Polar covalent apex-base bonding in borapyramidanes probed by solid-state NMR and DFT calculations

Pyramidane molecules have attracted chemists for many decades due to their regular shape, high symmetry and their correspondence in the macroscopic world. Recently, experimental access to a number of examples has been reported, in particular the rarely reported square pyramidal bora[4]pyramidanes. To describe the bonding situation of the nonclassical structure of pyramidanes, we present solid-state Nuclear Magnetic Resonance (NMR) as a versatile tool for deciphering such bonding properties for three now accessible bora[4]pyramidane and dibora[5]pyramidane molecules. 11 B solid-state NMR spectra indicate that the apical boron nuclei in these compounds are strongly shielded (around -50 ppm vs. BF3 -Et2 O complex) and possess quadrupolar coupling constants of less than 0.9 MHz pointing to a rather high local symmetry. 13 C-11 B spin-spin coupling constants have been explored as a measure of the bond covalency in the borapyramidanes. While the carbon-boron bond to the -B(C6 F5 )2 substituents of the base serves as an example for a classical covalent 2-center-2-electron (2c-2e) sp2 -carbon-sp2 -boron σ-bond with 1 J(13 C-11 B) coupling constants in the order of 75 Hz, those of the boron(apical)-carbon(basal) bonds in the pyramid are too small to measure. These results suggest that these bonds have a strongly ionic character, which is also supported by quantum-chemical calculations.

Molecular pyramids - from tetrahedranes to [6]pyramidanes

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.

Boraalkenes: Metal Coordination and Reactions with BH-boranes

The neutral N-heterocyclic carbene stabilized boraalkene 1 conveniently prepared by a BH borenium/hydroboration route, forms stable copper, gold or palladium π-complexes. The polar boraalkene 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.

Boraalkenes Made by a Hydroboration Route: Cycloaddition and B=C Bond Cleavage Reactions

Hydroboration of styrene or vinylcyclohexane with the IMes(C₆ F₅ )BH⁺ cation followed by deprotonation provided a convenient synthetic entry to the [B]=CHCH₂ R 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]=CHCH₂ R+CO₂ cycloadducts 13 a and 13 b added the borane HB(C₆ F₅ )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.

A deprotonation pathway to reactive [B]CH2 boraalkenes

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. The BCH₂ boraalkenes reacted with elemental sulfur to give a thiaborirane product. They underwent [2+2] cycloaddition reactions with carbon dioxide or sulfur dioxide to give four-membered boron containing heterocycles. The boraalkenes added strongly Lewis acidic boranes at their CH₂ carbon atoms. The corresponding HB(C₆F₅)₂/boraalkene adduct reduced carbon monoxide to a -OCH(C₆F₅)- moiety inside a five-membered heterocycle at the B-CH₂-B template. The boraalkenes reacted with the [(Me₂S)AuCl] reagent to form the corresponding (boraalkene)AuCl complexes.

Borole/Borapyramidane Relationship

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.

Proton-phosphorous connectivities revealed by high-resolution proton-detected solid-state NMR

Proton-detected solid-state NMR enables atomic-level insight in solid-state reactions, for instance in heterogeneous catalysis, which is fundamental for deciphering chemical reaction mechanisms. We herein introduce a phosphorus-31 radiofrequency channel in...

Generation of Boryl-nitroxide Radicals from a Boraalkene via the Nitroso Ene Reaction

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 NHC-stabilized boraalkene reacts with nitrosobenzene to give a [2+2] cycloaddition product plus a pair of persistent borylnitroxide radicals. These are thought to be formed by means of a bora nitroso ene reaction followed by H-atom transfer.

An Olefin-based Multi-component Reaction to 1,2-Azaborolidine Derivatives

Reaction of the borane FmesBH2·SMe2 [Fmes: 2,4,6-tris(trifluoro-methyl)phenyl] with two molar equivs of a small series of 1-alkenes followed by treatment with two molar equivalents of the bulky isonitrile CN-Xyl (Xyl:...

