Selective hydroxylation

The efficiency of n-octane hydroxylation was optimized and a mutant called variant 139-3 was found to perform this oxygenation 38 times faster than the wild-type enzyme. This variant is also very active for other substrates such as propane, hexane, cyclohexane, heptane, nonane and decane. Even ethane can be hydroxylated selectively with this engineered biocatalyst [121]. First reports on the control of... 

The Mo(VI)-peroxide complex, MoOs(Py)(HMPA), has been used424 to hydroxylate selectively enolizable esters, lactones, and ketones, presumably via epoxidation of the enolate ... 

This catalyst is known as a highly active catalytic system for the nonradical hydroxylation reaction of adamantane, in which turnovers per hour of 48,000 are observed and only C3-H is hydroxylated selectively. Its high activity makes the oxidation of the stable C-H bond of benzene possible. However, it was impossible to terminate the oxidation at phenol, and quinone was the final product observed. 

Microderivatization or chemical modification of analytical samples can be useful in enhancing the ion response, and hence the LC-MS sensitivity, for any particular analyte. In the case of alkaloids, which are already capable of ready ionization, chemical modification can be more valuable in elucidating structures by providing tandem MS evidence for positions of substitution in the parent molecule. Such derivatization or chemical modification for HPLC-MS can include H/D exchange by using deuterated mobile phases, Jones oxidation of aliphatic hydroxyls, selective acetylation of hydroxyl and amine groups, and N-oxide reduction [19,20]. 

Figure 1 shows the evolution of the selectivity to oxidation in the ring and in the alkyl chain during the first hours of the reaction. A very high selectivity to side-chain oxidation is observed in methanol - up to 97 % at the 4th hour. The main product is 1-phenylethanol which amounts to 93 % of the entire conversion. The maximal conversion of ethylbenzene to 1-phenylethanol achieved after 4 h reaction is 4.7 % (Fig.2). The ring hydroxylation selectivity...

Qualitatively, the selectivity of ethylbenzene oxidation in ethanol is the same as in methanol. However, in ethanol the ring hydroxylation selectivity increases up to 12 % (with more than 90 % p-selectivity), and selectivity to 1-phenylethanol decreases, especially for the reaction in dilute solution. 

The catalytic reaction is performed by varying the peroxide and catalyst concentrations. The TON (Fig. 3) increases using a smaller amoimt of the catalyst and for 1.0 mmol/1 Ti ions (1/4 of the standard amount) TON 35.6 is greater than in methanol (with 4.1 mmol/1 Ti ions). Ring hydroxylation selectivity in the experiments with lower catalyst concentration is lower, compared to the reaction under the standard conditions, but higher than that in methanol. 

Phenols can be hydroxylated selectively (9.10), reactions that are hard to accomplish by chemical means.157... 

Having established the identity of synthetic material, Gates turned to the introduction of a Cg oxygen functionality. A series of unsuccessful studies culminated in a low yielding acid-catalyzed hydroxylation. Selective demethylation and oxidation gave (5-dihydrothebainone 54, Scheme 3, and most other subsequent formal syntheses intercept... 

Figure 13 Hydroxylation selectivity of a catalyst appended to a hydrogen bond receptor, (a) Molecular model showing the preorganization of a H-bound ibuprofene molecule next to a dinuclear Mn-(0)2-Mn reactive site, leading to its regioselective hydroxylation. (b) Proposed mechanism for the rationalization of the hydroxylation stoeoselectivity. ...

Toluene, tert-Butyl Benzene Selective aromatic-ring hydroxylation of benzene and alkylbenzenes to produce corresponding phenols over Pt-loaded Ti02 photocatalyst can be conducted by using water molecule as an oxidant The hydroxylation selectivity is 97 % for toluene and more than 99 % for tert-butyl benzene after photoirradiation for 4 h. The phenols are hardly converted further, resulting in the high selectivity [4]. 

A pletliora of different SA systems have been reported in tire literature. Examples include organosilanes on hydroxylated surfaces, alkanetliiols on gold, silver, copper and platinum, dialkyl disulphides on gold, alcohols and amines on platinum and carboxyl acids on aluminium oxide and silver. Some examples and references can be found in [123]. More recently also phosphonic and phosphoric esters on aluminium oxides have been reported [124, 125]. Only a small selection out of tliis number of SA systems can be presented here and properties such as kinetics, tliennal, chemical and mechanical stability are briefly presented for alkanetliiols on gold as an example. 

Periodic acid has a selective oxidising action upon compounds having two hydroxyl groups or a hydroxyl and an amino group attached to adjacent carbon atoms and is characterised by the cleavage of the carbon - carbon bond (Malaprade reaction) ... 

The hydroxyl groups of glucose (and, of course, other saccharides) must be regio- and stereo-selectively attacked, if this most abundant natural carbon compound is to be used as starting material. We shall first show with a few selected examples, how this can be achieved (A.H. Haines, 1976 J. Lehmann, 1976 L. Hough, 1979). 

