Bicyclo[1.1.0]butane remains a fascinating compound. Stretching of its central CΓC s-bond culminates in a planar singlet diradical transition structure for inversion, [1] whereas stretching of the peripheral bonds leads to valence isomerization. [2] Bridging heteroatoms affect both processes. They enable the coexistence of the puckered bicyclic and planar diyl structures, and have thereby sparked intensive studies dominated by recent discoveries of diphosphorus analogues. Niecke et al. synthesized a crystalline 2,4-diphosphacyclobutane-1,3-diyl A [3] that isomerizes photolytically to 2,4-diphosphabicyclo[1.1.0]butane B [4] and thermally to 1,2-dihydro-1,2diphosphete C. [5] A still more congested diyl A that does not isomerize was reported by Yoshifuji and co-workers. [6] The group of Bertrand obtained both stable planar diyl and puckered forms for the isoelectronic 1,3-dibora-2,4-diphosphoniocyclobutanes. [7] 2,4-Diphosphabicyclo[1.1.0]butane is puckered when the phosphorus atoms are endo,endo substituted as in dihydro diphosphabenzvalene 1, which Mathey and co-workers obtained by an intramolecular phosphinidene addition to the C=C bond of a 1H-phosphirene. [8] Without geometrical constraints, such additions invariably lead to 1,2-diphosphetes of type C by isomerization or direct CΓP insertion. [9] Data on 2,4-disilabicyclo[1.1.0]butanes [10] is limited to two disilabenzvalenes [11] and is absent for the corresponding planar diyls. Only a single 2-silabenzvalene is known, generated by photolysis of a silabenzene. [12] For the synthesis of the 2,4-disila derivatives Ando et al. applied valence isomerization of bis(silirene) that involves an intramolecular silylene addition to the C = C bond of a silirene. [11] The use of stable silylenes might simplify this approach to a broader array of hetero derivatives, which we decided to explore for the unknown 2-phospha-4-silabicyclo[1.1.0]butanes 4.
Reaction of silylene Si{1,2-(NCH 2 tBu) 2 C 6 H 4 } (Si(NN); 3) [13] with 1H-phosphirene 2 a [14] in n-hexane at room temperature results in red crystalline 2,3-dihydro-1,3-phosphasilete 5 a instead of the desired 4 (Scheme 1). [15] Evidently, two Scheme 1. Synthesis of 2,3-dihydro-1,3-phosphasiletes 5.