Making Base-Assisted C–H Bond Activation by Cp*Co(III) Effective: A Noncovalent Interaction-Inclusive Theoretical Insight and Experimental Validation (original) (raw)
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Journal of the American Chemical Society, 2010
Recently, transition-metal-boryl compounds have been reported that selectively functionalize primary C-H bonds in alkanes in high yield. We have investigated this process with one of the welldefined systems that reacts under photochemical conditions using both density functional theory calculations and pico-through microsecond time-resolved IR spectroscopy. UV irradiation of Cp*W(CO) 3 (Bpin) (Cp*) C 5 (CH 3) 5 ; pin) 1,2-O 2 C 2-(CH 3) 4) in neat pentane solution primarily results in dissociation of a single CO ligand and solvation of the metal by a pentane molecule from the bath within 2 ps. The spectroscopic data imply that the resulting complex, cis-Cp*W(CO) 2 (Bpin)(pentane), undergoes C-H bond activation by a σ-bond metathesis mechanismsin 16 µs, a terminal hydrogen on pentane appears to migrate to the Bpin ligand to form a σ-borane complex, Cp*W(CO) 2 (H-Bpin)(C 5 H 11). Our data imply that the borane ligand rotates until the boron is directly adjacent to the C 5 H 11 ligand. In this configuration, the B-H σ-bond is broken in favor of a B-C σ-bond, forming Cp*W(CO) 2 (H)(C 5 H 11-Bpin), a tungsten-hydride complex containing a weakly bound alkylboronate ester. The ester is then eliminated to form Cp*W(CO) 2 (H) in approximately 170 µs. We also identify two side reactions that limit the total yield of bond activation products and explain the 72% yield previously reported for this complex. alize C-H bonds of alkane solvents at room temperature, but the reactions are generally not selective for a specific type of C-H bond. 7 Recently, however, one of the authors' laboratories discovered transition-metal-boryl complexes that selectively 2,6,8-10 functionalize primary C-H bonds by both thermal 2,8 and photochemical 6,9,10 reactions with yields as high as 92%. The mechanism of C-H bond cleavage and functionalization by these compounds has been investigated experimentally 2,6,8-10 and theoretically, 8,11,12 but complexes proposed to be intermediates have not been observed previously by spectroscopic
Organometallics, 2021
Recent reports have identified Cp*Co-based complexes as powerful catalysts for aromatic C-H bond activation under oxidative conditions. However, little is known about the speciation of Cp*Co species during catalysis. We now show that key intermediates, Cp*Co(III) metallacycles derived from 2-phenylpyridine (phpy-H), react swiftly in solution with one-electron oxidants to irreversibly collapse by a cyclocondensation of the organic ligands to afford cationic alkaloids in yields of >70 %. Low temperature EPR analysis of a mixture of cobaltacycle with the tritylium cation reveals the signatures of trityl and Co(IV)-centred radicals. Electrochemical analyses show that the oxidation of these cobaltacycles is irreversible and gives rise to several products in various amounts, among which the most salient ones are a cationic alkaloid resulting from the cyclocondensation of the phpy and Cp* ligands, and the dimeric cation {[Cp*Co] 2 (-I) 3 } +. DFT investigations of relevant noncovalent interactions using QTAIMbased NCI plots and Intrinsic Bond Strength Index suggest a ligand-dependent predisposition by "NCI-coding" for the Co(IV)templated cyclocondensation, the computed reaction network energy profile for which supports the key roles of a short lived Co(IV) metallacycle and of a range of triplet state organocobalt intermediates. These data are provided free of charge by the joint Cambridge Crystallographic Data Centre and Fachinformationszentrum Karlsruhe Access Structures service www.ccdc.cam.ac.uk/structures. Full experimental procedures and details, voltammograms, EPR, Mass and NMR spectra, energies and Cartesian coordinates, high resolution NCI figures. This material is available free of charge via the Internet at http://pubs.acs.org.
