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){(R_C,S_N)-Pro[CH(OH)₂]O}]Cl₂·2H₂O (22) was produced stereoselectively (85% ee) from the (S)-prolinato complex, (+)₅₇₈-p-[Co(tren){(S_C,S_N)-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){(R_C,S_N)-Pro(CH₂OH)O}]Cl₂·2H₂O (23) resulted from borohydride reduction of 22. The molecular structures of (+)₅₇₈-p-[Co(tren){(R_C,S_N)-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){(R_C,S_N)-Pro(CH₂OH)O}]Cl₂·2H₂O (23) were established by X-ray crystallography.