1,2,3‐Triphosphetenes, First Examples of a New Class of Phosphorus Heterocycles

Hitherto unknown 1,2,3‐triphosphetenes RCPPCF3PCF3 (3a–d) are formed as main products in reactions of phosphaalkynes RCP [R = iPr2N (1a), tBu (1b), Me2EtC (1c), 1‐methylcyclohexyl (1d)] with the cyclotetraphosphane (PCF3)4 (2). According to NMR results the CF3 groups in 3a–d have a trans disposition; an X‐ray diffraction study of 3a confirms this structure. The PP bond lengths in 3a are equal [2.201 (2) and 2.204 (2) Å] and correspond to single bonds. A considerable shortening is observed for the sp2‐C–N bond (1.336 Å) which, together with the elongation of the PC bond (1.746 Å), indicates effective π donation of the lone pair on nitrogen. Surprisingly, 3a can be prepared in quantitative yields by reaction of the PP ylide Me3PPCF3 (6) with 1 a (molar ratio: 2:1). In contrast, the corresponding reactions of 6 with the alkyl‐substituted phosphaalkynes 1b–d lead to the novel phosphorus ylides Me3PC(R)PPCF3PCF3PCF3 [R = tBu (10a), Me2EtC (10b), 1‐methylcyclohexyl (10c)] in good yields. In their molecular groundstate structures, determined by X‐ray diffraction, the lone pair on the phosphano P atom prefers the syn position with respect to the ylidic PC bond. An unusual lengthening of the sp2‐C–C bond [1.553 (4) (10a), 1.543 (6) (10b), 1.551 (4) Å (10c)] to values typical for sp3‐C/sp3‐C distances is observed.