Oxygen with aldehydes

Several ruthenium complexes bearing chiral Schiff s base ligands have been published. RuL(PPh3)(H20)2], complex C (Fig. 11), with PhIO produced (S)-styrene oxide in 80% ee [61]. Chiral Schiff s base complex D was examined using molecular oxygen with aldehyde, with or without 2,6-dichloropyridine N-oxide as an axial ligand. Styrene oxide was produced in up to 24% ee[62]. A chiral bis(oxazolinyl)pyridine ruthenium complex E with iodosylbenzene diacetate PhI(OAc)2 produced (lS,2S)-fra s-stilbene oxide in 74% ee [63]. Similarly, chiral ruthenium bis(bipyridine) sulfoxide complex F [64] was effective in combination with PhI(OAc)2 as an oxidant and resulted in in 33% ee for (R,R) trans-stilbene oxide and 94% ee for (R,R) trans-/i-Me-styrene (after 75 h at 25 °C). 

If the carboxylic acids on the cellulosic chain are not the major cause of the thermooxidative decay of old cellulosic textiles, one must consider the carbonyl species, particularly the aldehydes on the C2 and Q) carbons. Nikitin (14) noted that the primary autoxidation process is a reaction of molecular oxygen with aldehyde groups, which initiates a chain reaction resulting in more profound changes and decomposition of the molecule . Thus, reduction of the aldehyde groups should lead to improved stability of degraded cellulose. 

The carbonyl oxygen of aldehydes and ketones can form hydrogen bonds with the pro tons of OH groups This makes them more soluble m water than alkenes but less solu ble than alcohols... 

Sulfur tetrafluoride [7783-60-0] SF, replaces halogen in haloalkanes, haloalkenes, and aryl chlorides, but is only effective (even at elevated temperatures) in the presence of a Lewis acid catalyst. The reagent is most often used in the replacement of carbonyl oxygen with fluorine (15,16). Aldehydes and ketones react readily, particularly if no alpha-hydrogen atoms are present (eg, benzal fluoride [455-31-2] from benzaldehyde), but acids, esters, acid chlorides, and anhydrides are very sluggish. However, these reactions can be catalyzed by Lewis acids (HP, BF, etc). 

Reactions at G-5. The C-5 atom of hydantoins can be considered as an active methylene group, and therefore is a suitable position for base-cataly2ed condensation reactions with aldehydes (44). 2-Thiohydantoins give the reaction more readily than their oxygen counterparts ... 

Bacterial concentrations have also been determined by using the enzyme-catalyzed chemiluminescent reaction of reduced flavin mononucleotide (FMN) with oxygen and aldehydes. The detection limit was reported to be 10 ceUs of E. coli, which contains 7 x 10 g of FMN per ceU (303). 

The dianions derived from furan- and thiophene-carboxylic acids by deprotonation with LDA have been reacted with various electrophiles (Scheme 64). The oxygen dianions reacted efficiently with aldehydes and ketones but not so efficiently with alkyl halides or epoxides. The sulfur dianions reacted with allyl bromide, a reaction which failed in the case of the dianions derived from furancarboxylic acids, and are therefore judged to be the softer nucleophiles (81JCS(Pl)1125,80TL505l). 

Exchange of oxygen atom between the C=0 group of luciferin and the solvent water. The exchange is slow with carboxylic acids but fairly rapid with aldehydes and ketones (Samuel, 1962). 

The lower diastereoselectivity found with aldehyde 15 (R = CH3) can be explained by the steric influence of the two methyl substituents in close vicinity to the stereogenic center, which probably diminishes the ability of the ether oxygen to coordinate. In contrast, a significant difference in the diastereoselectivity was found in the additions of phenyllithium and phenylmagnesium bromide to isopropylidene glyceraldehyde (17)58 (see also Section 1.3.1.3.6.). Presumably the diastereo-sclcctivity of the phenyllithium addition is determined by the ratio of chelation-controlled to nonchelation-controlled attack of the nucleophile, whereas in the case of phenylmagnesium bromide additional chelation with the / -ether oxygen may occur. Formation of the -chelate 19 stabilizes the Felkin-Anh transition state and therefore increases the proportion of the anZz -diastereomeric addition product. 

An intermediate allyltin trichloride, possibly stabilized by an oxygen-tin hypervalent interaction, which then reacts with the aldehyde via a chair-like six-membered ring transition state with the substituent a to tin in an axial position, may be involved94. On heating with aldehydes, the 4-benzyloxypentenylstannane shows only modest diastereofacial selectivity22. 

Caprolactam is a thermally unstable compound which on distillation may form methyl-, ethyl-, propyl-, and n-amylamines. Also, at high temperatures, CL reacts widi oxygen to form hydroperoxides which in the presence of iron or cobalt ions are converted into adipimide. /V-alkoxy compounds are also formed by the reaction of CL with aldehydes during storage. 

The Mukaiyama aldol reaction refers to Lewis acid-catalyzed aldol addition reactions of silyl enol ethers, silyl ketene acetals, and similar enolate equivalents,48 Silyl enol ethers are not sufficiently nucleophilic to react directly with aldehydes or ketones. However, Lewis acids cause reaction to occur by coordination at the carbonyl oxygen, activating the carbonyl group to nucleophilic attack. 

Allylic boranes such as 9-allyl-9-BBN react with aldehydes and ketones to give allylic carbinols. The reaction begins by Lewis acid-base coordination at the carbonyl oxygen, which both increases the electrophilicity of the carbonyl group and weakens the C-B bond to the allyl group. The dipolar adduct then reacts through a cyclic TS. Bond formation takes place at the 7-carbon of the allyl group and the double bond shifts.36 After the reaction is complete, the carbinol product is liberated from the borinate ester by displacement with ethanolamine. Yields for a series of aldehydes and ketones were usually above 90% for 9-allyl-9-BBN. 

A. y-Oxygen-Substituleel Stannanes. Oxygenated allylic stannanes have been synthesized and used advantageously in several types of syntheses. Both a- and y-alkoxy and silyloxy stannane can be prepared by several complementary methods.177 C-y-Alkoxy and silyloxy allylic stannanes react with aldehydes to give primarily syn... 

More remote oxygen substituents can also influence stereochemistry. 4-Benzyloxy-2-pentenyl tri-n-butylstannane exhibits excellent enantioselectivity in reactions with aldehydes.186 This reaction is believed to involve chelation of the... 

It is well known that strong electrophiles such as carbocations are reduced by organosilicon hydrides (Eq. 1).3,70,71 On the other hand, simple mixtures of organosilicon hydrides and compounds with weakly electrophilic carbon centers such as ketones and aldehydes are normally unreactive unless the electrophilicity of the carbon center is enhanced by complexation of the carbonyl oxygen with Brpnsted acids3,70 73 or certain Lewis acids (Eq. 2).1,70,71,74,75 Using these acids, hydride transfer from the silicon center to carbon may then occur to give either alcohol-related or hydrocarbon products. 

Reduction to Alcohols. The organosilane-mediated reduction of ketones to secondary alcohols has been shown to occur under a wide variety of conditions. Only those reactions that are of high yield and of a more practical nature are mentioned here. As with aldehydes, ketones do not normally react spontaneously with organosilicon hydrides to form alcohols. The exceptional behavior of some organocobalt cluster complex carbonyl compounds was noted previously. Introduction of acids or other electrophilic species that are capable of coordination with the carbonyl oxygen enables reduction to occur by transfer of silyl hydride to the polarized carbonyl carbon (Eq. 2). This permits facile, chemoselective reduction of many ketones to alcohols. 

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