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Diastereoselective Assembly of Helicates Incorporating a Hexadentate Chiral Scaffold
Author(s) -
Constable Edwin C.,
Zhang Guoqi,
Housecroft Catherine E.,
Neuburger Markus,
Zampese Jennifer A.
Publication year - 2010
Publication title -
european journal of inorganic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 1434-1948
DOI - 10.1002/ejic.201000206
Subject(s) - chemistry , enantiopure drug , diastereomer , stereochemistry , chirality (physics) , stereoselectivity , crystallography , zinc , hydrogen bond , denticity , metal , crystal structure , enantioselective synthesis , molecule , organic chemistry , catalysis , quantum mechanics , quark , nambu–jona lasinio model , chiral symmetry breaking , physics
The condensation of enantiopure (1 R ,2 R )‐(–)‐1,2‐diaminocyclohexane with two equivalents of 2,2′‐bipyridine‐6‐carbaldehyde followed by reduction with NaBH 4 allows the facile synthesis of the enantiopure hexadentate ligand ( R , R )‐ 3 . Each tridentate metal‐binding domain in ( R , R )‐ 3 exhibits a degree of flexibility arising from inversion at the amine nitrogen centre and rotation about N–C and C–C single bonds. This leads to the formation of either M ‐[M{( R , R )‐ 3 }] 2+ or P ‐[M{( R , R )‐ 3 }] 2+ . The solid‐state structures of the copper(II), iron(II) and zinc(II) complexes of ( R , R )‐ 3 all show a preference for M ‐[M{( R , R )‐ 3 }] 2+ . In solution, [Fe{( R , R )‐ 3 }] 2+ exists predominantly as one diastereoisomer (assumed to be the M ‐form). The preference for the M ‐ over P ‐form is rationalized in terms of a favourable anti configuration of the NH and cyclohexane‐1,2‐diyl CH hydrogen atoms in the M ‐form, and an unfavourable syn ‐configuration in the P ‐form. When ( R , R )‐ 4 , the Schiff base analogue of ( R , R )‐ 3 , combines with zinc(II) or silver(I), [2+2] double helicates with M ‐chirality assemble in the solid state. With ( S , S )‐ 4 , iron(II) also assembles into a dinuclear, double helicate with M ‐handedness. Complete stereoselectivity is observed in solution with NMR spectroscopic data indicating the presence of one diastereoisomer. The preference for a dinuclear over mononuclear helicate can be traced to a decrease in ligand flexibility on going from 3 (saturated backbone) to 4 (unsaturated imine backbone).