Single Bond Activation Research Articles

Zirconium and hafnium catalyzed C–C single bond hydroboration

Selective cleavage and subsequent functionalization of C−C single bonds present a fundamental challenge in synthetic organic chemistry. Traditionally, the activation of C−C single bonds has been achieved using stoichiometric transition-metal complexes. Recently, examples of catalytic processes were developed in which use is made of precious metals. However, the use of inexpensive and Earth-abundant group IV metals for catalytic C−C single-bond cleavage is largely underdeveloped. Herein, the zirconium-catalyzed C−C single-bond cleavage and subsequent hydroboration reactions is realized using Cp2ZrCl2 as a catalytic system. A series of structures of various γ-boronated amines are readily obtained, which are otherwise difficult to obtain. Mechanistic studies disclose the formation of a N–ZrIV species, and then a β-carbon elimination route is responsible for C–C single bond activation. Besides zirconium, hafnium exhibits a similar performance for this transformation.

Pyridine-directed carbon–carbon single bond activation: Rhodium-catalyzed decarbonylation of aryl and heteroaromatic ketones

The decarbonylation of 2-pyridyl-substituted ketones via transition metal-catalyzed carbon–carbon bond activation provides ready access to a variety of biaryl compounds. The highly efficient and general method provides reliable decarbonylation of benzophenones including a range of functional groups and substitution patterns. The methodology has also proven highly efficient for heteroaromatic substrates, including those containing thiophenyl, indolyl, quinolinyl, and pyridine substitution.

ChemInform Abstract: Metal‐Organic Cooperative Catalysis in C—C Bond Activation

AbstractReview: 40 refs.

Metal-ligand cooperation by aromatization-dearomatization as a tool in single bond activation.

Metal-ligand cooperation (MLC) plays an important role in bond activation processes, enabling many chemical and biological catalytic reactions. A recent new mode of activation of chemical bonds involves ligand aromatization-dearomatization processes in pyridine-based pincer complexes in which chemical bonds are broken reversibly across the metal centre and the pincer-ligand arm, leading to new bond-making and -breaking processes, and new catalysis. In this short review, such processes are briefly exemplified in the activation of C-H, H-H, O-H, N-H and B-H bonds, and mechanistic insight is provided. This new bond activation mode has led to the development of various catalytic reactions, mainly based on alcohols and amines, and to a stepwise approach to thermal H2 and light-induced O2 liberation from water.

Application of Automated Reaction Path Search Methods to a Systematic Search of Single-Bond Activation Pathways Catalyzed by Small Metal Clusters: A Case Study on H-H Activation by Gold.

A new theoretical approach to find metal-cluster-catalyzed single bond activation pathways is introduced. The proposed approach combines two automated reaction path search techniques: the anharmonic downward distortion following (ADDF) and the artificial force induced reaction (AFIR) methods, developed in our previous works [Maeda, S.; Ohno, K.; Morokuma, K. Phys. Chem. Chem. Phys. 2013, 15, 3683-3701]. A simple model reaction of the H-H bond activation catalyzed by Aun (n = 7, 8) clusters is considered as an example. We have automatically found 33 and 20 transition-state (TS) structures for H2 dissociation on Au7 and Au8 clusters, respectively, and successfully identified the best dissociation pathways with the lowest barrier. Systematic analysis of the structure-dependent reactivity of small gold clusters is performed. It is demonstrated that the most stable structures of the gold clusters are not always highly reactive and several isomeric structures must be taken into account for adequate description of the reaction rates at finite temperatures. The proposed approach can serve as a promising tool for investigation of the chemical reactions catalyzed by small metal clusters.

Steric and Electronic Effects Influencing β-Aryl Elimination in the Pd-catalyzed Carbon–Carbon Single Bond Activation of Triarylmethanols

An analysis of the palladium-catalyzed activation of carbon-carbon single bonds within triarylmethanols has led to a greater understanding of factors influencing the β-aryl elimination process responsible for C-C bond cleavage. A series of competition reactions were utilized to determine that β-aryl elimination of aryl substituents containing ortho-substitution proceeds with significant preference to unsubstituted phenyl rings. Further experiments indicate that substrates containing either strongly donating or withdrawing substituents are cleaved from triarylmethanols more readily than relatively neutral species.

