Aldehydes selectivity

The 4-hydroxy-1-alkene (homoallylic alcohol) 81 is oxidized to the hetni-acetal 82 of the aldehyde by the participation of the OH group when there is a substituent at C3. In the absence of the substituent, a ketone is obtained. The hemiacetal is converted into butyrolactone 83[117], When Pd nitro complex is used as a catalyst in /-BuOH under oxygen, acetals are obtained from homoallylic alcohols even in the absence of a substituent at C-3[l 18], /-Allylamine is oxidized to the acetal 84 of the aldehyde selectively by participation of the amino group[l 19],... 

Interestingly, while diastereoselectivity is dependent on the relative size of the two non-hydro-gen substituents a to the aldehyde, selectivity does not appear to be sensitive to increases in the steric requirements of the substituent on the allylboronate21. 

If a chiral aldehyde, e.g., methyl (27 ,4S)-4-formyl-2-methylpentanoate (syn-1) is attacked by an achiral enolate (see Section 1.3.4.3.1.), the induced stereoselectivity is directed by the aldehyde ( inherent aldehyde selectivity ). Predictions of the stereochemical outcome are possible (at least for 1,2- and 1,3-induction) based on the Cram—Felkin Anh model or Cram s cyclic model (see Sections 1.3.4.3.1. and 1.3.4.3.2.). If, however, the enantiomerically pure aldehyde 1 is allowed to react with both enantiomers of the boron enolate l-rerr-butyldimethylsilyloxy-2-dibutylboranyloxy-1-cyclohexyl-2-butene (2), it must be expected that the diastereofacial selec-tivitics of the aldehyde and enolate will be consonant in one of the combinations ( matched pair 29), but will be dissonant in the other combination ( mismatched pair 29). This would lead to different ratios of the adducts 3a/3b and 4a/4b. 

The 1-octene conversions averaged 50% at the current flow rate (residence time 30 minutes). We believe the scatter in the data is due to the drift in the pump flow rate, which alters the residence time, and not to a change in the catalyst itself. In all cases the linear to branch aldehyde selectivity was very high in the range of 5 1 linear to branch aldehyde. The reaction was ran under thermomorphic conditions for over 400 hours and we found that we maintained good conversion and good selectivity. 

On the other hand, the oxidation of the alkyl substituent in alkyl aromatic compounds can be carried out by various methods efficiently. For example, CAN has been used to oxidize substituted toluene to aryl aldehydes. Selective oxidation at one methyl group can be achieved (Eq. 7.19).44 The reaction is usually carried out in aqueous acetic acid. 

Selective addition to aldehydes. Organomanganese halides add to aldehydes selectively in the presence of ketones. This is true for RMnI, RMnBr, prepared from RLi and MnBr2, and for RMnCl, prepared from RLi or RMnX and MnCl2. ... 

Turn over frequency (TOP) = [Product]Moi/([catalyst]MoixThour) [Pyc] = 70.56 pMol./gram of NPyc ScHo = percentage of aldehyde selectivity by GLC. 

The beer aldehydes selectively reacted with the PFBOA in the fiber. 

The most convenient and commonly employed method for promoting the generation of TEMPO-oxoammonium is instead the so-called Anelh procedure It resorts to NaClO as the regenerating oxidant in 1 1 molar ratio with the substrate, under co-catalysis by EUlr. In a two-phase CH2Cl2-water system at 0°C, the oxidation of a primary alkanol does take place, to give the aldehyde selectively and in good yields (entry 3) after a few minutes. In contrast, if the reaction is run in aqueous solution with 2 molar equivalents of... 

The initial study on the MeO-TEMPO / Mg(N03)2 / NBS triple catalyst system in the oxidation of 1 indicated the necessity of all three components the TEMPO based catalyst, the nitrate source (MNT) and the bromine source (NBS). A large number of metal nitrates and nitrites were screened initially and the highest activity and aldehyde selectivity under comparable reaction conditions were recorded using Mg(N03)2 as the nitrate component. A number of organic and inorganic bromides soluble in HOAc were also screened and high reaction rates were found when NBS was used as the bromide source. The effect of the concentration of the individual components of the new triple catalyst system on the reaction rate, on the conversion of 1 and on the selectivity to 2 over 60 min reaction time is shown in Figure 1. 

Another interesting observation from the data in Figure 2 was the effect of the catalyst concentration on the aldehyde selectivity (curves 2 in 2a-c). As mentioned earlier, at this moderate reaction temperature, the only by-product present in measurable quantities was hexanoic acid, formed as a product of the over-oxidation of 2. Contrary to what was reported in the literature for other TEMPO based oxidations of alcohols (20,21), the current catalyst system, particularly at higher... 

TEMPO concentrations is able to promote the further oxidation to an acid once the alcohol is completely converted to 2. The same is true for the best performing system in Figure 2c. At the optimum concentration for the AA-TEMPO catalyst of 0.6 mmol (3.7 mol%), the alcohol substrate is completely consumed at an aldehyde selectivity of 95+ %. Further increase in the AA-TEMPO concentration doesn t lead to further rate gains and the only visible result is the deterioration in the aldehyde selectivity. 

In the reaction of chiral enolates (Scheme 4) with chiral aldehydes, the intrinsic diaste-reofadal selectivities of the two components either match or mismatch. Reagent control can be obtained only if the enolate selectivity overcomes the aldehyde selectivity. 

Sodium borohydride, 277 Sodium dithionite, 277 Reagents which can be used to reduce cyclic ketones to equatorial (more stable) alcohols Potassium-f-Butanol, 277 Sodium-Propanol, 277 Sodium triacetoxyborohydride, 283 Reagents which can be used to reduce aldehydes selectively Dichlorotris(triphenylphosphine)-ruthenium(II), 107... 

The aldehyde selectivity is high at low conversions, but acetal and ester are also formed. The acid is obtained only at longer reaction times. The ester is evidently not formed from the acid, and is probably the product of a fast oxidation of a hemiacetalic intermediate. The reaction sequence is represented in Scheme 1. 

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