Oleg V. Ozerov

Hydrodefluorination of Perfluoroalkyl Groups Using Silylium-Carborane Catalysts

Carbon-fluorine bonds are among the most unreactive functionalities in chemistry. Interest in their activation arises in part from the high global warming potentials of anthropogenic polyfluoroorganic compounds. Conversion to carbon-hydrogen bonds (hydrodefluorination) is the simplest modification of carbon-fluorine bonds, but efficient catalytic hydrodefluorination of perfluoroalkyl groups has been an unmet challenge. We report a class of carborane-supported, highly electrophilic silylium compounds that act as long-lived catalysts for hydrodefluorination of trifluoromethyl and nonafluorobutyl groups by widely accessible silanes under mild conditions. The reactions are completely selective for aliphatic carbon-fluorine bonds in preference to aromatic carbon-fluorine bonds.

Hydrodefluorination and Other Hydrodehalogenation of Aliphatic Carbon−Halogen Bonds Using Silylium Catalysis

Trialkylsilylium cation equivalents partnered with halogenated carborane anions (such as Et(3)Si[HCB(11)H(5)Cl(6)]) function as efficient and long-lived catalysts for hydrodehalogenation of C-F, C-Cl, and C-Br bonds with trialkylsilanes as stoichiometric reagents. Only C(sp(3))-halogen bonds undergo this reaction. The range of C-F bond-containing substrates that participate in this reaction is quite broad and includes simple alkyl fluorides, benzotrifluorides, and compounds with perfluoroalkyl groups attached to an aliphatic chain. However, CF(4) has proven immune to this reaction. Hydrodechlorination was carried out with a series of alkyl chlorides and benzotrichlorides, and hydrodebromination was studied only with primary alkyl bromide substrates. Competitive experiments established a pronounced kinetic preference of the catalytic system for activation of a carbon-halogen bond of a lighter halide in primary alkyl halides. On the contrary, hydrodechlorination of C(6)F(5)CCl(3) proceeded much faster than hydrodefluorination of C(6)F(5)CF(3) in one-pot experiments. A solid-state structure of Et(3)Si[HCB(11)H(5)Cl(6)] was determined by X-ray diffraction methods.

Carbon-carbon coupling of C(sp3)-F bonds using alumenium catalysis.

Dialkylalumenium cation equivalents coupled with the hexabromocarborane anion function as efficient and long-lived catalysts for alkylation of aliphatic C-F bonds (alkylative defluorination or AlkDF) by alkylaluminum compounds. Only C(sp(3))-F bonds undergo AlkDF; C(sp(2))-F bonds are unaffected. Examples of compounds undergoing AlkDF include monofluoroalkanes, gem-difluorocyclopentane, and compounds containing a CF(3) group attached to either an aryl or an alkyl substituent. Conversion of C-F bonds to C-Me bonds is accomplished with high fidelity using Me(3)Al as the stoichiometric reagent. In reactions with Et(3)Al, hydrodefluorination of the C-F bonds is competitive with alkylation, indicative presumably of competitive hydride vs alkyl transfer from Et(3)Al. In a trialkylaluminum reagent, 1.1-1.4 alkyl groups per Al can be used to replace C-F bonds. Organoaluminum compounds efficiently remove water from the reaction mixture, obviating the need for rigorously dry solvents. Some organoaluminum compounds, especially methylaluminoxane, are capable of AlkDF with more reactive substrates, but catalysis by alumenium offers an advantage over the uncatalyzed C-F activation in terms of both increased rate and, in some cases, a dramatically increased selectivity.

Metallaboratranes Derived from a Triphosphanyl–Borane: Intrinsic C3 Symmetry Supported by a Z‐Type Ligand

Following the pioneering contributions of Knowles, Kagan, and Noyori, C1 and C2 chiral ligands have played a prominent role in asymmetric catalysis with transition-metal complexes. Over the last few years, increasing attention has been devoted to C3-symmetric derivatives, [2] and spectacular achievements have been reported using facially coordinating tripodal ligands assembled around a remote junction point (which does not enter the coordination sphere), an L-type coordination site, or an X-type coordination site. The trisoxazoline, trisamido, and triphosphane complexes A–C are archetypal examples of these three different situations (Scheme 1). Our interest in ambiphilic ligands that combine donor and acceptor moieties prompted us to investigate the ability of s-acceptor (Z-type) coordination sites to also support such a three-fold geometry. Accordingly, an L3Z tetradentate triphosphanyl–borane (TPB) ligand is reported herein to afford gold and platinum metallaboratranes D featuring dative M!B interactions and exhibiting C3 symmetry. [11–14] Such helical geometry has been shown to result from the tendency of the PCCBM metalla-

A Lanthanide Phosphinidene Complex: Synthesis, Structure, and Phospha-Wittig Reactivity

The first lanthanide complex featuring a phosphinidene functional group has been prepared and isolated. Preliminary reactivity studies demonstrate that the lutetium(III) phosphinidene complex, [{2-(iPr2P)-4-Me-C6H3}2NLu]2(μ-PMes)2, behaves as a phospha-Wittig reagent with aldehydes and ketones to give the corresponding phosphaalkenes. Attempts to use the bulky phosphine H2P-2,4,6-tBu3-C6H2 to kinetically stabilize a terminal phosphinidene resulted in C−H activation of an ortho-tBu group and formation of a phosphaindole.