Rhodium-Catalyzed Acylation with Quinolinyl Ketones: Carbon−Carbon Single Bond Activation as the Turnover-Limiting Step of Catalysis

The rhodium-catalyzed intramolecular carboacylation of quinolinyl ketones serves as an ideal subject for the mechanistic study of carbon-carbon bond activation. Combined kinetic and NMR studies of this reaction allowed the identification of the catalytic resting state and determination of the rate law, (12)C/(13)C kinetic isotope effects, and activation parameters. These results have identified the activation of a ketone-arene carbon-carbon single bond as the turnover-limiting step of catalysis and provided quantitative detail into this process.

Investigation of the Thermal Decomposition of [biphen(QPiPr)Pt(alk)2]: An Entry to C–C Single Bond Activation?

AbstractThe reaction of [(COD)PtCl2] with biphen(QPiPr)(Q′PiPr) (Q, Q′ = O, CH2O) yields cis‐[{biphen(QPiPr)(Q′PiPr)}PtCl2]. Treatment of trans‐[PtCl2(ethene)(py)] with biphen(QPiPr)(Q′PiPr) (Q, Q′ = O, CH2O) gives trans‐[{biphen(QPiPr)(Q′PiPr)}PtCl2] for Q or Q′ ≠ O. No trans/cis isomerization can be observed for these compounds after 24 h at 105 °C in [D8]toluene. In the case of Q = Q′ = O a trans/cis isomerization takes place at room temperature. X‐ray structures of a molecule in the solid state were determined for the compounds cis/trans‐[{biphen(QPiPr)(Q′PiPr)}PtCl2] with Q or Q′ ≠ O. In the case of Q = O and Q′ = CH2O the closer of the two bridging C atoms in the biphenyl fragment of the ligand is only 3.42 Å away from the Pt center but does not show a coupling to it in the 13C NMR spectrum. Treatment of both cis‐ and trans‐[{biphen(QPiPr)(Q′PiPr)}PtCl2] with two equivalents of ethyl‐ or n‐propylmagnesium chloride gives cis‐[{biphen(QPiPr)(Q′PiPr)}PtAlk2]; the X‐ray structures of cis‐[{biphen(CH2OPiPr)2}PtEt2] and cis‐[{biphen(OPiPr)2}Pt(nPr)2] are discussed. These complexes give the related alkene complexes as a result of β‐H elimination/reductive elimination if heated to temperatures above 90 °C in [D8]toluene, but none of the complexes with Q or Q′ ≠ O is stable at 95 °C. The thus‐formed Pt(alkene) complex loses the alkene (ethene or propene) in a consecutive reaction. For [{biphen(CH2OPiPr)2}PtEt2], selective deuteration of the CH2O group and the Pt‐ethyl group proves that scrambling of deuterium into the iPr methyl groups occurs (by a CH activation/insertion/β‐H elimination/reductive elimination sequence) after the β‐H elimination and before the loss of ethene, which at 95 °C is almost as fast as the loss of ethene. In the final alkene‐free 14 e complex a fast dynamic C–H activation of the iPr CH3 protons by the Pt center (one H at a time) can be detected [an Isotopic Perturbation of Resonance (IPR) is found for the partially deuterated compound], which most probably prevents a further reaction of the complex (e.g. activation of the bridging C–C single bond in the biphenyl backbone or permanent coordination of PPh3). H2, however, is activated by the final complex, and the use of D2 proves that deuterium is incorporated into the iPr groups of the ligand. The reaction of [(COD)PtCl2] with o‐Br‐phen(QPiPr) gives cis‐[PtCl2{o‐Br‐phen(QPiPr)}2] (X‐ray structure reported), which forms cis‐[Pt{phen(QPiPr)}2] upon treatment with sBuLi. The reaction of trans‐[PtCl2(ethene)(py)] with o‐I‐phen(QPiPr) gives trans‐[PtCl2{o‐I‐phen(QPiPr)}2] (X‐ray structure reported), which also forms cis‐[Pt{phen(QPiPr)}2] upon treatment with sBuLi. cis‐[Pt{phen(CH2OPiPr)}2] (X‐ray structure reported) slowly isomerizes quantitatively at 95 °C in toluene to the trans complex. No product of reductive elimination can be detected up to 150 °C in mesitylene. cis‐[Pt{phen(OPiPr)}2] (X‐ray structure reported) forms two products ― the trans isomer (X‐ray structure reported) and most likely a dimeric trans‐configured complex ― upon heating to 95 °C. Again, no reductive elimination is observed regardless of whether the reaction is performed under argon or ethene atmosphere up to 150 °C in mesitylene. All materials were characterized by means of 1H, 2H, 13C, 31P, and 195Pt NMR spectroscopy, FAB mass spectrometry, FT‐IR spectroscopy, elemental analysis, and X‐ray crystallography. DFT calculations (B3PW91/LANL2DZ) are also included for the most important compounds. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Mechanistic investigation of the thermal decomposition of Biphen(OP i-Pr)PtEt 2: An entrance into C–C single bond activation?

