A detailed kinetic study is described for the insertion of carbenes from methyl diazoacetate into the N–H bond of aniline, using Ir(TTP)CH3 (TTP = tetratolylporphyrinato) as a catalyst. Aniline strongly coordinates to the Ir center with a binding constant of K = (2.5 ± 0.5) × 104 at 296 K, forming an inactive hexacoordinate complex, (aniline)Ir(TTP)CH3. The rate of N–H insertion is first order in both diazo ester and catalyst. When the true amount of active, five-coordinate Ir(TTP)CH3 is taken into account, the insertion rate is found to be independent of the aniline concentration. This indicates that the rate-limiting step in the catalytic cycle occurs prior to the nucleophilic attack of aniline on an Ir carbene complex to generate a coordinated ylide. Thus, with N–H insertion catalyzed by Ir(TTP)CH3, aniline is both a strong ligand and a potent nucleophile. This is in contrast to the analogous catalytic insertion of carbenes into S–H bonds. p-Toluenethiol is a more weakly binding ligand toward Ir(TTP)CH3 (K = 680 ± 20 at 296 K). Moreover, the rate of S–H insertion is first order in diazo reagent, catalyst, and thiol concentrations. In this case, the slow step is nucleophilic attack of the thiol on the Ir carbene complex to form a coordinated sulfonium ylide intermediate. In comparison to aniline, p-toluenethiol is a weaker ligand and a poorer nucleophile. The consequence of these differences is that the rate of aniline attack on the carbene intermediate is much faster than the rate of formation of the intermediate carbene complex; whereas the rate of nucleophilic addition of the thiol is slower that the rate of carbene complex formation.

The insertion of carbenes derived from ethyl diazoacetate (EDA), methyl diazoacetate (MDA), methyl phenyldiazoacetate (MPDA), and methyl (p-tolyl)diazoacetate (MTDA) into the S–H bonds of aromatic and aliphatic thiols was catalyzed by (5,10,15,20-tetratolylporphyrinato)methyliridium(III), Ir(TTP)CH3, at ambient temperatures. Yields of the resulting thioether products were as high as 97% for aromatic thiols, with catalyst loadings as low as 0.07 mol %. Thiol binding to Ir(TTP)CH3 was measured at 23 °C by titration studies, providing equilibrium constants, Kb, ranging from 4.25 × 102 to 1.69 × 103 and increasing in the order p-nitrobenzenethiol < p-chlorobenzenethiol < benzenethiol < p-methylbenzenethiol < p-methoxybenzenethiol < benzyl mercaptan. Hammett plots were generated from the relative rates of S–H insertion, using different para-substituted benzenethiols in substrate competition experiments. In the presence of MDA and MTDA, the Hammett plots had slopes of −0.12 ± 0.01 and −0.78 ± 0.11, respectively. The Hammett data and kinetic studies are consistent with a mechanism that involves a rate-limiting nucleophilic attack of thiols on an iridium-carbene species, where the major species present in the reaction solution is an inactive, hexacoordinate Ir-thiol complex.

The oxidative transformation of cyclic amines to lactams, which are important chemical feedstocks, is efficiently catalyzed by CeO2-supported gold nanoparticles (Au/CeO2) and Aerosil 200 in the presence of an atmosphere of O2. The complete conversion of pyrrolidine was achieved in 6.5 h at 160 °C, affording a 97 % yield of the lactam product 2-pyrrolidone (γ-butyrolactam), while 2-piperidone (δ-valerolactam) was synthesized from piperidine (83 % yield) in 2.5 h. Caprolactam, the precursor to the commercially important nylon-6, was obtained from hexamethyleneimine in 37 % yield in 3 h. During the oxidation of pyrrolidine, two transient species, 5-(pyrrolidin-1-yl)-3,4-dihydro-2H-pyrrole (amidine-5) and 4-amino-1-(pyrrolidin-1-yl)butan-1-one, were observed. Both of these compounds were oxidized to 2-pyrrolidone under catalytic conditions, indicating their role as intermediates in the reaction pathway. In addition to the reactions of cyclic secondary amines, Au/CeO2 also efficiently catalyzes the oxidation of N-methyl cyclic tertiary amines to the corresponding lactams at 80 and 100 °C.

