Reductive dimerisation

Pinacols, e.g. (28), are much easier to make as the two hydroxyl groups come from reductive dimerisation of carbonyl groups (p T 201). A modern method uses V(II) as the reducing agent,... 

Reversing the rearrangement and keeping the symmetry gives a pinacol (22) which can be made by reductive dimerisation of (23) but not from selective hydroxylation of (24),... 

The participation of surface radical species is, then, strongly suspected in the Kotbe reaction, but there are other radical reactions, such as the reductive dimerisation of C02, that are thought to be homogeneous, particularly in non-aqueous solvents. The basic mechanism in this latter case is thought to be ... 

The synthesis of indigo was much more difficult than that of alizarin (6.2) [43]. In 1865 von Baeyer first attempted to obtain indigo by reductive dimerisation of isatin (6.116) he finally achieved a seven-step synthesis from phenylglycine via isatin in 1878. Many syntheses have been developed subsequently for indigo, but very few of these have achieved industrial importance. 

The reductive dimerisation of organic halides is, to our knowledge, the first reported reaction involving both a faradaic electroreduction and a catalysis by... 

A synthesis of great industrial interest is the electrochemical anodic reductive dimerisation of two molecules of acrylonitrile to give adiponitrile, from which adipic acid and 1,6-hexanediamine are prepared by hydrolysis and reduction, respectively, of the two nitrile groups. Polycondensation of the resulting products leads to Nylon 66 (Scheme 5.27). 

Intramolecular cyclisation of a 4-arylbutanoic acid system is also an important step in a convenient synthesis of the polycyclic system, chrysene, which is formulated and described in Expt 6.12. Here, methyl cinnamate is first subjected to reductive dimerisation to give methyl meso-ft,y-diphenyladipate, which is accompanied by some of the ( + )-form. The meso isomer (16) is the most easily isolable and cyclisation occurs smoothly in sulphuric acid to yield the diketone 2,1 l-dioxo-l,2,9,10,ll,18-hexahydrochrysene, which is obtained as the trans form (17) as shown in the following formulation. Clemmensen reduction of this ketone followed by dehydrogenation (in this case using selenium) completes the synthesis of chrysene. 

However, perfluoro-cyclobutene (15) and -cyclopentene (18) behave quite differently [28] because electropolymerisation occurs at the cathode, to give interesting conducting materials (17), (20) respectively, whose properties have not been explored. It seems most likely that the propagation process in each case involves nucleophilic attack on the perfluorocycloalkene by an intermediate carbanion, (16) or (19). The likely propagating processes are outlined in Scheme 8. In contrast, cathodic reductive dimerisation of the trifluoromethyl derivative (21) occurs [29]. 

The reaction of non-enethiolisable dithiobenzoic esters with LDA led to reductive dimerisation [139]. In the presence of methyl iodide, 1,2-bis(methylthio)stilbenes are formed, probably through single electron transfer from LDA, formation of a radical anion, dimerisation to a dithiolate and alkylation. 

Sml2 can effectively promote the intermolecular reductive dimerisation of aldehydes or ketones giving rise to symmetrical diols.7 9 Generally, arylalde-hydes and aryl ketones couple within seconds in THF at room temperature. Aliphatic aldehydes and ketones react considerably more slowly several hours are required for the aldehydes, whereas for ketones reaction times of 24 h are usually needed. Nevertheless, these slower couplings can be greatly accelerated by the addition of additives such as HMPA.10... 

An alternative synthesis, suitable for bulky wingtip groups, was introduced by Mercer et al. [53]. The key feature of the synthesis is the reductive dimerisation of a Schiff base by a base metal (manganese, magnesium or zinc) in the presence of trifluoroacetic acid (see Figure 5.13). 

Figure 5.13 Synthesis of N-substituted chiral f 2-diamines by reductive dimerisation.

Metallic lanthanum effects the stereoselective reductive dimerisation of 3-iodoflavanones to 3,3 -biflavanones as exemplified by the first synthesis of cf/-chamaejasmine <05OL271> and a Pd-catalysed diastereoselective cyclisation features in a route to the marine metabolite (-)-15-oxopuupehenol <05OL1477>. 

The reductive dimerisation of N-methyl acridinium ion to give (125) proceeds under ultra-violet light illumination in aqueous methanol or aqueous acetonitrile in the presence of triphenyl-phosphine here electron transfer from the triphenylphosphine to the excited state of the acridinium ion to produce the N-methylacridinyl radical is implicated. 

In the chapter on radical reactions (Chapter 24) we saw how to make pinacols , e.g. (18), by reductive dimerisation of ketones. These pinacols rearrange in acid to give r-alky) ketones (19). 

With this strategy, tert-butylmethylphenylphosphine borane and PAMP-BH3 were obtained, although the latter presented low optical purity (41% ee) due to racemisation in the decarboxylation step, which had been performed in xylene at 130 °C for 1 h. The menthyl ester intermediate 60, for R = o-An, was also reduced to the alcohol, which was mesylated and iodinated affording the (I-iodophosphine borane 62 in 62% combined yield. This compound was reductively dimerised by activated copper to diphosphine borane 63 (64% yield), which is similar to DiPAMP but with a butyl bridge. 

The effect of crown ethers on the reductive dimerisation of Schiff bases has been studied. The reduced base (191), with potassium in the presence of... 

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