Premium
5‐Benzyl‐3‐methylimidazolidin‐4‐one‐Derived Reactive Intermediates of Organocatalysis – A Comforting Resemblance of X‐Ray, NMR, and DFT Solid‐Phase, Liquid‐Phase, and Gas‐Phase Structures
Author(s) -
Grošelj Uroš,
Schweizer W. Bernd,
Ebert MarcOlivier,
Seebach Dieter
Publication year - 2009
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/hlca.200800432
Subject(s) - chemistry , iminium , ring (chemistry) , substituent , derivative (finance) , organocatalysis , stereochemistry , crystallography , crystal structure , x ray , phase (matter) , organic chemistry , catalysis , enantioselective synthesis , financial economics , economics , physics , quantum mechanics
The X‐ray crystal structures of three ( E )‐1‐cinnamoylidene iminium PF 6 salts of 5‐benzyl‐3‐methylimidazolidin‐4‐ones (2,2‐dimethyl‐, cis ‐2‐( tert ‐butyl)‐, and cis ‐2‐styryl‐substituted; 2 – 4 , resp.) are reported ( Figs. 3–5 ). In the 2,2‐dimethyl and in the cis ‐2‐styryl derivative, 2 and 4 , respectively, a CH bond of the cis ‐substituent in 2‐position points to the center of the benzene ring of the benzyl group above the five‐membered ring ( Fig. 6 , a and b ). NMR Measurements ( Fig. 8 ) provide evidence that the same structure is present in solution, and that a fourth derivative of this type, 5 ( Scheme ), has ( Z )‐ instead of ( E )‐configuration around the CN bond. In the cis ‐2‐( tert ‐butyl) derivative 3 , the benzyl group is located over the iminium π ‐system ( Figs. 4 and 6 , c ). Overlays with DFT‐calculated crotonylidene analogs, A and B , show that the theoretical and experimental structures are almost superimposable ( Fig. 9 and Table ). The structures are discussed in view of their role as reactive intermediates in organocatalysis and in view of the help synthetic organic chemists may experience from theory.