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Abstract
This study is concerned with the reductive cleavage of tetracoordinated organophos-phorus compounds (either quaternary phosphonium salts R4P+ X or tertiary phosphine oxides R3P=O) with either the naphthalene radical (naphthalenide) anion or lithium aluminium hy-dride in THF solution at room temperature (RT). Part 1 examines the reaction of lithium naphthalenide with both phosphonium salts and phosphine oxides. The reaction was dem-onstrated to cleave phenyl groups from both bis-salts and bis-oxides in the presence of 1,2-ethylene bridges; based upon this, parallel syntheses of either 1,4-diphosphabicyclo[2.2.2]oc-tane or its P,P'-dioxide were attempted by using the commercially available ethane-1,2-bis-(diphenylphosphine) as the starting material in each case. Examination of the products ofreductive cleavage of the series of benzylphenylphosphonium bromide [PhnP(CH2Ph)4-n]+ Br (where n = 0 to 3) with lithium naphthalenide leads to the proposal of a mechanism.
Part 2 describes hydridic reductions of both quaternary phosphonium salts and ter-tiary phosphine oxides. Examination of the lithium aluminium hydride reduction of qua-ternary phosphonium salts using 31P-NMR has confirmed tetraorganophosphoranes (R4PH; R = Ph, alkyl) as intermediates in the reaction; in addition, two previously unknown classes of compounds, the triorganophosphoranes R3PH2 and the tetraorganophosphoranates R4PH2 , were also found to be intermediates. The behaviour of bis-phosphonium salts where the phosphonium centres are separated by either 1,2-ethylene or 1,3-propylene bridges are also examined. Formation of a monocation exhibiting a bridging hydride occurs when the cyclic bis-phosphonium salt 1,1,5,5-tetraphenyl-1,5-diphosphocanium dibromide is reacted with li-thium aluminium hydride. Mechanisms are proposed which are consistent with the observed experimental results.