Coordination stabilization

Exposure of ferri-heme-hemopexin to imidazole or KCN can displace one or both of the heme coordinating His residues, but millimolar concentrations are required (138). Other potential ligands such as azide or fluoride are inactive. This coordination stability of the ferri-heme-hemopexin bis-histidyl complex, despite the exposed heme site, is home out by thermal imfolding studies (Section IV,F). Reduction of the heme-hemopexin complex, however, has dramatic effects on its stability. 

Sulfine complexes of platinum(II) can be formed by oxidative addition to Pt(PPh3)3. The initial step involves the formation of an rj2-CS complex which undergoes intramolecular oxidative addition of a C—S bond (equation 542).1869 Use of Pt(cod)2 and PCy3 gives the tricyclohexylphosphine analogue.1870 The reaction gives two stereoisomers.1871 The coordination stabilization of sulfines allows their synthesis in the coordination sphere of platinum, but the cyclic process is not very efficient.1872... 

Variable coordination numbers. The coordinating stabilization energy (about 4.18 kJ-mol ) of lanthanide ions is much smaller than the crystal field stabilization energy of transition metals (typically >418 kJ moP ). Therefore, the coordinating bonds of lanthanide complexes are not directional and the coordination number varies from 3 to 12. 

A reaction pathway involving heterolytic cleavage of the B—B bond might be expected to yield trans addition products if intermediates such as a coordinated boronium ion (VI) or a coordinatively stabilized carbonium ion (VII) were involved or nonspecific addition if a carbonium ion intermediate such as (VIII) were produced. The formally similar electrophilic addition of halogen to simple olefins is, of course, well known to lead to... 

However, studies on the scope of this sequence revealed that the substrate has to be an AT-tosyl sulfonamide and that certain boronic acids are not trans-metallated but rather give rise to the formation of the pyrrole 21 or a pyridine derivative 22 (Scheme 7). The peculiar outcome as a carbopalladation-Suzuki sequence is rationalized by coordinative stabilization of the insertion intermediate 18 by the sulfonyl oxygen atom, as represented in structure 19, now suppressing the usual -hydride elimination. If the transmetallation is rapid the Suzuki pathway is entered leading to product 17. However, if the transmetallation is slow, as for furyl or ferrocenyl boronic acid, either -hydride elimination or a subsequent cyclic carbopalladation occurs. The former leads to the formation of the diene 20 that is isomerized to the pyrrole 21. The latter furnishes the cyclopropylmethyl Pd species 23, which rearranges with concomitant ring expansion to furnish piperidyl-Pd intermediate 24 that suffers a -hydride elimination to give the methylene tetrahydro pyridine 22. 

The selenoxide 107 as well as its benzyl (3 = 890) and phenyl analogues (3 = 879) were separated into the enantiomers and their chiroptical properties investigated intramolecular nitrogen coordination stabilizes the selenoxide so that no bulky substituents were required Another selenoxide with N-stabilization is 108 ... 

In addition to the hemes, the enzyme contains an iron—sulfur protein with an 2he-2S center. This center, termed the Rieske center, is unusual in that one ot the iron ions is coordinated by two histidine residues rather than two cysteine residues. This coordination stabilizes the center in its reduced form, raising its reduction potential so that it can readily accept electrons from... 

Coupling reactions. Coordinative stabilization of the arylmanganese species by an imino nitrogen atom greatly facilitates their formation via metal insertion of an oC-H bond. o-Functionalization of the aryl residue in a 2-arylimidazole by reaction with electrophiles such as PhCHO after treatment with Mn(CO)sBr meets expectation. ... 

This compound displays a slightly pyramidal geometry at the a-carbons, and the phenyl rings are not coplanar, which is typical for sterically congested multiphenyl-substituted ligands of this type. The increased steric demand of the ligand together with the electronic and coordinative stabilization provided by the crown ether enable increased stability as compared to the dibenzyl derivative. Remarkably, crown ether-stabilized barium diphenylmethanide is sufficiently stable to allow for recrystallization from hot THF, whereas dibenzylbarium reacts readily with THF at temperature above -40 °C. 

The conversion of the allylic enantioface is most likely to proceed via the tx riHCO-octadienediyl-Ni" transition state TSefc[2] (Fig. 5). In accordance with evidence provided from both experimental [17,18] and theoretical [19] studies, allylic isomerization is most Ukely to occur via the ix, ri (C )-octa-dienediyl-Ni" transition state TSiso[3], which constitutes the internal rotation of the vinyl group around the formal C -C single bond (Fig. 5). The explicit investigation of monomer-assisted reaction paths revealed that additional ethylene or butadiene does not accelerate either allyhc isomerization or enantioface conversion by coordinative stabilization of the corresponding transition state. 

The decatrienyl-Ni complex is seen to be preferably generated through ethylene insertion into the q -allyl-Ni" bond of 2 via a square-planar transition state TS[2-5] that occurs at a distance of 1.8-1.9 A for the emerging C-C o-bond (Fig. 6). This leads to the [Ni"(q q A,-decatrienyl)j species 5 as the kinetic insertion product, which represents the thermodynamically favorable form of the decatrienyl-Ni" configurations 5-7. Furthermore, additional ethylene or butadiene monomers are indicated to not accelerate the insertion via coordinative stabilization of any of the involved key species and are therefore not likely to assist the decatrienyl-Ni formation along 2- 5. 

Recently, r 2-diazoalkane complexes X have been synthesized and fully characterized. However, this mode of coordination stabilizes the diazoalkane against N2 evolution and thereby hinders carbene complex formation [9, 10]. 

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