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
✍ Scribed by David A. Buckingham; Werner Marty; Alan M. Sargeson
- Publisher
- John Wiley and Sons
- Year
- 1978
- Tongue
- German
- Weight
- 894 KB
- Volume
- 61
- Category
- Article
- ISSN
- 0018-019X
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✦ Synopsis
Acid hydrolysis of the ester function in
A-( -)589-fi2-(RR)-[Co (trien) (glyOEt)C1I2+ (( -)-1) produces optically pure A-( -)589$2-(RR)-[CO (trien) (glyOH)C1I2+ (( -)-4). Hg2+ induced removal of chloride in (-)-4 follows the rate law kobs=kHg [Hg2+] with kHg=(1.36+0.03)x 1 0 -2 ~-l s -' , 25", p = 1.0, and produces optically pure A-( -)5@-/)2-(RR)-[Co (trien)(gly0)l2+ (( -)-2). Competition by NO, occurs in this reaction ([NO,]= l ~, 3%) indicating a path whereby external nucleophiles (Y = NO,, H20) compete with the intramolecular carboxylate function for an intermediate of reduced coordination number. Rapid ring closure to 2 must ensue for Y = H20. Base hydrolysis of chloride in (+)-1 produces (zk )-2 together with its diastereoisomer P2-(RS, SR)-[Co (trien) ( g l ~O ) ] ~+ , (( k )-3), in which one secondary amine function has an inverted configuration. 2 and 3 incoporate lSO-labelled solvent into the Co-0 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 P2-hydroxo ethylglycinate intermediates (Fig. 4). Base hydrolysis of (-)-4 follows the rate law kobs= k , , [OH-] with koH= (6.3 kO.6) x 1 0 -4 ~-' s -' , 25", ,LL= 1.0 producing (-)-2 (37-45%) and (-)-3 (63-55%), the ratio being somewhat medium dependent. Competition by added N3 (1M) occurs using (k)-4 forming P2-(RR, SS)-[Co (trien) (glyO)N3]+ (-2%) and P2-(RS, SR)-[Co (trien) (glyO)N3]+ (- 13%). Mutarotation at the secondary nitrogen centre is shown to occur after the rate determining loss of C1-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.