Akshai Kumar

Dehydrogenation of n-Alkanes by Solid-Phase Molecular Pincer-Iridium Catalysts. High Yields of α-Olefin Product.

We report the transfer-dehydrogenation of gas-phase alkanes catalyzed by solid-phase, molecular, pincer-ligated iridium catalysts, using ethylene or propene as hydrogen acceptor. Iridium complexes of sterically unhindered pincer ligands such as (iPr4)PCP, in the solid phase, are found to give extremely high rates and turnover numbers for n-alkane dehydrogenation, and yields of terminal dehydrogenation product (α-olefin) that are much higher than those previously reported for solution-phase experiments. These results are explained by mechanistic studies and DFT calculations which jointly lead to the conclusion that olefin isomerization, which limits yields of α-olefin from pincer-Ir catalyzed alkane dehydrogenation, proceeds via two mechanistically distinct pathways in the case of ((iPr4)PCP)Ir. The more conventional pathway involves 2,1-insertion of the α-olefin into an Ir-H bond of ((iPr4)PCP)IrH2, followed by 3,2-β-H elimination. The use of ethylene as hydrogen acceptor, or high pressures of propene, precludes this pathway by rapid hydrogenation of these small olefins by the dihydride. The second isomerization pathway proceeds via α-olefin C-H addition to (pincer)Ir to give an allyl intermediate as was previously reported for ((tBu4)PCP)Ir. The improved understanding of the factors controlling rates and selectivity has led to solution-phase systems that afford improved yields of α-olefin, and provides a framework required for the future development of more active and selective catalytic systems.

Dehydrogenation of Alkanes and Aliphatic Groups by Pincer-Ligated Metal Complexes

The alkyl group is the most common component of organic molecules and the most difficult to selectively functionalize. The development of catalysts for dehydrogenation of alkyl groups to give the corresponding olefins could open almost unlimited avenues to functionalization. Homogeneous systems, or more generally systems based on molecular (including solid-supported) catalysts, probably offer the greatest potential for regio- and chemoselective dehydrogenation of alkyl groups and alkanes. The greatest progress to date in this area has been achieved with pincer-ligated transition-metal-based catalysts; this and related chemistry are the subject of this review. Chemists are still far from achieving the most obvious and perhaps most attractive goal in this area, the dehydrogenation of simple alkanes to yield alkenes (specifically monoenes) with high yield and selectivity. Greater progress has been made with tandem catalysis and related approaches in which the initial dehydrogenated product undergoes a desirable secondary reaction. Also reviewed is the substantial progress that has been made in the closely related area of dehydrogenation of alkyl groups of substrates containing heteroatoms.

Recent Advances in Alkane Dehydrogenation Catalyzed by Pincer Complexes

Olefins are ubiquitous intermediates in the production of fuels and commodity chemicals. Accordingly, the development of methods for the selective dehydrogenation of alkanes to give olefins is a goal with great potential value. Molecular (homogeneous) catalysts appear quite promising in this respect. Great advances have been seen with pincer-ligated catalysts since the mid-1990s, particularly (but not exclusively) with iridium complexes. In this chapter we give an overview of this productive area of research, with emphasis on recent progress.

β-Hydride Elimination and C–H Activation by an Iridium Acetate Complex, Catalyzed by Lewis Acids. Alkane Dehydrogenation Cocatalyzed by Lewis Acids and [2,6-Bis(4,4-dimethyloxazolinyl)-3,5-dimethylphenyl]iridium

NaBArF4 (sodium tetrakis[(3,5-trifluoromethyl)phenyl]borate) was found to catalyze reactions of (Phebox)IrIII(acetate) (Phebox = 2,6-bis(4,4-dimethyloxazolinyl)-3,5-dimethylphenyl) complexes, including (i) β-H elimination of (Phebox)Ir(OAc)(n-alkyl) to give (Phebox)Ir(OAc)(H) and the microscopic reverse, alkene insertion into the Ir-H bond of (Phebox)Ir(OAc)(H), and (ii) hydrogenolysis of the Ir-alkyl bond of (Phebox)Ir(OAc)(n-alkyl) and the microscopic reverse, C-H activation by (Phebox)Ir(OAc)(H), as indicated by H/D exchange experiments. For example, β-H elimination of (Phebox)Ir(OAc)(n-octyl) (2-Oc) proceeded on a time scale of minutes at -15 °C in the presence of (0.4 mM) NaBArF4 as compared with a very slow reaction at 125 °C in the absence of NaBArF4. In addition to NaBArF4, other Lewis acids are also effective. Density functional theory calculations capture the effect of the Na+ cation and indicate that it operates primarily by promoting κ2-κ1 dechelation of the acetate anion, which opens the coordination site needed to allow the observed reaction to proceed. In accord with the effect on these individual stoichiometric reactions, NaBArF4 was also found to cocatalyze, with (Phebox)Ir(OAc)(H), the acceptorless dehydrogenation of n-dodecane.

