Diverse Reactivity of Amidinate-Supported Boron Centers with the Hypersilyl Anion and Access to a Monomeric Secondary Boron Hydride

Diverse reactivity of the bulky tris(trimethylsilyl)silyl substituent [Si(SiMe3)3], also known as the hypersilyl group, was observed for amidinate-supported dichloro- and phenylchloroborane complexes. Treatment of the dichloroborane with potassium tris(trimethylsilyl)silyl led to the activation of the backbone β-carbon center and formation of saturated four-membered heterocyclic chloroboranes R'{Si(SiMe3)3}C(NR)2BCl [R' = Ph, R = Cy (3); R' = Ph, R = iPr (6); R' = tBu, R = Cy (8)], whereas the four-membered amidinate hypersilyl-substituted phenyl borane 4 {PhC(NCy)2B(Ph)[Si(SiMe3)3]} was observed for the case of an amidinate-supported phenylchloroborane. The highly deshielded 11B NMR spectroscopic resonance and the distinct difference in the 29Si NMR spectrum confirmed the presence of a σ-donating hypersilyl effect on compounds 3, 6, and 8. Reaction of 3 with the Lewis acid AlCl3 led to the formation of complex 11 in which an unusual cleavage of one of the C-N bonds of the amidinate backbone is observed. Nucleophilic substitution at the boron center of saturated chloroborane 3 with phenyllithium generated the phenylborane derivative 12, whereas the secondary monomeric boron hydride 13 was observed after treatment with alane (AlH3). All compounds (2-13) have been fully characterized by NMR spectroscopy and single-crystal X-ray structure determination studies.

Magnetically Recyclable Borane Lewis Acid Catalyst for Hydrosilylation of Imines and Reductive Amination of Carbonyls

Fluorinated arylborane-based Lewis acid catalysts have shown remarkable activity and serve as ideal examples of transition metal-free catalysts for diverse organic transformations. However, their homogeneous nature poses challenges in terms of recyclability and separation from reaction mixtures. This work presents an efficient technique for the heterogenization of boron Lewis acid catalysts by anchoring Piers' borane to allyl-functionalized iron oxide. This catalyst demonstrates excellent activity in the hydrosilylation of imines and the reductive amination of carbonyls using various silanes as reducing agents under mild reaction conditions. The catalyst exhibits broad tolerance towards a wide range of functional substrates. Furthermore, it exhibits good recyclability and can be easily separated from the products using an external magnetic field. This work represents a significant advance in the development of sustainable heterogenous metal-free catalysts for organic transformations.

Stereocontrol via Propeller Chirality in FLP-Catalyzed Asymmetric Hydrogenation

Utilization of chiral frustrated Lewis pairs as catalysts in enantioselective hydrogenation of unsaturated molecules represents a promising approach in asymmetric synthesis. In our effort to improve our current understanding of the factors governing the stereoselectivity in these catalytic processes, herein we examined the mechanism of direct hydrogenation of aromatic enamines catalyzed by a binaphthyl-based chiral amino-borane. Our computational analysis reveals that only one particular conformer of the key borohydride reaction intermediate can be regarded as a reactive form of this species. This borohydride conformer has a well-defined chiral propeller shape, which induces facial selectivity in the hydride transfer to pro-chiral iminium intermediates. The propeller chirality of the reactive borohydride conformer is generated by the axially chiral binaphthyl scaffold of the amino-borane catalyst through stabilizing π-π stacking interactions. This new computational insight can be readily used to interpret the high degree of stereoinduction observed for these reactions. We expect that the concept of chirality relay could be further exploited in catalyst design endeavors.

