olga, Rothchild Robert, Bynum Kevin
New York University, Chemistry Department, New York, New York 10003, USA.
Appl Spectrosc. 2005 Mar;59(3):354-65. doi: 10.1366/0003702053585426.
Phencyclone, 1, a potent Diels-Alder diene, reacts with a series of N-alkylmaleimides, 2, to form hindered adducts, 3. The 300 MHz 1H and 75 MHz 13C NMR studies of these adducts at ambient temperatures have demonstrated slow rotations on the nuclear magnetic resonance (NMR) timescales for the unsubstituted bridgehead phenyl groups, and have revealed substantial magnetic anisotropic shielding effects in the 1H spectra of the N-alkyl groups of the adducts. The selected N-alkyl groups for the target compounds emphasized smaller branched alkyls, including C3 (isopropyl, a); C4 (isobutyl, b; and t-butyl, c); C5 (n-pentyl, d; isopentyl [isoamyl], e; 1-ethylpropyl, f; t-amyl, g;) and a related C8 isomer (1,1,3,3-tetramethylbutyl ["t-octyl"], h). The straight-chain n-pentyl analog was included as a reference. This present work on the branched N-alkylmaleimide adducts appreciably extends our earlier compilation on the N-n-alkylmaleimide adducts. Key methods for proton assignments included "high-resolution" 1H-1H chemical shift correlation spectroscopy, COSY45. 13C NMR of the adducts, 3, verified the expected number of aryl carbons for slow exchange limit (SEL) spectra of the bridgehead phenyl groups. The synthetic routes involved reaction of the corresponding amines, 4, with maleic anhydride to give the N-alkylmaleamic acids, 5, which underwent cyclodehydration to form the maleimides, 2. Magnetic anisotropic shielding magnitudes for alkyl group protons in the adducts were calculated relative to corresponding proton chemical shifts in the maleimides. Geometry optimizations for the above adducts (and for the N-n-butylmaleimide adduct) were performed at the Hartree-Fock level with the 6-31G* basis set. The existence of different contributing conformers for the adducts is discussed with respect to their calculated energies and implications regarding experimentally observed anisotropic shielding magnitudes.
1,苯基环戊烯，一种有效的狄尔斯-阿尔德二烯，与一系列N-烷基马来酰亚胺（2）反应，形成受阻加合物（3）。在室温下对这些加合物进行的300兆赫1H和75兆赫13C核磁共振研究表明，未取代的桥头苯基在核磁共振时间尺度上旋转缓慢，并揭示了加合物中N-烷基的1H谱中存在显著的磁各向异性屏蔽效应。目标化合物所选的N-烷基强调较小的支链烷基，包括C3（异丙基，a）；C4（异丁基，b；叔丁基，c）；C5（正戊基，d；异戊基[异戊基]，e；1-乙基丙基，f；叔戊基，g）；以及一种相关的C8异构体（1,1,3,3-四甲基丁基["叔辛基"]，h）。直链正戊基类似物作为参考。目前关于支链N-烷基马来酰亚胺加合物的这项工作显著扩展了我们早期关于N-正烷基马来酰亚胺加合物的汇编。质子归属的关键方法包括“高分辨率”1H-1H化学位移相关光谱法，COSY45。加合物（3）的13C核磁共振验证了桥头苯基慢交换极限（SEL）光谱中预期的芳基碳原子数。合成路线包括相应的胺（4）与马来酸酐反应生成N-烷基马来酰胺酸（5），后者经环脱水形成马来酰亚胺（2）。相对于马来酰亚胺中相应的质子化学位移，计算了加合物中烷基质子的磁各向异性屏蔽大小。使用6-31G*基组在哈特里-福克水平上对上述加合物（以及N-正丁基马来酰亚胺加合物）进行了几何优化。根据加合物计算出的能量以及与实验观察到的各向异性屏蔽大小的关系，讨论了不同贡献构象的存在情况。
