Barium Manganate Oxidation in Organic Synthesis: Part—IV: Oxidation of Aldehydes to Carboxylic Acids

The oxidation of aldehydes to corresponding carboxylic acids by barium manganate under heterogeneous conditions is described. Aromatic aldehydes were smoothly converted to carboxylic acids while aliphatic aldehydes underwent polymerisation.     

Direct catalytic asymmetric alpha-amination of aldehydes

The first direct catalytic asymmetric alpha-amination of aldehydes is described herein. alpha-Unbranched aldehydes react in this novel proline-catalyzed reaction with dialkyl azodicarboxylates to give alpha-amino aldehydes in excellent yields and enantioselectivities.     

Fluorometric determination of aromatic aldehydes with 1,2-Diaminonaphthalene

A new fluorometric method for the determination of aldehydes is presented. 1,2-Diaminonaphthalene reacts with aldehyde in dilute sulphuric acid to give a compound which fluoresces intensely in alkaline medium. The fluorescences produced from aromatic aldehydes in this method are fairly characteristic of individual aldehydes and their intensities are generally higher than those of fluorescences from aliphatic aldehydes. The only interference is from 2-oxo acids. The method may be suitable for the determination of aldehyde in complex samples.     

Lewis base activation of lewis acids. Addition of silyl ketene acetals to aldehydes

The weak Lewis acid silicon tetrachloride can be activated by catalytic amounts of the chiral bisphosphoramide (R,R)-3 to form a highly reactive, chiral trichlorosilyl cation which is an extremely effective promoter of aldol addition reactions between aldehydes and silyl ketene acetals. The tert-butyldimethylsilyl ketene acetal of methyl acetate adds nearly instantaneously to aromatic and olefinic aldehydes as well as aliphatic aldehydes (albeit more slowly) with excellent enantioselectivity. The homologous tert-butyldimethylsilyl ketene acetal of tert-butyl propanoate adds with nearly exclusive anti diastereoselectivity to a similar range of aldehydes also with excellent enantioselectivity. The origin of the slower reaction rate with aliphatic aldehydes is revealed to be the formation of chlorosilyl ether adducts.     

Highly enantioselective phenylacetylene additions to both aliphatic and aromatic aldehydes

The readily available and inexpensive BINOL in combination with Ti(O(i)Pr)(4) is found to catalyze the reaction of an alkynylzinc reagent with various types of aldehydes including aliphatic aldehydes, aromatic aldehydes, and other alpha,beta-unsaturated aldehydes to generate chiral propargyl alcohols with 91-99% ee at room temperature. No previous chiral catalysts have exhibited such a broad scope of enantioselectivity with respect to the type of aldehydes for this reaction. [reaction: see text]     

Analytical applications of crown ethers: II. Oxidation and determination of aldehydes

The oxidation of aldehydes by dicyclohexyl-18-crown-6-potassium permangante complex is presented. Results obtained by this procedure showed that the presence of crown ether (DCC) and the catalyst play an important role in the oxidation of the aldehydes. The stoichiometry found corresponds to 2 μeq of permanganate per 1 μmol of aldehydes in all cases. Alcohols and ketones are not oxidized. Finally, a method for quantitative evaluation of aldehydes is proposed.     

Analytical applications of crown ethers : III. Extraction and determination of nitrophenol isomers

The oxidation of aldehydes by dicyclohexyl-18-crown-6-potassium permangante complex is presented. Results obtained by this procedure showed that the presence of crown ether (DCC) and the catalyst play an important role in the oxidation of the aldehydes. The stoichiometry found corresponds to 2 μeq of permanganate per 1 μmol of aldehydes in all cases. Alcohols and ketones are not oxidized. Finally, a method for quantitative evaluation of aldehydes is proposed.     

Stereochemistry of the allylation and crotylation reactions of alpha-methyl-beta-hydroxy aldehydes with allyl- and crotyltrifluorosilanes. Synthesis of anti,anti-dipropionate stereotriads and stereodivergent pathways for the reactions with 2,3-anti- and 2,3-syn-alpha-methyl-beta-hydroxy aldehydes

