Exploring the Limits of Frustrated Lewis Pair Chemistry with Alkynes: Detection of a System that Favors 1,1-Carboboration over Cooperative 1,2-P/B-Addition

1,2-Carboboration of Arylallenes by In Situ Generated Alkenylboranes for the Synthesis of 1,4-Dienes

We herein report a novel method for the coupling of unactivated alkynes and arylallenes, which relies on an unprecedented and regioselective 1,2‐carboboration of the allene by an alkenylborane. The alkenylborane is conveniently prepared in situ by hydroboration of an alkyne with Piers’ borane, i. e., HB(C₆F₅)₂. The boryl‐substituted 1,4‐dienes that are formed by this carboboration are well‐suited for a subsequent Suzuki‐Miyaura coupling with aryl iodides. This allowed us to develop a three‐step, one‐pot protocol for the synthesis of aryl‐substituted 1,4‐dienes. The generality of the reaction was demonstrated by the synthesis of twenty dienes with modular variations of all three reaction partners. The mechanism of the new 1,2‐carboboration was investigated using dispersion corrected double‐hybrid DFT computations that allowed us to rationalize the chemo‐ and regioselectivity of this key step.

Piers' Borane-Induced Tetramerization of Arylacetylenes

We herein report that the reaction of Piers′ borane, i. e. HB(C₆F₅)₂, with an excess of arylacetylenes at room temperature leads to tetramerization of the acetylene and the diastereoselective formation of boryl‐substituted tetra‐aryl‐tetrahydropentalenes. The reaction mechanism was investigated by isotope labeling experiments and DFT computations. These investigations indicate that a series of 1,2‐carboboration reactions form an octatetraene that undergoes an electrocyclization. Two skeletal rearrangements then presumably lead to the formation of the tetrahydropentalene core. Overall, this intricate and unprecedented transformation comprises five carbon‐carbon bond formations in a single reaction.

Borane-catalyzed selective dihydrosilylation of terminal alkynes: reaction development and mechanistic insight

Here, we describe simple B(C₆F₅)₃-catalyzed mono- and dihydrosilylation reactions of terminal alkynes by using a silane-tuned chemoselectivity strategy, affording vinylsilanes and unsymmetrical geminal bis(silanes). This strategy is applicable to the dihydrosilylation of both aliphatic and aryl terminal alkynes with different silane combinations. Gram-scale synthesis and conducting the reaction without the exclusion of air and moisture demonstrate the practicality of this methodology. The synthetic utility of the resulting products was further highlighted by the structural diversification of geminal bis(silanes) through transforming the secondary silane into other silyl groups. Comprehensive theoretical calculations combined with kinetical isotope labeling studies have shown that a prominent kinetic differentiation between the hydrosilylation of alkynes and vinylsilane is responsible for the chemoselective construction of unsymmetrical 1,1-bis(silanes).

A B(C₆F₅)₃/silane-based system enables the chemoselective dihydrosilylation of terminal alkynes. Using a combination of different types of hydrosilanes, a series of unsymmetrical or symmetrical 1,1-bis(silanes) could be constructed.

1,2-Carbopentafluorophenylation of Alkynes: The Metallomimetic Pull-Push Reactivity of Tris(pentafluorophenyl)borane

We report the novel single-step 1,2-dicarbofunctionalization of an arylacetylene with an allylsilane and tris(pentafluorophenyl)borane [B(C₆ F₅ )₃ ] involving C-C bond formation with C-H bond scission at the β-position to the silicon atom of an allylsilane and B→C migration of a C₆ F₅ group. The 1,2-carbopentafluorophenylation occurs smoothly without the requirement for a catalyst or heating. Mechanistic studies suggest that the metallomimetic "pull-push" reactivity of B(C₆ F₅ )₃ imparts consecutive electrophilic and nucleophilic characteristics to the benzylic carbon of the arylacetylene. Subsequent photochemical 6π-electrocyclization affords tetrafluoronaphthalenes, which are important in the pharmaceutical and materials sciences. Owing to the unique reactivity of B(C₆ F₅ )₃ , the 1,2-carbopentafluorophenylation using 2-substituted furan proceeded with ring opening, and the reaction using silyl enolates formed a C-C bond with C-O bond scission at the silyloxy-substituted carbon.

Indene formation upon borane-induced cyclization of arylallenes, 1,1-carboboration, and retro-hydroboration

We herein report the reaction of arylallenes with tris(pentafluorophenyl)borane that yields pentafluorophenyl substituted indenes. The tris(pentafluorophenyl)borane induces the cyclization of the allene and transfers a pentafluorophenyl ring in the course of this reaction. A Hammett plot analysis and DFT computations indicate a 1,1-carboboration to be the C-C bond-forming step.