N-Heterocyclic Carbene Stabilized 1-Bora-1,3-butadienes

Deprotonation of [(NHC)(Fmes)B-allyl]⁺ borenium cations (NHC, IMes (a) or IMe₂ (b); Fmes, 2,4,6-(CF₃)₃C₆H₂) provides an easy entry to the NHC-stabilized 1-bora-1,3-butadienes. They feature a planar s-trans-conformation just like 1,3-butadiene. The 1-borabutadiene 7a undergoes hydroboration reactions; the HB(C₆F₅)₂ hydroboration product is trapped with CO or an isonitrile to give the respective cyclic zwitterionic borenium-borate enolate or enamide products. 1-Borabutadiene 7b undergoes 1,4-chalcogenation with elemental sulfur or selenium, and it gives the six-membered heterocyclic 1,4-addition product with the S═O bond of sulfur dioxide. Compound 7b served as a precursor for the formation of a borylated η³-allyl ligand at Ru. 7b formed a Rh complex by reaction with [Rh(ethylene)₂Cl]₂. It subsequently underwent an intramolecular C-H activation reaction to a mixture of η³-methyl-boraallyl Rh complex isomers.

Three-Component Reaction to 1,4,2-Diazaborole-Type Heteroarene Systems

The borane FmesBH₂ reacts in a three-component reaction with an isonitrile and a small series of organonitriles to give rare examples of the class of dihydro-1,4,2-diazaborole derivatives. In a related way, annulated BN-indolizine derivatives became conveniently available, as were dihydro-1,4,2-oxaza- or thiazaborole derivatives. The nucleophilic framework of a dihydro-1,4,2-diazaborole example allowed for an uncatalyzed acylation reaction. It also served as a 1,3-dipolar reagent and underwent a [3+2] cycloaddition/[4+2] cycloreversion sequence when treated with methyl propiolate to give the respective pyrrole product. The [3+2] cycloaddition product of a dihydro-1,4,2-diazaborole derivative with N-phenylmaleimide was isolated and its heterobicyclo[2.2.1]heptane derived structure characterized by X-ray diffraction.

The Bis(η6 -benzene)lithium Cation: A Fundamental Main-Group Organometallic Species

The synthesis and characterization of the bis(η⁶ -benzene)lithium cation, the benzene metallocene of the lightest metal, is reported. The boron compound FmesBCl₂ [Fmes: 2,4,6-tris(trifluoromethyl)phenyl] reacted with three molar equivalents of the lithio-acetylene reagent Li-C≡C-Fmxyl [Fmxyl: 3,5-bis(trifluoromethyl)phenyl]. Subsequent crystallization from benzene gave the [bis(η⁶ -benzene)Li]⁺ cation with the [{FmesB(-C≡C-Fmxyl)₃ }₂ Li]^- anion. This parent [(arene)₂ Li]⁺ cation shows an eclipsed arrangement of the pair of benzene ligands at the central lithium cation with uniform carbon-lithium bond lengths. The corresponding [(η⁶ -toluene)₂ Li]⁺ and [(η⁶ -durene)₂ Li]⁺ containing salts were similarly prepared. The bis(arene)lithium cations were characterized by X-ray diffraction, by solid-state ⁷ Li MAS NMR spectroscopy and their bonding features were analyzed by DFT calculations.

Carbon Monoxide Coupling Reactions via a Frustrated Lewis Pair-Derived η2-Formyl Borane

The η²-formyl borane system 3 is readily available by carbonylation of the vicinal P/BH frustrated Lewis pair (FLP) 1. It serves as a frustrated C/B Lewis pair toward carbon dioxide or phenylisocyanate. In the presence of B(C₆F₅)₃, it forms the coupling product between two CO-derived units. The resulting compound 13 rearranged to a doubly O-borylated endiolate, with both of the central carbon atoms originating from carbon monoxide. The subsequent treatment with a silane gave a rare macrocyclic silicon endiolate.

