Bronsted base ligands

Bronsted base Bronsted base Ligand Lewis base Anion receptor Guest... 

Metal ion complexation rates have been studied by the T-jump method. ° Divalent nickel and cobalt have coordination numbers of 6, so they can form complexes ML with monodentate ligands L with n = 1—6 or with bidentate ligands, n = 1-3. The ligands are Bronsted bases, and only the conjugate base form undergoes coordination with the metal ion. The complex formation reaction is then... 

Aromatization of the ligand is a major driving force in the C-C bond-cleavage reaction. For example, sp C-sp C bond cleavage in (pentamethylcydohexadienyl)ruthe-nium is reported to give ruthenocene derivatives (Scheme 14.23) [54]. In this reaction, a Bronsted base is believed to promote demethylation from the exo face similar reactions are documented for cationic ruthenium(II) complexes [55-57]. 

Von Zelewsky A, Barbossa L, Schlapfer C. Poly(ethylenimines) as Bronsted bases and as ligands for metal ions. Coordin Chem Rev 1993 123 229-235. 

Metal ions were simply one group of electrophiles. Class (a) metal ions reacted most rapidly, and more strongly, with nucleophiles which were very basic to the proton. Class (b) metal ions reacted most rapidly with nucleophiles that were easly oxidized. Metal ions are also one class of Lewis acids. The ligands with which they form complexes are simply Lewis (and Bronsted) bases. 

Calcium isopropoxide complexed to the simplest Ph-BOX ligand serves as a Bronsted base and chiral catalyst for rendering glycine t-butyl ester into a nucleophile toward acrylic esters. 

Using the pK values listed in Appendix B for various ligands which are also Bronsted bases, develop spreadsheets and EquiligrapHs for a number of common ligands. Preserve these for use in conjunction with questions concerning the role of pH in metal complex formation that will be introduced in Chapter 5. 

The titration data enable the investigator to describe the extent of reaction and therefore values of all the variables needed to calculate the K. Let us consider two cases, a simple acid-base titration for the determination of pK values and a metal complexation reaction involving a ligand which is also a Bronsted base. The second example is based on measuring the protons that are released when the conjugate acid of the ligand reacts with a metal ion. 

A ternary soft Lewis-acid/hard Bronsted-base/hard Lewis-base catalytic system for the direct catal5rtic enantioselective addition of allyl cyanide 37 to ketones to give tertiary alcohols with a Z-olefin (38) has been developed by Shibasaki (Scheme 2.24). Mechanistic studies revealed that Cu(i)/chiral phosphine ligand 35 and Li(OC6H4-p-OMe) serve as a soft Lewis acid and a hard Bronsted base, respectively, allowing for deprotonative activation of 37 as the rate-determining step. The hard Lewis base, bis(phosphine oxide) 36,... 

Free alkoxide and aryloxide anions are Bronsted bases with pK values of the corresponding alcohols ranging from 5 to 20 in water. The basicity is highly dependent on the electronic properties of the alkyl or aryl moieties. For example, the pK value of hexafluoro-tert-butanol, (CF3)jMeCOH, is 9.6, which is considerably lower than the pK value of tert-butanol (19.2), but roughly the same as that of phenol (9.9). Such differences in electronic, as well as steric, environments often leads to the different structures and reactivity patterns for compounds containing similar ancillary ligands, but different alkoxides or aryloxides. 

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