Synthesis of Zwitterionic Cobaltocenium Borate and Borata-alkene Derivatives from a Borole-Radical Anion

Three-component carboformylation: α-quaternary aldehyde synthesis via Co(iii)-catalysed sequential C-H bond addition to dienes and acetic formic anhydride

All carbon α-quaternary aldehydes are prepared via Co(iii)-catalysed sequential C-H bond addition to dienes and acetic formic anhydride, representing a rare example of intermolecular carboformylation. A wide range of internally substituted dienes containing diverse functionality can be employed in this reaction, affording complex α-quaternary aldehydes that would not be accessible via hydroformylation approaches. Mechanistic investigations, including control reactions and deuterium labeling studies, establish a catalytic cycle that accounts for formyl group introduction with an uncommon 1,3-addition selectivity to the conjugated diene. Investigations into the role of the uniquely effective additive Proton Sponge® were also conducted, leading to the observation of a putative, intermediate Co(i) tetramethylfulvene complex at low temperatures via NMR spectroscopy. The synthetic utility of the aldehyde products is demonstrated by various transformations, including proline-catalysed asymmetric aldol addition, reductive amination, and the asymmetric synthesis of amines using tert-butanesulfinamide technology.

One electron reduction transforms high-valent low-spin cobalt alkylidene into high-spin cobalt(ii) carbene radical

One electron reduction of formally CoIV(OR)2(CPh2) forms the [CoII(OR)2(CPh2)]- anion. Whereas low-spin Co(OR)2([double bond, length as m-dash]CPh2) demonstrated significant alkylidene character, the high-spin [Co(OR)2(CPh2)]- anion features a rare Co(ii)-carbene radical. Treatment of [Co(OR)2(CPh2)][CoCp*2] with xylyl isocyanide triggers formation of two new C-C bonds, and is likely mediated by nucleophilic attack of deprotonated CoCp*2+ on a transient ketenimine.

Redox-Controlled Reactivity at Boron: Parallels to Frustrated Lewis/Radical Pair Chemistry

We report the synthesis of new Lewis-acidic boranes tethered to redox-active vanadium centers, (Ph₂N)₃V(μ-N)B(C₆F₅)₂ (1a) and (N(CH₂CH₂N(C₆F₅))₃)V(μ-N)B(C₆F₅)₂ (1b). Redox control of the V^IV/V couple resulted in switchable borane versus "hidden" boron radical reactivity, mimicking frustrated Lewis versus frustrated radical pair (FLP/FRP) chemistry, respectively. Whereas heterolytic FLP-type addition reactions were observed with the V^V complex (1b) in the presence of a bulky phosphine, homolytic peroxide, or Sn-hydride bond cleavage reactions were observed with the V^IV complex, [CoCp₂^*][(N(CH₂CH₂N(C₆F₅))₃)V(μ-N)B(C₆F₅)₂] (3b), indicative of boron radical anion character. The extent of radical character was probed by spectroscopic and computational means. Together, these results demonstrate that control of the V^IV/V oxidation states allows these compounds to access reactivity observed in both FLP and FRP chemistry.

The 9-Borataphenanthrene Anion

The 9-borataphenanthrene anion is easily accessed by deprotonation of a 9,10-dihydro-9-boraphenanthrene and its diverse reactivity is investigated. Alkylation occurs at the carbon atom adjacent to boron, and room temperature hydroboration occurs across the B=C bond. The π-manifold of the central BC₅ ring coordinates to chromium in an η⁶ fashion while only the B=C unit binds η² to gold, indicating versatility of the 9-borataphenanthrene anion as a ligand. Supporting calculations rationalize the reactivity and aromaticity is corroborated by nucleus-independent chemical shift (NICS) indices.

Photo electron transfer induced desilylation of N,N-bis(trimethylsilyl)aminodibenzoborole to aminodibenzoborole

Photoinduced desilylation proceeds by single electron transfer and yields the first example of an unsubstituted aminoborole derivative.

Stepwise N-H Bond Formation From N2-Derived Iron Nitride, Imide and Amide Intermediates to Ammonia

Reduction of N₂ to ammonia in nature and in electrocatalysis takes place through 1-proton/1-electron steps, motivating efforts to experimentally study the steps during proton/electron transfer to well-characterized N₂-derived species with bridging nitrides. We report here the protonation and reduction reactions of an N₂-derived iron bis(nitride) complex (Rodriguez et al., Science, 2011, 334, 780). We isolate and definitively characterize triiron imido and amido intermediates that lie along the path to ammonia formation, and Mössbauer spectroscopy shows the oxidation level of iron atoms in these mixed-valence clusters. The first two H atoms add to one of the two nitrides of the bis(nitride) complex, and the proton-coupled electron transfer in the second step can be concerted or stepwise depending on the sources of protons and electrons. The characterization of partially protonated nitrides and their mechanisms of formation are expected to guide efforts to convert N₂ to ammonia with mild acids.

