Phosphaaluminirenes: Synthons for Main Group Heterocycles.

The phosphaaluminirenes HC[(CMe)(NDipp)]2Al[C(R)═P] (Dipp = 2,6-i-Pr2C6H3, R = tBu or adamantyl) 2 and 3, 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)]2Al (1) with phosphaalkynes. Computational investigations infer three-centered 2π-electron aromaticity of the AlCP rings. Compound 3 is readily protonated by tBuOH to induce a ring-opening σ-bond metathesis, giving an alumina-substituted P-hydrogeno phosphaalkene 4. Remarkably, the high strain of the AlCP ring of 3 allows for facile ring enlargement in reactions with CyNC, bis(diisopropylamino) cyclopropenylidene (BAC), elemental Se, Ph2CO, PhCH═CHCOPh, and PhCN at room temperature. These furnish a series of unprecedented main group heterocycles 5-10 with the C═P unsaturated bonds remaining intact. The mechanisms are considered in light of thorough density functional theory (DFT) calculations.

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 analogous deprotonation reaction of the less bulky phosphepinium cation 3 affords a reversible equilibrium between the phosphaalkene 4 and the corresponding cycloaddition product 5. This latter observation represents the first reversible cycloaddition of a main group multiply bonded species with an arene ring. The bicyclic species 2 was also shown to be oxidized or alkylated in reactions with S8 and MeOTf, affording 6 and 7, respectively. This finding and its implications for related cycloadditions of other main group multiply bonded species are considered computationally.

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

While (Ph2 CN2 )B(C6 F5 )3 is unstable, single electron transfer from Cp*2 Co affords the isolation of stable products [Cp*2 Co][Ph2 CNNHB(C6 F5 )3 ] 1 and [Cp*Co(C5 Me4 CH2 B(C6 F5 )3 )] 2. The analogous combination of Ph2 CN2 and BPh3 showed no evidence of adduct formation and yet single electron transfer from Cp*2 Cr affords the species [Cp*2 Cr][PhC(C6 H4 )NNBPh3 ] 3 and [Cp*2 Cr][Ph2 CNNHBPh3 ] 4. Computations showed both reactions proceed via transient radical anions of the diphenyldiazomethane-borane adducts to effect C-H bond activations.

Base-Stabilized [PO]+ /[PO2 ]+ Cations.

The salts [(BAC)2 PO][BF4 ] (5) and [(BAC)2 PO2 ][BF4 ] (4) (BAC=bis(diisopropylamino) cyclopropenylidene), consisting of the PO+ and PO2 + 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 K2 PtCl4 to furnish [[(BAC)2 PO]2 PtCl2 ][BF4 ]2 (6) and KCl. Remarkably, both 5 and 4 have shown to act as electrophiles undergoing reactions with fluoride anion, leading to [OPF2 ]- and (BAC)PO2 F, respectively.

The Arene-Stabilized η5 -Pentamethylcyclopentadienyl Arsenic Dication [(η5 -Cp*)As(toluene)]2.

Double chloride abstraction of Cp*AsCl2 gives the dicationic arsenic species [(η5 -Cp*)As(tol)][B(C6 F5 )4 ]2 (2) (tol=toluene). This species is shown to exhibit Lewis super acidity by the Gutmann-Beckett test and by fluoride abstraction from [NBu4 ][SbF6 ]. Species 2 participates in the FLP activation of THF affording [(η2 -Cp*)AsO(CH2 )4 (THF)][B(C6 F5 )4 ]2 (5). The reaction of 2 with PMe3 or dppe generates [(Me3 P)2 As][B(C6 F5 )4 ] (6) and [(σ-Cp*)PMe3 ][B(C6 F5 )4 ] (7), or [(dppe)As][B(C6 F5 )4 ] (8) and [(dppe)(σ-Cp*)2 ][B(C6 F5 )4 ]2 (9), respectively, through a facile cleavage of C-As bonds, thus showcasing unusual reactivity of this unique As-containing compound.

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 S8 , [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.

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 C6Cl4O2 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. The latter reaction is the first example of a phosphorus analog of the Büchner ring expansion reaction.

