Halides silylation

The alcohols used may also contain fairly sensitive functional groups, I. l.. esters, halides, silyl ethers, etc. In this work, therefore, tert-butyl toth ioacetate is behaving as a synthetic equivalent to diketene. When this... 

The lithio derivatives of dibenzofuran undergo the expected reactions. Carbonation yields carboxylic acids,reaction with sulfur dioxide yields sulfinic acids, and reaction with methyl sulfate yields methyl compounds. Alkylation can also be achieved by treatment of 4-lithio-dibenzofuran with alkyl halides. Silylation can be achieved with chlorotrimethyl- or chlorotriphenylsilane. " " Reaction of lithiodibenzo-furans with acetaldehyde and with benzonitrile take the expected course. 

Enamines are among the most powerful neutral nucleophiles and react spontaneously with alkyl halides. Silyl enol ethers are less reactive and so require a more potent electrophile to initiate reaction. Carbocations will do, and they can be generated in situ by abstraction of a halide or other leaving group from a saturated carbon centre by a Lewis acid. 

Organozinc addition reactions to unsaturated systems lead to an intermediate zinc compound which is in most cases further hydrolyzed to give the corresponding protonated organic compound. This intermediate can also be trapped by reaction with electrophiles (see Electrophile) (E+) such as aldehydes, acyl chlorides, organic halides, silyl halides, and so on. Depending on the nature of the electrophile (see... 

Further typical reactions of imidoylstannanes are shown in Scheme 6-5.71 Butyl-lithium reacts to form the imidoyllithium, which reacts with electrophiles such as alkyl halides, silyl chlorides, aldehydes, or chloroformates,75 and the Stille reaction with an acyl halide gives an a-carbonylimide which can be hydrolysed to an a-dicarbonyl compound.75... 

The synthesis of compounds 55 started from //-phosphinic acid 49b. The formation of the second P-C bond was achieved by in situ generation of the phosphonite species (TMSCl or BSA) followed by a conjugate addition of the appropriate Michael acceptor, or alkyl halides (silyl-Arbuzov reaction), or... 

Although the reduction potentials of aUcenes are too high to permit reduction, it is possible to reduce polycyclic aromatic hydrocarbons and alkenes bearing groups that can stabilize anions in aprotic media to afford the corresponding anion radicals and dianions. These species are good nucleophiles and can be captured by added electrophiles such as protons, alkyl halides, silyl halides, and carbon dioxide (Fig. 7) [11],... 

Alkyl halides and sulfonates are the most frequently used alkylating acceptor synthons. The carbonyl group is used as the classical a -synthon. O-Silylated hemithioacetals (T.H. Chan, 1976) and fomic acid orthoesters are examples for less common a -synthons. In most synthetic reactions carbon atoms with a partial positive charge (= positively polarized carbon) are involved. More reactive, "free carbocations as occurring in Friedel-Crafts type alkylations and acylations are of comparably limited synthetic value, because they tend to react non-selectively. 

An Q-arylalkanoate is prepared by the reaction of aryl halide or triflate with the ketene silyl acetal 74 as an alkene component. However, the reaction is explained by transmetallation of Ph - Pd—Br with 74 to generate the Pd eno-late 75, which gives the a-arylalkanoate by reductive elimination[76]. 

The reaction of a halide with 2-butene-1,4-diol (104) affords the aldehyde 105, which is converted into the 4-substituted 2-hydroxytetrahydrofuran 106, and oxidized to the 3-aryl-7-butyrolactone 107[94], Asymmetric arylation of the cyclic acetal 108 with phenyl triflate[95] using Pd-BINAP afforded 109, which was converted into the 3-phenyllactone 110 in 72% ee[96]. Addition of a molecular sieve (MS3A) shows a favorable effect on this arylation. The reaction of the 3-siloxycyclopentene 111 with an alkenyl iodide affords the. silyl... 

Conversion to a more facile, sulfur-derived, leaving group can be achieved by treatment with sodium thiosulfate or salts of thio and dithio acids (75,87). Under anhydrous conditions, boron tribromide converts the 3 -acetoxy group to a bromide whereas trimethyl silyl iodide gives good yields of the 3 -iodide (87,171,172). These 3 -halides are much more reactive, even when the carboxyl group is esterified, and can be displaced readily by cyano and by oxygen nucleophiles (127). 

