Rhodium(I)‐Catalyzed Enantioselective C?C Bond Activation

Relieving the strain : The rhodium(I)‐catalyzed activation of C C bonds in functionalized cyclobutanes opens a novel route to highly substituted carbo‐ and heterocycles. Particularly intriguing is the differentiation of enantiotopic C C bonds, which leads to the formation of highly enantiomerically enriched lactones, cyclopentanones, and cyclohexenones (see scheme).

     

Rhodium(I)‐Catalyzed Cycloisomerization of Benzylallene‐Alkynes through C?H Activation

The efficient Rh^I‐catalyzed cycloisomerization of benzylallene‐alkynes produced the tricyclo[9.4.0.0^3,8]pentadecapentaene skeleton through a C H bond activation in good yields. A plausible reaction mechanism proceeds via oxidative addition of the acetylenic C H bond to Rh^I, an ene‐type cyclization to the vinylidenecarbene–Rh^I intermediate, and an electrophilic aromatic substitution with the vinylidenecarbene species. It was proposed based on deuteration and competition experiments.     

The Liebeskind–Srogl C?C Cross‐Coupling Reaction

New kid on the block : The cross‐coupling of thioorganic compounds with boronic acids under neutral conditions in the presence of catalytic palladium(0) and a stoichiometric amount of a copper(I) oxygenate has emerged as a very useful method for the construction of C C bonds (see scheme). This intriguing and mechanistically unprecedented base‐free coupling has distinct advantages, in particular when traditional Pd⁰‐catalyzed cross‐coupling is not possible.

     

Aldehyde‐Assisted Ruthenium(II)‐Catalyzed C?H Oxygenations

Versatile ruthenium(II) complexes allow for site‐selective C H oxygenations with weakly‐coordinating aldehydes. The challenging C H functionalizations proceed with high chemoselectivity by rate‐determining C H metalation. The new method features an ample substrate scope, which sets the stage for the step‐economical preparation of various bioactive heterocycles.     

Catalytic C?C,C?N,and C?O Ullmann‐Type Coupling Reactions

Copper‐catalyzed Ullmann condensations are key reactions for the formation of carbon–heteroatom and carbon–carbon bonds in organic synthesis. These reactions can lead to structural moieties that are prevalent in building blocks of active molecules in the life sciences and in many material precursors. An increasing number of publications have appeared concerning Ullmann‐type intermolecular reactions for the coupling of aryl and vinyl halides with N, O, and C nucleophiles, and this Minireview highlights recent and major developments in this topic since 2004.     

Amide‐Directed Tandem C?C/C?N Bond Formation through C?H Activation

The transformation of C? H bonds into other chemical bonds is of great significance in synthetic chemistry. C? H bond‐activation processes provide a straightforward and atom‐economic strategy for the construction of complex structures; as such, they have attracted widespread interest over the past decade. As a prevalent directing group in the field of C? H activation, the amide group not only offers excellent regiodirecting ability, but is also a potential C? N bond precursor. As a consequence, a variety of nitrogen‐containing heterocycles have been obtained by using these reactions. This Focus Review addresses the recent research into the amide‐directed tandem C? C/C? N bond‐formation process through C? H activation. The large body of research in this field over the past three years has established it as one of the most‐important topics in organic chemistry.     

Homolytic S?S Bond Dissociation of 11 Bis(thiocarbonyl)disulfides R‐C(S)?S?S?C(S)R and Prediction of A Novel Rubber Vulcanization Accelerator

The structures and energetics of eight substituted bis(thiocarbonyl)disulfides (RCS₂)₂, their associated radicals RCS₂^., and their coordination compounds with a lithium cation have been studied at the G3X(MP2) level of theory for R=H, Me, F, Cl, OMe, SMe, NMe₂, and PMe₂. The effects of substituents on the dissociation of (RCS₂)₂ to RCS₂^. were analyzed using isodesmic stabilization reactions. Electron‐donating groups with an unshared pair of electrons have a pronounced stabilization effect on both (RCS₂)₂ and RCS₂^.. The S? S bond dissociation enthalpy of tetramethylthiuram disulfide (TMTD, R=NMe₂) is the lowest in the above series (155 kJ mol^?1), attributed to the particular stability of the formed Me₂NCS₂^. radical. Both (RCS₂)₂ and the fragmented radicals RCS₂^. form stable chelate complexes with a Li⁺ cation. The S? S homolytic bond cleavage in (RCS₂)₂ is facilitated by the reaction [Li(RCS₂)₂]⁺+Li⁺→2 [Li(RCS₂)]^.+. Three other substituted bis(thiocarbonyl) disulfides with the unconventional substituents R=OSF₅, Gu¹, and Gu² have been explored to find suitable alternative rubber vulcanization accelerators. Bis(thiocarbonyl)disulfide with a guanidine‐type substituent, (Gu¹CS₂)₂, is predicted to be an effective accelerator in sulfur vulcanization of rubber. Compared to TMTD, (Gu¹CS₂)₂ is calculated to have a lower bond dissociation enthalpy and smaller associated barrier for the S? S homolysis.     

