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On the mechanism of the Hg 2+ and base induced hydrolysis reactions of the β 2 ‐(RR, SS)‐Co (trien) (glyOR)Cl 2+ ions (R = H, C 2 H 5 ), evidence for the site of deprotonation in the reactive conjugate base
Author(s) -
Buckingham David A.,
Marty Werner,
Sargeson Alan M.
Publication year - 1978
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/hlca.19780610624
Subject(s) - chemistry , nucleophile , hydrolysis , carboxylate , moiety , medicinal chemistry , intramolecular force , base (topology) , stereochemistry , amine gas treating , chloride , ion , diastereomer , crystallography , catalysis , organic chemistry , mathematical analysis , mathematics
Acid hydrolysis of the ester function in Δ‐(−) 589 ‐β 2 ‐(RR)‐[Co (trien) (glyOEt) Cl] 2+ ((−)‐ 1 ) produces optically pure Δ‐(−) 589 ‐(RR)‐[Co (trien) (glyOH)Cl] 2+ ((−)‐ 4 ). Hg 2+ induced removal of chloride in (−)‐ 4 follows the rate law k obs = k Hg [Hg 2+ ] with k Hg = (1.36 ± 0.03) × 10 −2 M −1 s −1 , 25°, μ=1.0, and produces optically pure Δ‐(−) 589 ‐β 2 ‐(RR)‐[Co(trien) (glyO)] 2+ ((−)‐ 2 ). Competition by NO 3 −occurs in this reaction ([NO 3 − ], 1M, 3%) indicating a path whereby external nucleophiles (YNO 3 − , H 2 O) compete with the intramolecular carboxylate function for an intermediate of reduced coordination number. Rapid ring closure to 2 must ensue for Y H 2 O. Base hydrolysis of chloride in (±)‐ 1 produces (±)‐ 2 together with its diastereoisomer β 2 ‐(RS, SR)‐[Co(trien) (glyO)] 2+ , ((±)‐ 3 ), in which one secondary amine function has an inverted configuration. 2 and 3 incorporate 18 O‐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 β 2 ‐hydroxo ethylglycinate intermediates ( Fig. 4). Base hydrolysis of (−)‐ 4 follows the rate law K obs = k OH [OH − ] with k OH = (6.3 ± 0.6) × 10 −4 M −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 3 −(1M) occurs using (±) ‐ 4 forming β 2 ‐(RR, SS)‐[Co (trien) (glyO)N 3 ] + (∼2%) and β 2 ‐(RS, SR)‐[Co (trien) (glyO)N 3 ] + (∼ 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.