Hydrolysis silyl vinyl ethers

Kresge and Tobin48 also studied the hydrolysis of vinyl ethers and found a rate ratio of 130 between methyl vinyl ether and ethyl cA-trimethylsilylvinyl ether, corresponding to a stabilization of the /J-silyl carbocation of 2.9 kcal mol-1. In this case the small rate acceleration (compared to the cyclohexyl systems studied by Lambert) can be attributed to the unfavourable dihedral angle. The dihedral angle in the vinyl ether is 90° (24), and on protonation it drops to 60° (25), whereas maximum hyperconjugative interaction requires a dihedral angle of 0°. 

Kresge and Tobin80 investigated the /1-silicon effect on the hydrolysis of vinyl ethers (equation 29) and found a rate acceleration on the hydrolysis of 175 compared with 176, and hence a stabilizing effect of the /1-silyl group on the intermediate -ethoxy carbocation 177 compared with 178. The acceleration is small the rate factor (175) (176) of 129 is equivalent to a free energy of activation difference AAG of 2.9 kcalmol-1,... 

Aldol group transfer polymerization of ferf-butyldimethylsilyl vinyl ether [62] was initiated by pendant aldehyde functions incorporated along a poly(methyl methacrylate) (PMMA) backbone [63]. This backbone was a random copolymer prepared by group transfer polymerization of methyl methacrylate (MMA) and acetal protected 5-methacryloxy valeraldehyde. After deprotection of the aldehyde initiating group, polymerization proceeded by activation with zinc halide in THF at room temperature. The reaction led to a graft copolymer with PMMA backbone and poly(silyl vinyl ether) or, upon hydrolysis of the ferf-butyldimethylsilyl groups, poly(vinyl alcohol) branches. 

Addition of a silyl substituent into a-position of the a-(benzotriazol-l-yl)alkyl ether brings additional possibilities. Thus, lithiation of silyl ether 770 followed by treatment with an aldehyde or ketone gives unstable P-hydroxy-a-silyl-a-(benzotriazol-l-yl)alkyl ether 771 that spontaneously eliminates silanol to give vinyl ether 772 (Scheme 121). Treatment with ZnBr2 followed by hydrolysis with a diluted acid removes both the benzotriazolyl and the methyl groups to furnish carboxylic acid 773. In this way, in a simple manner, aldehydes and ketones are converted to one-carbon homologated carboxylic acid <1996S1425>. 

S-Substituted a-lithiated silyl enol ether 557 has been prepared by reductive lithia-tion of vinyl tellurides834 and sulfides835,836 with lithium 1 -(dimethylamino)naphthalenide (LDMAN). This intermediate 557 gave, after inverse Brook rearrangement, the enolate 558 and after hydrolysis the corresponding acylsilane (Scheme 151). 

Allenic silylated ethers are efficient precursors of H-a,/S-unsaturated aldehydes and acyl silanes (Scheme 34), " and unsaturated acyl derivatives of silicon, germanium, and tin have been prepared from 1-lithio-derivatives of conjugated vinyl ethers (Scheme 35). " Acid-catalysed hydrolysis of the metallovinyl ethers formed in the latter sequence leads to the acylmetallanes. However, 3-substituted vinyl ethers do not give exclusively the conjugated enones. 

C-C double bond cleavage of an enyne derivative using a metallocene derivative was reported by Takahashi and coworkers [34]. When an alkyne was treated with Cp2ZrEt2 and vinyl bromide or vinyl ether in this order after hydrolysis a 2,3-disubstituted diene derivative was obtained (Eqs. 22 and 23). Bis-silyl acetylene, aryl-substituted acetylene or cyclohexyl-substituted alkyne could be used as internal alkynes. 

Palladium-mediated addition of silyl stannane reagents to alkynyl ethers has been employed for the synthesis of aliphatic acyl silanes in very good yields via the intermediate a-alkoxy-/J-stannyl vinyl silanes (enol ethers of acyl silanes)82. In a second palladium-catalysed step, the vinyl stannane moiety could be coupled to suitable halides before hydrolysis to the acyl silanes with trifluoroacetic acid (Scheme 11). 

Dialkylvinylsilyl halides are readily prepared, for example, by hydrosilylation of acetylenes indeed, the most simple dimethyl- and diphenylvinylsilyl chlorides are now commercially available. Substitution of the halide with the appropriate hydroxydiene then generates the desired triene precursor. Vinyl silyl ethers are also much more stable than silyl acetals, especially towards hydrolysis nevertheless, a number of mild methods are still available for subsequent removal of the tether. 

If the substituents are not stable during acid hydrolysis or methanolysis, the complementary substitution pattern can be determined after permethylation, as already noted. Examples are organic (acetates) and inorganic esters (sulfates), and benzyl and silyl ethers. " Such an approach was applied as early as 1964 to vinyl starches."" In contrast to the direct determination of the substituent pattern, incomplete methylation and consequently erroneous evaluations cannot be recognized from the product pattern obtained in the indirect methods. If the average DS is known from an independent determination, it allows at least a certain control. Therefore, the absence of OH absorption should be carefully monitored by IR spectroscopy. 

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