Alkene-alkyl interconversion: an experimental and computational study of the olefin insertion and β-hydride elimination processes

Advances in Rhodium-Catalyzed Oxidative Arene Alkenylation

ConspectusAlkyl and alkenyl arenes are of substantial value in both large-scale and fine chemical processes. Billions of pounds of alkyl and alkenyl arenes are produced annually. Historically, the dominant method for synthesis of alkyl arenes is acid-catalyzed arene alkylation, and alkenyl arenes are often synthesized in a subsequent dehydrogenation step. But these methods have limitations that result from the catalytic mechanism including (1) common polyalkylation, which requires an energy intensive transalkylation process, (2) quantitative selectivity for Markovnikov products for arene alkylation using α-olefins, (3) for substituted arenes, regioselectivity that is dictated by the electronic character of the arene substituents, (4) inability to form alkenyl arenes in a single process, and (5) commonly observed slow reactivity with electron-deficient arenes. Transition-metal-catalyzed aryl-carbon coupling reactions can produce alkyl or alkenyl arenes from aryl halides. However, these reactions often generate halogenated waste and typically require a stoichiometric amount of metal-containing transmetalation reagent. Transition-metal-catalyzed arene alkylation or alkenylation that involves arene C-H activation and olefin insertion into metal-aryl bonds provides a potential alternative method to prepare alkyl or alkenylation arenes. Such reactions can circumvent carbocationic intermediates and, as a result, can overcome some of the limitations mentioned above. In particular, controlling the regioselectivity of the insertion of α-olefins into metal-aryl bonds provides a strategy to selectively synthesize anti-Markovnikov products. But, previously reported catalysts often show limited longevity and low selectivity for anti-Markovnikov products.In this Account, we present recent developments in single-step arene alkenylation using Rh catalyst precursors. The reactions are successful for unactivated hydrocarbons and exhibit unique selectivity. The catalytic production of alkenyl arenes operates via Rh-mediated aromatic C-H activation, which likely occurs by a concerted metalation-deprotonation mechanism, olefin insertion into a Rh-aryl bond, β-hydride elimination from the resulting Rh-hydrocarbon product, and net dissociation of alkenyl arene with formation of a Rh hydride. Reaction of the Rh hydride with Cu(II) oxidant completes the catalytic cycle. Although Rh nanoparticles can be formed under some conditions, mechanistic studies have revealed that soluble Rh species are likely responsible for the catalysis. These Rh catalyst precursors achieve high turnovers with >10,000 catalytic turnovers observed in some cases. Under anaerobic conditions, Cu(II) carboxylates are used as the oxidant. In some cases, aerobic recycling of Cu(II) oxidant has been demonstrated. Hence, the Rh arene alkenylation catalysis bears some similarities to Pd-catalyzed olefin oxidation (i.e., the Wacker-Hoechst process). The Rh-catalyzed arene alkenylation is compatible with some electron-deficient arenes, and they are selective for anti-Markovnikov products when using substituted olefins. Finally, when using monosubstituted arenes, consistent with a metal-mediated C-H activation process, Rh-catalyzed alkenylation of substituted arenes shows selectivity for meta- and para-alkenylation products.

Cp*Co(III)-Catalyzed γ-Selective C-H Allylation/Hydroamination Cascade for the Synthesis of Dihydroisoquinolines

A Cp*Co(III)-catalyzed γ-selective C-H allylation/hydroamination cascade toward the synthesis of 3,4-dihydroisoquinolines (DHIQs) has been successfully developed, starting from NH ketimines and allyl carbonates. Notably, highly efficient and γ-selective C-H allylations were accomplished using γ-substituted allyl reagents, thus overcoming the issues of poor α/γ selectivity and low reactivity of previous transition metal-catalyzed C-H allylations. The stereochemistry of allyl carbonates was a crucial factor, and synthesis of the DHIQs was achieved using (Z)-allyl carbonates.