Journal of the American Chemical Society, 2009
To capture the powerful potential of metal-mediated carbonhydrogen (C-H) bond activation, it is essential to develop compatible reactions that convert the resulting metal-alkyl (M-R) intermediates into useful functionalized products. 1 For alkane oxidation reactions, Pt, Pd, Hg, and Au metal catalysts have been exploited to break C-H bonds by electrophilic activation. 2,3 Because of the highly electrophilic C-H activation reactions, the resulting M-R intermediates react with weak O-nucleophiles to generate oxygenated products. 2 It has also recently been demonstrated that M-R intermediates can be functionalized with Oelectrophiles. 4 This type of M-R functionalization would be most useful if coupled to a nucleophilic C-H activation reaction.
Journal of the American Chemical Society, 2013
Tp′Rh(PMe 3)(CH 3)H was synthesized as a precursor to produce the coordinatively unsaturated fragment [Tp′Rh(PMe 3)], which reacts with benzene, mesitylene, 3,3dimethyl-1-butene, 2-methoxy-2-methylpropane, 2-butyne, acetone, pentane, cyclopentane, trifluoroethane, fluoromethane, dimethyl ether, and difluoromethane at ambient temperature to give only one product in almost quantitative yield in each case. All of the complexes Tp′Rh(PMe 3)(R)H were characterized by NMR spectroscopy, and their halogenated derivatives were fully characterized by NMR spectroscopy, elemental analysis, and X-ray crystallography. The active species [Tp′Rh(PMe 3)] was also able to activate the alkynyl C−H bond of terminal alkynes to give activation products of the type Tp′Rh(PMe 3)(CCR)H (R = t-Bu, SiMe 3 , hexyl, CF 3 , Ph, p-MeOC 6 H 4 , and p-CF 3 C 6 H 4). The measured relative rhodium−carbon bond strengths display two linear correlations with the corresponding carbon−hydrogen bond strengths, giving a slope of 1.54 for α-unsubstituted hydrocarbons and a slope of 1.71 for substrates with α-substitution. Similar trends of energy correlations were established by DFT calculated metal−carbon bond strengths for the same groups of substrates.
Kinetics and thermodynamics of intra- and intermolecular carbon-hydrogen bond activation
Journal of the American Chemical Society, 1985
The preference for intra-and intermolecular C-H bond activation has been determined by equilibration of the complex (C5Me5)Rh(PMe2CH2C6H5)(C6H5)H and its cyclometalated analogue (C5Me5)Rh(PMe,CH2C6H4)H in neat benzene at 51.2 OC ( K , = 36.7, AGO = -2.32 kcal/mol). By monitoring the approach to equilibrium over a 40 O C temperature range, the difference between the activation parameters for intra-and intermolecular activation by the 16-electron intermediate [(C5Me5)Rh(PMe2CH2C6HS)1 can be obtained (intra-inter): AAH' = 1.7 f 0.8 kcal/mol; AAS' = 4.5 f 2.5 eu. At 25 OC, this corresponds to a 1.86:l kinetic preference for intermolecular activation of the neat benzene solvent by the coordinatively unsaturated intermediate [(C5Me5)Rh(PMe2CH2C6H5)] over intramolecular cycloaddition. The effect of solvent concentration on activation selectivity is discussed. A comparison with intra-and intermolecular alkane activation is made by equilibrating the complex (C5Me5)Rh(PMezCH2CH2CH2)H with benzene and by examining the kinetics of cyclometalation vs. alkane activation. These studies reveal the same general trend with regard to thermodynamic and kinetic selectivity in alkanes and arenes: while there is little kinetic selectivity between intra-and intermolecular reactions involving neat solvent, there is a moderate thermodynamic preference for the intramolecular activation. . The activation of carbon-hydrogen bonds by homogeneous transition-metal complexes is a topic that has received a great deal of attention recently. Much of this interest arises from the recent reports that indicate that even the C-H bonds of alkanes can 0002-7863/85/1507-0620$01.50/0 (1) (a) Crabtree, R. H.; Mihelcic, J. M.; Quirk, J. M. J. Am. Chem. SOC. 1979, 101, 7738-7740. Crabtree, R. H.; Mellea, M. F.; Mihelcic, J. M.; Quirk, J. M. J. Am. Chem. SOC. 1982, 104, 107-113. Crabtree, R. H.; Demou, P. C.; Eden, D.; Mihelcic, J. M.; Parnell, C. A.; Quirk, J. M.; Morris, G. E. J. Am. Chem. SOC. 1982,104,6994-7001. (b) Baudry, D.; Ephritikhine, M.; Felkin, H. J . Chem. Soc., Chem. Commun. 1980, 1243-1244. Baudry, D.; Ephritikhine, M.; Felkin, H.; Zakrzewski, J.
