Chemoselective reactions oxidation

A zirconium complex, bis(cyclopenta(Uenyl)zirconium(IV) hydride will function as a catalyst for the chemoselective Oppenauer oxidation of primary alcohols in the presence of a hydrogen acceptor (cyclohexanone, benzaldehyde or benzophenone). This method appears to be of some value, since it also allows for the selective monooxidation of primary (and secondary) diols (Scheme 3). 1,2-Diols are not cleaved under these conditions and retro-aldol reactions appear not to be a problem. 

Scheme 8.3. Some examples of V+5-mediated reactions of allylic alcohols with r-BuOOH. (a) A chemoselective reaction [8]. (b) Stereoselective reactions of acyclic allylic alcohols, compared to results obtained using m-CPBA [9]. Note that better selectivity is usually obtained using the metal-based oxidation system, but not always with the same relative topicity as observed using a peracid.

Oxidation of primary and secondary alcohols by oxoammonium salts derived from nitroxides has become very popular because of the very mild and chemoselective reaction conditions available (Scheme 13). The stoichiometric oxidant can often be an inexpensive reagent, such as hypochlorite (bleach), O2 with a metal catalyst, electrochemical anodic oxidation, peracid, or bromine. The oxoammonium salt can be either pre-formed and used stoichiometrically or generated catalytically from the nitroxide in situ. The mechanism of the reactions is pH dependent strongly acidic conditions chemoselectively oxidize secondary alcohols with accelerated rates over primary alcohols, whereas basic or mildly acidic conditions provide chemoselective oxidation of primary alcohols in the presence of secondary alcohols. A compre-... 

Baeyer-Villiger oxidation (p. 853) catalytic hydrogenation (p. 844) chemoselective reaction (p. 848) dissolving-metal reduction (p. 846) enantioselective reaction (p. 857) epoxidation (p. 855) functional group interconversion (p. glycol (p. 858)... 

We hope that our survey of the important methods for reduction and oxidation has shown you that, by choosing the right reagent, you can often get reaction only at the functional group you want. The chemoselectivity you obtain is kinetic chemoselectivity—reaction at one functional group is simply faster than at another. 

Allylic carbonates are more reaetive than acetates. In addition, reaction of carbonates proceeds in the absence of bases [6]. Formation of jr-allylpalladium 9 from allyl methyl carbonates 8 proceeds by oxidative addition, followed by decarboxylation, and TT-allylpalladium methoxide 9 is generated at the same time, which abstracts a proton from a pronucleophile to form 10. In situ formation of methoxide is a key in the allylation under neutral conditions. Allylation under neutral conditions is useful for the reaction of base-sensitive compounds. For example, exclusive chemoselective reaction of the carbonate group in 4-acetoxy-2-butenyl methyl carbonate (11) occurred in the absence of a base to yield 12. Similar chemoselective reaction of the allyl carbonate group in the chiral cyclopentenyl methyl carbonate 13 with the jS-keto ester 14 without attacking the allylic acetate group to give 15 was observed even in the presence of NaH. As expected, retention of stereochemistry (see Chapter 4.2.1) was observed in this substitution [7]. 

Interpretation of the stereo- and chemoselectivity of oxidation reactions using thianthrene oxide must consider its low barrier to ring-inversion, steric effects, and the role that solvents may play in the product distribution. All of these possibilities could influence the Xso value of oxidants. Specifically, mechanistic consideration of electrophilic oxidation of thianthrene oxide in the presence of protic solvents or acid leads to diminished yields of SSO2. Despite aU of these concerns, benzothianthrene oxide is still considered a reliable probe for the electronic character of oxidants. 

Optically active a-substituted phenyloxyacyloxy and aryloxy phosphonates have been synthesized via catalytic asymmetric hydrogenation of the corresponding prochiral a,P-unsaturated phosphonates using Rh(i)/(7, i )-Me-DuPhos as the catalyst. The reactions exhibit excellent enantioselectivity with ee up to 96%. A new kinetic resolution process for a-hydroxyphosphonates with the assistance of N-salicylidene-L-tert-leucine-based vanadyl(v) methoxide complexes (209), achieving highly enantioselective and chemoselective aerobic oxidation at ambient temperature (Scheme 78). ... 

Interestingly, the reaction is chemoselective, and oxidation of aminoalkenes gave selectivily the N-oxides without any epoxide formation. One example is given in Eq. (8.22). A number of substituted pyridines were also oxidized to the pyridine N-oxides by DMD in quantitative yields [114]. 

The observed types of reactions include alkene epoxidation, allylic and benzylic oxidation, and alkane hydroxylation. The ligands on the cobalt atom control the chemoselectivity of oxidation. 

With higher alkenes, three kinds of products, namely alkenyl acetates, allylic acetates and dioxygenated products are obtained[142]. The reaction of propylene gives two propenyl acetates (119 and 120) and allyl acetate (121) by the nucleophilic substitution and allylic oxidation. The chemoselective formation of allyl acetate takes place by the gas-phase reaction with the supported Pd(II) and Cu(II) catalyst. Allyl acetate (121) is produced commercially by this method[143]. Methallyl acetate (122) and 2-methylene-1,3-diacetoxypropane (123) are obtained in good yields by the gas-phase oxidation of isobutylene with the supported Pd catalyst[144]. 

A synthetically useful virtue of enol triflates is that they are amenable to palladium-catalyzed carbon-carbon bond-forming reactions under mild conditions. When a solution of enol triflate 21 and tetrakis(triphenylphosphine)palladium(o) in benzene is treated with a mixture of terminal alkyne 17, n-propylamine, and cuprous iodide,17 intermediate 22 is formed in 76-84% yield. Although a partial hydrogenation of the alkyne in 22 could conceivably secure the formation of the cis C1-C2 olefin, a chemoselective hydrobora-tion/protonation sequence was found to be a much more reliable and suitable alternative. Thus, sequential hydroboration of the alkyne 22 with dicyclohexylborane, protonolysis, oxidative workup, and hydrolysis of the oxabicyclo[2.2.2]octyl ester protecting group gives dienic carboxylic acid 15 in a yield of 86% from 22. 

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