Jennifer L. McBee

Mechanistic Studies of the Hydroamination of Norbornene with Electrophilic Platinum Complexes: The Role of Proton Transfer

Hydroaminations of norbornene with arylsulfonamides and weakly basic anilines were achieved using electrophilic Pt(II) bis(triflate) complexes of the type L2Pt(OTf)2 (L2 = (t)Bu2bpy, (t)BuC6H4N== C(CH3)C(CH3)==NC6H4(t)Bu, (C6H5)2PCH2CH2P(C6H5)2, (C6F5)2PCH2CH2P(C6F5)2, S-BINAP). Pseudo-first-order kinetics reveal little to no dependence of the reaction rate on the ancillary ligand. Mechanistic studies do not favor an olefin coordination mechanism but are instead consistent with a mechanism involving sulfonamide coordination and generation of an acidic proton that is transferred to the norbornene. It is postulated that the resulting norbornyl cation is then attacked by free sulfonamide, and loss of proton from this adduct completes the hydroamination. The platinum-sulfonamide complex readily undergoes deprotonation to give a mu-amido platinum-bridged dimer that was isolated from the reaction solution. These studies also involve use of Me3SiPh and Me3SnPh as non-nucleophilic proton traps. Cleavage of the Ph-E bonds was used to detect the acidic, catalytically active species.

High oxidation state rhodium and iridium bis(silyl)dihydride complexes supported by a chelating pyridyl-pyrrolide ligand.

New rhodium and iridium complexes containing the bidentate ligand 3,5-diphenyl-2-(2-pyridyl)pyrrolide (PyPyr) were prepared. The bis(ethylene) complex (PyPyr)Rh(C(2)H(4))(2) (3) reacted with HSiEt(3), HSiPh(3), and HSi(t)BuPh(2) to produce the 16-electron Rh(V) bis(silyl)dihydrides (PyPyr)Rh(H)(2)(SiEt(3))(2) (8), (PyPyr)Rh(H)(2)(SiPh(3))(2) (9), and (PyPyr)Rh(H)(2)(Si(t)BuPh(2))(2) (10), respectively. The analogous Ir(V) bis(silyl)dihyride (PyPyr)Ir(H)(2)(SiPh(3))(2) (11) has also been synthesized. X-ray crystallography reveals that 9-11 adopt a coordination geometry best described as a bicapped tetrahedron. Silane elimination from 9 and 10 occurred in the presence of either HSiEt(3) or PPh(3). Mechanistic studies of the silane exchange process involving 10 and free HSiEt(3) (to give 8) indicate that this process occurs by rate-limiting reductive elimination of HSi(t)BuPh(2) from 10 to generate a 14-electron Rh(III) intermediate of the type (PyPyr)Rh(H)(Si(t)BuPh(2)).

9,10-Disubstituted Octafluoroanthracene Derivatives via Palladium-Catalyzed Cross-Coupling

9,10-Dichlorooctafluoroanthracene (1) reacts with aryl boronic acids and terminal alkynes under palladium-catalyzed cross-coupling conditions to afford 9,10-diaryloctafluoroanthracenes (2a-e) and 9,10-dialkynyloctafluoroanthracenes (6a,b), respectively. Optical spectroscopy and cyclic voltammetry indicate that octafluoro-9,10-di(thiophen-2-yl)anthracene (2d) exhibits donor-acceptor character and a LUMO energy level of -3.27 eV relative to vacuum. A functionalized 5-bromothiophen-2-yl derivative (2e) was obtained in high yield by bromination of 2d with NBS. X-ray crystallographic analysis of octafluoro-9,10-bis[(trimethylsilyl)ethynyl]anthracene (6a) reveals a solid-state structure that mimics the packing of columnar liquid crystals, with a pi stacking distance of 3.39 A between the octafluoroanthracene cores. In addition, octafluoro-9,10-bis(mesitylethynyl)anthracene (6b) displays a LUMO energy level of -3.50 eV, which approaches the value of -3.65 eV measured for perfluoropentacene, making 9,10-dialkynyloctafluoroanthracenes a promising new class of n-type organic materials.

Mechanism of reversible alkyne coupling at zirconocene: ancillary ligand effects.

