Stereoretentive formylation of (S)-proline: new application of the self-regeneration of stereo-centres (SRS) principle via chelation to cobalt(iii)

Abstract

In a Vilsmeier-Haack-type formylation reaction the α-(dihydroxymethyl)-(S)-prolinato complex (+)578-p-[Co(tren){(RC,SN)-Pro[CH(OH)2]O}]Cl2·2H2O (22) was produced stereoselectively (85% ee) from the (S)-prolinato complex, (+)578-p-[Co(tren){(SC,SN)-ProO}]2(H3O)2(HOEt2)(O3SCF3)7 (18). Similar reaction of the (S)-alaninato complex, (-)578-p-[Co(tren)(S-AlaO)](H3O)(O3SCF3)3 (13), produced the racemate, rac-p-[Co(tren)(Ala{CH(OH)2}O)]SO4·2H2O (17). The contrasting stereochemical outcomes of the formylation reaction with 18versus13 were ascribed to the stereogenic character of the coordinated sec. amine of the (S)-prolinate chelate in 18, which serves to uphold a chiral environment during reaction, whereas reaction intermediates derived from 13 lack this stereochemical feature. The stereoselective formylation of (S)-proline, relying on coordination to an inert metal centre, as conducted here, constitutes a novel application of the concept of Self-Regeneration of Stereocentres (SRS). The α-(hydroxymethyl)-(S)-prolinato complex, (+)578-p-[Co(tren){(RC,SN)-Pro(CH2OH)O}]Cl2·2H2O (23) resulted from borohydride reduction of 22. The molecular structures of (+)578-p-[Co(tren){(RC,SN)-Pro[CH(OH)2]O}]Cl2·2H2O (22), rac-p-[Co(tren)-(Ala{CH(OH)2}O)]I2·H2O (17) and (+)578-p-[Co(tren){(RC,SN)-Pro(CH2OH)O}]Cl2·2H2O (23) were established by X-ray crystallography.

1,1′-Biisoquinolines—Neglected Ligands in the Heterocyclic Diimine Family That Provoke Stereochemical Reflections

“…Rigorous examination and consideration of the nomenclature challenges inherent in these situations has been limited, and the development and codification of required nomenclature has also lagged. As a consequence, ad hoc and mutually inconsistent approaches have been developed by individual research groups to address their particular needs [115][116][117][118][119][120][121][122][123]. It is clear that these situations are certainly not adequately covered in the standard recommendations from IUPAC [20,124].…”