Aldehydes overview

During our research in this field of small-ring heterocycles we found that functionahzed aziridines are attractive chiral catalysts, e.g., in the diethylzinc addition to aldehydes. Aspects of such uses of aziridines will be discussed as well. This overview does not pretend to be an exhaustive coverage of all existing literature on small-ring aza-heterocycles as that would require a separate monograph. Instead, emphasis is put on functionahzed three-membered aza-heterocycles, that were investigated in the author s laboratory [1], and relevant related literature. The older literature on these heterocycles is adequately summarized in some extensive reviews [2]. Chiral aziridines have been reviewed recently by Tanner [3], by Osborn and Sweeney [4], and by McCoull and Davis [5]. 

A variety of double bonds give reactions corresponding to the pattern of the ene reaction. Those that have been studied from a mechanistic and synthetic perspective include alkenes, aldehydes and ketones, imines and iminium ions, triazoline-2,5-diones, nitroso compounds, and singlet oxygen, 10=0. After a mechanistic overview of the reaction, we concentrate on the carbon-carbon bond-forming reactions. The important and well-studied reaction with 10=0 is discussed in Section 12.3.2. 

A broad array of mechanistic pathways may be considered in the different variants of nickel-catalyzed reductive couplings of aldehydes and alkynes, and a generalized overview of possible mechanisms has been previously described [10]. Whereas a comprehensive mechanistic study has not been presented, a number of key observations have been illustrated that provide insight into how the nickel-catalyzed reductive couplings of aldehydes and alkynes proceed. It should be stressed at the outset that the different reaction variants may proceed by different mechanisms. 

A variety of alternate methods for the reductive coupling of aldehydes and alkynes have been developed. A number of important hydrometallative strategies have been developed, although most of these methods require the stoichiometric formation of a vinyl metal species or metallacycle. A very attractive hydrogenative coupling method has recently been developed, and its scope is largely complementary to the nickel-catalyzed methods. A very brief overview of these methods is provided below. 

This chapter aims to provide an overview of the current state of the art in homogeneous catalytic hydrogenation of C=0 and C=N bonds. Diastereoselec-tive or enantioselective processes are discussed elsewhere. The chapter is divided into sections detailing the hydrogenation of aldehydes, the hydrogenation of ketones, domino-hydroformylation-reduction, reductive amination, domino hydroformylation-reductive amination, and ester, acid and anhydride hydrogenation. 

In addition to the reactions discussed in the present section, specific reactions caused by the presence of some excipients may occur in solids. A detailed overview would be beyond the scope of this work, and we will simply present here two non-redox breakdown reactions that involve reducing sugars (i.e., sugars that contain an aldehyde function). The two reactions, however, are of a non-redox nature in that the aldehyde group does not act as a reducing reagent. 

The biocatalytic reduction of carboxylic acids to their respective aldehydes or alcohols is a relatively new biocatalytic process with the potential to replace conventional chemical processes that use toxic metal catalysts and noxious reagents. An enzyme known as carboxylic acid reductase (Car) from Nocardia sp. NRRL 5646 was cloned into Escherichia coli BL21(DE3). This E. coli based biocatalyst grows faster, expresses Car, and produces fewer side products than Nocardia. Although the enzyme itself can be used in small-scale reactions, whole E. coli cells containing Car and the natural cofactors ATP and NADPH, are easily used to reduce a wide range of carboxylic acids, conceivably at any scale. The biocatalytic reduction of vanillic acid to the commercially valuable product vanillin is used to illustrate the ease and efficiency of the recombinant Car E. coli reduction system." A comprehensive overview is given in Reference 6, and experimental details below are taken primarily from Reference 7. 

In Part 111 we cover that broad category of organic compounds called the carbonyls. First we give you an overview of Ccirbonyl basics, including structure, reactivity, and spectroscopy. Then we go into more detail on aldehydes and ketones, enols and enolates, and carboxylic acids and their derivatives. 

Abstract The purpose of this chapter is to present a survey of the organometallic chemistry and catalysis of rhodium and iridium related to the oxidation of organic substrates that has been developed over the last 5 years, placing special emphasis on reactions or processes involving environmentally friendly oxidants. Iridium-based catalysts appear to be promising candidates for the oxidation of alcohols to aldehydes/ketones as products or as intermediates for heterocyclic compounds or domino reactions. Rhodium complexes seem to be more appropriate for the oxygenation of alkenes. In addition to catalytic allylic and benzylic oxidation of alkenes, recent advances in vinylic oxygenations have been focused on stoichiometric reactions. This review offers an overview of these reactions... 

Another way to construct alkenes is by the addition of carbon radicals to nitrostyrenes such as 5. Ching-Fa Yao of National Taiwan Normal University in Taipei has reported (J. Org. Chem. 2004,69, 3961) an extension of this work, generating the acyl radical from the aldehyde 6, cyclizing it to generate a new radical, then trapping that radical with 5 to give 7. This article includes an overview of the several ways of adding radicals to 5. 

Amidocarbonylation aldehydes, 11, 512 enamides, 11, 514 overview, 11, 511-555 Amido complexes with bis-Cp titanium, 4, 579 Group 4, surface chemistry on oxides, 12, 515 Group 5, surface chemistry on oxides, 12, 524 with molybdenum mono-Cp, 5, 556 with mono-Cp titanium(IV) alkane elimination, 4, 446 amine elimination, 4, 442 characteristics, 4, 413 via dehalosilylation reactions, 4, 448 HCL elimination, 4, 446 metathesis reactions, 4, 438 miscellaneous reactions, 4, 448 properties, 4, 437... 

Dithianes are readily prepared from aldehydes (for an overview, see 1,3-dithianes as protecting group) and offer high stability towards acids and bases. Therefore, use of the S,S-acetal unit is especially useful in multistep synthesis. A crucial step is the hydrolysis of S,S-acetals, the difficulty of which is due to the excellent nucieophilicity of sulfur. 

In this paper we give an overview on the partial oxidation of primary and secondary alcohols to ketones, aldehydes or acids. The influence of promotion on catalyst deactivation will be illustrated using the example of a 5 wt% Pt/alumina partially covered by Bi. 

Much effort has been devoted to finding synthetically useful methods for the palladium-catalyzed aerobic oxidation of alcohols. For a detailed overview the reader is referred to several excellent reviews [163]. The first synthetically useful system was reported in 1998, when Peterson and Larock showed that simple Pd(OAc)2 in combination with NaHC03 as a base in DMSO as solvent catalyzed the aerobic oxidation of primary and secondary allylic and benzylic alcohols to the corresponding aldehydes and ketones, respectively, in fairly good yields [164, 165]. Recently, it was shown that replacing the non-green DMSO by an ionic liquid (imidazole-type) resulted in a three times higher activity of the Pd-catalyst [166]. 

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