4, 4 -bipyridine type ligands

Maikov et al. [37] prepared a series of C2-symmetric bipyridine-type ligands, the chiral moieties arising from the isoprenoid chiral pool (/3-pinene, 3-carene, 2-carene, or a-pinene, for example). Some representative examples are drawn in Scheme 16 (see 25, 26, 27) and were used as copper ligands of a copper(I) species obtained by an in-situ reduction of Cu(OTf )2 with phenyl-hydrazine. The use of the resulting catalysts in enantioselective cyclopropana-tion proceeded with up to 76% ee (for ligand 27) and high diastereoselectivity (up to 99 1). 

One of the most active and well-studied catalytic borylation systems is that generated from iridium(l) precursors such as [lr(COD)Cl]2 or [lr(COD)(OMe)]2 and bipyridine type ligands such as 2,2 -bipyridine or 4,4 -di-ferf-butyl-2,2 -bipyridine (dtbpy). In 2002, Ishiyama, Miyaura, and Hartwig et al. reported that the combination of [lr(COD)Cl]2 and 2,2 -bipyridine catalyzes arene borylation in the presence of excess arene under mild conditions (80°C). When the catalyst is generated from [lr(COE)2Cl]2 and dtbpy, the reaction proceeds even at room temperature [78, 79]. The same groups optimized conditions and found that the combination of [Ir(COD) (OMe)]2 and dtbpy (10) is a highly effective catalyst in the borylation of arenes so that reactions can be successfully performed with equimolar ratio of arenes and... 

A difficulty in isolation and clarification of the active Cu(I)—bipyridine complexes was overcome with the use of a silyl-containing bipyridyl ligand (L-6), which generates an ionic complex (Cu-1) on mixing with an equimolar amount of CuBr.89 90 The complex as well as an equimolar mixture of CuBr and L-6 showed levels of control similar those with L-3 and CuBr. An active Cu(I) species with bipyridine-type ligands presumably has a tetrahedral 18-electron species, as suggested for the similar complexes isolated after the polymerization of MA with... 

A Cu(II) complex with a bipyridine-type ligand (Cu-4) is effective in the controlled polymerization of styrene and acrylates in the presence of Al(0-i-Pr)3, which most probably serves as a reducing agent of Cu(II) into Cu(I).93-94 A fluoroalkyl-substituted bipyridine ligand (L-7) was also employed in supercritical carbon dioxide for the polymerization of fluorinated acrylates and methacrylates.95 Similar pyridine-based bidentate ligands, 1,10-phenanthroline and its... 

Shirakawa and Hayashi reported the iron-catalyzed oxidative coupling of arylboronic acids with arenes and heteroarenes (Eq. 28) [66]. They used iron(lll) triflate, a bipyridine-type ligand, and a peroxide as an oxidant. For substituted arenes, a mixture of ortho-, meta-, and para-substituted compounds was obtained, with modest selectivity for the ortho-isomer. The authors propose that Fe(lll) mediates generation of t-BuO radical from the peroxide, which oxidizes the arylboronic acid to generate an aryl radical that adds to the arene substrate. 

Nakamura observed the a-arylation of THE by an diorganozinc reagent in the presence of an iron/bipyridine-type ligand and 4-iodotoluene that presumably acted... 

Metal bipyridine complexes are classical mononuclear coordination compounds of photophysical interest [13]. Related binuclear (Fig. 11) and polynuclear species can be designed using a "double-bipyridine" ligand, obtained by linking together two bipyridine-type ligands via a short polymethylene chain [78-81]. 

Figure 2 DPPP and its derivatives and bipyridine-type ligands employed in CO/olefin copolymerization.

These ligands were active for allyhc substitutions but the process was not enantioselective in the benchmark reaction (88, in Scheme 49). More structurally constrained chelates led, however, to measurable enantioselectivities 40% ee for 89, 50% ee for 90, and 64% ee for 91 in the test reaction. By further modifications in the structure of these bipyridine-type hgands (see 92 in Scheme 51, a chiral Ci-symmetric 2,2 -bipyridine) [126], enantioselectivities up to 89% were obtained. 

