Alkane Coordination Selectivity in Hydrocarbon Activation by [Tp‘Rh(CNneopentyl)]: The Role of Alkane Complexes

supporting

confidence: 71%

The role of alkane coordination in C–H bond cleavage at a Pt(II) center

Chen

Labinger

Bercaw

2007

“…Consequently, this substrate is less effective at producing an oxidative addition product. By using the earlier defined relative rates for coordination, migration, dissociation, and activation for pentane in a kinetic simulation (48), and assuming the same rate constants apply to hexanenitrile, it can be predicted that the preference for pentane over hexanenitrile would be 1.75:1, in good agreement with experiment. Furthermore, because the rates of reductive elimination from both of these activation products is about the same, the kinetic selectivity of 0.27 kcal/mol also corresponds to the thermodynamic selectivity for pentane over hexanenitrile.…”

Section: Discussionmentioning

confidence: 60%

“…Once again, by using the earlier defined relative rates for coordination, migration, dissociation, and activation for alkanes in a kinetic simulation (48), and assuming the same rate constants apply to the alkylnitriles, one can predict the preference for alkylnitrile over acetonitrile. For propionitrile, butyronitrile, and valeronitrile, these simulations give preferences of 1.74:1, 2.02:1, …”

Section: Discussionmentioning

confidence: 99%

The activation of alkyl cyanides using a rhodiumtrispyrazolylborate complex

Vetter

Rieth

Jones

2007

Self Cite

O rganometallic complexes of the cyclopentadienyl (Cp) and trispyrazolylborate (Tp) type have been shown to activate a variety of COH bonds in alkanes and arenes (1-9). However, studies on the activation of alkanes and arenes containing reactive functional groups have received far less attention in the literature. Hartwig and coworkers (10-12) have been successful in the hydroborylation of mono-and disubstituted arenes using transition metal catalysts. The borylation of the arene ring is believed to occur after oxidative addition of the ring COH bond without interference of reaction with the functional group. Smith (13,14) has even shown that haloarenes can be borylated and functionalized without affecting the carbon-halogen bond. Although numerous examples are presented for the functionalization of substituted arenes (15), the functionalization of alkanes containing reactive functional groups is somewhat restricted, perhaps because some of the functional groups on the alkanes are more reactive toward the catalysts and do not allow for COH activation. In addition, some functional groups (e.g., OH) are known to make the adjacent COH bonds more reactive in alkanes (16).We recently reported the activation of 1-, 2-, and 3-chloropentanes using a trispyrazolylborate rhodium complex (17). It was determined that the [TpЈRh(L)] fragment did not oxidatively cleave the COCl bond of the chloroalkane, as would have been expected given the relative bond strengths of COH (Ϸ100 kcal) vs. COCl (80 kcal) bonds (18) and the literature precedence that exists for the oxidative addition of COCl bonds to Rh(I) and Ir(I) metal systems (15,19). Surprisingly, the [TpЈRh(L)] fragment reacted exclusively with the terminal COH bonds of the alkane to give the substituted haloalkyl hydride complex. Here, we extend the investigation of the activation of hydrocarbon COH bonds to include substrates containing the nitrile functional group (OC'N).The COH bond activation of acetonitrile has been documented in the past. Teuben and coworkers (20) have reported the COH activation of acetonitrile via a -bond metathesis using Cp* 2 LnCH(SiMe 3 ) 2 . Jesson and coworkers (21) have also documented the formation of the cyanomethyl hydride as a result of activation of acetonitrile using (dmpe) 2 M(Np)H (M ϭ Fe, Ru; Np ϭ 2-naphthyl). The scission of the COCN bond of acetonitrile has also been reported by Parkin and coworkers (22), Brookhart and coworkers (23, 24), Nakazawa et al. (25), and Jones and coworkers (26). Of particular interest is the work by Brookhart using a similar Cp*Rh(PMe 3 ) system (23), which is known to undergo COH bond activation in the presence of alkanes and arenes (3,27). However, in the presence of nitriles and triphenylsilane, the complex promotes the cleavage of the COCN bond.To demonstrate the diversity of nitrile reactivity with low valent metals, our group has also investigated the reactions of aromatic, allylic, and alkyl nitriles using [(dippe)NiH] 2 (28-30). In these studies, 2 coordination of the CN bond to the Ni metal cent...

“…When compared with a statistical ratio of 6:4:2, this observed ratio corresponds to a thermodynamic preference of 0.13 kcal/mol for CH 2 binding over CH 3 binding. Also, Vetter et al (38) observed a 1.5ϫ kinetic preference for coordination at a secondary COH bond rather than at a primary COH bond for TpЈRhL(alkane) complexes (TpЈϭTris-(3,5-dimethylpyrazolyl) borate; LϭCNCH 2 CMe 3 ).…”

Section: Resultsmentioning

confidence: 99%

Theoretical study of the rhenium–alkane interaction in transition metal–alkane σ-complexes

Cobar

Khaliullin

Bergman

et al. 2007

Metal-alkane binding energies have been calculated for [CpRe(CO) 2](alkane) and [(CO)2M(C5H4)C'C(C5H4)M(CO)2](alkane), where M ‫؍‬ Re or Mn. Calculated binding energies were found to increase with the number of metal-alkane interaction sites. In all cases examined, the manganese-alkane binding energies were predicted to be significantly lower than those for the analogous rhenium-alkane complexes. The metal (Mn or Re)-alkane interaction was predicted to be primarily one of charge transfer, both from the alkane to the metal complex (70 -80% of total charge transfer) and from the metal complex to the alkane (20 -30% of the total charge transfer).binding energy ͉ COH activation ͉ DFT calculations ͉ manganese C arbon-hydrogen (COH) bond activation reactions are important from a fundamental point of view, as well as in more practical terms to medicine, academia, and industry, as they are being used for the conversion of common, inexpensive alkanes into reactive (and physiologically active) molecules (1, 2). Many transition metal complexes are now known that can be used to activate COH bonds in alkanes (3). With low-valent metal complexes, the first bond-breaking step involves oxidative addition of an alkane to an unsaturated metal center (Fig. 1). The goal of subsequent steps is to convert the alkyl group R into an alcohol ROH or other potentially useful functionalized organic molecule.Many researchers today are focusing on the development of industrially practical organometallic oxidation catalysts (5-8), but there remain fundamental aspects of the COH activation process that are not fully understood. For instance, it is widely accepted that COH activation reactions proceed via alkane -complex intermediates (3) (Fig. 1, compound 3), and such species have recently been detected and studied in lowtemperature NMR experiments (9-11). Although complexes with intramolecular COH/metal (so-called agostic) interactions have been isolated and crystallographically characterized (3), in systems where strong evidence has been provided for intermolecular metal-alkane coordination in solution (9-11), isolation and solid state structural characterization have not yet been accomplished.† † Intermolecular metal-alkane complexes are therefore one of the most important targets in the study of COH activation; an understanding of the mechanisms of COH activation, which will allow researchers to better control and manipulate the outcome of the reactions, is crucial for the advancement of this area of chemistry.Quantum chemical methods have become useful and practical tools in study of TM complexes (14-17). Particularly, advancements in density functional theory (DFT) (16) and the use of effective core potentials (14, 15) have made qualitatively accurate predictions of the structures and chemistry of TM complexes possible at a reasonable computational cost (17). The goals of the present work are twofold: to make computational predictions of the relative stabilities of a selection of alkane -complexes, which will aid synthetic chemists in th...