Catalytic Reactions of Titanium Alkoxides with Grignard Reagents and Imines: A Mechanistic Study

The reactivity of Grignard reagents towards imines in the presence of catalytic and stoichiometric amounts of titanium alkoxides is reported. Alkylation, reduction, and coupling of imines take place. Whereas reductive coupling is the major reaction in stoichiometric reactions, alkylation is favored in catalytic reactions. Mechanistic studies clearly indicate that intermediates involved in the two reactions are different. Catalytic reactions involve a metal-alkyl complex. This has been confirmed by reactions of deuterium-labeled substrates and different alkylating agents. Under the stoichiometric conditions, however, titanium olefin complexes are formed through reductive elimination, probably through a multinuclear intermediate.

Electrochemical and chemical routes to hydride loss from an iridium dihydride

With a view towards replacing sacrificial hydrogen acceptors in alkane dehydrogenation catalysis, electrochemical methods for oxidative activation of a pincer-ligated iridium hydride intermediate were explored. A 1H(+)/2e(-) oxidation process was observed in THF solvent, with net hydride loss leading to a reactive cationic intermediate that can be trapped by chloride. Analogous reactivity was observed with the concerted hydride transfer reagent Ph3C(+), connecting chemical and electrochemical hydride loss pathways.

Computational tools for mechanistic discrimination in the reductive and metathesis coupling reactions mediated by titanium(IV) isopropoxide

AbstractA theoretical study has been carried out at the B3LYP/LANL2DZ level to compare the reactivity of phenyl isocyanate and phenyl isothiocyanate towards titanium(IV) alkoxides. Isocyanates are shown to favour both mono insertion and double insertion reactions. Double insertion in a head-to-tail fashion is shown to be more exothermic than double insertion in a head-to-head fashion. The head-to-head double insertion leads to the metathesis product, a carbodiimide, after the extrusion of carbon dioxide. In the case of phenyl isothiocyanate, calculations favour the formation of only mono insertion products. Formation of a double insertion product is highly unfavourable. Further, these studies indicate that the reverse reaction involving the metathesis of N,N′-diphenyl carbodiimide with carbon dioxide is likely to proceed more efficiently than the metathesis reaction with carbon disulphide. This is in excellent agreement with experimental results as metathesis with carbon disulphide fails to occur. In a second study, multilayer MM/QM calculations are carried out on intermediates generated from reduction of titanium(IV) alkoxides to investigate the effect of alkoxy bridging on the reactivity of multinuclear Ti species. Bimolecular coupling of imines initiated by Ti(III) species leads to a mixture of diastereomers and not diastereoselective coupling of the imine. However if the reaction is carried out by a trimeric biradical species, diastereoselective coupling of the imine is predicted. The presence of alkoxy bridges greatly favours the formation of the d,l (±) isomer, whereas the intermediate without alkoxy bridges favours the more stable meso isomer. As a bridged trimeric species, stabilized by bridging alkoxy groups, correctly explains the diastereoselective reaction, it is the most likely intermediate in the reaction. Graphical AbstractComputational studies carried out at the B3LYP/LANL2DZ level throw light on the contrasting reactivity of isocyanates and isothiocyanates towards titanium(IV) alkoxides. While both mono and double insertion reactions are feasible with isocyanates, only the mono insertion reaction appears to be thermodynamically feasible with isothiocyanates. Multilayer calculations are also performed on multinuclear titanium intermediates in diastereoselective coupling reactions initiated by low valent titanium species. These studies clearly predict that the coupling of a trinuclear titanium complex involvng alkoxy bridges gives rise to diastereoselective coupling. Non bridged forms and dimeric species would not result in the observed diastereoselectivity.