Imines electron donor-acceptor

While lanthanide triflates have been demonstrated to promote the reaction of indoles with imines <99SL498>, Johannsen has developed a new synthesis of optically active p-indolyl N-tosyl a-amino acids 110 via the enantioselective addition of A-tosylimnio esters of ethyl glyoxylate 109 to indoles 108 bearing both electron-donor and electron-acceptor substituents at C-5 using 1-5 mol% of a chiral copper(I)-Tol-BINAP catalyst <99CC2233>. 

Crabtree and coworkers used the disappearance of color to monitor the activity of 12 potential catalysts for hydrosilation reactions [6], The group used alkenes and imines with ferrocenyl and pyrimidyl substituents at either end as colored reactants. On hydrosilation the color is bleached, because conjugation between the electron donor and acceptor groups is reduced. The bleaching is easily observed by eye and can be recorded quantitatively by means of a digital camera (Figure 5.4.1). By use of this method, the known Wilkinson catalyst was identified in a proof-of-concept experiment. A palladacycle, [ Pd (o-tolylfi PQ H4 OAc 2, usually used in Heck reactions, was also found to be catalytically active. 

An unstable adduct between triphenylphosphine and a photochemically-generated dimethylgermylene has been characterised spectrophotometrically.The first 2,3-dihydro-l,3,2-X -benzodiazaphospholes (73) have been formed in the reactions of triphenylphosphine with g-benzoquinone di-imines stabilised by coordination.A complex of phenylnitrene with a tungsten pentacarbonyl acceptor has been trapped using triphenylphosphine. A kinetic study of the reactions of diazoalkanes with triphenylphosphine, leading to the phosphazenes (74), indicates a biphilic mechanism, the dominant interaction in the transition state involving the diazoalkane as a net electron donor,... 

Analogous evaluation of other substituted s-triazole limine NMR spectra shows87 that the carbamoylimino, the sulfonylimino, and the arylimino groups86 act as donors, whereas the nitroimino group acts as an electron acceptor. From the NMR data, the arylimino group in pyridine A-imines is also an electron donor.70-71 (For quantitative constants of these substituents see Section III,C.)... 

The use of chloromethyl phenyl sulfone as a carbon nucleophile in the VNS reaction of 17 and 18 allowed one to obtain substitution products 27 and 28. In this case, the undesirable para-substituted regioisomer of 27 was formed in only 4% yield. Reduction of the nitro group in sulfones 27, 28 provided corresponding anilines 29 and 30, which gave good yields of imines 31, 32 with both electron-acceptor and electron-donor benzalde-hydes. Further cychzation under basic conditions provided access to 2-aryl substituted 5(6)-SF5-indoles 33 and 34 (Scheme 7). 

As synthetic chemists desired to tune the optoelectronic and redox properties of conjugated polymers in a fine manner, more complicated conjugated systems were required. The two fused heterocycles-substituted polythiophenes 16 and 17 (Chapter 18) illustrate this as electron-poor imine functionality in 16 brings donor-acceptor character to the material, while the more electron-rich thio-based system 17 provides for especially easy oxidation. Polymerization of these complicated bis-2-thienyl monomers by electrochemical methods paves the way for fundamental structure-property relationships to be understood, ultimately directing the synthetic chemist towards soluble polymers (Chart 1.6). 

Substituted benzylideneanilines form molecular complexes187 with 106 and with 2,5-dichloro-3,6-dihydroxy-p-benzoquinone (110) (chloranilic acid). These complexes were investigated by IR and 111 NMR. The complexes between benzylideneanilines and 106 are CT complexes (111), while complexes with 110 are formed through electron and proton transfer187, as illustrated in 112. In 111, a refers lo n -> jr interaction and b is a jr -> jr interaction, sp2 N-atoms of some heterocyclic imines (such as 2,2 -bipyridyl, 1,10-phenantroline) are also n-donors towards acceptors (tetracyanoethylene, p-benzoquinone, chloranil)188. 

Figure 5.27 Schematic representation of the polymeric Jelley or Scheihe assemblies of isocyanine dyes. An imine carbon atom acts as electron acceptor (a), an enamine nitrogen as donor (d). If the angle a is much below 54°, long wavelength polymer bands are observed. The rigid polymer fluoresces.

Imidazole, like water, is both a good donor and a good acceptor of hydrogen bonds the imine nitrogen donates an electron pair and the iV-hydrogen, being appreciably acidic (see 24.4), is an acceptor. This property is central to the mode of action of several enzymes that utilise the imidazole ring of a histidine... 

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