Carbonyl compounds, reductive etherification

Oriyama and coworkers reported an iron-catalyzed reductive etherification of carbonyl compounds with triethylsilane and alkoxytriaUcylsilane [149, 150] and alcohols (Scheme 48) [151]. 

Scheme 48 Iron-catalyzed reductive etherification of carbonyl compounds...

Although some methods for reductive etherifications of carbonyl compounds have been reported [152-162], the iron-catalyzed version possesses several advantages (1) fairly short reaction times are needed, (2) not only trimethylsilyl ether but also triethylsilyl and butyldimethylsilyl ethers and alcohols are adaptable, and (3) a broad substrate scope. 

Triethylsilane can also facilitate the high yielding reductive formation of dialkyl ethers from carbonyls and silyl ethers. For example, the combination of 4-bromobenzaldehyde, trimethylsi-lyl protected benzyl alcohol, and EtsSiH in the presence of catalytic amounts of FeCls will result in the reduction and benzylation of the carbonyl group (eq 32). Similarly, Cu(OTf)2 has been shown to aid EtsSiH in the reductive etherification of variety of carbonyl compounds with w-octyl trimethylsilyl ether to give the alkyl ethers in moderate to good yields. Likewise, TMSOTf catalyzes the conversion of tetrahydrop)ranyl ethers to benzyl ethers with Ets SiH and benzaldehyde, and diphenylmethyl ethers with EtsSiH and diphenylmethyl formate. Symmetrical and unsymmetrical ethers are afforded in good yield from carbonyl compounds with silyl ethers (or alcohols) and EtsSiH catalyzed by bismuth trihalide salts. An intramolecular version of this procedure has been nicely applied to the construction of cA-2,6-di- and trisubstituted tetrahydropyrans. ... 

Sol 9. (c) Sodium borohydride in the presence of CeCls selectively reduces a,P-unsaturated carbonyl compounds into allylic alcohols. It is called Luche s reduction. The selective 1,2-addition is facilitated by the strongly oxophilic Ce " " ion, which coordinates with the carbonyl oxygen. The allylic alcohol I then undergoes mercury catalyzed fran -etherification to give allyl vinyl ether (Claisen substrate), which readily undergoes [3,3] shift on heating to give II. 

Another example where a bismuth tribromide salt was found efficient is in the synthesis of the diastereomers of (6-methyltetrahydropyran-2-yl) acetic acid. Cycliza-tions of two different 5-silyloxy carbonyl compounds under either BiBrs-initiated reductive etherification (leading to the cis isomer) or tandem cyclization-addition (trans-isomer) etherification protocols from the corresponding P-keto ester or aldehyde respectively, has efficiently provide the target molecules [47]. 

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