Biphen(OP i-Pr) and (COD)PtCl 2 give Biphen(OP i-Pr)PtCl 2 which upon treating with ethyl Grignard forms Biphen(OP i-Pr)PtEt 2. The thermal decomposition of Biphen(OP i-Pr)PtEt 2 was investigated in the temperature range of 353–383 K. The clean and quantitative formation of the Pt(Ethene) adduct was observed. X-ray structures of a molecule in the solid state of all three reaction products and two further related complexes with phenyl fingers instead of i-Pr have been determined. For the complexes with i-Pr fingers a decisive deviation from a square plane is observed in contrast to the complexes with phenyl fingers. The P–Pt–P angle increases from about 95° in Biphen(OP i-Pr)PtCl 2 to about 120° in Biphen(OP i-Pr)Pt(Ethene), forcing the bridging C–C single bond of the biphenyl fragment as near as 4.17 Å to the Pt center. No through-space coupling between the bridging C atoms and the Pt center could be observed in 13C NMR spectroscopy. No bond lengthening of the bridging C–C single bond in the biphenyl fragment was observed in Biphen(OP i-Pr)Pt(Ethene) in comparison to the precursor complexes. The thermal decomposition of Biphen(OP i-Pr)PtEt 2 can be described by a first-order kinetic and the activation parameters were determined (temperature range: 353–383 K; Δ H ‡ = 173.8 ± 16.2 kJ/mol and Δ S ‡ = 104.7 ± 44.1 J/(mol K)). The reaction kinetics were also measured for perdeuterated ethyl groups yielding in a kinetic isotopic effect of 1.56 ± 0.14 which was almost temperature-independent. Selective deuteration at α and β position of the ethyl group, respectively, showed that β-H elimination takes place fast in comparison to the complete thermolysis. In the temperature range of 333–353 K only a scrambling of the deuterium atoms was found without further decomposition (temperature range: 333–353 K; Δ scram H ‡ = 76.1 ± 15.2 kJ/mol, Δ scram S ‡ = −80.7 ± 45.5 J/(mol K) for Biphen(OP i-Pr)PtEt 2-d 6). The ethene is not lost during the scrambling process. The scrambling process is connected with a primary KIE decisively larger than 1.56. Biphen(OP i-Pr)Pt(Ethene) exchanges the coordinated ethene with ethene in solution as proven by labeling experiments. Both a dissociative and an associative mechanism could be shown to take place as ethene exchange reaction by means of VT 1H NMR spectroscopy via line shape analysis (temperature range: 333–373 K; Δ ass H ‡ = 26.9 ± 29.6 kJ/mol, Δ ass S ‡ = −148.0 ± 87.5 J/(mol K), Δ diss H ‡ = 86.0 ± 6.5 kJ/mol, Δ diss S ‡ = 5.4 ± 17.8 J/(mol K)). The Pt(0) complex formed during the dissociative loss of ethene activates several substrates among them: O 2, H 2, H 2SiPh 2 via Si–H activation, MeI presumably via forming a cationic methyl adduct and ethane via C–H activation but it was proven that the bridging C–C single bond of the biphenyl fragment is not even temporarily broken. The materials were characterized by means of 1H NMR, 13C NMR, 31P NMR, 195Pt NMR, EA, MS, IR, X-ray analysis and polarimetric measurement where necessary.

Twofold CC Single Bond Activation and Cleavage in the Reaction of Octatetraynes with Titanocene and Zirconocene Complexes

Twofold CC Single Bond Activation and Cleavage in the Reaction of Octatetraynes with Titanocene and Zirconocene Complexes