Phospholes ,synthesised by an improved procedure, undergo Dials-Alder reactions at high pressure but not dipolar cycloadditions to nitrile oxides (which deoxygenate) or diaxomethanes ( which can, however, trap the phosphole oxides) . Diphenylketene gives an unexpected spirophosphorane with phospholes at 9 kbar.
Phospholes have been known only since 1959 1 and 1-monosubstituted analogues only since the synthesis of 1-methylphosphole by Quin 2 in 1967. There is considerable controversy as to the extent of delocalisation of the phosphorus lone pair and the degree of aromaticity to be accorded to this ring system but the consensus of opinion seems to lie in favour of there being a small amount of aromatic character. Thus, the P-Co bond length is about 7 pm shorter than normal 3. The 31P-NMR chemical shift is 30-50 ppm to the low-field side of typical resonances for phospholenes and phosphines, supportive though not conclusive of the presence of a ring current 4p5. On the other hand, the geometry of the bonds at phosphorus is pyramidal indicating aromatic stabilisation being insufficient to overcome the strong barrier to inversion of phosphorus-(3). Recently a Diels-Alder reaction has been reported between I-phenyl-2,3-dimethylphosphole,lc, and N-phenylmaleimide which argues in favour of diene character of the ring 6. In an aromaticity index proposed by Bird 7, phosphole appears to be the least aromatic of the heterocycles; thiophene,pyrrole, furan, phosphole and this is supported by ab initio calculations of resonance energies -149, -124. -89 and -54 kJmol-', respectively *pg. The calculated inversion barrier of 66 kJmol-1 is less than that of phosphines (ca. 150 kJmol-1) which presumably results from a degree of overlap of np electrons with the ring. In view of these considerations it seemed likely that phospholes would show considerable diene character and that a variety of (4+2) and other cycloadducts would be possible especially under high pressure conditions. This communication reports on the extent to which this is borne out. Experimental 3,4-Dimethyl-1-phenylphosphole was prepared by a modification of the method of McCormack, Scheme 1. lo. Phenyldibromophosphine (5.570. 20 mmol) was mixed with 2,3-dimethylbuta-1,3diene (1.75g, 20 mmol) in hexane (10 mL). The mixture was maintained at a pressure of 4-5 kbar at 25OC for 10h using the apparatus previously described ll. The product was a