Bruce M. Foxman

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.

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-

Asymmetric Vinylogous Aldol Reaction of Silyloxy Furans with a Chiral Organic Salt

Despite their synthetic significance there is a general lack of asymmetric vinylogous aldol reactions that tolerate variations of both the silyloxy furans and aldehydes. We have developed a new chiral organic catalyst based on a carboxylate-ammonium salt prepared from a thiourea-amine and a carboxylic acid. This new catalyst enabled us to develop an efficient asymmetric vinylogous aldol reaction of unprecedented scope with respect to both 2-trimethylsilyloxy furans and aldehydes.

Activation of CO2 by a heterobimetallic Zr/Co complex.

At room temperature, the early/late heterobimetallic complex Co((i)Pr(2)PNMes)(3)Zr(THF) has been shown to oxidatively add CO(2), generating (OC)Co((i)Pr(2)PNMes)(2)(μ-O)Zr((i)Pr(2)PNMes). This compound can be further reduced under varying conditions to generate either the Zr oxoanion (THF)(3)Na-O-Zr(MesNP(i)Pr(2))(3)Co(CO) or the Zr carbonate complex (THF)(4)Na(2)(CO(3))-Zr(MesNP(i)Pr(2))(3)Co(CO). Additionally, reactivity of the CO(2)-derived product has been observed with PhSiH(3) to generate the Co-hydride/Zr-siloxide product (OC)(H)Co((i)Pr(2)PNMes)(3)ZrOSiH(2)Ph.

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.

Multielectron Redox Activity Facilitated by Metal−Metal Interactions in Early/Late Heterobimetallics: Co/Zr Complexes Supported by Phosphinoamide Ligands

To assess the effect of dative M-->M interactions on redox properties in early/late heterobimetallic complexes, a series of Co/Zr complexes supported by phosphinoamide ligands have been synthesized and characterized. Treatment of the Zr metalloligands (Ph(2)PN(i)Pr)(3)ZrCl (1), ((i)Pr(2)PNMes)(3)ZrCl (2), and ((i)Pr(2)PN(i)Pr)(3)ZrCl (3) with CoI(2) leads to reduction from Co(II) to Co(I) and isolation of the heterobimetallic complexes ICo(Ph(2)PN(i)Pr)(3)ZrCl (4), ICo((i)Pr(2)PNMes)(3)ZrCl (5), and ICo((i)Pr(2)PN(i)Pr)(3)ZrCl (6), respectively. Interestingly, treatment of CoI(2) with the phosphinoamine Ph(2)PNH(i)Pr in the absence of a bound Zr center leads to the disubstituted Co(II) complex (Ph(2)PNH(i)Pr)(2)CoI(2) (7). The tris(phosphinoamine) Co(I) complex (Ph(2)PNH(i)Pr)(3)CoI (8) can only be generated in the presence of an added reductant such as Zn(0), indicating that the reduction of Co(II) to Co(I) only occurs in the presence of Zr in the formation of complexes 4-6. Structural characterization of 4-6 reveals a Zr-Co interaction, with interatomic distances of 2.7315(5) A, 2.6280(5) A, and 2.6309(5) A, respectively. This distance appears to decrease as the phosphine donors on Co become more electron-releasing, strengthening the dative Co-->Zr interaction. Cyclic voltammetry of 4-6 shows that all three compounds can be further reduced by two electrons at relatively mild reduction potentials (-1.65 V to -2.07 V vs Fc/Fc(+)). The potentials at which these reductions occur in each of these complexes are largely governed by the extent to which electron-density is donated to Zr, as well as the electron-donating ability of the phosphine substituents. Moreover, cyclic voltammetry of complex 8 reveals that in the absence of Zr, the Co center is significantly more electron rich, and thus more difficult to reduce. Chemical reduction of 5 leads to the isolation of the two-electron reduced dinitrogen complex [N(2)Co((i)Pr(2)PNMes)(3)ZrX][Na(THF)(5)] (9). X-ray crystallography of 9 reveals that two-electron reduction is accompanied by a significant contraction of the Co-Zr interatomic distance from 2.6280(5) A to 2.4112(3) A. These heterobimetallic complexes have been studied computationally using density functional theory to examine the nature of the metal-metal interactions in these species.

Metal-metal multiple bonds in early/late heterobimetallics support unusual trigonal monopyramidal geometries at both Zr and Co.

Reduction of Zr/Co heterobimetallic complexes ICo(MesNP(i)Pr(2))(3)ZrCl (1) and ICo((i)PrNP(i)Pr(2))(3)ZrCl (2) with excess Na/Hg under N(2), followed by subsequent benzene extraction to remove coordinated Na halide salts, leads to neutral two-electron reduced, dinitrogen-bound complexes (THF)Zr(MesNP(i)Pr(2))(3)Co-N(2) (4) and Zr((i)PrNP(i)Pr(2))(3)Co-N(2) (5). Upon halide loss, a THF solvent molecule coordinates to the axial site of the Zr center in 4, while this axial site remains unoccupied in 5. X-ray crystallography reveals short Co-Zr distances in 4 and 5, indicative of metal-metal multiple bonding, and an unprecedented trigonal monopyramidal geometry about the Zr center in 5. Reduction of 4 under an Ar atmosphere (in the absence of N(2)) results in another unusual structure type: an unoccupied axial Co coordination site and a trigonal monopyramidal Co center in (THF)Zr(MesNP(i)Pr(2))(3)Co (6). X-ray crystallography reveals that, in the absence of coordinated N(2), the Co-Zr bond can attain full triple bond character with a Co-Zr distance of 2.14 A, the shortest M-M distance in an early/late heterobimetallic complex reported to date. To further assess the electronic structure and bonding in 4, 5, and 6, calculations were performed on these molecules using DFT and the results of these theoretical investigations will be discussed.

Synthesis of the tricyclic triamine core of martinelline and martinellic acid

Abstract The tricyclic triamine core27 of martinellic acid (2) has been prepared stereospecifically in eight steps from 2-hydroxymethylaniline in 11% overall yield. The key steps are the addition of 2-hydroxymethylaniline to vinylcyclopropane14 to prepare cycloaddition precursor19 in only two steps and an intramolecular [3+2] azomethine ylide cycloaddition reaction to produce tetracycle20 with>9:1 diastereoselectivity. Download : Download full-size image

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.

Structural Study-Guided Development of Versatile Phase Transfer Catalysts for Asymmetric Conjugate Additions of Cyanide

New phase transfer catalysts are reported for the first example of an organocatalytic asymmetric conjugate addition of cyanide with acetone cyanohydrin. Utilizing an accessible cupreidinium salt and a cyanation reagent suitable for industrial scale, this reaction holds significant promise for practical asymmetric synthesis. Additionally, the reported catalysts were developed as a result of gaining key structural insights via X-ray analysis of a series of catalysts of varying efficiencies and asymmetric induction.