Steric effects on regioselectivity in 1.3-Dipolar cycloaddition of C,N-dialyl nitrones with acceptor-substituted alkynes
摘要:
The 1.3 dipolar cycloaddition of alkynes 4-6 with acyclic nitrones 1-3, 23 and 24 as well as with cyclic nitrones 27 is studied. As was found for the reaction of the aldonitrones 1-3 an increased portion of 5-regioisomers is formed with increasing steric demand of either of the two substituents, the C-alkyl substituent R2 and the N-alkyl substituent R1. Thus the conclusion is drawn that cycloaddition of 1-3 proceeds not only via transition states arising from Z-nitrones but also via transition states developing from E-nitrones, although in the ground-state the Z-isomer is favored to a considerable extent. In this context steric destabilization of the transition states is discussed qualitatively.
New 2-substituted pyrroline-N-oxides: An EPR solvent study of the radical spin adducts
作者:Edward G. Janzen、Yong-Kang Zhang、D. Lawrence Haire
DOI:10.1002/mrc.1260321203
日期:1994.12
water) were measured and the solvent effect on these parameters was evaluated. It was found that for the various 2‐substituted DMPO‐type spin adducts both the nitrogen and β‐hydrogen EPR hyperfinesplittings correlated linearly (r2 ⩾ 0.90) with typical solventpolarity parameters such as ET(30). The correlation between the nitrogen and β‐hydrogen hyperfinesplittingconstants were even more linear (r2