Lewis acid-base exchange reactions of polypnicogens
Cribbs, Leonard V
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The insertion of a perfluoroalkylphosphinidene moiety RfP from perfluoroalkyclcycloplyphosphines (RfP)n into the pnicogen-pnicogen bond of tetraalkyl dipnicogens appears to be general. Thus, a CFo unit from (CFoP)/ c reacts with R2P-PR2 (where R = Me, Et, Bu) to form the tripnicogen compounds R2P-P(CF3)-PR-2. These tripnicogens undergo neutral water hydrolysis surprisingly easily to form chiral dipnicogens. For example, Me2P-P(CF3)-PMe2 hydrolyzes immediately at ambient temperature with an equimolar quantity of water to yield the new chiral secondary phosphine Me2P-P(CF3)H plus Me2P(O)OH and Me2PH. A second equivalent of water hydrolyzes the dipnicogens to the primary perfluoroalkylphosphine. To further investigate the generality of the "insertion" reaction, cyclic polyarsines were studied to ascertain the possibility of (RfAs)n or (RAs)n providing a source of the arsinidene species RfAs or RAs. Addition of an equimolar quantity of Me2As-AsMe2 to (CF3As)4,5 in C6D6 does form the tripnictide Me2As-As(CF3)-AsMe2. The reaction of a secondary alkyl or aryl phosphine or arsine with the cyclic polyphosphine (CF3P)4,5 or cyclic polyarsine (CF3As)4,5 produces the chiral dipnictides of the type R2E-E(CF3)H, E = P or As. In addition, a number of other products are produced in a complex equilibrium mixture. The origin of the complex equilibrium has been studied by examining the reaction of pairs of reactants and/or products and a reaction mechanism has been deduced. Exchange reactions of the pnicogen-pnicogen bond have also been investigated. Thus, unsymmetrical diphosphines of the type R2P-PR2 typically disproportionate to form the symmetrical species R2P-PR2 and R2P-PR2. However, an exception to this behavior is noted when the relative basicities of the two pnicogen atoms in a dipnicogen differ dramatically. Generality of the Lewis Acid-Base exchange reactions was observed by studying molecules in which P-P bond breakage of the diphosphine was associated with scission of a P-P, P-H, and/or P-X bond in the second reactant. In all cases studied in which reaction occurred, product formation could be explained as proceeding through a four-centered intermediate in which nucleophilic attack by a relatively basic pnicogen atom occurs on a relatively acidic pnicogen atom. A variety of di- and tripnictides were prepared by this method and their NMR parameters determined.