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.

Bora-alkenes: Metal Coordination and Reactions with BH-boranes.

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 (C6 F5 )2 BH or C6 F5 BH2 ⋅ 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-ß-lactone type intermediate was isolated in the case of the isothiocyanato-boraalkene reaction and characterized by X-ray diffraction.

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.

Stereochemical Behavior of Pairs of P-stereogenic Phosphanyl Groups at the Dimethylxanthene Backbone.

The P-stereogenic bis(phosphanes) 7 and 9, featuring pairs of P(Mes)-ethynyl or vinyl substituents at the dimethyl xanthene backbone show rather low barriers of stereochemical inversion at phosphorus. π-Conjugative effects are probably causing these low inversion barriers. Compound 7 reacted with B(C6 F5 )3 to form the nine-membered heterocyclic product 10, featuring a [P]-C≡C-B(C6 F5 )3 substituent. Compound 7 was converted to the bis[P(Mes)vinyl] xanthene derivative 9, which gave the zwitterionic P(H)(Mes)-CH=CH-B(C6 F5 )3 containing product 16 upon treatment with B(C6 F5 )3 . Thermally induced epimerization barriers at phosphorus of ca. 20 to 27 kcal mol-1 were calculated by DFT for the alkenyl- and alkynyl-P derived systems 6 to 9, 15 and 16 and experimentally determined for the examples 7 and 16.

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 proton-detected solid-state NMR at fast magic-angle spinning. We demonstrate our approach using solid-state 1H/31P and 1H/13C correlation experiments at high magnetic fields (850 and 1200 MHz) and high spinning frequencies (100 kHz) to characterize four selected PH-containing compounds from the chemistry of phosphane-borane frustrated Lewis pairs. Frustrated Lewis pairs have gained high interest in the past years, particularly due to their capabilities of activating and binding small molecules, such as di-hydrogen, however, their analytical characterization especially in the solid state is still limited. Our approach reveals proton-phosphorus connectivities providing important information on spatial proximity and chemical bonding within such compounds. We also identify protons that show strongly different chemical-shift values compared to the solution state, which we attribute to intermolecular ring-current effects. The most challenging example presented herein is a cyclotrimeric frustrate Lewis pair-associate comprising three crystallographically distinct phosphonium entities that are unambiguously distinguished by our approach. Such 31P spin-filtered proton-detected NMR can be easily extended to other material classes and can strongly impact the structural characterization of reaction products of hydrogen-activated phosphane/borane FLPs, heterogeneous catalysts and solid-state reactions in general.

Formation of a Hybrid 1-Bora-3-boratabenzene Heteroarene Anion Derivative.

The B(C6 F5 )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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

Cycloaddition Reactions of an Active Cyclic Phosphane/Borane Pair with Alkenes, Alkynes, and Carbon Dioxide.

The active six-membered cyclo-FLP 6 undergoes a rapid P/B addition reaction to carbon dioxide. At elevated temperature, the resulting heterobicyclo[2.2.2]octane derived product 7 undergoes ring opening and equilibrates with the cyclotetramer (7)4 . In the large macrocyclic structure, four monomeric six-membered cyclo-FLP units are connected by four CO2 molecules to form the supramolecular ring system. The P/B cyclo-FLP 6 undergoes a variety of additional cycloaddition reactions.