An arene-stabilized η-pentamethylcyclopentadienyl antimony dication acts as a source of Sb or Sb cations.

Double chloride abstraction from Cp*SbCl2 (Cp* = pentamethylcyclopentadienyl) affords an arene-stabilized η5-Cp*-bound antimony dicationic compound [(η5-Cp*)Sb(tol)][B(C6F5)4]2 (1) (tol = toluene), which exhibits strong Lewis acidity, abstracting chloride from Ph3CCl. 1 reacts with different Lewis bases via divergent pathways, giving rise to release of either Sb+ or Sb3+ cations.

Reactions of carbene-stabilized borenium cations.

In this paper we probe the reactivity of the borenium cations [CH(NCHCH)(NCHPh)BH][B(CF)] 2 and [CH(NCHCH)B][B(CF)] 3. The reactions of 2 with cyclohexene or 3,3-dimethyl-1-butene gave the alkyl-aryl borenium salts [PhCH(CHN)CCHCHBR][B(CF)] (R = Cy 4, CHCHtBu 5) while the corresponding reactions with diphenylacetylene, 1-hexyne and 1-phenyl-1-propyne gave the aryl-alkenyl borenium cation salts [PhCH(CHN)CCHCHBC(R)C(H)R][B(CF)] (R = R = Ph 6, R = H, R = CH7, R = Me, R = Ph 8a, R = Ph, R = Me 8b). In contrast, the reaction of 2 with ethynyldiphenylphosphane or 2-vinylpyridine lead to the formation of the adducts, [PhCH(CHN)CCHCHB(H)P(Ph)CCH][B(CF)] 9, [PhCH(CHN)CCHCHB(H)NCHC(H)CH][B(CF)] 10, respectively, while the more bulky donor HC[double bond, length as m-dash]C(Ph)PMes gave 1,2-hydroboration of the phosphinoalkene affording [PhCH(CHN)CCHCHBCHCH(Ph)PMes][B(CF)] 11.

Base-Stabilized [PO] /[PO ] Cations.

The salts [(BAC) PO][BF ] (5) and [(BAC) PO ][BF ] (4) (BAC=bis(diisopropylamino) cyclopropenylidene), consisting of the PO and PO cations, respectively, coordinated to the singlet carbenes, have been prepared. Computational investigations reveal that the electronic structure of the PO cation is a hybrid between the charge-localized and charge-delocalized resonance forms, resulting in ambiphilic reactivity. Compound 5 reacts as a donor with the transition-metal complex K PtCl to furnish [[(BAC) PO] PtCl ][BF ] (6) and KCl.

Single Electron Transfer to Diazomethane-Borane Adducts Prompts C-H Bond Activations.

While (Ph CN )B(C F ) is unstable, single electron transfer from Cp* Co affords the isolation of stable products [Cp* Co][Ph CNNHB(C F ) ] 1 and [Cp*Co(C Me CH B(C F ) )] 2. The analogous combination of Ph CN and BPh showed no evidence of adduct formation and yet single electron transfer from Cp* Cr affords the species [Cp* Cr][PhC(C H )NNBPh ] 3 and [Cp* Cr][Ph CNNHBPh ] 4. Computations showed both reactions proceed via transient radical anions of the diphenyldiazomethane-borane adducts to effect C-H bond activations.

Phosphaaluminirenes: Synthons for Main Group Heterocycles.

The phosphaaluminirenes HC[(CMe)(NDipp)]Al[C(R)═P] (Dipp = 2,6--PrCH, R = Bu or adamantyl) and , featuring an unsaturated three-membered AlCP ring, have been synthesized as crystalline solids via a [1 + 2] cycloaddition reaction of the aluminum(I) complex HC[(CMe)(NDipp)]Al () with phosphaalkynes. Computational investigations infer three-centered 2π-electron aromaticity of the AlCP rings. Compound is readily protonated by BuOH to induce a ring-opening σ-bond metathesis, giving an alumina-substituted -hydrogeno phosphaalkene .

Oligomerization of phosphaalkynes mediated by bulky N-heterocyclic carbenes: avenues to novel phosphorus frameworks.

Reactions of RC[triple bond, length as m-dash]P (R = tBu or Ad (admantyl)) with NHCs (SIMes, 1a; IMes, 1b and IDipp, 1d), leading to 1,2,3-triphosphetenes 2 and 3, a triphosphole 4, and a di-1,2-dihydro-1,2-diphosphete-substituted diphosphene 5, are reported. Compound 5 represents a novel P chain framework, as well as a newly discovered phosphaalkyne hexamer stabilized by two NHCs. Computational investigations suggest that the weak π-accepting ability of NHCs results in low stability of the initially formed 2H-phosphirenes, thus facilitating spontaneous oligomerization reactions.