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.

Characterization of the Zwitterionic Intermediate in 1,1-Carboboration of Alkynes

The reaction of a Lewis acidic borane with an alkyne is a key step in a diverse range of main group transformations. Alkyne 1,1-carboboration, the Wrackmeyer reaction, is an archetypal transformation of this kind. 1,1-Carboboration has been proposed to proceed through a zwitterionic intermediate. We report the isolation and spectroscopic, structural and computational characterization of the zwitterionic intermediates generated by reaction of B(C₆ F₅ )₃ with alkynes. The stepwise reactivity of the zwitterion provides new mechanistic insight for 1,1-carboboration and wider B(C₆ F₅ )₃ catalysis. Making use of intramolecular stabilization by a ferrocene substituent, we have characterized the zwitterionic intermediate in the solid state and diverted reactivity towards alkyne cyclotrimerization.

Metallomimetic Chemistry of Boron

The study of main-group molecules that behave and react similarly to transition-metal (TM) complexes has attracted significant interest in recent decades. Most notably, the attractive idea of replacing the all-too-often rare and costly metals from catalysis has motivated efforts to develop main-group-element-mediated reactions. Main-group elements, however, lack the electronic flexibility of TM complexes that arises from combinations of empty and filled d orbitals and that seem ideally suited to bind and activate many substrates. In this review, we look at boron, an element that despite its nonmetal nature, low atomic weight, and relative redox staticity has achieved great milestones in terms of TM-like reactivity. We show how in interelement cooperative systems, diboron molecules, and hypovalent complexes the fifth element can acquire a truly metallomimetic character. As we discuss, this character is powerfully demonstrated by the reactivity of boron-based molecules with H₂, CO, alkynes, alkenes and even with N₂.

Metal-free gem selective dimerization of terminal alkynes catalyzed by a pyridonate borane complex

The metal-free dimerization of alkynes catalyzed by a pyridonate borane complex via a 1,2-carboration is reported.

Stoichiometric and catalytic isomerization of alkenylboranes using bulky Lewis bases

1,1-Carboboration of alkynes with boranes R'B(C₆F₅)₂ (R' = (CH₂)₃Ph, C₆F₅) affords mixtures of the E and Z isomers of RCH = CR'B(C₆F₅)₂ (R = (CH₂)₂Me, (CH₂)₃Me, (CH₂)₂Ph, CH₂CHMe₂, CHMe₂, CMe₃, CH₂OMe, CH₂N(CO)₂C₆H₄), treatment of these alkenylboranes with stoichiometric or catalytic amounts of tBu₂PH or other donors are shown to induce isomerization. The proposed mechanism involving a zwitterionic borataalkene intermediate is supported by computations.

Tris(2,4,6-trifluorophenyl)borane: An Efficient Hydroboration Catalyst

The metal-free catalyst tris(2,4,6-trifluorophenyl)borane has demonstrated its extensive applications in the 1,2-hydroboration of numerous unsaturated reagents, namely alkynes, aldehydes and imines, consisting of a wide array of electron-withdrawing and donating functionalities. A range of over 50 borylated products are reported, with many reactions proceeding with low catalyst loading under ambient conditions. These pinacol boronate esters, in the case of aldehydes and imines, can be readily hydrolyzed to leave the respective alcohol and amine, whereas alkynyl substrates result in vinyl boranes. This is of great synthetic use to the organic chemist.

Reactivity of the geminal phosphinoborane tBu2PCH2BPh2 towards alkynes, nitriles, and nitrilium triflates

The reactivity of the geminal phosphinoborane tBu₂PCH₂BPh₂ towards terminal alkynes, nitriles and nitrilium salts is investigated.

Borane-catalyzed indole synthesis through intramolecular hydroamination

Catalytic metal-free intramolecular hydroamination for the synthesis of indoles and tetrahydroisoquinolines was achieved.

