Direct Access to Fluorene by Successive C–O/C–H Bond Activations of 2‑Phenylbenzyl Ester

Catalytic formation of fluorene has been achieved from 2-phenylbenzyl trifluoroacetate via successive C–O and C–H bond cleavage reactions by Pd­(OAc)₂/PPh₃ in 97% yield. This reaction involves the oxidative addition of ester to give (carboxylato)­(2-phenylbenzyl)­palladium­(II) species and deprotonation from the 2-phenylbenzyl group by the cleaved carboxylato group via an internal electrophilic substitution mechanism

Regio- and Enantioselective Linear Cross-Dimerizations between Conjugated Dienes and Acrylates Catalyzed by New Ru(0) Complexes

New naphthalene complexes of Ru(0) with various Ru­(η⁶-naphthalene)­(cyclic diene) (<b>3</b>) ligands catalyze linear cross-dimerization between conjugated dienes and acrylates. One of the noteworthy catalysts is the dibenzocyclooctatetraene complex <b>3d</b>, which shows high catalytic activity for the cross-dimerization between 1,3-pentadiene and methyl acrylate to give the cross-dimers in 99% yield (branch/linear = 77/23) within 1 h at 50 °C with 1 mol % catalyst loading. When Ru­(η⁶-naphthalene)­[(−)-Ph-bod*] (<b>3f</b>) was used as the catalyst, treatment of 2,4-dimethylhexa-2,4-diene with <i>tert-</i>butyl acrylate produced the chiral cross-dimer in 44% yield with 49% ee. This is the first example of enantioselective cross-dimerization between conjugated dienes and substituted alkenes

Stoichiometric Carbon–Carbon Bond Forming Reaction of 1,3-Diene with 1,2-Diene in a Ruthenium(0) Complex

A stoichiometric reaction of Ru­(η⁴-<i>cisoid-</i>2,3-dimethyl-1,3-butadiene)­(η⁴-1,5-COD)­(NCMe) (<b>1a</b>) with ethyl 2,3-butadienoate produces <i>rac-prone,supine-</i>(1<i>S</i>,2<i>R,</i>4<i>S</i>,5<i>R</i>)-1-<i>anti-</i>4-<i>anti-</i>Ru­(η³:η³-1-ethoxycarbonyl-2-methylidene-4,5-dimethylhex-4-ene-1,6-diyl)­(η⁴-1,5-COD) (<b>2aa</b>) in 90% yield and in 52% isolated yield. The stereochemistry of this and the related products shows prior coordination of the 1,2-diene followed by the nucleophilic attack of the coordinated 1,3-diene, where the Ru(0) complex distinguishes two different orthogonal π planes and the prostereogenic face, depending on the steric congestion, regardless of their electronic properties

Markovnikov-Selective Hydrosilylation of Electron-Deficient Alkenes with Arylsilanes Catalyzed by Mono(phosphine)palladium(0)

Markovnikov-selective hydrosilylation of electron-deficient alkenes with HSiPh₃ is catalyzed by a mono­(phosphine)­palladium(0) complex, Pd­(η²:η²-C₆H₁₀O)­(PMe₃) (<b>1a</b>). The hydrosilylation of acrylonitrile with HSiPh₃ at 30 °C proceeds to completion within 40 min in the presence of 5 mol % of <b>1a</b>. The complex <b>1a</b> also shows the catalytic activity for the hydrosilylation with mono- and diarylsilanes and monochlorosilane such as HSiPhMe₂, HSiPh₂Me, and HSiClMe₂. The hydrosilylation using para-substituted styrenes clearly shows the electron-withdrawing substituent promoting the reaction. Mechanistic studies indicate that the reaction is proceeding by a Chalk–Harrod mechanism with the reductive elimination of an (alkyl)­(silyl)­palladium­(II) intermediate being the rate-determining step

Regio- and Enantioselective Linear Cross-Dimerizations between Conjugated Dienes and Acrylates Catalyzed by New Ru(0) Complexes