Organoboron-mediated polymerizations

The scientific community has witnessed extensive developments and applications of organoboron compounds as synthetic elements and metal-free catalysts for the construction of small molecules, macromolecules, and functional materials over the last two decades. This review highlights the achievements of organoboron-mediated polymerizations in the past several decades alongside the mechanisms underlying these transformations from the standpoint of the polymerization mode. Emphasis is placed on free radical polymerization, Lewis pair polymerization, ionic (cationic and anionic) polymerization, and polyhomologation. Herein, alkylborane/O2 initiating systems mediate the radical polymerization under ambient conditions in a controlled/living manner by careful optimization of the alkylborane structure or additives; when combined with Lewis bases, the selected organoboron compounds can mediate the Lewis pair polymerization of polar monomers; the bicomponent organoboron-based Lewis pairs and bifunctional organoboron-onium catalysts catalyze ring opening (co)polymerization of cyclic monomers (with heteroallenes, such as epoxides, CO2, CO, COS, CS2, episulfides, anhydrides, and isocyanates) with well-defined structures and high reactivities; and organoboranes initiate the polyhomologation of sulfur ylides and arsonium ylides providing functional polyethylene with different topologies. The topological structures of the produced polymers via these organoboron-mediated polymerizations are also presented in this review mainly including linear polymers, block copolymers, cyclic polymers, and graft polymers. We hope the summary and understanding of how organoboron compounds mediate polymerizations can inspire chemists to apply these principles in the design of more advanced organoboron compounds, which may be beneficial for the polymer chemistry community and organometallics/organocatalysis community.

Accessing Highly Substituted Indoles via B(C6F5)3-Catalyzed Secondary Alkyl Group Transfer

Herein, we report a synthetic method to access a range of highly substituted indoles via the B(C6F5)3-catalyzed transfer of 2° alkyl groups from amines. The transition-metal-free catalytic approach has been demonstrated across a broad range of indoles and amine 2° alkyl donors, including various substituents on both reacting components, to access useful C(3)-alkylated indole products. The alkyl transfer process can be performed using Schlenk line techniques in combination with commercially available B(C6F5)3·nH2O and solvents, which obviates the requirement for specialized equipment (e.g., glovebox).

Catalytic hydroboration of aldehydes and ketones with an electron-rich acyclic metallasilylene

The application of main group metal complexes in catalytic reactions is of increasing interest. Here we show that the electron-rich, acyclic metallasilylene L'(Cl)GaSiL C (L' = HC[C(Me)NDipp]2, Dipp = 2,6-iPr2C6H3; L = PhC(NtBu)2) acts as a precatalyst in the hydroboration of aldehydes with HBPin. Mechanistic studies with iso-valeraldehyde show that silylene C first reacts with the aldehyde with [2 + 1] cycloaddition in an oxidative addition to the oxasilirane 1, followed by formation of the alkoxysilylene LSiOCH[Ga(Cl)L']CH2CHMe2 (2), whose formation formally results from a reductive elimination reaction at the Si center. Alkoxysilylene 2 represents the active hydroboration catalyst and shows the highest catalytic activity with n-hexanal (reaction time: 40 min, yield: >99%, TOF = 150 h-1) at room temperature with a catalytic load of only 1 mol%. Furthermore, the hydroboration reaction catalysed by alkoxysilylene 2 is a living reaction with good chemoselectivity. Quantum chemical calculations not only provide mechanistic insights into the formation of alkoxysilylene 2 but also show that two completely different hydroboration mechanisms are possible.

Diiminium Nucleophile Adducts Are Stable and Convenient Strong Lewis Acids

Strong Lewis acids are essential tools for manifold chemical procedures, but their scalable deployment is limited by their costs and safety concerns. We report a scalable, convenient, and inexpensive synthesis of stable diiminium-based reagents with a Lewis acidic carbon centre. Coordination with pyridine donors stabilises these centres; the 2,2'-bipyridine adduct shows a chelation effect at carbon. Due to high fluoride, hydride, and oxide affinities, the diiminium pyridine adducts are promising soft and hard Lewis acids. They effectively produce acylpyridinium salts from carboxylates that can acylate amines to give amides and imides even from electronically intractable coupling partners.

The Effect of Carborane Substituents on the Lewis Acidity of Boranes

The Lewis acidity of primary, secondary, and tertiary boranes with phenyl, pentafluorophenyl, and all three isomers of the C-substituted icosahedral carboranes (ortho, meta, and para) was investigated by computing their fluoride, hydride, and ammonia affinities as well as their global electrophilicity indices and LUMO energies. From these calculations, it was determined that the substituent effects on the Lewis acidity of these boranes follow the trend of ortho-carborane > meta-carborane > para-carborane > C6F5 > C6H5.