Formation and Cycloaddition Reactions of a Reactive Boraalkene Stabilized Internally by N-Heterocyclic Carbene

The synthesis of element-carbon double bonds is of great importance for the development and understanding of reactive π-bonded systems in chemistry. The seven-membered heterocyclic system 4 b is readily made by internal C-H activation at a pendent isopropyl methyl group of the respective [(IPr)C₆ F₅ BH]⁺ borenium ion. Subsequent deprotonation with the IMes carbene gives the neutral cyclic boraalkene system 5 b. The B=C double bond in compound 5 b adds carbon dioxide, CS₂ , sulfur dioxide, phenyl isocyanate, an acetylenic ester or two NO molecules to give the corresponding four-membered ring annulated heterocycles. With sulfur or selenium 5 b gives the respective three-membered ring systems. N₂ O reacts with 5 b to give a mixture of the related oxaborirane 18 and a unique [B]OH containing diazoalkane product 19.

Frustrated Lewis-Pair Neighbors at the Xanthene Framework: Epimerization at Phosphorus and Cooperative Formation of Macrocyclic Adduct Structures

Attachment of a pair of P-stereogenic mesityl(alkynyl)phosphanyl groups at the 4- and 5-positions of a 9,9-dimethylxanthene framework gave mixtures of the respective rac- and meso-bisphosphanyl diastereoisomers. They slowly epimerized in a thermally induced reaction with Gibbs activation barriers of about 25 kcal mol^-1 at room temperature (measured and DFT calculated). The reaction of the meso-mesityl(tert-butylethynyl)phosphanyl derivative with two molar equivalents of Piers' borane [HB(C₆ F₅ )₂ ] led to the formation of the alkylidene-bridged geminal bisphosphane/borane-frustrated Lewis pair system. The compound was obtained enriched (>85 %) in the rac diastereoisomer. With a variety of bifunctional donor substrates, the rac-bis-P/B FLP formed macrocyclic compounds. They were all formally derived from meso-configurated diastereoisomers of the bisphosphanylxanthene backbone.

Multi-component synthesis of dihydro-1,3-azaborinine derived oxindole isosteres

We report on the synthesis of the first examples of 1,3-azaborinine derived oxindole systems, the BN-isosteres of the important compound class of the oxindoles. Hydroboration of terminal aryl acetylenes with FmesBH2, followed by treatment with 2 equiv. of a glycine ester derived isonitrile gave a small series of 1,3-azaborinine derived oxindoles. The new BN-oxindoles show interesting photophysical behavior.

Formation of amidino-borate derivatives by a multi-component reaction

Cyclohexene reacts with the (Fmes)BH2·SMe2 borane reagent and three molar equivalents of the isonitrile CN-Xyl to give the five membered 1,3-BN heterocyclic product 7 that contains a zwitterionic borata-amidinium moiety and a cyclohexenyl substituent. The analogous five-component coupling between cyclopentene, (Fmes)BH2·SMe2 and CN-Xyl in a 1 : 1 : 3 molar ratio gives the related cyclic amidino-borate derivative 10. The reaction of the (Fmes)BH2 derived frustrated Lewis pair 12, in situ generated or employed as the isolated dimer, reacts with 3 CN-Xyl equivs. at elevated temperature (60 °C) to yield the analogous coupling product 13.

Introducing the Dihydro-1,3-azaboroles: Convenient Entry by a Three-Component Reaction, Synthetic and Photophysical Application

The (Fmes)BH₂·SMe₂ reagent (7) reacts sequentially with an acetylene and, e.g., xylylisonitrile in a convenient three-component reaction to give a series of unprecedented dihydro-1,3-azaborole derivatives 16. The tolane-derived example 16a was deprotonated and used as a ligand in organometallic chemistry. Compounds 16 served as the starting materials for the straightforward synthesis of various dihydro-1,3-azaborinine derivatives by treatment with an isonitrile. Several diaryldihydro-1,3-azaboroles showed interesting photophysical properties such as aggregation-induced emission and high fluorescence quantum yields.

