Levy L. Cao

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.

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.

S(VI) Lewis acids: fluorosulfoxonium cations

Avenues to S-based Lewis acids were developed via the oxidation of aryl-sulfoxides with XeF2, 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.

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].

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.