Oxygenation oxidative allylic

The allylic hydroperoxides generated by singlet oxygen oxidation are normally reduced to the corresponding allylic alcohol. The net synthetic transformation is then formation of an allylic alcohol with transposition of the double bond. 

Stille coupling (12, 56).1 The key step in a synthesis of (E)-neomanoalide (4) involves palladium-catalyzed coupling of an allyl halide with an a-stannylfuran. Thus 1 and 2 couple in the presence of Pd(dba)2 and P(C6H5), to form 3 as a 1 1 mixture of (E)- and (Z)-isomers in 66% yield. Conversion of 3 to 4 involves reduction (DIBAH) and selective singlet oxygen oxidation of a 2-silylfuran to a butenolide. 

Oxidation of allylic andhomallylic acetates (cf. 10,175-176).1 This system is an efficient catalyst for oxygenation of terminal alkenes to methyl ketones (Wacker process). Similar oxidation of internal olefins is not useful because it is not regioselective. However, this catalyst effects oxygenation of allylic ethers and acetates regioselectively to give the corresponding /i-alkoxy ketones in 40-75% yield. Under the same conditions, homoallylic acetates are oxidized to y-acetoxy ketones as the major products. 

Oxygenation of allylic alcohols. This Ru(II) complex, as well as RuBr2[P(C6H5)3]3 and RuH(OAc)[P(C6H5)3]3, catalyzes the oxidation of allylic alcohols to 2,/J-unsaturated ketones or aldehydes by molecular oxygen with retention of configuration. The oxidation of retinol to retinal (second example) requires addition of 2,6-dimethylpyridine to prevent side reactions.2... 

The well-known Sharpless system for the enantioselective epoxidation of allyl alcohols has been investigated [23]. This system employs a tetra-alkoxy titanium precursor, a dialkyltartrate as an auxiliary, and an alkyl hydroperoxide as oxidant, to effect the enantioselective epoxidation. The key intermediate is thought to be a dimeric complex in which titanium is simultaneously coordinated to the chelating tartarate ligand, the substrate in the form of an oxygen bound / -allyl-oxide and an -tert-butylperoxide. 

Palladium-catalyzed oxidative allylic C-H functionalization provides attractive methods for the transformations of olefins, and their utility can be further enhanced by the development of more effective ways to use molecular oxygen (or air) to promote the catalytic cycle. The results outlined in this chapter summarize significant progress in the coupling reaction between terminal alkene and various types of nucleophiles. Further studies will be directed to explorations of the scope of nucleophilic reagents and olefins, and elucidation of the mechanisms of those reactions. Such studies will play an important role in the ongoing development of Pd-catalyzed C-H bond activations. 

Following the work by Li, Shi and co-workers reported both an inter- and intramolecular oxidative allylic alkylation reaction utilizing palladium(II) complex 49 as a catalyst and using benzoquinone (BQ) with oxygen gas as the oxidant (Scheme 28) [51]. 

Oxidation of allylic alcohols. The catalyst system (dba)2Pd-Ph,P in the presence of oxygen is effective to oxidize allylic alcohols. Interestingly, with exclusion of O2 and in the presence of an acid, certain substrates containing a remote double bond undergo cyclodehydration. The oxidation can also be effected with (PhiP>4Pd, NH4PF6, and O2. 

Hydroxy-l-alkenyl carbamates. Transpositional oxygenation of allylic carbamates with SeOj cannot be achieved directly. However, after stannylation, the oxidation proceeds well (using SeOj/r-BuOOH or SeOj alone). 

Tial oxygenation of allylic carba-p a.-. er. after stannylation, the oxidation... 

SmithKline chemists adapted singlet oxygen oxidation methodology to prepare ( )-methyl (5S), (6S)-oxido-7-hydroxyheptanoate 730 (n=l), a key fragment of leukotrienes A-E (Scheme 1.201). 4,5-Diphenyl-2-methyloxazole 231a was converted to the allylic alcohol 727 in five or six straightforward steps. 

The singlet oxygen oxidation differs from autoxidation in several important respects (1) it is an ene and not a radical chain reaction, (2) it gives products which are similar in type but not identical in structure to those obtained by autoxidation, and (3) it is a quicker reaction and its rate is related to the number of double bonds rather than the number of doubly activated allylic groups. " ... 

Often, the hydroperoxides generated in the oxidation of alkenes are not the ultimately desired product. They can be reduced without extensive prior purification to the corresponding allylic alcohol. The sequence of singlet-oxygen oxidation followed by reduction constitutes a reasonably general method of synthesis of allylic alcohols from olefins with allylic migration of the double bond. Scheme 9.14 records some specific examples. 

Nickel sulfamate aerospace chemical Chlorotrifluoromethane aerospace components Propylene oxide/allyl glycidyl ether copolymer Tetrafluoroethylene/propylene copolymer aerospace gas component Oxygen... 

RuClg.(PPhg) catalyses the homogeneous oxidation by molecular oxygen of allylic alcohols (e.g., geraniol and nerol)... 

NiCl(allyl)(NHG) (75, NHC = A-heterocyclic carbene) have been prepared in very high yields from the reaction of [Ni(/x-Cl)(77 -allyl)]2 with various NHC (Scheme 23) the solid structure of one derivative has been described. These complexes bind oxygen reversibly and with a fairly high equilibrium constant (Xeq 2,100 100 1 mol ). The resulting Ni-02 adduct decomposes to a hydroxyl-bridged species with the loss of the oxidized allyl moiety K2 4 X 10 0.2 X 10 s ) the half-life of this process is 7 s. The interesting ability of the Ni center in 75 to split oxygen and form C-O bonds is of potential value for Ni-catalyzed aerobic oxidations. 

On a basic oxide, one expects the allylic species, formed by the first H absttac-tion, to be anionic, and tt-bonded to a Lewis acid site. It is then susceptible to an attack at the a-hydrogen by neighboring lattice oxygen. The allyl species can be side-on or end-on bonded depending on the metal. The former may undergo nucleophilic attack and give a ketone or an aldehyde, while the latter may dimerise. 

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