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.

Reaction of carbon oxides with an ethylene-bridged PH/B Lewis pair

The reaction of 2,4,6-tri(tert-butyl)phenyl vinyl phosphane with Piers' borane [HB(C6F5)2] gave the ethylene-bridged PH/B frustrated Lewis pair (FLP) system. It is a monomer at high temperature (>323 K), but exists as an associated 12-membered macrocyclic trimer below 273 K. The PH/B FLP splits dihydrogen and serves as a metal-free hydrogenation catalyst. It adds carbon dioxide. It serves as a PH/B template for the reduction of carbon monoxide by the HB(C6F5)2 borane to the formyl stage. The resulting six membered P/B/O containing heterocycle is opened upon treatment with pyridine and it reacts with benzaldehyde in a boron mediated Claisen-Tishchenko reaction.

The [(NHC)B(H)C6 F5 ]+ Cations and Their [B](H)-CO Borane Carbonyls

Hydride abstraction from the heterocyclic carbene borane adducts (NHC)BH₂ C₆ F₅ (NHC: IMes or IMe₄ ) gave the B-H containing [(NHC)B(H)C₆ F₅ ]⁺ borenium cations. They added carbon monoxide to give the respective [(NHC)B(H)(C₆ F₅ )CO]⁺ boron carbonyl cations. Carbon nucleophiles add to the boron carbonyl to give [B](H) acyls. Hydride reduced the [B]CO cation to hydroxymethylborane derivatives.

A BH Borenium-Derived Thioxoborane, Its Persulfide, and Their Li+-Induced Reactions with Alkynes and with Carbon Dioxide

Insertion of sulfur into the B-H bond of the BH borenium salt [IMes(C₆F₅)BH]⁺ followed by deprotonation gave the thioxoborane IMes(C₆F₅)B═S. Subsequent treatment with additional sulfur gave the corresponding boron persulfide, a NHC-stabilized boradithiirane. The B═S compound reacted with carbon dioxide in the presence of the lithium salt Li[B(C₆F₅)₄] by formal [2+2] cycloaddition to give a boron thiocarbonate-type product. The boron persulfide formally inserted phenyl acetylene into the B-S bond in the presence of Li[B(C₆F₅)₄] to give the respective five-membered heterocycle.

Using the FpXylBH2•SMe2 reagent for the regioselective synthesis of cyclic bis(alkenyl)boranes

The reactive borane reagent FpXylBH2•SMe2 was prepared from 1,4-bis(trifluoromethyl)benzene by treatment with n-BuLi, followed by H3B·SMe2 and subsequent removal of hydride. It undergoes a regioselective hydroboration reaction with 1,2-bis(trimethylsilylethynyl)benzene to give the "dimeric" product 13a featuring a conjugated 14-membered core heterocyclic structure that contains a pair of FpXylB units.

Reductive Cleavage of the CO Molecule by a Reactive Vicinal Frustrated PH/BH Lewis Pair

A dimeric ethylene-bridged PH/BH system reduced carbon monoxide to the -CH₂-O- state. In the presence of B(C₆F₅)₃, the frustrated PH/BH Lewis pair reacted with carbon monoxide by reductive coupling of two CO molecules at the template. Removal of the B(C₆F₅)₃ borane with pyridine liberated one equiv of carbon monoxide to give a cyclic five-membered P(═O)-CH₂-B compound, the product of reductive cleavage of carbon monoxide. It reacted as a borylated Horner P(═O)CH₂B carbon nucleophile with carbon dioxide to give a bicyclic product by P-CH₂ attack on CO₂ combined with internal P═O to boron coordination.

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.