New naphthalene complexes of Ru(0) with various Ru­(η⁶-naphthalene)­(cyclic diene) (<b>3</b>) ligands catalyze linear cross-dimerization between conjugated dienes and acrylates. One of the noteworthy catalysts is the dibenzocyclooctatetraene complex <b>3d</b>, which shows high catalytic activity for the cross-dimerization between 1,3-pentadiene and methyl acrylate to give the cross-dimers in 99% yield (branch/linear = 77/23) within 1 h at 50 °C with 1 mol % catalyst loading. When Ru­(η⁶-naphthalene)­[(−)-Ph-bod*] (<b>3f</b>) was used as the catalyst, treatment of 2,4-dimethylhexa-2,4-diene with <i>tert-</i>butyl acrylate produced the chiral cross-dimer in 44% yield with 49% ee. This is the first example of enantioselective cross-dimerization between conjugated dienes and substituted alkenes

Stoichiometric Carbon–Carbon Bond Forming Reaction of 1,3-Diene with 1,2-Diene in a Ruthenium(0) Complex

A stoichiometric reaction of Ru­(η⁴-<i>cisoid-</i>2,3-dimethyl-1,3-butadiene)­(η⁴-1,5-COD)­(NCMe) (<b>1a</b>) with ethyl 2,3-butadienoate produces <i>rac-prone,supine-</i>(1<i>S</i>,2<i>R,</i>4<i>S</i>,5<i>R</i>)-1-<i>anti-</i>4-<i>anti-</i>Ru­(η³:η³-1-ethoxycarbonyl-2-methylidene-4,5-dimethylhex-4-ene-1,6-diyl)­(η⁴-1,5-COD) (<b>2aa</b>) in 90% yield and in 52% isolated yield. The stereochemistry of this and the related products shows prior coordination of the 1,2-diene followed by the nucleophilic attack of the coordinated 1,3-diene, where the Ru(0) complex distinguishes two different orthogonal π planes and the prostereogenic face, depending on the steric congestion, regardless of their electronic properties

Selective Alkene Insertion into Inert Hydrogen–Metal Bonds Catalyzed by Mono(phosphorus ligand)palladium(0) Complexes

Isolated mono­(phosphorus ligand)­palladium(0) complexes catalyzed alkene insertions into hydrogen–tungsten bonds. These insertions using WHCp­(CO)₃ with ethyl acrylate and dimethyl fumarate smoothly gave the corresponding alkyltungsten complexes. Kinetic studies involving the stoichiometric reactions and DFT calculations suggest the following steps: (i) formation of a mono­(phosphorus ligand)­mono­(alkene)­palladium(0) species, (ii) subsequent reaction of a metal hydride with the palladium(0), (iii) insertion of the coordinated alkene into the resulting palladium hydride, and (iv) reductive elimination between the alkyl and metal on the palladium center to release the alkylmetal species with regeneration of a palladium(0) by a reaction with alkene

Acid-Promoted sp³ C–H Bond Cleavage in a Series of (2-Allylphenoxo)ruthenium(II) Complexes. Mechanistic Insight into the Aryloxo–Acid Interaction and Bond Cleavage Reaction

