Search

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(C

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

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.

Deprotonation of [(NHC)(Fmes)B-allyl]

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.

Hydroboration of styrene or vinylcyclohexane with the IMes(C

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.

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.

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.

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.

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.

The B(C

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.

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.

An intramolecular ethylene-bridged PH/BH system, isolated as a dimer, reduces carbon monoxide to the -CH2-O- state. In the presence of B(C6F5)3 the frustrated PH/BH Lewis pair reacts with carbon mo...

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(C

A five-component one-pot sequence between FmesBH2, alkene and isonitrile was developed to give 1,2-azaborolidine derivatives.

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., Physical Chemistry Chemical Physics, 24 (13), ISSN:1463-9084, ISSN:1463-9076

The synthesis and characterization of the bis(η

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., The borane induced cyclization of the bis-allene followed by reaction with two molar equivalents of allene gives the functionalized decalin product.

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.

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.

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.

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.

Treatment of 1,3,5-tris(trifluoromethyl)benzene with n-BuLi, followed by H3 B⋅SMe2 and subsequent hydride removal gave the (Fmes)BH2 reagent, which was isolated as a SMe2 stabilized monomer or a ligand free (μ-H)2 -bridged dimer. Reaction with Mes2 P(vinyl) gave the respective ethylene-bridged P/B(Fmes)H FLP. It reduced carbon monoxide to the formyl stage and carbon dioxide to the formaldehyde oxidation state. Most new compounds were characterized by X-ray diffraction.

Several dienamines R2N-CH=CH-CH=CMe2 react with Piers' borane by HB(C6F5)2 addition to the terminal -CH=CMe2 unit to form the isopropyl substituted products 14. The strongly electrophilic -B(C6F5)2 functionality at the C3 position of the chain shows a marked contact to the adjacent enamine β-carbon atom C2. This generates a pronounced C/B FLP character which chemically comes to bear in the 1,2-addition reactions to the C=O bond of benzaldehyde or the -C≡N bonds of a series of alkylcyanides. In some cases, the formation of uncommon rearrangement products is observed.

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.

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 pairs (FLPs) show some small molecule binding features that are remotely reminiscent of metal/ligand coordination. This is illustrated by examples of synergic binding of carbon monoxide (CO) or nitric oxide (NO) to the Lewis acid/Lewis base pairs of some intramolecular FLP systems. Various pathways of carbon monoxide reduction at FLP frameworks leading to the formyl stage and beyond are described. CO reactions at extended FLP systems containing multiple Lewis acid/Lewis base combinations lead to unusually structured products. The formation of macrocyclic oligomers upon treatment of some P/B/B FLPs with carbon monoxide are typical examples. Some carbon monoxide reactions of specifically substituted d0-configurated Group 4 bent metallocene complexes show remarkable similarities to typical reactions of the metal-free main group element FLPs.

Insertion of sulfur into the B-H bond of the BH borenium salt [IMes(C

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.

(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.

Hydride abstraction from the heterocyclic carbene borane adducts (NHC)BH

International audience; Tame d0 phosphidotitanocene cations stabilized with a pendant tertiary phosphane arm are reported. These compounds were obtained by one-electron oxidation of d1 precursors with [Cp2Fe][BPh4]. The electronic structure of these compounds was studied experimentally (EPR, UV/Vis, and NMR spectroscopy, X-ray diffraction analysis) and through DFT calculations. The theoretical analysis of the bonding situation by using the electron localization function (ELF) shows the presence of π-interactions between the phosphido ligand and Ti in the d0 complexes, whereas dπ–pπ repulsion prevents such interactions in the d1 complexes. In addition, CH–π interactions were observed in several complexes, both in solution and in the solid state, between the phosphido ligand and the phosphane arm. The d0 complexes were found to be light sensitive, and decompose through Ti−P bond homolysis to give TiIII species. A naked d0 phosphidotitanocene cation has been trapped by reaction with diphenylacetylene, yielding a Ti/P frustrated Lewis pair (FLP), which was found to be less reactive than a previously reported Zr analog.

1-(Trimethylsilyl)propyne 2a reacts with halogenoboranes XB(C6F5)2 (X: Cl, Br) in a 1 : 1 molar ratio at r.t. to give the respective alkenylboranes 3. These undergo a subsequent 1,1-alkenylboration reaction with additional alkyne 2a to give the persubstituted 3-boryl-2,4-hexadiene products 4. The conjugated diyne bis(trimethylsilyl)butadiyne 2d reacts with ClB(C6F5)2 in a 2 : 1 molar ratio by a sequence of consecutive 1,1-carboboration reactions to give the persubstituted borylated octadiendiyne product 6.

