Phospha[3]radialenes. Syntheses, structures, strain energies, and reactions

In situ-generated terminal phosphinidene complex PhPW(CO)(5) adds in a 1.2- fashion to the terminal double bond of tetramethylcumulene and cyclic 1,2,3 -cyclodecatriene. The resulting alkenylidenephosphiranes 19A and 20A, which are three-membered phosphiranes containing an exocyclic allenic group, sub sequently rearrange to the corresponding phospha[3]radiales 19B and 20B, wh ich are phosphiranes having two exocyclic double bonds. All four organophos phorus compounds were characterized by single-crystal X-ray structure deter minations. Bicyclic 20A contains a significantly bent (171.5(7)degrees) and twisted (14.2(8)degrees) allenic unit in contrast to 19A. The rearrangemen t to the thermodynamically favored radialenes is considered to occur bg a p hosphirane ring opening/closure sequence. On using a second equivalent of P hPW(CO)5, insertion takes place into a PC bond of 20B, but not of 19B, to g ive two new phospha[4]radialene isomers. viz. cis-20C and trans-20C, both o f which were characterized by crystal structure determinations. The PPCC ri ng in these systems is significantly puckered (similar to 150 degrees), cau sing the olefinic bonds of the butadiene unit to be much twisted from plana rity. Both phospha[3]radialenes undergo Diels-Alder reactions with methyl-1 ,2,4-triazole-3,5-dione (MTAD), resulting in the case of the acyclic cumule ne in the expected addition product 19D of which the phosphirane ring easil y hydrolyzes. Cycloaddition of MTAD to 20B does not occur at the radialene' s diene unit but rather invokes one of its PC bonds, possibly in a concerte d [(sigma (2) + pi (2)) + pi (2)] mechanism, to give the unexpected adduct 20F. Ab initio theoretical studies on the parent systems, using the G3(MP2) method, show phospha[3]radialene to be 3.6 kcal/mol more stable than ethen ylidenephosphirane. Their strain energies (SE) are calculated to be 32.3 an d 29.7 kcal/mol, respectively. The 22.2 kcal/mol SE of phosphirane: increas es by 5.9 kcal/mol on introducing one exocyclic double bond and by another 4.2 kcal/mol on introducing the second one. Still, the SE of phospha[3]radi alene is less than the 39.0 kcal/mol of the more condensed phosphirene.