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

Hitherto unknown 1,2,3‐triphosphetenes RCPPCF 3 PCF 3 ( 3a–d ) are formed as main products in reactions of phosphaalkynes RCP [R = i Pr 2 N ( 1a ), t Bu ( 1b ), Me 2 EtC ( 1c ), 1‐methylcyclohexyl ( 1d )] with the cyclotetraphosphane (PCF 3 ) 4 ( 2 ). According to NMR results the CF 3 groups in 3a–d have a trans disposition; an X‐ray diffraction study of 3a confirms this structure. The PP 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 sp 2 ‐C–N bond (1.336 Å) which, together with the elongation of the PC 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 Me 3 PPCF 3 ( 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 Me 3 PC(R)PPCF 3 PCF 3 PCF 3 [R = t Bu ( 10a ), Me 2 EtC ( 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 PC bond. An unusual lengthening of the sp 2 ‐C–C bond [1.553 (4) ( 10a ), 1.543 (6) ( 10b ), 1.551 (4) Å ( 10c )] to values typical for sp 3 ‐C/sp 3 ‐C distances is observed.