Bis(1-Methyl-ortho-Carboranyl)Borane

The Lewis superacid, bis(1-methyl-ortho-carboranyl)borane, is rapidly accessed in two steps. It is a very effective hydroboration reagent capable of B-H addition to alkenes, alkynes, and cyclopropanes. To date, this is the first identified Lewis superacidic secondary borane and most reactive neutral hydroboration reagent.

Examining the reactivity of tris(ortho-carboranyl)borane with Lewis bases and application in frustrated Lewis pair Si-H bond cleavage

Reactions of tris(ortho-carboranyl)borane with Lewis bases reveals only small bases bind. The tremendous bulk and Lewis acidity is leveraged in frustrated Lewis pair Si-H cleavage with a wider range of Lewis bases and greater efficacy than B(C₆F₅)₃.

Structure and reactivity studies of the aluminum analogue of Piers' borane [HAl(C6F5)2]3

The aluminum analogue of Piers' borane, [HAl(C₆F₅)₂]₃1, is prepared on a gram-scale. Density functional theory (DFT) calculations reveal 1 has a higher fluoride ion affinity (FIA) than Piers' borane, while the Al-H moiety proved to be a strong hydride donor, reacting with alcohol and terminal alkyne to give the corresponding dehydrogenative products 3 and 4. Hydroalumination product 5 was prepared via reaction of 1 with aldehyde. In addition, 1 catalyzes the hydrosilylation of alkynes and alkenes.

The phosphinoboration of thiosemicarbazones

This study reports on the exploration of the phosphinoboration reaction with several thiosemicarbazones (R5R4NC(S)NR3N=CR1R2). Reactions between either Ph2PBpin (pin = 1,2-O2C2Me4) or Ph2PBcat (cat = 1,2-O2C6H4) with thiosemicarbazones containing a terminal primary or secondary amine afforded boron-containing heterocyclic 1,3,4-thiadiazoline products in excellent yield. The addition of Ph2PBpin to thiosemicarbazones containing an NMe2 group in the terminal position generated novel five-membered heterocycles in moderate yield, which included boron, sulfur, and nitrogen atoms. Heterocyclization of the thiosemicarbazones occurs preferentially in the presence of functional groups such as acetyl and pyridyl groups.

Tris(ortho-carboranyl)borane: An Isolable, Halogen-Free, Lewis Superacid

The synthesis of tris(ortho-carboranyl)borane (BoCb₃ ), a single site neutral Lewis superacid, in one pot from commercially available materials is achieved. The high fluoride ion affinity (FIA) confirms its classification as a Lewis superacid and the Gutmann-Beckett method as well as adducts with Lewis bases indicate stronger Lewis acidity over the widely used fluorinated aryl boranes. The electron withdrawing effect of ortho-carborane and lack of pi-delocalization of the LUMO rationalize the unusually high Lewis acidity. Catalytic studies indicate that BoCb₃ is a superior catalyst for promoting C-F bond functionalization reactions than tris(pentafluorophenyl)borane [B(C₆ F₅ )₃ ].

B(C6 F5 )3 -Catalyzed E-Selective Isomerization of Alkenes

Herein, we report the B(C₆ F₅ )₃ -catalyzed E-selective isomerization of alkenes. The transition-metal-free method is applicable across a diverse array of readily accessible substrates, giving access to a broad range of synthetically useful products containing versatile stereodefined internal alkenes. The reaction mechanism was investigated by using synthetic and computational methods.

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.

