On the mechanism of the Hg2+ and base induced hydrolysis reactions of the β2‐(RR, SS)‐Co (trien) (glyOR)Cl2+ions (R = H, C2H5), evidence for the site of deprotonation in the reactive conjugate base

Abstract

AbstractAcid hydrolysis of the ester function in Δ‐(−)589‐β2‐(RR)‐[Co (trien) (glyOEt) Cl]2+ ((−)‐1) produces optically pure Δ‐(−)589‐(RR)‐[Co (trien) (glyOH)Cl]2+ ((−)‐4). Hg2+ induced removal of chloride in (−)‐4 follows the rate law kobs = kHg [Hg2+] with kHg = (1.36 ± 0.03) × 10−2 M−1s−1, 25°, μ=1.0, and produces optically pure Δ‐(−)589‐β2‐(RR)‐[Co(trien) (glyO)]2+ ((−)‐2). Competition by NO occurs in this reaction ([NO], 1M, 3%) indicating a path whereby external nucleophiles (YNO, H2O) compete with the intramolecular carboxylate function for an intermediate of reduced coordination number. Rapid ring closure to 2 must ensue for Y  H2O. Base hydrolysis of chloride in (±)‐1 produces (±)‐2 together with its diastereoisomer β2‐(RS, SR)‐[Co(trien) (glyO)]2+, ((±)‐3), in which one secondary amine function has an inverted configuration. 2 and 3 incorporate 18O‐labelled solvent into the Co‐O position of the coordinated carboxylate moiety (2: 9.0%; 3: 12.3%) indicating that at least part of the product arises via intramolecular hydrolysis in β2‐hydroxo ethylglycinate intermediates (Fig. 4). Base hydrolysis of (−)‐4 follows the rate law Kobs = kOH[OH−] with kOH = (6.3 ± 0.6) × 10−4M−1 S−1, 25°, μ = 1.0 producing (−)‐2 (37‐45%) and (−)‐3 (63‐55%), the ratio being somewhat medium dependent. Competition by added N (1M) occurs using (±) ‐4 forming β2‐(RR, SS)‐[Co (trien) (glyO)N3]+ (∼2%) and β2‐(RS, SR)‐[Co (trien) (glyO)N3]+ (∼ 13%). Mutarotation at the secondary nitrogen centre is shown to occur after the rate determining loss of Cl− in 1 and 4 and before the formation of 2 and 3. It is concluded that this secondary nitrogen is the site of deprotonation in the reactive conjugate bases of 1 and 4, and possible mechanisms for the mutarotation process are considered.