Reactions of an anionic chelate phosphane/borata-alkene ligand with [Rh(nbd)Cl]2, [Rh(CO)2Cl]2 and [Ir(cod)Cl]2

Borata-alkenes can serve as anionic olefin equivalent ligands in transition metal chemistry. A chelate ligand of this type is described and used for metal coordination. Deprotonation of the Mes₂P(CH₂)₂B(C₆F₅)₂ frustrated Lewis pair in the α-CH[B] position gave the methylene-bridged phosphane/borata-alkene anion. It reacted with the [Rh(nbd)Cl] or [Rh(CO)₂Cl] dimers to give the respective neutral chelate [P/C[double bond, length as m-dash]B][Rh] complexes. The reaction of the [P/C[double bond, length as m-dash]B]^- anion with [Ir(cod)Cl]₂ proceeded similarly, only that the complex underwent a subsequent oxidative addition reaction at the mesityl substituent. Both the resulting Ir(iii)hydride complex 15 and the P/borata-alkene Rh system 12 were used as hydrogenation catalysts. The [P/C[double bond, length as m-dash]B(C₆F₅)₂]Rh(nbd) complex 12 served as a catalyst for arylacetylene polymerization.

Total Chemical Syntheses of the GM3 and F-GM3 Ganglioside Epitopes and Comparative Pre-Clinical Evaluation for Non-Invasive Imaging of Oligodendrocyte Differentiation

Gangliosides are intimately involved in a plenum of (neuro)inflammatory processes, yet progress in establishing structure-function interplay is frequently hindered by the availability of well-defined glycostructures. Motivated by the ubiquity of the ganglioside GM₃ in chemical neurology, and in particular by its conspicuous presence in myelin, the GM₃ epitope was examined with a view to preclinical validation as a tracer. The suitability of this scaffold for the noninvasive imaging of oligodendrocyte differentiation in Multiple sclerosis is disclosed. The stereocontrolled synthesis of a site-selectively fluorinated analogue (F-GM₃) is also disclosed to enable a comparative analysis in oligodendrocyte (OL) differentiation. Whereas the native epitope caused a decrease in the viability in a dose-dependent manner, the addition of distinct F-GM₃ concentrations over 48 h had no impact on the OL viability. This is likely a consequence of the enhanced hydrolytic stability imparted by the fluorination and highlights the potential of fluorinated glycostructures in the field of molecular imaging. Given the predominant expression of GM₃ in oligodendrocytes and the capacity of GM₃ to interact with myelin-associated proteins, this preclinical evaluation has revealed F-GM₃ to be an intriguing candidate for neurological imaging.

Using the Secondary PH/BH Functional Groups of an Active Geminal Frustrated Lewis Pair for Carbon Monoxide Reduction and Reactions with Nitriles and Isonitriles

Reaction of the secondary alkynyl(Mes*)PH phosphane 2 with (Fmes)BH₂ ⋅SMe₂ gives the geminal PH/BH frustrated Lewis pair (FLP) 3. The PH and the BH functions are jointly used in the reduction of carbon monoxide under mild reaction conditions to give the [P]-CH₂ -O-[B] product. A subsequent cycloaddition sequence results in the liberation of formaldehyde. The FLP 3 reacts with benzonitrile to give a P-benzamidine, and it couples two isonitriles at the FLP framework.

Halogen-directed chemical sialylation: pseudo-stereodivergent access to marine ganglioside epitopes

Sialic acids are conspicuous structural components of the complex gangliosides that regulate cellular processes. Their importance in molecular recognition manifests itself in drug design (e.g. Tamiflu®) and continues to stimulate the development of effective chemical sialylation strategies to complement chemoenzymatic technologies. Stereodivergent approaches that enable the α- or β-anomer to be generated at will are particularly powerful to attenuate hydrogen bond networks and interrogate function. Herein, we demonstrate that site-selective halogenation (F and Br) at C3 of the N-glycolyl units common to marine Neu2,6Glu epitopes enables pseudo-stereodivergent sialylation. α-Selective sialylation results from fluorination, whereas traceless bromine-guided sialylation generates the β-adduct. This concept is validated in the synthesis of HLG-1 and Hp-s1 analogues.