Aqueous Sonogashira coupling between lipophilic terminal alkynes and aryl bromides or iodides gave moderate to high yields at 40 °C using readily available and inexpensive surfactants (2.0 w/v% in water) such as SDS and CTAB. The catalyst precursor was 2 mol% Pd(PPh3)2Cl2, and included a 5 mol% Cu(I) co-catalyst for aryl iodide substrates. Aryl-bromide reagents were found to be inhibited by iodide and Cu(I). Studies under Cu(I)-free conditions reveal two competing pathways. A deprotonation pathway gives rise to the traditional Sonogashira product (3), while a carbopalladation pathway produces enyne, 5. The surfactant solution (SDS or CTAB) can be recycled up to three times for coupling between 1-octyne and 1-iodonapthalene in the presence of CuI before the yields decrease.

Modification of a triarylphosphine with a cholate moiety affords a new ligand, 1, which is effective in palladium-catalyzed Heck cross-couplings between acrylates and aryl iodides under mild, aqueous reaction conditions. High yields, up to 99%, were achieved in water at 40 °C. In competition studies, a more hydrophobic substrate (n-Bu acrylate) was preferred over the least hydrophobic substrate (methyl acrylate), supportive of a localized hydrophobic microenvironment near the catalytic center. The enhanced reactivity and selectivity for hydrophobic substrates disappeared when the local hydrophobicity was eliminated using a standard water-soluble phosphine or in organic solvents.

Iridium meso-tetratolylporphyrinato (TTP) mono- and bis-diaminocarbene complexes, [Ir(TTP)[═C(NHBn)(NHR)]2–x(C≡NBn)x]BF4, where R = Bn, n-Bu and x = 1, 0, were synthesized by nucleophilic addition of amines to the bis-isocyanide complex [Ir(TTP)(C≡NBn)2]BF4. Rhodium and iridium porphyrinato N-heterocyclic carbene (NHC) complexes M(TTP)CH3(NHC), where NHC = 1,3-diethylimidazolylidene (deim) or 1-(n-butyl)-3-methylimidazolylidene (bmim), were prepared by the addition of the free NHC to M(TTP)CH3. The NHC complexes displayed two dynamic processes by variable-temperature NMR: meso-aryl–porphyrin C–C bond rotation and NHC exchange. meso-Aryl–porphyrin C–C bond rotation was exhibited by both rhodium and iridium complexes at temperatures ranging between 239 and 325 K. Coalescence data for four different complexes revealed ΔG⧧ROT values of 59 ± 2 to 63 ± 1 kJ·mol–1. These relatively low rotation barriers may result from ruffling distortions in the porphyrin core, which were observed in the molecular structures of the rhodium and iridium bmim complexes. Examination of NHC exchange with rhodium complexes by NMR line-shape analyses revealed rate constants of 3.72 ± 0.04 to 32 ± 6 s–1 for deim displacement by bmim (forward reaction) and 2.7 ± 0.4 to 18 ± 2 s–1 for bmim displacement by deim (reverse reaction) at temperatures between 282 and 295 K, corresponding to ΔGf⧧ of 65.2 ± 0.6 kJ·mol–1 and ΔGr⧧ of 66.2 ± 0.5 kJ·mol–1, respectively. Rates of NHC exchange with iridium were far slower, with first-order dissociation rate constants of (1.75 ± 0.04) × 10–4 s–1 for the forward reaction and (1.2 ± 0.1) × 10–4 s–1 for the reverse reaction at 297.1 K. These rate constants correspond to ΔG⧧ values of 94.2 ± 0.6 and 95.2 ± 0.2 kJ·mol–1 for the forward and reverse reactions, respectively. Equilibrium constants for the exchange reactions were 1.6 ± 0.2 with rhodium and 1.56 ± 0.04 with iridium, favoring the bmim complex in both cases, and the log(K) values for NHC binding to M(TTP)CH3 were 4.5 ± 0.3 (M = Rh) and 5.4 ± 0.5 (M = Ir), as determined by spectrophotometric titrations at 23 °C. The molecular structures also featured unusually long metal–Ccarbene bonds for the bmim complexes (Rh–CNHC: 2.255(3) Å and Ir–CNHC: 2.194(4) Å).

