SYNTHESIS OF TRANSIENT SILENES BY A MODIFIED PETERSON REACTION

Tris(trimethylsilyl)silylmagnesium bromide, obtained in situ from tris(trimethylsilyl)silyllithium and magnesium bromide, reacts with acetone, pivalaldehyde, or 2,4,6-trimethylbenzaldehyde to give the (1-hydroxyalkyl)tris (trimethylsilyl)silanes (Me(3)Si)(3)SiC(OH)Me(2) (1a), (Me(3)Si)(3)CH(OH)tBu (1b), and (Me(3)Si)(3)SiCH(OH)Mes (1c), resp. After deprotonation with methyllithium in ether at -78 degrees C 1a-c eliminate trimethylsilanolate according to a modified Peterson mechanism to form transient silenes (Me(3)Si)(2)Si=CR(1)R(2) (6a: R(1)= R(2) = Me; 6b: R(1) = H, R(2) = tBu; 6C: Ri = H, R(2) = Mes). In the absence of trapping agents these silenes dimerize, 6a leading to the linear dimer 1-isopropenyl-2-isopropyl-1,1,2,2-tetrakis methylsilyl)disilane (7) and 6b giving the head-to-head cyclodimerization product (E)-3,4-di-tert-buty1-1,1,2,2-tetrakis(trimethylsilyl) -1,2-disilacyclobutane (8), whereas 6c in a very unusual cyclodimerization step affords (E)-1,2,3,8a-tetrahydro-1-mesityl-5,7,8a-trimethyl-2,2,3,3-tetrakis(trimeth-ylsilyl)-2,3-disilanaphthalene (9). Compound 9 is the result of an unexpected [2 + 4] reaction, in which the silene formally acts as the monoene and - involving the aromatic substituent - simultaneously also as the diene. The reaction of 1a-c with methyllithium in THF at low temperature initiates 1,3-Si,O-trimethylsilyl migrations leading to (trimethylsiloxy)[bis (trimethylsilyl) silyl] alkanes (Me(3)Si)(2)SiH-CR(1)R(2)OSiMe(3) 3a-c. Reaction of 1a-c with an excess of methyllithium, tert-butyllithium, or phenyllithium, leads to trisilanes (Me(3)Si)(2)-SiR(3)-CHR(1)R(2) 11a-e, formed by the addition of the organolithium reagent to the Si=C bond of the transient silenes 6a-c. Deprotonation of 1b and 1c in the presence of 2,3-dimethyl-1,3-butadiene gives the [2 + 4] cycloadducts 6-tert-butyl-3,4-dimethyl- 1,1-bis(trimethylsilyl)-1-sila-3-cyclohexene (12a) and 6-mesityl-3,4-dimethyl-1,1-bis(trimethylsi lyl)-1-sila-3-cyclohexene (12b), For 8 and 9 the results of the X-ray analyses are given.