Cyclohexene, addition

Exercise 22-37 A side reaction when reducing benzene derivatives to 1,4-cyclo-hexadienes with lithium or sodium in liquid ammonia is over-reduction to give cyclohexenes. Addition of ethanol greatly reduces the importance of this side reaction. Explain what role ethanol plays in preventing over-reduction. 

Diacetoxylation of 1,3-dienes. Palladium-catalyzed oxidation of 1,3-cyclo-hcxadicnc with bcnzoquinone (used in catalytic amounts with MnO, as the external oxidant) in acetic acid gives a 1 1 mixture of cis- and //-an.v-1,4-diacctoxy-2-cyclohexene. Addition of LiCI or LiOAc has a profound effect on the stereochemistry. Oxidation in the presence of lithium acetate results in selective tran. -diacetoxylation. whereas addition of lithium chloride results in selective r/.v-diacetoxylation (equation I). ... 

Carbohydrate-derived enolones are a particular case of chiral cyclohexenes. Addition of tert-butyl radicals occurs with high stereoselectivity trans to the C-5 substituent at both x- and /f-anomers8I5. The primary adduct radical rearranges through transfer of the benzoyl group and final hydrogen atom abstraction to yield the product as a mixture of isomers at C-2. Treatment of the mixture with pyridine leads to the formation of one enolone wdth elimination of benzoic acid. 

Addition of carbon tetrachloride to cyclohexene. Addition of carbon tetrachloride to cyclohexene catalyzed by this Ru(Il) complex results in formation of franj-l-trichloromethyl-2-chlorocyclohexane (63% yield, pure) as the major product. Catalysis by dibenzoyl peroxide results in a 53 47 mixture of the trans-and cir-adducts (low yield). 

It is reported that unless special precautions are taken for the vacuum distillation of crude -ionylideneacetaldehyde extensive rearrangement is observed and the tricyclic ketone (327) may be isolated in about 50% yield. " The formation of (327) can be rationalized by a scheme involving successive E-Z isomerization, [1, 5] shift of aldehyde hydrogen, and intramolecular cycloaddition of the unsaturated ketene so formed to the cyclohexene part of the molecule. A second product is formed in the course of the reaction and the compound (328) is a tentative suggestion for its structure since a second mode for the ketene-cyclohexene addition is possible. 

What is the stereochemistry of bromination Do the two bromine atoms add from the same side of the double bond (syn, as in catalytic hydrogenation) or from opposite sides (see margin) Let us examine the bromination of cyclohexene. Addition on the same side should give ciA-1,2-dibromocyclohexane the alternative would result in trans-1,2-dibromocyclohexane. The second pathway is borne out by experiment—only anti addition is observed. Because anti addition to the two reacting carbon atoms can take place with equal probability in two possible ways—in either case, from both above and below the TT bond—the product is racemic. 

In a 500 ml. three-necked flask, equipped with a mechanical stirrer, thermometer and dropping funnel, place 300 ml. of 88-90 per cent, formic acid and add 70 ml. of 30 per cent, hydrogen peroxide. Then introduce slowly 41 g. (51 ml.) of freshly distilled cyclohexene (Section 111,12) over a period of 20-30 minutes maintain the temperature of the reaction mixture between 40° and 45° by cooling with an ice bath and controlling the rate of addition. Keep the reaction mixture at 40° for 1 hour after all the cyclohexene has been added and then allow to stand overnight at room temperature. Remove most of the formic acid and water by distillation from a water bath under reduced pressure. Add an ice-cold solution of 40 g. of sodium hydroxide in 75 ml. of water in small portions to the residual mixture of the diol and its formate take care that the tempera... 

The oxidation of simple internal alkenes is very slow. The clean selectiv oxidation of a terminal double bond in 40, even in the presence of an internt double bond, is possible under normal conditions[89,90]. The oxidation c cyclic alkenes is difficult, but can be carried out under selected condition Addition of strong mineral acids such as HCIO4, H2S04 and HBF4 accelerate the oxidation of cyclohexene and cyclopentene[48,91], A catalyst system 0 PdSO4-H3PM06W6Oii(j [92] or PdCF-CuCF m EtOH is used for the oxidatioi of cyclopentene and cyclohexene[93]. 

