Synthesis of Heterocycle-Containing 9,9-Diarylfluorenes Using Superelectrophiles.

A superacid-promoted method for the synthesis of 9,9-diarylfluorenes is described. The chemistry involves cyclizations and arylations with biphenyl-substituted heterocyclic ketones and a mechanism is proposed involving superelectrophilic intermediates. The key reactive intermediates-dicationic and trication fluorenyl cations have been observed by low-temperature NMR and the mechanism has been further studied using DFT calculations.

Intramolecular Conjugate Additions with Heterocyclic Olefins.

The intramolecular reactions of olefinic N-heterocycles have been studied. In triflic acid-promoted reactions, conjugate addition is observed with pyrazine-, 2-pyrimidine-, and 2-quinoxaline-based olefins and a phenyl group nucleophile. Markovnikov addition is observed with pyridine and 5-quinoxaline-based olefins. These results are in accordance with previous observations relating the type of addition-conjugate or Markovnikov-to the positions of olefinic substituents of the N-heterocycle.

Cyclodehydrations leading to indene products having N-heterocyclic substituents.

In this Note, we describe superacid-promoted cyclodehydrations leading to functionalized indenes. The product indenes are synthesized having N-heterocyclic substituents, including pyridyl, imidazolyl, pyrimdyl, and other groups. A mechanism is proposed involving dicationic, superelectrophilic intermediates. The protonated N-heterocyclic rings are shown to dramatically lower the LUMO energy level of the carboxonium electrophile (compared to that of a similar uncharged system).

Superacid-promoted additions involving vinyl-substituted pyrimidines, quinoxalines, and quinazolines: mechanisms correlated to charge distributions.

The superacid-promoted reactions of vinyl-substituted N-heterocycles have been studied. Diprotonated pyrimidines, quinoxalines, and quinazolines exhibit an unusual regioelectronic effect that controls the type of addition reaction observed. Depending on the ring position of the vinyl substituent, either conjugate addition or Markovnikov addition occurs. The mode of addition has been shown to correlate well to NBO calculated charges.

Superelectrophiles and the effects of trifluoromethyl substituents.

Trifluoromethyl-substituted superelectrophiles were generated in superacid (CF(3)SO(3)H), and their chemistry was examined. The strong electron-withdrawing properties of the trifluoromethyl group are found to enhance the electrophilic character at cationic sites in superelectrophiles. This leads to greater positive-charge delocalization in the superelectrophiles. These effects are manifested by the unusual chemo-, regio-, and stereoselectivities shown by the superelectrophiles in chemical reactions.