SILA HETEROCYCLES .22. DIELS-ALDER REACTIONS OF DICHLORO(NEOPENTYL)SILAETHENE WITH FURANS AND REARRANGEMENT OF THE RESULTING BICYCLES INTO MONOCYCLIC SI-O 7-MEMBERED RING COMPOUNDS

The reaction of trichloro(vinyl)silane (1) and LitBu (2) initially leads to the alpha-lithio adduct Cl3SiCH(Li)CH2tBu (3) which can be trapped by trimethylsilyl triflate yielding the C-silylated product [Cl3SiCH(SiMe3)CH2tBu (5)]. 1,2-LiCl elimination from 3 gives dichloro(neopentyl)silaethene [Cl2Si = CHCH2tBu (4)] as intermediate. This can be established by insertion of 4 into the Si-O bond of Me3SiOMe to Cl2Si(OMe)CH(SiMe3)CH2tBu (6). 4 is a dienophile of high activity and reacts with the furans 7-9 to give the [4 + 2] cycloaddition compounds 10-12. These Diels-Alder products are thermally unstable and decompose slowly at room temperature into the starting furans and tetrachlorodisilacyclobutane (15). Under thermolysis conditions (3 d/170-degrees-C) they rearrange into the monocyclic Si-O seven-membered ring compounds 16-18. A mechanism for the 7-Oxa-2-silabicyclo[2.2.1]hex-5-en --> 2-Oxa-2-silacyclohepta-4,6-diene rearrangement is proposed.