On the Origin of Regio- and Stereoselectivity in the Rhodium-Catalyzed Vinylarenes Hydroboration Reaction

Investigation of the rhodium-catalyzed hydroboration of NHC-boranes: the role of alkene coordination and the origin of enantioselectivity

The mechanism of the intramolecular enantioselective rhodium(i)-catalyzed hydroboration of NHC-boranes is investigated by experiments and calculations, using Density Functional Theory and Random Phase Approximation methods.

Mechanism and Origin of the Stereoselectivity in the Palladium-Catalyzed trans Hydroboration of Internal 1,3-Enynes with an Azaborine-Based Phosphine Ligand

An azaborine-based phosphine-Pd catalyst was introduced by the Liu group to promote trans hydroboration of the C≡C triple bond of internal 1,3-enyne substrates. Despite the excellent yield and selectivity observed experimentally, the mechanism and the origin of this special trans selectivity remained unknown. Herein, a comprehensive theoretical investigation was performed to clarify these issues. Accordingly, two main mechanisms (inner- and outer-sphere) were proposed and examined. Different from the conventional inner-sphere mechanism, in which the transition metal is involved in H-B bond cleavage, this reaction follows an outer-sphere mechanism, in which Pd does not directly participate in H-B bond cleavage. More specifically, the favorable pathway followed a Tsuji-Trost type reaction, in which the H-B bond was weakened by the formation of a four-coordinate boron intermediate (i.e., the boron is attached to the terminal carbon of the alkyne group). It then underwent a hydride-transfer process with the assistance of a second borane molecule, and finally reductive elimination generated the trans hydroboration product. Further analysis ascribed the origin of the special trans selectivity to the unique steric effect and electronic effect introduced by the special κ¹ -P-η² -BC coordination pattern.

Mechanistic Insights into Carbonyl-Directed Rhodium-Catalyzed Hydroboration: ab Initio Study of a Cyclic γ,δ-Unsaturated Amide

A two-point binding mechanism for the cationic rhodium(I)-catalyzed carbonyl-directed catalytic asymmetric hydroboration of a cyclic γ,δ-unsaturated amide is investigated using density functional theory. Geometry optimizations and harmonic frequency calculations for the model reaction are carried out using the basis set 6-31+G** for C, O, P, B, N, and H and LANL2DZ for Rh atoms. The Gibbs free energy of each species in THF solvent is obtained based on the single-point energy computed using the PCM model at the ECP28MWB/6-311+G(d,p) level plus the thermal correction to Gibbs free energy by deducting translational entropy contribution. The Rh-catalyzed reaction cycle involves the following sequence of events: (1) chelation of the cyclic γ,δ-unsaturated amide via alkene and carbonyl complexation in a model active catalytic species, [Rh(L2)₂S₂]⁺, (2) oxidative addition of pinacol borane (pinBH), (3) migratory insertion of the alkene double bond into Rh–H (preferred pathway) or Rh–B bond, (4) isomerization of the resulting intermediate, and finally, (5) reductive elimination to form the B–C or H–C bond with regeneration of the catalyst. Free energy profiles for potential pathways leading to the major γ-borylated product are computed and discussed in detail. The potential pathways considered include (1) pathways proceeding via migratory insertion into the Rh–H bond (pathways I, I-1, and I-2), (2) a potential pathway proceeding via migratory insertion into the Rh–B bond (pathway II), and two potential competing routes to a β-borylated byproduct (pathway III). The results find that the Rh–H migratory insertion pathway I-2, followed in sequence by an unanticipated isomerization via amide rotation and reductive elimination, is the most favorable reaction pathway. A secondary consequence of amide rotation is access to a competing β-hydride elimination pathway. The pathways computed in this study are supported by and help explain related experimental results.

QM/MM Modeling of Enantioselective Pybox–Ruthenium- and Box–Copper-Catalyzed Cyclopropanation Reactions: Scope, Performance, and Applications to Ligand Design

An extensive comparison of full-QM (B3LYP) and QM/MM (B3LYP:UFF) levels of theory has been made for two enantioselective catalytic systems, namely, Pybox-Ru and Box-Cu complexes, in the cyclopropanation of alkenes (ethylene and styrene) with methyl diazoacetate. The geometries of the key reaction intermediates and transition structures calculated at the QM/MM level are generally in satisfactory agreement with full-QM calculated geometries. More importantly, the relative energies calculated at the QM/MM level are in good agreement with those calculated at the full-QM level in all cases. Furthermore, the QM/MM energies are often in better agreement with the stereoselectivity experimentally observed, and this suggests that QM/MM calculations can be superior to full-QM calculations when subtle differences in inter- and intramolecular interactions are important in determining the selectivity, as is the case in enantioselective catalysis. The predictive value of the model presented is validated by the explanation of the unusual enantioselectivity behavior exhibited by a new bis-oxazoline ligand, the stereogenic centers of which are quaternary carbon atoms.

Transition States of Binap–Rhodium(I)-Catalyzed Asymmetric Hydrogenation: Theoretical Studies on the Origin of the Enantioselectivity

By using the hybrid IMOMM(B3LYP:MM3) method, we examined the binap-Rh(I)-catalyzed oxidative-addition and insertion steps of the asymmetric hydrogenation of the enamide 2-acetylamino-3-phenylacrylic acid. We report a path that is energetically more favorable for the major enantiomer than for the minor enantiomer. This path follows the "lock-and-key" motif and leads to the major enantiomeric product via an energetically favorable binap-dihydride-Rh(III)-enamide complex. Our theoretical results are consistent with the mechanism that takes place via Rh(III) dihydride formation, that is, oxidative addition of H2 followed by enamide insertion.

