Rh-H Bond Research Articles

Sequential H/D Exchanges Resulting from Rollover-Cyclometallation during Photoirradiation of Rhodium(III) Complex in Methanol-d4.

We present the photoreaction of newly prepared bis(6,6'-dimethyl-2,2'-bipyridine)(oxalato)rhodium(III) ([Rh(N^N)2(ox)]+) in CD3OD. Photoirradiation of this complex causes the dissociation of ox, followed by the formation of the unprecedented Rh(III) complex with Rh-H and Rh-C s bonds, [Rh(N^N)(C^N)(H)(CD3OD)]+ (C^N = [6,6'-dimethyl-2,2'-bipyridine]-3-yl-κC3,κN1'). This hydride formation and cyclometallation spontaneously proceed owing to the conflict between the steric hindrance arising from the methyl groups of N^N and the driving force for the structural change due to [Rh(N^N)2]+ formation. Although [Rh(N^N)(C^N)(H)(CD3OD)]+ is initially converted to [Rh(N^N)2]+ by photoirradiation, it is immediately regenerated by the rollover cyclometallation of the [Rh(N^N)2]+ complex. [Rh(N^N)(C^N)(H)(CD3OD)]+ undergoes H/D exchange for the H atoms in the Rh-H bond and at the 3, 3'-positions of the N^N ligand during the photoirradiation. DFT calculations predict with reasonable certainty the spontaneous structural change of [Rh(N^N)2]+ to [Rh(N^N)(C^N)(H)(CD3OD)]+ and the subsequent photodriven Rh-C bond rupture leading to the formation of [Rh(N^N)2]+ accompanied by H/D exchange reactions.

Chemo-, regio- and enantioselective hydroformylation of trisubstituted cyclopropenes: access to chiral quaternary cyclopropanes

Catalytic asymmetric synthesis of polysubstituted chiral cyclopropane presents a significant challenge in organic synthesis due to the difficulty in enantioselective control. Here we report a rhodium-catalyzed highly chemo-, regio- and enantioselective hydroformylation of trisubstituted cyclopropenes affording chiral quaternary cyclopropanes. Importantly, the easy made sterically bulky ligand L1 can effectively suppress hydrogenation and decomposition reactions and give quaternary cyclopropanes with high regio- and enantioselectivities for both aryl and alkyl functionalized substrates. Control experiments and computational studies reveal the sterically hindered well-defined chiral pocket instead of the substrates bearing electron-withdrawing diester groups is important for controlling the enantioselectivity and regioselectivity. Scale-up reaction and follow-up diverse transformations are also presented. Density Functional theory (DFT) computations suggest that the regio- and enantio-selectivities originate from the cyclopropene insertion to the Rh-H bond. The high regioselectivity is found to benefit from the presence of more efficient noncovalent interactions (NCIs) manifesting in the form of C–H···Cl, C–H···N, and l.p(Cl)···π contacts.

Dirhodium(II)/DPPM Catalyzed 1,2-Hydrosilylation of Conjugated Dienes with Tertiary Silanes.

A simple and efficient Rh2(OAc)4/DPPM (bis(diphenylphosphanyl)methane) catalyzed regioselective 1,2-anti-Markovnikov hydrosilylation of conjugated dienes with various tertiary silanes gave homoallylic silanes in acetonitrile, which tolerate broad functional groups. Control experiments proved that no π-allyl transition metal intermediates were involved in this 1,2-anti-Markovnikov hydrosilylation. Dirhodium hydride species was observed in hydrosilylation, suggesting that a direct insertion of the terminal double bond into a Rh-H bond is involved in this reaction.

Dirhodium(II) complex catalyzed dehydrosilylation of styrenes: theoretical investigations on the mechanism, selectivity, and ligand effects.

