The renaissance of Brønsted superbases is primarily based on their pronounced capacity for a large variety of chemical transformations under mild reaction conditions. Four major set screws are ...available for the selective tuning of the basicity: the nature of the basic center (N, P, …), the degree of electron donation by substituents to the central atom, the possibility of charge delocalization, and the energy gain by hydrogen bonding. Within the past decades, a plethora of neutral electron‐rich phosphine and phosphazene bases have appeared in the literature. Their outstanding properties and advantages over inorganic or charged bases have now made them indispensable as auxiliary bases in deprotonation processes. Herein, an update of the chemistry of basic phosphines and phosphazenes is given. In addition, due to widespread interest, their use in catalysis or as ligands in coordination chemistry is highlighted.
The hunt is still on for the most powerful neutral superbase. The steadily growing scope of applications has resulted in a renaissance of highly electron‐rich phosphorus‐containing Lewis and Brønsted bases. Strongly π‐donating substituents at the center of basicity can be employed to increase the proton affinity and the σ‐donation ability. This minireview highlights recently published achievements focusing especially on the past five years.
The introduction of a number of unsaturated fluorinated and chlorinated substituents onto gallium and indium was investigated. Gallate, GaR4−, and indate, InR4−, anions with trifluorovinyl (CF=CF2), ...1‐chloro‐2,2‐difluorovinyl (CCl=CF2), (E)‐pentafluoropropen‐1‐yl (E‐CF=CFCF3), 3,3,3‐trifluoropropynyl (C≡C−CF3) and dichloromethyl (CHCl2) substituents were synthesized and isolated as the respective tetraphenylphosphonium salts. The respective gallate and indate salts were characterized by various methods, including NMR spectroscopy, mass spectrometry and X‐ray diffraction analysis. The generation of the respective organyllithium reagents, required for the syntheses of the gallate and indates, was performed starting from commercially available materials. This series of novel gallate and indate salts extends the chemistry of electron withdrawing groups to gallium and indium compounds.
A host of perfluorinated unsaturated electron withdrawing groups are introduced to gallium and indium in the form of novel gallate and indate salts. These gallates and indates constitute an important first step for future investigations into the respective strong Lewis acids.
Herein, ring‐opening of mesoionic carbenes (iMICs) (iMIC=ArC{N(Dipp)}2C(SiMe3)C:) (Dipp=2,6‐iPr2C6H3, Ar=Ph, 4‐Me2NC6H4 or 4‐PhC6H4) based on an 1,3‐imidazole scaffold to yield N‐ethynylformimidamide ...(eFIM) derivatives as crystalline solids (eFIM={(Dipp)N=C(Ar)N(Dipp)}C≡CSiMe3) is reported. eFIMs are thermally stable under inert gas atmosphere and show moderate air stability (t1/2=3 h for Ar=Ph). eFIMs are excellent surrogates of iMICs, which generally have a limited shelf‐life, and readily undergo ring‐closing click reactions with a variety of main‐group as well as transition metal Lewis acids to form hitherto challenging iMIC‐compounds in good to excellent yields. In addition to the relevance of eFIMs in the synthesis of iMIC‐compounds, quantification of the stereoelectronic properties of a representative iMIC (Ar=Ph) by experimental and theoretical methods suggests remarkably σ‐donor property and steric profile of these new ligand sets.
Stable unmasked iMICs are extremely scarce and have limited shelf‐life. An unprecedented ring‐opening of iMICs to N‐ethynylformimidamides (eFIMs) has been reported. eFIMs are thermally stable under an inert atmosphere and show moderately air‐stability. They are excellent iMIC‐surrogates and readily undergo ring‐closing‐clicks with a variety of (non)metal Lewis acids, affording iMIC‐compounds (iMIC)M with zero waste and easy workup.
Cyclic organoalane compounds (ADCAr)AlH22 (ADCAr = ArC{(DippN)C}2; Dipp = 2,6‐iPr2C6H3; Ar = Ph or 4‐PhC6H4 (Bp)) based on anionic dicarbene (ADC) frameworks have been reported as crystalline solids. ...Treatments of Li(ADCAr) with LiAlH4 at room temperature afford (ADCAr)AlH22 with the concomitant release of LiH. Compounds (ADCAr)AlH22 are stable crystalline solids and are freely soluble in common organic solvents. They are annulated tricyclic compounds with an almost planar central C4Al2‐core embedded between two peripheral 1,3‐imidazole (C3N2) rings. At room temperature, (ADCPh)AlH22 readily reacts with CO2 to form two‐ and four‐fold hydroalumination products (ADCPh)AlH(OCHO)2 and (ADCPh)Al(OCHO)22, respectively. Further hydroalumination reactivity of (ADCPh)AlH22 has been shown with isocyanate (RNCO) and isothiocyanate (RNCS) species (R=alkyl or aryl group). All compounds have been characterized by NMR spectroscopy, mass spectrometry, and single‐crystal X‐ray diffraction.
