Ethers, relative cleavage rates

Relative Cleavage Rates for Selected Ethers of a Primary Alcohol... 

Relative Cleavage Rates of Various Ethers of a Secondary Alcohol... 

The Ic2 values (M s ) of the solvolysis of various RsSiOPh in aqueous ethanol at 25 °C were determined under acidic and basic conditions . The size of the silyl groups influences the rate and follows the order listed in entries 55 and 56 of Table 1. The correlation between the Si—O—C angle and the oxygen basicities may be completely different, or even reversed, in comparison with trimethylsilyl and rert-butyldimethylsilyl ethers. The relative hydrolysis rates may be simply controlled by steric hindrance to solvent assistance for the Si—O bond cleavage following the protonation step. 

Several methoxy-substituted benzyl ethers have been prepared and used as protective groups. Their utility lies in the fact that they are more readily cleaved oxidatively than the unsubStituted benzyl ethers. The table below gives the relative rates of cleavage with dichlorodicyanoquinone (DDQ). ... 

The rate of cleavage is inversely proportional to the bulk of the silyl ether and so trimethylsilyl groups are removed relatively easily whilst tert-butyldimethylsilyl groups are more resistant to removal. 

These isomerizations, rearrangements, and cleavages are best explained by a carbonium-ion mechanism. Vapor-phase dehydration of alcohols over aluminum oxide greatly reduces the tendency for isomerization and rearrangement. The alcohol vapors are passed over the catalyst at 300-420°. In this manner, pure 1-butene is prepared from re-butyl alcohol and t-butylethylene is obtained from methyl-/-butylcatbinol (54%). The relative rates of dehydration of the simpler alcohols over alumina have been studied. The main side reaction is dehydration to ethers (method 118). 

Relative Rates for Substituted Benzyl Ether Cleavage... 

The relative rates of benzyl either cleavage using these conditions is as follows PMB>Bn (85%) DMBN>Bn (95% ) DOB > Bn (98% ) DOB>DMBN (85%). The reagent also does not cleave A-benzylamines. Benzyl groups are readily cleaved by hydrogenolysis in the presence of a DOB ether (Pd/C, EtOAc, hexane, 12 h, rt, H2(l atm), >95% yield." ... 

Boraadamantane ate-complexes (e.g., (126)) are readily formed when treating (5) (or its THF complex (5a)) with lithium alkyls or alkali metal alcoholates (Scheme 49). The process can conveniently be monitored by the value of the B chemical shift, because a strong high-field shift is characteristic for the ate-complexes (to — 20 ppm in the case of RLi, and to — 3 ppm in the case of ROMe). Protolysis of the ate-complexes with HCl in ether proceeds under very mild conditions to give a borabicyclic compound, THF complex (5a) and methane (Scheme 49). The ratio of the products attests the relative rates of cleavage of the B—and B—Me bonds <83JOM(246)l29>. 

Selective cleavage of silyl ethers. - T CH2CI2 or CH3CN at 25" in rates that are The relative rates are in the order Et-.Si 5 phenols are stable to Sip4 in CH CI at 23 selective cleavage of primary or secondary... 

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