Complexes cerium borohydride

A possible explanation is based on the formation of cerium borohydride which would complex with oxygen, delivering the hydride from the ft side of the molecule (Figure 1), while NaBH4 would attack preferably from the less hindered a side. The resulting anions would be successively protonated by methanol preferably from the same side (/ in the former case, a in the latter case), to give products in which the two hydrogens are in cis relationship. 

The mechanistic study of the reaction (46) has eliminated the formation of a cerium borohydride. The cerium trichloride enhances the acidity of methanol (through complexation) and promotes its reaction with NaBH to (OCH ) - species. CeCl simultaneously activates the carbonyl... 

The regiochemistry of borohydride reduction of cyclic enediones such as (99) was markedly affected by addition of cerium(m) whedier a complexation site was accessible to the Lewis acid appears to be the main factor involved.159... 

Recent methods for the cleavage of allyl ethers that have that have yet to be tested on the anvil of complex target synthesis include (a) diborane generated in situ by reaction of sodium borohydride with iodine in THF at 0 °C (cyanoT ester, nitro, acetonide and tetrahydropyranyl groups survive) 434 (b) cerium(Ill) chloride and sodium iodide in refluxing acetonitrile (benzyl. THP and Boc groups survive) 435 (c) iodotrimethylsilane in acetonitrile at room temperature 436 and (d) DDO in wet dichloromethane (secondary allyl ethers, benzyl, acetate and TBS groups survive).437... 

The subsequent transformation of these cycloadducts further illustrates the versatility of this approach in the construction of guaianolide sesquiterpenes (Scheme 1)7 The moiety incorporated in the adduct allows for the introduction of diverse functionalities at C-S and C-2. For example, reduction of the cycloadduct (11) with sodium borohydride produces a good yield of the iron complex (16). Oxidation of this complex with cerium ammonium nitrate gives methyl ester (17). In addition, (11) reacts with nucleophiles such as lithium dimethylmalonate and methanol to give the alkylated complex (18) and the meth-oxylated complex (19), respectively. Oxidative demetallation of (11) with bromine leads to selective replacement of the moiety at C-2 by bromide. ... 

The application of a sodium borohydride/cerium(III) nitrate reagent to a pyrone reduction produced results that departed somewhat from the 1,2-additions observed by Luche. Catalytic reductions of y-pyrones can lead to complex mixtures of products via nonselective processes however, the lanthanide/borohydride system provided a good yield of the specifically reduced dihydropyrone, Q, from the unsaturated precursor, while preserving the carbonyl function adjacent to the double bond (Poulton and Cyr, 1980). 

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