A doubly base-stabilized diborane based on a benzylphosphine linker was prepared by a salt elimination reaction between 2-LiC6H4CH2PCy2.Et2O and B2Br4. This compound was reduced with KC8 to its corresponding diborene, with the benzylphosphine forming a five-membered chelate. The diborene reacts with butadiene, 2-trimethylsiloxy-1,3-butadiene and isoprene to form 4,5-diboracyclohexenes, which interconvert between their 1,1-(geminal) and 1,2-(vicinal) chelated isomers. The 1,1-chelated diborene undergoes a halide-catalysed isomerisation into its thermodynamically favoured 1,2-isomer, which undergoes Diels-Alder reactions more slowly than the kinetic product. Cycloaddition reactions of unsaturated compounds are the most powerful tools for the synthesis of organic ring systems. [1] Almost a century has passed since the discovery of [4+2] cycloaddition reactions, [2] and a vast array of derivatives of this reaction is now known. While the majority of the research has naturally been confined to organic chemistry, inorganic chemists have often sought to extend these reactions to other main group elements (Scheme 1). In 1972, Roark et al. used anthracene and 1,4diphenyl-1,3-butadiene to trap a proposed fleeting disilene via Diels-Alder cycloadditions. [3] Although the subsequently discovered stable disilenes typically did not undergo reactions with butadienes, [4] a tetrasilyldisilene was reported to react with 2,3-dimethyl-1,3-butadiene to afford a [4+2]-cycloaddition product. [5] Diels-Alder reactions with digermenes followed a similar path, with 1,3-dienes initially being used to trap otherwise unstable R2Ge=GeR2 species. [6] These reactions were, however, often low yielding, and hampered by the propensity of disilenes and digermenes to dissociate into R2E: species. These inorganic Diels-Alder reactions could in some cases be reversed by pyrolysis (500-600 °C). [7] Ando and Tsumuraya then showed that the digermene Mes2Ge=GeMes2, which is stable enough to be observed in solution, reacts with 2,3-dimethyl-1,3-butadiene to give the digermacyclohexene product in good yield. [8] The equivalent reactions for group 13 doubly-bonded compounds have been significantly limited by the instability of RE=ER compounds. To the best of our knowledge, the only examples of [4+2]-cycloadditions of isolated group 13 dimetallenes are the reactions by Power and co-workers of the isolable terphenyl-substituted digallene ArGa=GaAr (Ar = 2,6-(2,6i Pr2C6H3)2C6H3) with cyclopentadiene and cycloheptatriene (Scheme 1). [9] The same group reported the trapping of the analogous dialumene, ArAl=AlAr, as its Diels-Alder adduct with toluene, [10] while analogous arene cycloaddition products have also been shown to act as sources of the dialumene unit for reactions with other substrates. [11] Related reactions of boronboron doubly-bonded compounds have never been reported. Scheme 1. Examples of Diels-Alder reactions of homoatomic main-group alkene derivatives.