Palladium catalysts, generated from Pd(OAc)2 and 2 equiv of N,N-dialkylbenzimidazolium iodide, are effective for the alkoxycarbonylation of olefins in high yields (>90%). Alkoxycarbonylation of 1-hexene in dimethylacetamide is achieved within 24 h at 110 °C using 1 mol % catalyst, 1000 psi CO, and ethanol. Reactions can be prepared in air, without the need of an acid additive to produce ethyl 2-methylhexanoate and ethyl heptanoate in approximately a 2:1 ratio.

Ir(TTP)CH3 catalyzed N–H insertion reactions between ethyl diazoacetate (EDA) or methyl phenyldiazoacetate (MPDA) and a variety of aryl, aliphatic, primary, and secondary amines to generate substituted glycine esters with modest to high yields. Aniline substrates generally gave yields above 80%, with up to 105 catalyst turnovers, and without slow addition of the diazo reagent. Good yields were also achieved with aliphatic amines, though higher catalyst loadings and slow addition of the amine were necessary in some cases. Primary amines reacted with EDA to generate both single- and double-insertion products, either of which could be produced selectively in high yield with the proper choice of stoichiometric ratios and reaction temperature. Notably, mixed trisubstituted amines, RN(CH2CO2Et)(CHPhCO2Me), were generated from the insertion of 1 equiv of EDA and 1 equiv of MPDA into primary amines. The N–H insertion mechanism was examined using substrate competition studies, trapping experiments, and multiple spectroscopic techniques. Substrate competition studies using pairs of amines with EDA or MPDA revealed Hammett correlations with respective slopes of ρ = 0.15 and ρ+ = −0.56 as well as kinetic isotope ratios of kH/kD = 1.0 ± 0.2 and 2.7 ± 0.2. Competitive amine binding to the iridium center was demonstrated by kinetics and equilibrium binding studies. Equilibrium binding constants ranged from 102 to 105. Monitoring the reaction by absorption spectroscopy revealed a transient metalloporphyrin complex. The lifetime of this species was dependent on the nature of the amine substrate, which suggests that the catalytic cycle proceeds through a metal–ylide intermediate.

C–H insertion reactions between different substrates and diazo reagents were catalyzed by tetratolylporphyrinato methyliridium (Ir(TTP)CH3). The highest yields were achieved for reactions between the bulky diazo reagent methyl 2-phenyldiazoacetate (MPDA) and substrates containing electron-rich C–H bonds. An intermediate metalloporphyrin complex was identified as a metal–carbene complex, Ir(TTP)(═C[Ph]CO2CH3)(CH3) (4), using 1H NMR and UV/vis absorption spectroscopy. The presence of 4 was further supported by computationally modeling the absorption spectra with time-dependent DFT (6-31G(d,p)/SBKJC basis set, PBE0 functional). Kinetic studies for C–H insertion reactions using different substrates showed substantial differences in the rate of MPDA consumption, suggesting that carbene transfer is rate-limiting. Furthermore, primary kinetic isotope effects of 3.7 ± 0.3 and 2.7 ± 0.4 were measured using toluene and cyclohexane, respectively. These data are consistent with a mechanism that involves direct C–H insertion rather than a radical rebound pathway.