A new method for the stereoselective synthesis of the anti,anti-dipropionate stereotriad via the reaction of alpha-methyl-beta-hydroxy aldehydes with (Z)-crotyltrifluorosilane (24) is described. These reactions were designed to occur through bicyclic transition states (e.g., 31) in which the silane reagent is covalently bound to the beta-hydroxyl group of the aldehyde and the crotyl group is transferred intramolecularly. This methodology was used to synthesize the C(7)-C(16) segment (58) of zincophorin, which contains a synthetically challenging all-anti stereopentad unit. Surprisingly, 2,3-anti- and 2,3-syn-alpha-methyl-beta-hydroxy aldehydes react in a stereodivergent manner with 24: 2,3-anti-beta-hydroxy aldehydes give the targeted anti,anti-dipropionate adducts with high selectivity, but the reactions of 2,3-syn-beta-hydroxy aldehydes are poorly selective. The stereodivergent behavior of 2,3-syn- vs 2,3-anti-alpha-methyl-beta-hydroxy aldehydes is also exhibited in their reactions with the allyl- (68) and (E)-crotyltrifluorosilanes (27). Competition experiments performed with beta-hydroxy aldehydes 37a (anti) and the corresponding p-methoxybenzyl (PMB) ether 48, and between aldehyde 39 (syn) and the PMB ether 90, established that the 2,3-anti-beta-hydroxy aldehydes react predominantly through bicyclic transition states while the 2,3-syn aldehydes react predominantly through conventional Zimmerman-Traxler transition states. NMR studies established that both the 2,3-syn and the 2,3-anti aldehydes form stable, pentavalent silicate intermediates (98 and 100) with PhSiF(3), but chelated structures 99 and 101 could not be detected. The activation energies for the competing bicyclic and conventional Zimmerman-Traxler transition states were calculated by using semiemperical methods (MNDO/d). These calculations indicate that the stereodivergent behavior of the 2,3-syn-beta-hydroxy aldehydes and the 2,3-anti-beta-hydroxy aldehydes is due to differences in nonbonded interactions in the bicyclic transition states. Specifically, nonbonded interactions in the bicyclic transition states for the allylation/crotylation reactions of the 2,3-syn-beta-hydroxy aldehydes permits the traditional Zimmerman-Traxler transition states to be preferentially utilized.     

Direct organocatalytic asymmetric alpha-chlorination of aldehydes

The direct organocatalytic enantioselective alpha-chlorination of aldehydes has been developed. The reaction proceeds for a series of different aldehydes with NCS as the chlorine source using easily available catalysts such as l-proline amide and (2R,5R)-diphenylpyrrolidine. The alpha-chloro aldehydes are obtained in up to 99% yield and up to 95% ee. The synthetic utility of the enantioselective alpha-chlorination of aldehydes is demonstrated by transformation of the alpha-chloro aldehydes to the corresponding alpha-chloro alcohols (>90% yield) by standard reduction and further transformation to both a terminal epoxide and amino alcohol, both obtained without loss of optical purity. Oxidation of the alpha-chloro aldehydes followed by esterification gave optically active alpha-chloro esters without loss of optical purity. It is demonstrated that these optically active alpha-chloro esters can be converted into nonproteinogenic amino acids in overall high yields, maintaining the enantiomeric excess obtained in the catalytic enantioselective alpha-chlorination step.     

The oxidation of aldehydes to acids with calcium hypochlorite [Ca(OCl)2]

Calcium hypochlorite is an efficient reagent for the oxidation of aldehydes to the corresponding acids. Reactions are carried out at ambient temperature in aqueous acetonitrile-acetic acid solution. Aliphatic aldehydes and aromatic aldehydes with electron withdrawing groups afford good to excellent yields. Nuclear chlorination is the preferred reaction in aromatic aldehydes with electron donating groups.     

Organocatalytic SOMO reactions of copper(I)-acetylide and alkylindium compounds with aldehydes

The derivatisation of aldehydes in their α-position is an important facet of organic synthesis. Organocatalytic radical reactions afford α-functionalised aldehydes via a SOMO activation pathway. New organo-SOMO reactions of aldehydes with copper(I)-acetylide and alkylindium reagents are detailed. These reactions proceed well under the catalysis of chiral imidazolidinones. The corresponding functionalised aldehydes were obtained with acceptable yields, but with only low enantiomeric ratios.     

Automated HPLC-HRGC: A powerful method for essential oil analysis. Part III. Aliphatic and terpene aldehydes of orange oil

The application of a new method, computer-controlled on-line HPLC-HRGC, to the analysis of the components of sweet orange oil has enabled the determination of aldehydes without interferences. HPLC was used to separate the oil aldehydes into three fractions, aliphatic aldehydes, sesquiterpene aldehydes, and monoterpene aldehydes. These fractions were transferred automatically to the GC, where the individual aldehydes were separated into well resolved chromatographic peaks.     

First Magnesium-mediated Carbonyl Benzylation in Water

Catalyzed by AgNO3, Mg was found for the first time to be able to mediate the coupling reaction between aromatic aldehydes and benzyl bromide or chloride in water. The yields were slightly higher than the recent results for Mg-mediated allylation despite the fact that aqueous benzylation is intrinsically much harder than allylation. It was also found that the coupling reaction was chemoselective for aromatic aldehydes over aliphatic aldehydes, and chemoselective for aromatic aldehydes over aromatic ketones.     

Silica Phosphoric Acid: An Efficient and Recyclable Catalyst for the Solvent-Free Synthesis of Acylals and Their Deprotection in MeOH

Silica phosphoric acid was used as an efficient, mild, and recyclable solid catalyst for the synthesis of acylals from various structurally diverse aldehydes and acetic anhydride under solvent-free conditions. The acylation of aldehydes was highly chemoselective, and no ketone was acylated, which provided a method for the synthesis of acylals from aldehydes in the presence of ketones. Silica phosphoric acid–catalyzed deprotection of acylals to the corresponding aldehydes in MeOH has also been developed with excellent yield. The deprotection of the acylals of aromatic aldehydes took priority over those of aliphatic aldehydes.     

