Scrutinizing formally NiIV centers through the lenses of core spectroscopy, molecular orbital theory, and valence bond theory.

Nickel K- and L2,3-edge X-ray absorption spectra (XAS) are discussed for 16 complexes and complex ions with nickel centers spanning a range of formal oxidation states from II to IV. K-edge XAS alone is shown to be an ambiguous metric of physical oxidation state for these Ni complexes. Meanwhile, L2,3-edge XAS reveals that the physical d-counts of the formally NiIV compounds measured lie well above the d6 count implied by the oxidation state formalism. The generality of this phenomenon is explored computationally by scrutinizing 8 additional complexes. The extreme case of NiF62- is considered using high-level molecular orbital approaches as well as advanced valence bond methods. The emergent electronic structure picture reveals that even highly electronegative F-donors are incapable of supporting a physical d6 NiIV center. The reactivity of NiIV complexes is then discussed, highlighting the dominant role of the ligands in this chemistry over that of the metal centers.

Nickel-Borolide Complexes and Their Complex Electronic Structure.

Borolides (BC42-) can be considered as dianionic heterocyclic analogues of monoanionic cyclopentadienides. Although both are formally six-π-electron donors, we herein demonstrate that the electronic structure of their corresponding transition metal complexes differs significantly, leading to altered properties. Specifically, the 18-electron sandwich complex Ni(iPr2NBC4Ph2)2 (1) features an ∼90° angle between the Ni-B-N planes and is best described as a combination of three limiting resonance structures with the major contribution stemming from a formally Ni2+ species bound to two monoanionic radical (BC4•-) ligands. Compound 1 displays two sequential one-electron oxidation events over a small potential range of <0.2 V, which strikingly contrasts the large potential separations between redox partners in the family of metallocenes, and the potential reasons for this unusual observation are discussed.

9-Borafluorenes: Synthesis, Properties, and Reactivity.

This review covers all aspects of 9-borafluorene chemistry, from the first attempted synthesis in 1960 to the present. This class of molecules consists of a tricyclic system featuring a central antiaromatic BC4 ring with two fused arene groups. The synthetic routes to all 9-borafluorenes and their adducts are presented. The Lewis acidity and photophysical properties outlined demonstrate potential utility as sensors and in electronic materials. The reactivity of borafluorenes reveals their prospects as reagents for the synthesis of other boron-containing molecules. The appealing traits of 9-borafluorenes have stimulated investigations into analogues that bear different aromatic groups fused to the central BC4 ring. Finally, we offer our views on the challenges and future of borafluorene chemistry.

Total Synthesis of (±)-Phyllantidine: Development and Mechanistic Evaluation of a Ring Expansion for Installation of Embedded Nitrogen-Oxygen Bonds.

The development of a concise total synthesis of (±)-phyllantidine (1), a member of the securinega family of alkaloids containing an unusual oxazabicyclo[3.3.1]nonane core, is described. The synthesis employs a unique synthetic strategy featuring the ring expansion of a substituted cyclopentanone to a cyclic hydroxamic acid as a key step that allows facile installation of the embedded nitrogen-oxygen (N-O) bond. The optimization of this sequence to effect the desired regiochemical outcome and its mechanistic underpinnings were assessed both computationally and experimentally. This synthetic approach also features an early-stage diastereoselective aldol reaction to assemble the substituted cyclopentanone, a mild reduction of an amide intermediate without N-O bond cleavage, and the rapid assembly of the butenolide found in (1) via use of the Bestmann ylide.

Synthesis of 9-borafluorene analogues featuring a three-dimensional 1,1'-bis(o-carborane) backbone.

The synthesis of [1,1'-bis(o-carboranyl)]boranes was achieved through the deprotonation of 1,1'-bis(o-carborane) reagents followed by salt metathesis with (iPr)2NBCl2. X-ray crystallography confirms planar central BC4 rings and Gutmann-Beckett studies reveal an increase in Lewis acidity at the boron center in comparison to their biphenyl congener, 9-borafluorene.

Iron(II)-Catalyzed Azidotrifluoromethylation of Olefins and N-Heterocycles for Expedient Vicinal Trifluoromethyl Amine Synthesis.

