Gerald Kehr

The study of 3D architectures at a molecular scale has fascinated chemists for generations. This includes molecular pyramids with all-carbon frameworks, such as trigonal, tetragonal and pentagonal pyramidal geometries. A small number of substituted tetrahedranes and all-carbon [4]–[5]pyramidanes have been experimentally generated and studied. Although the hypothetical unsubstituted parent [3]–[6]pyramidanes have only been explored computationally, the formal replacement of carbon vertices with isolobal main group element fragments has provided a number of examples of stable hetero[m]pyramidanes, which have been isolated and amply characterized. In this Review, we highlight the synthesis and chemical properties of [3]–[6]pyramidanes and summarize the progress in the development of chemistry of pyramid-shaped molecules.

The negatively charged borataalkenes and the neutral boraalkenes are isolable organoelement compounds that contain BC double bonds. The use of strongly electron withdrawing fluoroaryl substituents at boron greatly facilitates the preparation of these systems. The (C₆F₅)₂B containing boranes show greatly enhanced α-CH acidicties. This favours the formation of the corresponding borataalkenes by deprotonation. Several synthetic routes to reactive boraalkenes have been developed that make use of fluoroaryl containing B–H borenium cations. The borataalkenes serve as active borata-Wittig olefination reagents. Many boraalkenes undergo [2 + 2] cycloaddition reactions at the BC double bond with various unsaturated organic reagents. Both the borataalkenes and boraalkenes may serve as π-ligands in organometallic and coordination chemistry.

The neutral N‐heterocyclic carbene stabilized bora‐alkene 1, conveniently prepared by a BH borenium/hydroboration route, forms stable copper, gold or palladium π‐complexes. The polar bora‐alkene B=C system undergoes regioselective hydroboration reactions with the (C₆F₅)₂BH or C₆F₅BH₂ ⋅ SMe₂ boranes. The latter reaction involves a subsequent rearrangement that leads to internal hydride vs. isothiocyanate substituent exchange at the borane pair.

Hydroboration of styrene or vinylcyclohexane with the IMes(C₆F₅)BH⁺ cation followed by deprotonation provided a convenient synthetic entry to the [B]=CHCH₂R boraalkenes 9 a and 9 b. The in situ generated IMes(SCN)BH⁺ system reacted similarly with 1,1‐diphenylethene followed by deprotonation to give the isothiocyanato substituted boraalkene 9 c. The boraalkenes underwent [2+2] cycloaddition reactions with a small series of heterocumulenes to give the respective four‐membered heterocycles. The [B]=CHCH₂R+CO₂ cycloadducts 13 a and 13 b added the borane HB(C₆F₅)₂ with cleavage of the central B−C σ‐bond. CS₂ underwent an unusual reaction with the boraalkenes, namely insertion into the B=C bond with formation of the borylated dithioketene acetal under complete rupture of the strong B=C double bond. The intermediate dithiobora‐β‐lactone type intermediate was isolated in the case of the isothiocyanato‐boraalkene reaction and characterized by X‐ray diffraction.

Hydroboration of styrene or vinylcyclohexane with the IMes(C₆F₅)BH⁺ cation followed by deprotonation provided a convenient synthetic entry to the [B]=CHCH₂R boraalkenes 9 a and 9 b. The in situ generated IMes(SCN)BH⁺ system reacted similarly with 1,1‐diphenylethene followed by deprotonation to give the isothiocyanato substituted boraalkene 9 c. The boraalkenes underwent [2+2] cycloaddition reactions with a small series of heterocumulenes to give the respective four‐membered heterocycles. The [B]=CHCH₂R+CO₂ cycloadducts 13 a and 13 b added the borane HB(C₆F₅)₂ with cleavage of the central B−C σ‐bond. CS₂ underwent an unusual reaction with the boraalkenes, namely insertion into the B=C bond with formation of the borylated dithioketene acetal under complete rupture of the strong B=C double bond. The intermediate dithiobora‐β‐lactone type intermediate was isolated in the case of the isothiocyanato‐boraalkene reaction and characterized by X‐ray diffraction.

