Reduction of ethers

Because of the high stability of the triphenylmethyl carbocation, the reductive ether cleavage of trityl ethers with EtySiH/trimethylsilyl triflate (TMSOTf) is highly successful. This reaction even occurs in the presence of highly reactive sugar ketals, leaving the ketals intact (Eq. 126).269 

The combination of PMHS and Pd(PPh3)4 reduces allyl ethers to propene and alcohols.270 The best combination for the reductive cleavage of ethers appears to be Et3SiH/(C6F5)3B, which gives excellent yields of the alcohol (via the silyl ether) and alkane (Eq. 127).145 

Dialkyl ethers are reduced with the combination of Et3SiU/TFA, although the yields vary.144,271 tert-Butyl triphenylcyclopropenyl ether is reduced to the corresponding cyclopropene (Eq. 128),272 and a dibenzyl-like ferrocene-derived ether is reduced to the corresponding alkane (Eq. 129).179 

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Because the reduction potential of ether is usually more negative than that of halides, examples that belong to this category are rather rare. Generally, cathodic reduction of ethers is similar to that of alcohols, and nonactivated ethers are not reducible under the conditions of electroreduction. Activated ethers such as benzylic and allylic ethers are elec-trochemically reduced to a limited extent (Scheme 7) [1, 15, 16]. Reduction of epoxides is usually difficult however, electroreductive cleavage of activated epoxides to the corresponding alcohols is reported [17, 18]. The cleavage of the C—O bond of ethers is more easily accomplished in anodic oxidation than in cathodic reduction, which is stated in Chapter 6. 

Radical-anion complexes Scope of this review 91 Thermodynamic and kinetic methodologies Voltammetric methods 92 Homogeneous redox catalysis 94 Convolution analysis 98 Laser flash photolysis 102 Photoacoustic calorimetry 103 Thermochemical estimates 105 Fleduction of C—O and O O bonds 106 Reduction of ethers 107 Reduction of peroxides and endoperoxides Reduction of S—S and C—S bonds 136 Reduction of disulfides 137 Reduction of sulfides 150 Concluding remarks 157 Fleferences 160... 

The position of the heteroatom may also strongly affect the global reactivity of the molecule. Thus, ethers, esters, thioethers, and even amines, that are normally weakly reactive toward the electron, can afford cleavage at the condition to be in an activated position (benzylic or allylic) to the X group (a fast cleavage of the radical anion results in a large shift to less cathodic potentials when the global bielectronic step is considered). In a different way, the reduction of ethers and amines could be achieved only under the conditions of the Birch reaction (i.e. reduction by Li metal in amines or by a solvated electron). These conditions can be electrochemically... 

In the first case, the chelate-controUed reduction of ether 379 yields a large surplus of the erythro-diol while the very bulky silylether 380 precludes any chelation and mainly affords the threo-didl 382 [125]. 

The combination of hydrosilanes with silicon Lewis acids is valuable for reduction of ethers. For instance, trityl-protected alcohols and carbohydrates can be deprotected efficiently and selectively by the Et3SiOTf-catalyzed reduction with EtsSiH [154]. 

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