Steric and Electronic Effects in the Dimerization of Wanzlick Carbenes: The Alkyl Effect

The steric and electronic influence of N ‐alkyl substituents on the dimerization energies Δ G ° of Wanzlick carbenes (imidazolidin‐2‐ylidenes) was investigated experimentally and through DFT methods for a series of non‐symmetrically substituted Wanzlick carbenes. A series of 3‐alkyl‐1‐ tert ‐butylimidazolidin‐2‐ylidenes with decreasing steric demand of the alkyl substituent (isopropyl, ethyl and methyl) were obtained in four steps from the commercially available N ‐alkylaminoethanol compounds. The carbenes are hydrolytically sensitive, colorless oils that can be distilled without decomposition and show no sign of dimerization to the respective enetetramines, even after prolonged heating. Calculations at the B98/6‐31G(d) level confirm that the dimerization of all three carbenes is thermodynamically unfavorable. To separate the steric and electronic stabilization of Wanzlick carbenes by N ‐alkyl substituents, the formation energies of R,H 3 mono‐alkyl enetetramines were used to derive electronic increments for the N ‐alkyl substituents. The computational data show that all alkyl substituents electronically stabilize Wanzlick carbenes vs. their dimerization products with increments ranging from 2.97 kcal mol –1 ( N ‐methyl) to as high as 6.28 kcal mol –1 ( N ‐ tert ‐butyl). For combinations of N ‐methyl, N ‐ethyl and N ‐isopropyl substituents, the increments are additive and the dimerization energies were found to be free of noticeably steric effects. Significant steric strain was found for all t Bu‐substituted carbenes with strain energies of the dimerization products ranging from 6.92 kcal mol –1 [formation of ( E )‐Me 2 t Bu 2 ‐enetetramine] to 24.23 kcal mol –1 (formation of t Bu 4 enetetramine). The tert ‐butyl substituent thus assumes a unique position by strongly stabilizing the carbenes electronically as well as sterically.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)