Structures, Lewis Acidities, Electrophilicities, and Protecting Group Abilities of Phenylfluorenylium and Tritylium Ions

Abstract

The isolation, characterization, and the first X-ray structures of a fluorenylium ion and its Lewis adducts with nitrogen- and phosphorus-centered Lewis bases are reported. Kinetics of the reactions of a series of fluorenylium ions with reference π-, σ-, and n-nucleophiles of various sizes and nucleophilicities allowed the interplay between electronic and structural parameters on the electrophilicities of these planarized tertiary carbenium ions to be elucidated. Structure-reactivity correlations and extensive comparisons of their reactivities with those of di- and triarylcarbenium ions are described. Quantitative determination of the electrofugalities of fluorenylium ions revealed to which extent they are complementing tritylium ions as protecting groups and how their tuning is possible. Determination of the equilibrium constants of the Lewis adducts formation between pyridines of calibrated Lewis basicities and phenylfluorenylium and tritylium ions allowed the determination of their Lewis acidities and to showcase the potential of these carbon-centered Lewis acids in catalysis.

Methyl Anion Affinities of the Canonical Organic Functional Groups

“…To date, Mayr and Ofial have reported Mayr E parameters for over 319 electrophiles . The set includes molecules from over 30 different classes of functional groups: (i) empty p orbitals with π conjugation: benzylic cations, − ,− allyl cations, , and the tropylium ion; (ii) empty π* orbitals: ketones, iminium ions, , oxacarbenium ions, sulfacarbenium ions, acyl imines, tosyl imines, phosphoryl imines, quinone methides, , indolylmethylium ions, , alpha, beta-unsaturated ketones, ,,− acrylates, acrylamides, cinnamates, acrylonitriles, alkylidenemalonates and related compounds, − maleates and related compounds, alkylidene malonitriles, nitrostyrenes, cinnamyliminium ion derivatives, − benzyl­idene malonitriles, quinones, , sulfonyl substituted ethylenes, ,, electrophilic arenes, − and azocarboxylates; , and (iii) empty σ* orbitals: chlorinating agents (σ C–Cl * ), electrophilic fluorinating agents (σ N–F * ), and electrophilic trifluoromethylsulfenylating agents (σ X–S * ) . Cationic pi-metal complexes ,,, can be represented as empty p orbitals or empty σ*.…”

Brønsted Acid-Catalyzed Intramolecular Hydroarylation of β-Benzylstyrenes

“…Notably, other tritylium salts, including Ph 3 CBF 4 , Ph 3 CSnCl 5 , and Ph 3 CPF 6 , resulted in little or no conversion of 1a at 75 °C, presumably due to their visible insolubility. No product was detected in the presence of triethylamine (entry 12), which suggests that this variant is catalyzed by the in situ generated Brønsted acid H−TPFPB . Finally, no conversion was observed when the tritylium salt was used at ambient temperature (entry 13).…”

Carbocation Organocatalysis in Interrupted Povarov Reactions to cis‐Fused Pyrano‐ and Furanobenzodihydropyrans

“…On the basis of the above experimental facts and prior work by others,, a plausible mechanism of an interrupted Povarov reaction was proposed (Scheme ). Initially, salicylaldimine 1a is activated by complexation with a Lewis acidic tritylium ion to form intermediate I ; next, complex I is attacked nucleophilically by the electron‐rich alkene 2,3‐dihydrofuran ( 2 ) to form the corresponding intermediate II ; then an intramolecular nucleophilic attack within II from the hydroxyl oxygen to the oxocarbenium carbon gives intermediate III ; finally, proton transfer of intermediate III releases the final product 3 , and the tritylium (Tr + ) is regenerated.…”

“…Albeit carbocations were discovered early in 1901, stable carbocations have been pursued as conceptually attractive highly powerful and tunable organic Lewis acid catalysts only in the last couple of years . Herein, we report triphenylmethylium (tritylium) tetrafluoroborate (TrBF 4 ) as an organocatalyst in the synthesis of cis ‐4‐aminobenzodihydropyrans with excellent yields (> 90 %) within 10 min by a remarkably low catalyst loading (1 mol‐%).…”