ORGANOPHOSPHORUS COMPOUNDS .119. THE PHOSPHAALKYNE CYCLOTETRAMER SYSTEM - SYNTHESES, VALENCE ISOMERIZATIONS, AND REACTIONS

In contrast to their all-carbon analogues, phosphaalkyne cyclooligomer s only became accessible a few years ago. A milestone in the chemistry of cyclotetramers was the synthesis and structural characterization o f the tetraphosphacubane 5, obtained as a thermolysis product of tert- butylphosphaacetylene (1). Since then, the construction of eleven cycl otetramers has been achieved, covering seven different frameworks (A-G ). As a consequence of the use of kinetically stabilized phosphaalkyne s as starting materials, all known cyclotetramers bear sterically dema nding substituents (tert-butyl, tert-pentyl, l-adamantyl). Cyclotetram erizations are not only achieved by thermolysis of 1, but also by alte rnative and selective routes such as transition-metal-mediated, Lewis acid- and base-induced processes, as well as by cycloadditions to phos phaalkyne cyclotrimers. Interestingly, these tetramers can be intercon verted by various valence isomerizations. The results of thermal and p hotochemically-induced rearrangements are in good agreement with MO ca lculations carried out for the parent compounds. Phosphaalkyne cyclote tramers exhibit a highly interesting reactivity and other peculiar fea tures. One outstanding example is the tetraphosphacubane 5, which show s unusual structural and spectroscopic properties as a result of its u nique bonding arrangement.