Affecting a Carbonyl Group

2-Acetylbenzotellurophene was reduced to 2-(l-hydroxyethyl)-benzotellurophene by lithium aluminum hydride in diethyl ether. The alcohol was not purified but immediately acetylated.  

2-Formylbenzotellurophene formed a 2,4-dinitrophenylhydrazone (m.p. 268°) and was reduced to 2-methylbenzotellurophene by hydrazine.  

2-Methylbenzotellurophene 1.0 g (3.9 mmol) of 2-formylbenzotellurophene is dissolved in 15 m/ of 1,2-ethanediol, 1.5 ml (1.6 g, 31 mmol) of hydrazine hydrate are added, and the mixture is heated under reflux for 20 min to dissolve the hydrazone that has formed. The mixture is then cooled, 1.0 g of potassium hydroxide is added in small portions, the resultant mixture is heated under reflux for 0.5 h, and then poured into ice/water. The mixture is extracted with diethyl ether, the organic phase is separated, dried with anhydrous sodium sulfate, filtered, and evaporated. The residue is recrystallized from ethanol yield 0.76 g (80%) m.p. 58°. 

2-Carboxybenzotellurophene was reduced by lithium aluminum hydride to 2-hydroxy-methylbenzotellurophene and converted to 2-(chlorocarbonyl)-benzotellurophene.  

2-Hydroxymethylbenzotellurophene A solution of 1.0 g (3.7 mmol) of 2-carboxybenzotellurophenc in toluene is added dropwise to a suspension of 0.2 g (5.3 mmol) of lithium aluminum hydride in tetrahydrofuran and the resultant mixture is heated under reflux on a water bath for 0.5 h. The product is isolated in the usual manner and recrystallized from hexane yield 0.2 g (62%) m.p. 100°. 

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Reactions at an sp1-Carbon Atom 4.4.2.1. Affecting a Carbonyl Group... 

In the preceding chapter you learned that nucleophilic addition to the carbonyl group IS one of the fundamental reaction types of organic chemistry In addition to its own reactivity a carbonyl group can affect the chemical properties of aldehydes and ketones m other ways Aldehydes and ketones having at least one hydrogen on a carbon next to the carbonyl are m equilibrium with their enol isomers... 

The way in which various substituents affect the polarization of a carbonyl group is similar to the way they affect the reactivity of an aromatic ring toward electrophilic substitution (Section 16.5). A chlorine substituent, for example, inductively withdraws electrons from an acyl group in the same way that it withdraws elections from and thus deactivates an aromatic ring. Similarly, amino, methoxvl, and methylthio substituents donate electrons to acyl groups by resonance in the same way that they donate electrons to and thus activate aromatic rings. 

Introduction of a carbonyl group next to a double bond seems to be affected by sterie factors since the eyclo-heptene (19) gives only ketone (2o5 on allylio oxidation with SeOr Synthesis... 

In aryl alkyl ketones, there are two types of C-F bonds to reduce. Fluorine atoms in the a-positions are usually more easily replaced than those on the aromatic ring. It is also possible to reduce a-C-F bonds selectively without affecting the carbonyl group. 

The only known method enabling a selective replacement of a-fluorine substituents in aliphatic polyfluorinated esters without affecting the carbonyl group is pholoinduced reduction (e. g., 1 - 2) the reaction yield is dependent on the solvent (hydrogen donor) used, e. g. propan-2-ol > tetrahydrofuran > 1,3-dioxolane.106 The reactivity of fluorinated esters is also dependent on the group attached to the /(-carbon, e. g. 3 > 4 > 5.106... 

How -elimination reactions and subsequent processes under alkaline conditions affect the carbonyl groups and the Mw of an oxidized cotton linters is shown in Fig. 2. The initial concentration of carbonyls is 30 pmol/g, introduced by hypochlorite treatment at pH 7. The sample was treated with a buffer solution of pH 11 at 40 °C. The Mw suffers a steep decrease within the first 5 hours, the drop in carbonyls proceeds at a significantly lower rate. While the changes in Mw level off after approx. 20 hours of treatment, the carbonyl groups are slightly decreasing further (unpublished results). 

This is quite different to the reactivity of a C=C double bond conjugated with a carbonyl group, which, as you have just seen, reacts with nucleophiles such as cyanide, amines, and alcohols. The conjugated system is different from the sum of the isolated parts, with the C=0 group profoundly affecting the reactivi- true electJ on distribution lies somewhere ty of the C=C double bond. To show why, we can use... 

The last stages arc shown below. The ketone is protected, and the alkene oxidized to a carbonyl group, cleaving off one of the C atoms (you will meet this reaction—ozonolysis—in Chapter 35). The diester can be cyclized by a Claisen ester condensation. The stereogenic centres in the ring are not affected by any of these reactions so a Irans ring junction must result from this reaction. >... 

The alkylthio group in an aromatic ketone may be removed with Raney Ni without affecting the carbonyl group. Thus, treatment of desyl thioethers (a-alkylthiodesoxy-benzoins) with Raney Ni, which had been deactivated by refluxing in acetone,149 gave desoxybenzoins in yields of 55-90% (eq. 13.73).150... 

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