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 [C H N (tBu)C(C H )][BF ], 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 . Reaction of 1 with PMe afforded the N-bound phosphine adduct [C H N(tBu)N(PMe )C(C H )]BF ] 3.

Addition reactions and diazomethane capture by the intramolecular P-O-B FLP: tBuPOBcat.

FLPs, R2POBcat (R = tBu 1, Mes 2), are readily derived from the reactions of the corresponding phosphine oxides, nBuLi and ClBcat. Despite the poor Lewis acidity of boron, these species react with PhOH, CO2, CS2, PhNCO, MesCNO, O2, 9,10-phenanthrenedione, and diazomethanes to effect FLP addition reactions affording a series of heterocycles. The reaction of 1 with EtO2CCHN2 gives the bicyclic product, EtO2CCHN2(tBu2POBcat)2.

Electrophilic boron carboxylate and phosphinate complexes.

The reactions of a series of carboxylic acids with H2B(C6F5)·SMe2 are shown to afford species of the form [RC(O)OB(C6F5)]2O, (R = Tol 1, Ph 2, C6F53, Me2BrC 4, Me 5) in 87-95% yields with the concurrent reduction of the carboxylic acid to the corresponding aldehyde. A mechanism for the formation of 1-5 is proposed to proceed via a cyclic eight-membered ring species. Analogues of these species were prepared via reactions of carboxylic and phosphinic acids with HB(C6F5)2 and H2B(C6F5)·SMe2, respectively, to give [TolC(O)OB(C6F5)2]26, [(C6F5)C(O)OB(C6F5)2]27, and [Ph2P(O)OBH(C6F5)]28.

Interception of intermediates in phosphine oxidation by mesityl nitrile-N-oxide using frustrated Lewis pairs.

Phosphine oxidation by MesCNO is rapid; however, an FLP strategy intercepts the 1,3 addition products including [MesC(R3P)NOB(C6F5)3] (R = Ph 1, p-tol 2), [MesC(Mes2PH)NOB(C6F5)3] 3 (MesC(NOB(C6F5)3)Ph2P)2(CH2)n (n = 2: 4, 3: 5) and [MesC(Ph3P)NOB(C6F4H)3] 6. These species are shown to react with tBuOK or [Bu4N]F permitting the oxidation to proceed via a process involving borane dissociation. Similarly, the equilibrium established by 1 with B(C6F4H)3 and 6 with B(C6F5)3 provides experimental support for the "Cummins mechanism" for these phosphine oxidations.

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).

The phosphinoboration of carbodiimides, isocyanates, isothiocyanates and CO.

The transition metal-free addition of phosphinoboronate ester PhPBpin (pin = 1,2-OCMe) to heterocumulenes including carbodiimides, isocyanates, isothiocyanates and carbon dioxide has been investigated. The corresponding 1,2-addition products were readily prepared at room temperature without the need of a catalyst or added base. Addition of methanol to the compounds derived from addition of PhPBpin to carbodiimides, isocyanates, and isothiocyanates resulted in traditional hydrophosphination products.

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.