Synthesis of Transient Silenes by a Modified Peterson Reaction

Abstract

Abstract 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) t Bu ( 1b ), and (Me 3 Si) 3 SiC(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 (Me 3 Si) 2 Si=CR 1 R 2 ( 6a : R 1 = R 2 = Me; 6b : R 1 = H, R 2 = t Bu; 6c : R 1 = 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(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 (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 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.