Triphenylmethyl cation reduction

An alternative approach to pK determination for very weak hydrocarbon acids is the electrochemical method of Breslow " . This method is thermodynamic in origin employing voltammetric reduction/oxidation of the cation (or anion) to radical thence to anion (or cation) and comparing the energetics of these steps to the triphenylmethyl system (including bond dissociation energies of the respective hydrocarbons). Values of p aS obtained for some weak carbon acids by this method are given in Table 1 for comparison with the Streitwieser results. 

A comparison of the reduction potentials of triphenylcyclopropenyl (40, Ar = Ph) and triphenylmethyl (42) cations indicates that the former is harder to reduce ... 

However, the acidities of the benzyltriphenylphosphonium cation (pATa = 17.4) and of diethyl benzylphosphonate (pATa = 27.6) are sufficiently different to throw into question the likelihood of deprotonation of diethyl benzylphosphonate by the radical anion of 1. On the basis of a detailed study of the processes associated with the reduction of 1 (discussed later), it is likely that the effective base may well be the much more basic triphenylmethyl-derived carbanion, 4 in Scheme 20 (pATa of PI13CH = 30.6). This may possibly be produced by adventitious protonation of the fuchsone radical anion. Alternatively, the active EGB may be the dianion formed by disproportionation. 

An effective route to the parent cation (33) using Meerwein s reagent, borohydride reduction followed by reaction with triphenylmethyl tetrafluoroborate is illustrative (Scheme 6) <86ZC138>. 

Hydride abstraction from dienyl tricarbonyl iron complexes furnishes cationic dienyl tricarbonyl iron complexes. For example, reaction of the diene - iron tricarbonyl complex (115) with triphenylmethyl hexafluorophosphate followed by trimethylsilyl cyanide furnished with excellent regio- and stereoselectivity a new diene-iron tricarbonyl complex (116) (Scheme 170). Excellent regio- and stereoselectivity is seen upon reaction of the cationic complex (116) with trimethylsilyl cyanide (TMS-CN) (Scheme 170). Reduction of the nitrile affords a spirocyclic lactam complex. Intramolecular cyclization of in situ formed enols furnishes spirocyclic compounds again with excellent stereocontrol (Scheme 171). An interesting example of hydride transfer from a cyclohexadiene ring to a pendant aldehyde followed by nucleophilic addition is seen in Scheme 172. 

Reduction of thiopyrylium iodide with zinc, and treatment of the product with triphenylmethyl fluoroborate, perchlorate, or iodide, yields the bisthiopyrylium salts (50 X = BF4, CIO4, or I), which are relatively stable in air but decompose rapidly in contact with water. 4-4-Coupling in the dication is indicated by the very simple n.m.r. spectrum. The dication is reduced with zinc in acetonitrile to the radical cation (51), the e.s.r. 

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