B(C6F5)3-guided cyclotrimerization-rearrangement of phenylacetylene. Evidence of the (C6F5)3B−–C(H)=C+Ph intermediate in a 1,1-carboboration reaction

In presence of 2,2,6,6-tetramethylpiperidinium ([TMPH]+) chlorotris(pentafluorophenyl)borate ([TMPH]+[ClB(C6F5)3]−, 3), phenylacetylene undergoes an unusual cyclotrimerization-rearrangement leading to tris(pentafluorophenyl)(3,4,5-triphenylphenyl)borate anion (1) as a minor product which can be isolated and purified in a form of salts [1·(TMPH) n ·Cl ( n –1)] (n=3 or 5). A variable temperature and concentration NMR spectroscopy study of 3 in CDCl3 unambiguously demonstrated its ability to liberate free B(C6F5)3, which initiates cyclotrimerization and guides rearrangements towards formation of the tetraarylborate anion 1. For the previously studied “spectator” reaction between phenylacetylene and B(C6F5)3 in CDCl3, 1H, 19F, and 11B NMR-spectral evidence of the (C6F5)3B−–C(H)=C+Ph zwitterionic intermediate of the 1,1-carboboration reaction has been demonstrated. The crystal structures of [1·(TMPH) 3 ·Cl 2], the salt 3, and a 1:1 adduct of 1,3,5-tris(4-fluorophenyl)benzene and 2,4,6-tris(pentafluorophenyl)-1,3,5,2,4,6-trioxatriborinane (2) have been established by X-ray diffraction analysis.

Frustrated Lewis Pair-Catalyzed Cycloisomerization of 1,5-Enynes via a 5-endo-dig Cyclization/Protodeborylation Sequence

The first frustrated Lewis pair-catalyzed cycloisomerization of a series of 1,5-enynes was developed. The reaction proceeds via the π-activation of the alkyne and subsequent 5-endo-dig cyclization with the adjacent alkene. The presence of PPh3 was of utmost importance on the one hand to prevent side reactions (for example, 1,1-carboboration) and on the other hand for the efficient protodeborylation to achieve the catalytic turnover. The mechanism is explained on the basis of quantum-chemical calculations, which are in full agreement with the experimental observations.

Highly selective catalytic trans-hydroboration of alkynes mediated by borenium cations and B(C6F5)3

Transition metal free alkyne trans-hydroboration is achieved using strong boron electrophiles in the presence of a B–H moiety.

The trans-hydroboration of terminal alkynes mediated by borenium cations [NHC(9-BBN)]⁺ (NHC = N-heterocyclic carbene, 9-BBN = 9-borabicyclo(3.3.1)nonane) exclusively affords Z-vinylboranes. NHCs and chelating dialkyl substituents on the borenium cation and “non”-basic anions were essential to preclude alternative reactions including dehydroboration. Deuterium labelling studies indicate the mechanism involves addition of the boron electrophile to the alkyne and transfer of hydride to the opposite face of the activated alkyne. trans-Hydroboration proceeds with only catalytic amounts of B(C₆F₅)₃ or [Ph₃C][B(C₆F₅)₄] to activate the (NHC)9-BBN(H) precursor with the borenium regenerated in the hydride transfer step. The NHC can be removed from the trans-hydroborated products by the addition of Et₂O–BF₃ providing access to vinylBBN species effective for Suzuki–Miyaura couplings to generate Z-alkenes. Combinations of catalytic B(C₆F₅)₃ and stoichiometric [HB(C₆F₅)₃]^– also lead to trans-hydroboration of terminal alkynes to form Z-isomers of [arylCH Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> CHB(C₆F₅)₃]^–.

Phosphole formation by 1,1-carboboration – reactions of bis-alkynyl phosphanes with a frustrated P/B Lewis pair

The reaction of arylbis(alkynyl)phosphanes with the frustrated P/B Lewis pair mes₂PCH₂CH₂B(C₆F₅)₂gives phosphane/borane functionalized phospholes by a sequence of 1,1-carboboration reactions.

Cyclopropanation/Carboboration Reactions of Enynes with B(C6F5)3

Stoichiometric reaction of B(C6F5)3 with 1,6-enynes is shown to proceed via initial cyclopropanation and formal 1,1-carboboration. Depending on the substitution on the alkene moiety, subsequent ring-opening of the cyclopropane affords either cyclopentane or cyclohexane derivatives in which the C6F5 and B(C6F5)2 adopt a 1,4-positioning. Mechanistically, this transformation involves π-activation of the alkyne moiety, which triggers cyclopropanation, followed by carboboration. Both the cyclopropanation and subsequent ring-opening are shown to be stereospecific. Both cyclopropanation and 1,4-carboborated products were employed as Lewis acid components in frustrated Lewis pair activation of H2 and CO2.

Advanced 1,1-carboboration reactions with pentafluorophenylboranes

The advanced 1,1-carboboration reaction converts alkynes to alkenylboranes and allows a variety of sequential ring forming reactions of bis(alkynyl) compounds.

