Trityl Cation Research Articles

Single C−F Bond Cleavage of Trifluoromethylarenes with an ortho‐Silyl Group

AbstractThe transformation of a single C−F bond of trifluoromethylarenes bearing a hydrosilyl group at the ortho position was achieved. The activation of the hydrosilyl group with a trityl cation in the presence of nucleophiles allowed for selective C−F bond functionalization, for example, by allylation, carboxylation, or chlorination. Further derivatization of the resulting fluorosilylarenes afforded various aromatic difluoromethylene compounds.

Single C-F Bond Cleavage of Trifluoromethylarenes with an ortho-Silyl Group.

The transformation of a single C-F bond of trifluoromethylarenes bearing a hydrosilyl group at the ortho position was achieved. The activation of the hydrosilyl group with a trityl cation in the presence of nucleophiles allowed for selective C-F bond functionalization, for example, by allylation, carboxylation, or chlorination. Further derivatization of the resulting fluorosilylarenes afforded various aromatic difluoromethylene compounds.

ChemInform Abstract: Carbocation Catalysis: Oxa‐Diels—Alder Reactions of Unactivated Aldehydes and Simple Dienes.

AbstractThe versatility of the trityl cation as an efficient Lewis acid organocatalyst is demonstrated in the title process.

Stabilization and Transfer of the Transient [Mes*P4](-) Butterfly Anion Using BPh3.

The transient bicyclo[1.1.0]tetraphosphabutane anion, generated from white phosphorus (P4) and Mes*Li (Mes*=2,4,6-tBu3C6H2), can be trapped by BPh3 in THF. This Lewis acid stabilized anion can be used as an [RP4](-) transfer agent, reacting cleanly with neutral Lewis acids (B(C6F5)3, BH3, and W(CO)5) to afford unique singly and doubly coordinated butterfly anions, and with the trityl cation to form a neutral, nonsymmetrical, all-carbon-substituted P4 derivative. This reaction path enables a simple, stepwise functionalization of white phosphorus.

Stabilization and Transfer of the Transient [Mes*P4]− Butterfly Anion Using BPh3

AbstractThe transient bicyclo[1.1.0]tetraphosphabutane anion, generated from white phosphorus (P4) and Mes*Li (Mes*=2,4,6‐tBu3C6H2), can be trapped by BPh3 in THF. This Lewis acid stabilized anion can be used as an [RP4]− transfer agent, reacting cleanly with neutral Lewis acids (B(C6F5)3, BH3, and W(CO)5) to afford unique singly and doubly coordinated butterfly anions, and with the trityl cation to form a neutral, nonsymmetrical, all‐carbon‐substituted P4 derivative. This reaction path enables a simple, stepwise functionalization of white phosphorus.

Carbocation Catalysis: Oxa‐Diels–Alder Reactions of Unactivated Aldehydes and Simple Dienes

AbstractThe versatility of the trityl cation (TrBF4) as a highly efficient Lewis acid organocatalyst is demonstrated in the oxa‐Diels–Alder reaction of various unactivated aromatic and aliphatic aldehydes and simple unactivated dienes, such as isoprene and 2,3‐dimethylbutadiene. The transformation proceeds smoothly to give 3,6‐dihydropyrane adducts in high to moderate yields with catalyst loadings down to 1.0 mol‐% under mild reaction conditions. In contrast to most previously reported strategies, this protocol does not require substrate functional group activation, neither by electron‐deficient aldehydes (2‐oxo aldehydes) or electron‐rich dienes (methoxy or amino‐butadiene).

Stoichiometric reactions and catalytic hydrogenation with a reactive intramolecular Zr(+)/amine frustrated Lewis pair.

