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B(C6F5)3 and the corresponding anion [B(C6F5)4]− are ubiquitous in main group and transition metal chemistry. Known derivatives are generally limited to the incorporation of electron donating substituents. Herein we describe electrophilic fluorination and dearomatization of such species using XeF2 in the presence of BF3 or Lewis acidic cations. In this fashion the anions [HB(C6F5)3]−, [B(C6F5)4]− and [(C6F5)3BC≡NB(C6F5)3]−, are converted to [FB(C6F7)3]−, [B(C6F7)4]−, and [(C6F7)3BC≡NB(C6F7)3]−, respectively. Similarly, the borane adducts (L)B(C6F7)3 (L=MeCN, OPEt3) are produced. These rare examples of electrophilic attack of electron deficient rings proceed as [XeF][BF4] acts as a frustrated Lewis pair effecting fluorination and dearomatization of C6F5 rings. [ABSTRACT FROM AUTHOR]

Described is a kinetics and computational study of the reaction of NO with the intramolecular bridged P/B frustrated Lewis pair (FLP) endo-2-(dimesitylphosphino)-exo-3-bis(pentafluorophenyl)boryl-norbornane to give a persistent FLP-NO aminoxyl radical. This reaction follows a second-order rate law, first-order in [FLP] and first-order in [NO], and is markedly faster in toluene than in dichloromethane. By contrast, the NO oxidation of the phosphine base 2-(dimesitylphosphino)norbornene to the corresponding phosphine oxide follows a third-order rate law, first-order in [phosphine] and second-order in [NO]. Formation of the FLP-NO radical in toluene occurs with a ΔH(‡) of 13 kcal mol(-1), a feature that conflicts with the computation-based conclusion that NO addition to a properly oriented B/P pair should be nearly barrierless. Since the calculations show the B/P pair in the most stable solution structure of this FLP to have an unfavorable orientation for concerted reaction, the observed barrier is rationalized in terms of the reversible formation of a [B]-NO complex intermediate followed by a slower isomerization-ring closure step to the cyclic aminoxyl radical. This combined kinetics/theoretical study for the first time provides insight into mechanistic details for the activation of a diatomic molecule by a prototypical FLP.

Recent reports of radical formation within frustrated Lewis pairs (FLPs) suggested that single‐electron transfer (SET) could play an important role in their chemistry especially for C−C coupling. In sharp contrast, our extensive dispersion‐corrected DFT calculations show that although reactive benzhydryl radical along with phosphine radical cation species can be kinetically generated from bulky phosphines and benzhydryl cation, direct P−C hetero‐coupling may lead to bulky phosphonium cation as reactive carbocation transfer reagents to styrene substrates, which is kinetically much more favorable than the recently proposed radical C−C coupling between benzhydryl radical and styrene. Similarly, meta‐stable radical cation Mes3P+⋅ salt is also kinetically accessible via SET reactions of Mes3P and B(C6F5)3 with 0.5 equivalent of p‐O2C6Cl4. [ABSTRACT FROM AUTHOR]

Recently, it was shown that Piers' borane [HB(C6F5)2]2 can induce rapid oligomerization reactions of arylacetylenes and arylallenes. Using phenylallene PhCH=C=CH2 and phenylacetylene PhCCH as typical substrates, the detailed mechanisms of borane‐induced oligomerization are disclosed by extensive state‐of‐the‐art DFT calculations. Following initial regioselective hydroboration of unsaturated C=C or C≡C bonds, further oligomerization and structural rearrangement reactions may occur through unique boryl‐enabled conversions such as 1,3‐boryl‐shift, allylboration, 1,2‐carboboration, and 1,5‐electrocyclization. Together with stepwise transborylation with hydroboranes such as pinacolborane HBpin, such fundamental reactions may pave a catalytic way to borane‐induced oligomerization of arylacetylenes and arylallenes. [ABSTRACT FROM AUTHOR]

