Sepulchrates formation

Figure 7-16. A partial mechanism for the stepwise formation of a capping group from the reaction of [Co(en)3]3+ with formaldehyde and ammonia. A similar process leads to the formation of the second cap and the encapsulating sepulchrate ligand.

Fig. 8 Template synthesis of the cobalt(III) sepulchrate complex, [Coffl(sep)]3+. Step i complex formation and oxidation by air step ii Schiff base condensation step iii nucleophilic attack to the C=N double bond by the deprotonated forms of ammonia...

The template synthesis of the nickel sepulchrate proved to be rather complicated because of macrocyclic and acyclic amines (Scheme 69), competitive formation reactions occurred upon refluxing ethylenediamine, formaldehyde, and ammonia in the presence of Ni + ion. 

The fact that the synthesis of sarcophaginates and sepulchrates proceeds via the formation of an imine complex was also confirmed by isolation of the semiclathrochelate [Co(sen)]- + complex (as a by-... 

The mechanism proposed for the formation of the sepulchrates accounts for the fact that, despite seven chiral centres and six nitrogen-metal bonds, the synthesis of [Co(sep)] + cation yields only one isomer. After the formation of the first heminal diamine and second imine centre, the chiral nitrogen centre must be oriented so that the methylene unit of the amine fragment and the proton are apical and equatorial, respectively, before intramolecular cycli-zation. In this case, six-membered ring closure takes place. The same apical orientation of the six-membered chelate cycle must be retained to complete the formation of a capping group. After the completed encapsulation, the methylene unit and the proton cannot... 

Not only does chelation make the complex more stable, but it also forces the donor atoms to take up adjacent or cis sites in the resulting complex. Equation (2) shows how displacement of a chelating carbonate ion gives the unusual cis dichloride product instead of the thermodynamically more stable trans dichloride. Polydentate chelating ligands with three or more donor atoms also exist. Macrocyclic ligands, such as (4) and (5), confer an additional increment in the formation constant (the macrocyclic effect) they have been given trivial names, such as cryptates (4) and sepulchrates (5).i... 

The inner-sphere reductions of [Co(NH3)5(SCONHR)] and [Co(NH3)5 (OCSNHR)] by Gr involve attack at the remote oxygen and sulfur atoms, respectively, with a subsequent isomerization of the 0-bonded ehromium(III) product in the former reaction. The unusually rapid reactions of the S-bonded cobalt(III) complexes are attributed to a structural tran -effect on the Co—N bond length, reducing the reorganization energy needed to form the transition state. A kinetic study of the Cr reduction of [Co(NH3)5(pyruvate)] reveals that the rate of reduction is dependent on the nature of pyruvate ligand, with the keto form about 400 times as reactive as the hydrated form. An inner-sphere mechanism has be postulated for the Cr reduction of [Co(NH3)5(pyridine N-oxide)] on the basis of the rate and activation parameters. The outer-sphere Cr reduction of [Co(sepulchrate)] is catalyzed by halide ions, with the ion-pair formation constants for [Co(sep), estimated to be 5.5, 2.3, and 1.7 M" for Cl", Br", and I", respectively. ... 

Another way to achieve indirect chemical reduction in solution is represented by the use of organometallic complexes (e g., cobalt(lll) sepul-chrate trichloride). Often also redox partners and sometimes NAD(P)H are included in the reaction mixtures along with mediators [231, 232]. For example, application of a platinum wire working electrode supported the hydroxylation of lairric acid by recombinant CYP4A1 in the presence of rat CPR and cobalt(lll) sepulchrate trichloride in solution. The product formation rate obtained was comparable to that obtained with NADPH... 

---