ArP=C=Si(Ph)Tip: The first allenic compound with doubly bonded phosphorus and silicon

The first allenic compound with doubly bonded silicon and phosphorus, the 3-phospha-1-silaallene 1, has been obtained by dechlorination of the corresponding (chlorosilyl)chlorophosphaalkene 10 with tert-butyllithium at low temperature. Compound 1 was characterized by C-13 NMR (delta = 269.1 for the allenic carbon), Si-29 NMR (delta=75.7), and P-31 NMR spectroscopy (delta = 288.7), and by addition of methanol to the Si=C double bond. In the absence of a trapping agent, 1 dimerizes above - 30 degrees C to afford two types of dimers: 14, formed over one Si=C and one P=C double bond, and 15, formed over two Si=C double bonds, probably according to a concerted pathway. A thermodynamic equilibrium has been observed between the two stereoisomers 15 and 16 involving an isomerization around the P=C double bond. Compound 16 has been structurally characterized, displaying a nearly planar 1,3-disilacyclobutane ring with the two phenyl (Ph) and the two 2,4,6-tri-tert-butylphenyl (Ar) groups on the same side of this ring and the two 2,4,6-triisopropylphenyl (Tip) groups on the other side.