Ethers, aryl oxidation

A catalytic enantio- and diastereoselective dihydroxylation procedure without the assistance of a directing functional group (like the allylic alcohol group in the Sharpless epox-idation) has also been developed by K.B. Sharpless (E.N. Jacobsen, 1988 H.-L. Kwong, 1990 B.M. Kim, 1990 H. Waldmann, 1992). It uses osmium tetroxide as a catalytic oxidant (as little as 20 ppm to date) and two readily available cinchona alkaloid diastereomeis, namely the 4-chlorobenzoate esters or bulky aryl ethers of dihydroquinine and dihydroquinidine (cf. p. 290% as stereosteering reagents (structures of the Os complexes see R.M. Pearlstein, 1990). The transformation lacks the high asymmetric inductions of the Sharpless epoxidation, but it is broadly applicable and insensitive to air and water. Further improvements are to be expected. 

Allylic acetates are widely used. The oxidative addition of allylic acetates to Pd(0) is reversible, and their reaction must be carried out in the presence of bases. An important improvement in 7r-allylpalladium chemistry has been achieved by the introduction of allylic carbonates. Carbonates are highly reactive. More importantly, their reactions can be carried out under neutral con-ditions[13,14]. Also reactions of allylic carbamates[14], allyl aryl ethers[6,15], and vinyl epoxides[16,17] proceed under neutral conditions without addition of bases. 

The hydroxyl groups can be alkylated with the usual alkylating agents. To obtain aryl ethers a reverse treatment is used, such as treatment of butynediol toluenesulfonate or dibromobutyne with a phenol (44). Alkylene oxides give ether alcohols (46). 

Decafluorobiphenyl [434-90-2] C F C F (mol wt, 334.1 mp, 68°C bp, 206°C), can be prepared by I Jllmann coupling of bromo- [344-04-7] chloro- [344-07-0] or iodopentafluorobenzene [827-15-6] with copper. This product shows good thermal stabiHty decafluorobiphenyl was recovered unchanged after 1 h below 575°C (270). Decafluorobiphenyl-based derivatives exhibit greater oxidative stabiHty than similar hydrocarbon compounds (271). Therm ally stable poly(fluorinated aryl ether) oligomers prepared from decafluorobiphenyl and bisphenols show low dielectric constant and moisture absorption which are attractive for electronic appHcations (272). 

Ultraviolet radiation causes cleavage of the aryl ether linkage (23). DMPPO undergoes oxidation when exposed to ultraviolet light and oxygen by direct attack on the aromatic ring to produce a variety of ring-cleaved and quinoidal stmctures (24). 

With Phenols. The 2-hydroxylethyl aryl ethers are prepared from the reaction of ethylene oxide with phenols at elevated temperatures and pressures (78,79). 2-Phenoxyethyl alcohol is a perfume fixative. The water-soluble alkylphenol ethers of the higher poly(ethylene glycol)s are important surface-active agents. They are made by adding ethylene oxide to the alkylphenol at ca 200°C and 200—250 kPa (>2 atm), using sodium acetate or... 

Electrochemical oxidation of alkyl aryl ethers results m oxidative dealkylation and coupling of the intermediate radicals ElectrooxidaUon m the presence of hydrogen fluonde salt leads to fluonnated dienones [66] (equation 58)... 

Methyl aryl ethers, such as anisole, are cleaved to iodomethane and a phen-oxide ion by treatment with Li) in hot DMR Propose a mechanism for this reaction. 

The bipyridyl ligand, with 80 %, gives the best catalyst, probably because it affords the best balance between the a-donor and rt-acceptor characters which favours respectively the oxidative addition of the aryl bromide and the reductive elimination of the aryl ether at the nickel center. 

Hammel KE, MD Mozuch, KA Jensen, PJ Kersten (1994) HjOj recycling during oxidation of the arylglycerol P-aryl ether lignin structure by lignin peroxidase and glyoxal oxidase. Biochemistry 33 13349-13354. 

Quideau, S. Ralph, J. A biomimetic route to lignin model compounds via silver(I) oxide oxidation. 1. Synthesis of dilignols and non-cyclic benzyl aryl ethers. Holzforschung 1994, 48, 12-22. 

Allyl aryl ethers undergo accelerated Claisen and [1,3] rearrangements in the presence of a mixture of trialkylalanes and water or aluminoxanes. The addition of stoichiometric quantities of water accelerates both the trimethylaluminum-mediated aromatic Claisen reaction and the chiral zirconocene-catalyzed asymmetric carboalumination of terminal alkenes. These two reactions occur in tandem and, after oxidative quenching of the intermediate trialkylalane, result in the selective formation of two new C-C bonds and one C-0 bond (Eq. 12.70).153 Antibodies have also been developed to catalyze Claisen154 and oxy-Cope155 rearrangements. 