Skeletal change in the PNP pincer ligand leads to a highly regioselective alkyne dimerization catalyst

A Rh complex of a bulky diarylamino-based PNP pincer ligand is a robust catalyst for the dimerization of terminal alkynes and highly selective for the trans-enyne product.

Ligand Reactivity in Diarylamido/Bis(Phosphine) PNP Complexes of Mn(CO)3 and Re(CO)3

The syntheses of meridionally ligated tricarbonyl complexes (PNP)Mn(CO)(3) and (PNP)Re(CO)(3) are described (PNP = [2-P(CHMe(2))(2)-4-MeC(6)H(3)](2)N(-)). Cyclic voltammograms show reversible one-electron redox couples for both parent compounds (-0.34 V vs Cp(2)Fe(0/+) for (PNP)Mn(CO)(3), -0.25 V vs Cp(2)Fe(0/+) for (PNP)Re(CO)(3)), and chemical oxidation with AgOTf results in formation of the corresponding paramagnetic triflate salts [(PNP)Mn(CO)(3)]OTf and [(PNP)Re(CO)(3)]OTf. Electron paramagnetic resonance spectra and computational results indicate that this event is primarily ligand centered; allylation of the organic ligand moiety of [(PNP)Mn(CO)(3)]OTf with allyltributylstannane is consistent with this assignment. The oxidation (PNP)Mn(CO)(3) to [(PNP)Mn(CO)(3)]OTf results in a shift in average CO stretching frequency of 30 cm(-1); protonation of (PNP)Mn(CO)(3) with TfOH to form [(PNHP)Mn(CO)(3)]OTf results in a similar magnitude shift.

Catalytic dehydrogenative borylation of terminal alkynes by a SiNN pincer complex of iridium.

Compounds with carbon-boron bonds are versatile intermediates for building more complex molecules via the elaboration of the carbon-boron bonds into other carbon-element bonds. The synthesis of carbon-boron bonds by catalytic dehydrogenative borylation of carbon-hydrogen bonds with dialkoxyboranes (RO)2BH is particularly attractive. It has been demonstrated for a variety of carbon-hydrogen bond types but not for the C(sp)-H bonds of terminal alkynes, for which hydroboration of the triple bond is a competing process. We report a new iridium catalyst that is strictly chemoselective for C-H borylation of terminal alkynes. The key to the success of this catalyst appears to be the new ancillary SiNN pincer ligand that combines amido, quinoline, and silyl donors and gives rise to structurally unusual Ir complexes. A variety of terminal alkynes (RC≡C-H) can be converted to their alkynylboronates (RC≡C-Bpin, where pin = pinacolate) in high yield and purity within minutes at ambient temperature.

Reactivity of a Pd(I)−Pd(I) Dimer with O2: Monohapto Pd Superoxide and Dipalladium Peroxide in Equilibrium

The Pd(I)-Pd(I) dimer [((F)PNP)Pd-](2) reacts with O(2) upon exposure to light to produce either the superoxide ((F)PNP)PdO(2) or the peroxide [((F)PNP)PdO-](2), which exist in equilibrium with free O(2). Both complexes contain square-planar Pd(II) centers. The unpaired electron density in ((F)PNP)PdO(2) is localized on the superoxide ligand.

Exhaustive Chlorination of [B12H12]2− without Chlorine Gas and the Use of [B12Cl12]2− as a Supporting Anion in Catalytic Hydrodefluorination of Aliphatic C−F Bonds

The fully chlorinated closo-dodecaborate salt Cs(2)[B(12)Cl(12)] was prepared in high yield from Cs(2)[B(12)H(12)] and SO(2)Cl(2) in acetonitrile at refluxing temperature. [Ph(3)C](2)[B(12)Cl(12)] was obtained by simple metathesis reactions. Catalytic hydrodefluorination of benzotrifluoride sp(3) C-F bonds was accomplished using [Ph(3)C](2)[B(12)Cl(12)] as a precatalyst and Et(3)SiH as a stoichiometric reagent. Full consumption of the sp(3) C-F bonds in p-FC(6)H(4)CF(3) and C(6)F(5)CF(3) with a turnover number up to 2000 was achieved.