Methyl anion abstraction from Cp*2Zr(CH3)OCH2CH2N(i)Pr2 (13) with trityl cation generates [Cp*2Zr(OCH2CH2N(i)Pr2)](+) (14). Complex 14 behaves as a reactive Zr(+)/amine frustrated Lewis pair (FLP). It reacts with dichloromethane to give the [Zr]Cl[OCH2CH2N(CH2Cl)(i)Pr2](+) cation (15), it slowly loses H2 upon standing at 60 °C to give a metallacyclic iminium cation product 18, and it reacts with terminal alkynes to give the [Zr]-alkynyl/ammonium systems 19. The organometallic FLP 14 cleaves dihydrogen heterolytically at near to ambient conditions to give the [Zr]H[OCH2CH2NH(i)Pr2](+) complex 20, which reduces benzaldehyde to the respective [Zr]OCH2Ph product 21 and is able to transfer the H(+)/H(-) pair to styrene to give ethylbenzene. Consequently, the Zr(+)/amine FLP 14 was used as an active hydrogenation catalyst for a series of alkenes and internal alkynes. The catalytic hydrogenation reactions were carried out under mild conditions (r.t., 1.5 bar of H2) using between 1 and 4 mol % FLP catalyst 14.

Cyclic silylated onium ions of group 15 elements.

Five- and six-membered cyclic silylated onium ions of group 15 elements I were synthesized by intramolecular cyclization of transient silylium ions II. Silylium ions II were prepared by the hydride transfer reaction from silanes III using trityl cation as hydride acceptor. It was found that smaller ring systems could not be obtained by this approach. In these cases tritylphosphonium ions IV were isolated instead. Cations I and IV were isolated in the form of their tetrakispentafluorphenyl borates and characterized by multinuclear NMR spectroscopy and, in two cases, by X-ray diffraction analysis. Cyclic onium ions I showed no reactivity similar to that of isoelectronic intramolecular borane/phosphane frustrated Lewis pairs (FLPs). The results of DFT computations at the M05-2X level suggest that the strength of the newly formed Si-E linkage is the major reason for inertness of I[B(C6F5)4] versus molecular hydrogen.

Metal-free Lewis acid mediated dehydrocoupling of phosphines and concurrent hydrogenation

The stoichiometric reaction of trityl cation with two equivalents of Ph2PH affords the phosphine stabilized phosphenium salt [Ph2(H)PPPh2][B(C6F5)4] via hydride abstraction, while catalytic amounts of B(p-HC6F4)3 effects catalytic phosphine dehydrocoupling with the liberation of H2. This reaction is accelerated by the presence of olefin or imine, effecting concurrent hydrogenation.

A Late Stage Strategy for the Functionalization of Triazolium-based NHC catalysts.

A strategy for the diversification of triazolium-based catalysts is presented. This method is based on the reduction to the triazoline, which serves as a suitable and stable substrate for Pd-mediated cross-coupling, followed by trityl cation mediated reoxidation to the triazolium.

Di- and triarylmethylium ions as probes for the ambident reactivities of carbanions derived from 5-benzylated Meldrum's acid.

The kinetics of the reactions of carbocations with carbanions 1 derived from 5-benzyl-substituted Meldrum's acids 1-H (Meldrum's acid = 2,2-dimethyl-1,3-dioxane-4,6-dione) were investigated by UV/Vis spectroscopic methods. Benzhydryl cations Ar2CH(+) added exclusively to C-5 of the Meldrum's acid moiety. As the second-order rate constants (kC) of these reactions in DMSO followed the linear free-energy relationship lg k = sN (N+E), the nucleophile-specific reactivity parameters N and sN for the carbanions 1 could be determined. In contrast, trityl cations Ar3C(+) reacted differently. While tritylium ions of low electrophilicity (E<-2) reacted with 1 through rate-determining β-hydride abstraction, more Lewis acidic tritylium ions initially reacted at the carbonyl oxygen of 1 to form trityl enolates, which subsequently reionized and eventually yielded triarylmethanes and 5-benzylidene Meldrum's acids by hydride transfer.

Dinuclear μ-fluoro cations of copper, silver and gold

Terminal fluoride complexes of the monovalent group 11 metals, supported by N-heterocyclic carbene (NHC) ligands, react with one-half equivalent of triphenylmethyl cation to form fluoride-bridged dinuclear cations. An improved preparation of the starting terminal fluorides is introduced. The crystal structures of the cations display bent MFM arrangements with large intermetallic separations. The bridging fluorides are highly labile, undergoing facile hydrolysis. The fluoride-bridged digold complex adds across an allene CC bond to form a diaurated allylic fluoride.

