Benzenes reaction with dienes

The Pt(CH2 = CH2)(PPh3)2-catalyzed dehydrogenative double silylation of olefins and dienes with o-bis(dimethylsilyl)benzene was also examined by Tanaka and co-workers.61 The major product of the reaction with dienes, such as isoprene and penta-1,2-diene, is a result of 1,2-addition to the less substituted double bond. The reaction pathway for simple alkenes, shown in Eq. (19), appears to be dependent on the alkene substrate and, in some cases, on reaction temperature. Products resulting from 1,2-addition, 1, and 1,1-addition, 2, are detected for various substrates. In addition, hydrosilylation may occur to give the simple hydrosilylated product, 3, or a by-product, 4, derived from 1,4-migration of a methyl group in 3. 

The reaction with dienes can be carried out either in benzene at 60-80 °C using a,a -dibromo ketone, diene, and Fe2(CO)g... 

The process may be used to form linear polymers. Nuyken and Volkel224 225 described a method for tclcchclic production, based on the radical initiated reaction of difunctional transfer agents with dienes (e.g. divinyl benzene (13), dimethacrylate esters). However, currently the most common use of thiol-ene... 

The synthesis of chaparrinone and other quassinoids (naturally occurring substances with antileukemic activity) is another striking example [16a-c]. The key step of synthesis was the Diels-Alder reaction between the a,/l-unsaturated ketoaldehyde 1 (Scheme 6.1) with ethyl 4-methyl-3,5-hexadienoate 2 (R = Et). In benzene, the exo adduct is prevalent but it does not have the desired stereochemistry at C-14. In water, the reaction rate nearly doubles and both the reaction yield and the endo adduct increase considerably. By using the diene acid 2 (R = H) the reaction in water is 10 times faster than in organic solvent and the diastereoselectivity and the yield are satisfactory. The best result was obtained with diene sodium carboxylate 2 (R = Na) when the reaction is conducted 2m in diene the reaction is complete in 5h and the endo adduct is 75% of the diaster-eoisomeric reaction mixture. 

Even simple dienes and polyenes are difficult to classify in comparison with alkenes. Whereas bromination, oxidation and reaction with tetranitromethane (TNM) can identify the number of double bonds and their location in the molecular structure, conjugated double bonds produce very complex mixtures. Furthermore, many of the tests based on 7r-complexation can also apply for aromatic moieties. An example is the TNM 7r-complex which is yellow with benzene and orange with naphthalene and the tests are therefore non-specific. 

The presence of hetero-atoms within the system, remote from the alkene double bonds, does not have an adverse influence on the SET processes that occur. Thus irradiation of the diene 33 in benzene solution with 1,4-dicyanonaphthalene as the electron-transfer sensitizer affords the cyclobutane 34 in 78% yield. Various examples of the reaction were described giving cyclobutane derivatives in 54-69% yield. Benzene, or an arene solvent, is vital for the success of the reaction. When acetonitrile is used, allylation of the sensitizer (akin to the photo-NOCAS reaction) results in the formation of the three products 35-3718. (2 + 2)-Cyclization of this type described for 33 is also seen with the dialkenyl ether 38. When 38 is irradiated using X > 350 nm or X > 450 nm in acetonitrile... 

Regio- and stereoselective dihydroxylation of dienes functionalized at the allylic position with a benzene sulfone group has been reported42. Osmylation of dienic sulfones 33, a potential key synthon for forskolin, occurred exclusively on the A6-7 double bound and preferentially from the a-face of the traws-fused bicyclic molecule, presumably due to a combination of steric and electronic factors (equation 25). While the reaction of diene sulfones proceeded sluggishly under catalytic conditions, treatment of 33a with a stoichiometric amount of OSO4 resulted in quantitative yield of diastereomeric diols 34a and 35 in a 9 1 ratio, respectively. Protecting the hydroxy group of the dienol as its t-butyldimethylsilyl ether (33b) affords diol 34b exclusively. 