Reversible Intramolecular Cycloaddition of Phosphaalkene to an Arene Ring.

The phosphepinium cation 1 is deprotonated by base generating a phosphaalkene that undergoes cycloaddition to the N-bound aromatic ring affording the 2-phosphabicyclo[2.2.2]octa-5,7-diene 2.

The Arene-Stabilized η -Pentamethylcyclopentadienyl Arsenic Dication [(η -Cp*)As(toluene)].

Double chloride abstraction of Cp*AsCl gives the dicationic arsenic species [(η -Cp*)As(tol)][B(C F ) ] (2) (tol=toluene). This species is shown to exhibit Lewis super acidity by the Gutmann-Beckett test and by fluoride abstraction from [NBu ][SbF ]. Species 2 participates in the FLP activation of THF affording [(η -Cp*)AsO(CH ) (THF)][B(C F ) ] (5).

Facile Cleavage of the P=P Double Bond in Vinyl-Substituted Diphosphenes.

The reactions of the cyclic alkyl amino carbene (CAAC) 1 with phosphaalkynes generate the kinetically unstable CAAC-derived phosphirenes 4 and 5, which undergo rearrangement/dimerization reactions to give the vinyl-substituted diphosphenes 2, 3, and 6. The P=P double bond scission of 2 or 3 is unprecedentedly effected by S , [AuCl(tht)], or MeOTf at room temperature, which affords a dithiophosphorane 7, a phosphepine Au complex 8, or phosphepinium cations 9 and 10, respectively. The cationic species feature little homoaromaticity while representing the first examples of the phosphorus-containing analogue of the tropylium ion.

Reversible 1,1-hydroaluminations and C-H activation in reactions of a cyclic (alkyl)(amino) carbene with alane.

Varying the reaction ratio of cyclic (alkyl)(amino) carbene (cAACEt) with AlH3·NEtMe2 leads to the isolation of (cAACEtH)AlH2·NEtMe21 and (cAACEtH)2Al(μ-H)2AlH2·NEtMe22 and the first example of a monomeric dialkyl-aluminum hydride (cAACEtH)2AlH 3. VT and 1H-1H EXSY NMR experiments of 3 demonstrated the isomerization of the diastereomers of 3via the first instance of reversible hydride migration between the Al and the C center. In addition, heating solutions of 3 at 100 °C affords (cAACEtH)Al(CHC(Et)2CH2C(Me)2NC6H3(iPr)C(Me)CH2) 4 with loss of H2.

Homolytic cleavage of peroxide bonds via a single electron transfer of a frustrated Lewis pair.

Single electron transfer (SET) reactions are effected by the combination of a MesP/B(CF) frustrated Lewis pair with benzoyl peroxide derivatives. The resulting homolytic cleavage of the peroxide bonds affords the radical salts [MesP˙][RCOOB(CF)] (R = Ph 1, p-BrCH2, p-MeCH3). These species react with PhSnH to give [MesPH][RCOOB(CF)] (R = Ph 10, p-BrCH11, p-MeCH12) and (PhSn).

Reductive Coupling and Loss of N from Magnesium Diazomethane Derivatives.

The reductive coupling of two diazomethanes is affected by reaction with [(NacNac )Mg] affording the species [(NacNac )Mg(N CPh )] 2 and [(NacNac )Mg(N C(C H ) )] 3. These species containing N linkages readily evolve the central N at 50 and 75 °C to give the Mg-imide products [(NacNac )Mg(NCPh )] (4) and [(NacNac )Mg(NC(C H ) )] (5), respectively. The mechanism for the loss of N was considered computationally.

An umpolung of Lewis acidity/basicity at nitrogen by deprotonation of a cyclic (amino)(aryl)nitrenium cation.

A cyclic (amino)(aryl)nitrenium cation 2 has been achieved by treatment of spiro[fluorene-9,3'-indazole] (1) with Ph2CHCl and AgBF4. This cation 2 is Lewis acidic at nitrenium N1, reacting with PMe3 affording a Lewis acid/base adduct 3. In contrast, deprotonation of 2 with other bases provides a neutral compound 4 which is Lewis basic at N1, reacting with electrophiles including GaCl3, MeOTf and PhNCO.

Zinc-Containing Radical Anions via Single Electron Transfer to Donor-Acceptor Adducts.