The dirhodium(II) complexes with bridging phosphine and OAc ligands showed high reactivity and selectivities in olefin dehydrosilylation. In order to determine the structure of the actual catalyst which cannot be determined experimentally, the geometries of the dirhodium catalyst, the detailed catalytic mechanism, and the stereo- and chemo-selectivities of the title reaction were studied using DFT calculations. The results showed that one OAc group is monodentate and the other is bidentate in the dirhodium catalyst C'. The determined catalytic cycle consists of four processes: Rh-H bond activation in C', Si-H bond activation in alkoxysilane, alkylene insertion into the Rh-Si bond, followed by β-H elimination or σ-metathesis reaction. Among them, the alkylene insertion process is the rate-determining step. The stereoselectivity of the title reaction is controlled by the steric effect and orbital interactions between the alkyene and dirhodium catalysts in the β-H elimination process. The chemoselectivity is regulated by the presence of the axial ligand in the dirhodium catalyst, when there is an axial ligand coordinated to the Rh atom, E-alkene is the main product, whereas alkane would be obtained in the absence of an axial ligand. Our work determines the structure of the actual catalyst, and provides explanations and predictions for the activity, and chemo- and stereo-selectivity control of olefin dehydrosilylation.

Mechanistic insight into formaldehyde hydroformylation catalyzed by rhodium complexes: A theoretical and experimental study

The aldehyde hydroformylation is of critical importance for synthesis of higher aldehydes and polyhydric alcohols, but the reaction mechanism has not been clearly resolved in the existing literature. In this paper, we have established the dissociation and anion mechanism for formaldehyde hydroformylation using DFT calculation and experimental study. In the dissociation mechanism, HCHO insertion into the Rh-H bond is the rate-controlling step and CH3OH, HCOOCH3 are preferentially produced via the CH3O-Rh route. The CO coordination reaction of CH3O-Rh(CO)2(PPh3) is not favored energetically leading to facile generation of CH3OH relative to HCOOCH3. In the anion mechanism, the nucleophilic addition of HCHO to the HOCH2-Rh species is more energetically favorable and the rate-determining step turns to H2 oxidation addition. The effective energy barrier of hydrogenolysis for CH3OH is higher than that for HOCH2CHO, accounting for higher selectivity of HOCH2CHO than CH3OH observed in the reaction. The calculations also predict the strong electron-donating ligand serves to give rise to higher reactivity in the anion mechanism.

Catalytic Hydrogenation of a Manganese(V) Nitride to Ammonia.

The catalytic hydrogenation of a metal nitride to produce free ammonia using a rhodium hydride catalyst that promotes H2 activation and hydrogen-atom transfer is described. The phenylimine-substituted rhodium complex (η5-C5Me5)Rh(MePhI)H (MePhI = N-methyl-1-phenylethan-1-imine) exhibited higher thermal stability compared to the previously reported (η5-C5Me5)Rh(ppy)H (ppy = 2-phenylpyridine). DFT calculations established that the two rhodium complexes have comparable Rh-H bond dissociation free energies of 51.8 kcal mol-1 for (η5-C5Me5)Rh(MePhI)H and 51.1 kcal mol-1 for (η5-C5Me5)Rh(ppy)H. In the presence of 10 mol% of the phenylimine rhodium precatalyst and 4 atm of H2 in THF, the manganese nitride (tBuSalen)Mn≡N underwent hydrogenation to liberate free ammonia with up to 6 total turnovers of NH3 or 18 turnovers of H• transfer. The phenylpyridine analogue proved inactive for ammonia synthesis under identical conditions owing to competing deleterious hydride transfer chemistry. Subsequent studies showed that the use of a non-polar solvent such as benzene suppressed formation of the cationic rhodium product resulting from the hydride transfer and enabled catalytic ammonia synthesis by proton-coupled electron transfer.

Theoretical Study on the Rhodium-Catalyzed Hydrosilylation of C═C and C═O Double Bonds with Tertiary Silane

