Eine Geschichte über zwei Elemente: die Umpolung der Lewis-Acidität/Basizität von Bor und Phosphor

Bis(amidophenolato)phosphonium: Si-H Hydride Abstraction and Phosphorus-Ligand Cooperative Activation of C-C Multiple Bonds

The first bis(amidophenolato)phosphonium salts are prepared and fully characterized. The perfluorinated derivative represents the strongest monocationic phosphorus Lewis acid on the fluoride and hydride ion affinity scale isolable to date. This affinity enables new reactions, such as hydride abstraction from Et₃ SiH, the first phosphaalkoxylation of an alkyne or a phosphorus catalyzed intramolecular hydroarylation. All properties and reactions are scrutinized by theory and experiment. Substantial σ- and π-acidity provides the required affinity for substrate activation, while phosphorus-ligand cooperativity substantially enriches the reactivity portfolio of phosphonium ions.

One-step Oxidative Monofluorination of Electron-Deficient Sulfoxides to Access Highly Lewis Acidic Sulfur(VI) Cations

The strongly oxidizing, powerful electrophilic fluorination reagent [FXe][OTf] is shown to effect direct oxidative monofluorination of sulfoxides. This one-step, chloride promoter-free methodology provides access to so far inaccessible, yet highly desirable strongly Lewis acidic fluorosulfoxonium cations from electron-deficient and/or sterically demanding sulfoxides that are shown to be practically unreactive towards the previously reported XeF₂ /NEt₄ Cl system. Experimental and density functional theory studies have been conducted to assess the Lewis acidities of the prepared sulfur(VI) cations. Preliminary results obtained with chiral sulfoxides provide early insights into the mechanism of these fluorination reactions.

The Reactivity of Isomeric Nitrenium Lewis Acids with Phosphines, Carbenes, and Phosphide

Alkylation of spiro[fluorene-9,3'-indazole] at N(1) and N(2) with tBuCl affords the nitrenium cations [C6 H4 N2 (tBu)C(C12 H8 )][BF4 ], 1 and 2, respectively. Compound 1 converts to 2 over the temperature range 303-323 K with a free energy barrier of 28±5 kcal mol-1 . Reaction of 1 with PMe3 afforded the N-bound phosphine adduct [C6 H4 N(tBu)N(PMe3 )C(C12 H8 )]BF4 ] 3. However, phosphines attack 2 at the para-carbon atom of the aryl group with concurrent cleavage of N(2)-C(1) bond and proton migration to C(1) affording [(R3 P)C6 H3 NN(tBu)CH(C12 H8 )][BF4 ] (R=Me 4, nBu 5). Analogous reactions of 1 and 2 with the carbene SIMes prompt attack at the para-carbon with concurrent loss of H. affording the radical cation salts [(SIMes)C6 H3 N(tBu)NC(C12 H8 ). ][BF4 ] 6 and [(SIMes)C6 H3 NN(tBu)C(C12 H8 ). ][BF4 ] 7, whereas reaction of 2 with BAC gives the Lewis acid-base adduct, [C6 H4 N(BAC)N(tBu)C(C12 H8 )][BF4 ] 8. Finally, reactions of 1 and 2 with KPPh2 result in electron transfer affording (PPh2 )2 and the persistent radicals C6 H4 N(tBu)NC(C12 H8 ). and C6 H4 NN(tBu)C(C12 H8 ). . The detailed reaction mechanisms are also explored by extensive DFT calculations.

Facile Synthesis of a Stable Dihydroboryl {BH2 }- Anion

While the one-electron reduction of (CAAC^Me )BH₂ Br (CAAC^Me =1-(2,6-diisopropylphenyl)-3,3,5,5-tetramethylpyrrolidin-2-ylidene) yields a hydride-shift isomer of the corresponding tetrahydrodiborane, a further reversible reduction leads to the first stable parent boryl anion, [(CAAC^Me )BH₂ ]^- , which acts as a powerful boron nucleophile.

