Aldehydes oxidative esterification

Oxidation. Oxidation of the -amyl alcohols produces aldehydes, which after continued oxidation can yield acids. This route to aldehydes has httle merit. However, oxidative esterifications with alkah metal hypohaUtes (eg, calcium chlorite, Ca(OCl)2) (49), bromates (eg, sodium bromate, NaBrO )... 

P,S -Unsaturated alcohols undergo an oxidative esterification with aliphatic aldehydes in the presence of an iridium(I) catalyst and potassium carbonate.330 Precoordination of the ene-alkoxide with iridium is proposed, followed by reaction with aldehyde. Although the ester yield is high, a mixture of unsaturated and saturated esters is typically obtained, except for secondary alcohols. 

Pincer-ligated iridium complexes have been used as homogeneous catalysts for the dehydrogenation of aliphatic polyalkenes to give partially unsaturated polymers. The catalyst appears to be selective for dehydrogenation in branches as compared with the backbone of the polymer.56 The mechanism shown in Scheme 1 has been suggested for an [IrCl(cod)]2-catalysed oxidative esterification reaction of aliphatic aldehydes and olefinic alcohols.57... 

Description The common production method of DMT from paraxylene and methanol is through successive oxidations in four major steps oxidation, esterification, distillation and crystallization. A mixture of p-xy-lene and methyl p-toluate (MPT) is oxidized with air using a heavy-metal catalyst. All organics are recovered from the offgas and recycled to the system. The acid mixture from the oxidation is esterified with methanol and produces a mixture of esters. The crude ester mixture is distilled to remove all heavy boilers and residue produced lighter esters are recycled to the oxidation section. Raw DMT is then sent to the crystallization section to remove DMT isomers and aromatic aldehydes. 

The oxidation of aldehydes to carboxylic acids has been most extensively investigated with horse liver alcohol dehydrogenases (65-67, 69, 73). There are two distinct reactions the direct oxidation of aldehydes as their hydrated gem-diol form [reaction (9)] and the oxidation of hemiacetals to esters [oxidative esterification, reaction (10)]. 

Oxidative esterification. The H2O2-V2O5 reagent converts aromatic aldehydes to methyl esters in methanol (perchloric acid is also present). 

Palladium catalysts are widely used in liquid phase aerobic oxidations, and numerous examples have been employed for large-scale chemical production (Scheme 8.1). Several industrially important examples are the focus ofdedicated chapters in this book Wacker and Wacker-type oxidation of alkenes into aldehydes, ketones, and acetals (Scheme 8.1a Chapters 9 and 11), 1,4-diacetoxylation of 1,3-butadiene (Scheme 8.1b Chapter 10), and oxidative esterification of methacrolein to methyl methacrylate (Scheme 8.1c Chapter 13). In this introductory chapter, we survey a number of other Pd-catalyzed oxidation reactions that have industrial significance, including acetoxylation of ethylene to vinyl acetate (Scheme 8. Id), oxidative carbonylation of alcohols to dialkyl oxalates and carbonates (Scheme 8.1e), and oxidative coupling of dimethyl phthalate to 3,3, 4,4 -tetramethyl biphenylcarboxy-late (Scheme 8.1f). 

Esters are very useful chemical intermediates, in terms of atom economy and versatility, and can be helpful in further transformations. Esterification is one of the fundamental transformations in organic synthesis and is widely used in laboratories and industry [1]. Oxidative esterification of aldehydes with alcohols is an attractive method for the synthesis of esters because aldehydes are readily available raw materials on a commercial scale. Although several facile and selective esterification reactions have been reported [2], the development of a catalytic method for the direct oxidative esterification of aldehydes with alcohols under mild and neutral conditions in the presence of molecular oxygen as the terminal oxidant is highly desirable from both economic and environmental aspects. 

I 13 Aerobic Oxidative Esterification of Aldehydes with Alcohols... 