Sialic acids are conspicuous structural components of the complex gangliosides that regulate cellular processes.

Solid-State NMR Techniques for the Structural Characterization of Cyclic Aggregates Based on Borane-Phosphane Frustrated Lewis Pairs

Modern solid-state NMR techniques offer a wide range of opportunities for the structural characterization of frustrated Lewis pairs (FLPs), their aggregates, and the products of cooperative addition reactions at their two Lewis centers. This information is extremely valuable for materials that elude structural characterization by X-ray diffraction because of their nanocrystalline or amorphous character, (pseudo-)polymorphism, or other types of disordering phenomena inherent in the solid state. Aside from simple chemical shift measurements using single-pulse or cross-polarization/magic-angle spinning NMR detection techniques, the availability of advanced multidimensional and double-resonance NMR methods greatly deepened the informational content of these experiments. In particular, methods quantifying the magnetic dipole-dipole interaction strengths and indirect spin-spin interactions prove useful for the measurement of intermolecular association, connectivity, assessment of FLP-ligand distributions, and the stereochemistry of adducts. The present review illustrates several important solid-state NMR methods with some insightful applications to open questions in FLP chemistry, with a particular focus on supramolecular associates.

Borane-induced ring closure reaction of oligomethylene-linked bis-allenes

The trimethylene-linked bis-allene 3a reacts with Piers' borane [HB(C₆F₅)₂] by a hydroboration/allylboration sequence to generate the cyclization product 5a. Its pyridine adduct was isolated and characterized by X-ray diffraction. Compound 5a undergoes a typical frustrated Lewis pair 1,2-P/B alkene addition reaction with PPh₃ to give the heterobicyclic bridged olefinic zwitterionic product 9a. The tetramethylene-linked bis-allene 3b and its phenylene annulated analogue 3c react with HB(C₆F₅)₂ to give the analogous seven-membered ring products 5b,c under mild conditions. The cyclization product 5a undergoes a series of sequential allylboration reactions with two equivalents of allene followed by ring-closure to give the four-component coupling product 12a. It undergoes FLP addition to an exo-methylene group upon treatment with PPh₃. Compound 12a is oxidatively converted to the boron-free alcohol.

Formation of Active Cyclic Five-membered Frustrated Phosphane/Borane Lewis Pairs and their Cycloaddition Reactions

The cyclic five-membered frustrated phosphane/borane Lewis pairs 11 a, b featuring the bulky octaethylhydrindacenyl- (Eind) substituent or its mono-bromo derivative (^Br Eind) at phosphorus are monomeric at room temperature. The reactive frustrated Lewis pairs (FLPs) cleave dihydrogen. The cyclic FLP 11 b (^Br Eind) undergoes 1,2-P/B addition to ethylene to give the zwitterionic heteronorbornane derivative 14 b. It reacts similarly with the carbon-carbon double bond of norbornene. With a variety of organic π-reagents, the cyclic FLP 11 b often undergoes reaction sequences reminiscent of the Alder-Rickert reaction: the cycloaddition reaction is followed by rapid cycloreversion to form new five-membered heterocyclic FLP products with extrusion of ethene. Reactions of 11 b with benzaldehyde or with acetylenes follow this reaction pattern.

Multi-Component Synthesis of Rare 1,3-Dihydro-1,3-azaborinine Derivatives: Application of a Bora-Nazarov Type Reaction

The twofold hydroboration products of (Fmes)BH₂ ⋅SMe₂ with a series of alkynes (2-butyne, arylethynes) react with two molar equiv of 2,6-dimethylphenyl isocyanide (CN-Xyl) at 80 °C to give rare examples of 1,3-azaborinine derivatives. A mechanistic study revealed a reaction course involving insertion of one isonitrile followed by a bora-Nazarov type ring-closure reaction and subsequent isonitrile insertion to give the respective 1,3-dihydro-1,3-azaborinines 5.