Formation, Stability, and Structures of Borenium and Boronium Cations Derived from Pentamethylazaferrocene–Boranes by Hydride or Chloride Abstraction Reactions

Hydride abstraction reactions from 1′,2′,3′,4′,5′-pentamethylazaferrocene–boranes (PMAF-boranes, where borane = BH3, 9-bora[3.3.1]bicyclononane) using trityl cation in CH2Cl2 yielded the corresponding borenium cations, whose stability depends on the counteranion of the trityl salt. If the [BF4]− salt was employed, then fluoride abstraction from the anion occurred immediately, yielding PMAF-BF3 and other identifiable byproducts. If the [B(C6F5)4]− salt was employed, then the borenium ions were stable in solution at room temperature and could be characterized by 1H, 13C, and 11B NMR spectroscopy. Double hydroboration of [PMAF-BH2]+ with 1,5-cycloooctadiene yields the borenium ion of PMAF-9-bora[3.3.1]-bicyclononane, [PMAF-9BBN]+. Reaction of [PMAF-BH2]+ with excess PMAF yields the boronium cation [(PMAF)2BH2]+, which was isolated and crystallographically characterized as its [B(C6F5)4]− salt. Reaction of [PMAF-9BBN]+ with 2 equiv of N-methylimidazole leads to the boronium cation [(N-methylimidazole)2(BBN)]+, which was also isolated and crystallographically characterized as its [B(C6F5)4]− salt. Chloride abstraction from PMAF-BCl3 with AlCl3 in CH2Cl2 yielded the borenium cation [PMAF-BCl2]+, which was characterized in solution by 1H, 13C, and 11B NMR spectroscopy. A single-crystal X-ray structure was also obtained of [PMAF-BCl2]+[AlCl4]−, revealing structural changes upon borenium ion formation, including rehybridization of the B center and a bending of the BCl2 moiety toward the Fe center. The Lewis acidity of all three borenium cations was assessed by the Gutmann–Beckett method, and [PMAF-BH2]+ was found to be more acidic than B(C6F5)3. DFT calculations were performed on [PMAF-BH2]+ using the BP86 method with the LANL2DZ basis set for Fe and the TZVP basis set for C, H, N, and B. The optimized structure revealed substantial bending of the BH2 moiety toward Fe with a dip angle of −23.7°. Inspection of the frontier MOs reveal that the electrophilic B center is stabilized through delocalized interactions that involve orbitals on Fe, the pyrrolyl ring, and the pentamethylcyclopentadienyl ring.

Is the Perfluorinated Trityl Cation Worth a Revisit? A Theoretical Study on the Lewis Acidities and Stabilities of Highly Halogenated Trityl Derivatives

AbstractThe relative Lewis acidities of a series of known, unknown and observed but unstable halogenated derivatives of the trityl cation have been probed in a computational study. The Lewis acidity was investigated from hydride, fluoride and methide affinities. The results indicate that the perhalogenated trityl cation [C(C6F5)3]+ is a desirable synthetic target, especially in the modern age of weakly coordinating anions. Our results also indicate that the stable [C(C6Cl5)3]+ cation may be an equally interesting Lewis acid.

Carbocations as Lewis Acid Catalysts in Diels–Alder and Michael Addition Reactions

In general, Lewis acid catalysts are metal-based compounds that owe their reactivity to a low-lying empty orbital. However, one potential Lewis acid that has received negligible attention as a catalyst is the carbocation. We have demonstrated the potential of the carbocation as a highly powerful Lewis acid catalyst for organic reactions. The stable and easily available triphenylmethyl (trityl) cation was found to be a highly efficient catalyst for the Diels-Alder reaction for a range of substrates. Catalyst loadings as low as 500 ppm, excellent yields, and good endo/exo selectivities were achieved. Furthermore, by changing the electronic properties of the substituents on the tritylium ion, the Lewis acidity of the catalyst could be tuned to control the outcome of the reaction. The ability of this carbocation as a Lewis acid catalyst was also further extended to the Michael reaction.