The reaction mechanism that proceeds via the hemiacetal is considered less likely since the Pd-Pb-catalyzed oxidative esterification proceeds in neutral to basic conditions rather than in acidic conditions, and an alkali is also integrated as the third component of the catalyst thus, a mechanism involving an acyl palladium intermediate is favored. The reaction mechanism in Figure 13.3 rationalizes the observation that the presence of aldehydes suppresses the production of MF. It is presumed that alkali is effective in increasing the adsorption equilibrium of the alcohol. 

The oxidative esterification of aldehydes in the presence of alcohols to produce saturated and unsaturated aliphatic esters has been developed at the Nippon Shokubai laboratories a great result is represented by the new synthesis of methylmethacrylate (MMA) from methacrolein, methanol and molecular oxygen, which allows yields up to 50 mol MMA (kg cat)" h" using supported bimetallic gold catalysts under relatively mild conditions (80 °C and 3 MPa) [7]. 

Oxidative esterification, Japanese chemists have reported a novel oxidation discovered in the carbohydrate field. Reaction of the tri-n-butylstannyl alkoxide (D with 1 equiv. of NBS in CCU at 20° gives the ester (2) and succinimide (3) in high yield. The reaction involves oxidation of (1) to an aldehyde, which then reacts with (1) to form the ester (2). 

The preparation of a,p-epo)ty esters was developed by Yan and coworkers via one-pot epoxidation and subsequent oxidative esterification. The product was applied in the synthesis of (—)-clausenamide (Scheme 8.42). At the same time, Jorgensen also published a one-pot Michael addition/oxidative esterification of a,p-unsaturated aldehydes. ... 

Enantioselective oxidative esterification, thioesterification, and amidation of aldehydes have been achieved using a combined organocatalyst system of a chiral NHC and riboflavin, with dioxygen as terminal oxidant." ... 

An unexpected oxidative esterification of cinnamaldehydes by cinnamyl bromides is catalysed by a simple NHC, A,A -dibenzyl-benzimidazolium chloride, in up to 86% yield in DBU, using dioxygen or Mn02 as oxidant. The mechanism has been probed by 0-labelling, and neither cinnamyl nor cinnamate moieties undergo cis-trans isomerization during the process. The reaction has been successfully extended to other aldehydes, although electron-rich cases are poor substrates. ... 

Cinnamyl cinnamate derivatives were obtained in good to excellent yields by esterification of a,j8-unsaturated aldehydes, including aromatic aldehydes, with cinnamyl bromides in the presence of DBU, K2CO3, THF, and f-BuOH using either atmospheric O2 or Mn02 as the oxidant and IV-heterocyclic carbene (NHC) as the catalyst. The use of 02 indicated that the path of the O2 oxidation via Breslow intermediate (119) (Scheme 12) differs from that for aldehyde oxidation by Mn02 via a tetrahedral intermediate (124) (Scheme 13). ... 

Furthermore, the first enantioselective oxidative esterification of simple aldehydes... 

This is the first report of the iodine-catalyzed selective synthesis of iodo-pyrano [4,3-Z)]-quinolines and ortho-alkynyl esters from ortho-alkynyl aldehydes. The developed novel oxidative esterification process provides a powerful tool for the preparation of a wide range of functionalized pyranoquinolinones as well as isocoumarins. 

Verma et al. [62] observed a chemoselective behavior of iodine in diSeient solvents in the electrophilic iodocyclization of o-alkynyl aldehydes. o-Alkynyl aldehydes on reaction with in CH C with appropriate nucleophiles provided pyrano[4,3-6]quinolines 40 via the formation of cyclic iodonium intermediate 39. In case of using alcohols as a solvent as well as nucleophile, o-alkynyl esters 42 were obtained selectively in good to excellent yields via the formation of hypoio-dide intermediate 41. Subsequently, o-alkynyl esters 42 were converted into py-ranoquinolinones and isocoumarins by electrophilic iodocyclization. The developed oxidative esterification provides a novel access for the chemoselective synthesis of esters 43 from aldehydes without oxidizing primary alcohol present in the substrate (Scheme 10.28). 

