Pyridyl bipyridyls from

Homochiral pyridyl, bipyridyl, and phosphino derivatives of 2,2-dimethyl-1,3-dioxolane (113)-(115) were prepared from T-(+)-tartrate. These compounds were assessed in metal-catalyzed asymmetric hydroformylation of styrene enantioselectivity was generally low.342... 

Nevertheless, positions and differences of Ej and E2 allow some qualitative interpretations. The vinylene bridge in 5,6-position seems to be inert not only in ii, which resembles the 4,4 -bipyridyl system i/(c.f. 3.1) but also 13B2 and 13B3 which behave similar to the 2,2 -bipyridyl systems 2B2 and 2B3. In 13B3 the vinylene group prevents the two pyridyl units from distortion by the Ca-brit e between the nitrogen atoms compared to 2B3. Thus its Ksem stays closer to that of 13B2. 

The presence of the nitrogen atom presumably gives rise to an (relatively small) increase of the acidity of the ring protons, compared to benzene. The pK-values of the pyridine protons are not known, but it may be assumed that these are a few units lower than those of benzene (pK 43) but higher than 36, the value of diisopropylamine. Attempts to metallate pyridine did not result in any accumulation of metallated intermediates [146]. The isolation of 2,2 -bipyridyl from this reaction was considered as evidence for the intermediate occurrence of 2-pyridyl-lithium. Even pyridinecarboxamides Py—C(=0)NR2 are incompletely metallated by LDA [147]. 

Polymeric compounds are formed from 1,3,5-benzene tricarboxylate and zinc with 2,2 -bipyridyl, phenanthroline, or a pyridyl-2-(l-methyl-lH-pyrazol-3-yl) derivative. A number of compounds were characterized with varying ligand protonation and some, such as [ZnL(pyridine-2-(l-methyl-4-nitro-lH-pyrazol-3-yl))(H20)], containing single-stranded coordinative chains.383... 

Suitably substituted diazines also form chelate complexes, e.g. 2,3,5,6-tetrakis(a-pyridyl)pyrazine yields red tridentate complexes with Fe11. In the fast development of metallosupramolecular chemistry, many other polydentate ligands, based on 2,2 -bipyridyl units, have been obtained and studied (94CI(L)56). Thus, two molecules of oligopyridine (58) interact with various metal ions (Fe2+, Co2+, Cu2+ etc.) to form a double-helical [M2L2]4+ complex in which each metal is bonded to a tridentate region from each ligand. 

Photolysis of aryl or pyridyl oxime esters in pyridine provides a-phenylpyridines as the major products together with bipyridyls (84TL3887). Rate constants for the addition of phenyl radical to protonated and non-protonated 4-substituted pyridines have been determined by studing the competition between phenyl radical addition and chlorine abstraction from carbon. The 4-arylpyridines were the major products, and no 3-substituted pyridines were observed. Among the solvents studied (MeCN, DMF, DMSO, and HMPA), MeCN gave the highest yields and selectivity (910PP438). 

Additional work by the Forster group, making use of transient emission spectroscopy, probed the rate of photoinduced electron transfer between metal centers within a novel trimeric complex [Os(II)(bpy)2(bpe)2 ] [Os(II) (bpy)2Cl]2 4+, where bpy is 2,2/-bipyridyl and bpe is fra s-l,2-bis-(4-pyridyl) ethylene. Transient emission experiments on the trimer, and on [Os(bpy)2(bpe)2]2+ in which the [Os(bpy)2Cl]+ quenching moieties are absent, reveal that the rate of photoinduced electron transfer (PET) across the bpe bridge is 1.3 0.1 x 108s-1. The electron transfer is believed to be from the peripheral Os(II)Cl metal centers to the [Os(bpy)2(bpe)2]2+ chro-mophore. Comparison to rate constants for reduction of monolayers at a Pt electrode reveals that the photoinduced process is significantly faster [39]. 

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