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 (Me₃Si)₃SiC(OH)Me₂ (1a), (Me₃Si)₃CH(OH)tBu (1b), and (Me₃Si)₃SiC(OH)Mes (1c), resp. After deprotonation with methyllithium in ether at ‐78°C 1a‐c eliminate trimethylsilanolate according to a modified Peterson mechanism to form transient silenes (Me₃Si)₂Si=CR¹R² (6a: R¹ = R² = Me; 6b: R¹ = H, R² = tBu; 6c: R¹ = H, R² = 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(trimethylsilyl)disilane (7) and 6b giving the head‐to‐head cyclodimerization product (E)‐3,4‐di‐tert‐butyl‐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(trimethylsilyl)‐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 (Me₃Si)₂SiH–CR¹R²OSiMe₃ 3a‐c. Reaction of 1a‐c with an excess of methyllithium, tert‐butyllithium, or phenyllithium, leads to trisilanes (Me₃Si)₂‐SiR³–CHR¹R² 11a‐e, formed by the addition of the organolithium reagent to the Si=C bond of the transient silences 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(trimethylsilyl)‐l‐sila‐3‐cyclohexene (12b). For 8 and 9 the results of the X‐ray analyses are given.