The 1,1 carboboration reaction of a variety of metal-substituted alkynes with simple trialkylboranes R₃B yields the respective alkenylboranes (Wrackmeyer reaction). The use of the strongly electrophilic R-B(C₆F₅)₂ reagents allows for much milder reaction conditions and gives good yields of the respective bulky alkenylboranes from conventional terminal alkynes by means of 1,2-hydride migration. Even internal alkynes undergo 1,1-carboboration with the R-B(C₆F₅)₂ reagents, in this case yielding alkenylboranes by means of C–C bond cleavage. Phosphorus, sulfur or even boron containing substituents can serve as the migrating alkynyl substituents in the advanced 1,1-carboboration reactions using the R-B(C₆F₅)₂ reagents. Sequential 1,1-carboboration of geminal bis(alkynyl) derivatives of these elements with the R-B(C₆F₅)₂ boranes yields boryl substituted phospholes, thiophenes or even boroles in quite a variety. Vicinal bis(alkynyl)arenes or heteroarene substrates undergo benzannulation reactions in this way. Many of the -B(C₆F₅)₂ substituted 1,1-carboboration products can be used as reagents in cross coupling reactions. A recently disclosed organometallic analogue, namely a 1,1-carbozirconation reaction is described.

Functionalization of Intramolecular Frustrated Lewis Pairs by 1,1-Carboboration with Conjugated Enynes

The vicinal P/B frustrated Lewis pair (FLP) Mes2PCH2CH2B(C6F5)2 undergoes 1,1-carboboration reactions with the Me3Si-substituted enynes to give ring-enlarged functionalized C3-bridged P/B FLPs. These serve as active FLPs in the activation of dihydrogen to give the respective zwitterionic [P]H(+)/[B]H(-) products. One such product shows activity as a metal-free catalyst for the hydrogenation of enamines or a bulky imine. The ring-enlarged FLPs contain dienylborane functionalities that undergo "bora-Nazarov"-type ring-closing rearrangements upon photolysis. A DFT study had shown that the dienylborane cyclization of such systems itself is endothermic, but a subsequent C6F5 migration is very favorable. Furthermore, substituted 2,5-dihydroborole products are derived from cyclization and C6F5 migration from the photolysis reaction. In the case of the six-membered annulation product, a subsequent stereoisomerization reaction takes place and the resultant compound undergoes a P/B FLP 1,2-addition reaction with a terminal alkyne with rearrangement.

A 1,1-Carboboration Route to Bora-Nazarov Systems

Hydroboration of the conjugated enynes 1 a and 1 b with Piers' borane [HB(C6F5)2] gave the respective dienylboranes trans-2 c and trans-2 d. Their photolysis resulted in the formation of the dihydroborole products 3 c and 3 d. Both were converted to their pyridine adducts 5 c and 5 d, respectively. Compounds 3 c and 5 c,d were characterized by X-ray diffraction. The reaction of the bis(enynyl)boranes 6 a and 6 b with B(C6F5)3 resulted in the formation of the dihydroboroles 7 a and 7 b, respectively. This reaction is thought to proceed by 1,1-carboboration of one of the enynyl substituents at boron to generate the dienylborane intermediates 8 a/8 b, followed by thermally induced bora-Nazarov ring-closure and subsequent stabilizing 1,2-pentafluorophenyl group migration from boron to carbon. Compound 7 a was characterized by X-ray diffraction and solid-state (11)B NMR spectroscopy.

Σ- versus π-activation of alkynyl benzoates using B(C₆F₅)₃

We have prepared a range of alkynyl benzoates in high yields and have investigated their reactivities with the strong Lewis acid B(C6F5)3. In such molecules both σ-activation of the carbonyl and π-activation of the alkyne are possible. In contrast to the reactivity of propargyl esters with B(C6F5)3 which proceed via 1,2-addition of the ester and B(C6F5)3 across the alkyne, the inclusion of an additional CH2 spacer switches off the intramolecular cyclization and selective σ-activation of the carbonyl group is observed through adduct formation. This change in reactivity appears due to the instability of the species which would be formed through B(C6F5)3 activation of the alkyne.

Thiophene synthesis via 1,1-carboboration

Reaction of bis(tert-butylethynyl)sulfide with the boron Lewis acid reagents X–B(C₆F₅)₂ (X = CH₃, Cl, C₆F₅) in pentane at r.t. gave the respective borylated thiophenes in a sequence of 1,1-carboboration reactions.