A related tert-butylation procedure in which the silyl enol ether is added to a mixture of titanium tetrachloride and tert-butyl chloride gives rise to distinctly lower yields. This is also the case if the tertiary halide is added to a mixture of silyl enol ether and titanium tetrachloride. ... 

Volatile silylamides are readily prepared by reacting a silyl halide with NH3, RNH2 or R2NH in the vapour phase or in Et20, e.g. ... 

Variations and Improvements on Alkylations of Chiral OxazoUnes Metalated chiral oxazolines can be trapped with a variety of different electrophiles including alkyl halides, aldehydes,and epoxides to afford useful products. For example, treatment of oxazoline 20 with -BuLi followed by addition of ethylene oxide and chlorotrimethylsilane yields silyl ether 21. A second metalation/alkylation followed by acidic hydrolysis provides chiral lactone 22 in 54% yield and 86% ee. A similar... 

Symmetric allylsilanes and unsymmetric allylsilanes, in which the silyl substituent is at the less substituted end of the allyl fragment, are available from allyl halides and trimethylsilylmetal reagents13. 2-Chloro-l-cyclohexenes react with inversion of configuration and a 1,3-shift, with better results in the presence of coppcr(l) iodide14. 

Reactions of Silyl Anions with Metal Halides... 

An interesting variant of metal-silicon bond formation is the combination of metal halides with silyl anions. Since silyl dianions are not available, only one metal-silicon bond can be formed directly. The silylene complexes are then accessible by subsequent reaction steps [113], An example of this approach is given by the reaction of cis-bistriethylphosphaneplatinumdichloride 25 with diphenylsilylli-thium, which yields, however, only dimeric platinadisilacyclosilanes 26a-c [114]. 

A number of approaches have been tried for modified halo-de-diazoniations using l-aryl-3,3-dialkyltriazenes, which form diazonium ions in an acid-catalyzed hydrolysis (see Sec. 13.4). Treatment of such triazenes with trimethylsilyl halides in acetonitrile at 60 °C resulted in the rapid evolution of nitrogen and in the formation of aryl halides (Ku and Barrio, 1981) without an electron transfer reagent or another catalyst. Yields with silyl bromide and with silyl iodide were 60-95%. The authors explain the reaction as shown in (Scheme 10-30). The formation of the intermediate is indicated by higher yields if electron-withdrawing substituents (X = CN, COCH3) are present. In the opinion of the present author, it is likely that the dissociation of this intermediate is not a concerted reaction, but that the dissociation of the A-aryl bond to form an aryl cation is followed by the addition of the halide. The reaction is therefore mechanistically not related to the homolytic halo-de-diazoniations. 

An extensive study (6) of Sn displacement reactions of allyl halides using silyl anions/anionoids has provided the following regioselective alternatives ... 

To a solution of the silylated glycinate (50 mmol) in ether (50 ml), cooled to —10 to 0°C, was added a solution of sodium hexamethyldisilazide (55 mmol) in ether (100ml) with stirring. Stirring was continued at ambient temperature for a short time, and then the alkyl halide (50 mmol) was added dropwise. The mixture was heated under reflux for 10-15 h, cooled, filtered, and the product was distilled directly (52-70%). 

Kim and coworkers introduced silyl radical mediated addition of alkyl radical to silyloxy enamine 76. The silyloxy enamine moiety is readily accessible from a variety of functionalities. The mechanistic concept is illustrated in the Scheme 12 and involves the addition of R radical to 76 to give the radical adduct 77 and the subsequent homolytic cleavage of N-O bond to yield the desired product 78 and a silyloxy radical 79. The latter undergoes 1,2-phenyl migration to give the silyl radical 80 that abstracts halogen from the alkyl halide to regenerate the R radical. 

Other indirect methods are treatment of the halide with silyl-thiols and KH, followed by treatment with fluoride ion and water, and hydrolysis of Bunte salts (see 10-41) is another method. 

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