The selective transformation of ubiquitous but inert C H bonds to other functional groups has far-reaching practical implications, ranging from more efficient strategies for fine chemical synthesis to the replacement of current petrochemical feedstocks by less expensive and more readily available alkanes. The past twenty years have seen many examples of C-H bond activation at transition-metal centres, often under remarkably mild conditions and with high selectivity. Although profitable practical applications have not yet been developed, our understanding of how these organometallic reactions occur, and what their inherent advantages and limitations for practical alkane conversion are, has progressed considerably. In fact, the recent development of promising catalytic systems highlights the potential of organometallic chemistry for useful C-H bond activation strategies that will ultimately allow us to exploit Earth's alkane resources more efficiently and cleanly.

Other way round: 1H and 13C NMR spectroscopy on isotopically labeled glucose reveals that in the presence of tin‐containing zeolite Sn‐Beta, the isomerization reaction of glucose in water proceeds by way of an intramolecular hydride shift (see scheme) rather than proton transfer. This is the first mechanistic demonstration of Sn‐Beta acting as a Lewis acid in a purely aqueous environment.

The protonolysis mechanisms of several alkylplatinum(II) complexes [(tmeda)PtMeCl (2) (tmeda = N,N,N‘,N‘-tetramethylethylenediamine), (tmeda)Pt(CH2Ph)Cl (5), (tmeda)PtMe2 (11), and trans-(PEt3)2Pt(CH3)Cl (15)] in CD2Cl2 and CD3OD have been investigated. These reactions model the microscopic reverse of C−H activation by aqueous Pt(II). Each of the four systems (2 in CD3OD, 5 in CD2Cl2, 11 in CD3OD, and 15 in CD3OD) exhibits different behavior in the protonolysis reaction as observed by low-temperature 1H NMR spectroscopy. Protonolysis of 2 in methanol-d 4 proceeds with no observable intermediates. Reversible reaction between 5 and HCl in CD2Cl2 at −78 °C produces (tmeda)Pt(CH2Ph)(H)Cl2 (6), which undergoes reductive elimination of toluene at higher temperatures. Treatment of 11 with HCl in methanol at −78 °C generates (tmeda)PtMe2(H)Cl (12), which incorporates deuterium from solvent (CD3OD) into the methyl groups prior to reductive elimination of methane. Finally, 15 reacts with H+ in methanol to liberate methane with no intermediates observed. However, hydrogen/deuterium exchange takes place between the solvent (CD3OD) and Pt−Me prior to methane loss. Each of these reactions was evaluated further to determine the kinetics of the reaction, activation parameters, and isotope effects. Based on the results, a common mechanistic sequence is proposed to operate in all the reactions:  (1) chloride- or solvent-mediated protonation of Pt(II) to generate an alkylhydridoplatinum(IV) intermediate, (2) dissociation of solvent or chloride to generate a cationic, five-coordinate platinum(IV) species, (3) reductive C−H bond formation producing a platinum(II) alkane σ-complex, and (4) loss of alkane either through an associative or dissociative substitution pathway. The characteristics of each system differ due to changes in the relative stabilities of the intermediates and/or transition states upon varying the solvent or alkylplatinum species.

A series of bis(aryl)diimine-ligated methyl complexes of Pt(II) with various substituted aryl groups has been prepared. The cationic complexes [(ArN=CR [bond] CR=NAr)PtMe(L)](+)[BF(4)](-) (Ar = aryl; R = H, CH(3); L = water, trifluoroethanol) react smoothly with benzene at approximately room temperature in trifluoroethanol solvent to yield methane and the corresponding phenyl Pt(II) cations, via Pt(IV)-methyl-phenyl-hydrido intermediates. The reaction products of methyl-substituted benzenes suggest an inherent reactivity preference for aromatic over benzylic C [bond] H activation, which can however be overridden by steric effects. For the reaction of benzene with cationic Pt(II) complexes bearing 3,5-disubstituted aryl diimine ligands, the rate-determining step is C [bond] H activation, whereas for the more sterically crowded analogues with 2,6-dimethyl-substituted aryl groups, benzene coordination becomes rate-determining. This switch is manifested in distinctly different isotope scrambling and kinetic deuterium isotope effect patterns. The more electron-rich the ligand is, as assayed by the CO stretching frequency of the corresponding carbonyl cationic complex, the faster the rate of C [bond] H activation. Although at first sight this trend appears to be at odds with the common description of this class of reaction as electrophilic, the fact that the same trend is observed for the two different series of complexes, which have different rate-determining steps, suggests that this finding does not reflect the actual C [bond] H activation process, but rather reflects only the relative ease of benzene displacing a ligand to initiate the reaction; that is, the change in rates is mostly due to a ground-state effect. The stability of the aquo complex ground state in equilibrium with the solvento complex increases as the diimine ligand is made more electron-withdrawing. Several lines of evidence, including the mechanism of degenerate acetonitrile exchange for the methyl-acetonitrile Pt(II) cations in alcohol solvents, suggest that associative substitution pathways operate to get the hydrocarbon substrate into, and out of, the coordination sphere; that is, the mechanism of benzene substitution proceeds by a solvent (TFE)-assisted associative pathway.

The oligomerization of ethylene typically leads to a broad range of R-olefins; catalytic systems that are selective for specific desirable alkenes would be of great interest. There have been several recent reports of trimerization of ethylene to 1-hexene with high selectivity. 1 While some ethylene trimerization catalysts are based on titanium and tantalum, 1j,k chromium-based systems generally display higher activity, selectivity, and thermal stability. 1b-i A recent communication describes a catalyst, generated in situ by treating Cr(III) salts with methylalumoxane in the presence of the diphosphine PNP OMe [1, PNP OMe ) (o-MeO-C 6 H 4 ) 2 PN(Me)P(o-MeO-C 6 H 4 ) 2 ], that trimerizes ethylene to 1-hexene with unprecedented selectivity and productivity. 1c,d Herein we report the synthesis and characterization of chromium complexes of 1, 2 along with some mechanistic investigations of the ethylene trimerization reaction they catalyze.Reactions of diphosphine 1 (Scheme 1) with suitable chromium-(III) etherate complexes afford compounds (PNP OMe )CrCl 3 (2) and (PNP OMe )CrPh 3 (3); 2 reacts with o,o′-biphenyldiyl Grignard to give (PNP OMe )Cr(o,o′-biphenyldiyl)Br (4). All three are hexacoordinate, displaying (P,P,O)-κ 3 coordination of the diphosphine, established by single-crystal X-ray diffraction as well as 2 H NMR. 3,4 The structures of 3 and 4 (Figure 1) reveal long Cr-O bonds (2.293(2) Å in 3 and 2.337(2) Å in 4) and significant differences between the two Cr-P bond lengths. The ether-chelated phosphine is closer to chromium by 0.16 Å (3) and 0.23 Å (4).