Reaction mechanisms of hydrosilylation of ketone and alkene with tertiary silane using the Wilkinson-type catalyst were theoretically investigated on the basis of density functional calculations using ωB97XD functional. Previously proposed three mechanisms, the Chalk-Harrod (CH) mechanism, the modified Chalk-Harrod (mCH) mechanism, and the outer-sphere mechanism were examined. Besides, we also found two mechanisms, the alternative CH (aCH) mechanism and the double hydride (DH) mechanism. In the aCH mechanism, a four-coordinate rhodium hydride complex formed through the elimination of R3Si-Cl is a catalytically active species. In the DH mechanism, the active species is a six-coordinate complex with two Rh-H bonds. For the C═O double bond hydrosilylation, the rate-determining steps of the aCH and DH mechanisms are both acetone insertion into the Rh-H bond, and the order of the activation barrier is DH < aCH ≈ CH < mCH. For the C═C double bond hydrosilylation, except for the mCH pathway whose rate-determining step is the hydrosilane addition reaction, the rate-determining steps of the CH, aCH, and DH pathways are Si-C reductive elimination reactions. The order of the energy barrier is DH ≈ mCH < aCH ≈ CH. In the outer-sphere mechanism, no stable intermediate or transition state was found. Consequently, we concluded that the DH mechanism is adopted as the mechanism for the Rh-catalyzed hydrosilylation of the carbonyl group while the mCH or DH mechanism is adopted as the mechanism for alkenes under conditions where their active intermediates are formed. The present result revises a hypothesis that the hydrosilylation of the carbonyl group is in general accomplished by the mCH mechanism. The active species in the DH mechanism has one more extra Rh-H bond compared to that of the other pathways, and its interaction with a silyl group, trans-influence, and small steric effect are the origin of the highly efficient catalytic activity, which was not reported before.

Controlling Hydrogen Evolution during Photoreduction of CO2 to Formic Acid Using [Rh(R-bpy)(Cp*)Cl]+ Catalysts: A Structure-Activity Study.

The photochemical reduction of CO2 to formic acid catalyzed by a series of [Rh(4,4'-R-bpy)(Cp*)Cl]+ and [Rh(5,5'-COOH-bpy)(Cp*)Cl]+ complexes (Cp* = pentamethylcyclopentadienyl, bpy = 2,2'-bipyridine, and R = OCH3, CH3, H, COOC2H5, CF3, NH2, or COOH) was studied to assess how modifications in the electronic structure of the catalyst affect its selectivity, defined as the HCOOH:H2 product ratio. A direct molecular-level influence of the functional group on the initial reaction rate for CO2 versus proton reduction reactions was established. Density functional theory computations elucidated for the first time the respective role of the [RhH] and [Cp*H] tautomers, recognizing rhodium hydride as the key player for both reactions. In particular, our calculations explain the observed tendency of electron-donating substituents to favor CO2 reduction by means of decreasing the hydricity of the Rh-H bond, resulting in a lower hydride transfer barrier toward formic acid production as compared to substituents with an electron-withdrawing nature that favor more strongly the reduction of protons to hydrogen.

A DFT study on the mechanism of rhodium-catalyzed regioselective hydrothiolation of the allyl amine

The reaction mechanisms of Rh-catalyzed regioselective hydrothiolation of the allyl amine employing four bidentate phosphine ligands are investigated with DFT calculations. The free energy profiles of anti-Markovnikov and Markovnikov pathways arising from different alkene insertion types are computed to elucidate the ligand-controlled regioselectivity. For 1,2-bis(diphenylphosphino)benzene (dppbz) and 1,3-bis(diphenylphosphino)propane (dppp) ligands with small nature bite angle (βn ≤ 86°), the anti-Markovnikov pathway that features the 1,2-alkene insertion into Rh-H bond is favored by 2 ˜ 4 kcal/mol in barriers of elementary steps. While for 1,4-bis(diphenylphosphino)butane (dppb) and bis(2-diphenylphosphinophenyl)ether (DPEphos) ligands with large nature bite angle (βn ≥ 99°), the Markovnikov pathway with 1,2-alkene insertion into Rh-S bond is preferential by 2 ˜ 7 kcal/mol in barriers. The P-Rh-P bite angle is a reliable predictor and regulator of the regioselectivity of reaction as evidenced by good correlations between reaction barrier and P-Rh-P bite angle. Smaller P-Rh-P bite angle in TSs is generally found for small nature bite angle ligand dppbz and dppp in preferential anti-Markovnikov pathway, while TSs with larger P-Rh-P bite angle are favored by large nature bite angle ligand DPEphos and dppb. Larger difference in P-Rh-P bite angles of TSs between Markovnikov and anti-Markovnikov pathway generally leads to the greater disparity in barrier heights of two pathways, and hence greater regiodivergency of reaction.

Preparation, Characterization, and Electrochemical Activation of a Model [Cp*Rh] Hydride.

