Regio selectivity

It is also possible to screen catalysts according to their selectivities (regio- and stereoselectivity), their stabilities, and rates, since kinetic investigations can be carried out as well [23]. In addition, the application of especially designed NMR pulse sequences further extends the possibilities of the PHIP method [19]. For all of these applications, the main advantage of PHIP is the remarkable signal enhancement which is combined with a spectroscopic in situ method of observation. 

More selective (types of selectivity chemo-selectivity, regio-selectivity, diastereo-selectivity, and enantio-selectivity)... 

Glycosidase Oligosaccharide formation Anomeric selectivity Regio- and diastereo-selectivity, limited yields... 

Working with Chiral Selectivity Regio-, Stereo-, or Enantioselectivity... 

This section will review the connectivity of some catalytic reaction networks starting from isolated one-product unicyclic mechanisms, connected unicyclic mechanisms showing selectivities (regio-, chemo- and stereoselectivities) and systems possessing more than one uncoupled unicyclic mechanisms. All of these systems exhibit various forms of linear rate increases versus metal loading. Subsequently, mechanisms exhibiting quadratic, linear-quadratic, bilinear and linear-bilinear rate dependences will be explored. CBER systems can exhibit any one of these four complex rate dependences. 

As indicated by radical trap experiments, various types of selectivity (regio-, bond-, and stereo-selectivity), and kinetic and theoretical studies [18-24, 27, 29, 33-40], these peroxidative oxidations of alkanes occur via radical mechanisms. They are believed to proceed via free hydroxyl radical (HO ) that acts as an H-abstractor from the alkane RH to yield the corresponding alkyl radical R. Fast reaction of R with O2 generates the alkylperoxyl radical ROO, which, following known pathways [38, 39, and references therein], lead to the formation of the final alcohol (ROH) and the corresponding ketone via the alkylperoxide ROOH. 

Lipase Fatty acid selectivity Regio- selectivity Properties/applications References... 

The catalytic activation of alkynes and alkenes by (CsR5)Ru complexes has been extensively explored during the past decade and can lead to the creation of carbon-carbon bonds, often via the formation of a ruthenacycle intermediate after an oxidative coupling process. [2+2+2] Cycloadditions, cycloisomerizations, or dienes formation are examples of the versatility of theses complexes. The diversity and the selectivity (regio- and often stereoselectivity) of these reactions, which can proceed under mild conditions [134—136], show the interest of (CsR5)Ru catalysts for new synthetic methodologies and the future potential in organic synthesis. 

Alkene metathesis has significant advantages in natural product synthesis for several reasons. First, alkene moieties broadly exist in numerous natural products, and alkene metathesis allows facile access from the readily available or easily prepared olefins to those that are difficult to access. Second, alkene metathesis reactions either do not produce any by-product or only generate the volatile ethylene. Third, alkenes are relatively stable in the multistep synthesis, and are sufficiently reactive to be used in a wide range of transformations to generate other functionalities under specific reaction conditions. Finally, and most importantly, with the help of effective catalysts, alkene metathesis can provide remarkable selectivities (regio-, chemo-, and stereoselectivity) in the challenging synthetic operations. 

Lipases can be divided into three classes based on their specificity and/or selectivity regio- or positional specific lipases, fatty acid—type specific lipases, and specific lipases for a certain class of acylglycerols (mono-, di-, or triglycerides). In terms of regioselectivity, lipases have been divided into three types sn-1,3-specific (hydrolyze ester bonds in positions R1 or R3), sn-2-specific (hydrolyze ester bond in position R2), and nonspecific (do not distinguish between positions of ester bonds to be cleaved). Most known lipases are 1,3-regiospecific with activity on terminal positions. 

Svntliesis The reactions were actually carried out like tliis (Stereo- and regio-selectivity turn out all right, J. Amer. Chem. Soc.. 1924, 4, 783). 

Oxidation of olefins and dienes provides the classic means for syntheses of 1,2- and 1,4-difunctional carbon compounds. The related cleavage of cyclohexene rings to produce 1,6-dioxo compounds has already been discussed in section 1.14. Many regio- and stereoselective oxidations have been developed within the enormously productive field of steroid syntheses. Our examples for regio- and stereoselective C C double bond oxidations as well as the examples for C C double bond cleavages (see p. 87f.) are largely selected from this area. 

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). 

Hard carbon nucleophiles of organometallic compounds react with 7r-allyl-palladium complexes. A steroidal side-chain is introduced regio- and stereo-selectively by the reaction of the steroidal 7T-allylpalladium complex 319 with the alkenylzirconium compound 320[283]. 

