Reactions of Silicon Atoms with Conjugated π‐Systems: A Matrix‐Spectroscopic Study

The reaction of silicon atoms with cyclopentadiene and butadiene in argon at 10 K has been studied. Addition to the conjugated π‐system of cyclopentadiene occurs in a [1,4]‐fashion. Upon irradiation the π‐adduct 3 rearranges to the formal insertion product 2 which has an unusual structure. Remarkably, a photoequilibrium between the two exo , endo isomers 2 and 4 can be observed. In the case of butadiene the addition is less stereospecific. The planar [1,4]‐adduct 11 is formed together with the [1,2]‐adduct 14 . Surprisingly, besides 11 a second, nonplanar [1,4]‐adduct 10 can be detected. The isomers 11 and 14 are transformed into 10 upon irradiation. The final photoproduct is silole 9 . The structural elucidation of the new species is based on isotopic labelling and a comparison of the experimental observations with the results from density functional calculations. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)