Ferrocene tethered boramidinate frustrated Lewis pairs: stepwise capture of CO2 and CO

The synthesis and reactivity of novel ferrocene tethered boramidinate frustrated Lewis pairs (FLPs), capable of the sequential capture of small molecules, is reported. Reactions of 1,1'-dicarbodiimidoferrocenes with different boranes provides access to metallocene tethered FLPs. The reactivity of the boramidinate moieties can be tuned by the nature of the carbodiimido substituents (alkyl vs. aryl) and the borane used in the reduction (9-borabicyclo[3.3.1]nonane [(C₈H₁₄)₂BH]₂vs. bis-pentafluorophenyl borane [(C₆F₅)₂BH]₂). The boramidinate FLP arms do not engage in intramolecular reactions, allowing for independent small molecule capture by each FLP. By careful synthetic control, sequential capture of different gaseous small molecules (CO₂ and CO or CO₂ and CNtBu) by the same bis(boramidinate)ferrocene molecule has been demonstrated.

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.

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.

Preparation of Structurally and Electronically Diverse N → B-Ladder Boranes by [2 + 2 + 2] Cycloaddition

We report the synthesis of a series of eight N → B-ladder boranes through cobalt-mediated cyclotrimerization of (2-cyanophenyl)-dimesitylborane with different dialkynes. The resulting tetracoordinate boranes show variable electrochemical and optical properties depending on the substitution pattern in the backbone of the coordinating pyridine-derivatives. While boranes containing alkyl-substituted pyridines show lower electron affinities than the known parent compound, boranes featuring π-extended pyridine derivatives show higher electron affinities in the range of acceptor substituted triarylboranes. All derivatives show larger Stokes shifts (8790-6920 cm^-1) compared to the N → B-ladder borane coordinated by an unsubstituted pyridine.

A strongly Lewis-acidic and fluorescent borenium cation supported by a tridentate formazanate ligand

Lewis acids are highly sought after for their applications in sensing, small-molecule activation, and catalysis. When combined with π-conjugated molecular frameworks, Lewis acids with unique optoelectronic properties can be realized. Here, we use a tridentate formazanate ligand to create a planar, redox-active, fluorescent, and strongly Lewis-acidic borenium cation. We also demonstrate that this compound can act as a colourimetric probe for reactivity.

Cyclic Ether Triggers for Polymeric Frustrated Lewis Pair Gels

Sterically hindered Lewis acid and base centers are unable to form Lewis adducts, instead forming frustrated Lewis pairs (FLPs), where latent reactivity can be utilized for the activation of small molecules. Applying FLP chemistry into polymeric frameworks transforms this chemistry into responsive and functional materials. Here, we report a versatile synthesis strategy for the preparation of macromolecular FLPs and explore its potential with the ring-opening reactions of cyclic ethers. Addition of the cyclic substrates triggered polymer network formation, where the extent of cross-linking, strength of network, and reactivity are tuned by the steric and electronic properties of the ethers. The resultant networks behave like covalently cross-linked polymers, demonstrating the versatility of FLPs to simultaneously tune both small-molecule capture and mechanical properties of materials.

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.

On 1,3-phosphaazaallenes and their diverse reactivity

1,3-Phosphaazaallenes are heteroallenes of the type RP Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> CNR′ and little is known about their reactivity. In here we describe the straightforward synthesis of ArPCNR (Ar = Mes*, 2,4,6-tBu-C₆H₂; ^MesTer, 2.6-(2,4,6-Me₃C₆H₂)–C₆H₃; ^DipTer, 2.6-(2,6-iPr₂C₆H₂)–C₆H₃; R = tBu; Xyl, 2,6-Me₂C₆H₃) starting from phospha-Wittig reagents ArPPMe₃ and isonitriles CNR. It is further shown that ArPCNtBu are thermally labile with respect to the loss of iso-butene and it is shown that the cyanophosphines ArP(H)CN are synthetically feasible and form the corresponding phosphanitrilium borates with B(C₆F₅)₃, whereas deprotonation of ^DipTerP(H)CN was shown to give an isolable cyanidophosphide. Lastly, the reactivity of ArPCNR towards Pier's borane was investigated, showing hydroboration of the CN bond in Mes*PCNtBu to give a hetero-butadiene, while with ^DipTerPCNXyl the formation of the Lewis acid–base adduct with a B–P linkage was observed.