The P/B FLP 6, formed by the 1,1-hydroboration of dimesitylphosphino(trimethylsilyl)acetylene with HB(C6F5)2 forms the Au(I)X complexes 9a (X: Cl) and 10a (X: NTf2), respectively. Both show marked Au⋯B interactions. The P/B FLP isomer 8, featuring the bulky SiMe3 substituent at the carbon adjacent to boron forms gold complexes 9b and 10b, both of which show weaker Au⋯B interactions. Complex 10a was employed in the catalytic hydroamination of a series of alkynes with p-toluidine. Complexes 9 and 10 were characterized by X-ray diffraction.

Phenylallene reacts rapidly with ClB(C6F5)2 to give the respective 2-borylindene. Several substituted allenylarenes form the respective 2-B(C6F5)2 boryl-substituted indenes upon treatment with ClB(C6F5)2 or BrB(C6F5)2 as well. Bis- and tris-allenylarenes form the corresponding products featuring multiple five-membered ring annulations, including a symmetrical tris-borylated dihydro-1H-trindene derivative. The B(C6F5)2 borylindenes show fluorescence properties.

The charge density distribution of the intramolecular frustrated Lewis pair (FLP) Mes2PCH2CH2B(C6F5)2 (1), the phosphinimine HNPMes2CH2CH2B(C6F5)2 (2), as well as a FLP homologue with nitrogen NEt2CHPhCH2B(C6F5)2 (3) were investigated with Bader’s quantum theory of atoms in molecules (QTAIM). The charge densities were derived from both experimental high-resolution X-ray diffraction data (2, 3) and theoretical calculations (1, 3). The QTAIM analysis for the FLPs 1 and 3 showed the prominent B-pnictogen interaction to be weak dative bonds without significant charge-transfer. This holds also true for the B–N–bond of 2. The nitrogen atom is negatively charged, due to a charge transfer from phosphorous and shows features of a sp2-hybridization. The bond is therefore best described as a non-hypervalent Pδ+–Nδ− moiety.

Piers’ borane HB(C6F5)2 reacted with the alkyne Mes2P–C≡C–SiMe3 by a rarely observed 1,1-hydroboration reaction under kinetic control to give the unsaturated vicinal frustrated phosphane/borane Lewis pair 6, featuring both the PMes2 and SiMe3 groups at the same carbon atom C1. Compound 6 is a reactive P/B FLP which splits dihydrogen under mild conditions. Thermolysis at 100 °C converts it to the markedly less reactive P/B FLP regioisomer which bears the −SiMe3 substituent at carbon atom C2 adjacent to the B(C6F5)2 group. Most new compounds were characterized by X-ray diffraction.

(C6 F5 )2 B-halides were conveniently prepared by treatment of (C6 F5 )2 BH with tritylchloride or -bromide, respectively. With cyclopropylacetylene, (C6 F5 )2 BBr underwent sequential cis-1,2-halogenoboration followed by 1,2-carboboration to give the 4-bromo-2,4-dicyclopropylbutadienyl-B(C6 F5 )2 product. It reacted further with additional cyclopropylacetylene to give the linear triene and tetraene products in a metal-free alkyne oligomerization reaction. The pyridine adduct of the initial diene product was characterized by X-ray diffraction. (C6 F5 )2 BCl reacted analogously. Similar (C6 F5 )2 BX induced oligomerization reactions were carried out with two conjugated enynes.

Hydroboration of N-allyl-cis-2,6-dimethylpiperidine with HB(C6F5)2 gave the trimethylene-bridged frustrated N/B Lewis pair 7. It featured a trans-2,6-dimethyl substitution pattern at the piperidine unit which indicated preceding equilibration with its iminium cation/hydridoborate isomer 6 by means of an internal hydride transfer. In situ generated compound 6 is essential for the reaction with CO/HB(C6F5)2 to give the borataepoxide product 12 at the [N]–(CH2)3–[B] framework. The borataepoxide 12 reacts rapidly with CO2, cleaves the acidic C–H bond of a terminal alkyne, splits dihydrogen, and reacts with nitriles and benzaldehyde. Most products were characterized by X-ray diffraction.

A series of phosphanyl substituted alkynes R12P–CC–R2 [R12P: Mes2P or tBu2P; R2: SiMe3, Ph, tBu] reacts with the X–B(C6F5)2 reagents [X: Cl, Br] to give thermally rather stable neutral zwitterionic phosphirenium borate products.