Enantioselective CpxRhIII-Catalyzed Carboaminations of Acrylates

Enantioselective carboaminations of olefins constitute an attractive strategy for a rapid increase in molecular complexity from readily available starting materials. Reported here is an intermolecular asymmetric carboamination of acrylates using rhodium(III)-catalyzed alkenyl C−H activations of N-enoxysuccinimides to generate the nitrogen and carbon portion for the transfer. A rhodium complex equipped with a tailored bulky trisubstituted chiral Cp^x ligand ensures carboamination chemoselectivity as well high levels of enantioinduction. The transformation operates under mild reaction conditions at ambient temperatures and provides access to a variety of α-amino esters in good yields and excellent enantiomeric ratios of >99.5:0.5.     

Rhodium-Catalyzed C8-Alkenylation of Isoquinolones with Maleimides

An efficient Rh(III)-catalyzed C−H alkenylation of N-protected isoquinolone with maleimides is reported. The carbonyl group of isoquinolone acts as an inherent directing group. Various N-substituents in the maleimide, including alkyl, aryl, and even H and −OH, were well tolerated under the developed reaction condition. This protocol showed broad substrate scope, good selectivity, and excellent yields. Hammett plot is also drawn to check the effect of substituents on the reaction progress.     

Discrete Supramolecular Stacks Based on Multinuclear Tweezer-Type Rhodium Complexes

By taking advantage of self-complementary π–π stacking and CH–π interactions, a series of discrete quadruple stacks were constructed through the self-aggregation of U-shaped dirhodium metallotweezer complexes featuring various planar polyaromatic ligands. By altering the conjugate stacking strength and bridging ligands, assemblies with a range of topologies were obtained, including a binuclear D-shaped macrocycle, tetranuclear open-ended cagelike frameworks, and duplex metallotweezer stacking structures. Furthermore, a rare stacking interaction resulting in selective C−H activation was observed during the self-assembly process of these elaborate architectures.     

Pd-Catalyzed Annulation of 1-Halo-8-arylnaphthalenes and Alkynes Leading to Heptagon-Embedded Aromatic Systems

A palladium-catalyzed heptagon-forming annulation reaction between 1-halo-8-arylnaphthalene and diarylacetylene is reported. The reaction is promoted using a catalytic system comprised of Pd(OAc)₂, moderately electron-deficient triarylphosphine P(4-ClC₆H₄)₃, and Ag₂CO₃ to afford benzo[4,5]cyclohepta[1,2,3-de]naphthalene derivatives in moderate to good yields, in preference to fluoranthene as a competing byproduct. Twofold annulation can also be achieved to access a novel heptagon-embedded polycyclic aromatic hydrocarbon compound.     

Rhodium(II)‐Catalyzed Nondirected Oxidative Alkenylation of Arenes: Arene Loading at One Equivalent

A bimetallic Rh^II catalyst promoted the C? H alkenylation of simple arenes at 1.0 equivalent without the use of a directing group. A phosphine ligand as well as cooperative reoxidation of Rh^II with Cu(TFA)₂ and V₂O₅ proved essential in providing monoalkenylated products in good yields and selectivities, especially with di‐ and trisubstituted arenes.     

Synthesis of Spirocyclic Enones by Rhodium‐Catalyzed Dearomatizing Oxidative Annulation of 2‐Alkenylphenols with Alkynes and Enynes

The dearomatizing oxidative annulation of 2‐alkenylphenols with alkynes and enynes proceeds with high yields and regioselectivities under Rh^III catalysis. These reactions are successful using Cu(OAc)₂ or air as the stoichiometric oxidant, and provide spirocyclic enones, the basic ring system of which appears in several natural products. Application of this process to the preparation of a highly functionalized tetracycle is also demonstrated.     

Rhodium‐Catalyzed Cyclization of Diynes with Nitrones: A Formal [2+2+5] Approach to Bridged Eight‐Membered Heterocycles

N‐aryl‐substituted nitrones were employed as five‐atom coupling partners in the rhodium‐catalyzed cyclization with diynes. In this reaction, the nitrone moiety served as a directing group for the catalytic C? H activation of the N‐aryl ring. This formal [2+2+5] approach allows rapid access to bridged eight‐membered heterocycles with broad substrate scope. The results of this study may provide new insight into the chemistry of nitrones and find applications in the synthesis of other heterocycles.     

