1,1-Carboboration through Activation of Silicon-Carbon and Tin-Carbon Bonds

Observation of Borane-Olefin Proximity Interaction Governing the Structure and Reactivity of Boron-Containing Macrocycles

While attractive interactions between borane and olefin have been postulated to trigger various boron-mediated organic transformations, proximity structures of these functional groups, other than the formation of weak van der Waals complexes, have never been directly observed. Here we show that a close intramolecular borane-olefin interaction operates in macrocyclic systems containing borane and olefinic groups obtained by multi-step 1,2-carboboration between a strained alkyne and 9-borafluorene derivatives. Depending on Lewis acidity of the borane moiety and the size of the macrocycles, the magnitude of interaction changes, resulting in different reaction modes. The whole picture of the multi-step reactions has been revealed experimentally with theoretical supports. The present finding may not only provide a deeper understanding of the fundamental boron-mediated interaction but also lead to the development of new organic transformations involving molecular activation by boranes.

Alkyne 1,1-Hydroboration to a Reactive Frustrated P/B-H Lewis Pair

The Mes₂ P-C≡C-SiMe₃ alkyne reacts with the borane H₂ B-Fmes by means of a rare 1,1-hydroboration reaction to give an unsaturated C₂ -bridged frustrated P/B-H Lewis pair. Most of its reactions are determined by the presence of the B-H functionality at the FLP function and the activated connecting carbon-carbon double bond. It reduces carbon monoxide to the formyl stage. With nitriles it reacts in an extraordinary way: it undergoes a reaction sequence that eventually results in the formation of a P-substituted dihydro-1,2-azaborole derivative. Several similar examples were found. In one case a P-ylide was isolated that was related to an intermediate of the reaction sequence. It subsequently opened in an alternative way to give an alkenyl borane product.

Experimental and Theoretical Studies of a Spirostannole and Formation of a Pentaorganostannate

A new spirostannole, 1,1',3,3'-tetrakis(5-methylthiophen-2-yl)-4,4',5,5',6,6',7,7'-octahydro-2,2'-spirobi[benzo[c]stannole] (4), is synthesised and the molecular structure is compared with the optimised geometry from DFT calculations. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are twice degenerated and show a small HOMO-LUMO energy gap of 3.2 eV. In addition, cyclic voltammetry measurements are conducted and three redox processes are observed. Absorption and emission spectra show maxima at λ_abs,max 436 nm and λ_em,max 533 nm, respectively. Spirostannole 4 is a strongly absorbing material, but an extremely weak emitter in solution at 295.15 K. However, when the solution is cooled from 280 to 80 K, the emission becomes visible. The reaction of spirostannole 4 with methyllithium is monitored by NMR spectroscopy at 238.15 K. The ¹¹⁹Sn{¹H} NMR signal shifts from -36.0 (4) to -211.0 ppm, which is indicative of the formation of the lithium pentaorganostannate 5. The complex is thermally instable at 295.15 K, but insights into the molecular structure and electronic behaviour are obtained by DFT and TD-DFT calculations.

Boron and Silicon-Substituted 1,3-Dienes and Dienophiles and Their Use in Diels-Alder Reactions

Boron and silicon-substituted 1,3-dienes and boron and silicon-substituted alkenes and alkynes have been known for years and the last 10 years have seen a number of new reports of their preparation and use in Diels-Alder reactions. This review first covers boron-substituted dienes and dienophiles and then moves on to discuss silicon-substituted dienes and dienophiles.

A rare olefin 1,1-carboboration reaction opens a synthetic pathway to an unusually structured frustrated Lewis pair

(2,6-Dimesitylphenyl)P(vinyl)₂5d reacts with HB(C₆F₅)₂ in a sequence involving a rare example of a 1,1-carboboration of an olefin to give the borylated tetrahydrophosphole derivative 6d. Compound 6d is an active frustrated Lewis pair that splits dihydrogen under mild conditions and serves as a metal-free hydrogenation catalyst. It also adds to carbon dioxide. Compound 6d serves as an intermediate in the HB(C₆F₅)₂ catalyzed aryl(divinyl)phosphane (5d) to dihydrophosphole conversion.

Consecutive intermolecular 1,1-carboboration reactions of Me3Si-substituted alkynes with the halogeno-B(C6F5)2 reagents

1-(Trimethylsilyl)propyne 2a reacts with halogenoboranes XB(C₆F₅)₂ (X: Cl, Br) in a 2 : 1 molar ratio under mild conditions by consecutive 1,1-carboboration reactions to give the highly substituted boryldienes 4.

Transition-Metal-Free Cleavage of CO

Tertiary silane 1H , 2-[(diphenylsilyl)methyl]-6-methylpyridine, reacts with tris(pentafluorophenyl)borane (BCF) to form the intramolecular pyridine-stabilized silylium 1+ -HBCF. The corresponding 2-[(diphenylsilyl)methyl]pyridine, lacking the methyl-group on the pyridine ring, forms classic N(py)→B adduct 2H -BCF featuring an intact silane Si-H fragment. Complex 1+ -HBCF promotes cleavage of the C≡O triple bond in carbon monoxide with double C-Csp2 bond formation, leading to complex 3 featuring a B-(diarylmethyl)-B-aryl-boryloxysilane fragment. Reaction with pinacol generates bis(pentafluorophenyl)methane 4 as isolable product, proving the transition-metal-free deoxygenation of carbon monoxide by this main-group system. Experimental data and DFT calculations support the existence of an equilibrium between the silylium-hydroborate ion pair and the silane-borane mixture that is responsible for the observed reactivity.

Stoichiometric and catalytic isomerization of alkenylboranes using bulky Lewis bases

1,1-Carboboration of alkynes with boranes R'B(C₆F₅)₂ (R' = (CH₂)₃Ph, C₆F₅) affords mixtures of the E and Z isomers of RCH = CR'B(C₆F₅)₂ (R = (CH₂)₂Me, (CH₂)₃Me, (CH₂)₂Ph, CH₂CHMe₂, CHMe₂, CMe₃, CH₂OMe, CH₂N(CO)₂C₆H₄), treatment of these alkenylboranes with stoichiometric or catalytic amounts of tBu₂PH or other donors are shown to induce isomerization. The proposed mechanism involving a zwitterionic borataalkene intermediate is supported by computations.

Imidazole-stabilized, electron-deficient boron cations

Highly Lewis acidic borenium cations were prepared and are shown to activate H₂ and effect 1,1-carborations of alkynes.

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.