Caleb D. Martin

Ring Expansion Reactions of Pentaphenylborole with Dipolar Molecules as a Route to Seven-Membered Boron Heterocycles

Reactions of pentaphenylborole with isocyanates, benzophenone, and benzaldehyde produced new seven-membered heterocycles in high yields. For 1-adamantyl isocyanate, a BNC5 heterocycle was obtained from the insertion of the C-N moiety into the five-membered borole, whereas for 4-methoxyphenyl isocyanate, a BOC5 heterocycle was generated from the insertion of the C-O unit. These reactions are believed to occur via a mechanism wherein coordination of the nucleophile to the borole (1-adamantyl, N-coordination or O-coordination for 4-methoxyphenyl) is followed by ring expansion to afford the observed seven-membered heterocycles. The selectivity to form B-O- or B-N-containing heterocycles is based on the polarization of the isocyanate implying tunable reactivity for the system. Having observed that isocyanates react as 1,2-dipoles with pentaphenylborole, we examined benzophenone and benzaldehyde, which both reacted to insert C-O units into the ring. This represents a new efficient method for preparing rare seven-membered boracycles.

Remarkably Stable Chalcogen(II) Dications

Air-stable, chalcogen-centered dications have been synthesized and comprehensively characterized. These represent the first diiminopyridine (DIMPY) complexes of the chalcogens as well as the single nonmetallic (sulfur) complex of this ubiquitous ligand. Their stability under ambient conditions is a distinct contrast to other highly charged main-group cations.

1,2‐Phosphaborines: Hybrid Inorganic/Organic P–B Analogues of Benzene

Photolysis of the cyclic phosphine oligomer [PPh]5 in the presence of pentaarylboroles leads to the formation of 1,2-phosphaborines by the formal insertion of a phenylphosphinidene fragment into the endocyclic CB bond. The solid-state structure features a virtually planar central ring with bond lengths indicating significant delocalization. Appreciable ring current in the 1,2-phosphaborine core, detected in nuclear independent chemical shift (NICS) calculations, are consistent with aromatic character. These products are the first reported 1,2-BPC4 conjugated heterocycles and open a new avenue for BP as a valence isoelectronic substitute for CC in arene systems.

Reactions of imines, nitriles, and isocyanides with pentaphenylborole: coordination, ring expansion, C-H bond activation, and hydrogen migration reactions.

The reactions of pentaphenylborole with imines, isocyanides, and acetonitrile were investigated experimentally and theoretically. On the basis of literature precedent, we envisioned that the dipolar substrates would undergo facile ring expansion reactions to yield new BNC5 heterocycles. For acetonitrile and one particular imine, this ring expansion process was observed. However, in many cases, unexpected reactivity occurred. This included hydride migration of an imine ring expanded product and the ortho C-H bond activation of an aryl group of an imine if two phenyl groups were present on the α-carbon. A bulky group on the nitrogen atom of an imine prevented coordination to the boron center, and no reaction was observed, indicating that coordination to the borole is a critical step for any type of reaction to occur. Isocyanides made coordination complexes, but heating to induce further reactivity resulted in mixtures. The mechanisms were elucidated via DFT calculations, which complement the experimental findings.

Peculiar Reactivity of Isothiocyanates with Pentaphenylborole

The reactions of isothiocyanates with the antiaromatic pentaphenylborole were investigated, revealing significantly different outcomes than the analogous reactions with isocyanates. The 1:1 stoichiometric reaction products isolated include a seven-membered BNC5 heterocycle and a fused bicyclic 4/5-ring system. Studies suggest that the seven-membered ring undergoes an intramolecular [2 + 2] electrocyclic ring closure to produce the bicyclic system. The only derivative for which stoichiometry influenced the reaction outcome was 4-methoxyphenylisothiocyanate. The reaction of borole with an excess of 4-methoxyphenylisothiocyanate resulted in the formation of a fused tetracyclic species with two equivalents of isothiocyanate incorporated into the product. Rational pathways for these unusual transformations are presented.

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.

Synthesis and Reactivity of Sulfur(II) Dications Stabilized Using Monodentate Ligands

The stoichiometric 1:2:2 reaction of SCl(2), trimethylsilyl trifluoromethanesulfonate, and pyridine produced sulfur(II) dications featuring two pyridine donors. The complexes were reacted with unsaturated organic substrates, which displayed addition with carbon-carbon and carbon-nitrogen double bonds. The substitution on the para position of the ligand was varied by using electron donating, electron withdrawing, and neutral groups. The electronics at this position were determined to have a substantial effect on the outcome of the reaction. The presence of an electron donating group increased the Lewis basicity sufficiently to completely stop the reaction, whereas an electron withdrawing group decreased the reaction time. These observations are unique to the chemistry of sulfur(II) dications and main group polycations as a whole.

Cross-coupling reactions between stable carbenes.

By utilizing stable carbenes with low-lying LUMOs, coupling with the stable nucleophilic diaminocyclopropenylidene was achieved. This reaction resulted in the formation of two new and rare examples of a bent allene as well as the isolation of the first carbene-carbene heterodimer.

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.

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.