Theoretical studies on the iminium ion-forming reaction

✍ Scribed by Chang Kon Kim; In Young Lee; Chan Kyung Kim; Ikchoon Lee

• Publisher: John Wiley and Sons
• Year: 1999
• Tongue: English
• Weight: 89 KB
• Volume: 12
• Category: Article
• ISSN: 0894-3230
• DOI: 10.1002/(sici)1099-1395(199906)12:6<479::aid-poc148>3.0.co;2-k

✦ Synopsis

The iminium ion forming reactions of YC 6 H 4 NHCH 2 NH 3 (I) and the corresponding carbon analog, YC 6 H 4 CH 2 CH 2 NH 3 (II) were studied theoretically at the MP2/6-31G*//RHF/6-31G* level. The iminium ionforming reactions are favored mainly due to a large amount of electron donation (0.05-0.06e) from the nitrogen to the antibonding orbital of the cleaving bond, n N → s*(r 3 ), by a first-neighbor vicinal charge-transfer interaction. At ca 50% progress of reaction, i.e. bond cleavage in the S N 1 transition state (TS), bond contraction of the geminal bond (r 2 ) is greater by ca 10% (60%), which is also greater than bond stretching of a vicinal bond (50%). The reaction through the imbalanced TS provides the minimum energy path in accordance with the basic laws of nature. The Hammett r values for the iminium ion-forming activation and equilibrium processes are ca À1.3 and À4.3, respectively. The negative activation energies for the S N 2 processes with I in the gas phase are consistent with the experimental direct nucleophilic displacement by a strong nucleophile at the iminium ion-forming substrate in water.