Pyridine reductive dimerization

Treatment of UCI4 with the lithium complex obtained from dicyclohexylcar-bodiimide followed by crystallization from pyridine afforded a dinuclear uranium(rV) oxalamidinate complex in the form of dark green crystals in 94% yield (Scheme 191). The same compound could also be obtained by first reducing UCI4 to LiUCli (or UQs+LiCl) followed by reductive dimerization of di(cyclo-hexyl)carbodiimide as shown in Scheme 191. The molecular structure of this first oxalamidinato complex of an actinide element is depicted in Figure 31. ° ... 

Reductive dimerization of substituted pyridines to yield bipyridyls by zinc is catalysed by nickel salts under phase-transfer catalytic conditions [25]. 

In deference to their biological significance much work has been done on this class of compound a model studies to analyze or mimic the behavior of pyridine nucleotides and bipyridinium herbicides, py ridinium ions (132 Scheme 26) being more susceptible to reduction. Dimerization is a common fate o the neutral radical formed. The most thermodynamically stable product, the 4,4 -bipyridyl species pre dominate, and other dimers can sometimes rearrange to the 4,4 -bipyridyl isomer. Radicals with mere... 

Reductive dimerization of benzene [246], cyano biphenyl ether [247], pyridine [248], and acridine [249] derivatives has also been investigated. Radical anions of diesters of pyridine and benzene undergo rapid reversible dimerization (A dim = 10 -10 M" s" ) [250]. 

The kinetics of the reductive dimerization of NAD" in aqueous buffer (pH 9.1) have been studied by DPSC, CV, and LSV (Table 22) [302]. Reduction of NADP" on a preparative scale in 0.1 M NH4CI/NH3 buffer (pH 9.3) leads to a mixture of dimers formed by coupling of the pyridine ring systems. The three diasteromeric 4,4 -dimers were the major products ( 70%) formed along with minor amounts of the 4,6 -dimers [303]. Reduction of A-methylacridinium ion (105) in MeCN gives the expected dimer, 10,10 -dimethyl-9,9 -biacridine [300]. The value of for 105 was found [300,304] to be about an order of magnitude smaller than that for the radical derived from 104 (Table 22), probably due to the greater delocalization in the radical derived from 105. 

Pinacolization. In dichloromethane, aromatic aldehydes undergo reductive dimerization under the influence of TiClj. The reaction is highly stereoselective, giving the iy/j-l,2-diols predominantly syn anti 200 1). A cross-coupling is the major reaction when methyl phenylglyoxylate and pyridine are present, and p-amino-a-hydroxy esters are obtained with the further introduction of an amine. ... 

The dimeric 2,3-dihydro-l,2,4-oxadiazole palladium(n) complex 182 (Equation 27) reacts with aqueous methylamine to liberate the ligand 183 <2003JCD2544>. A similar process has also been applied to platinum(lv)-bound complexes 184, using pyridine to liberate the 2,3-dihydro-l,2,4-oxadiazole <2000JA3106>. Reduction of the platinum(iv) complexes 184 (Equation 28) gives the corresponding platinum(n) complexes 185 <2001IC264>. 

Free radical attack at the pyridine ring is noted for its low selectivity and substituents have little effect. Arylation takes place at all three positions, but halogen atoms preferentially attack the a-, and alkyl radicals the a- and y-positions. Metals such as sodium and zinc transfer a single electron to pyridine to form anion radicals. These can dimerize by reaction at the a- or y-position to yield dipyridyls by loss of hydride ion. Thus, reduction of pyridine by chemical and catalytic means is easier than reduction of benzene. 

Bipyridyls are by-products in the Chichibabin reaction. The dimerization of methyl-pyridinium salts has been patented, as it provides a route to the important herbicide paraquat (69USP3478042). Pyridine may undergo reduction in the presence of other nftetals as mentioned in Sections 2.05.4.7.5 and 2.05.4.7.6. 

These dimerizations are analogous to those of the radical anions R2C - 0 which are intermediates in the reduction of ketones to pinacols. Indeed, in the presence of magnesium amalgam, pyridine... 

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