Lauren E. Longobardi

Hydrogen Activation by an Aromatic Triphosphabenzene

Aromatic hydrogenation is a challenging transformation typically requiring alkali or transition metal reagents and/or harsh conditions to facilitate the process. In sharp contrast, the aromatic heterocycle 2,4,6-tri-tert-butyl-1,3,5-triphosphabenzene is shown to be reduced under 4 atm of H2 to give [3.1.0]bicylo reduction products, with the structure of the major isomer being confirmed by X-ray crystallography. NMR studies show this reaction proceeds via a reversible 1,4-H2 addition to generate an intermediate species, which undergoes an irreversible suprafacial hydride shift concurrent with P-P bond formation to give the isolated products. Further, para-hydrogen experiments confirmed the addition of H2 to triphosphabenzene is a bimolecular process. Density functional theory (DFT) calculations show that facile distortion of the planar triphosphabenzene toward a boat-conformation provides a suprafacial combination of vacant acceptor and donor orbitals that permits this direct and uncatalyzed reduction of the aromatic molecule.

Stable Borocyclic Radicals via Frustrated Lewis Pair Hydrogenations

The synthesis and isolation of stable main group radicals remains an ongoing challenge. Here we report the application of frustrated Lewis pair chemistry to the synthesis of boron-containing radicals. H2 activation with polyaromatic diones and B(C6F5)3 leads to radical formation in good yields. These radicals are robust; they do not decompose on silica gel or react with O2 and are stable at 35 °C under N2 indefinitely. The mechanism of formation is explored experimentally, with support from DFT calculations. EPR and UV/vis spectroscopy as well as cyclic voltammetry data are provided, and the radicals are shown to react with cobaltocenes in one-electron chemical reductions to their corresponding borate anions.

Frustrated Lewis Pair Activation of an N‐Sulfinylamine: A Source of Sulfur Monoxide

Inter- and intramolecular P/B frustrated Lewis pairs are shown to react with an N-sulfinylamine to form PNSOB linakages. These species can be regarded as phosphinimine-borane-stabilized sulfur monoxide complexes, and indeed these species act as sources of SO, effecting the oxidation of PPh3 and delivering SO to [RhCl(PPh3)3] and an N-heterocyclic carbene.

Reactions of Boron-Derived Radicals with Nucleophiles

Reactions of phenanthrenedione- and pyrenedione-derived borocyclic radicals, CnH8O2B(C6F5)2• (n = 14 (1), 16 (3)), with a variety of nucleophiles have been studied. Reaction of 1 with P(t-Bu)3 affords the zwitterion 3-(t-Bu)3PC14H7O2B(C6F5)2 (5) in addition to the salt [HP(t-Bu)3][C14H8O2B(C6F5)2] (6). In contrast, the reaction of 1 with PPh3 proceeds to give two regioisomeric zwitterions, 1-(Ph3P)C14H7O2B(C6F5)2 (7a) and 3-(Ph3P)C14H7O2B(C6F5)2 (7b), as well as the related boronic ester C14H8O2B(C6F5) (2). In a similar fashion, 3 reacted with PPh3 to give 3-(Ph3P)C16H7O2B(C6F5)2 (8a), 1-(Ph3P)C16H7O2B(C6F5)2 (8b), and boronic ester C16H8O2B(C6F5) (4). Reactions of secondary phosphines Ph2PH and tBu2PH with 3 yield 3-(R2PH)C16H7O2B(C6F5)2 (R = Ph (9), t-Bu (10)). The reaction of 1 with N-heterocyclic carbene IMes afforded 3-(IMes)C14H7O2B(C6F5)2 (11) and [IMesH][C14H8O2B(C6F5)2] (12), while the reactions with quinuclidine and DMAP afforded the species 3-(C7H13N)C14H7O2B(C6F5)2 (13) and [H(NC7H13)2][C14H8O2B(C6F5)2] (14), and the salt [9,10-(DMAP)2C14H8O2B(C6F5)2][C14H8O2B(C6F5)2] (15), respectively. These products have been fully characterized, and the mechanism for the formation of these products is considered in the light of DFT calculations.

Electron spin relaxation of a boron-containing heterocyclic radical

Preparation of the stable boron-containing heterocyclic phenanthrenedione radical, (C6F5)2B(O2C14H8), by frustrated Lewis pair chemistry has been reported recently. Electron paramagnetic resonance measurements of this radical were made at X-band in toluene:dichloromethane (9:1) from 10 to 293K, in toluene from 180 to 293K and at Q-band at 80K. In well-deoxygenated 0.1mM toluene solution at room temperature hyperfine splittings from 11B, four pairs of 1H, and 5 pairs of 19F contribute to an EPR spectrum with many resolved lines. Observed hyperfine couplings were assigned based on DFT calculations and account for all of the fluorines and protons in the molecule. Rigid lattice g values are gx=2.0053, gy=2.0044, and gz=2.0028. Near the melting point of the solvent 1/Tm is enhanced due to motional averaging of g and A anisotropy. Increasing motion above the melting point enhances 1/T1 due to contributions from tumbling-dependent processes. The overall temperature dependence of 1/T1 from 10 to 293K was modeled with the sum of contributions of a process that is linear in T, a Raman process, spin rotation, and modulation of g anisotropy by molecular tumbling. The EPR measurements are consistent with the description of this compound as a substituted aromatic radical, with relatively small spin density on the boron.