The mechanism of reversible alkyne coupling at zirconium was investigated by examination of the kinetics of zirconacyclopentadiene cleavage to produce free alkynes. The zirconacyclopentadiene rings studied possess trimethylsilyl substituents in the alpha-positions, and the ancillary Cp2, Me2C(eta(5)-C5H4)2, and CpCp* (Cp* = eta(5)-C5Me5) bis(cyclopentadienyl) ligand sets were employed. Fragmentation of the zirconacyclopentadiene ring in Cp2Zr[2,5-(Me3Si)2-3,4-Ph2C4] with PMe3, to produce Cp2Zr(eta(2)-PhC[triple bond]CSiMe3)(PMe3) and free PhC[triple bond]CSiMe3, is first-order in initial zirconacycle concentration and zero-order in incoming phosphine (k(obs) = 1.4(2) x 10(-5) s(-1) at 22 degrees C), and the activation parameters determined by an Eyring analysis (DeltaH(double dagger) = 28(2) kcal mol(-1) and DeltaS(double dagger) = 14(4) eu) are consistent with a dissociative mechanism. The analogous reaction of the ansa-bridged complex Me2C(eta(5)-C5H4)2Zr[2,5-(Me3Si)2-3,4-Ph2C4] is 100 times faster than that for the corresponding Cp2 complex, while the corresponding CpCp* complex reacts 20 times slower than the Cp2 derivative. These rates appear to be largely influenced by the steric properties of the ancillary ligands.

Mesityl Alkyne Substituents for Control of Regiochemistry and Reversibility in Zirconocene Couplings: New Synthetic Strategies for Unsymmetrical Zirconacyclopentadienes and Conjugated Polymers

Reaction of 2 equivs of MesC[triple bond]CPh with Cp(2)Zr(eta(2)-Me(3)SiC[triple bond]CSiMe(3))(pyr) afforded the zirconacyclopentadiene Cp(2)Zr[2,5-Ph(2)-3,4-Mes(2)C(4)]. The regiochemistry of this isomer (betabeta with respect to the mesityl substituents) was determined through single-crystal X-ray analysis and 2D (NOESY, HSQC, HMBC) NMR experiments. This selectivity is attributed largely to a steric-based directing effect of the o-methyl ring substituents since coupling of 1,3-dimethyl-2-(phenylethynyl)benzene with zirconocene gave a single regioisomer (o-xylyl groups in both beta-positions) while coupling of 1,3-dimethyl-5-(phenylethynl)benzene gave a statistical distribution of zirconacyclopentadiene regioisomers. The coupling reaction of 2 equivs of MeC[triple bond]CMes or PrC[triple bond]CMes with Cp(2)Zr(eta(2)-Me(3)SiC[triple bond]CSiMe(3))(pyr) at ambient temperature gave the betabeta regioisomers, Cp(2)Zr[2,5-Me(2)-3,4-Mes(2)C(4)] and Cp(2)Zr[2,5-Pr(2)-3,4-Mes(2)C(4)], respectively, as the major products. Heating solutions of these zirconacycles at 80 degrees C for several hours resulted in an increase in the amount of the unsymmetrical product. For reaction mixtures of PrC[triple bond]CMes and Cp(2)Zr(eta(2)-Me(3)SiC[triple bond]CSiMe(3))(pyr) the major (and apparently thermodynamic) product under these reaction conditions was Cp(2)Zr[2,4-Mes(2)-3,5-Pr(2)C(4)]. The steric strain in the mesityl-substituted zirconacycles allowed for facile substitution reactions of MesC[triple bond]CPh or PrC[triple bond]CMes by less bulky alkynes (i.e., tolan and 3-hexyne) to give the unsymmetrical ziconacyclopentadienes Cp(2)Zr[2,4,5-Ph(3)-3-MesC(4)], Cp(2)Zr[2-Ph-3-Mes-4,5-Et(2)C(4)], and Cp(2)Zr[2-Pr-3-Mes-4,5-Ph(2)C(4)]. Reaction of a mesityl-terminated diyne containing a rigid dihexylfluorenylene spacer with zirconocene afforded poly(p-fluorenylenedienylene) after demetalation with benzoic acid.

Synthesis and characterization of 2,7-bis(pentafluorophenylethynyl)-hexafluoroheterofluorenes : new materials with high electron affinities

The synthesis, optical, electrochemical and physical characterization of 2,7-bis(pentafluorophenylethynyl)hexafluoroheterofluorenes is described along with a preliminary evaluation of their performance in photovoltaic applications.