Primary alcohols electrophiles from

Assuming that Ti(IV) is distributed statistically in all tetrahedral positions, it can be easily seen that even for crystallite sizes of 0,2 m the great majority of T1(IV) is located inside the pore structure. Assuming that every Ti(IV) is a catalytic centre with equal activity, diffusion limitations for molecules of different sizes should be observed. This is in fact the case. It has been shown [27] that the rate of oxidation of primary alcohols decreases regularly as the chain length increases, while for iso-butyl alcohol a sudden drop in the rate is observed. Also the reactivity order of olefins on TS-1 is different from the order observed with homogeneous electrophilic catalysts, while as already indicated very bulky molecules are unreactive when TS-1 is used as the catalyst. All these facts can only be interpreted as due to diffusion limitations of the larger molecules, which means that the catalytic sites are located inside the pore structure of the solid. 

The activation of DMSO by electrophilic reagents such as oxallyl chloride or trifluoroacetic anhydride (TFAA) (among many others) produces an oxidant capable of oxidizing primary alcohols to aldehydes in high yields. This oxidation is called the Swern oxidation and yields the aldehyde (oxidized product) by reductive elimination of dimethylsulfide (reduced product) and proceeds under mild, slightly basic conditions. It is a second widely used and effective oxidative method for the production of aldehydes from primary alcohols. 

Asymmetric aikyiation of imide etiolates.1 The sodium enolates of 3 and 7 are alkylated with marked but opposite diastereoselectivity by alkyl halides. The selectivity is improved by an increase in the size of the electrophile, with methylation being the least stereoselective process. The asymmetric induction results from formation of (Z)-enolates (chelation) with the diastereoselectivity determined by the chirality of the C4-substituent on the oxazolidone ring (equations I and II). The products can be hydrolyzed to the free carboxylic acids or reduced by LiAlH4 to the corresponding primary alcohols and the unreduced oxazolidone (1 or 2). 

Amides differ from carboxylic acids and other acid derivatives in their reaction with Li A1H4 Instead of forming primary alcohols, amides are reduced to amines (Fig.P). The mechanism (Fig.Q) involves addition of the hydride ion to form an intermediate that is converted to an organoaluminium intermediate. The difference in this mechanism is the intervention of the nitrogen s lone pair of electrons. These are fed into the electrophilic centre to eliminate the oxygen that is then followed by the second hydride addition. 

Product 24 contains a secondary alcohol at the position where the primary alcohol was in substrate 23. From that it can be seen that 23 is first converted into the corresponding aldehyde, which then acts as electrophile in the following aldol reaction. Here, oxidation to the aldehyde is performed with the iodo(V)-reagent IBX (o-iodoxy-benzoic acid, 51). After addition-elimination, species 52 is formed, which disproportionates to the iodo(III)-compound 53 and the desired aldehyde 38. 

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 C-C reductive coupling of r-unsaturated compounds with carbonyl electrophiles by ruthenium-catalysed transfer hydrogenation leading to carbonyl allylation, vinylation, and propargylation has been reviewed. The ability of primary alcohols to function both as hydrogen donors and as aldehyde precursors, enabling carbonyl addition directly from the alcohol oxidation level, has been discussed. ... 

Preparative scale photochemical reactions of 1,4-diazo-oxides yield products expected from an electrophilic carbene intermediate. The singlet carbene is stabilized in polar solvents. It reacts with primary alcohols as an electrophile, giving the corresponding hydroquinone mono-ether. With isopropanol, and probably other secondary alcohols, hydrogen atom abstraction occurs, giving the phenoxyl radical. ... 

Primary benzylic alcohols are oxidized to aldehydes in good yields without overoxidation (entry 1) lowering the pH from 5 to 3.5 increases the conversion, for reasons not fnUy understood yet (entry 2) . The aminoxyl radical is an electrophilic species" ... 

Taddei has developed a soluble PEG supported scavenger 53 to capture a variety of nucleophilic functional groups (Scheme 13) [21]. This scavenger was based on an electrophilic dichlorotriazine core and relied on selective precipitation (by the addition of ether to acetonitrile) to remove it from the reaction mixture. This scavenger 53 is particularly versatile, and has been used to remove primary, secondary and tertiary alcohols, diols and thiols... 

Primary and secondary alcohols are readily converted to mesylate or tosylate esters by reaction with the corresponding sulfonyl chloride. The mesylate and tosylate esters derived from tertiary alcohols are too reactive and cannot be isolated. (Although we will not go into the mechanism of these reactions in detail at this point, the reactions involve the attack of the oxygen [the nucleophile] of the alcohol at the sulfur [the electrophile], ultimately displacing chloride [the leaving group].) Pyridine is often used as a solvent for these preparations in order to react with the HC1 that is produced as a by-product. An example of the preparation of a methanesulfonate (mesylate) ester is shown in the following equation ... 

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