Ethers, benzylic hydrogenation

Under certain conditions surface catalytic deuterations can lead to the exchange of benzylic hydrogens. An example in the steroid field is the exchange of the benzylic hydrogens in estrone methyl ether (42) with deuterium in the presence of palladized charcoal." " According to mass spectrometric analysis, the product (43) contains three deuteriums (83 %), which have been assigned to the 6- and 9a-positions on the basis of NMR evidence." " ... 

The proposed mechanism for the photochemical cleavage of nBn ethers (see Scheme 3) involves a n tt transition of the nitrogroup and an intramolecular, benzylic hydrogen abstraction by the excited nitro group. Rearrangement leads to a hemiacetal that decomposes to a free alcohol and to 2-nitrosobenzaldehyde that undergoes further thermal and photochemical reactions. 

A number of substrates having a benzylic ether moiety were reacted with 51 to afford the corresponding benzylic esters in good yields (equation 84). For evaluating the effects of p-substiments on the oxidation of a series of benzylic ethers, a competitive oxidation of p-substimted benzylic propyl ethers with 51 was carried out. The Hammett correlation plot for the oxidation reaction gave a better correlation of the relative ratio factors with the a rather than with the a+ substituent constants and afforded a reaction constant p+ = —0.57 (r = 0.99). This p+ value shows that 51 is an electrophilic species and appears to be comparable to the p+ value of —0.65 for benzylic hydrogen abstraction from dibenzyl ethers by the benzoyloxy radicaP . 

In the complexes of w/Ao-substituted benzyl ethers, the benzylic hydrogen atoms are diastereotopic. Alkylation of the derived lithium compounds occurs completely stereoselective-ly anti to the tricarbonylchromium face from a rotamer in which the benzylic methoxy group is anti to the or/Ao-substituent2,3. The stereochemistry of the alkylation was confirmed unambiguously by X-ray analysis of the product3. 

Cleavage of benzyl ethers Benzyl ethers are cleaved in high yield by catalytic transfer hydrogenation with 20% palladium on carbon (1, 782)4 as catalyst and cyclohexcne as hydrogen donor. 

Mechanistically, the absence of enol ether in the reaction is of interest because it implied that the palladated syn addition product A (Figure 2) underwent an unusual trans elimination of palladium, possibly due to more acidic character of the benzylic hydrogen. Alternatively, such a result was the consequence of the intervention of palladocycle B as the intermediate from which the C-C bond formation occurred with the extrusion of Pd(0). 

Abnormal olefin arylation reactions which are of interest mechanistically and preparatively occur with some allylically substituted compounds. The ailylic esters and ethers appear normal and produce cinnamyl derivatives exclusively while ailylic alcohols and chlorides are abnormal. Ailylic alcohols and "arylpalladium acetates form 3-arylaldehydes from primary ailylic alcohols and 3-arylketones from secondary alcohols 3°). The mechanism of reaction apparently involves anti-Markovnikov addition of the palladium compound to the double bond followed by elimination of the hydrogen atom on the hydroxyl-bearing carbon rather than the benzylic hydrogen. This again would be elimination of the more electronegative hydrogen atom. 

Oxidation of benzyl ethers. Benzyl ethers are oxidized to benzoate esters in yields of 54-96% by ruthenium tetroxide using the Sharpless conditions (11, 462). This reaction provides a useful method for cleavage of benzyl ethers, which is usually effected by hydrogenation. 

As described in the previous section, substitution at the benzylic position always takes place together with nuclear substitution if the aromatic substrates possess replaceable benzylic hydrogens (equation 39). Benzylic alcohols, esters and ethers can all be oxidized at the benzylic position to yield the corresponding carbonyl compounds. ... 

A new cortisone derivative, 3,20-dioxo-lljS,17a,21-trihydroxypregna-l,4,7-triene (298), has been prepared from the intermediate (297) by a classical combination of chemical and microbiological reactions.The chemistry of 17,20-and 20,21-acetonides, epimeric at C(20), has been described. A deprotection of masked steroidal alcohols by hydride transfer has been reported.Several benzyl ethers and bismethylenedioxy-steroids have been prepared and treated with trityl fluoroborate. The results demonstrated that the benzyl hydrogen atoms are sufficiently basic to give a benzyloxonium ion, as shown in the conversion of the 17,20 20,21-bismethylenedioxy-corticoid (299) into its free analogue (302) (see Part II, Chapter 1, refs. 120 and 323). 

Hydrogenolysis of the C-0 bond is not observed when the benzene rings of benzyl alcohols and ethers are hydrogenated under these conditions, particularly when a small amount of acetic acid is added to the reaction mixture.4.5 Under these conditions the hydrogenation of the benzene ring of... 

Benzyl ethers are quite stable under both acidic and basic conditions and toward a wide variety of oxidizing and reducing reagents. Hence, they are frequently used in organic syntheses as protecting groups. It should be noted, however, that n-BuLi may deprotonate a benzylic hydrogen, especially in the presence of TMEDA (tetra-methylethylenediamine) or HMPA (hexamethylphosphoramide). 

Potassium Aluminum Sulfate 103-50-4 Benzyl Ether Ammonium Hydrogen Carbonate... 

Organic Reactions contains a review of the fission of benzyl ethers, including cyclic benzyl ethers, by hydrogenation.43 Ring fission of furans has been reviewed by Jones and Taylor.44 Burwell45 has written an excellent and detailed report on the cleavage of ethers, in which he discusses also the theoretical problems involved he concludes with some indications for the fission of sulfides, a subject that is treated also by Tarbell and Hamish.46... 

Bromomethylation is also possible, e.g.y the preparation of benzyl bromide in 86.5% yield from benzene, formaldehyde, and hydrogen bromide in acetic plus sulfuric acid.684,685 So is iodomethylation of aromatic compounds, by reaction of bis(chloromethyl) ether and hydrogen iodide in glacial acetic acid.686... 

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