Synthesis and Reactions of Novel Boron-containing Double-bond Compounds by Taking Advantage of Steric Protection

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Very little is reported for the synthesis of stable boron-containing double-bond compounds because they are highly reactive, On the other hand, we have succeeded in the synthesis of highly reactive species by taking advantage of kinetic stabilization using effective steric protection group, 2,4,6-tris[bis(trimethylsilyl)-methyl]phenyl (Tbt) group. In this project, as the first stage for the development of the chemistry of multiple-bonded compounds containing group 13 elements, we planned the synthesis of stable thioxoborane 1 [TbtB=S] and diborene 2 [TbtB=BTbt] by taking advantage of Tbt group leading to the elucidation of their properties.We succeeded in the synthesis of the first stable dimercaptoborane 3 [TbtB(SH)_2]. Reactions of 3 with various group 4, 14 or 15 metal dihalides gave the corresponding 1,3,2,4-dithiametallaboretane derivatives 4, and the differences in their structures and properties were systematically investigated. Thermolysis of 1,3,2,4-dithiastannaboretane 4a [TbtBS_2SnPh_2] resulted in the formation of the expected thioxoborane 1 via [2+2]retrocycloaddition. Thioxoborane 1 reacted with 1 , 3-dienes to afford the corresponding [2+4]cycloadducts 5, which was found to regenerate 1 via [2+4]retrocycloaddition on heating.Reaction of 4a with dimethylsulfoxide (DMSO) gave the oxoborane-DMSO complex 6 [TbtB=O-DMSO], the first stable oxoborane-Lewis base complex. Thermolysis of 6 resulted in the formation of the corresponding oxoborane [TbtB=O].We have also succeeded in the generation of selenoxoborane [TbtB=Se] by thermolysis of the corresponding 1,3,2,4-diselenastannnaboretane derivative.Although we have not succeeded in the synthesis of diborene 2, the other target molecule in this project, yet, we believe that the synthesis of 2 can be achieved on the basis of the knowledge about boron-group 16 element double-bond compounds obtained in this project.