Monomeric half-sandwich rhodium hydride complexes are often proposed as intermediates in catalytic cycles, but relatively few such compounds have been isolated and studied, limiting understanding of their properties. Here, we report preparation and isolation of a monomeric rhodium(III) hydride complex bearing the pentamethylcyclopentadienyl (Cp*) and bis(diphenylphosphino)benzene (dppb) ligands. The hydride complex is formed rapidly upon addition of weak acid to a reduced precursor complex, Cp*Rh(dppb). Single-crystal X-ray diffraction data for the [Cp*Rh] hydride, which were previously unavailable for this class of compounds, provide evidence of the direct Rh-H interaction. Complementary infrared spectra show the Rh-H stretching frequency at 1986 cm-1. In contrast to results with other [Cp*Rh] complexes bearing diimine ligands, treatment of the isolated hydride with strong acid does not result in H2 evolution. Electrochemical studies reveal that the hydride complex can be reduced only at very negative potentials (ca. -2.5 V vs ferrocenium/ferrocene), resulting in Rh-H bond cleavage and H2 generation. These results are discussed in the context of catalytic H2 generation, and development of design rules for improved catalysts bearing the [Cp*] ligand.

Understanding the influence of hydrogen pressure on the enantioselectivity of hydrogenation: A combined theory-experiment approach

Why would the pressure of hydrogen affect the enantioselectivity of hydrogenation in an opposite way for two enamide reactants, M-acrylate (methyl 2-acetamidoacrylate) and E-emap (ethyl 4-methyl-3-acetamido-2-pentanoate), with rhodium (I) complexes coordinated to a diphosphine ligand? This question was answered by a combination of experiments and DFT calculations. The activation free energies, the kinetic constants and the enantiomeric excess ee have been calculated. In the static pathways, the two substrates show significant differences. From the square-planar substrate-metal complex, M-acrylate (resp. E-emap) prefers the attack of the hydrogen on the side opposite to (resp. of) the amide group. Both substrates however show lower transition states on the pathway starting from the minor stereoisomer of the substrate-metal complex. The turnover-limiting step for M-acrylate is the oxidative addition of hydrogen on the substrate-metal complex, while it is the migratory insertion of the substrate in the Rh-H bond for E-emap. However, the energy profiles are not sufficient to understand the enantioselectivity of the reaction: the kinetic simulations lead to different conclusions and show a marked influence of the hydrogen pressure. For both substrates, the R isomer is obtained in the realistic pressure range, coming from the minor isomer for M-acrylate and from the major isomer for E-emap. Hence, depending on the reactant, the preferred pathway follows either the major/minor or the lock-and-key concept. The importance of combining DFT calculations with kinetic simulations in unraveling the mechanism is underlined. In particular, they reveal that the rate of formation of the metal-substrate complex plays a key role for the enantioselectivity.

Kinetics and mechanisms of homogeneous catalytic reactions. Part 14. Hydroformylation of 1-hexene with formaldehyde catalyzed by a cationic bis(diphosphine)rhodium complex

Complex [Rh(κ2-P,P-dppe)2]acac ([1a]acac, dppe: 1,2-bis(diphenylphosphino)ethane) showed to be a highly efficient precatalyst for the hydroformylation of 1-hexene with formaldehyde under relatively mild reaction conditions (353–403K) in 1,4-dioxane as the solvent; the corresponding C7 aldehydes were obtained in linear to branched ratios (l/b) close to 2. A kinetic study for this reaction lead to the experimentally determined rate law: ri={K1k2/(1+K1[CH2O])}[Rh][C6H12][CH2O] [K1=(1.47±0.04)M−1 and k2=(0.47±0.01)M−1s−1 at 373K). The experimental activation parameters for the overall reaction were also calculated [Ea=(20.1±0.9)kcalmol−1, ΔH≠=(19.5±0.9)kcalmol−1, ΔS≠=(−9.3±0.5)calK−1mol−1 and ΔG≠=(22±2)kcalmol−1]. The kinetic study together with results on coordination chemistry performed with 1a and with the BF4 salt of its iridium analogue, [Ir(κ2-P,P-dppe)2]BF4 ([1b]BF4), as well as theoretical DFT-calculations allowed us to propose a mechanism for this reaction. The catalytic cycle involves the reversible oxidative addition of formaldehyde to 1a to yield [Rh(H)(CHO)(κ2-P,P-dppe)2]+, followed by the insertion of the olefin into the Rh-H bond to generate [Rh(alkyl)(CHO)(κ2-P,P-dppe)2]+, which is considered as the rds of the process [Ea2=(23.9±0.8) kcalmol−1]; the cycle is completed by the reductive elimination of the aldehydes to regenerate 1a and restart the cycle.

