Magnesiums silyl

Table 46 the Vcn couplings connected with carbons 3, 4, and 5 are given. In Table 47, the C NMR chemical shifts for the lithium, magnesium, silyl, and 7-butoxy derivatives (compounds 79, 80, 81, 82, and 88) are included. 

The combined solutions were dried over magnesium sulfate and subsequently concentrated in a water-pump vacuum. Distillation of the residue through a 30-cm Vigreux column gave the silylated propargyl sulfide, b.p. 65°C/18 mmHg, n ... 

By application of the most common procedure—i.e by using an a-silylated organolithium or magnesium reagent—the /3-hydroxysilane 5a/5b can be isolated. However in the case of M = Na or K, the alkoxide oxygen in 4a/4b is of strong ionic character, and a spontaneous elimination step follows to yield directly the alkene 3. 

To a mixture of vinyl bromide (40 mmol) and the catalyst dichloro-[(R)-Af,N-dimethyl-l-[(.S)-2-(diphenylphosphino)ferrocenyl]ethylamine]-palladium(n) (0.2 mmol) was added an ethereal solution of [a-(trimethyl-silyl)benzyl]magnesium bromide (0.6-1 m, 80 mmol) at —78 °C. The mixture was stirred at 30 °C for 4 days, and then cooled to 0 °C and hydrolysed with dilute aqueous HC1 (3 m). The organic layer was separated, and the aqueous layer was re-extracted with ether. The combined organic extracts were washed with saturated sodium hydrogen carbonate solution and water, and dried. Concentration and distillation gave the chiral allylsilane (79%, 66% ee), b.p. 55°C/0.4mmHg. 

Among the preformed enol derivatives used in this way have been enolates of magnesium, lithium, titanium, zirconium, and tin, ° silyl enol ethers, enol borinates,and enol borates, R CH=CR"—OB(OR)2. The nucleophilicity of silyl enol ethers has been examined. In general, metallic Z enolates give the syn (or erythro) pair, and this reaction is highly useful for the diastereoselective synthesis of these products. The ( ) isomers generally react nonstereoselectively. However, anti (or threo) stereoselectivity has been achieved in a number of cases, with titanium enolates, with magnesium enolates, with certain enol bor-inates, and with lithium enolates at — 78°C. ... 

This must reflect activation of the carbonyl group by magnesium ion, since ketones are less reactive to pure dialkylzinc reagents and tend to react by reduction rather than addition.141 The addition of alkylzinc reagents is also promoted by trimethylsilyl chloride, which leads to isolation of silyl ethers of the alcohol products.142... 

Selective silylation of polychloromethanes using reactive metal electrodes such as zinc and magnesium has also been reported as shown in Scheme 37 [76, 77]. The electroreduction of carbon tetrachloride and chloroform in the presence of chlorotrimethylsilane affords the monosilylated and disilylated products. The product selectivity seems to depend upon the electrode material. 

The process is initiated by a double Grignard reaction of 4-pentynoic acid 153, first with 3-butenyl magnesium bromide and subsequently magnesium acetylide followed by silylation of the formed ter-tiaiy hydroxyl function. The cobalt induced polycy-clization leads directly to the fenestrane 157 interestingly, the reaction halts at the stage of 156 when employing the unprotected alcohol. 

The addition of carbonyl compounds towards lithiated 1-siloxy-substituted allenes does not proceed in the manner described above for alkoxyallenes. Tius and co-work-ers found that treatment of 1-siloxy-substituted allene 67 with tert-butyllithium and subsequent addition of aldehydes or ketones led to the formation of ,/i-unsaturated acyl silanes 70 (Scheme 8.19) [66]. This simple and convenient method starts with the usual lithiation of allene 67 at C-l but is followed by a migration of the silyl group from oxygen to C-l, thus forming the lithium enolate 69, which finally adds to the carbonyl species. Transmetalation of the lithiated intermediate 69 to the corresponding zinc enolate provided better access to acylsilanes derived from enolizable aldehydes. For reactions of 69 with ketones, transmetalation to a magnesium species seems to afford optimal results. 

Similar yields can be obtained in this silylation by using the chloromagnesium salt (from butyl magnesium chloride) as described in Org. Synth. 1987, 6S, 61. 

Ab initio calculations also confirm that the use of an allyl magnesium alkoxide in place of the alcohol functionality will lead to high or complete stereoselectivity (138). When homoallylic alcohols are used, the Kanemasa protocol afforded the respective isoxazolines with poor stereoselectivity ( 55 45) in the case of terminal aUcenes, but with very high diastereoselectivity (up to 96 4) in the reaction of cis-1,2-disubstituted olefins (136). Extension of this concept to the reaction of a-silyl allyl alcohols also proved feasible and produced the syn (threo) adducts as nearly pure diastereomers (>94 6) (137). Thus, the normal stereoselectivity of the cycloaddition to the Morita-Baylis-Hillman adducts (anti > syn, see above) can be reversed by prior addition of a Grignard reagent (176,177). Both this reversal... 

The trimethylsilylation of the di-Grignard derivative of acetylene has to be carried out at temperatures that are considerably higher than those necessary for the silylation of ethynyl mono-magnesium bromide. This decreased reactivity may be ascribed to the slight solubility of the intermediate. 

Moreover, in the divided cell the exo.endo ratio of bromosilanes was 91 9 in the anode compartment bnt only 52 48 in the cathode compartment. Thus, the nature of the ultrasonic effect was explained assuming that beside the electrochemical silylation at the cathode, a parallel silylation process occurs at a magnesium anode, namely the silylation by 70 of an intermediate Grignard reagent produced from dibromide 69. It appears as a rare example of the anodic reduction However, the increase in the current density dnring electrolysis cansed a decrease in the apparent current efficiency. This observation indicates a chemical natnre of the anodic process. Of course, the ultrasonic irradiation facihtates the formation of the organomagnesium intermediate at the sacrificial anode and the anthors reported a similar ultrasonic effect for the nonelectrochemical but purely sonochemical... 

To develop new electrophilic reagents, Ricci and coworkers have described the synthesis of trimethylsilyloxy and hydroxy compounds from magnesium enolates and bis(trimethyl-silyl)peroxide. Magnesium enolates, generated using magnesium diisopropylamide, (DA)2Mg, give the hydroxycarbonyl compounds in excellent yields (equation 72, Table 8). 

The additions of alkyl or aryl silyl chlorides to 1 -ethoxycarbonyl- 1H-azepine in HMPA solution in the presence of magnesium proceed via anion radicals to yield trans adducts... 

The air-sensitive violet complex thus formed was treated with methyl-magnesium bromide and then hydrolyzed to give /rans-Ph(Me3Si)C= CPh(SiMe3) (90, 91). Apparently, this is the first example of transfer of silyl groups from a metal to a coordinated hydrocarbon ligand. It seems that a similar step is involved in the dehydrogenative cis double silylation of acetylenes catalyzed by diethyl(bipyridyl)nickel(II) (156). 

After optical resolution by lipase, the silylated furanol with a trifluoromethyl group, was converted into the corresponding butenolide by oxidation with magnesium monoperoxyphthalate (MMPP). Ring opening of this butenolide... 

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