The activation of alkyl cyanides using a rhodiumtrispyrazolylborate complex

Abstract

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...

Activation of sp3 Carbon–Hydrogen Bonds by a Ruthenium(II) Complex and Subsequent Metal‐Mediated CC and CN Bond Formation

“…Extension of these reactions to substrates that possess sp 3 CH bonds could be of utility, but the activation of sp 3 CH bonds can be challenging, owing to the weakly coordinating nature of aliphatic moieties. While a functional group can provide a method to pre‐coordinate the substrate, the presence of heterogroups or halides can also prevent CH activation, because such moieties might either coordinate too strongly or provide a thermodynamically or kinetically more accessible site for metal‐mediated bond activation 10. 11 Herein, we report initial fundamental studies of stoichiometric sp 3 CH bond activation by [TpRu(PMe 3 )(NCR)Me] (R=Me or C 6 F 5 ).…”

Activation of sp3 Carbon–Hydrogen Bonds by a Ruthenium(II) Complex and Subsequent Metal‐Mediated CC and CN Bond Formation

“…While a functional group can provide a method to precoordinate the substrate, the presence of heterogroups or halides can also prevent C À H activation, because such moieties might either coordinate too strongly or provide a thermodynamically or kinetically more accessible site for metal-mediated bond activation. [10,11] Herein, we report initial fundamental studies of stoichiometric sp 3 C À H bond activation by [TpRu(PMe 3 )(NCR)Me] (R = Me or C 6 F 5 ).…”

“…While a functional group can provide a method to precoordinate the substrate, the presence of heterogroups or halides can also prevent C À H activation, because such moieties might either coordinate too strongly or provide a thermodynamically or kinetically more accessible site for metal-mediated bond activation. [10,11] Herein, we report initial fundamental studies of stoichiometric sp 3 Mechanistic studies suggest that aromatic C À H activations by [TpRu(L)(NCMe)Me] (L = CO or PMe 3 ) to release methane and produce [TpRu(L)(NCMe)Ar] (Ar = aryl) involve dissociation of NCMe, coordination of the aromatic substrate, and subsequent CÀH activation of the substrate. [6,7,9,12] Heating [TpRu(PMe 3 )(NCMe)Me] (1) at 608C in CH 3 CN results in C À H activation of acetonitrile to release methane and produce [TpRu(PMe 3 )(NCMe)(CH 2 CN)] [2, Eq.…”

“…In a recent report that focused on successful rhodium-based C À H activation of nitriles, Jones and co-workers note the challenge of selectively cleaving C À H bonds in the presence of functional groups, including cyano groups. [10] Even though the nitrile ligand of 2 is labile, attempts to observe benzene C À H(D) activation by 2 in C 6 D 6 at 60 8C resulted in decomposition to multiple undetermined complexes after 20 h. Neither the previously reported [TpRu-(PMe 3 )(NCMe)([D 5 ]Ph)] complex nor H/D exchange between benzene and the cyanomethyl ligand was observed, [16] which is consistent with Jones and co-workers report that the presence of a cyano group may provide ground-state stabilization relative to alkyl ligands. [10] Furthermore, attempts at degenerate 5 ]2) resulted in no observable reaction by 1 H NMR spectroscopy after 15 h at 100 8C.…”

“…[10] Even though the nitrile ligand of 2 is labile, attempts to observe benzene C À H(D) activation by 2 in C 6 D 6 at 60 8C resulted in decomposition to multiple undetermined complexes after 20 h. Neither the previously reported [TpRu-(PMe 3 )(NCMe)([D 5 ]Ph)] complex nor H/D exchange between benzene and the cyanomethyl ligand was observed, [16] which is consistent with Jones and co-workers report that the presence of a cyano group may provide ground-state stabilization relative to alkyl ligands. [10] Furthermore, attempts at degenerate 5 ]2) resulted in no observable reaction by 1 H NMR spectroscopy after 15 h at 100 8C. These results suggest that replacing the methyl ligand of 1 with the less basic cyanomethyl ligand of 2 increases the free energy of the transition state, DG°, for C À H activation.…”