Annulenes, reduction

Reduction of the nonaromatic polyene [12]annulene, either electrochemically or with lithium metal, generates a 14-7c-electron dianion" ... 

The anomalously reduced stabilities of certain nonalternant hydrocarbons and higher members of [4n+2] annulenes arise from their seemingly peculiar geometrical structures in which a strong bond distortion often accompanied by a molecular-symmetry reduction occurs. 

Crystal data and details of data collection, data reduction and final refinement are reported in Table 1. The procedure for data collection and processing, which included a correction for scan-truncation effects, were similar to those recently described for syn-l,6 8,13-biscarbonyl[14]annulene [10] and citrinin [11], Figure 1 shows the numbering scheme adopted in the present analysis. 

What experimental evidence is available to clarify the resulting mode of spin-density and charge-density distribution If one characterizes the reduction of (i) a single annulene, (ii) a doubly layered and (iii) a triply layered analogue by cyclic voltammetry, one derives a first criterion (Alexander et al., 1989 Bohnen et al., 1992 Fry et a/., 1985). The potential... 

Consequently, the reduction of [4n]annu-lenes should be observed at relatively positive potentials with small AE separations for the dianion formation, while the reduction of the [4n + 2]annulenes should occur at more negative potentials with large AE separations for the dianion formation. This is exactly what is observed in (see Table 1). Although benzene [37, 38], the classical Hiickel aromatic with 4 1 + 2 = 6 jr-electrons, is reduced at —3.42 V (vs. Ag/AgCl), the reduction of the [Sjannulene COT occurs at —1.81V. Similarly, the [16]annulene (3) [67, 68] is more easily reduced than the corresponding [ISjannulene (4) [69], although the reduction of the larger jr-systems should be more favorable for electrostatic reasons. 

For larger annulenes, even the reduction to tetraanions is possible [47, 75-77]. This is especially favorable for the neutral 4 + 2 species, which, after the injection of four electrons, reform a stabilized 4 + 2 jr-system. On the other hand, the existence of four excess charges in one molecule gives rise to strong electron repulsion. 

Reductive Generation of Anionic Species 1101 Tab. 2 Structure formula of annulenes and cyclic conjugated systems (1-17)... 

The redox behavior of [MnLY], where L is the dianionic form of dibenzotetramethyltetra-aza[14]annulene and Y is tetrahydrofuran or nothing, has been investigated." A two-electron oxidation and one- and two-electron reductions have been determined for this complex. The... 

The reduction of several annelated corannulene derivatives has been performed using lithium and potassium metals.7 It has been found that annelation affects the annulenic character of corannulene by changing its charge distribution the dianions of derivatives that are annelated with six-membered rings have less annulenic character and are less paratropic than corannulene dianion. 

Oxidation and reduction of all-carbon annulenes are known to result in cationic or anionic species with aromatic properties different from those of the neutral ring, offering a textbook example of the validity of Hiickel rules [87], hi pyrrole-containing porphyrinoids, such oxidations and reductions can be realized with retention of charge neutrality because the charge can be adjusted by protonation or... 

Among mixed-ligand cyclo-octatetraene complexes, the U, Th, Pa, Np, and Pu complexes [(r -Cx 11x)An(p-SPr)2]2 were reported, which can be prepared by reacting the thiol with U(COT)(BH4) [399a], Reduction of the bulky 8-annulene thorium complex [Th COT(TBS)2 2] [COT(TBS)2 = CxIIfi(/-BuMc2Si)2-l,4] by potassium... 

Busch et al.102 have reported the electrochemical reduction of complexes of tetra-aza[16]annulene (20), to the porphyrin-like dianion. The Con(taab)2+ ion can be... 

Electrochemical reduction of Ni(taab)2+ [taab = (20), see p. 231] occurs in two one-electron steps, to complexes formulated as [Nim(taab)] + and [Niu(taab)]°. The relationship between the annulene taab and the two-electron reduction product, the porphyrin-like taab2-, is discussed.102 The preparation of macrocycles of type (87) by a template synthesis requires a minimum ring size of x = y = 3 and depends upon the strong complexing of the metal ion at the pH of the reaction, otherwise the metal hydroxide precipitates. The NiL(C104)2,nH20 (n = 0, 1, or 2) species have been prepared for x = 3, y = 4.479... 

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