Cerium borohydrides

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... 

Alternatively, the hydroxy group can be converted to the tosyl ester168 or replaced by chlorine108,169 followed by a base-catalyzed elimination. 1-Benzoxepin-5-(4//)-one is reduced with cerium(III) chloride/sodium borohydride to the hydroxy derivative. After conversion to the p-toluenesulfonate, the double bond is formed in 2 upon treatment with potassium tert-pen-toxide.168... 

In the special case of a-alkyl-a -nitro-substituted imines, reduction with sodium borohydride/ cerium(III) chloride and elimination of the chiral auxiliary generated the nitroalkene without racemization22. 

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... 

Oxo-4//-pyrido[l,2-a]pyrimidine-7-carboxylates 500 were obtained when 7-iodo-4T/-pyrido[ 1,2-a]pyrimidines 499 reacted with carbon monoxide in the presence of bis(triphenylphosphine)palladium(II) chloride catalyst and triethylamine (Scheme 29) (94MI2). From 7-iodo derivatives 499, 7-vinyl derivatives 501 and 7-(l-ethoxyvinyl) derivatives 502 were prepared with vinyltributyltin in the presence of Pd(PPh3)4 and with (1-ethoxy-vinyl)tributyltin in the presence of bis(triphenylphosphine)palladium(II) chloride catalyst, respectively, in toluene at 80°C. 7-(l-Ethoxy vinyl)pyr-ido[l, 2-a]pyrimidin-4-ones 502 were hydrolyzed to yield 7-acetyl derivatives 503. The acetyl group of compounds 503 was reduced with sodium borohydride in the presence of cerium(III) chloride in ethanol to give 7-(1 -hydroxyethyl)pyridopyrimidin-4-ones 504. 

Methadone hydrochloride (600.0 mg,1.74 mmol) was dissolved in ethanol (10 ml) and the solution was stirred whilst sodium borohydride (3.47 mmol) was added portion-wise over a period of 5 min. When the addition was complete a spatula end of cerium (III) chloride heptahydrate was added. The resultant solution was allowed to stir at room temperature for 30 min then the ethanol was removed under reduced pressure. The residue was portioned between diethyl ether (40 ml) and water (40 ml). The aqueous layer was extracted with more diethyl ether (2 times 20 ml) and then the combined organics were washed with brine (40 ml) and dried (MgS04). The ether was removed under reduced pressure to leave 435.0 mg, (80%) of 6-dimethylamino-4,4-diphenyl-3-heptanol. 

To a stirred solution of 122 mg of ll,15-dideoxy-ll-acetoxy-16-methyl-15-oxo-18,19-tetradehydro-5,6,7-trinor- 4,8-inter-m-phenylene PGI2 in 10 ml of methanol was added 150 mg of cerium chloride heptahydrate, and then the solution was cooled in an ice bath, and 15 mg of sodium borohydride was added to the solution. After 10 min, to the mixture was added 2 ml of aqueous saturated solution of sodium hydrogen carbonate, and the mixture was further stirred for 10 min. 

Previously, LAAM has been prepared from rf-methadone (24) (Scheme 29.8) via catalytic reduction using hydrogen or, alternatively, using sodium and propanol.29 However, the most convenient method for the reduction was using sodium borohydride in the presence of cerium(III)... 

Under appropriate experimental conditions, hydrogen telluride, phenyl tellurol, and sodium hydrogen telluride perform the selective reduction of double bonds of a,/3-unsaturated carbonyl systems.89 90 This reaction with a,j3-unsaturated ketones complements the cerium trichloride/sodium borohydride method, which reduces the carbonyl group, keeping the carbon-carbon double bond intact92 (Scheme 18). 

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... 

Sodium bis(2-methoxyethoxy)aluminum) hydride. Sodium borohydride-Cerium trichloride. Sodium cyanoborohydride. Sodium dithionite. Sodium hydroxy-... 

Sodium aluminum chloride, 435 Sodium amalgam, 310, 416-417 Sodium amide-Sodium r-butoxide, 417-418 Sodium azide-Triphenylphosphine, 418 Sodium bis(2-methoxyethoxy)aluminum hydride, 166,418-420 Sodium borohydiidc, 323,420-421 Sodium borohydride-Alumina, 421 Sodium borohydride-Bis(2,4-pentane-dionato)copper(ll), 421 Sodium borohydride-Cerium(Ill) chloride,... 

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. ... 

Correct reagent selection allowed reduction of steroidal enone (74) to either diastereoisomeric allylic alcohol, uncontaminated by its isomer. Sodium borohydride/cerium chloride in methanol-THF gave the equatorial alcohol (73), while L-selectride produced the axial isomer (75) via equatorial attack (Scheme 12). Unexpected axial attack on diketone (76) to give equatorial alcohol (77 equation 19) led to the proposition that for hydride additions to decalones two 1,3-diaxial interactions override one peri interaction which in turn takes precedence over a single 1,3-diaxial interaction. ... 

In spite of substantial evidence, the tendency for sodium borohydride to reduce enones in a conjugate sense is often ignored, but the need for reduction to the corresponding allylic alcohols has led to the development of several new specific reagent combinations. The most widely accepted of these involves sodium borohydride in the presence of cerium chloride, which has been optimized to give excellent... 

As discussed above, the absolute stereochemistry of halenaquinone (+)-l and halenaquinol (+)-2 has been theoretically determined by the calculation of the CD spectra of naphthalene-diene derivatives by means of the tt-electron SCF-CI-DV MO method. 18 To apply the same method to these xestoquinone compounds, xesto-quinol dimethyl ether (+)-74 was converted to naphthalene-diene derivative 75 by reduction with sodium borohydride in the presence of cerium(IIl) chloride23 and methanol, followed by treatment with pyridinium p-toluenesulfonate and methanol (Scheme 16).20 The product obtained was a mixture of two stereoisomers of the methoxyl group at the 4-position, from which a single isomer 75 was isolated as... 

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