Search

A non-planar triarylborane and a new member of the boratriptycene family bearing a selenium atom in bridgehead position of triptycene scaffold was generated and isolated as a boron-‘ate’ complex paired with a weakly coordinating anion. With similar electronegativity while possessing longer atom radius with respect to sulfur, the introduction a selenium atom, in the form of a selenenium moiety, at the bridgehead of a triptycene scaffold allows a very precise modification of the pyramidalization of the boron atom environment. Experimental and computational evaluation of the Lewis acidity of this new boratriptycene derivative gave qualitative information on how a modification of the pyramidalization of the boron environment affects alone Lewis acidity parameters of such pyramidal triarylborane.

Although boron Lewis acids commonly adopt a trigonal planar geometry, a number of compounds in which the trivalent boron atom is located in a pyramidal environment have been described. This review will highlight the recent developments of the chemistry and applications of non-planar boron Lewis acids, including a series of non-planar triarylboranes derived from the triptycene core. A thorough analysis of the properties and of the influence of the pyramidalization of boron Lewis acids on their stereoelectronic properties and reactivities is presented based on recent theoretical and experimental studies.1 Non-planar Trialkylboranes2 Non-planar Alkyl and Aryl-Boronates3 Non-planar Triarylboranes and Alkenylboranes3.1 Previous Investigations on Bora Barrelenes and Triptycenes3.2 Recent Work on Boratriptycenes from Our Research Group4 Applications of Non-planar Boranes4.1 Non-planar Alkyl Boranes and Boronates4.2 Non-planar Triarylboranes (Boratriptycenes)5 Other Non-planar Group 13 Lewis Acids6 Further Work and Perspectives

Kinetic and thermodynamic measurements of the ease of covalent hydration of 4-nitrobenzodifuroxan (NBDF) to give the corresponding hydroxy adduct has been carried out over a large pH range of 0.82–12.23 in aqueous solution. A most important result is that water is the sole efficient nucleophile contributing to the hydration of this peculiar nitroolefin in the pH range 4–8. Based on this finding as well as a pK a H2O value of 2.85 for the complexation process there is no doubt that the electrophilic character of NBDF falls in the domain of superelectrophilicity defined with reference to covalent nucleophilic additions to 4,6-dinitrobenzofuroxan (DNBF; pK a H2O = 3.75) and related heterocycles. This also corresponds to a positioning of NBDF at the top of the electrophilicity scale E introduced by Mayr to describe the feasibility of nucleophilic-electrophilic combinations. Returning to the hydration of the series of activated olefins, it has been possible to expand the domain of reactivity of Michael acceptors by six orders of magnitude, going from benzylidenemalonitrile (pK a H2O = 10.70; E = –9.42) to the para-nitro-substituted benzylidene Meldrum’ s acid (pK a H2O = 3.46; E = –5.49). The positioning of these olefins on the pK a scale shows that not only 4-nitrobenzodifuroxan but also the unsubstituted Meldrum’s acid are located in the superelectrophilic region.

The structures of the covalent Lewis adducts and/or frustrated Lewis pairs derived from 2- and 2,6-substituted pyridines with diaryl (Ar2CH+) and with the more bulky triaryl (Ar3C+) carbenium ions were analyzed by UV-vis and NMR spectroscopy. Thermodynamics (equilibrium constants) and kinetics (rate constants) of the associations of the carbon-centered Lewis acids Ar2CH+ with a series of sterically hindered pyridines were investigated and used for the determination of the Lewis basicities and nucleophilicities, on the basis of the Mayr electrophilicity/nucleophilicity and Lewis acidity/basicity linear free energy relationships. In addition, methyl and benzhydryl cation affinities were computed to elucidate the respective steric and electronic contributions of the substituents to the nitrogen atom Lewis basicity. The influence of the size of the reference carbenium ion on the magnitude of the repulsion induced by the pyridine substituents (Me, tBu in 2- or 2,6-positions) was also analyzed. Cumulated steric repulsion was found to decrease the reactivity of the nitrogen atom by up to 10 orders of magnitude.