Electron-donating groups and high ring strain promoted ring opening of methylenecyclopropanes catalyzed by rhodium and iridium complexes

The possible ring-opening pathways of X-substituted MCPs (X = NO2, CF3, F, H, Ph, CH3, OH, NH2) promoted by MH(CO)(PY3)3 (M = Rh, Ir, Y = H, Ph) and Wilkinson's catalyst-[RhCl(PY3)3] (Y = H, Ph) were studied by density functional theory. The ring-opening reaction was initiated by the orbital interaction between the transition metal of catalyst and reactant MCP. The strong coordination ability of Ir with CC double bond and the inertness of complex intermediate caused the difficulty in further reaction steps of catalyst dissociation and hydride transfer, rationalizing that IrH(CO)(PPh3)3 catalyzed ring-opening reaction occurred at higher temperature (50 °C) than the RhH(CO)(PPh3)3 catalyzed one (room temperature). Without the medium hydride in RhCl(PPh3)3 catalyst, the CC bond of MCP cannot insert into the RhH bond and the hydride cannot be borrowed from RhCl(PPh3)3 like RhH(CO)(PPh3)3 catalyst, hence making the RhCl(PPh3)3-catalyzed reaction much more difficult to occur. The ring-opening step is predicted to be the rate-determining step for MH(CO)(PPh3)3 catalyst, which is different from the Cp2LnH (Ln = La, Lu) catalyzed reactions. Multifaceted factors of substituents including ring strain, electron-donating ability, and the steric effect of the substituents were revealed for MH(CO)(PPh3)3 catalyzed reactions when M = Rh and Ir. The hydroxyl substituted MCP (X = OH) bears not only higher ring strain but also the stronger electron-donating ability, leading to the lowest activation energy among the studied systems. Understanding of the multifaceted factors of substituents is useful for regulating the ring-opening reaction of substituted MCP promoted by transition metal complexes.

Mechanism of Rhodium-Catalyzed Formyl Activation: A Computational Study.

Metal-catalyzed transfer hydroformylation is an important way of cleaving C-C bonds and constructing new double bonds. The newly reported density functional theory (DFT) method, M11-L, has been used to clarify the mechanism of the rhodium-catalyzed transfer hydroformylation reported by Dong et al. DFT calculations depict a deformylation and formylation reaction pathway. The deformylation step involves an oxidative addition to the formyl C-H bond, deprotonation with a counterion, decarbonylation, and β-hydride elimination. After olefin exchange, the formylation step takes place via olefin insertion into the Rh-H bond, carbonyl insertion, and a final protonation with the conjugate acid of the counterion. Theoretical calculations indicate that the alkalinity of the counterion is important for this reaction because both deprotonation and protonation occur during the catalytic cycle. A theoretical study into the formyl acceptor shows that the driving force of the reaction is correlated with the stability of the unsaturated bond in the acceptor. Our computational results suggest that alkynes or ring-strained olefins could be used as formyl acceptors in this reaction.

Nitrous Oxide as a Hydrogen Acceptor for the Dehydrogenative Coupling of Alcohols.

The oxidation of alcohols with N2O as the hydrogen acceptor was achieved with low catalyst loadings of a rhodium complex that features a cooperative bis(olefin)amido ligand under mild conditions. Two different methods enable the formation of either the corresponding carboxylic acid or the ester. N2 and water are the only by-products. Mechanistic studies supported by DFT calculations suggest that the oxygen atom of N2O is transferred to the metal center by insertion into the Rh-H bond of a rhodium amino hydride species, generating a rhodium hydroxy complex as a key intermediate.