Aromatic Homologation by Non‐Chelate‐Assisted RhIII‐Catalyzed CH Functionalization of Arenes with Alkynes

Larger condensed arenes are of interest owing to their electro‐ and photochemical properties. An efficient synthesis is the catalyzed aromatic annulation of a smaller arene with two alkyne molecules. Besides difunctionalized starting materials, directed C? H functionalization can be used for such aromatic homologation. However, thus far the requirement of either pre‐functionalized substrates or suitable directing groups were limiting this approach. Herein, we describe a rhodium(III)‐catalyzed method allowing the use of completely unbiased arenes and internal alkynes. The reaction works best with copper(II) 2‐ethylhexanoate and decabromodiphenyl ether as the oxidant combination. This aromatic annulation tolerates a variety of functional groups and delivers homologated condensed arenes. Aside from simple benzenes, naphthalenes and higher condensed arenes provide access to highly substituted and highly soluble acenes structures having important electronic and photophysical properties.     

Preparation of Butadienylpyridines by Iridium-NHC-Catalyzed Alkyne Hydroalkenylation and Quinolizine Rearrangement

Iridium(I) N-heterocyclic carbene complexes of formula Ir(κ²O,O’-BHetA)(IPr)(η²-coe) [BHetA=bis-heteroatomic acidato, acetylacetonate or acetate; IPr=1,3-bis(2,6-diisopropylphenyl)imidazolin-2-carbene; coe=cyclooctene] have been prepared by treating Ir(κ²O,O’-BHetA)(η²-coe)₂ complexes with IPr. These complexes react with 2-vinylpyridine to afford the hydrido-iridium(III)-alkenyl cyclometalated derivatives IrH(κ²O,O’-BHetA)(κ²N,C-C₇H₆N)(IPr) through the iridium(I) intermediate Ir(κ²O,O’-BHetA)(IPr)(η²-C₇H₇N). The cyclometalated IrH(κ²O,O’-acac)(κ²N,C–C₇H₆N)(IPr) complex efficiently catalyzes the hydroalkenylation of aromatic and aliphatic terminal alkynes and enynes with 2-vinylpyridine to afford 2-(4R-butadienyl)pyridines with Z,E configuration as the major reaction products (yield up to 89 %). In addition, unprecedented (Z)-2-butadienyl-5R-pyridine derivatives have been obtained as minor reaction products (yield up to 21 %) from the elusive 1Z,3gem-butadienyl hydroalkenylation products. These compounds undergo a thermal 6π-electrocyclization to afford bicyclic 4H-quinolizine derivatives that, under catalytic reaction conditions, tautomerize to 6H-quinolizine to afford the (Z)-2-(butadienyl)-5R-pyridine by a retro-electrocyclization reaction.     

Rhodium(III)‐Catalyzed Alkenylation–Annulation of closo‐Dodecaborate Anions through Double B−H Activation at Room Temperature

1,2,3‐Trisubstituted closo‐dodecaborates with B?O, B?N, and B?C bonds as well as a fused borane oxazole ring have been synthesized by rhodium‐catalyzed direct cage B?H alkenylation and annulation of ureido boranes in the first reported example of regioselective B?H bond functionalization of the [B₁₂H₁₂]^2? cage by transition‐metal catalysis. This reaction proceeded at room temperature under ambient conditions and exhibited excellent selectivity for efficient monoalkenylation with good functional‐group tolerance. The urea moiety enabled B?H activation by acting as a directing group, was incorporated in the oxazole ring in situ, and also avoided multiple alkenylation. A possible mechanism is proposed on the basis of the isolation of a rhodium agostic intermediate and control experiments.     

Rhodium(III)-Catalyzed Asymmetric [4+1] and [5+1] Annulation of Arenes and 1,3-Enynes: A Distinct Mechanism of Allyl Formation and Allyl Functionalization

We report chiral Rh^III cyclopentadienyl-catalyzed enantioselective synthesis of lactams and isochromenes through oxidative [4+1] and [5+1] annulation, respectively, between arenes and 1,3-enynes. The reaction proceeds through a C−H activation, alkenyl-to-allyl rearrangement, and a nucleophilic cyclization cascade. The mechanisms of the [4+1] annulations were elucidated by a combination of experimental and computational methods. DFT studies indicated that, following the C−H activation and alkyne insertion, a Rh^III alkenyl intermediate undergoes δ-hydrogen elimination of the allylic C−H via a six-membered ring transition state to produce a Rh^III enallene hydride intermediate. Subsequent hydride insertion and allyl rearrangement affords several rhodium(III) allyl intermediates, and a rare Rh^III η⁴ ene-allyl species with π-agostic interaction undergoes SN₂′-type external attack by the nitrogen nucleophile, instead of C−N reductive elimination, as the stereodetermining step.     

