Cristiano Zonta

Substituent effects on cation–π interactions: A quantitative study

A synthetic supramolecular complex has been adapted to quantify cation–π interactions in chloroform by using chemical double-mutant cycles. The interaction of a pyridinium cation with the π-face of an aromatic ring is found to be very sensitive to the π-electron density. Electron-donating substituents lead to a strong attractive interaction (−8 kJ/mol−1), but electron-withdrawing groups lead to a repulsive interaction (+2 kJ/mol−1).

Synthesis of Benzotri(benzonorbornadienes) (BTBNDs): Rigid, Cup-Shaped Molecules with High Electron Density within the Cavity

Cyclotrimerisation of benzo-polycyclic bromostannylalkenes 8a−d with Cu(NO3)2·3H2O in THF affords benzotri(benzonorbornadienes) 3a−d as a mixture of the syn and anti isomers. The ratio of syn to anti is close to the 1:3 statistical value in most cases (i.e. in cyclotrimers 3a,b,d), but highly in favour of the anti isomer in 3c, where steric hindrance by the methoxy groups plays an important role in the stereochemistry of the cyclotrimerisation. The substrates for the cyclotrimerisation, i.e. the bromostannyl alkenes 8a−d, were prepared from bromoalkenes 7a−d by treatment with base (LDA) and quenching with trimethyltin chloride. In turn, bromoalkenes 7a−d were prepared from alkenes 5a−d by radical bromination-elimination. The reaction conditions used were designed to minimise Wagner-Meerwein rearrangements that would lead to unwanted bromo isomers. The cup-shaped syn cyclotrimers 3a−d exhibit high electron density within the cavity as determined by AM1 semiempirical calculations of their electrostatic potential surfaces and are valuable substrates for supramolecular chemistry. As an example, it is shown that fullerene C60 is drawn into solution in acetonitrile by complexation with both the syn and anti trimer of benzonorbornadiene 3a.

Stereochemistry of the cyclotrimerisation of enantiopure polycyclic bromostannylalkenes: Mechanistic considerations on the coupling of alkenyl stannanes by copper(II) nitrate

The cyclotrimerisation of enantiopure 1-bromo-2-trimethylstannylbenzonorbornadiene 2. contrary to the expectations, affords predominantly the trimer anti-4. This observation suggests that the reaction proceeds mainly via a SnSn coupling to produce the dimer anti-5 and a tin-copper product that triggers halogen-metal exchange on the dimer thus allowing a second coupling with the starting reagent eventually leading to the anti- trimer. The little but consistent formation of the isomer syn-4 and meso dimer 5 can arise from a racemisation of the bromine-copper-tin intermediate possibly via an alkyne structure.

anti-Selective Heck-type cyclotrimerization of polycyclic bromoalkenes

Abstract Vinyl halides can react as both substrate and reagent in the Heck reaction. In the case of bicyclic vinyl halides, the reaction leads to cyclotrimers through an anti -selective process.

Synthesis and Structure of D3h-Symmetric Triptycene Trimaleimide

A new D3h symmetric triptycene derivative has been synthesized with the aim of obtaining molecules that are able to assemble into porous structures, and can be used in the development of new ligands. The synthesis involves a Diels-Alder reaction as the key step, followed by an oxidation and the formation of a maleimide ring. Triptycene trimaleimide furnished single crystals which have been analyzed by means of X-ray diffraction.

Palladium-catalysed cyclotrimerisation reactions of polycyclic alkenes under the Stille and Grigg coupling conditions

Preformed or in situ generated polycyclic bromostannylalkenes react under palladium catalysis under Stille or Grigg reaction conditions to afford cyclotrimerised adducts via a three-fold carbon–carbon coupling reaction.