The Family of Ferrocene‐Stabilized Silylium Ions: Synthesis, 29Si NMR Characterization, Lewis Acidity, Substituent Scrambling, and Quantum‐Chemical Analyses

The purpose of this systematic experimental and theoretical study is to deeply understand the unique bonding situation in ferrocene-stabilized silylium ions as a function of the substituents at the silicon atom and to learn about the structure parameters that determine the (29)Si NMR chemical shift and electrophilicity of these strong Lewis acids. For this, ten new members of the family of ferrocene-stabilized silicon cations were prepared by a hydride abstraction reaction from silanes with the trityl cation and characterized by multinuclear (1)H and (29)Si NMR spectroscopy. A closer look at the NMR spectra revealed that additional minor sets of signals were not impurities but silylium ions with substitution patterns different from that of the initially formed cation. Careful assignment of these signals furnished experimental proof that sterically less hindered silylium ions are capable of exchanging substituents with unreacted silane precursors. Density functional theory calculations provided mechanistic insight into that substituent transfer in which the migrating group is exchanged between two silicon fragments in a concerted process involving a ferrocene-bridged intermediate. Moreover, the quantum-chemical analysis of the (29)Si NMR chemical shifts revealed a linear relationship between δ((29)Si) values and the Fe···Si distance for subsets of silicon cations. An electron localization function and electron localizability indicator analysis shows a three-center two-electron bonding attractor between the iron, silicon, and C'(ipso) atoms, clearly distinguishing the silicon cations from the corresponding carbenium ions and boranes. Correlations between (29)Si NMR chemical shifts and Lewis acidity, evaluated in terms of fluoride ion affinities, are seen only for subsets of silylium ions, sometimes with non-intuitive trends, indicating a complicated interplay of steric and electronic effects on the degree of the Fe···Si interaction.

Formal meta-specific intramolecular Friedel–Crafts allylic alkylation of phenols through a spirocyclization–dienone–phenol rearrangement cascade

Formal meta-specific intramolecular Friedel–Crafts allylic alkylation of phenols was achieved based on spirocyclization–dienone–phenol rearrangement cascades. Systematic screening of acid catalysts revealed that Sc(OTf)3 was a highly effective catalyst for dienone–phenol rearrangement of spiro[4.5]cyclohexadienones. Using 5 mol % of Sc(OTf)3 as the promoter, various spirocyclic substrates were transformed into the corresponding phenol derivatives in good to excellent yield. Furthermore, the one-pot sequential spirocyclization–dienone–phenol rearrangement proceeded using a palladium and scandium multi-catalytic system or a triphenylmethyl cation single-catalyst system, providing the corresponding meta-allylated phenol derivatives in excellent yield.

The triazolyl analogue of the trityl cation

Abstract The formation of the tris(3-benzyl-triazol-5-yl)carbenium cation, a heterocyclic analogue of the trityl cation, from tris(3-benzyl-triazol-5-yl)methanol, itself generated by Cu(I)-catalyzed Huisgen reaction of benzyl azide with triethynylmethanol, in the presence of trifluoroacetic anhydride is evidenced through a selective S N 1 reactivity with various O -, N -, S -, and C -nucleophiles, giving the corresponding triazolylmethane derivatives in moderate to high yields.

Synthesis of Silylium and Germylium Ions by a Substituent Exchange Reaction

The reaction of diarylalkylsilanes and -germanes with trityl cation in the presence of a weakly coordinating anion to give the corresponding triarylsilylium or -germylium ions was investigated. This reaction provides a facile access to a larger range of these sterically highly hindered Lewis acids. The factors that promote the substituent exchange were studied, and significant effects of the substituent, of the solvent, and of the group 14 element were revealed. A combined solid-state XRD and NMR investigation of the tris(pentamethylphenyl) silylium borate [(Me5C6)3Si]2[B12Cl12] disclosed the trigonal planar coordination environment of the silicon atom in this silylium ion. NMR investigations indicate for 2,4,6-triisopropylphenyl-substituted silylium and germylium ions the onset of C–H···E+ three-center interactions (E = Si, Ge) between the distant CH bond of the isopropyl group and the positively charged group 14 element atom.

Glycosylation of alcohols using glycosyl boranophosphates as glycosyl donors

Abstract A novel glycosylation that uses glycosyl boranophosphate triesters as glycosyl donors and trityl cation (Tr+) as an activator was developed. Two types of reactions were studied: (1) the boranophosphate triester was activated with TrNTf2 to react with an alcohol and (2) O-trityl ethers worked as both glycosyl acceptors and Tr+ sources. The latter gave better results and the desired O-glycosylation products were rapidly generated and isolated in moderate to good yields.