Nickel-Catalyzed α-Allylation of Aldehydes and Tandem Aldol Condensation/Allylation Reaction with Allylic Alcohols

An additive-free nickel-catalyzed α-allylation of aldehydes with allyl alcohol is reported. The reaction is promoted by 1 mol % of in situ formed nickel complex in methanol, and water is the sole by-product of the reaction. The experimental conditions allow the conversion of various α-branched aldehydes and α,β-unsaturated aldehydes as nucleophiles. The same catalyst and reaction conditions enabled a tandem aldol condensation of aldehyde/α-allylation reaction.     

Aldehyde separation by polymer-supported oligo(ethyleneimines)

Chlorosulfonated styrene (10%) divinylbenzene resin beads reacted with an excess of ethylenediamine (EDA), diethylenetriamine (DETA), and triethylenetetramine (TETA) to give the corresponding sulfonamides with pendant oligo(ethyleneimines). The resulting modified resins are useful in the separation of aldehydes from hydrocarbon mixtures. Sorption of aldehydes occurs through formation of both Schiff base and five-membered (imidazoline) rings. Sorbed aldehydes can readily be stripped from the resins by treating with dilute acid solutions. Since the sulfamide bond has a reasonable stability toward acid-base hydrolysis, the loaded resins can be regenerated and recycled by simple acid-base washings, without losing their activity. In the present study, sorption and desorption kinetics of acetaldehyde, benzaldehyde, and salicylaldehyde have been investigated under different conditions. The aldehyde sorption obeys second-order kinetics. The method presented is applicable for all aromatic aldehydes. However, in the case of aliphatic aldehydes carrying an α-hydrogen, aldol condensation products form in solution. So aliphatic aldehydes and their aldol products are sorbed together by the resins. This limits the recovery of aliphatic aldehydes. Consequently, the resins described are cost effective sorbents for the removal and recovery of aromatic aldehydes from various mixtures. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35 : 2857–2864, 1997     

Organocatalytic asymmetric alpha-bromination of aldehydes and ketones

The first organocatalytic enantioselective alpha-bromination of aldehydes and ketones is presented; a C2-symmetric diphenylpyrrolidine catalyst afforded the alpha-brominated aldehydes in good yields and up to 96% ee, while ketones were alpha-brominated by a C2-symmetric imidazolidine in up to 94% ee; furthermore, the organocatalytic enantioselective alpha-iodination of aldehydes is also demonstrated to proceed with up to 89% ee.     

Aerobic oxidation of N-alkylamides catalyzed by N-hydroxyphthalimide under mild conditions. Polar and enthalpic effects

The oxidation of N-alkylamides by O(2), catalyzed by N-hydroxyphthalimide (NHPI) and Co(II) salt, leads under mild conditions to carbonyl derivatives (aldehydes, ketones, carboxylic acids, imides) whose distribution depends on the nature of the alkyl group and on the reaction conditions. Primary N-benzylamides lead to imides and aromatic aldehydes at room temperature without any appreciable amount of carboxylic acids, while under the same conditions nonbenzylic derivatives give carboxylic acids and imides with no trace of aldehydes, even at very low conversion. These results are explained through hydrogen abstraction by the phthalimide-N-oxyl (PINO) radical, whose reactivity with benzyl derivatives is governed by polar effects, so that benzylamides are much more reactive than the corresponding aldehydes. The enthalpic effect is, however, dominant with nonbenzylic amides, making the corresponding aldehydes much more reactive than the starting amides. The importance of the bond dissociation energy (BDE) of the O-H bond in NHPI is emphasized.     

General and practical conversion of aldehydes to homologated carboxylic acids

The reaction of aldehydes with trichloromethide followed by sodium borohydride or sodium phenylseleno(triethyl)borate under basic conditions affords homologated carboxylic acids in high yields. This operationally simple procedure provides a practical, efficient alternative to other homologation protocols. The approach is compatible with sensitive aldehydes including enals and enolizable aldehydes. It also offers convenient access to alpha-monodeuterated carboxylic acids.     

Nickel‐Catalyzed α‐Allylation of Aldehydes and Tandem Aldol Condensation/Allylation Reaction with Allylic Alcohols

An additive‐free nickel‐catalyzed α‐allylation of aldehydes with allyl alcohol is reported. The reaction is promoted by 1 mol % of in situ formed nickel complex in methanol, and water is the sole by‐product of the reaction. The experimental conditions allow the conversion of various α‐branched aldehydes and α,β‐unsaturated aldehydes as nucleophiles. The same catalyst and reaction conditions enabled a tandem aldol condensation of aldehyde/α‐allylation reaction.