We report herein an iron-catalyzed azidotrifluoromethylation method for expedient vicinal trifluoromethyl primary-amine synthesis. This method is effective for a broad range of olefins and N-heterocycles, and it facilitates efficient synthesis of a wide variety of vicinal trifluoromethyl primary amines, including those that prove difficult to synthesize with existing approaches. Our preliminary mechanistic studies revealed that the catalyst-promoted azido-group transfer proceeds through a carbo-radical instead of a carbocation species. Characterization of an active iron catalyst through X-ray crystallographic studies suggests that in situ generated, structurally novel iron-azide complexes promote the oxidant activation and selective azido-group transfer.

Intermolecular insertion reactions of azides into 9-borafluorenes to generate 9,10-B,N-phenanthrenes.

The reactions of 9-borafluorenes with organic azides result in either the insertion of the α-nitrogen and elimination of N2 or the insertion of the γ-nitrogen to generate the corresponding phenanthrene analogues with boron and nitrogen in the 9- and 10-positions, respectively.

Expedient Synthesis of 1,2-Thiaborines by Means of Sulfur Insertion into Boroles.

The propensity of boroles to undergo ring expansion reactions has been exploited as a route to generate 1,2-thiaborines, molecules that can be viewed as hybrid inorganic/organic analogues of benzene. Computational studies as well as structural data indicate that the species reported here have a high degree of aromatic character.

Ring expansion reactions of anti-aromatic boroles: a promising synthetic avenue to unsaturated boracycles.

Conjugated boron heterocycles have emerged as attractive synthetic targets due to their potential in medicinal chemistry and as electronic materials. However, the development of unsaturated boracycles has been hampered by difficulties in their preparation. Recently, a new synthetic avenue to access these species has been developed that takes advantage of the high reactivity of boroles. These five-membered anti-aromatic heterocycles can react with substrates to furnish ring expansion products via the insertion of one, two, or three atoms into the boracyclic ring. The ring expansion can occur via two pathways, the first exploits the activated diene moiety of the heterocycle in Diels-Alder chemistry with the resulting bicyclic species undergoing further rearrangements. The second reaction pathway is initiated by the coordination of the Lewis basic site of a substrate to the highly Lewis acidic boron center rendering the endocyclic B-C bond of the borole nucleophilic, inducing the formation of larger boracycles via attack at the electrophilic site of the substrate. This review summarizes the current state of this chemistry and details the mechanisms leading to the products. The methodologies described herein could very well be extended to other substrates, as well as applied to other anti-aromatic heterocycles.

Oxygen insertion into boroles as a route to 1,2-oxaborines.

The synthesis of 1,2-oxaborines is accomplished via the reaction of pentaarylboroles with N-methylmorpholine-N-oxide via a 1,1-insertion reaction. The aromatic nature of 1,2-oxaborines was evaluated by computing nuclear independent chemical shift (NICS) values. Collectively, the experimental and computational studies indicate the unsaturated central BOC4 ring has appreciable aromatic character.

Probing the reactivity of pentaphenylborole with N-H, O-H, P-H, and S-H bonds.

The reactions of molecules containing E-H functionalities (E = Group 15 or 16 element) and pentaphenylborole were investigated revealing diverse outcomes. For aniline and water, protodeborylation ring opening reactions occurred via the N-H or O-H bonds. Pentaphenylborole reacted with water in a 1 : 1 or 2 : 1 ratio to yield the corresponding boroxane and diboroxane, respectively, whereas aniline reacted strictly in a 1 : 1 ratio. Interestingly, 1-naphthalenethiol reacted to produce a 1-bora-cyclopent-3-ene heterocycle. The reaction with a primary phosphine generated an adduct which was resilient, even at elevated temperatures. DFT calculations provide support for the observed reaction products, and identify the initial adduct as a key intermediate in determining the final product. In particular, ring opening may be linked to the lability of the hydrogen in the initial adduct. Collectively, these reactions provide insight into new reaction pathways, the stability of boroles, as well as mechanistic insight into previously reported transformations.