Tetratolylporphyrinato (TTP) iridium complexes were shown to be extremely active and robust catalysts for the cyclopropanation of olefins using diazo compounds as carbene sources. Ir(TTP)CH3 (1) catalyzed the cyclopropanation of styrene with ethyl diazoacetate (EDA) at −78 °C and achieved 4.8 × 105 turnovers in three successive reagent additions with no sign of deactivation. High yields and moderate trans selectivities were attained for electron-rich and sterically unhindered substrates. A Hammett ρ+ value of −0.23 was determined by competition experiments with para-substituted styrenes. Furthermore, competitive cyclopropanation of styrene and styrene-d8 with EDA and 1 demonstrated a moderate inverse secondary isotope effect of 0.86 ± 0.03. These data are consistent with a catalytic cycle that proceeds through a metalloporphyrin carbene intermediate. Carbene transfer to olefin substrates appears to be rate limiting, as indicated by kinetic studies. Hexavalent iridium halogenato tetratolylporphyrinato complexes of the form Ir(TTP)X(L), where X = Cl, Br, I, NCS and L = CO, NMe3 (2–6), and cationic analogues, where X = BF4 and L = CO or vacant site (7, 8), also demonstrated high catalytic cyclopropanation activity.

Bulk gold powder (∼5–50 μm particles) catalyzes the reactions of isocyanides with amines and amine N-oxides to produce ureas. The reaction of n-butyl isocyanide (nBu–N≡C) with di-n-propylamine and N-methylmorpholine N-oxide in acetonitrile, which was studied in the greatest detail, produced 3-butyl-1,1-dipropylurea (O═C(NHnBu)(NnPr2)) in 99% yield at 60 °C within 2 h. Sterically and electronically different isocyanides, amines, and amine N-oxides react successfully under these conditions. Detailed studies support a two-step mechanism that involves a gold-catalyzed reaction of adsorbed isocyanide with the amine N-oxide to form an isocyanate (RN═C═O), which rapidly reacts with the amine to give the urea product. These investigations show that bulk gold, despite its reputation for poor catalytic activity, is capable of catalyzing these reactions.

Bulk gold powder (~50 μm) catalyzes the oxidative dehydrogenation of amines to give imines using amine N-oxides (R3N-O) as the oxidant. The reaction of dibenzylamine (PhCH2–NH–CH2Ph) with N-methylmorpholine N-oxide (NMMO) in the presence of gold powder at 60 °C produced N-benzylidenebenzylamine (PhCH=N–CH2Ph) in 96% yield within 24 h. Benzyl alcohol was oxidized by NMMO to benzaldehyde in >60% yield in the presence of gold powder. Although O2 was previously shown to oxidize amines in the presence of bulk gold, it is surprising that gold is also capable of catalyzing the oxidation of amines using amine oxides, which are chemically so different from O2.

Bulk gold powder (∼50 μm) and alumina-supported gold catalyzed the oxidative dehydrogenation of 5-, 6-, and 7-membered cyclic amines to amidines. These amidines were hydrolyzed upon treatment with Aerosil 200 (fumed silica gel) and water, producing lactams in 42–73% yields and amines in 36–63% yields. The gold and Aerosil 200 catalysts could also be combined in a one-pot reaction to catalyze the conversion of cyclic amines to lactams in yields up to 51%.Keywords: amidine; amine; caprolactam; catalysis; gold; hydrolysis; nylon; oxidative-dehydrogenation

Bulk gold metal powder, consisting of particles (5−50 μm) much larger than nanoparticles, catalyzes the coupling of carbenes generated from diazoalkanes (R2C═N2) and 3,3-diphenylcyclopropene (DPCP) to form olefins. It also catalyzes cyclopropanation reactions of these carbene precursors with styrenes. The catalytic activity of the gold powder depends on the nature of the gold particles, as determined by TEM and SEM studies. The reactions can be understood in terms of mechanisms that involve the generation of carbene R2C: intermediates adsorbed on the gold surface.