Nature chemistry, 2018
C-H functionalization represents a promising approach for the synthesis of complex molecules. Instead of relying on modifying the functional groups present in a molecule, the synthetic sequence is achieved by carrying out selective reactions on the C-H bonds, which traditionally would have been considered to be the unreactive components of a molecule. A major challenge is to design catalysts to control both the site- and stereoselectivity of the C-H functionalization. We have been developing dirhodium catalysts with different selectivity profiles in C-H functionalization reactions with donor/acceptor carbenes as reactive intermediates. Here we describe a new dirhodium catalyst capable of the functionalization of non-activated primary C-H bonds with high levels of site selectivity and enantioselectivity.
Room temperature C–H bond activation on a [PdI–PdI] platform
Chemical Communications, 2013
All manipulations were carried out under an inert atmosphere with the use of standard Schlenkline techniques. Glass wares were flame-dried under vacuum prior to use. Solvents were dried by conventional methods prior to use. Pd sponge, Pd(OAc) 2 were purchased from Arora Matthey, India. NOBF 4 , NaNH 2 and B(C 6 F 5) 3 were purchased from Sigma-Aldrich. Pd(CH 3 CN) 4 (BF 4) 2 1 , Pd 2 (dba) 3 .CHCl 3 2 , [Pd 2 (CH 3 CN) 6 ](BF 4) 2 3 and 2-aminonicotinaldehyde 4 were synthesized following the literature procedure. 2-Substituted-1,8-Naphthyridine ligands L 1 H and L 2 H were prepared by Friedlander condensation between 2-aminonicotinaldehyde and corresponding acetyl substituted five membered heterocycles. 5 1 H and 13 C NMR spectra were obtained on a JEOL JNM-LA 500 MHz spectrometer. 1 H NMR chemical shifts were referenced to the residual hydrogen signal of the deuterated solvents. ESI-MS were recorded on a Waters Micro mass Quattro Micro triple-quadruple mass spectrometer using acetonitrile as solvent. Infrared spectra were recorded on a Bruker Vertex 70 FTIR spectrophotometer in the ranges from 400 to 4000 cm-1 using KBr pellets. Elemental analyses were performed on a Thermoquest EA1110 CHNS/O analyzer. The crystallized compounds were powdered, washed several times with dry diethyl ether and dried in vacuum for at least 48 h prior to elemental analyses. X-ray data collection and refinement. Single crystal X-ray structural studies were performed on a CCD Bruker SMART APEX diffractometer equipped with an Oxford instruments low-temperature attachment. All the data were collected at 100(2) K using graphite-monochromated Mo-Kα radiation (λ α = 0.71073 Å). The frames were indexed, integrated and scaled using SMART and SAINT software package 6 and the data were corrected for absorption using the SADABS program. 7 The structures were
Nature Reviews Methods Primers, 2021
Transition metal-catalysed C-H activation has emerged as an increasingly powerful platform for molecular syntheses, enabling applications to natural product syntheses, latestage modification, pharmaceutical industries and material sciences, among others. This Primer summarizes representative best practices for the experimental setup and data deposition for C-H activation, as well as discussing key developments including recent advances in asymmetric, photoinduced and electrocatalytic C-H activation. Likewise, strategies for applications of C-H activation towards the assembly of structurally complex (bio)polymers and drugs in academia and industry are discussed. In addition, current limitations in C-H activation and possible approaches for overcoming these shortcomings are reviewed.
Long-range C-H bond activation by Rh(III)-carboxylates
Journal of the American Chemical Society, 2014
Traditional C-H bond activation by a concerted metalation-deprotonation (CMD) mechanism involves precoordination of the C-H bond followed by deprotonation from an internal base. Reported herein is a "through-arene" activation of an uncoordinated benzylic C-H bond that is 6 bonds away from a Rh(III) ion. The mechanism, which was investigated by experimental and DFT studies, proceeds through a dearomatized xylene intermediate. This intermediate was observed spectroscopically upon addition of a pyridine base to provide a thermodynamic trap.