A theoretical study on the mechanisms of intermolecular hydroacylation of aldehyde catalyzed by neutral and cationic rhodium complexes

In this paper, we used density functional theory (DFT) computations to study the mechanisms of the hydroacylation reaction of an aldehyde with an alkene catalyzed by Wilkinsons catalyst and an organic catalyst 2-amino-3-picoline in cationic and neutral systems. An aldehydes hydroacylation includes three stages: the C-H activation to form rhodium hydride (stage I), the alkene insertion into the Rh-H bond to give the Rh-alkyl complex (stage Ⅱ), and the C-C bond formation (stage Ⅲ). Possible pathways for the hydroacylation originated from the trans and cis isomers of the catalytic cycle. In this paper, we discussed the neutral and cationic pathways. The rate-determining step is the C-H activation step in neutral system but the reductive elimination step in the cationic system. Meanwhile, the alkyl group migration-phosphine ligand coordination pathway is more favorable than the phosphine ligand coordination-alkyl group migration pathway in the C-C formation stage. Furthermore, the calculated results imply that an electron-withdrawing group may decrease the energy barrier of the C-H activation in the benzaldehyde hydroacylation.

Kinetics and Thermodynamics of H–/H•/H+ Transfer from a Rhodium(III) Hydride

The thermodynamics and kinetics of all three cleavage modes for Rh-H, the transfer of H(-), H(+), or H•, have been studied for the Rh(III) hydride complex Cp*Rh(2-(2-pyridyl)phenyl)H (1a). The thermodynamic hydricity, ΔG°H(-), for 1a has been measured (49.5(1) kcal/mol) by heterolytic cleavage of H2 with Et3N in CH3CN. The transfer of H(-) from 1a to 1-(1-phenylethylidene)pyrrolidinium is remarkably fast (kH(-) = 3.5(1) × 10(5) M(-1) s(-1)), making 1a a very efficient catalyst for the ionic hydrogenation of iminium cations. The pKa of 1a in CH3CN has been measured as 30.3(2) with (tert-butylimino)tris(pyrrolidino)phosphorane (12), and the rate constant for H(+) transfer from 1a to 12 has been estimated (kH(+) = 5(1) × 10(-4) M(-1) s(-1)) from the half-life of the equilibration. Thus, 1a is a poor H(+) donor both thermodynamically and kinetically. However, 1a transfers H• to TEMPO smoothly, forming a stable Rh(II) radical Cp*Rh(2-(2-pyridyl)phenyl)• (14a) that can activate H2 at room temperature and 1 atm. The metalloradical 14a has a g value of 2.0704 and undergoes reversible one-electron reduction at -1.85 V vs Fc(+)/Fc in benzonitrile, implying a bond-dissociation enthalpy for the Rh-H bond of 1a of 58.2(3) kcal/mol--among the weakest Rh(III)-H bonds reported. The transfer of H• from 1a to Ar3C• (Ar = p-(t)BuC6H4) is fast, with kH• = 1.17(3) × 10(3) M(-1) s(-1). Thus, 1a is a good H(-) and H• donor but a poor H(+) donor, a combination that reflects the high energy of the Rh(I) anion [Cp*Rh(2-(2-pyridyl)phenyl)](-).

Propagation and termination steps in Rh-mediated carbene polymerisation using diazomethane

Rh-mediated carbene (co)polymerisation of diazomethane works best in the presence of Rh(III) catalyst precursors, the use of which leads to a significant increase in polymer yield and molecular weight. Chain termination via β-hydride elimination is severely suppressed for these species, although this process does still occur leading to unsaturated chain ends. Subsequent chain walking leading to the formation of branched polymers seems not to occur. Computational studies describing pathways for both chain propagation and chain termination using a (cycloocta-2,5-dien-1-yl)Rh(III)(alkyl) species as a representative model for the active species revealed that chain propagation is favoured for these species, although β-hydride elimination is still viable at the applied reaction temperatures. The computational studies are in excellent agreement with all experimental results, and further reveal that chain propagation via carbene insertion (leading to linear poly-methylene) occurs with a much lower energy barrier than insertion of 1-alkenes into either the Rh-H bond after β-hydride elimination or into the Rh-C bond of the growing polymer chain (leading to branched polymers). These energetic differences conveniently explain why experimentally the formation of branches is not observed in (co)polymerisation reactions employing diazomethane. The formation of substantial amounts of low-M(w) oligomers and dimers in the experimental reactions can be ascribed to the presence of (1,5-cyclooctadiene)Rh(I) species in the reaction mixture, for which chain termination via β-hydride elimination is clearly favoured over chain propagation. These two species stem from a non-selective catalyst activation process during which the catalyst precursors are in situ activated towards carbene polymerisation, and as such the results in this paper might contribute to further improvements of this reaction.