1,1-Alkenylboration of diarylphosphino-enynes: convenient synthetic entry to vicinal P/B Lewis pairs at extended conjugated π-frameworks

Alkenylboranes R-CHCH-B(C₆F₅)₂ undergo carbon–carbon coupling by means of 1,1-alkenylboration with diarylphosphino-enynes to give conjugated hexatriene derivatives that bear a vicinal pair of B(C₆F₅)₂ and PAr₂ functionalities at the framework.

syn-1,2-carboboration of alkynes with borenium cations

The reaction of 8-(trimethylsiloxy)quinoline (QOTMS) with BCl₃ and (aryl)BCl₂ forms QOBCl₂ and QOBCl(aryl). The subsequent addition of stoichiometric AlCl₃ follows one of two paths, dependent on the steric demands of the QO ligand and the electrophilicity of the resulting borenium cation. The phenyl- and 5-hexylthienylborenium cations, QOBPh⁺ and QOBTh⁺, are formed, whereas QOBCl⁺ is not. Instead, AlCl₃ preferentially binds with QOBCl₂ at oxygen, forming QOBCl₂⋅AlCl₃, rather than abstracting chloride. A modest increase in the steric demands around oxygen, by installing a methyl group at the 7-position of the quinolato ligand, switches the reactivity with AlCl₃ back to chloride abstraction, allowing formation of QOBCl⁺. All the prepared borenium cations are highly chlorophilic and exhibit significant interaction with AlCl₄⁻ resulting in an equilibrium concentration of Lewis acidic “AlCl₃” species. The presence of “AlCl₃^” species limits the alkyne substrates compatible with these borenium systems, with reaction of [QOBPh][AlCl₄] with 1-pentyne exclusively yielding the cyclotrimerised product, 1,3,5-tripropylbenzene. In contrast, QOBPh⁺ and QOBTh⁺ systems effect the syn-1,2-carboboration of 3-hexyne. DFT calculations at the M06-2X/6-311G(d,p)/PCM(DCM) level confirm that the higher migratory aptitude of Ph versus Me leads to a lower barrier to 1,2-carboboration relative to 1,1-carboboration.

The B(C6F5)3 boron Lewis acid route to arene-annulated pentalenes

4,5-Dimethyl-1,2-bis(1-naphthylethynyl)benzene (12) undergoes a rapid multiple ring-closure reaction upon treatment with the strong boron Lewis acid B(C6F5)3 to yield the multiply annulated, planar conjugated π-system 13 (50 % yield). In the course of this reaction, a C6F5 group was transferred from boron to carbon. Treatment of 12 with CH3B(C6F5)2 proceeded similarly, giving a mixture of 13 (C6F5-transfer) and the product 15, which was formed by CH3-group transfer. 1,2-Bis(phenylethynyl)benzene (8 a) reacts similarly with CH3B(C6F5)2 to yield a mixture of the respective C6F5- and CH3-substituted dibenzopentalenes 10 a and 16. The reaction is thought to proceed through zwitterionic intermediates that exhibit vinyl cation reactivities. Some B(C6F5)3-substituted species (26, 27) consequently formed by in situ deprotonation upon treatment of the respective 1,2-bis(alkynyl)benzene starting materials (24, 8) with the frustrated Lewis pair B(C6F5)3/P(o-tolyl)3. The overall formation of the C6F5-substituted products formally require HB(C6F5)2 cleavage in an intermediate dehydroboration step. This was confirmed in the reaction of a thienylethynyl-containing starting material 21 with B(C6F5)3, which gave the respective annulated pentalene product 23 that had the HB(C6F5)2 moiety 1,4-added to its thiophene ring. Compounds 12-14, 23, and 26 were characterized by X-ray diffraction.

Reaction of strongly electrophilic alkenylboranes with phosphanylalkynes: rare examples of intermolecular 1,1-alkenylboration reactions

Simple alkenylbis(pentafluorophenyl)boranes undergo 1,1-alkenylboration with phosphanylacetylenes to give phosphane-borane Lewis pairs with a conjugated diene backbone.

Carbonylation reactions of intramolecular vicinal frustrated phosphane/borane Lewis pairs

The intramolecular frustrated Lewis pair (FLP) Mes2PCH2CH2B(C6F5)2 4 adds cooperatively to carbon monoxide to form the five-membered heterocyclic carbonyl compound 5. The intramolecular FLP 7 contains an exo-3-B(C6F5)2 Lewis acid and an endo-2-PMes2 Lewis base functionality coordinated at the norbornane framework. This noninteracting FLP adds carbon monoxide in solution at -35 °C cooperatively to yield a five-membered heterocyclic FLP-carbonyl compound 8. In contrast, FLP 7 is carbonylated in a CO-doped argon matrix at 25 K to selectively form a borane carbonyl 9 without involvement of the adjacent phosphanyl moiety. The free FLP 7 was generated in the gas phase from its FLPH2 product 10. A DFT study has shown that the phosphonium hydrido borate zwitterion 10 is formed exergonically in solution but tends to lose H2 when brought into the gas phase.