Synthesis and reactivity at the Ir-MeTpm platform: from κ1-N coordination to κ3-N-based organometallic chemistry

Reaction of [Ir(μ-Cl)(COE)2]2 (COE = cis-cyclooctene) with tris(3,5-dimethylpyrazol-1-yl)methane (MeTpm) affords [IrCl(κ1-N-MeTpm)(COD)] (1) (COD = 1,5-cyclooctadiene). The formation of 1 implies the transfer dehydrogenation of a COE ligand to give COD and COA (cyclooctane). A mechanistic proposal based on DFT calculations that explains this iridium promoted process has been disclosed. Additionally, reactivity studies have allowed the preparation and characterization, including determination of the molecular structures of a number of iridium complexes with the MeTpm ligand in κ1, κ2 or κ3-N coordination modes. Moreover, the first example of an Ir-cyclooctyl complex featuring hydride and carbonyl ligands, whose solid state structure has been determined by X-ray diffraction methods, is reported.

(Diphenylphosphino)alkylaldehyde affords hydride- or alkyl-[(diphenylphosphino)alkylacyl]rhodium(iii) or (diphenylphosphino)alkylester complexes: theoretical and experimental diastereoselectivity

The reaction of [RhCl(COD)]2 (COD = 1,5-cyclooctadiene) with racemic PPh2(CH(Ph)CH2CHO) and pyridine (py) led to the oxidative addition of the aldehyde, and a single geometric isomer of [RhHCl(PPh2(CH(Ph)CH2CO))(py)2] (1), with hydride trans to chloride, was obtained as a mixture of two diastereomers in a 95 : 5 ratio; this was in agreement with density functional theory (DFT) calculations. In a chloroform solution, the exchange of hydride by chloride yielded [RhCl2(PPh2(CH(Ph)CH2CO))(py)2] (2) as a mixture of a kinetically preferred species, trans-py-2a, and two diastereomers, cis-Cl-2b' and cis-Cl-2b, with cis pyridines and a chloride trans to acyl; as predicted by the DFT calculations, the latter was the major species. Complex 1 reacted with racemic PPh2(CH(Ph)CH2CHO) or PPh2(o-C6H4CHO) to afford [RhHCl(PPh2(CH(Ph)CH2CO))(κ1-PPh2(CH(Ph)CH2CHO))(py)] (3) or [RhHCl(PPh2(o-C6H4CO))(κ1-PPh2(CH(Ph)CH2CHO))(py)] (4), respectively, both with a dangling alkylaldehyde. Diastereomeric mixtures with the ratios 3a/3a' = 80 : 20 and 4a/4a' = 50 : 50 were obtained. Complex 4 reacted with N-donors to afford cationic [RhH(NN)(PPh2(o-C6H4CO))(κ1-PPh2(CH(Ph)CH2CHO))]BPh4 (NN = 1,10-phenanthroline, 5; 2,2'-bipyridine, 6) or with 8-aminoquinoline (aqui) or 2-(aminomethyl)pyridine to yield imination products with terdentate ligands: [RhH(PPh2(o-C6H4CO))(κ3-PNN)]BF4 (PNN = PPh2(CH(Ph)CH2CNC9H6N), 7 and PPh2(CH(Ph)CH2CNCH2C5H4N), 8, respectively. Compounds 5-8 were obtained as equimolar a/a' mixtures of diastereomers. Moreover, 5a and 5a' could be separated. [RhCl(NBD)]2 reacted with racemic PPh2(CH(Ph)CH2CHO) and N-donors to provide nortricyclyl (Ntyl) derivatives [RhCl(NN)(Ntyl)(PPh2CH(Ph)CH2CO)] (NN = phen, 9 and bipy, 10) as an a/a' = 75 : 25 mixture of diastereomers. By reacting [RhCl(NBD)]2 with PPh2(CH(Ph)CH2CHO) and quinoline-8-carbaldehyde in methanol, the phosphino-ester complex [RhCl(Ntyl)(C9H6NCO)(κ2-PPh2CH(Ph)CH2CO(OCH3)] 11 was obtained. The initial equimolar mixture of two diastereomers readily transformed into a single diastereomer, which was found to be thermodynamically most stable by the DFT calculations. Furthermore, single crystal X-ray diffraction analysis of cis-Cl-2b, 5a, 7a, 10a and 11 is reported.