Tietze and coworkers [60] observed a combination of a Heck reaction and a C-H-activation by treatment of the alkyne 6/1-111 with Pd°. These authors aimed at compound 6/1-112, but 6/1-110 was obtained as a single product in high yield (Scheme 6/1.29). It can again be assumed that after oxidative addition a cis-carbopalladation of the triple bond takes place to give an alkenyl Pd intermediate which undergoes the C-H-insertion into the neighboring naphthalene and not into the aryl ether moiety. 

More recently, reductive elimination of aryl ethers has been reported from complexes that lack the activating substituent on the palladium-bound aryl group (Equation (55)). These complexes contain sterically hindered phosphine ligands, and these results demonstrate how steric effects of the dative ligand can overcome the electronic constraints of the reaction.112,113 Reductive elimination of oxygen heterocycles upon oxidation of nickel oxametallacycles has also been reported, but yields of the organic product were lower than they were for oxidatively induced reductive eliminations of alkylamines from nickel(II) mentioned above 215-217... 

The high-valent metal species required for activation of an alkyne has also been generated by the oxidative addition to an allylic or propargylic system. For example, with an allyl aryl ether as the substrate, this type of reaction achieves a cycloisomerization that occurs through an 0- to C-allyl migration (Equation (92)) 323,324 similarly, (9-propargyl derivatives lead to a mixture of allenyl and propargyl products (Equation (93)).325,326... 

Further insight into the carbon-oxygen reductive elimination from Pt(IV) and the involvement of five-coordinate Pt(IV) intermediates has been provided recently. The first direct observation of high-yield C-0 reductive elimination from Pt(IV) was described and studied in detail (50,51). Carbon-oxygen coupling to form methyl carboxylates and methyl aryl ethers was observed upon thermolysis of the Pt(IV) complexes ( P2 )PtMe3(OR) ( P2 =bis(diphenylphosphino)ethane or o-bis(diphenyl-phosphino)benzene OR=carboxylate, aryl oxide). As shown in Scheme 47, competitive C-C reductive elimination to form ethane was also observed. 

The highest enantioselectivity in the dialkyl-substituted olefines has been obtained with the aryl ethers of DHQD 94a and DHQ 94b. With potassium ferri-cyanide as secondary oxidant, it is possible to carry out the reaction at room temperature, and slow addition of the olefins is not required. Under these conditions, the diols can be obtained in 85-90% yield and excellent enantioselectivity. 

Six novel fluorinated poly(aryl ether)s containing 1,4-naphthalene moieties were synthesized in high yield using 2,2-bis[4-( 1 -naphthoxy)phenyl]hexafluoro-propane (1). Oxidative coupling ofl yielded a polymer with high 7, low moisture absorption, and low dielectric constant that could be cast into flexible films. The low dielectric constant and low moisture absorption of 6FNE may make it useful as a dielectric insulator in microelectronics applications. 

Allyl and aryl ethers produce carboxylic esters in good yields (60-80%) upon oxidation by benzyltriethylammonium permanganate in dichloromethane [33], e.g. dibenzyl ether gives benzyl benzoate (80%). 

Complex iron(III) salts are frequently used in oxidative arene coupling reactions and quinone formation and tetra-n-butylammonium hexacyanoferrate(III) has several advantages in it use over more conventional oxidative procedures. When used as the dihydrogen salt, Bu4N[H2Fe(CN)6], it oxidizes 2,6-di-z-buty 1-4-methylphenol (1) to the coupled diarylethane (2), or aryl ethers (3) and (4) (Scheme 10.4), depending on the solvent. It is noteworthy that no oxidation occurs even after two days with the tris-ammonium salt. 

The anodic coupling of aryl ethers is reviewed in Ref. [180]. Aryl ethers are more selectively coupled than phenols for the following reasons The carbon-oxygen coupling is made impossible and the ortho-coupling and the oxidation to quinones become more difficult. A mixture of triflu-oroacetic acid (TFA) and dichloromethane proved to be the most suitable electrolyte [181]. TFA enhances the radical cation stability and suppresses the nucle-ophilicity of water. Of further advantage is the addition of alumina or trifluo-roacetic anhydride [182]. Table 12 compiles representative examples of the aryl ether coupling. 

The grafting of poly(ethylene oxide) chains onto PES and a poly(aryl ether) (PAE) has been used to increase the hydrophilicity of these non-ionic poly-mers. These systems were then blended with SPES. SEM analysis showed... 

The allyl aryl ether 183 is converted into the 2-methylbenzofuran in a one-pot procedure via sequential Pd-catalyzed Claisen rearrangement and oxidative cyclization mediated by stoichiometric quantities of 1,4-benzoqui-none (BQ) (Equation 119) <2005OL3355>. 

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