Dewar-triphosphinines

When the salts 30 are not isolated but rather directly hydrolyzed under mild conditions the 1,4-dihydrophosphinines 32 - or in the case of the sterically demanding isopropyl group the dihydro-Dewar-triphosphinine 34 - are obtained [33]. 1,4-Dihydrotriphosphinines of the type 32 are also obtained together with other products on hydrolysis of the hafnium complex 10 [34]. Crystal structure analyses of 32a and 34 confirmed the structures and revealed the steric arrangement of the substituents shown in Scheme 8. We assume that the hydrolysis proceeds through the common intermediate 31, formed by protonation of the A o -phosphorus atom the existence of the latter as the only plausible intermediate has been confirmed by ab initio calculations [35]. 

Scheme 29. Homo-Diels-Alder reactions of 1,3,5-Dewar-triphosphinines...

According to calculations, the parent 1,3,5-triphosphinine is 25 kcal mol more stable than its Dewar (triphosphabicyclo[2.2.0]hexadiene) isomer <95UP 625-01 >. 

The synthesis of tri-/-butyl-lP,32, 5A -triphosphinine arose from studies of the behavior of t-butylphosphaethyne (206) in the coordination sphere of transition metals. The first indication of the transition metal template catalyzed trimerization of (206) was the isolation of the vanadium-complexed 1,3,5-triphospha-Dewar benzene (207) <87AG(E)908>. The formation of the tetra-phosphabarrelene complexed to zirconium (208) implied the intermediate formation of the com-plexed tri-(-butyltriphosphinine derivative. The free barrelene could be obtained by oxidative decomplexation using C2CI6 <95AG(E)8i>. 

Of the possible valence isomers of 1,3,5-triphosphinines the Dewar-triphos-phinines and the triphosphabenzvalenes are known. The former are obtained by expulsion of the metal fragment in the transition metal-mediated cyclotrimer-ization of phosphaalkynes (see Scheme 4) and the latter by photolysis of 1,3,5-triphosphinines. 

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