A series of RuCp­[OC₆H₃(CH₂CHCH₂-2)­(R)]­(PPh₃)_<i>n</i> complexes (<i>n</i> = 2, R = H (<b>1a</b>); <i>n</i> = 1, R = 4-OMe (<b>2b</b>), 4-Me (<b>2c</b>), 4-Ph (<b>2d</b>), 4-Br (<b>2e</b>), 4-NO₂ (<b>2f</b>), 6-OMe (<b>2g</b>), 6-Me (<b>2h</b>), 6-Ph (<b>2i</b>)) have been prepared in 27–76% yields. These 2-allylaryloxo complexes <b>1a</b> and <b>2b</b>–<b>f</b> are in equilibrium between RuCp­[OC₆H₃(CH₂CHCH₂-2)­(R)-κ¹<i>O</i>]­(PPh₃)₂ (<b>1</b>) and RuCp­[OC₆H₃(CH₂CHCH₂-2)­(R)-κ¹<i>O</i>,η²<i>C</i>,<i>C</i>′]­(PPh₃) (<b>2</b>) in solution, and <b>2g</b>–<b>i</b> do not react with PPh₃. The equilibrium constant <i>K</i>₁ (<i>K</i>₁ = [<b>2</b>]­[PPh₃]/[<b>1</b>]) is about the same for <b>1a</b> and <b>2b</b>–<b>f</b> (<i>K</i>₁ = 0.07–0.31 M). In contrast to the conventional aryloxo complexes of the late transition metals, treatment of <b>1a</b> and <b>2a</b>–<b>g</b> with weak Brϕnsted acids (HOR) gives a rapid equilibrium with <b>2</b>·HOR. The association constant <i>K</i>₂ (<i>K</i>₂ = [<b>2</b>·HOR]/([<b>2</b>]­[HOR])) increases on decreasing the p<i>K</i>_a value of the acid employed and on increasing the induction effect of substituents at the 4-position in the aryloxo group. These features suggest present association being regarded as a simple acid–base interaction. Interestingly, further association of <b>2</b>·HOR with the second acid leads to the cleavage of the benzylic C–H bond, giving RuCp­[C₃H₄{1-C₆H₃(OH-2)­(R)}-η³<i>C</i>,<i>C</i>′,<i>C</i>″]­(PPh₃) (<b>3</b>). The thermodynamic and kinetic studies suggest formation of hydrogen bonds among two Brϕnsted acid molecules, lone-pair electrons in the aryloxo oxygen, and a benzylic methylene proton. Such association makes the Ru­(II) center more electrophilic to attack the benzylic carbon to give <b>3</b>

Stoichiometric and Catalytic Cross Dimerization between Conjugated Dienes and Conjugated Carbonyls by a Ruthenium(0) Complex: Straightforward Access to Unsaturated Carbonyl Compounds by an Oxidative Coupling Mechanism

A series of stoichiometric and catalytic cross dimerizations between conjugated dienes and conjugated carbonyls are studied. The reaction of Ru­(η⁴-<i>cisoid-</i>1,3-butadiene)­(η⁴-1,5-COD)­(NCMe) (<b>2a</b>) with methyl acrylate gives a Ru(0) complex, Ru­[methyl η⁴-<i>cisoid-</i>(2<i>E</i>,4<i>E</i>)-hepta-2,4-dienoate]­(η⁴-1,5-COD)­(NCMe) (<b>3aa</b>) in 97% yield. Similar treatments of <b>2a</b> with a series of <i>tert-</i>butyl acrylate, methyl crotonate, 3-buten-2-one, and <i>N</i>,<i>N-</i>dimethylacrylamide produce similar analogues of <b>3ac</b>. When (<i>E</i>)-1,3-pentadiene complex <b>2d</b> is employed in the reaction with methyl acrylate, the branched coupling product Ru­[methyl η⁴-<i>cisoid-</i>(2<i>E</i>,4<i>E</i>)-4-methylhepta-2,4-dienoate]­(η⁴-1,5-COD)­(NCMe) (<b>3da-</b><i><b>b</b></i>) is dominantly obtained in 65% yield along with the linear product in 19% yield. In the case of the (<i>E</i>)-2,5-dimethylhexa-1,3-diene complex <b>2e</b>, the corresponding branch product is exclusively obtained in 86% yield. The catalytic cross dimerizations between conjugated dienes and conjugated carbonyls are established by <b>2</b>. The origin of the present chemoselectivity is the η⁴-coordination of a conjugated diene and η²-coordination of an electron-deficient alkene to formal 6<i>e</i> coordination sites at Ru(0), and the regioselectivity being prone to giving branched products is interpreted as an oxidative coupling mechanism, involving nucleophilic attack of the coordinated diene to the coordinated electron-deficient alkene

Ru(0)-Catalyzed Direct Coupling of Internal Alkynes with Conjugated Dienes: An Efficient Access to Conjugated Trienes

Ru­(0)-catalyzed direct coupling of internal alkynes with conjugated dienes enables a direct access to conjugated trienes, where the reaction is formally regarded as a stereoselective syn alkyne insertion into the terminal C–H bond in the conjugated diene. The reaction is catalyzed by Ru­(η⁶-naphthalene)­(η⁴-1,5-COD) (<b>1</b>; 3–10 mol %) with high regio- and stereoselectivities