Ketones half-wave potentials

Ketonate complexes of Ru are reported in a number of papers. The parent complex [Ru(acac)3] has been subject to a polarized neutron diffraction study at 4.18 K, to powder neutron diffraction studies and to single-crystal structure determinations at 293 K, 92 K, and 10.5 K. The structure is disordered at all temperatures. Measurements of the magnetic susceptibilities (at 2.5 K and 300 K) have been made along different crystal axis directions, and the results analyzed. An investigation of the relationships between ionization potentials and half-wave potentials of a series of tris(/3-ketonate)Ru complexes has been reported, and the electrochemical properties of [Ru(acac)3] in chloroaluminate molten salt media have been reported. The reduced species [Ru(acac)3] can react with AICI4 reduction by bulk electrolysis of a small amount of [Ru-(acac)3] in the melt yields [RuClg]. ... 

In acid solution the half-wave potentials for these processes are pH dependent. The overall reaction involves two electrons and is irreversible. Bond cleavage is believed to lead to the enol as shown in Scheme 5.4. Where, as with acetophenone, the ketone product is electroactive at more negative potentials, the wave height for ketone reduction is less than expected and is limited by the rate of enol to ketone tautomerism. This is because the enol is not electroactive. 

Figure 5.1. Variation of half-wave potential (E ) with pH for a-subslituted ketones (a) N-methyl-N-phenacylpiperidinium cation, (b) N-methyl-N-(2-oxopropy )piperidimum cation, (c)diethyl-(phenacyl)-sulphonium cation. Data from refs. (90] and [91].

An effect of the ring-size in addition to that observed in the elimination taking place in the reduction of vicinal dibromides, has been observed for medium-sized ring compounds which carry the electro-active group either in the side-chain or directly attached to the ring. The half-wave potentials of cycloalkyl bromides can be correlated with the half-wave potentials of cyclic ketones and their derivatives (Fig. 29). The observed relation indicates that the relative effect of ring-size on the reactivity is similar in all the reaction series compared. 

The hydantoins (116 and 117) have been prepared from DL-j8-ferrocenyl-a-alanine and N - ferrocenyl methyl-AT-(a-carbethoxy-benzyl)urea, respectively.68 The Friedel Crafts reaction of ferrocene and 2-pyrroylchloride gave the ketone (118).112,113 The polarographic half-wave potentials of 118 and other related ferrocenes were measured113 and it was concluded that the ferrocene radical was more electropositive than the phenyl radical. 

In the polarography area, half-wave potentials have been ranployed as a tool for predicting the scope and limitation of the reaction of a,P-unsaturated ketones... 

The carbonyl group, including aldehydes, ketones, and quinones, produce voltammetric waves. In general, aldehydes are reduced at lower potentials than ketones conjugation of the carbonyl double bond also results in lower half-wave potentials. 

Two waves arise with the indicated half-wave potentials. The wave height for the process P2 + Zb -> P3 occurring at the more negative potential will obviously depend on the rate at which electroactive P2 is generated from Pi, which is itself assumed to be electroinactive. Examples where this type of behavior arises are the reductions of a-substituted ketones, aromatic bicarbonyl compounds such as p-diacetylbenzene, terephthaldialdehyde and a,/8-unsaturated ketones. Restricting ourselves to one type of example discussed by Zuman, " the following is the scheme for reduction of an a-substituted... 

In the second group in Table 2.15, two reduction waves were obtained for each of the three peroxides, the first at -0.60 to -0.70 V, the second at -1.0 to -1.26 V. Methyl ethyl ketone peroxide in dimethyl phthalate showed three half-wave potentials, one of which (-1.82 V) was attributed to the phthalate ester, the first reduction wave (0.60 V) was observed only when the concentration of peroxide was below 0.01 M, and the relationship between diffusion current and concentration was nonlinear, since the diffusion current showed a maximum at a concentration of 2.1 x 10 M. The second reduction wave (half-... 

The electronics and structure of the acceptors, especially a, 3-unsaturated ketones, is also a determinant in 1,2- vs. 1,4-addition processes. In general, substitution of aryl or large groups at the carbonyl unit increases the preference for 1,4-addition,16ab while a, 3-unsaturated aldehydes afford exclusive 1,2-addition and [3,(3-disubstitution suppresses 1,4-addition,l6c presumably due to steric hinderance. House and Seyden-Penne have established good correlations between chemoselectivity and either the half-wave electrolytic reduction potentials,17 or the energy levels of the LUMO of various a, 3-unsaturated ketones.17b... 

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