Dendrimer 1 + is a classical example of a dendrimer containing a luminescent metal complex core. In this dendrimer the 2,2 -bipyridine (bpy) ligands of the [Ru(bpy)3] +-type core carry branches containing 1,2-dimethoxybenzene- and 2-naphthyl-type chromophoric units [15]. 

Pyridyl-containing ligands have already been discussed but the large body of work involving bipyridyl type ligands is represented here. Bipyridine and polybipyridine ligands in particular have found application with zinc in the formation of supramolecular structures and examples will be mentioned in Section 6.8.4.9. 

Il)cycles formed with 2,2 -bipyridine or 1,10-phenanthroline type ligands to give a variety of target compounds (Scheme 18). These syntheses are more feasible than previous, where the complexes cis-[RuCl2(bpy)2] or fRuCl .(tpy)] have been used to cyclometalate 2-phenyl-pyridine and its derivatives (230-233). A bunch of diverse compounds have been prepared in good yields. Crystal structures of some of them are shown in Fig. 21 and their useful properties are summarized in Table IX. 

Scheme 9 illustrates the sequence of events that occur when these Ru(II)-pyridinium type 1 dyads (16) are photoexcited. Visible light excitation produces the MLCT excited state, 17. Forward ET occurs via transfer of an electron from the bipyridine acceptor ligand to the covalently linked pyridinium acceptor to produce charge separated state 18, which features a d5 Ru(III) ion linked to the reduced pyridinium acceptor. Finally, back ET occurs by transfer of the odd electron from the pyridinium radical to the Ru(III) center. 

Rare Earth Complexes with 2,2 -Bipyridine (9) Type Ligands... 

Next to the rich oxygenation chemistry of Mn in Mn-Pc and Mn-POR complexes, there exists catalytic chemistry of Mn with non-heme-type ligands, mostly bioinspired. In Photosystem II, a non-heme multinuclear Mn redox center allows to oxidize water, while in catalase the active center is a dinuclear Mn species [34], Biomimctic models for these biological redox centers use ligands such as 2,2 -bipyridine (BPY), triaza- and tetraazacycloalkanes and Schiff bases such as Me(Salen) and Mc(saloph) (structure sec below) [23J. Usually, the complexes activate heterolytically peroxides, with Mn valency changes such as ... 

More synthetic interest is generated by the potentially very useful hydration of dienes. As shown on Scheme 9.6, methylethylketone (MEK) can be produced from the relatively cheap and easily available 1,3-butadiene with combined catalysis by an acid and a transition metal catalyst. Ruthenium complexes of several N-N chelating ligands (mostly of the phenanthroline and bipyridine type) were found active for this transformation in the presence of Bronsted acids with weakly coordinating anions, typically p-toluenesulfonic acid, TsOH [18,19], In favourable cases 90 % yield of MEK, based on butadiene, could be obtained. 

Rate parameters for the reaction of nickel(II) with pyridine, bipyridine, phenanthroline, and terpyridine are available [118] for dimethyl-sulphoxide and acetonitrile solvents along with similar data for terpyridine in methanol and ethylene glycol (Table 12). To test whether or not substitution is normal in type, ligand substitution rate coefficients (fej) are compared with the respective solvent exchange rates (fes) by means of a ratio R defined as... 

Emission-titration experiments for the Eu-(1) diacetate chloride in methanol solution were carried out with the following compounds in neutral or anionic form 1,10-phenanthroline 2,2 -bipyridine tribenzylphosphine oxide benzoic, 2-pyridine-carboxylic, 3-pyridinecarboxylic, 2-furancarboxylic, 2-thiophenecarboxylic, and 2-pyr-rolecarboxylic acids various pyridinedicarboxylic adds (2,3- 2,4- 2,5- 2,6- and 3,5-). These experiments showed that neutral ligands of the phenanthroline and bipyridine type do not enhance the Eu emission even though they produce intensely luminescent complexes with the free Eu(III) ion. Aromatic monocarboxylato ligands without heteroatoms readily replace the acetate(s) in the original substrate, as shown by the... 

---