The carbocyclic aluminium hydrides (ADCAr)AlH22 are readily accessible as crystalline solids on treatment of anionic dicarbenes Li(ADCAr) with LiAlH4. Hydroalumination of CO2 (and other organic substrates) with (ADCPh)AlH22 has been shown to access (ADCPh)Al(OCH(O))22 (and other products).
A bidentate boron Lewis acid based on 1,8‐diethynylanthracene has been studied in detail with respect to its adduct formation with diamines and diphosphanes of different linker lengths between the ...donor functions. A clear correlation between the linker length of the bifunctional base and the formation of 1 : 1 adducts, 1 : 2 adducts or oligomers was found. The adducts were characterized in solution by NMR titration experiments and structurally by X‐ray diffraction. In addition, adduct formation and competition experiments of the host system with ZR3 (Z=N, P; R=H, Me) demonstrated the generally higher stability of alkylphosphane adducts compared to alkylamine adducts with boron functions. The results provide a general insight into the adduct formation of bidentate Lewis acids with guests of different sizes as well as the differences in stability between borane‐amine and borane‐phosphane adducts.
The adduct formation of a bidentate boron Lewis acid with diamines and ‐phosphines with an increasing linker length was observed in solution by NMR experiments and in the solid state by X‐ray diffraction. This study offers insight into the selectivity of the host‐guest adduct formation of bidentate Lewis acids with host molecules of varying size.
We report the room temperature dimerization of carbon monoxide mediated by C4/C5‐vicinal anionic dicarbenes Li(ADC) (ADC = ArC{(Dipp)NC}2; Dipp = 2,6‐iPr2C6H3; Ar = Ph, DMP (4‐Me2NC6H4), Bp ...(4‐PhC6H4)) to yield (E)‐ethene‐1,2‐bis(olate) (i.e. −O−C=C−O− = COen) bridged mesoionic carbene (iMIC) lithium compounds COen‐(iMIC)Li2 (COen‐iMIC2 = ArC{(Dipp)NC}2(CO)2) in quantitative yields. COen‐(iMIC)Li2 are highly colored stable solids, exhibit a strikingly small HOMO–LUMO energy gap, and readily undergo 2e‐oxidations with selenium, CuCl (or CuCl2), and AgCl to afford the dinuclear compounds COon‐(iMIC)E2 (E = Se, CuCl, AgCl) featuring a 1,2‐dione bridged neutral bis‐iMIC (i.e. COon‐iMIC2 = ArC{(Dipp)NC}2(C=O)2). COen‐(iMIC)Li2 undergo redox‐neutral salt metathesis reactions with LiAlH4 and (Et2O)2BeBr2 and afford COen‐(iMIC)AlH22 and COen‐(iMIC)BeBr2, in which the dianionic COen‐moiety remains intact. All compounds have been characterized by NMR spectroscopy, mass spectrometry, and X‐ray diffraction. Stereoelectronic properties of COon‐iMIC2 are quantified by experimental and theoretical methods.
Direct room temperature dimerization of carbon monoxide by anionic dicarbenes Li(ADC) has been reported to quantitatively yield (E)‐ethene‐1,2‐bis(olate) bridged mesoionic carbene (iMIC) lithium compounds, COen‐(iMIC)Li2. They undergo 2e‐oxidation to afford 1,2‐dione bridged bis iMIC, COon‐(iMIC)2 containing compounds while redox neutral salt metatheses yield COen‐(iMIC)E2 compounds (E=main group species).
Carbocyclic aluminium halides (ADC)AlX22 (2‐X) (X=F, Cl, and I) based on an anionic dicarbene (ADC=PhC{N(Dipp)C}2, Dipp = 2,6‐iPr2C6H3) framework are prepared as crystalline solids by ...dehydrohalogenations of the alane (ADC)AlH22 (1). KC8 reduction of 2‐I affords the peri‐annulated Al(III) compound (ADCH)AlH2 (4) (ADCH=PhC{N(Dipp)C2(DippH)N}, DippH=2‐iPr,6‐(Me2C)C6H3)) as a colorless crystalline solid in 76 % yield. The formation of 4 suggests intramolecular insertion of the putative bis‐aluminylene species (ADC)Al2 (3) into the methine C−H bond of HCMe2 group. Calculations predict singlet ground state for 3, while the conversion of 3 into 4 is thermodynamically favored by 61 kcal/mol. Compounds 2‐F, 2‐Cl, 2‐I, and 4 have been characterized by NMR spectroscopy and their solid‐state molecular structures have been established by single crystal X‐ray diffraction.
