|M.Sc Student||Maksymenko Shimon|
|Subject||Reactions of Silylithium Reagants with Ketones|
|Department||Department of Chemistry||Supervisors||? 18? Yitzhak Apeloig|
|Dr. Dmitry Bravo-Zhivotovs|
|Full Thesis text|
The sila-olefination reaction, a reaction between a ketone and a silyl lithium, is useful for the synthesis of silenes (R2Si=CR’2) with two silyl substituents on the Si-terminus. To better understand the sila-olefination reaction we studied: (a) the effect of the substituents on the carbonyl group; (b) the effect of the substituents on the silyl anion; (c) the effect of the solvating molecules and of the reaction solvent.
The reaction of (Me3Si)3SiLi•3THF with 2-adamantanone, which leads to formation of a transient silene, (Me3Si)2Si=(2-Ad), proceeds via formation of the alcoholate (Me3Si)3Si-(2-Ad)-(OLi), which could not be isolated. However, this alcoholate can be prepared in hexane from (Me3Si)3Si-(2-Ad)-(OH) and sila-olefination occurs by adding three equivalents of THF. We suggest that the THF molecules dissociate the alcoholate aggregates, triggering elimination of Me3SiOLi from (Me3Si)3Si-(2-Ad)-(OLi).
The sterically hindered aliphatic ketone, tBu(CO)Ad, and the less hindered 2-Ad=O react in hexane with tBuMe2SiLi yielding the corresponding alcoholates. On the other hand, in THF tBu(CO)Ad yields a 1,2-Brook rearrangement product, while 2-Ad=O yields the corresponding alcoholate. We propose that in the reaction of tBu(CO)Ad the alkoxysilane is formed via a “reverse” addition of the silyl moiety to oxygen. We suggest that in the first step the lithiosilane reduces the ketone, forming a ketyl anion-radical and a silyl radical, and then the radical pair undergoes coupling forming the alkoxysilane.
Solvating THF molecules on the silyllithium are not required in the sila-olefination reaction of (tBuMe2Si)3SiLi with 2-adamantanone unlike in the sila-olefination with the smaller (Me3Si)3SiLi. Based on DFT B3LYP-D3/TZVP calculations we conclude that intramolecular strain in the intermediate alcoholate (tBuMe2Si)3Si-(OLi)-(2-Ad) provides the driving force for elimination of tBuMe2SiOLi. Using this knowledge we synthesized the first H-substituted silene H(tBu2MeSi)Si=(2-Ad), which however dimerizes at room temperature to the corresponding 1,2-disilacyclobutane.
Solvation of (tBuMe2Si)3SiLi by 12-crown-4 leads to formation of dissociated ion-pairs. This dissociated lithiosilane reacts with 2-adamantanone to yield products different from those observed in the reactions of 2-adamantanone with (tBuMe2Si)3SiLi•3THF or non-solvated (tBuMe2Si)3SiLi. The first product after hydrolysis is (tBuMe2Si)3SiH, which is probably formed from radical (tBuMe2Si)3Si•. We observed by EPR spectroscopy the spectrum of the 2-adamantyl-centered radical R-Si(SiR3)2-(2-Ad)•, providing indirect evidence for the formation of (tBuMe2Si)3Si• and the of 2-adamantanone ketyl anion-radical. The second product (tBuMe2SiO)(tBuMe2Si)2Si-(2-Ad)H is formed via addition of tBuMe2OSiLi to silene (tBuMe2Si)2Si=(2-Ad). We propose that cleavage of tBuMe2OSiLi aggregates by 12-crown-4 leads to activation of the silanolate towards addition to the Si=C bond.