The benzylidene derivative above is used, if both hydroxyl groups on C-2 and C-3 are needed in synthesis. This r/vzns-2,3-diol can be converted to the sterically more hindered a-cpoxide by tosylation of both hydroxy groups and subsequent treatment with base (N.R. Williams, 1970 J.G. Buchanan, 1976). An oxide anion is formed and displaces the sulfonyloxy group by a rearside attack. The oxirane may then be re-opened with nucleophiles, e.g. methyl lithium, and the less hindered carbon atom will react selectively. In the following sequence starting with an a-glucoside only the 2-methyl-2-deoxyaltrose is obtained (S. Hanessian, 1977). 

A similar intramolecular oxidation, but for the methyl groups C-18 and C-19 was introduced by D.H.R. Barton (1979). Axial hydroxyl groups are converted to esters of nitrous or hypochlorous acid and irradiated. Oxyl radicals are liberated and selectively attack the neighboring axial methyl groups. Reactions of the methylene radicals formed with nitrosyl or chlorine radicals yield oximes or chlorides. 

Selectivity is not an issue m the conversion of alcohols to alkyl halides Except for certain limitations to be discussed m Section 8 15 the location of the halogen sub stituent m the product corresponds to that of the hydroxyl group m the starting alcohol... 

I60C-Hydroxy Derivatives of Gorticoids and their Acetonides. The preparation of 16a-hydroxy-9a-fluoroprednisolone (48) from the 3,20-bisethylene ketal of hydrocortisone acetate (49) has been reported (73). The latter was dehydrated with thionyl chloride in pyridine to yield the 4,9(11),16-triene (50). The 16,17-unsaturated linkage was selectively hydroxylated with OsO /pyridine to yield the 16a,17a-diol (51), which was converted... 

Activators enhance the adsorption of collectors, eg, Ca " in the fatty acid flotation of siUcates at high pH or Cu " in the flotation of sphalerite, ZnS, by sulfohydryl collectors. Depressants, on the other hand, have the opposite effect they hinder the flotation of certain minerals, thus improving selectivity. For example, high pH as well as high sulfide ion concentrations can hinder the flotation of sulfide minerals such as galena (PbS) in the presence of xanthates (ROCSS ). Hence, for a given fixed collector concentration there is a fixed critical pH that defines the transition between flotation and no flotation. This is the basis of the Barsky relationship which can be expressed as [X ]j[OH ] = constant, where [A ] is the xanthate ion concentration in the pulp and [Oi/ ] is the hydroxyl ion concentration indicated by the pH. Similar relationships can be written for sulfide ion, cyanide, or thiocyanate, which act as typical depressants in sulfide flotation systems. 

Pentafiuoropyridine—hexafiuorobenzene working fiuids show the requisite stabiUty at 382°C for automotive Rankiae-cycle power units (427). Hydroxyl and related functions ia steroids can be selectively protected as tetrafiuoro-4-pyridyl ethers by pentafiuoropyridine (428). 

The aromatic core or framework of many aromatic compounds is relatively resistant to alkylperoxy radicals and inert under the usual autoxidation conditions (2). Consequentiy, even somewhat exotic aromatic acids are resistant to further oxidation this makes it possible to consider alkylaromatic LPO as a selective means of producing fine chemicals (206). Such products may include multifimctional aromatic acids, acids with fused rings, acids with rings linked by carbon—carbon bonds, or through ether, carbonyl, or other linkages (279—287). The products may even be phenoUc if the phenoUc hydroxyl is first esterified (288,289). 

During the 1980s few innovations were disclosed in the Hterature. The hydroxylation of phenol by hydrogen peroxide has been extensively studied in order to improve the catalytic system as well as to master the ratio of hydroquinone to catechol. Other routes, targeting a selective access to one of the dihydroxyben2enes, have appeared. World production capacities according to countries and process types are presented in Table 1. 

Other Methods. A variety of other methods have been studied, including phenol hydroxylation by N2O with HZSM-5 as catalyst (69), selective access to resorcinol from 5-methyloxohexanoate in the presence of Pd/C (70), cyclotrimerization of carbon monoxide and ethylene to form hydroquinone in the presence of rhodium catalysts (71), the electrochemical oxidation of benzene to hydroquinone and -benzoquinone (72), the air oxidation of phenol to catechol in the presence of a stoichiometric CuCl and Cu(0) catalyst (73), and the isomerization of dihydroxybenzenes on HZSM-5 catalysts (74). 

Because lactic acid has both hydroxyl and carboxyl functional groups, it undergoes iatramolecular or self-esterificatioa and forms linear polyesters, lactoyUactic acid (4) and higher poly(lactic acid)s, or the cycUc dimer 3,6-dimethyl-/)-dioxane-2,5-dione [95-96-5] (dilactide) (5). Whereas the linear polyesters, lactoyUactic acid and poly(lactic acid)s, are produced under typical condensation conditions such as by removal of water ia the preseace of acidic catalysts, the formation of dilactide with high yield and selectivity requires the use of special catalysts which are primarily weakly basic. The use of tin and ziac oxides and organostaimates and -titanates has been reported (6,21,22). 

Weak base resins when in the free base (hydroxyl) form are not capable of splitting neutral salts such as sodium chloride. Salt forms of weak base resins release anions to the Hquid phase if other ions for which the resin has a greater selectivity are present. 

Facilitated transport membranes can be used to separate gases membrane transport is then driven by a difference in the gas partial pressure across the membrane. Metal ions can also be selectively transported across a membrane driven by a flow of hydrogen or hydroxyl ions in the other direction. This process is sometimes called coupled transport. 

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