Hydrogen Activation by an Aromatic Triphosphabenzene

Abstract

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.

A Stable but Highly Reactive Phosphine‐Coordinated Borenium: Metal‐free Dihydrogen Activation and Alkyne 1,2‐Carboboration

“…[16] Bertrand et al pioneered the field with ambiphilic acyclic and cyclic amino carbenes, [17] Aldridge and co-workers generalized the approach to aminoboryl and aminosilyl silylenes, [18] and Piers et al extended the field to highly electron-deficient antiaromatic boroles [2a] (see Refs. [19,20] for additional examples).…”

Following palladium catalyzed methoxycarbonylation by hyperpolarized NMR spectroscopy: a parahydrogen based investigation

“…68 Interesting, the corresponding 31 P coupling increases to around 70 Hz in related hexadiene based complexes. 69 In the case of related Pd(II) bis phosphine complexes containing H and MeOH ligands couplings of [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30] Hz are typical and so a virtual triplet would be expected for the alkenic protons in both of these types of product if such large couplings are retained. We therefore suggest in 10 that CO binding to the fourth site occurs, and that the palladium centre is charged, which means that an out-of-plane cis-stilbene ligand is indicated ( Figure 5).…”

“…[17][18][19][20] Workers have also used this method to examine not only a range of heterogeneous reactions [21][22][23][24][25] but some that don't involve a metal centre. [26][27][28][29] The PHIP approach has therefore developed substantially from the early starting point of Weitekamp, 30, 31 Eisenberg 32,33 and Bargon, 34,35 as illustrated in several reviews. 11,12 In order to understand the physical basis of PHIP, p-H 2 needs to be recognised as simply dihydrogen that exists in the anti-symmetric magnetic state that is represented by the nuclear wave function  { .…”

Non-conventional Lewis Acids and Bases in Frustrated Lewis Pair Chemistry

“…To the best of our knowledge, FLP chemistry with phosphenium cations has yet to be achieved. In related reactivity, however, we have recently described the reaction of the aromatic triphosphabenzene (42) with H 2 [94]. This compound has a resonance structure which generates a positive charge at a phosphorus atom (phosphenium cation) and negative charge on the para-carbon atom.…”