Digging the Sigma-Hole of Organoantimony Lewis Acids by Oxidation

The development of group 15 Lewis acids is an area of active investigation that has led to numerous advances in anion sensing and catalysis. While phosphorus has drawn considerable attention, emerging research shows that organoantimony(III) reagents may also act as potent Lewis acids. Comparison of the properties of SbPh₃ , Sb(C₆ F₅ )₃ , and SbAr^F ₃ with those of their tetrachlorocatecholate analogues SbPh₃ Cat, Sb(C₆ F₅ )₃ Cat, and SbAr^F ₃ Cat (Cat=o-O₂ C₆ Cl₄ , Ar^F =3,5-(CF₃ )₂ C₆ H₃ ) demonstrates that the Lewis acidity of electron deficient organoantimony(III) reagents can be readily enhanced by oxidation to the +V state-as verified by binding studies, organic reaction catalysis, and computational studies. The results are rationalized by explaining that oxidation of the antimony center leads to a lowering of the accepting σ* orbital and a deeper carving of the associated σ-hole.

Nitrogen-Based Lewis Acids Derived from Phosphonium Diazo Cations

Reaction of PPh₃ and [(p-ClC₆ H₄ )N₂ ][BF₄ ] affords [(p-ClC₆ H₄ )N(PPh₃ )N(PPh₃ )][BF₄ ] 1, while reaction with (Ph₂ PCH₂ )₂ gave [(p-ClC₆ H₄ )(NPh₂ PCH₂ )₂ )][BF₄ ] 2. These species confirm the Lewis acidity of [(p-ClC₆ H₄ )N₂ (PR₃ )][BF₄ ] cations at N. In contrast, use of bulky phosphines afford the species [ArN₂ (PR₃ )][BF₄ ] (R=tBu 3, Mes 4). Compound 3 undergoes one electron reduction to give the stable radical [(p-ClC₆ H₄ )N₂ (PtBu₃ )]^. 5. Combination of 3 and PtBu₃ acts as an FLP to effect (SPh)₂ cleavage, generating [PhSPtBu₃ ]⁺ and the radical [ArN₂ (PR₃ )]^. . Collectively, these data affirm the ability of the cations [ArN₂ (PR₃ )]⁺ to behave as one or two electron acceptors.

Nitrogen-Based Lewis Acids: Synthesis and Reactivity of a Cyclic (Alkyl)(Amino)Nitrenium Cation

A room-temperature-stable crystalline cyclic (alkyl)(amino)nitrenium cation 2 features cationic nitrogen atom with a smaller HOMO-LUMO gap compared to that of a 1,2,3-triazolium 5 (an N-heterocyclic nitrenium cation). The low-lying LUMO of 2 results in an enhanced electrophilicity, which allowed for the formation of Lewis adducts with neutral Lewis bases, such as Me₃ P, nBu₃ P, and IiPr. The N-based Lewis acid 2 also forms an FLP with tBu₃ P but subsequently reacts with (PrS)₂ to cleave the S-S bond. Both experimental and theoretical results suggest that the Lewis acidity of 2 is stronger than its N₃ analogues.

Reversible Oxidative Se-Se Coupling of Phosphine Selenides by Ph3 Sb(OTf)2

Salts of diphosphoniumdiselenide dications ([R₃ PSeSePR₃ ][OTf]₂ ) have been isolated from reactions of trialkylphosphine selenides with triphenylantimony bistriflate. The redox process is speculated to proceed via a cationic coordination complex [Ph₃ SbL₂ ][OTf]₂ (L=Me₃ PSe, iPr₃ PSe), which is also formed in the reaction of [R₃ PSeSePR₃ ][OTf]₂ with Ph₃ Sb. The observations indicate that the reductive elimination of [R₃ PSeSePR₃ ]²⁺ from [Ph₃ Sb(SePR₃ )₂ ]²⁺ is reversible through the oxidative addition of [R₃ PSeSePR₃ ]²⁺ to Ph₃ Sb.