Cyclobutadiene analogues

The cyclobutadiene analogue diazadiboretidine reacts with hexafluoroacetone to give a nng expansion product [776] (equation 90) 6-(3-Fluoroaryl)decaborane is formed by alkylation of decaboranyl anion and separation of the two isomers (5- and 6-benzyl) formed by reaction with dimethyl sulfide [777] (equation 91). 

Azete, azetine and azetidine are the aza analogues of cyclobutadiene, cyclobutene and cyclobutane, respectively (Box 8.1). 

Several sila-analogues of cyclobutadiene, the prototype of Htickel An antiaromatic systems, were studied computationally. The available studies on mono- and disilacy-clobutadiene are quite old and therefore use relatively low levels of theory, and their results should therefore be treated with caution. 

The PES of tetrasilacyclobutadiene, 144, the full silicon analogue of cyclobutadiene, was studied extensively using relatively high levels of theory which include the contributions of polarization functions and of electron correlation52 163-165. The calculations revealed a very complex SizjlI4 PES. 

Table 2. jt- and a- Distortion Energies (AE, AE ) of Analogues of Cyclobutadiene, and 3jtjf Excitation Energies of a. T-Bond (AEst)" ... 

Table 4 shows Bo values calculated for hexagons and squares using Kollmar s method.157 Comparison of entries 1—4 to entries 5—8 reveals that the benzene analogues, with the 4/V+ 2 electrons, have considerably larger Bo values than the 4 TV analogues of square cyclobutadiene. This trend is related to the accepted view that cyclic delocalization of 47V + 2 electrons possesses a higher resonance energy than cyclic delocalization of 47V electrons. 

Singlet generated cyclopropyl carbenes, e.g. 253 (Z = CR), give cyclobutadienes by Wolff-like rearrangement whereas alkyne products most likely result from the triplet carbene 34 4.-347 nitrene analogue 253 (Z = N ) is thought to behave similarly but azacyclobutadiene is not isolable from the Sj reaction as it fragments to alkyne and nitrile. 

Studies of the reactivity of diphosphenes and phospha-arsenes which bear a complexed transition metal substituent at phosphorus (or arsenic) continue to appear.Aspects of the coordination chemistry of diphosphenes also continue to receive attention.The phosphorus and arsenic analogues of cyclobutadiene, cyclo-P and cyclo-As, have been stabilised as ligands at a niobium centre. 

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