The allylic esters 189 and 191 conjugated with cyclopropane undergo regio-selective reactions without opening the cyclopropane ring. The soft carbon nucleophiles are introduced at the terminal carbon to give 190, and phenylation with phenylzinc chloride takes place on the cyclopropane ring to form 192[120]. 

Ailyl enol carbonates derived from ketones and aldehydes undergo Pd-cat-alyzed decarboxylation-elimination, and are used for the preparation of a, /3-unsaturated ketones and aldehydes. The reaction is regiospecific. The regio-isomenc enol carbonates 724 and 726, prepared from 723, are converted into two isomeric enones, 725 and 727. selectively. The saturated aldehyde 728 can be converted into the a,/3-unsaturated aldehyde 730 via the enol carbonate 729[459]. 

Carbonylation of alkenes bearing suitable functional groups proceeds regio-selectively. Carbonylation of vinylsilane and / -substituted vinylsilanes pro-... 

The mechanistic complexity of hydroboration-oxidation stands m contrast to the simplicity with which these reactions are carried out experimentally Both the hydrobo ration and oxidation steps are extremely rapid reactions and are performed at room tern perature with conventional laboratory equipment Ease of operation along with the fact that hydroboration-oxidation leads to syn hydration of alkenes and occurs with a regio selectivity opposite to Markovmkov s rule makes this procedure one of great value to the synthetic chemist... 

Hydrogen bromide is unique among the hydrogen halides m that it can add to alkenes either by electrophilic or free radical addition Under photochemical conditions or m the presence of peroxides free radical addition is observed and HBr adds to the double bond with a regio selectivity opposite to that of Markovmkov s rule... 

Tertiary, benzyl, and aHyhc nitro compounds can also be used as Friedel-Crafts alkylating agents eg, reaction of (CH2)3CN02 (2-nitro-2-methyl propane [594-70-7]) with anisole in the presence of SnCl gives 4-/-butylanisole [5396-38-3] (7). SoHd acids, such as perfluorodecanesulfonic acid [335-77-3], and perfluorooctanesulfonic acid [1763-23-1] have been used as catalysts for regio-selective alkylations (8). 

Preformed Carbocationic Intermediates. Propargyl cations stabilized by hexacarbonyl dicobalt have been used to effect Friedel-Crafts alkylation of electron-rich aromatics, such as anisole, /V, /V- dim ethyl a n il in e and 1,2,4,-trimethoxybenzene (24). Intramolecular reactions have been found to be regio and stereo-selective, and have been used ia the preparatioa of derivatives of 9JT- uoreaes and dibenzofurans (25). 

Carbocations generated from alkanes using superacids react with carbon monoxide under mild conditions to form carboxyUc acid (188). In this process isomeric carboxyUc acids are produced as a mixture. However, when the reaction is mn with catalytic amounts of bromine (0.3 mmol eq) in HF-SbF solution, regio-selective carboxylation is obtained. / -Propane was converted almost exclusively to isobutyric acid under these conditions. 

Displacement of the sulfinyl group of penems (85), obtained by regio selective oxidation of (74, R = R" = CH2CHCH2) (Fig. 5)... 

Under conditions similar to those already outlined, stable aziridin imine derivatives, e.g. (422) and (423), can be prepared in excellent yields (70-80%) by treating the appropriate a-bromoamidines (easily accessible from the amide precursor) with potassium t-butoxide in ether <70AG(E)38l). At low temperatures the elimination proceeds with high regio- and stereo-selectivity at -40 °C (421) yields predominantly (422). 

The success of the halo ketone route depends on the stereo- and regio-selectivity in the halo ketone synthesis, as well as on the stereochemistry of reduction of the bromo ketone. Lithium aluminum hydride or sodium borohydride are commonly used to reduce halo ketones to the /mm-halohydrins. However, carefully controlled reaction conditions or alternate reducing reagents, e.g., lithium borohydride, are often required to avoid reductive elimination of the halogen. 

It IS likely that the syn selectivity exhibited in cycloadditions of fluoroallene IS due to electrostatic interactions [23 25] As in the case of difluoroallene the reactions of fluoroallene with diazoalkanes and nitrile oxides are facile, but such reactions, other than that shown in equation 18, are neither regio nor stereospeutic [23, 25] Indeed, the addition of phenylnitrile oxide to fluoroallene occurs with preferential anti addition for both regioisomenc products (equation 20)... 

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