Halo aldehydes, formation

Regarding ozonation processes, the treatment with ozone leads to halogen-free oxygenated compounds (except when bromide is present), mostly aldehydes, carboxylic acids, ketoacids, ketones, etc. [189]. The evolution of analytical techniques and their combined use have allowed some researchers to identify new ozone by-products. This is the case of the work of Richardson et al. [189,190] who combined mass spectrometry and infrared spectroscopy together with derivatization methods. These authors found numerous aldehydes, ketones, dicarbonyl compounds, carboxylic acids, aldo and keto acids, and nitriles from the ozonation of Mississippi River water with 2.7-3 mg L 1 of TOC and pH about 7.5. They also identified by-products from ozonated-chlorinated (with chlorine and chloramine) water. In these cases, they found haloalkanes, haloalkenes, halo aldehydes, haloketones, haloacids, brominated compounds due to the presence of bromide ion, etc. They observed a lower formation of halocompounds formed after ozone-chlorine or chloramine oxidations than after single chlorination or chlorami-nation, showing the beneficial effect of preozonation. 

In the Perkow reaction, a trialkyl phosphite reacts with an a-halo-aldehyde or -ketone to yield an enol phosphate (i.e. [P—O] bond formation, e.g. Scheme 5).76 a-Haloaldehydes react cleanly but with a-haloketones the Michaelis-Arbuzov reaction usually competes with the product distribution depending on the reaction... 

The old method of heating the calcium salts of formic and a second carboxylic acid for aldehyde formation has been modified by the use of a catalytic decomposition technique. By this scheme, the acid vapors are passed over thorium oxide, titanium oxide, or magnesium oxide at 300° or the acids are heated under pressure at 260° in the presence of titanium dioxide. In the latter procedure, non-volatile acids can be used. With aliphatic acids over titanium oxide, reaction occurs only when more than seven carbon atoms are present, the yields increasing with increase in the molecular weight (78-90%). Aromatic-acids having halo and phenolic groups are converted in high yields to aldehydes, e.g., salicylaldehyde (92%) and p-chlorobenzaldehyde (8S>%). Preparation of a thorium oxide catalyst has been described (cf. method 186). 

The reaction is applicable to the formation of m and p-dialdehydes, but not the ortho isomer, from the fcis-(chloromethyl)-benzenes, as well as aldehyde esters, e.g., p-carbomethoxybenzaldehyde, and halo aldehydes, e.g., l-bromo-2-naphthaldehyde. ... 

Rovis and coworkers used a-halo aldehydes to generate the formal redox ester products in the presence of an N-Ph triazolium salt [77]. This methodology was later extended to the enantioselective formation of phenyl a-chloroesters 96 [78] and a-chloro-carboxylic acids [79] from a,a-dichloro-aldehydes 94 using chiral triazolium salt 95 (Scheme 18.16). Consistent with the simultaneous observations by Bode and coworkers, a nucleophilic co-catalyst was necessary for the formation of amides [80]. An a-chloroamide could be synthesized in good enantioselectivity (80% ee) by the use of chiral triazolium salt 95. 

Amino Acids. The formation of A/-halo-a-amino acids involves halogenation of the acid anion (13). /V-Cb1oro-CX-amino acids decompose to aldehydes and nitriles, the selectivity depending on pH and stoichiometry (110). For example, AJ-chloroalanine decomposes in the 6.5—10 pH range. 

The preparation of e/n-difluoro compounds by the oxidative fluorodesul-furization ot 1,3-dithiolanes readily proceeds by treatment with a pyridinium polyhydrogen fluoride-Af-halo compound reagent the latter serves as a bromonium ion source [2], l,3-Dibromo-5,5-dimethylhydantoin is the most effective of several At-halo oxidants. It is believed that /V-halo compounds combine with hydrogen fluoride to generate in situ halogen fluorides, the oxidants. Formation of gem-difluorides from dithiolanes derived from ketones is efficient and rapid, even at -78 °C, whereas the reaction of dithiolanes derived from aldehydes requires higher temperature (0 °C) (equation 4). 