The Breslow intermediates are quite unstable toward oxidative conditions, providing acyl imidazolinium species that can acylate several nucleophiles or suffer 1,4-conjugated additions. Gois et al. disclosed the oxidative esterification of aldehydes using an iron/NHC catalytic system to prepare benzoic and cinnamic esters using phenols as nucleophiles (Scheme 26.11)... 

Aldehydes (R-CHO) undergo an (9-selective addition of nitrosoarenes (0=N-Ar) catalysed by NHCs this oxidative esterification yields R-C02-NHAr product in fair to good yields, with no C-N product [RCON(OH)Ar] in most cases. ... 

Cinnamils (162) have surprisingly been found to evolve towards 2,3,8-triaryl vinyl fulvenes (163) and o-terphenyl derivatives (164) under NHC catalysis. The formation of these products has been rationalized via a mechanism involving a complex cascade process. Finally, a mechanistic investigation has been carried out on the NHC-catalysed aerobic oxidative esterification of aromatic aldehydes.Aryloin species have not only been identified as key intermediates of such oxidative reactions, but most importantly is that they have been shown to be the species which react with oxygen in the air and not the Breslow intermediates as previously suggested. 

Table 224. Oxidative esterification of alcohols/aldehydes using Au-based supported catalysts.

The resurgence of interest in NHC as catalysts in the last 6 years led several chemists to reinvestigate external oxidants for the azolium-catalyzed esterification of aldehydes. Using cyanide, which shares many similarities to azolium salts in its reactions with aldehyde, Corey had shown in 1968 that the combination of an aldehyde and sodium cyanide in the presence of Mn02 and an alcohol led to esters. Scheldt executed this work with triazolium carbene as a promoter, leading to similar results (Scheme 14.9). Other oxidants including TEMPO radicals (2,2,6,6-tetramethylpiperidine 1-oxyl) and azobenzene" were also found suitable for NHC-catalyzed oxidative esterifications. 

Our strategy for the synthesis of (+)-dactylolide (2.217) is outlined in Scheme 2.69. We envisioned that the 20-membered macrolactone in 2.332 could be constructed by intramolecular iV-heterocyclic carbene (NHC)-catalyzed oxidative macrolactonization of co-hydroxy aldehyde 2.333. Intramolecular NHC-catalyzed oxidative esterification reactions have been recognized as an attractive tool and rapidly growing area in the synthetic community. Indeed, several examples of these reactions have recently been reported [208-216], which clearly provide a new opportunity for the development of catalytic acyl transfer agents in macrolactonization reactions of co-hydroxy aldehydes in the presence of oxidants. The substrate for the macrolactonization reaction would be derived firom the cyanohydrin alkylation of 2,6-dr-tetrahydropyran enal 2.335 with dienyl chloride 2.334. 2,6 -di-tetrahydropyran enal would in turn be constructed by employing the 1,6-oxa conjugate addition reaction of co-hydroxy 2,4-dienal 2.336. Despite the... 

Oxidation. The oxidation of acid sensitive alcohols with Chromium(VI) Oxide in HMPA is one example of the use of HMPA in oxidation reactions. (CAUTION Do not crush CrOs prior to reaction since violent decomposition can occur. The use of DMPU has been reported to have a similar hazardous effect). Recently, a Bromine/NaHCOs/HMPA system was used for the oxidative esterification of alcohols with aldehydes, where the HMPA considerably accelerated the oxidation by bromine and lowered the rate of unwanted halo-genation. Epoxidations of alkenes or allylic alcohols have been accomplished using M0O5 HMPA-pyridine (Oxodiperoxymolyb-denum(pyridine)(hexamethylphosphoric triamide) MoOPH). ... 

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