A theoretical study of steric and electronic effects in the rhodium-catalyzed carbonylation reactions

Insights into the ligand effects of rhodium catalysts toward reductive carbonylation of methanol to ethanol

Steric congestion on the reactivity of Rh-based catalysts with diphosphine modified for reductive carbonylation of methanol to ethanol.

A DFT-Elucidated Comparison of the Solution-Phase and SAM Electrochemical Properties of Short-Chain Mercaptoalkylferrocenes: Synthetic and Spectroscopic Aspects, and the Structure of Fc-CH2CH2-S-S-CH2CH2-Fc

Facile synthetic procedures to synthesize a series of difficult-to-obtain mercaptoalkylferrocenes, namely, Fc(CH2)nSH, where n = 1 (1), 2 (2), 3 (3), or 4 (4) and Fc = Fe(η(5)-C5H5)(η(5)-C5H4), are reported. Dimerization of 1-4 to the corresponding disulfides 19-22 was observed in air. Dimer 20 (Z = 2) crystallized in the triclinic space group P1̅. Dimers 20-22 could be reduced back to the original Fc(CH2)nSH derivatives with LiAlH4 in refluxing tetrahydrofuran. Density functional theory (DFT) calculations showed that the highest occupied molecular orbital of 1-4 lies exclusively on the ferrocenyl group implying that the electrochemical oxidation observed at ca. -15 < Epa < 76 mV versus FcH/FcH(+) involves exclusively an Fe(II) to Fe(III) process. Further DFT calculations showed this one-electron oxidation is followed by proton loss on the thiol group to generate a radical, Fc(CH2)nS(•), with spin density mainly located on the sulfur. Rapid exothermic dimerization leads to the observed dimers, Fc(CH2)n-S-S-(CH 2)nFc. Reduction of the ferrocenium groups on the dimer occurs at potentials that still showed the ferrocenyl group ΔE = Epa,monomer - Epc,dimer ≤ 78 mV, indicating that the redox properties of the ferrocenyl group on the mercaptans are very similar to those of the dimer. (1)H NMR measurements showed that, like ferrocenyl oxidation, the resonance position of the sulfhydryl proton, SH, and others, are dependent on -(CH2)n- chain length. Self-assembled monolayers (SAMs) on gold were generated to investigate the electrochemical behavior of 1-4 in the absence of diffusion. Under these conditions, ΔE approached 0 mV for the longer chain derivatives at slow scan rates. The surface-bound ferrocenyl group of the metal-thioether, Fc(CH2)n -S-Au, is oxidized at approximately equal potentials as the equivalent CH2Cl2-dissolved ferrocenyl species 1-4. Surface coverage by the SAMs is dependent on alkyl chain length with the largest coverage obtained for 4, while the rate of heterogeneous electron transfer between SAM substrate and electrode was the fastest for the shortest chain derivative, Fc-CH2-S-Au.

Synthesis and structure of rhodium(i) silyl carbonyl complexes: photochemical C-F and C-H bond activation of fluorinated aromatic compounds

The rhodium(i) silyl carbonyl complexes [Rh{Si(OEt)3}(CO)(dippp)] () and [Rh{Si(OEt)3}(CO)(dippe)] () (dippp = 1,3-bis(diisopropylphosphino)propane, dippe = 1,2-bis-(diisopropylphosphino)ethane) were synthesized on treatment of the methyl compounds [Rh(CH3)(CO)(dippp)] () or [Rh(CH3)(CO)(dippe)] () with HSi(OEt)3 at low temperature. The methyl complexes and were prepared starting from the binuclear complexes [{Rh(μ-Cl)(dippp)}2] () and [{Rh(μ-Cl)(dippe)}2] (), respectively. The silyl complexes and as well as the precursors [{Rh(μ-I)(dippp)}2] (), [Rh(X)(CO)(dippp)] (: X = CH3, : X = I) and [Rh(X)(CO)(dippe)] (: X = CH3, : X = Cl) were characterized by NMR and IR spectroscopy and the structures in the solid state were determined by X-ray crystallography. The silyl complex converts into the carbonyl-bridged complex [{Rh(μ-CO)(dippp)}2] () above temperatures of -30 °C by loss of the silyl ligand, whereas is more thermally stable and a reaction to the binuclear complex [{Rh(μ-CO)(dippe)}2] () was observed at 50 °C. The silyl complex reacted under irradiation with hexafluorobenzene and pentafluoropyridine to give the C-F activation products [Rh(C6F5)(CO)(dippe)] () and [Rh(2-C5F4N)(CO)(dippe)] (), respectively. As additional products the silyl dicarbonyl complex [Rh{Si(OEt)3}(CO)2(dippe)] () and the cationic complex [Rh2(μ-H)(μ-CO)2(dippe)2](+)[SiF5](-) () were identified. Compound was synthesized independently by treatment of with gaseous CO. In a similar manner, the dippp analogue [Rh{Si(OEt)3}(CO)2(dippp)] () was also prepared starting from . Photochemical reaction of with pentafluorobenzene and 2,3,5,6-tetrafluoropyridine resulted selectively in C-H bond activation to afford and [Rh(4-C5F4N)(CO)(dippe)] (), respectively.