Reductive Coupling and Loss of N2 from Magnesium Diazomethane Derivatives.

The reductive coupling of two diazomethanes is affected by reaction with [(NacNacMes )Mg]2 affording the species [(NacNacMes )Mg(N2 CPh2 )]2 2 and [(NacNacMes )Mg(N2 C(C6 H4 )2 )]2 3. These species containing N4 linkages readily evolve the central N2 at 50 and 75 °C to give the Mg-imide products [(NacNacMes )Mg(NCPh2 )]2 (4) and [(NacNacMes )Mg(NC(C6 H4 )2 )]2 (5), respectively. The mechanism for the loss of N2 was considered computationally. Compounds 2 and 3 reacted with O2 to liberate the tetrazene (Ph2 N2 )2 6 and the hydrazine ((C6 H4 )2 CN)2 7, whereas reactions with Me3 SiOSO2 CF3 or Me3 SiCl with 2 and 3 provide the related silyl imines 8 and 9, respectively.

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

Reactions of [Cp*2 Fe] with the Lewis acid [Zn(C6 F5 )2 ] in the presence of [(PhC(S)S)2 ], 9,10-phenanthrenedione or 4,5-pyrenedione yield the salt [Cp*2 Fe][(PhC(S)S)Zn(C6 F5 )2 ] 1, [Cp*2 Fe][((C14 H8 O2 )Zn(C6 F5 )2 )⋅] 4, and [Cp*2 Fe][((C16 H8 O2 )Zn(C6 F5 )2 )⋅] 5, respectively. The latter two species represent the first examples of isolable zinc-containing radical anions. While [(PhC(S)S)2 ] binds weakly to [Zn(C6 F5 )2 ], the diones afford the isolable adducts [(C14 H8 O2 )Zn(C6 F5 )2 ] 2 and [(C16 H8 O2 )Zn(C6 F5 )2 ] 3. Cyclic voltammetry and computational studies support the view that 4 and 5 are formed via single electron transfer (SET) to the donor-acceptor adducts, 2 and 3, respectively. Subsequent treatment of 4 and 5 with [NC5 H4 NMe2 ] affords [Zn(C6 F5 )2 (NC5 H4 NMe2 )2 ] with liberation of the dione, and regeneration of [Cp*2 Fe].

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.

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(C6F5)3 E = B or Al) with a small molecule substrate and decamethylferrocene (Cp*2Fe). Initially, the corresponding reactions of (PhS)2 and (PhTe)2 were shown to give the species [Cp*2Fe][PhSB(C6F5)3] 1 and [Cp*2Fe][(µ-PhS)(Al(C6F5)3)2] 2 and [Cp*2Fe][(µ-PhTe)(Al(C6F5)3)2] 3, respectively. Analogous reactions with di-tert-butyl peroxide yielded [Cp*2Fe][(µ-HO)(B(C6F5)3)2] 4 with isobutene while with benzoyl peroxide afforded [Cp*2Fe][PhC(O)OE(C6F5)3] (E = B 5, Al 6). Evidence for a radical pathway was provided by the reaction of Ph3SnH and p-quinone afforded [Cp*2Fe][HB(C6F5)3] 7 and [Cp*2Fe]2[(µ-O2C6H4)(E(C6F5)3)2] (E = B 8, Al 9). In addition, the reaction of TEMPO with Lewis acid and Cp*2Fe afforded [Cp*2Fe][(C5H6Me4NOE(C6F5)3] (E = B 10, Al 11). Finally, reactions with O2, Se, Te and S8 gave [Cp*2Fe]2[((C6F5)2Al(µ-O)Al(C6F5)3)2]2 12, [Cp*2Fe]2[((C6F5)2Al(µ-Se)Al(C6F5)3)2]2 13, [Cp*2Fe][(µ-Te)2(Al(C6F5)2)3] 14 and [Cp*2Fe]2[(µ-S7)B(C6F5)3)2] 15, respectively. The mechanisms of these SET reactions are discussed, and the ramifications are considered.

