No abstract

No abstract

1), was prepared in the reaction between 2,2′dilithiobiphenyl and nickelocene in diethyl ether. Compound 1 was transformed to other 9-nickelafluorenyllithium compounds, 2) and (η 5 -(1-(η 5 -cyclopentadienyl))-1nickelafluorenyl)lithium(dimethoxyethane) (3), whose crystal and molecular structures were determined by single-crystal X-ray analysis.

The minimal H O values required for cyclohexene, a-methylstyrene, styrene, 2,4-diphenyl-4-methyl-1-pentene, 4-phenyl-1-butene, tert-butylbenzene and cumene to undergo the chosen test reactions over solid acids at 303 K have been established. The chosen reactions and the respective H O min values obtained are as follows: a-methylstyrene and styrene oligomerization (-3.3 and -5.3), 2,4-diphenyl-4-methyl-1-pentene and 4-phenyl-1-butene oligomerization and isomerization (-6.9 and -7.9), cyclohexene oligomerization (-7.8), tert-butylbenzene disproportionation (-10.8) and cumene disproportionation (-11.5). The above mentioned test reactions were used to study the acid strength of different solid acids.

A relation between the acid strength of the active centers of sulfated alumina catalysts and the results of transformation of styrene dimers was studied. The acid strength measurements were performed according to the test reaction method using a-methylstyrene, styrene, tert-butylbenzene and cumene reactions at 2) allowed to transform them selectively (96 %) into benzene and alkylbenzenes. When the acid strength of the catalyst was higher (H O B -3.5), the selectivity of the reaction changed. The selectivity of benzene and its derivatives diminished (to 65 %) and dicyclic compounds (indane and indene derivatives) appeared (selectivity 21 %) among the reaction products.

Owing to their wide reactivity profile, as well as a broad range of biological activity, organophosphorus compounds play a pivotal role in modern organic chemistry. Recently, much effort has been devoted to their preparation in an enantioselective fashion. This microreview summarizes the use of chiral organic molecules as catalysts in the synthesis of optically active organophosphorus compounds. Particular attention is given to the catalytic activation modes employed in the devised synthetic strategies, with special emphasis on factors governing stereochemical reaction outcomes. Furthermore, the importance of the products obtained and their selected applications are discussed.

(allylic anion generation); c-Al 2 O 3 -Cr: Brønsted acid centers originating from chromium ions at high oxidation states (alkenyl chain protonation) and Cr 3? redox sites (allylic cation generation); SiO 2 -Al 2 O 3 (13 %)-Cr: Brønsted acid centers of silica-alumina (alkenyl chain and aromatic ring protonation) and Cr 3? redox sites (allylic cation generation). The protonation of styrene dimer at the alkenyl chain leads to b-cleavage to toluene and a-methylstyrene as well as to styrene and ethylbenzene. The allylic species formed react to the different styrene dimer isomers and through diphenylbutadiene intermediate to coke. The use of methylcyclohexane as solvent for the styrene dimers in the presence of chromium containing catalysts results in the aromatization reaction leading to toluene and hydrogen. The presence of the later reactant in the system causes styrene and a-methylstyrene hydrogenation to ethylbenzene and cumene.

Reactions of 9-nickelafluorenyllithium with cobalt and nickel pentamethylcyclopentadienyl-acetylacetonates resulted in the formation of the novel nickelacyclic-cobaltocene 2 and nickelacyclic-nickelocene 3, respectively, in which the central metal atom is bonded to the nickelafluorenyl ring. On the basis of their redox propensity, compounds 2 and 3 were oxidized to the corresponding monocations [2](+) and [3](+). The crystal and molecular structures of both the redox couples were determined by single-crystal X-ray analysis. In spite of their structural similarity, they display a rather different electron transfer ability. To throw light on such an aspect, the pertinent redox couples have been examined by EPR spectroscopy and the nature of the frontier orbitals involved in the redox activity of the neutral precursors has been supported by extended Huckel theoretical calculations.