Rhodium‐Catalyzed ortho‐Selective C?F Bond Borylation of Polyfluoroarenes with Bpin?Bpin

An ortho‐selective C F bond borylation between N‐heterocycle‐substituted polyfluoroarenes and Bpin‐Bpin with simple and commercially available [Rh(cod)₂]BF₄ as a catalyst is now reported. The reaction proceeds under mild reaction conditions with high efficiency and broad substrate scope, even toward monofluoroarene, thus providing a facile access to a wide range of borylated fluoroarenes that are useful for photoelectronic materials. Preliminary mechanistic studies reveal that a Rh^III/V catalytic cycle via a key intermediate rhodium(III) hydride complex [(H)Rh^IIIL_n(Bpin)] may be involved in the reaction.     

The Morita?Baylis?Hillman Reaction of Chiral Highly Oxygenated Cyclopent‐2‐enones

The Morita? Baylis? Hillman (MBH) reactions of (4S,5R,7R,8R)‐ and (4R,5R,7R,8R)‐4‐hydroxy‐7,8‐dimethoxy‐7,8‐dimethyl‐6,9‐dioxaspiro[4.5]dec‐2‐en‐1‐ones ( 2 and 3 , resp.) with aldehydes using various catalysts were studied. A combination of Bu₃P/phenol in THF was found being optimum conditions giving the corresponding MBH adducts with high diastereoisomeric ratios. After separation, each stereomerically pure isomer of the MBH adducts was subjected to hydrolysis employing 1% aq. CF₃COOH (TFA) in a water bath of an ultrasonic cleaner to afford the corresponding polyhydroxylated cyclopentenones in good yields.     

Rhodium(III)‐Catalyzed C?C and C?O Coupling of Quinoline N‐Oxides with Alkynes: Combination of C?H Activation with O‐Atom Transfer

[Cp*Rh^III]‐catalyzed C H activation of arenes assisted by an oxidizing N O or N N directing group has allowed the construction of a number of hetercycles. In contrast, a polar N O bond is well‐known to undergo O‐atom transfer (OAT) to alkynes. Despite the liability of N O bonds in both C H activation and OAT, these two important areas evolved separately. In this report, [Cp*Rh^III] catalysts integrate both areas in an efficient redox‐neutral coupling of quinoline N‐oxides with alkynes to afford α‐(8‐quinolyl)acetophenones. In this process the N O bond acts as both a directing group for C H activation and as an O‐atom donor.     

Palladium(II)‐Catalyzed C?H Activation/C?C Cross‐Coupling Reactions: Versatility and Practicality

Pick your Pd partners : A number of catalytic systems have been developed for palladium‐catalyzed C? H activation/C? C bond formation. Recent studies concerning the palladium(II)‐catalyzed coupling of C? H bonds with organometallic reagents through a Pd^II/Pd⁰ catalytic cycle are discussed (see scheme), and the versatility and practicality of this new mode of catalysis are presented. Unaddressed questions and the potential for development in the field are also addressed.

     

Intramolecular Pnicogen Interactions in PHF?(CH2)n?PHF (n=2–6) Systems

A computational study of the intramolecular pnicogen bond in PHF? (CH₂)_n? PHF (n=2–6) systems was carried out. For each compound, two different conformations, (R,R) and (R,S), were considered on the basis of the chirality of the phosphine groups. The characteristics of the closed conformers, in which the pnicogen interaction occurs, were compared with those of the extended conformer. In several cases, the closed conformations are more stable than the extended conformations. The calculated interaction energies of the pnicogen contact, by means of isodesmic reactions, provide values between ?3.4 and ?26.0 kJ mol^?1. Atoms in molecules and electron localization function analysis of the electron density showed that the systems in the closed conformations with short P ??? P distances have a partial covalent character in this interaction. The calculated absolute chemical shieldings of the P atoms showed an exponential relationship with the P ??? P distance. In addition, a search in the Cambridge crystallographic database was carried out to detect those compounds with a potential intramolecular pnicogen bond in the solid phase.     

