Silyllithium reagents 1,4-addition

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. 

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. 

Triphenylsilylpotassium was intensively studied by Gilman before he made the silyllithium reagent, and he showed that it reacted with a variety of electrophilic substances in addition to carbonyl compounds and epoxides, such as silyl halides, pyridine, azo- and azoxybenzene, thiocyanates, and benzonitrile, as well as with far less electrophilic substances like stilbene, anthracene, diphenyl sulfide, and diphenyl sulfone. ... 

Reactions of silylcuprates provide additional examples of a silyl anion based mechanism (Scheme 3). The reagent (19), prepared from silyllithium and copper(I) cyanide, reacts with a C=C bond to give, after aqueous work-up, c/j-hydrosilylated products (20). Conjugate addition of (21) to a,P-unsaturated... 

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