Reactions of [Cp* Fe] with the Lewis acid [Zn(C F ) ] in the presence of [(PhC(S)S) ], 9,10-phenanthrenedione or 4,5-pyrenedione yield the salt [Cp* Fe][(PhC(S)S)Zn(C F ) ] 1, [Cp* Fe][((C H O )Zn(C F ) )⋅] 4, and [Cp* Fe][((C H O )Zn(C F ) )⋅] 5, respectively. The latter two species represent the first examples of isolable zinc-containing radical anions. While [(PhC(S)S) ] binds weakly to [Zn(C F ) ], the diones afford the isolable adducts [(C H O )Zn(C F ) ] 2 and [(C H O )Zn(C F ) ] 3.

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 Me P, nBu P, and IiPr. The N-based Lewis acid 2 also forms an FLP with tBu P but subsequently reacts with (PrS) to cleave the S-S bond.

A Transient Vinylphosphinidene via a Phosphirene-Phosphinidene Rearrangement.

A room-temperature-stable crystalline 2H-phosphirene (1) was prepared by treatment of an electrophilic diamidocarbene with tert-butylphosphaalkyne. Compound 1 is shown to react as a vinylphosphinidene generated via phosphirene-phosphinidene rearrangement. Thermolysis is shown to affect C-N bond scission while reactions with CClO or (tht)AuCl afford formal oxidation of the phosphindene center and the phosphinidene-insertion into an aromatic C-C bond of a mesityl group, respectively.

Single Electron Delivery to Lewis Pairs: An Avenue to Anions by Small Molecule Activation.

Single electron transfer (SET) reactions are effected by the combination of a Lewis acid (e.g., E(CF) E = B or Al) with a small molecule substrate and decamethylferrocene (Cp*Fe).

FLP reactivity of [PhC] and (o-tolyl)P and the capture of a Staudinger reaction intermediate.

The frustrated Lewis pair (FLP) derived from the trityl cation and (o-tolyl)P effects the activation of 1,4-cyclohexadiene and 1-bromo-4-ethynylbenzene and heterolytically cleaves the S-S bond of diphenyl disulfide. The FLP also captures pentafluorophenyl azide as the Staudinger reaction intermediate, a species that reacts with PhSiH to give the silyl analog.

N-Heterocyclic carbene stabilized parent sulfenyl, selenenyl, and tellurenyl cations (XH, X = S, Se, Te).

NHC-stabilized parent sulfenyl (H-S), selenenyl (H-Se) and tellurenyl (H-Te) cations have been achieved by treatment of NHC chalcogen adducts with trifluoromethanesulfonic acid. Computational investigations show that most of the positive charges are localized at chalcogen atoms and carbene carbons with strong donor acceptor interactions between the lone pair of chalcogen atoms and the vacant orbital of the carbene centre, accounting for its unexpected stability.

S(vi) Lewis acids: fluorosulfoxonium cations.

Avenues to S-based Lewis acids were developed via the oxidation of aryl-sulfoxides with XeF, giving difluorodiarylsulfoxides which react via fluoride abstraction to afford Lewis acidic fluorosulfoxonium cations; this acidity is derived from the S-F σ* orbital and has been probed both experimentally and computationally.

Cationic aluminum hydride complexes: reactions of carbene-alane adducts with trityl-borate.

Reaction of (Idipp)AlH3 with [Ph3C][B(C6F5)4] in toluene affords the dimeric aluminum dication [((Idipp)AlH(μ-H))2][B(C6F5)4]22. In contrast, the reaction of (IBn)AlH3 with [Ph3C][B(C6F5)4] in bromobenzene gives a redistribution product, the salt of a monomeric dication [(IBn)2AlH][B(C6F5)4]24.

Double FLP-Alkyne Exchange Reactions: A Facile Route to Te/B Heterocycles.

1-Bora-4-tellurocyclohexa-2,5-diene undergoes sequential [4 + 2] cycloadditions/alkyne-elimination reactions to incorporate 2 equiv of terminal alkyne with the loss of diarylalkyne, affording access to a series of 11 new tellurium-boron heterocycles. These alkyne exchange reactions proceed regioselectively and can tolerate a variety of functional groups, thus providing the potential for further derivatization. The mechanism of the exchange reaction is confirmed by a DFT study to involve the interaction of the Te and B with the alkyne in a frustrated Lewis pair fashion in the transition states.

Synthesis and properties of imidazolo-fused benzotriazinyl radicals.

1,3-Diphenylbenzo[e][1,2,4]triazin-7(1H)-one, the oxidation product of 1,3-diphenyl-1,4-dihydro-1,2,4-benzotriazin-4-yl (Blatter's radical), reacts with N'-arylbenzamidines in PhMe at ca. 100 °C in the presence of N,N-diisopropylethylamine (Hünig's base) (1 equiv.) to give N'-aryl-N-(1,7-dihydro-7-oxo-1,3-diphenylbenzo[e][1,2,4]triazin-6-yl)benzimidamides in 49-95% yield.