Diacetoxylation of various conjugated dienes including cyclic dienes has been extensively studied. 1,3-Cyclohexadiene was converted into a mixture of isomeric l,4-diacetoxy-2-cyclohexenes of unknown stereochemistry[303]. The stereoselective Pd-catalyzed 1,4-diacetoxylation of dienes is carried out in AcOH in the presence of LiOAc and /or LiCI and beiizoquinone[304.305]. In the presence of acetate ion and in the absence of chloride ion, /rau.v-diacetox-ylation occurs, whereas addition of a catalytic amount of LiCl changes the stereochemistry to cis addition. The coordination of a chloride ion to Pd makes the cis migration of the acetate from Pd impossible. From 1,3-cyclohexadiene, trans- and ci j-l,4-diacetoxy-2-cyclohexenes (346 and 347) can be prepared stereoselectively. For the 6-substituted 1,3-cycloheptadiene 348, a high diaster-eoselectivity is observed. The stereoselective cij-diacetoxylation of 5-carbo-methoxy-1,3-cyclohexadiene (349) has been applied to the synthesis of dl-shikimic acid (350). 

In MeOH, l,4-dimethoxy-2-cyclohexene (379) is obtainejl from 1,3-cydo-hexadiene[315]. Acetoxylation and the intramolecular alkoxylation took place in the synthesis of the naturally occurring tetrahydrofuran derivative 380 and is another example of the selective introduction of different nucleo-philes[316]. In intramolecular 1,4-oxyacetoxylation to form the fused tetrahy-drofurans and tetrahydropyrans 381, cis addition takes place in the presence of a catalytic amount of LiCI, whereas the trans product is obtained in its absence[317]. The stereocontrolled oxaspirocyclization proceeds to afford the Irons product 382 in the presence of Li2C03 and the cis product in the presence of LiCl[ 318,319]. 

Oxidative dimerization of various 2-benzyloxy-2-thiazoline-5-ones (222) catalyzed by iodine and triethylamine is another example of the nucleophilic reactivity of the C-4 atom (469) (Scheme 112). Treatment of 212 with pyrrolidinocyclohexene yields the amide (223) (Scheme 113). The mechanism given for the formation of 223 is proposed by analogy with the reactivitx of oxazolones with enamines (4701. 4-Substituted 2-phenylthiazol-5(4Hi-ones react with A -morphoiino-l-cyclohexene in a similar manner (562j. Recently. Barret and Walker have studied the Michael addition products... 

Write a structural formula for the compound formed on elec trophilic addition of sulfuric acid to cyclohexene (step 1 in the two step transfer mation shown in the preceding equation)... 

Construct a molecular model of the product formed by anti addition of Br2 to 1 2 dimethyl cyclohexene... 

Section 10 12 Conjugate addition of an alkene (the dienophile) to a conjugated diene gives a cyclohexene derivative in a process called the Diels-Alder reaction It is concerted and stereospecific substituents that are cis to each other on the dienophile remain cis m the product... 

Diels-Alder reaction (Section 10 12) Conjugate addition of an alkene to a conjugated diene to give a cyclohexene denva tive Diels-Alder reactions are extremely useful in synthesis... 

Wilkinson Hyd.rogena.tion, One of the best understood catalytic cycles is that for olefin hydrogenation in the presence of phosphine complexes of rhodium, the Wilkinson hydrogenation (14,15). The reactions of a number of olefins, eg, cyclohexene and styrene, are rapid, taking place even at room temperature and atmospheric pressure but the reaction of ethylene is extremely slow. Complexes of a number of transition metals in addition to rhodium are active for the reaction. 

Photolysis of pyridazine IV-oxide and alkylated pyridazine IV-oxides results in deoxygenation. When this is carried out in the presence of aromatic or methylated aromatic solvents or cyclohexane, the corresponding phenols, hydroxymethyl derivatives or cyclohexanol are formed in addition to pyridazines. In the presence of cyclohexene, cyclohexene oxide and cyclohexanone are generated. 