We report a class of compounds in which both PIII−X and PI forms featuring the same ligand are stable and readily cycled with each other. A series of PIII−X (X=Cl, Br, I) dicationic triflate salts supported by benzyl‐ and allyl‐substituted 2,6‐bis(benzimidazole‐2‐yl)pyridine (BZIMPY) ligands is synthesized. Surprisingly, treatment of these with R3PO (R=Et, Oct) results in reduction to BZIMPY‐ligated PI monocationic triflate salts while treatment with Ph3P reduces but also substitutes the compound to produce Ph3P‐BZIMPY‐ligated PI dicationic triflate salts. The mechanisms of these surprising reductions are probed experimentally and rationalized computationally. The PIII−X dications are shown to be strong Lewis acids both experimentally and computationally and to readily behave as X+, PX, and P+ transfer agents in reactions with phosphines, NHCs, and diazabutadienes. The PI mono‐ and dications are shown to be very effective P+ transfer agents when treated similarly. Oxidation from a monocationic PI salt back to the dicationic PIII−X (X=Cl, Br) salt was achieved by treatment with N‐chlorosuccinimide (NCS) or N‐bromosuccinimide (NBS). Full characterization is reported using multinuclear nuclear magnetic resonance spectroscopy, elemental analysis, and single‐crystal X‐ray diffractometry where suitable crystals were isolated. [ABSTRACT FROM AUTHOR]

Cyclopropenium cations incorporating electron deficient substituents are Lewis acidic despite the presence of π-electrons. The chloride and electron affinities are examined computationally and experimentally, respectively. These cations form classic Lewis acid-base adducts with PPh3, while sterically demanding phosphines yield frustrated Lewis pairs (FLPs) which participate in FLP additions. Depending on the basicity of the phosphine used, addition to alkynes or alkyne deprotonation is observed. In either case, new C–C bonds are formed, thus extending the utility of the concept of FLP chemistry to these delocalized π-cations. [ABSTRACT FROM AUTHOR]

Es wird eine enantioselektive Reduktion von einfachen Carbeniumionen mit Cyclohexadienen, die eine hydridische C‐H‐Bindung an einem asymmetrisch substituierten Kohlenstoffatom tragen, vorgestellt. Die grundlegende Reaktion ist eine Transferhydrierung von Alkenen (Styrolen), bei der ausschließlich chirale Cyclohexadiene als Diwasserstoffsurrogate Anwendung finden. Das Tritylkation wird als Initiator dieses Brønsted‐Säure‐katalysierten Verfahrens eingesetzt, bei dem ein schwieriges intermolekulares Abfangen einer Carbeniumionenzwischenstufe durch die bereits erwähnte Hydridquelle enantioselektivitätsbestimmend ist. Lediglich nichtkovalente Wechselwirkungen begünstigen einen der Übergangszustände energetisch, was zu guten Enantiomerenverhältnissen der Hydrierungsprodukte führt. Der berechnete Reaktionsmechanismus sowie frühere Erkenntnisse aus Studien zu anderen Transferhydrierungsmethoden auf der Grundlage der Cyclohexadienplattform stützen die vorliegenden Ergebnisse. [ABSTRACT FROM AUTHOR]

Recently, it was shown that the double Ca−H−Ca bridged calcium hydride cation dimer complex [LCaH2CaL]2+ (macrocyclic ligand L=NNNN‐tetradentate Me4TACD) exhibited remarkable activity in catalyzing the hydrogenation of unactivated 1‐alkenes as well as the H2 isotope exchange under mild conditions, tentatively via the terminal Ca−H bond of cation monomer LCaH+. In this DFT mechanistic work, a novel substrate‐dependent catalytic mechanism is disclosed involving cooperative Ca−H−Ca bridges for H2 isotope exchange, competitive Ca−H−Ca bridges and terminal Ca−H bonds for anti‐Markovnikov addition of unactivated 1‐alkenes, and terminal Ca−H bonds for Markovnikov addition of conjugation‐activated styrene. THF‐coordination plays a key role in favoring the anti‐Markovnikov addition while strong cation‐π interactions direct the Markovnikov addition to terminal Ca−H bonds. [ABSTRACT FROM AUTHOR]