Density functional theory calculations of Rh-β-diketonato complexes

Density functional theory results on the geometry, energies and charges of selected Rh-β-diketonato reactants, products and transition states.

Catalytic, asymmetric indolizidinone aza-quaternary stereocenter synthesis: expedient synthesis of the cylindricine alkaloid core

The Rh(I)•CKphos catalyzed [2 + 2 + 2] cycloaddition of 1,1-disubstituted alkenyl isocyanates and alkyl alkynes selectively forms previously inaccessible vinylogous amide indolizidinone cycloadducts, establishing an aza-quaternery stereocenter with excellent enantioselectivities (up to 98% ee). This advance enables a seven step catalytic, asymmetric synthesis of the tricyclic core of the cylindricine alkaloids with excellent control of product selectivity as well as regio- and enantioselectivity.

Oxidative addition of methyl iodide to [Rh(PhCOCHCOPh)(CO)(P(OCH2)3CCH3)]: an experimental and computational study

The reaction rate of the oxidative addition and CO insertion steps of methyl iodide with [Rh(PhCOCHCOPh)(CO)(P(OCH2)3CCH3)] are presented. Large negative experimental values for the activation entropy and results from a density functional theory computational chemistry study indicated trans addition of the CH3I to [Rh(PhCOCHCOPh)(CO)(P(OCH2)3CCH3)]. A study of the molecular orbitals gives insight into the flow of electrons during the oxidative addition reaction. CO insertion leads to a square pyramidal [Rh(PhCOCHCOPh)(P(OCH2)3CCH3)(COCH3)(I)] acyl product with the COCH3 moiety in the apical position. The strong electron donation of the P(OCH2)3CCH3 ligand accelerates the oxidation addition step of methyl iodide to [Rh(PhCOCHCOPh)(CO)(P(OCH2)3CCH3)] by ca. 265 times faster (at 35°C) than that of the Monsanto catalyst, but inhibits the CO insertion step.

Electrochemical illumination of intramolecular communication in ferrocene-containing tris-β-diketonato aluminum(III) complexes; cytotoxicity of Al(FcCOCHCOCF3)3

The series of ferrocene-containing tris-β-diketonato aluminum(III) complexes [Al(FcCOCHCOR)(3)] (R = CF(3), 1; CH(3), 2; C(6)H(5), 3; and Fc = ferrocenyl = Fe(η(5)-C(5)H(5))(η(5)-C(5)H(4)), 4) were synthesized and investigated structurally and electrochemically; complex 1 was subjected to cytotoxicity tests. (1)H NMR-spectroscopy distinguished between the mer and fac isomers of 2 and 3. Complex 1 existed only as the mer isomer. A single crystal X-ray crystallographic determination of the structure of a mer-isomer of Al(FcCOCHCOCF(3))(3), 1, (Z = 4, space group P2(1)2(1)2(1)) demonstrated extensive delocalization of all bonds which explained the pronounced electrochemically observed intramolecular communication between molecular fragments. In contrast to electrochemical studies in CH(2)Cl(2)/[N((n)Bu)(4)][PF(6)], the use of the supporting electrolyte [N((n)Bu)(4)][B(C(6)F(5))(4)] allowed identification of all Fc/Fc(+) electrochemical couples by cyclic and square wave voltammetry for 1-4. For R = Fc, formal reduction potentials of the six ferrocenyl groups were found to be E°' = 33, 123, 304, 432, 583, and 741 mV versus free ferrocene respectively. Complex 1 (IC(50) = 10.6 μmol dm(-3)) was less cytotoxic than the free FcCOCH(2)COCF(3) ligand having IC(50) = 6.8 μmol dm(-3) and approximately 2 orders of magnitude less toxic to human HeLa neoplastic cells than cisplatin (IC(50) = 0.19 μmol dm(-3)).

Relationship between electrochemical potentials and substitution reaction rates of ferrocene-containing β-diketonato rhodium(I) complexes; cytotoxicity of [Rh(FcCOCHCOPh)(cod)]