Examples of isolated boron substituted nitroxide radicals are rare. The reaction of the reactive cyclic boraalkene 3 with nitrosobenzene yields a mixture of the [2 + 2] cycloaddition product 4a, the B-nitroxide radicals 5a and 6a and the azoxybenzene co-product 7avia a bora nitroso ene reaction pathway, the boron analogue of the nitroso ene reaction. The products were separated by flash chromatography, and the B-nitroxide radicals were characterized by X-ray diffraction and EPR spectroscopy. Radical 5a was shown to be a hydrogen atom abstractor. Both the B-nitroxide radicals are more easily oxidized compared to e.g. TEMPO, as shown by cyclic voltammetry.

The B(C₆F₅)₂‐substituted borole 2⋅SMe₂ was obtained from the phenyl bis(trimethylsilylethynyl) borane SMe₂ adduct (1⋅SMe₂) by a synthetic sequence containing a rare 1,1‐hydroboration reaction. Subsequent thermolysis at 50 °C converted it to the bicyclic borenium/borate zwitterion 3⋅SMe₂. Photolysis of 3⋅SMe₂ gave the diboracyclohexadiene derivative 4⋅SMe₂, which after ligand exchange with N,N‐dimesitylimidazolylidene and deprotonation gave the 1‐bora‐3‐boratabenzene anion derivative as its potassium salt 6. Some unique follow‐up reactions of the unsaturated diboron containing six‐membered heterocycles were investigated.

The B(C₆F₅)₂‐substituted borole 2⋅SMe₂ was obtained from the phenyl bis(trimethylsilylethynyl) borane SMe₂ adduct (1⋅SMe₂) by a synthetic sequence containing a rare 1,1‐hydroboration reaction. Subsequent thermolysis at 50 °C converted it to the bicyclic borenium/borate zwitterion 3⋅SMe₂. Photolysis of 3⋅SMe₂ gave the diboracyclohexadiene derivative 4⋅SMe₂, which after ligand exchange with N,N‐dimesitylimidazolylidene and deprotonation gave the 1‐bora‐3‐boratabenzene anion derivative as its potassium salt 6. Some unique follow‐up reactions of the unsaturated diboron containing six‐membered heterocycles were investigated.

Boroles and borapyramidanes are classical and nonclassical constitutional isomers, respectively. It is here shown that they can indeed be interconverted. Treatment of the bis(alkynyl)B(C₆F₅) SMe₂ adduct 3·SMe₂ with HB(C₆F₅)₂ gave borole 1·SMe₂, featuring trimethylsilyl substituents in both α positions to boron, by means of a 1,1-hydroboration/alkenylboration sequence. Photolysis of the classical borole adduct 1·SMe₂ resulted in rearrangement to its nonclassical structural isomer, borapyramidane 2, in high yield, which exhibits a vicinal pair of trimethylsilyl substituents at the square pyramidane base. Neutral borapyramidane 2 is a rare example of an isoster of the (CH)₅⁺ pyramidane cation. Thermolysis of borapyramidane 2 in the presence of SMe₂ at 60 °C re-formed borole 1·SMe₂, which converted at 100 °C to 2,3-bis-silyl-substituted borole isomer 8·SMe₂. Its photolysis also gave borapyramidane 2. Prolonged photolysis of 2 at elevated temperatures converted this to borapyramidane isomer 10 containing a pair of trimethylsilyl groups in 1,3-position at its square C₄-pyramidal base. The borole and borapyramidane isomers were characterized by X-ray diffraction, and the system was analyzed by density functional theory (DFT) calculations.

The BH compounds IMes(Ar^F)BH(NTf₂) (Ar^F: C₆F₅ or FpXyl) were converted to the IMes(Ar^F)BCH₂ boraalkenes in a three step reaction sequence of B-methylation with methyllithium, hydride abstraction and deprotonation.