TADDOL-Derived Phosphites and Phosphoramidites for Efficient Rhodium-Catalyzed Asymmetric Hydroboration

[reaction: see text] Two simple TADDOL-derived monodentate ligands, the (1R,2S)-2-phenylcyclohexanol-derived phosphite and the N,N-(phenylbenzyl)phosphoramidite, give comparably high levels of enantioselectivity (90-96% ee) in the rhodium-catalyzed hydroborations of substituted styrenes bearing either electron-donating or electron-withdrawing substituents. Rhodium(I) chloride and tetrafluoroborate catalyst precursors give comparable results. Pinacolborane is superior to catecholborane in these reactions.

Rhodium-Catalyzed Asymmetric Cyclization/Hydroboration of 1,6-Enynes

[reaction: see text] Reaction of enyne 1 with catecholborane catalyzed by a 1:1 mixture of [Rh(COD)(2)](+)SbF(6)(-) and (S)-BINAP (5 mol %) followed by Pd-catalyzed arylation with p-IC(6)H(4)CF(3) gave benzylidenecyclopentane 5 in 65% yield with 88% ee. Rhodium-catalyzed asymmetric cyclization/hydroboration followed either by Pd-catalyzed arylation or by oxidation was applied to the synthesis of a number of chiral, nonracemic carbocycles and heterocycles.

Rhodium(I) acetylacetonato complexes containing phosphinoalkynes as catalysts for the hydroboration of vinylarenes

Three novel rhodium(I) acetylacetonato (acac) complexes bearing phosphinoalkynes (Ph2PC≡C-t-Bu, Ph2PC≡CPPh2, and Ph2PC≡CC≡CPPh2) have been prepared and characterized fully. Addition of B2cat3 (cat = 1,2-O2C6H4) to Rh(acac)(Ph2PC≡C-t-Bu)2 (1a) led to zwitterionic Rh(η6-catBcat)(Ph2PC≡C-t-Bu)2 (2a), the first example of this type of compound to contain monodentate phosphine ligands. All new rhodium complexes have been investigated for their ability to catalyse the hydroboration of vinylarenes.Key words: catalysis, hydroboration, phosphinoalkynes, regioselectivity, rhodium.

Origin of Stereoinduction by Chiral Aminophosphane Phosphinite Ligands in Enantioselective Catalysis: Asymmetric Hydroformylation

The origin of stereoinduction by chiral aminophosphane phosphinite (AMPP) ligands in asymmetric hydroformylation was investigated with a theoretical approach. The roles of the stereogenic center at the aminophosphane phosphorus atom (NP*) and of the chirality of the backbone were analyzed by considering three experimentally tested cases: 1) P-stereogenic yielding high ee, 2) P-nonstereogenic yielding low ee, and 3) P-stereogenic yielding low ee. We succeeded in reproducing the experimentally observed trends for the three studied AMPP ligands. Our results indicated that alkene insertion into the rhodium-hydride bond is the selectivity-determining step, and not alkene coordination. Additional calculations on model systems revealed that the different nonbonding weak-type interactions of styrene with the substituents of the NP* stereogenic center in an axial position is responsible for stereodifferentiation. The chirality of the AMPP backbone plays a secondary role. The rationalization of the stereochemical outcome is not straightforward, because two competitive equatorial/axial reaction paths, showing opposite asymmetric induction, must be considered. Construction of stereochemical models and evaluation of stereoinduction for novel ligand systems suggested that two prerequisites are required to improve the performance of AMPP-type ligands in asymmetric hydroformylation: 1) combination of stereorecognition and stereohindrance by substituents at the NP* atom, and 2) more rigid backbones.

Regiochemical control of the catalytic asymmetric hydroboration of 1,2-diarylalkenes

The hydroboration of stilbenes and related disubstituted alkenes catalysed by QUINAP complexes may proceed with high enantio- and regioselectivity; rhodium and iridium catalysts give the same product regioisomer but opposite enantiomers.

In Quest of Factors That Control the Enantioselective Catalytic Markovnikov Hydroboration/Oxidation of Vinylarenes

This study attempts to rationalise the unpredictable performance of transition metal catalysed asymmetric hydroboration of vinylarenes on varying the precursor of the catalyst from cationic to neutral species, [M(cod)(L-L)]BF4, [M(mu-Cl)(cod)]2/(L-L), the metal (M=Rh and Ir), and the hydroborating reagent (catecholborane, pinacolborane). The approaches are based on the agreement between experimental data provided by (R)-Binap and (R)-Quinap modified catalytic systems and computational data evidenced by DFT calculations and QM/MM strategies. Unprecedentedly high enantiomeric excesses in the hydroboration/oxidation of vinylarenes with both electron-withdrawing substituents ((R)-(+)-1-p-F-phenylethanol, ee up to 92 %) and electron-releasing substituents ((R)-(+)-1-p-MeO-phenylethanol, ee up to 98 %), can be attributed to a rhodium halide key intermediate.

Regio- and enantiocontrol in the room-temperature hydroboration of vinyl arenes with pinacol borane

The catalyzed hydroboration of vinyl arenes was carried out using pinacol borane instead of catechol borane, as the former reagent and the product boronates are significantly easier to handle. By careful choice of catalyst, either the branched or the linear product can be obtained in greater than 96% selectivity. Interestingly, common ligands such as BINAP and Josiphos give opposite asymmetric induction with pinacol borane as compared with catechol borane, while P,N-ligands such as Quinap gave the same sense of induction. The hydroboration of 6-methoxynaphthalene proceeded with the greatest regio- (95:5) and enantioselectivity (94:6) of all vinyl arenes examined. The hydroboration product was then employed in a concise synthesis of the nonsteroidal antiinflammatory agent, Naproxen.