Silanes silyl ketenes

Silyl group transfer can also be used to functionalize chain ends. For example, allyl silanes, silyl ketene acetals, and silyl enol ethers [301-304] generate polymers with terminal allyl and methacrylate groups [Eq. (103)]. This type of transfer becomes degradative (termination) if reinitiation with silyl halides is not possible. 

Energy loss spectroscopy, 30 305-306, 308 a, P-Enoates, Michael reaction of silyl ketene acetal, 38 275 Enol silanes... 

This reaction, also, has been performed diastereoselectively.461 Allylic silanes R.2C=CHCH2SiMe3 can be used instead of silyl enol ethers (the Sakurai reaction). 462 Similarly, silyl ketene acetals, e.g., 56, give 8-keto esters, in MeN02 as solvent, for example,463... 

Cu(II) and Sn(II) Bisoxazolinc Complexes. Evans has prepared and studied a family of Cu(II) complexes prepared from bisoxazoline ligands [8]. Utilizing these complexes a number of different addition reactions can be successfully conducted on pyruvate, benzyloxyacetalde-hyde, and glyoxylates. Whereas the focus of the work in the context of aldol addition reactions has been on the use of silyl ketene acetals (vide infra), the addition of ketone-derived enoxy silanes 8a-b with methyl pyruvate has been examined (Eq. 8B2.1). The additions of 8a-b proceed in the presence of 10 mol % Cu(II) catalyst at -78°C in CH2Cl2, affording adducts of acetophenone 9a and acetone 9b with 99% and 93% ee, respectively. 

The (dienyl)iron cations of type (248) and (265) are susceptible to reaction with nucleophiles. For the (cyclohexadienyl)iron cations, nucleophilic attack always occurs at a terminal carbon, on the face of the ligand opposite to the metal, to afford / -cyclohexadiene products. Typical nucleophiles used are malonate anions, amines, electron-rich aromatics, silyl ketene acetals, enamines, hydrides, and aUyl silanes intramolecular nucleophilic addition is also possible. The addition of highly basic organometaUic nucleophiles (Grignard reagents, organolithiums) is often problematic this may be overcome by replacing one of the iron carbonyl... 

The addition reaction of fert-butyl thioacetate-derived silyl ketene acetal produces the corresponding aldol adducts in 84% yield and up to 96% enantiomeric excess (Eq. 16). The enantioselectivity of the products was observed to be optimal with toluene as solvent the use of the more polar dichloromethane consistently produced adducts with 10-15% lower enantiomeric excess. The bulkier ferf-butylthioacetate-derived enol silane was found to lead to uniformly higher levels of enantioselectivity than the smaller S-ethyl thioketene acetal. This process is impressive in that it tolerates a wide range of aldehyde substrates for instance, the aldol addition reaction has been successfully conducted with aldehydes substituted with polar functionaUty such as N-Boc amides, chlorides, esters, and 0-benzyl ethers. A key feature of this system when compared to previously reported processes was the abiUty to achieve high levels of stereoselectivity at 0 °C, in contrast to other processes that commonly prescribe operating temperatures of -78 °C. 

The addition of propionate-derived enol silanes 140 deUvered 1,2-disubstitut-ed aldol adducts 141 and 142 in useful yields and selectivities (Eq. 17) [90]. As in the acetate-derived additions, the selectivity of the process was dependent on the thioalkyl substituent of the silyl ketene acetal 140. The 1,2-syn adduct was obtained from the addition of E-enolsilane and -butyl glyoxylate (Eq. 17, entry 3). Correspondingly, the formation of 1,2-anti adduct was observed in the addition of a-benzyloxy acetaldehyde and the Z-enol silane derived from the ferf-butyl thio ester. 

The use of these boryl complexes in catalytic, enantioselective additions to aldehydes by silyl ketene acetals has also been the subject of intense investigation by Yamamoto (Eq. 30) [108]. Although ethyl and benzyl acetate-derived enol silanes furnished racemic products, the phenyl acetate-derived trimethylsilyl ketene acetals proved optimal, giving adducts in up to 84% ee. Additionally, Yamamoto has documented the use of 184 in aldol addition reactions of propionate- and isobutyrate-derived enol silanes (Eqs. 31 and 32). Thus, the addition of the phenyl acetate derived (E)-enol silane afforded adducts as diastereomeric mixtures with the syn stereoisomer displaying up to 97% ee (Eq. 32). 

The TiCU-mediated reaction of enol silanes with imines was first introduced by Ojima and coworkers in 1977. The reaction was then extended to several similar substrates, i.e. nitrones, ot-methoxycarba-mates, aminals, 4-acetoxyazetidin-2-one, 40 anj to different Lewis acids, i.e. SnCU, TiCU-(0PH)2, catalytic ZnX2, catalytic TMSOTf, ° to give good yields of the addition products with low levels ( 80 20) or a complete lack of simple stereoselection. Moderate to good anti selectivities were reported in the addition of silyl ketene acetals to imines under particular reaction conditions (equation 9) significant results are summarized in Table 4. 

Chiral silyl ketene acetals (Il)-(20) were recently introduced for diastereoselective aldol-type additions. Camphor derivatives (11)-(16) are conformationally rigid with one diastereotopic face of the enol silane sterically shielded. - A -Methylephedrine derivatives (17)-(20) are likely to bind to TiCU through the NMe2 group with consequent dramatic conformational constraint.As a result the Lewis acid mediated additions to C=X occur in a highly stereoselective way. The chiral auxiliaries can then be removed (and recycled) by reduction, saponification or displacement with various nucleophiles to give useful synthetic intermediates. 

Enol silanes (181), as reported by Pilli and Russowsky, add to arylimines (182) in the presence of catalytic amounts of TMS-OTf to afford /V-aryl-p-amino ketones (183 equation 20). The reaction is carried out by adding the enol silane to the imine in the presence of IS mol % of TMS-OTf at 0 C in CH2CI2, followed by aqueous work-up. Yields range from 50% to 98% in a series of benzylideneaniline derivatives using the enol silanes derived from acetophenone and t-butyl methyl ketone. The extent to which the reaction may be limited to nonenolizable imines, as in the analogous TMS-OTf-promoted reactions of silyl ketene acetals (see Section is not repotted. The authors describe some... 

When R3 is replaced by a smaller group, and R2 is large (TMS, Bu1), then both Ai and S.i (gauche interaction between R and R2) are disfavored compared to Si, and the syn isomers are obtained (Table 1 entries 14-17). The same analysis applies to several other cases ketene bis(trimethylsilyl) acetals (R3 = OTMS) are anti selective when R2 is Me (anti 86-89%) and syn selective when R2 is Bu1 (syn 70-89%) 24 silyl ketene acetal (3) derived from butyrolactone is anti (R Ji ) selective when R = H (anti 70%) and syn (R, S ) selective when R = TMS (single isomer, equation 2) 25a c S-trimethylsilyl S,N-acetals (4) are anti selective when R = Me (anti 60-87%) and syn selective when R = Pr (syn 60%, equation 3).22 Cyclic enol silanes usually show poor selectivity,2 1026 apart from isolated cases where good anti. syn ratios were obtained by carefully choosing reagents and Lewis acids. Fair anti preferences were observed with the cyclopentenone-derived silyl enol ether and TiCU (equation 4 R = Pr1, Bn anti (R Ji ) syn(R, S ) >90 10)27 and with 2-trimethylsilyloxyfuran (5 equation 5 anti (R, R ) syn(R, S ) 76—88 24—12).17-27... 

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