Catalytic Non‐Conventional trans‐Hydroboration: A Theoretical and Experimental Perspective

We have studied the non-conventional trans-hydroboration reaction of alkynes both experimentally and theoretically. A catalytic system based on the in situ mixture of [{Rh(cod)Cl}(2)]/PCy(3) (cod=1,5-cyclooctadiene, Cy=cyclohexyl) has been able to activate pinacolborane and catecholborane and transfer boryl and hydride groups onto the same unhindered carbon atom of the terminal alkynes. The presence of a base (Et(3)N) favored the non-conventional trans-hydroboration over the traditional cis-hydroboration. Varying the substrate had a significant influence on the reaction, with up to 99% conversion and 94% regioselectivity observed for para-methyl-phenylacetylene. Both DFT and quantum mechanical/molecular mechanical ONIOM calculations were carried out on the [RhCl(PR(3))(2)] system. To explain the selectivity towards the (Z)-alkenylboronate we explored several alternative mechanisms to the traditional cis-hydroboration, using propyne as a model alkyne. The proposed mechanism can be divided into four stages: 1) isomerization of the alkyne into the vinylidene, 2) oxidative addition of the borane reagent, 3) vinylidene insertion into the Rh-H bond, and finally 4) reductive elimination of the C-B bond to yield the 1-alkenylboronate. Calculations indicated that the vinylidene insertion is the selectivity-determining step. This result was consistent with the observed Z selectivity when the sterically demanding phosphine groups, such as PCy(3) and PiPr(3), were introduced. Finally, we theoretically analyzed the effect of the substrate on the selectivity; we identified several factors that contribute to the preference for aryl alkynes over aliphatic alkynes for the Z isomer. The intrinsic electronic properties of aryl substituents favored the Z-pathway over the E-pathway, and the aryl groups containing electron donating substituents favored the occurrence of the vinylidene reaction channel.

Allylic C–H bond activation and functionalization mediated by tris(oxazolinyl)borato rhodium(i) and iridium(i) compounds

Allylic C-H bond oxidative addition reactions, mediated by tris(oxazolinyl)borato rhodium(I) and iridium(I) species, provide the first step in a hydrocarbon functionalization sequence. The bond activation products To(M)MH(η(3)-C(8)H(13)) (M = Rh (1), Ir (2)), To(M)MH(η(3)-C(3)H(5)) (M = Rh (3), Ir (4)), and To(M)RhH(η(3)-C(3)H(4)Ph) (5) (To(M) = tris(4,4-dimethyl-2-oxazolinyl)phenylborate) are synthesized by reaction of Tl[To(M)] and the corresponding metal olefin chloride dimers. Characterization of these group 9 allyl hydride complexes includes (1)H-(15)N heteronuclear correlation NMR experiments that reveal through-metal magnetization transfer between metal hydride and the trans-coordinated oxazoline nitrogen. Furthermore, the oxazoline (15)N NMR chemical shifts are affected by the trans ligand, with the resonances for the group trans to hydride typically downfield of those trans to η(3)-allyl and tosylamide. These group 9 oxazolinylborate compounds have been studied to develop approaches for allylic functionalization. However, this possibility is generally limited by the tendency of the allyl hydride compounds to undergo olefin reductive elimination. Reductive elimination products are formed upon addition of ligands such as CO and CN(t)Bu. Also, To(M)RhH(η(3)-C(8)H(13)) and acetic acid react to give To(M)RhH(κ(2)-O(2)CMe) (8) and cyclooctene. In contrast, treatment of To(M)RhH(η(3)-C(3)H(5)) with TsN(3) (Ts = SO(2)C(6)H(4)Me) gives the complex To(M)Rh(η(3)-C(3)H(5))NHTs (10). Interestingly, the reaction of To(M)RhH(η(3)-C(8)H(13)) and TsN(3) yields To(M)Rh(NHTs)(H)OH(2) (11) and 1,3-cyclooctadiene viaβ-hydride elimination and Rh-H bond amination. Ligand-induced reductive elimination of To(M)Rh(η(3)-C(3)H(5))NHTs provides HN(CH(2)CH=CH(2))Ts; these steps combine to give a propene C-H activation/functionalization sequence.