Electrochemical generation reactivity

Challenging applications, such as the synthesis of P-peptides [30] or examples of flash chemistry (like the reaction of electrochemically generated reactive cation pools [31]) have been successfully realized using microreactors. 

FIGURE 6.9 Time-of-flight mass spectra and deconvolution results of (a) unmodified (3-LGA, (b) (3-LGA after reaction with NAPQI, (c) (3-LGA after reaction with AQQI, and (d) (3-LGA after reaction with CLZox. The mass spectra of the modified protein were obtained after online reaction of the unmodified protein with electrochemically generated reactive metabolites. (From Lohmann, W. et al., Anal. Chem., 80, 9714, 2008. With permission.)... 

For microreactor synthesis using electrochemically generated reactive species, see Yoshida, J. (2005) Chem. Commun., 4509. 

The reactivity of electrochemically generated phosphonyl radicals has been recently reviewed by Kargin and Budnikova [8] and will not be considered here. The reactivity of phosphonyl radicals is mainly accounted for by the three processes [9] shown in Scheme 2 radical addition (1), atom transfer (2 and 3), and electron transfer (4). 

The electrochemical generation and reactivity of phosphoniumyl and related charged radicals have been recently reviewed by Kargin and Bunikova [8]. In 1995, Yasui reviewed the reactivity of trivalent phosphorus compounds in single electron transfer (SET) processes [41] and, in 1990, the EPR features and reactivity of phosphoniumyl radicals were reviewed by Tordo [42]. 

Rhodium Porphyrins. Chemical syntheses of [CPDRh32 and (P)Rh(R) complexes are well known(4-11). Electrochemical techniques have also been used to synthesize dimeric metal-metal bonded [(TPP)RhJ 2 as well as monomeric metal-carbon a-bonded (TPP)Rh(R) and (0EP)Rh(R)(12-16). The electrosynthetic and chemical synthetic methods are both based on formation of a highly reactive monomeric rhodium(II) species, (P)Rh. This chemically or electrochemically generated monomer rapidly dimerizes in the absence of another reagent as shown in Equation 1. 

The highly reactive nature of (P)Rh is perhaps best demonstrated by the reaction of electrochemically generated (TPP)Rh with terminal alkenes and alkynes. The overall reaction with alkynes is given by Equation 7 and the suggested mechanism... 

The work on the electrochemical generation of a solution of ceric sulphate from slurry of cerous sulphate in 1-2 M sulphuric acid was abandoned by BCR due to difficulties encountered in handling slurried reactants. A 6kW pilot reactor operated at 50 °C using a Ti plate anode and a tungsten wire cathode (electrolyte velocity about 2ms 1) produced 0.5 M Ce(S04)2 on the anode with a current efficiency of 60%. The usefulness of Ce(IV) has been limited by the counter anions [131,132], Problems include instability to oxidation, reactivity with organic substrates and low solubility. Grace found that use of cerium salts of methane sulfonate avoids the above problems. Walsh has summarized the process history, Scheme 6 [133],... 

We chose to study the generation of alkoxycarbenium ion 26 from thioacetal 28. The electrochemically generated ArS(ArSSAr)+, 37 which was well characterized by CSI-MS, was found to be quite effective for the generation of alkoxycarbenium ions, presumably because of its high thiophilicity (Scheme 17). The conversion of 28 to 26 requires 5 min at -78 °C. The alkoxycarbenium ion pool 26 thus obtained exhibited similar stability and reactivity to that obtained with the direct electrochemical method. The indirect cation pool method serves a powerful tool not only for mechanistic studies on highly reactive cations but also for rapid parallel synthesis. 

Scheme 1 Electrochemical generation of reactive intermediates for polar reactions.

Thus it has been shown that some metal-carbonyl compounds can be activated by electrochemical reduction generating reactive anionic species. Without going into details, it is worth pointing out that the synthesis of aldehydes can be obtained by electrolyzing a stoichiometric mixture of alkyl halides and ironpentacarbonyls (Eq. 17) [124, 125] ... 

Electrochemical oxidation is a means of generating reactive metabolites either in the absence of biological nucleophiles or through the addition of nucleophiles under controlled conditions. Mass spectrometric, NMR and IR evaluations can all be performed as described for amodiaquine [41] but recapitulation of microsomal metabolism under electrochemical conditions is not always possible and hence this technique is probably of limited applicability. 

II. ELECTROCHEMICAL GENERATION OF A REACTIVE ZINC FOR THE FORMATION OF STABLE OR TRANSIENT ORGANOZINC REAGENTS AND THEIR REACTIVITY... 

Modern electrochemical methods provide the coordination chemist with a powerful means of studying chemical reactions coupled to electron transfer and exploiting such chemistry in electrosynthesis. In addition, the electrochemical generation of reactive metallo intermediates can provide routes for the activation of otherwise inert molecules, as in the reduction of N2 to ammonia,50 and for electrocatalyzing redox reactions, such as the reduction of C02 to formate and oxalate,51 the oxidation of NH3 to N02-,52 and the technologically important oxidation of water to 02 or its converse, the reduction of 02 to water.53 Electrochemical reactions involving coordination compounds and organometallic species have been extensively reviewed.54-60... 

The reaction of alkyl isothiocyanates, RNCS, with diphenylphosphinic hydrazide (338) in benzene has been reported.308 The bis(diethylamino)[(methylthio)thiocarbon-yl]carbenium salts (339 X = I or BF4) display ambident reactivity and can react either at carbenium carbon (hard nucleophiles) or at the thiocarbonyl sulfur atom (soft nucleophiles).309 Electrochemically generated superoxide reacts with dithioic S,S -diesters (dicarbothiolates) (340 Ar = C5H3N or C6H4) to give the monocarboxylate anions in 100% yield before giving the dicarboxylate anions.310... 

The previous review in this series1 discussed the variations which can be expected in the structure and reactivity of electrochemically generated anion radicals and cation radicals as the group X in C=X is varied through C, N and O. The reader is referred to the previous review for that discussion, which will not be repeated here. 

---