- Dienes as Possible Intermediates in the Catalytic Hydrogenation of Aromatic Hydrocarbons. 1. Dienes Derived from 1,4-Di-tert-butylbenzene and a Rhodium Catalyst
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The evolution of the products upon hydrogenating three diene derivatives of 1,4-di-tert-butylbenzene (1) is compared with the formation of 1,4-di-tert-butylcyclohexene (2) and cis- and trans-1,4-di-tert-butylcyclohexane (3 and 4) from 1 on an alumina-supported rhodium catalyst to determine which diene (or dienes), upon adsorption on the catalyst, best represents the structure of the intermediate formed in the rate-determining surface reaction of 1.Of the dienes 1,4-di-tert-butyl-1,4-cyclohexadiene (6), 1,4-di-tert-butyl-1,3-cyclohexadiene (7), and 2,5-di-tert-butyl-1,3-cyclohexadiene (8), the last exhibits best the properties expected if it forms the same adsorbed intermediate as does 1 at the same hydrogen pressure.Unlike the arene, however, the dienes do not yield the cis-saturated isomer 3 as an initial product at low hydrogen pressures.Instead, cis-3,6-di-tert-butylcyclohexene (5) is formed along with ene 2 and arene 1; the dienes tend to exclude the cycloalkenes from the catalyst, their effectiveness increasing in the order 6 7 8.The result indicates that at low pressures little of arene 1 (less than 5percent) is transformed directly to cis-1,4-di-tert-butylcyclohexane (3); instead enes 2 and 5 are produced initially, and because the concentration of the intermediate adsorbed dienes remains low during the hydrogenation of the arene, cis ene 5 is rapidly converted to 3 in the presence of the arene.At high hydrogen pressures, the cis ane 3 is an initial hydrogenation product of the dienes as it is of 1.
- Outlaw, James F.,Cozort, James Ray,Garti, Nissim,Siegel, Samuel
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- Reactions of Alkynyldihaloboranes with 1,3-Dienes. 1,4-Alkynylborations and Stepwise Diels-Alder Reactions
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Alkynyldihaloboranes 1-6 are readily generated in situ from the boron-tin exchange reaction of BCl3 or BBr3 with the corresponding alkynylstannanes. The Diels-Alder reactions of 1-4 with isoprene in hexanes proceed rapidly at 25 °C, affording 1,4-cyclohexadiene products in high yield with high regioselectivity. Reactions carried out in CH2Cl2 exhibited an alternative product that results from the formal 1,4-alkynylboration of the diene. The alkynylboration intermediates can undergo further conversion to the Diels-Alder adducts under the reaction conditions. The mechanism of these reactions is discussed.
- Leung, Shun-Wang,Singleton, Daniel A.
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