Silyllithium reagent

Reduction of the Si-Si bond of oligosilanes with organo- or silyllithium reagents (Scheme 2.44, A,... 

Silenolates 26, i.e. silicon analogues of enolates, are formed, as shown by Ishikawa and coworkers, when sterically congested tris(trimethylsilyl)acylsilanes 25 are treated with silyllithium compounds (Scheme 10)87a b c The silyllithium reagent does not add to the C=0 bond but exclusively cleaves a Si—Si bond, yielding the corresponding silenolate 26. 

Three different routes to the key compounds for the sila-Peterson elimination, the a-alkoxydisilanes 157, are described in the literature, namely A, reaction of silyllithium reagents with ketones or aldehydes B, addition of carbon nucleophiles to acylsilanes C, deprotonation of the polysilylcarbinols. In addition, method D, which already starts with the reaction of 2-siloxysilenes with organometallic reagents, leads to the same products. The silenes of the Apeloig-Ishikawa-Oehme type synthesized so far are summarized in Table 4. 

Interestingly, silyllithium reagents substituted with aromatic electron-releasing groups (34f) and 34g) gave 35 (1,2-adduct), 36 (1,16- and 1,29-adduct), 37 (1,4-adduct) and 38 (dimer at 1,4-position) which can be separated by gel permeation chromatography (GPC). The 13C-NMR spectrum of 37f displays 60 signals for all quaternary carbons. 

The reactivity difference of the silyllithium reagents raises some questions concerning the mechanism. To clarify the mechanistic pathway, it was examined by semiempirical calculations using hydrosilanes instead of silyllithium compounds (Table 5)25. As deduced from the HOMO energies of Table 5, electron-releasing substituents favor the formation of adducts 35 and 36, which indicate that their formation should involve a radical reaction proceeding via electron transfer from the silyllithium to Cgo. 

Reductive Silylation of Benzamides Using Silyllithium Reagent... 

V,/V-Dialkyl-RSMAs have been prepared by addition of silyllithium reagents to iminium salts formed in situ by reacting aldehydes with silylamines in the presence of lithium perchlorate. Although excellent yields have been obtained, except when starting from enolizable aldehydes, the use of the dangerous reagent lithium perchlorate makes this procedure less attractive.197... 

An alternative disconnection of the alkoxide requires the addition of a silyllithium reagent to an enone. Addition of stoichiometric base to the alcohol 51 produces an alkoxide 52, but no evidence of Brook rearrangement to generate 53 was found on protonation of the product. However, alkoxide 52 must exist in equilibrium with some of the organolithium 53, since alkylation with a soft electrophile (Mel) produced 54.41 The equilibrium concentration of the organolithium 53 is lessened in this case by the impossibility of O-Li coordination. 

A different approach that even obviates the use of a preformed silyllithium reagent takes advantage of the cleavage of the Si-Si bond of a disilane by a copper salt. Hosomi and co-workers185 have reported on the reaction of various enones or enals 250 with hexamethyldisilane or l,l,2,2-tetramethyl-l,2-diphenyldisilane, catalyzed by copper(i) triflate-benzene complex (Scheme 61). The transformation requires heating to 80-100 °C in DMF or DMI and the presence of tri-/z-butylphosphine in order to stabilize the copper catalyst under these harsh conditions. The addition products 251 were obtained with high yield after acidic work-up. The application of the method to alkylidene malonates as the Michael acceptor was recently disclosed.1... 

Reaction of an excess of morpholine with C6o in the presence of oxygen afforded the dimeric 157/( )-158 as main product precipitating from the benzene solution.316 Dimeric 1,4-adducts were finally observed besides the monomeric species among the products isolated after addition of certain silyllithium reagents to C6o and subsequent quenching with ethanol (cf. Section IV.A.l.h).195... 

Conjugate additions. Enals undergo selective allylation with diallylcerium chloride in the presence of ATPH. Regioselective Robinson annulations can be initiated by the conjugated addition of lithium enolates to enones. - Remarkably, silyllithium reagents add to the p-position of aromatic carbonyl compounds in the presence of ATPH. ... 

The reactivity of silylcuprates with oxabicyclic compounds was also examined [218, 223]. With oxabicyclo[2.2.1] compounds, addition and Peterson elimination to produce cyclohexadienes occurred as with silyllithium reagents, Eq. 200. 

The formation of an asymmetric silyllithium reagent by lithium cleavage of the silicon-silicon bond of an optically active disilane 10 (eq. [7]) has been reported (26). Hydrolysis of the silyllithium reagent 11 yielded an optically active silicon hydride. This result demonstrates that silyllithium has considerably enhanced optical stability relative to acyclic alkyllithium compounds. 

A controlled opening of a,(3-epoxy ketones involves reaction with Ph(Me)2SiLi and mild hydrolysis. Attack of the silyllithium reagent on the ketone groups is followed by a Brook rearrangement and p-elimination. " ... 

The formation of an asymmetric silyllithium reagent from an optically active disilane has been reported92 (equation 25). 

The observed stereoselective lithium cleavage of a silicon-silicon bond and stereoselective protonolysis demonstrate the configurational stability of the silyllithium reagent. 

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