1,2,3-TRIPHOSPHETENES, FIRST EXAMPLES OF A NEW CLASS OF PHOSPHORUS-HETEROCYCLES

Hitherto unknown 1,2,3-triphosphetenes RC = P-PCF3-PCF3 (3 a-d) are fo rmed as main products in reactions of phosphaalkynes R-C = P [R = iPr( 2)N (1 a), tBu (1 b), Me(2)EtC (1 c), 1-methylcyclohexyl (1 d)] with t he cyclotetraphosphane (PCF3)(4) (2). According to NMR results the CF3 groups in 3 a-d have a trans disposition; an X-ray diffraction study of 3 a confirms this structure. The P-P bond lengths in 3 a are equal [2.201(2) and 2.204(2) Angstrom] and correspond to single bonds. A con siderable shortening is observed for the sp(2)-C-N bond (1.336 Angstro m) which, together with the elongation of the P = C bond (1.746 Angstr om), indicates effective pi donation of the lone pair on nitrogen. Sur prisingly, 3 a can be prepared in quantitative yields by reaction of t he PP ylide Me(3)P = PCF3 (6) with 1 a (molar ratio: 2:1). In contrast , the corresponding reactions of 6 with the alkyl-substituted phosphaa lkynes 1b-d lead to the novel phosphorus ylides Me(3)P = C(R)-P-PCF3-P CF3-PCF3 [R = tBu (10 a), Me(2)EtC (10 b), 1-methylcyclohexyl (10 c)] in good yields. In their molecular groundstate structures, determined by X-ray diffraction, the lone pair on the phosphano P atom prefers th e syn position with respect to the ylidic P = C bond. An unusual lengt hening of the sp(2)-C-C bond [1.553 (4) (10 a), 1.543 (6) (10 b), 1.55 1 (4) Angstrom, (10 c)] to values typical for sp(3)-C/sp(3)-C distance s is observed.