Dehalogenation.1 DMBI effects dehalogenation of a-halo carbonyl compounds in a variety of ethereal solvents with formation of DMBI+X in generally high yield. The order of relative reactivity is Br > Cl > F (halides) and primary > secondary > tertiary (for the a-substituted position). In combination with HO Ac (1 equiv.) the reagent also reduces acyl chlorides to aldehydes (70-90% yield). 

A conversion typical of a-halo-a-lithioaUcanes is the formation of epoxides that results from their reaction with aldehydes or ketones. As illustrated in equation 61, the bromo-lithium carbenoid is usually generated by halogen-lithium exchange. The intermediate lithium aUcoxide 113 undergoes an in situ ring closure to give the oxirane 114 . 

Glycidic esters can easily be converted to aldehydes (2-40). The reaction has been extended to the formation of analogous aziridines by treatment of an imine with an a-halo ester or an a-halo N,N-disubstituted amide and f-BuOK in the solvent 1,2-dimethoxy-ethane.636 However, yields were not high. Acid-catalyzed Darzens reactions have also been reported.637 See also 6-61. 

The proposed reaction mechanism for the destruction of aqueous solutions of TCE or PCE predicts the formation of stable oxidized polar organic compounds. These compounds consist of acids, aldehydes, and possibly halo-acetic acids. Three possible mechanisms have been proposed for the formation of by-products due to the irradiation of aqueous solutions containing TCE and PCE. The first is for the formation of formaldehyde, acetaldehyde, and glyoxal, which are formed at a concentration of approximately two orders of magnitude less than the influent solute concentration. Second, the formation of formic acid decreased with increasing radiation dose. The formic acid concentration was found to be higher for PCE than TCE. These results are most probably due to the slower reaction rate constants of PCE with e and OH, compared to TCE. The third possible reaction is the formation of haloacetic acids when TCE and OH react. The mechanism of decomposition of PCE by OH is shown in Equation (12.30) to Equation... 

The u halo natkm of an aldehyde or ketom is a tyi al or-substitution reaction that proceeds by acidenol intermediate, as shown in ngure 22.4 [Pg.905]

As a preparative method the direct decarboxylation of olefinic acids is almost limited to the formation of styrenes and stilbenes from substituted cinnamic acids. Thermal decomposition of cinnamic acid gives styrene (41%). The yield is nearly quantitative if the reaction is carried out in quinoline at 220° in the presence of a copper catalyst. The yields of substituted styrenes where the aryl radical contains halo, methoxyl, aldehyde, cyano, and nitro groups are in the range of 30-76%. cis-Stilbene and cis-p-nitrostilbene are prepared in this way from the corresponding a-phenylcinnamic acids (65%). One aliphatic compound worthy of mention is 2-ethoxypropene, prepared by heating -ethoxycro-tonic acid at 165° (91% yield). The mechanism of acid-catalyzed decarboxylations of this type has been studied. Isomerization of the double bond from the a,/5- to the /5, y-position before decarboxylation very likely occurs in many instances. ... 

Fukuzawa, S., Fujinami, T., Sakai, S. Carbon-carbon bond formation between a-halo ketones and aldehydes promoted by cerium(lll) iodide or cerium(lll) chloride-sodium iodide. J. Chem. Soc., Chem. Common. 1985, 777-778. 

Condensations. Alumina promotes the formation of a-hydroxyphosphonate esters from aromatic aldehydes and dialkyl phosphonates, and the adducts are converted to a-aminophosphonate esters on reaction with ammonia. A solvent-free synthesis of a-nitro ketones comprises mixing nitroalkanes, aldehydes, and neutral alumina and oxidizing the adducts with wet, alumina-supported CrOj (15 examples, 68-86%). The Knoevenagel reaction, the Michael addition of nitromethane to gcm-diactivated alkenes, and the formation of iminothiazolines from thioureas and a-halo ketones are readily effected with alumina under microwave irradiation. 

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