The addition of the anion of a-bromo-a-nitrotoluerie (564) to cyclohexene gave the hexahydro derivative (565) of 3-phenyl-l,2-benzisoxazole (75TL2131). An unusual hexahydro derivative (566) was produced by the bis addition of benzonitrile N-oxide to benzoquinone (67AHC(8)277). 

In addition to rhodium(III) oxide, cobalt(II) acetylacetonate or dicobalt octacarbonyl has been used by the submitters as catalyst precursors for the hydroformylation of cyclohexene. The results are given in Table I. 

Cyclohexenone has been prepared by dehydrohalogenation of 2-bromocyclohexanone, by the hydrolysis and oxidation of 3-chlorocyclohexene, by the dehydration of a-hydroxycyclohexa- ione, by the oxidation of cyclohexene with chromic acid or hydrogen peroxide in the presence of a vanadium catalyst, by I lie addition of acroleiti to ethyl acetoacctate followed by cycliza-lion, hydroly.sis, and decar])oxylation, by the reduction of N,N-dimelliyliiniline with sodium and ethanol itt liquid ammonia... 

The acetyl chloride obtained is yellow in color, probably because of the presence of the sulfenyl chlorides mentioned above. The addition of cyclohexene will discharge the color (although a darker color develops later) and redistillation then yields a stable water-clear product. The yield of acetyl chloride varies from 60% to 85%, depending on the care with which liquids are transferred and the vapors are trapped. O he amount... 

Moderate stereoselectivity is also seen in the addition of phenoxycarbene to cyclohexene (enby 4), in which the product ratio is apparently influenced by steric factors that favor introduction of the larger group (PhO versus H) in the less crowded exo position. 

Among the cases in which this type of kinetics have been observed are the addition of hydrogen chloride to 2-methyl-1-butene, 2-methyl-2-butene, 1-mefliylcyclopentene, and cyclohexene. The addition of hydrogen bromide to cyclopentene also follows a third-order rate expression. The transition state associated with the third-order rate expression involves proton transfer to the alkene from one hydrogen halide molecule and capture of the halide ion from the second ... 

Cycloaddition involves the combination of two molecules in such a way that a new ring is formed. The principles of conservation of orbital symmetry also apply to concerted cycloaddition reactions and to the reverse, concerted fragmentation of one molecule into two or more smaller components (cycloreversion). The most important cycloaddition reaction from the point of view of synthesis is the Diels-Alder reaction. This reaction has been the object of extensive theoretical and mechanistic study, as well as synthetic application. The Diels-Alder reaction is the addition of an alkene to a diene to form a cyclohexene. It is called a [47t + 27c]-cycloaddition reaction because four tc electrons from the diene and the two n electrons from the alkene (which is called the dienophile) are directly involved in the bonding change. For most systems, the reactivity pattern, regioselectivity, and stereoselectivity are consistent with describing the reaction as a concerted process. In particular, the reaction is a stereospecific syn (suprafacial) addition with respect to both the alkene and the diene. This stereospecificity has been demonstrated with many substituted dienes and alkenes and also holds for the simplest possible example of the reaction, that of ethylene with butadiene ... 

How do orbital symmetry requirements relate to [4tc - - 2tc] and other cycloaddition reactions Let us constmct a correlation diagram for the addition of butadiene and ethylene to give cyclohexene. For concerted addition to occur, the diene must adopt an s-cis conformation. Because the electrons that are involved are the n electrons in both the diene and dienophile, it is expected that the reaction must occur via a face-to-face rather than edge-to-edge orientation. When this orientation of the reacting complex and transition state is adopted, it can be seen that a plane of symmetry perpendicular to the planes of the... 

An orbital correlation diagram can be constructed by examining the symmetry of the reactant and product orbitals with respect to this plane. The orbitals are classified by symmetry with respect to this plane in Fig. 11.9. For the reactants ethylene and butadiene, the classifications are the same as for the consideration of electrocyclic reactions on p. 610. An additional feature must be taken into account in the case of cyclohexene. The cyclohexene orbitals tr, t72. < i> and are called symmetry-adapted orbitals. We might be inclined to think of the a and a orbitals as localized between specific pairs of carbon... 

The addition of bromotrichloromethane to cyclohexene gives a nearly 1 1 mixture of the two possible stereoisomers ... 

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