ABSTRACT
The nucleophilicity and Lewis basicity of sterically hindered phosphines, widely used in catalysis and in frustrated Lewis pair (FLP) chemistry, have been quantified by determining the rates and equilibrium constants of their associations with reference systems (benzhydrylium and tritylium ions) of calibrated electrophilicities and Lewis acidities. These structure-reactivity investigations allow a rationalization of the Lewis acid-base interactions all along the way from covalent Lewis adducts to FLPs. Comparisons of the association of phosphines of increasing sizes (Ph3 P, (o-tolyl)3 P, and tBu3 P) with the triarylborane B(C6 F5 )3 and with the isoelectronic tritylium ions Ar3 C+ provide detailed insights for the future fine-tuning of the reactivities of FLPs. As a proof of concept, tritylium-ion-derived FLPs were shown to react with alkynes, as reported for the FLPs derived from the benchmark triarylborane B(C6 F5 )3 .
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.
ABSTRACT
Eight substituted aryl(indol-3-yl)methylium tetrafluoroborates 3(a-h)-BF4 and three bis(indol-3-yl)methylium tetrafluoroborates 3(i-k)-BF4 have been synthesized and characterized by NMR spectroscopy and X-ray crystallography. Their reactions with π-nucleophiles 8(a-j) (silylated enol ethers and ketene acetals) were studied kinetically using photometric monitoring at 20 °C. The resulting second-order rate constants were found to follow the correlation log k(20 °C) = sN(N + E), in which nucleophiles are characterized by the two solvent-dependent parameters N and sN, and electrophiles are characterized by one parameter, E. From the previously reported N and sN parameters of the employed nucleophiles and the measured rate constants, the electrophilicities of the indol-3-ylmethylium ions 3(a-k) were derived and used to predict potential nucleophilic reaction partners. A discrepancy between published rate constants for the reactions of morpholine and piperidine with the (2-methylindol-3-yl)phenylmethylium ion 3h and those calculated from E, N, and sN was analyzed and demonstrated to be due to a mistake of the value reported in the literature.