KC8 reduction of 2‐I with a catalytic amount the NHC (IMe4) has been shown to afford the peri‐annulated compound 4 as a colorless crystalline solid. The formation of 4 suggests intramolecular insertion of the putative bis‐aluminylene 3 into the C−H bonds of HCMe2 groups. Calculations predict singlet ground state for 3 while the conversion of 3 into 4 is thermodynamically favored by 61 kcal/mol.
Herein, reactivity studies of a cyclic bis‐hydridostannylene (ADC)SnH2 (1‐H2) (ADC=PhC{(NDipp)C}2; Dipp=2,6‐iPr2C6H3) with various unsaturated organic substrates are reported. Reactions of terminal ...alkynes (RC≡CH) with 1‐H2 afford mixed acetylide‐vinyl‐functionalized bis‐stannylenes via dehydrogenation and hydrostannylation. Treatment of 1‐H2 with PhC≡CCH3 gives a unique distannabarrelene via dehydrogenative C(sp3)−H stannylation and hydrostannylation of the C≡CCH3 moiety. 1‐H2 undergoes dehydrogenative 2+2‐cycloaddition reactions with diphenylacetylene, azobenzene, acetone, benzophenone, and benzaldehyde to form the 1,4‐distannabarrelene derivatives. The elimination of H2 in these reactions suggests the masked‐diradical property of 1‐H2. In fact, these 2+2‐cycloaddition products are also accessible on treatments of the Sn(I) diradicaloid (ADC)Sn2 (1) with appropriate reagents. All compounds have been characterized by multinuclear NMR spectroscopy and single crystal X‐ray diffraction. Moreover, the catalytic activity of 1‐H2 has been shown for the hydroboration of unsaturated substrates.
Reactivity (including catalytic activity) of hydridostannylene (1‐H2) with a variety of unsaturated (alkyne, carbonyl, azo) substrates has been reported. 1‐H2 (e. g.) undergoes dehydrogenative 2+2‐cycloaddition with Ph2CO to form the 1,4‐distannabarrelene 1‐OCPh2, which readily liberates Ph2CO on exposure to H2 and regenerates 1‐H2. Thus, 1‐H2 behaves as a masked Sn‐diradicaloid 1 that also affords 1‐OCPh2 on treatment with Ph2CO.
The reaction of the oxygen‐bridged frustrated Lewis pairs (FLPs) tBu2P−O−Si(C2F5)3 (1) and tBu2P−O−AlBis2 (2) with azobenzene, promoted by UV irradiation, led to a selective complexation of the ...cis‐isomer. The addition product of 2 is stable, while the adduct of 1 isomerizes in solution in an ortho‐benzidine‐like 3,3‐rearrangement by cleavage of the N−N bond, saturation of the nitrogen atoms with hydrogen atoms and formation of a new bond between two phenyl ortho‐carbon atoms. Similar rearrangements take place with different para‐substituted azobenzenes (R=Me, OMe, Cl) and di(2‐naphthyl)diazene, while ortho‐methylated azo compounds do not form adducts with 1. All adducts were characterized by multinuclear NMR spectroscopy and elemental analyses and the mechanism of the rearrangement was explored by quantum‐chemical calculations.
C−C bond formation and complete N=N bond rupture in azobenzenes was induced by a heterosiloxane‐type frustrated POSi Lewis pair; the reactivity resembles that of the acid initiated classical benzidine rearrangement, but starting from an N=N double‐bonded system.
The geminal frustrated Lewis pair (FLP) (F5C2)3SnCH2P(tBu)2 (2) was prepared by reacting (F5C2)3SnCl with LiCH2P(tBu)2. It is neutral and contains an extremely electronegatively substituted, but ...relatively soft (hard–soft acid–base, HSAB) acidic tin function. Its FLP‐type reactivity was proven by reaction with a variety of small molecules (CO2, SO2, CS2, PhNCO, HCl, (Ph3P)AuCl). However, it shows no reaction in H/D scrambling experiments with H2/D2 mixtures and binds CO2 reversibly, as was observed by VT‐NMR spectroscopy. Compound 2 and all its adducts were completely characterized by means of multinuclear NMR spectroscopy, elemental analysis, and X‐ray diffraction experiments.
A soft binding site is provided by the tin atom in the intramolecular frustrated Lewis pair (F5C2)3SnCH2P(tBu)2, which forms adducts with a range of small molecules. The softer CS2 binds more strongly, whereas CO2 forms a weaker and reversibly bound adduct.
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