The combination of phospha-Wittig reagents with isonitriles affords 1,3-phosphaazaallenes and their diverse reactivity has been studied in detail.

Tropylium-Promoted Hydroboration Reactions: Mechanistic Insights Via Experimental and Computational Studies

Hydroboration reaction of alkynes is one of the most synthetically powerful tools to access organoboron compounds, versatile precursors for cross-coupling chemistry. This type of reaction has traditionally been mediated by transition-metal or main group catalysts. Herein, we report a novel method using tropylium salts, typically known as organic oxidants and Lewis acids, to promote the hydroboration reaction of alkynes. A broad range of vinylboranes can be easily accessed via this metal-free protocol. Similar hydroboration reactions of alkenes and epoxides can also be efficiently catalyzed by the same tropylium catalysts. Experimental studies and DFT calculations suggested that the reaction follows an uncommon mechanistic pathway, which is triggered by the hydride abstraction of pinacolborane with tropylium ion. This is followed by a series of in situ counterion-activated substituent exchanges to generate boron intermediates that promote the hydroboration reaction.

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.

Reversible Silylium Transfer between P-H and Si-H Donors

The Mo=PR₂ π* orbital in a Mo phosphenium complex acts as acceptor in a new P^III -based Lewis superacid. This Lewis acid (LA) participates in electrophilic Si-H abstraction from E₃ SiH to give a Mo-bound secondary phosphine ligand, Mo-PR₂ H. The resulting Et₃ Si⁺ ion remains associated with the Mo complex, stabilized by η¹ -P-H donation, yet undergoes rapid exchange with an η¹ -Si-H adduct of free silane in solution. The equilibrium between these two adducts presents an opportunity to assess the role of this new LA in catalytic reactions of silanes: is the LA acting as a catalyst or as an initiator? Preliminary results suggest that a cycle including the Mo-bound phosphine-silylium adduct dominates in the catalytic hydrosilylation of acetophenone, relative to a putative cycle involving the silane-silylium adduct or "free" silylium.

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.

B-H and O-H bonds activation via a single electron transfer of frustrated radical pairs

The rare examples of B-H bond activation in a frustrated radical pair regime have been observed by treatment of TEMPO radicals with Piers' borane HB(C6F5)2 or bis-borane, respectively. The resulting concomitant formation of zwitterionic products and geminal N/B frustrated Lewis pairs implied a one electron process. In addition, the reaction of a TEMPO/B(C6F5)3 pair with H2O·B(C6F5)3 was assumed to involve one-electron reduction of water. Our results provide insights into chemical bond (e.g. B-H and O-H) activation via a single electron transfer.

Transition metal-mediated dinitrogen functionalisation with boron

An overview of the available methods to functionalize dinitrogen with boron reagents using transition metal complexes is given.

Formation of Nucleophilic Allylboranes from Molecular Hydrogen and Allenes Catalyzed by a Pyridonate Borane that Displays Frustrated Lewis Pair Reactivity

Here we report the in situ generation of nucleophilic allylboranes from H₂ and allenes mediated by a pyridonate borane that displays frustrated‐Lewis‐pair reactivity. Experimental and computational mechanistic investigations reveal that upon H₂ activation, the covalently bound pyridonate substituent becomes a datively bound pyridone ligand. Dissociation of the formed pyridone borane complex liberates Piers borane and enables a hydroboration of the allene. The allylboranes generated in this way are reactive towards nitriles. A catalytic protocol for the formation of allylboranes from H₂ and allenes and the allylation of nitriles has been devised. This catalytic reaction is a conceptually new way to use molecular H₂ in organic synthesis.

Deoxygenative Divergent Synthesis: En Route to Quinic Acid Chirons

The installation of vicinal mesylate and silyl ether groups in a quinic acid derivative generates a system prone for stereoselective borane-catalyzed hydrosilylation through a siloxonium intermediate. The diversification of the reaction conditions allowed the construction of different defunctionalized fragments foreseen as useful synthetic fragments. The selectivity of the hydrosilylation was rationalized on the basis of deuteration experiments and computational studies.

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.