Treatment of meso(tetra-p-tolyl)porphryin, H2(TTP), or Li2(TTP) with OSeCl2 or OSe(NMe2)2 in toluene or THF resulted in the formation of the diprotonated porphyrin, [H4(TTP)]Cl2 and indicated the reluctance of selenium to insert into the porphyrin core. The molecular structure of [H4(TTP)]Cl2 was determined by single-crystal X-ray diffraction and exhibited the typical saddle-shape distortion of diprotonated porphyrins. The molecular structure of (p-MeO–Ph)2Te(salen) was reexamined by X-ray diffraction. The geometry of the Te(IV) center is strongly influenced by a stereochemically active lone pair and is best described as having an AX4E disphenoid structure in which the salen oxygen atoms occupy axial positions and the two anisyl ligands reside in equatorial sites. Distances between Te and the salen nitrogen atoms are 2.852(3) and 2.984(3) Å and are largely nonbonding.Oxo and diaryl derivatives of Se(IV) and Te(IV) are reluctant to bind all donor atoms of tetrachelate ligands. The molecular structure of (p-MeO–Ph)2Te(salen) reveals the coordination preference of Te(IV).

A method for the synthesis of dinuclear metal complexes has been developed which utilizes the multichelating porphyrin ligand, α,α-5,15-bis(o-nicotinoylphenyl)-2,8,12,18-tetraethyl-3,7,13,17-tetramethylporphyrin, H2[DPE]-(py)2 (di[o-nicotinoylphenyl]etioporphyrin). In addition to the porphyrin site, the two terminal pyridyl groups of the ligand serve as an additional chelate. The two metal binding sites are chemically different and can be used to bind different metal centers. The versatility of this ligand is demonstrated by fabricating a heterobimetallic complex, [Ni(DPE)-(py2)]Ru(COD)Cl2, composed of a porphyrin coordination complex linked to an organometallic fragment.A method for the synthesis of dinuclear metal complexes has been developed which utilizes the multichelating porphyrin ligand, α,α-5,15-bis(o-nicotinoylphenyl)-2,8,12,18-tetraethyl-3,7,13,17-tetramethylporphyrin, H2[DPE]-(py)2(di[o-nicotinoylphenyl]etioporphyrin). The versatility of this ligand is demonstrated by fabricating a heterobimetallic complexes, [Ni(DPE)-(py2)]Ru(COD)Cl2, composed of a porphyrin coordination complex linked to an organometallic fragment.

Sterically encumbered monomeric transition-metal complexes were examined as epoxidation catalysts. Bulky ligands such as tetradentate triamidoamines of the type [(RNCH2CH2)3N]3− and tridentate tris(3-t-butylpyrazolyl)borate were used to diminish the deactivation of the catalysts via μ-oxo dimerization. Several monomeric manganese and iron complexes, including M[(RNCH2CH2)3N] [R=trimethylsilyl: M=Mn3+ (1); R=C6F5: M=Fe3+ (2), Mn3+ (3)], {η3-HB(3-tBuPz)3}MCl [M=Fe2+ (4), Mn2+ (6)] and {η3-HB(3-tBuPz)3}FeOTf (5), were synthesized and examined as possible catalysts for the selective epoxidation of norbornylene and styrene. Oxygen atom sources were PhIO or molecular O2–isobutyraldehyde. With PhIO as the oxidant, complexes 1, 3, and 6 oxidized norbornylene and styrene catalytically to the corresponding epoxides in high selectivity, while complexes 2 and 4 exhibited low epoxidation activity. By using O2 with isobutyraldehyde as a co-reductant, complexes 1, 3, 4, and 6 showed efficient catalytic activity with high selectivity and reaction rate for epoxidation, in comparison to control experiments without a catalyst. Complex 4 with O2 and isobutyraldehyde also catalyzed the epoxidation of stilbenes to epoxides. Both cis- and trans-stilbenes are converted mainly to the trans-stilbene oxide. The possible role of the metal complexes as the active epoxidation catalysts is discussed.