Electrostatically Enhanced 3- and 4-Pyridyl Borate Salt Nucleophiles and Bases

Abstract

A variety of electrostatically enhanced 3-and 4pyridylborate salt catalysts are reported and show significant improvement over an activated noncharged neutral control compound. Their nucleophilicity in a stoichiometric S N 2 reaction and catalytic performance in a urethane synthesis are evaluated along with three methods for rapidly evaluating the basicity of these species. That is, qualitative titrations in CH 2 Cl 2 and CHCl 3 were carried out, two separate solution-state IR studies in CCl 4 and CDCl 3 are reported, and the proton affinities of the anionic components of the salts were computed. Charge differences between the anion and its protonated zwitterionic conjugate acid are evaluated along with the highest occupied molecular orbitals of the anions in relationship to some of the surprising reactivity findings that were observed in the two kinetic studies.

Anion-Activated Bases and Nucleophiles Characterized by Photoelectron Spectroscopy

“…Structures for 1 − −6 − were previously examined and provided the starting geometries for the optimizations and single-point energy calculations carried out in this work. 32 Interestingly, M06-2X/aug-cc-pVTZ geometries of the neutral structures derived from 1 − and 2 − have elongated C−B distances of 2.739 and 2.742 Å compared to 1.635 and 1.636 Å bond lengths in their respective anions (Figure 2). The BF 3 substituent also flattens out in the radical and becomes almost planar with FBFF dihedral angles of 118.6 and 175.3°in 1 − and 1 • , respectively, and 119.4 and 175.4°for 2 − and 2 • .…”

“…Geometry optimizations were carried out using the M06-2X − hybrid functional along with the aug- cc -pVDZ basis set starting with previously reported structures, and subsequently were reoptimized with the larger aug- cc -pVTZ basis set . Vibrational frequencies were computed for all of the aug- cc -pVDZ and some of the aug- cc -pVTZ structures to obtain zero-point energies and to confirm that they correspond to stationary points on the potential energy surface.…”

“…It also reacts 6 times faster than DMAP in their base-catalyzed urethane forming reaction with 1-butanol and 4-methylphenylisocyanate (eq 2). , These compounds represent a new class of reagents, and to increase the reactivity of such species, the interaction of the oppositely charged ions presumably needs to be weakened. Therefore, we decided to probe where the excess electron density resides in the free and unencumbered borate anions derived from salts 1–6 by carrying out DFT and CCSD­(T) computations in conjunction with negative ion photoelectron spectroscopy (NIPES) studies. …”

“…A series of tetraphenylphosphonium salts including 3-and 4pyridyltrifluoroborates (1 and 2, respectively) and their triaryl analogues 3−6 (Scheme 1) were prepared and studied. 32 The S N 2 reaction of the tetraphenylphosphonium salt of 3pyridyltri(4-t-butylphenyl)borate (3) with 1-iodooctane (eq 1) was found to proceed 10 times faster than the activated noncharged pyridine derivative known as DMAP (i.e., 4dimethylaminopyridine). It also reacts 6 times faster than DMAP in their base-catalyzed urethane forming reaction with 1-butanol and 4-methylphenylisocyanate (eq 2).…”

“…It also reacts 6 times faster than DMAP in their base-catalyzed urethane forming reaction with 1-butanol and 4-methylphenylisocyanate (eq 2). 32,33 These compounds represent a new class of reagents, and to increase the reactivity of such species, the interaction of the oppositely…”