The [4 + 2] cycloadducts of Me2E=C(SiMe3)2 (E = Si, Ge, Sn) and anthracene are prepared by reaction of an excess of anthracene in benzene with the [2 + 2] cycloadduct of Me2Si=C(SiMe3)2 and Ph2C=NSiMe3 at 130°C, with Me2Ge(OPh)-CLi(SiMe3)2 at 100°C, and with Me2SnBr-CNa(SiMe3)2 at 80°C, respectively. The mentioned adducts act as sources for the ethenes Me2E=C(SiMe3)2 above 100°C, the intermediate formation of which has been demonstrated by trapping experiments with 2,3-dimethylbutadiene (formation of a [4 + 2] and an ene reaction product). The half life time of the anthracene adducts with E = Si, Ge, and Sn in the presence of DMB (= 2,3-dimethylbutadiene) in benzene on thermolysis at 130°C (first order reactions) is found to be 141, 12, and 0.3 h, respectively. In the absence of DMB, thermolysis of the cycloadducts leads in benzene as reaction medium exclusively to the dimers of the Me2E=C(SiMe3)2 intermediates. In toluene as reaction medium ene products of Me2E=C(SiMe3)2 are observed in addition to the dimers. The ene products are not isolable as such, but only after migration of the Me2E-CH(SiMe3)2 substituents. The formed derivatives C6H5CH2EMe2CH(SiMe3)2 of toluene in part give a second ene reaction with another Me2E=C(SiMe3)2 molecule. X-ray structure analyses of the mentioned sources in fact prove the latter to be normal [4 + 2] cycloadducts of Me2E=C(SiMe3)2 and anthracene.Key words: silaethene, germaethene, stannaethene, [4 + 2] anthracene adducts, [4 + 2] cycloadditions and reversions, ene reactions, X-ray structure analyses.
Alder-ene reactions driven by high steric strain and bond angle distortion to form benzocyclobutenes