Stereoselective Synthesis of Tertiary Allylic Amines by Titanium-Catalyzed Hydroaminoalkylation of Alkynes with Tertiary Amines

Intermolecular hydroaminoalkylation reactions of symmetrical and unsymmetrical alkynes with tertiary amines take place in the presence of catalytic amounts of TiBn₄, Ph₃C[B(C₆F₅)₄], and a sterically demanding aminopyridinato ligand precursor. The resulting products, synthetically and pharmaceutically useful tertiary β,γ-disubstituted allylic amines, are formed in convincing yields and with excellent stereoselectivity. Particularly promising for future applications is the fact that even the industrial side product trimethylamine can be used as a substrate.     

Rhodium(III)‐Catalyzed Alkenylation Reactions of 8‐Methylquinolines with Alkynes by C(sp3)H Activation

The alkenylation reactions of 8‐methylquinolines with alkynes, catalyzed by [{Cp*RhCl₂}₂], proceeds efficiently to give 8‐allylquinolines in good yields by C(sp³)? H bond activation. These reactions are highly regio‐ and stereoselective. A catalytically competent five‐membered rhodacycle has been structurally characterized, thus revealing a key intermediate in the catalytic cycle.     

Front Cover: Synthesis and Characterization of Pd(0) Complexes with Electronically Differentiated Ferrocene Diphosphane Ligands (Eur. J. Inorg. Chem. 22/2023)

The strategy of modifying phosphane ligands through substituent variation has been widely applied in coordination chemistry and catalysis. This contribution focuses on unsymmetric ferrocene diphosphanes with electronically distinct phosphane moieties, Ph₂PfcCH₂PAr₂ (Ar=Ph, 1 ; 3,5-C₆H₃Me₂, 2 ; and 3,5-C₆H₃(CF₃)₂, 3 ; fc=ferrocene-1,1′-diyl), which were synthesized and converted to the corresponding selenides ( 1Se – 3Se ) and Pd(0) complexes [Pd(L-κ²P,P′)(η²-ma)] ( 5 – 8 for L= 1 – 3 and dppf, ma=maleic anhydride). All compounds were characterized by NMR spectroscopy, ESI MS and elemental analysis, and the structures of 2 , 1Se ⋅ CHCl₃, 2Se and 5 ⋅ PhMe were determined by X-ray diffraction analysis. In addition, the redox behavior of 1 – 3 and 5 – 8 was studied by cyclic voltammetry and rationalized through DFT calculations. The prepared Pd(0) complexes and their model compound [Pd(dppf-κ²P,P′)(η²-ma)] were employed in Pd-catalyzed C−H arylation of benzoxazole with chlorobenzene in n-butanol in the presence of K₃PO₄ as the base, and the catalytic results were compared with the collected characterization data, including the ¹J_PSe coupling constants determined for 1Se – 3Se , as a measure of ligand basicity.     

Rhodium(III)‐Catalyzed Annulation of 2‐Alkenyl Anilides with Alkynes through C−H Activation: Direct Access to 2‐Substituted Indolines

A Rh^III complex featuring an electron‐deficient η⁵‐cyclopentadienyl ligand catalyzed an unusual annulation between alkynes and 2‐alkenyl anilides to form synthetically appealing 2‐substituted indolines. Formally, the process can be viewed as an allylic amination with concomitant hydrocarbonation of the alkyne. Mechanistic experiments indicate that this transformation involves an unusual rhodium migration with a concomitant 1,5‐H shift.     

Chiral Cp‐Rhodium(III)‐Catalyzed Asymmetric Hydroarylations of 1,1‐Disubstituted Alkenes

Metal‐catalyzed functionalizations at the ortho position of a directing group have become an efficient bond‐forming strategy. A wide range of transformations that employ Cp*Rh^III catalysts have been described, but despite their synthetic potential, enantioselective variants that use chiral versions of the Cp* ligand remain scarce (Cp*=pentamethyl cyclopentadienyl). Cyclopentadienyl compounds with an atropchiral biaryl backbone are shown to be suitable ligands for the efficient intramolecular enantioselective hydroarylation of aryl hydroxamates. Dihydrofurans that bear methyl‐substituted quaternary stereocenters are thus obtained by C? H functionalization under mild conditions.     

Rhodium‐Catalyzed CH Annulation of Nitrones with Alkynes: A Regiospecific Route to Unsymmetrical 2,3‐Diaryl‐Substituted Indoles

The direct C? H annulation of anilines or related compounds with internal alkynes provides straightforward access to 2,3‐disubstituted indole products. However, this transformation proceeds with poor regioselectivity in the synthesis of unsymmetrically 2,3‐diaryl substituted indoles. Herein, we report the rhodium(III)‐catalyzed C? H annulation of nitrones with symmetrical diaryl alkynes as an alternative method to prepare 2,3‐diaryl‐substituted N‐unprotected indoles with two different aryl groups. One of the aryl substituents is derived from N?C‐aryl ring of the nitrone and the other from the alkyne substrate, thus providing the indole products with exclusive regioselectivity.