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. In neat AcOH heated at ca. 120 °C, N'­aryl-N-(1,7-dihydro-7-oxo-1,3-diphenylbenzo[e][1,2,4]triazin-6-yl)benzimidamides cyclodehydrate to give the novel 8-substituted 1,3,7-triphenyl-4,8-dihydro-1H-imidazo[4,5-g][1,2,4]benzotriazin-4-yls in 13-81% yield. During the optimization of this cyclodehydration an additional oxazole fused benzotriazinyl radical 1,3,7-triphenyl-1,4-dihydro[1,3]oxazolo[4,5-g][1,2,4]benzotriazin-4-yl was isolated as a side product and characterized. The CV and EPR data of the imidazolo- and oxazolo-fused radicals are presented as well as single crystal X-ray structures of 1,3,7-triphenyl-1,4-dihydro-[1,3]oxazolo[4,5-g][1,2,4]benzotriazin-4-yl and 1,3,7,8-tetraphenyl-4,8-dihydro-1H-imidazo[4,5-g][1,2,4]benzotriazin-4-yl.

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.

An arene-stabilized η5-pentamethylcyclopentadienyl antimony dication acts as a source of Sb+ or Sb3+ 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 [C3H2(NCH2C6H4)(NCH2Ph)BH][B(C6F5)4] 2 and [C3H2(NCH2C6H4)2B][B(C6F5)4] 3. The reactions of 2 with cyclohexene or 3,3-dimethyl-1-butene gave the alkyl-aryl borenium salts [PhCH2(CHN)2CCH2C6H4BR][B(C6F5)4] (R = Cy 4, CH2CH2tBu 5) while the corresponding reactions with diphenylacetylene, 1-hexyne and 1-phenyl-1-propyne gave the aryl-alkenyl borenium cation salts [PhCH2(CHN)2CCH2C6H4BC(R1)C(H)R2][B(C6F5)4] (R1 = R2 = Ph 6, R1 = H, R2 = C4H97, R1 = Me, R2 = Ph 8a, R1 = Ph, R2 = Me 8b). In contrast, the reaction of 2 with ethynyldiphenylphosphane or 2-vinylpyridine lead to the formation of the adducts, [PhCH2(CHN)2CCH2C6H4B(H)P(Ph2)CCH][B(C6F5)4] 9, [PhCH2(CHN)2CCH2C6H4B(H)NC5H4C(H)CH2][B(C6F5)4] 10, respectively, while the more bulky donor H2C[double bond, length as m-dash]C(Ph)PMes2 gave 1,2-hydroboration of the phosphinoalkene affording [PhCH2(CHN)2CCH2C6H4BCH2CH(Ph)PMes2][B(C6F5)4] 11. In another vein of reactivity, one or two equivalents of the FLP, PtBu3/B(C6F5)3 is shown to react with 3 to give the zwitterionic borenium-borate species [C2H2(NCH(BC(CHNCH2C6H4)2)C6H4)(NCH(B(C6F5)3)C6H4)CB] 12 and the anionic bis-borate species[tBu3PH][C2H2(NCH(B(C6F5)3)2C6H4)2CB] 13. The implications of these findings are discussed.

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 P6 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. Such oligomerizations are highly susceptible to the electronic property and steric bulk of NHCs.

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 Mes3P/B(C6F5)3 frustrated Lewis pair with benzoyl peroxide derivatives. The resulting homolytic cleavage of the peroxide bonds affords the radical salts [Mes3P˙]+[RCOOB(C6F5)3]- (R = Ph 1, p-BrC6H42, p-MeC6H43). These species react with Ph3SnH to give [Mes3PH]+[RCOOB(C6F5)3]- (R = Ph 10, p-BrC6H411, p-MeC6H412) and (Ph3Sn)2. The mechanism of these SET reactions is discussed in light of control experiments and DFT calculations.

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.

FLP reactivity of [Ph3C]+ and (o-tolyl)3P and the capture of a Staudinger reaction intermediate.

The frustrated Lewis pair (FLP) derived from the trityl cation and (o-tolyl)3P 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 Ph3SiH to give the silyl analog.