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.

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.

Ring Expansions of Boroles with Diazo Compounds: Steric Control of C or N Insertion and Aromatic/Nonaromatic Products

Access to novel imine-substituted 1,2-azaborinines, as well as highly arylated boracyclohexa-3,5-dienes has been developed by ring expansion of boroles with diazoalkanes with varying degrees of steric bulk. The formation of a diazoalkane intermediate is also discussed for the reaction of ortho-brominated p-tolyl-azide with 1,2,3,4,5-pentaphenylborole. Structural details as well as UV/Vis spectroscopic and cyclic voltammetric data are provided. These boron-containing heterocycles have the potential to serve as building blocks for boron-containing materials.

Investigating the ring expansion reaction of pentaphenylborole and an azide

The reaction between trimethylsilyl azide and pentaphenylborole was recently shown to produce the corresponding 1,2-azaborine. Investigating this transformation theoretically suggests that the reaction proceeds via coordination of the azide to the borole, rearrangement to a bicyclic species, and conversion to a kinetically favoured eight-membered BN3C4 heterocycle or expulsion of N2 to furnish the thermodynamically favoured 1,2-azaborine. The eight-membered species was structurally characterized as a borole adduct and represents an unusual analogue of cyclooctatetraene.

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.

A combined experimental and theoretical study on the isomers of 2,3,4,5-tetracarba-nido-hexaborane(6) derivatives and their photophysical properties

Building upon our ongoing studies on the high-yield synthetic route to 2,3,4,5-tetracarba-1,6-nido hexaborane derivative 5, we continue to explore the chemistry of this system to isolate a range of perphenylated C4 B2 -type carborane isomers. As a result, the photolysis of 5 for an elongated period yielded 6, which exhibits an intense luminescence upon excitation with a UV lamp (λex =366 nm). In contrast, the conversion of 5 under thermal conditions (microwave heating) generated the isomer 7, which does not show any luminescence. The structural disparity among the three isomers 5-7 is the position of the butadiene moiety with respect to the C4 B2 -carborane cage as deduced from single-crystal X-ray diffraction analyses. In an attempt to generate the classical structural motif of 5, an equimolar amount of KOtBu was added to a THF solution of 5, which immediately provided the boratacyclopentadiene derivative 8 in good yield. The observed rearrangement reactions were further investigated by quantum chemical calculations to suggest a plausible reaction pathway. According to the DFT calculations, the energetically favored reaction mechanism most likely involves tricyclic transition states and classical intermediate structures. In addition, the fluorescence emission properties of 6 were investigated in solution as well as in the solid state.

Antiaromaticity to aromaticity: from boroles to 1,2-azaborinines by ring expansion with azides

We have exploited the reactivity of antiaromatic boroles, gaining access to aryl-substituted monocyclic 1,2-azaborinines. The observed ring-expansion reaction of inherently electron-deficient boroles with organometallic and organic azides is demonstrated for representative examples. This substance class is expected to provide a new avenue into 1,2-azaborinine chemistry, especially in the area of functional organoboron materials. Our results are based on NMR and UV/Vis spectroscopy as well as single-crystal X-ray crystallography and provide a virtually quantitative approach that also offers numerous points of variation.

Redox chemistry of a hydroxyphenyl-substituted borane

Chemical reduction of a hydroxyphenyl-substituted borane triggers a sequential electron- and intramolecular hydrogen-atom-transfer process to afford a hydridoborate phenoxide dianion. On the other hand, hydrogen-atom abstraction of the borane leads to the isolation of a neutral borylated phenoxyl radical, which can be transformed to the corresponding benzoquinone borataalkene derivative by reduction with cobaltocene.

Evidence for extensive single-electron-transfer chemistry in boryl anions: isolation and reactivity of a neutral borole radical

Despite the synthesis of a boryl anion by Yamashita et al. in 2006, compounds that show boron-centered nucleophilicity are still rare and sought-after synthetic goals. A number of such boryl anions have since been prepared, two of which were reported to react with methyl iodide in apparent nucleophilic substitution reactions. One of these, a borolyl anion based on the borole framework, has now been found to display single-electron-transfer (SET) reactivity in its reaction with triorganotetrel halides, which was confirmed by the isolation of the first neutral borole-based radical. The radical was characterized by elemental analysis, single-crystal X-ray crystallography, and EPR spectroscopy, and has implications for the understanding of boron-based nucleophilic behavior and the emergent role of boron radicals in synthesis. This radical reactivity was also exploited in the synthesis of compounds with rare B-Sn and B-Pb bonds, the latter of which was the first isolated and structurally characterized compound with a "noncluster" B-Pb bond.