Iron porphryin complexes are active catalysts for the cyclopropanation of alkenes by ethyl diazoacetate. Fe(TIP) (TIP = meso-tetra-p-tolylporphyrin), an isolated iron(II) porphyrin complex, can be used as the catalyst, or the iron(III) complexes of several porphyrins can be reduced in situ. The reactions produce synthetically useful excesses of the trans cyclopropyl ester products. This stereoselectivity exhibits a modest solvent dependence, with donor solvents giving higher ratios of the trans cyclopropane products. The diastereoselectivity exhibits only a modest dependence on the steric bulk of the porphyrin. The reactions are selective for 1-alkenes and 1, 1-disubstituted alkenes. Conjugated substrates and enol ethers react more rapidly than simple aliphatic alkenes. A mechanistic model for the iron-mediated reactions is proposed which is consistent with the data presented herein. Disciplines Chemistry CommentsReprinted (adapted) Am. Chern. Soc. 1995, 117, 9194-9199 Shape Abstract: Iron porphryin complexes are active catalysts for the cyclopropanation of alkenes by ethyl diazoacetate. Fe(TIP) (TIP = meso-tetra-p-tolylporphyrin), an isolated iron(II) porphyrin complex, can be used as the catalyst, or the iron(III) complexes of several porphyrins can be reduced in situ. The reactions produce synthetically useful excesses of the trans cyclopropyl ester products. This stereoselectivity exhibits a modest solvent dependence, with donor solvents giving higher ratios of the trans cyclopropane products. The diastereoselectivity exhibits only a modest dependence on the steric bulk of the porphyrin. The reactions are selective for 1-alkenes and 1, 1-disubstituted alkenes. Conjugated substrates and enol ethers react more rapidly than simple aliphatic alkenes. A mechanistic model for the iron-mediated reactions is proposed which is consistent with the data presented herein.The metal-catalyzed cyclopropanation of substituted olefins by diazo esters is a reaction commonly employed in organic synthesis. 1 Chiral copper 2 -4 and rhodium 5 • 6 catalysts have recently been reported to effect cyclopropanation with high enantioselectivity. However, the diastereoselectivities obtained in these reactions are generally poor. Mixtures of the trans and cis cyclopropyl esters are produced, with the trans isomer obtained in only a slight excess. A notable exception is a recently reported ruthenium catalyst/ which provided high enantioselectivities and trans/cis ratios of 10:1 using ethyl diazoacetate (EDA) as the carbene source.We are interested in the development of metalloporphyrin cyclopropanation catalysts, since such reactions often exhibit unique stereoselectivities and exceedingly high catalyst turnover numbers. For example, rhodium(III) porphyrin-catalyzed cyclopropanation reactions using EDA as the carbene source produced the cis cyclopropyl ester as the major product when bulky ligands such as the tetramesitylporphyrin (TMP) (see Figure 1) were employed. 8 -11 To the best of our knowledge, this is the only exampl...
This paper describes preliminary results in the design, construction, and characterization of cobalt(Il) porphyrins derivatized with alkanethiol appendages. The use of the thiol appendages leads to the formation of a chemisorbed monolayer of the corresponding thiolate at gold electrodes. Our findings suggest that this approach may serve as a beginning for fabricating electrocatalytic monolayers with a preselected architecture through the manipulation of the number and location of the appendages. Voltammetric data indicate that monolayers from both I(Co) and II(Co) catalyze the two-electron reduction of 0 2 to H20 2• The monolayer from I(Co), however, has a lower electrocatalytic activity. Infrared, X-ray photoelectron, and visible spectroscopic data are presented that argue the difference in reactivity arises from a difference in interfacial architecture. The results from attempts to metalate monolayers from I(Hz) and II(H2) support this interpretation. Findings are also reported that indicate the preparation of mixed monolayers (e.g., twocomponent mono layers from I(Co) and CHs(CH2)aSH) may prove valuable to this area of research.
Fe(III), Cu(II), and Ag(II) porphyrin complexes are active catalysts for benzylic and ring C−H insertions by carbene fragments transferred from methyl diazomalonate, 2. Temperatures above 100 °C are required, and yields greater than 70% have been achieved. C−H insertions with cyclohexane and tetrahydrofuran are catalyzed at a lower temperature of 60 °C with 60% yields when para-substituted methyl 2-phenyldiazoacetates, 15a−d, are used as carbene sources. The rate for Fe(TPP)Cl-catalyzed insertion into the C−H bond of cyclohexane was found to be first-order in the concentration of methyl 2-(pchlorophenyl)diazoacetate, p-Cl-MPDA, indicating that formation of a carbene complex is the ratedetermining step. Competition reactions for cyclohexane insertion with para-substituted methyl 2-phenyldiazoacetates correlated linearly with σ+ Hammett parameters with a ρ value of -1.11 ± 0.05 when Fe(TPP)Cl was used as a catalyst, demonstrating that electron-donating para-substituents on the phenyl group of the methyl 2-aryldiazoacetates enhanced reactivity. These data are consistent with the involvement of an electrophilic iron−carbene complex in the catalytic cycle. A mechanistic model for the iron-mediated C−H insertion reactions is proposed. Disciplines Chemistry CommentsReprinted (adapted) with permission from Organometallics 27 (2008): 637, ABSTRACT: C−H insertion reactions between different substrates and diazo reagents were catalyzed by tetratolylporphyrinato methyliridium (Ir(TTP)CH 3 ). 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]CO 2 CH 3 )(CH 3 ) (4), using 1 H 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)CH 3 (1) catalyzed the cyclopropanation of styrene with ethyl diazoacetate (EDA) at −78°C and achieved 4.8 × 10 5 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-d 8 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, NMe 3 (2−6), and cationic analogues, where X = BF 4 and L = CO or vacant site (7,8), also demonstrated high catalytic cyclopropanation activity.
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. Disciplines Chemistry CommentsReprinted (adapted) Abstract: Bulk gold metal powder, consisting of particles (5-50 µm) much larger than nanoparticles, catalyzes the coupling of carbenes generated from diazoalkanes (R 2 CdN 2 ) 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 R 2 C: intermediates adsorbed on the gold surface.
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