A series of ferrocene-containing rhodium complexes of the type [Rh(FcCOCHCOR)(cod)] (cod = 1,5-cyclooctadiene) with R = CF(3), 1, (E(pa)(Rh) = 269; E(o)'(Fc) = 329 mV vs. Fc/Fc(+)), CCl(3), 2, (E(pa) = 256; E(o)' = 312 mV), CH(3), 3, (E(pa) = 177; E(o)' = 232 mV), Ph = C(6)H(5), 4, (E(pa) = 184; E(o)' = 237 mV), and Fc = ferrocenyl = (C(5)H(5))Fe(C(5)H(4)), 5, (E(pa) = 135; E(o)'(Fc1) = 203; E(o)'(Fc2) = 312 mV), have been studied electrochemically in CH(3)CN. Results indicated that the rhodium(I) centre is irreversibly oxidised to Rh(III) in a two-electron transfer process before the ferrocenyl fragment is reversibly oxidized in a one-electron transfer process. The peak anodic (oxidation) potential, E(pa), (in V vs. Fc/Fc(+)) of the rhodium core in 1-5 relates to k(2), the second-order rate constant for the substitution of (FcCOCHCOR)(-) with 1,10-phenanthroline in [Rh(FcCOCHCOR)(cod)] to form [Rh(phen)(cod)](+) in methanol at 25 °C with the equation lnk(2) = 39.5 E(pa)(Rh) - 3.69, while the formal oxidation potential of the ferrocenyl groups in 1-5 relates to k(2) by lnk(2) = 40.8 E(o)'(Fc)-6.34. Complex 4 (IC(50) = 28.2 μmol dm(-3)) was twice as cytotoxic as the free FcCOCH(2)COPh ligand having IC(50) = 54.2 μmol dm(-3), but approximately one order of magnitude less toxic to human HeLa neoplastic cells than cisplatin (IC(50) = 2.3 μmol dm(-3)).

Enantioselective rhodium-catalyzed [2 + 2 + 2] cycloadditions of terminal alkynes and alkenyl isocyanates: mechanistic insights lead to a unified model that rationalizes product selectivity

This manuscript describes the development and scope of the asymmetric rhodium-catalyzed [2 + 2 + 2] cycloaddition of terminal alkynes and alkenyl isocyanates leading to the formation of indolizidine and quinolizidine scaffolds. The use of phosphoramidite ligands proved crucial for avoiding competitive terminal alkyne dimerization. Both aliphatic and aromatic terminal alkynes participate well, with product selectivity a function of both the steric and electronic character of the alkyne. Manipulation of the phosphoramidite ligand leads to tuning of enantio- and product selectivity, with a complete turnover in product selectivity seen with aliphatic alkynes when moving from Taddol-based to biphenol-based phosphoramidites. Terminal and 1,1-disubstituted olefins are tolerated with nearly equal efficacy. Examination of a series of competition experiments in combination with analysis of reaction outcome shed considerable light on the operative catalytic cycle. Through a detailed study of a series of X-ray structures of rhodium(cod)chloride/phosphoramidite complexes, we have formulated a mechanistic hypothesis that rationalizes the observed product selectivity.

Multi-component cycloaddition approaches in the catalytic asymmetric synthesis of alkaloid targets

Cycloaddition reactions are attractive strategies for the rapid formation of molecular complexity in organic synthesis, as multiple bonds are formed in a single process. To this end, several research groups have been actively involved in the development of catalytic methods to activate readily accessible pi-components to achieve cycloadditions. However, the use of C-N pi-components for the formation of heterocycles by these processes is less well developed. It has been previously demonstrated that the combination of different isocyanates with two alkynes yields pyridones of several types by metal-catalyzed [2 + 2 + 2] cycloadditions. The potential of this chemistry has been extended to alkenes as C-C pi-components, allowing the formation of sp(3)-stereocenters. In this tutorial review directed towards [n + 2 + 2] cycloadditions of heterocumulenes, alkynes and alkenes, the recent advances in the catalytic asymmetric synthesis of indolizidine, quinolizidine and azocine skeletons are discussed.

Molecular Electrostatic Potential Approach to Determining the Steric Effect of Phosphine Ligands in Organometallic Chemistry

A two-layer ONIOM(B3LYP/6-31G(d,p):UFF) quantum mechanics (QM)-molecular mechanics (MM) optimization of PR3 ligands, where the QM layer is always constructed as PH3, followed by molecular electrostatic potential (MESP) analysis of the QM layer is suggested as a simple and effective method for evaluating the steric effect of PR3 ligands. The subtle variations in the electron distribution that arise as a result of the steric bulkiness as well as the conformational changes in the substituent groups is well reflected in the value of the MESP minimum (Vmin) located in the QM region. In general, a sterically bulky group has always shown a more negative Vmin than a sterically less bulky group. The difference between the Vmin value of free PH3 and the Vmin value at the QM region of PR3 is used as a measure of the steric effect of the PR3 ligand. This value, designated as MESPsteric, showed a good linear correlation with the cone angle values as well as the average of the intervalence HPH angles found in the QM layer. Further, the difference between the Vmin value at the QM region of PR3 (an indicator of the steric effect) and the Vmin value of the completely optimized PR3 ligand (an indicator of the combined effect of steric and electronic effects) showed a good linear correlation with the Hammett Sigmasigmap constants, which further confirmed the present approach to understanding the steric effect separately from the electronic effect.