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Intramolecular γ‐Hydroxylations of Nonactivated CH Bonds with Copper Complexes and Molecular Oxygen
Author(s) -
Schönecker Bruno,
Zheldakova Tatjana,
Lange Corinna,
Günther Wolfgang,
Görls Helmar,
Bohl Martin
Publication year - 2004
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200306054
Subject(s) - chemistry , hydroxylation , intramolecular force , molecule , denticity , stereochemistry , hydrogen bond , copper , hydrolysis , lone pair , nitrogen atom , crystallography , computational chemistry , organic chemistry , ring (chemistry) , enzyme , crystal structure
Copper( I ) complexes incorporating the isomeric bidentate ligands IMPY (iminomethyl‐2‐pyridines) or AMPY (aminomethylene‐2‐pyridines) are quite unusual in their ability to bind and activate molecular oxygen. Using these complexes, hydroxylations of nonactivated CH, CH 2 , or CH 3 groups in the γ‐position in relation to the imino‐nitrogen atom, and with a specific orientation of one H atom with respect to the binuclear CuO species, can be achieved in synthetically useful yields. Through mechanistic studies employing conformationally well‐defined molecules (for example, cyclic isoprenoids), coupled with solid‐state X‐ray structure analyses and force‐field calculations, we postulate a seven‐membered transition state for this reaction in which six atoms lie approximately in a plane. This plane is defined by the positions of the lone pairs on the nitrogen atoms, as well as the copper and the oxygen atoms. For a successful hydroxylation, one hydrogen atom should be located close to this plane. Prediction of the stereochemical course of these reactions is possible based on a simple geometrical criterion. The convenient introduction of IMPY and AMPY groups as auxiliaries into oxo and primary amino compounds and the simple hydrolysis after the hydroxylation procedure has allowed the synthesis of 3‐hydroxy‐1‐oxo and 3‐hydroxy‐1‐amino compounds. If desired, the 3‐hydroxy‐1‐IMPY and ‐1‐AMPY compounds can be reduced with NaBH 4 to obtain 3‐hydroxy‐1‐aminomethylpyridines. For a successful hydroxylation procedure, the method employed for the synthesis of the Cu I complexes is very important. Starting either from Cu I salts or from Cu II salts with a subsequent reduction with benzoin/triethylamine may turn out to be the better way, depending on the ligand and the molecular structure.