From the investigation of all these data it is clear that the aromaticity of phosphinine is nearly equal to that of benzene. Their chemical reactivity, however, is different. Most important is the effect of the in-plane phosphorus lone pair, which (i) is able to form a complex and (ii) can be attacked by electrophiles to form A -phosphinines. Thus, electrophilic substitution reaction on the ring carbon is impossible. In Diels—Alder reactions, phosphinines behave as dienes, providing similar products to benzene but under less forcing condition (the reaction with bis(trifluoromethyl) acetylene takes place at 100 °C with 3, while for benzene 200 °C is required). 

Isopropenylbenzofuran (124, Scheme 30) affords good yields of the adducts 123 and 125 on separate reaction with maleic anhydride and tetracyanoethylene. With but-3-en-2-one, 2-isopropenylbenzofuran (124, Scheme 31) affords the adducts 126 and 127 in a combined yield of 29%. When the crude product was dehydrogenated with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone in boiling benzene, the aromatized product 128 (6%) was obtained. It was accompanied by the dicyanodibenzofuran 129, which was found to arise from the excess diene present in the reaction mixture. A speculative mechanism is shown. 

Sundaram and co-workers [90TL(31)7357] prepared 1-methoxycarbonyl-substituted diene 354 and reported its reaction with dimethyl acet-eylenedicarboxylate in refluxing benzene affording cycloadduct 355 in 72% yield (Scheme 78) there is no comment in the report on the aromatiza-tion step, which should imply loss of both dimethylamino and methoxycar-bonyl groups. Similarly, the synthesis of pyrimidinone 356 (96% yield) was described to occur when 354 was stirred with phenylketene at room temperature. 

Reaction of pentafluoro-substituted benzene derivatives with xenon difluoride/boron trifluoride results in 1,4- and 1,2-fluorine addition with regiospecificity depending on the substituent.41 For example, the heptafluorocyclohexa-1,4-dienes 1 can be obtained in this way in high yield. 

The tetrafluorocobaltates of the alkali metals show12 considerable differences in their reactions with benzene. Lithium tetrafluorocobaltate(III) at 100-130 C gives 3,3,6,6-tetra-fluorocyclohexa-1,4-diene (13) of over 90% purity. This compound has long been postulated1,549 as a major intermediate in the fluorination with cobalt(III) fluoride. The sodium, potassium, and rubidium salts give similar product mixtures (ca. 8 compounds), most being polyfluoroenes (e.g. 14, 8% 15, 12% 16, 35%). 

In early studies of these reactions, the turnover efficiency was not always high, and stoichiometric amounts of the promoters were often necessary to obtain reasonable chemical yields (Scheme 105) (256). This problem was first solved by using chiral alkoxy Ti(IV) complexes and molecular sieves 4A for reaction between the structurally elaborated a,/3-unsaturated acid derivatives and 1,3-dienes (257). Use of alkylated benzenes as solvents might be helpiul. The A1 complex formed from tri-methylaluminum and a C2 chiral 1,2-bis-sulfonamide has proven to be an extremely efficient catalyst for this type of reaction (258). This cycloaddition is useful for preparing optically active prostaglandin intermediates. Cationic bis(oxazoline)-Fe(III) catalysts that form octahedral chelate complexes with dienophiles promote enantioselective reaction with cyclopentadiene (259). The Mg complexes are equally effective. 

However, benzene and its derivatives can be reduced to cyclohexa-dienes by solutions of metals such as Li, Na, K, Zn, and Hg in a weakly acidic solvent, such as liquid ammonia, amines, or ether-alcohol mixtures. This general type of reaction is known as the Birch reduction after the Australian chemist, A. J. Birch. With benzene, reduction with metals leads to 1,4-cyclo-hexadiene ... 

The transient thioaldehydes can also be trapped with dienes giving the corresponding Diels-Alder adducts in good yields. These reactions are carried out in toluene under reflux. Milder conditions (reflux in dichloromethane or benzene) have been reported by Sato and Satoh for a similar transformation which involves cyclic polysulfides instead of elemental sulfur238 (equation 18). The aldehydes obtained were trapped in Diels-Alder reactions. 

Vinylpyrroles 42 having electron-withdrawing substituents on the vinyl group are poor dienes for cycloaddition reactions with acetylenic esters. General reaction conditions such as heating at reflux in benzene or carbon... 

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