Ruthenium‐Catalyzed meta‐Selective C?H Bromination

The first example of a transition‐metal‐catalyzed, meta‐selective C H bromination procedure is reported. In the presence of catalytic [{Ru(p‐cymene)Cl₂}₂], tetrabutylammonium tribromide can be used to functionalize the meta C H bond of 2‐phenylpyridine derivatives, thus affording difficult to access products which are highly predisposed to further derivatization. We demonstrate this utility with one‐pot bromination/arylation and bromination/alkenylation procedures to deliver meta‐arylated and meta‐alkenylated products, respectively, in a single step.     

Redox‐Neutral Rhodium‐Catalyzed C?H Functionalization of Arylamine N‐Oxides with Diazo Compounds: Primary C(sp3)?H/C(sp2)?H Activation and Oxygen‐Atom Transfer

An unprecedented rhodium(III)‐catalyzed regioselective redox‐neutral annulation reaction of 1‐naphthylamine N‐oxides with diazo compounds was developed to afford various biologically important 1H‐benzo[g]indolines. This coupling reaction proceeds under mild reaction conditions and does not require external oxidants. The only by‐products are dinitrogen and water. More significantly, this reaction represents the first example of dual functiaonalization of unactivated a primary C(sp³) H bond and C(sp²) H bond with diazocarbonyl compounds. DFT calculations revealed that an intermediate iminium is most likely involved in the catalytic cycle. Moreover, a rhodium(III)‐catalyzed coupling of readily available tertiary aniline N‐oxides with α‐diazomalonates was also developed under external oxidant‐free conditions to access various aminomandelic acid derivatives by an O‐atom‐transfer reaction.     

Copper(I)‐Catalyzed Alkylation of Polyfluoroarenes through Direct C?H Bond Functionalization

The copper(I)‐catalyzed alkylation of electron‐deficient polyfluoroarenes with N‐tosylhydrazones and diazo compounds has been developed. This reaction uses readily available starting materials and is operationally simple, thus representing a practical method for the construction of C(sp²) C(sp³) bonds with polyfluoroarenes through direct C H bond functionalization. Mechanistically, copper(I) carbene formation and subsequent migratory insertion are proposed as the key steps in the reaction pathway.     

Highly Enantioselective Rhodium(I)‐Catalyzed Activation of Enantiotopic Cyclobutanone C?C Bonds

The selective functionalization of carbon–carbon σ bonds is a synthetic strategy that offers uncommon retrosynthetic disconnections. Despite progress in C C activation and its great importance, the development of asymmetric reactions lags behind. Rhodium(I)‐catalyzed selective oxidative additions into enantiotopic C C bonds in cyclobutanones are reported. Even operating at a reaction temperature of 130 °C, the process is characterized by outstanding enantioselectivity with the e.r. generally greater than 99.5:0.5. The intermediate rhodacycle is shown to react with a wide variety of tethered olefins to deliver complex bicyclic ketones in high yields.     

Diketopyrrolopyrrole?Thiophene‐Based Acceptor–Donor–Acceptor Conjugated Materials for High‐Performance Field‐Effect Transistors

We report the synthesis, morphology, and field‐effect‐transistor (FET) characteristics of new acceptor–donor–acceptor conjugated materials that consist of diketopyrrolopyrrole (DPP) acceptor groups and one of four different thiophene moieties, that is, dithiophene (2T), thieno[3,2‐b]‐thiophene (TT), dithieno[3,2‐b:2′,3′‐d]‐thiophene (DTT), and 5,5′′′‐di‐(2‐ethylhexyl)‐[2,3′;5′,2′′;4′′,2′′′]quaterthiophene (4T). The optical band gaps of the as‐prepared materials are smaller than 1.7 eV, which is attributed to the strong intramolecular charge transfer and the backbone coplanarity of the thiophene moieties. The order of both crystallinity and FET mobility (×10^?2–×10^?4 cm² V^?1 s^?1) is TT2DPP > 4T2DPP > 2T2DPP >DTT2DP, which differ in the structure of the π‐conjugated cores and core symmetry. Well‐ordered intermolecular chain packing was confirmed by the GIXD and AFM results. In particular, the FET hole mobility of TT2DPP was further improved to 0.1 cm² V^?1 s^?1, which was attributed to the well‐interconnected structure through solution‐shearing. These experimental results suggest the potential applications of the new DPP? thiophene? DPP conjugated materials for organic electronic devices.     

M4(CH2)4 Cubane‐Type Rare‐Earth Methylidene Complexes: Unique Reactivity toward Unsaturated C?O,C?N,and C?S Bonds

The reactivity of the cubane‐type rare‐earth methylidene complex [Cp′Lu(μ₃‐CH₂)]₄ ( 1 , Cp′=C₅Me₄SiMe₃) with various unsaturated electrophiles was investigated. The reaction of 1 with CO (1 atm) at room temperature gave the bis(ketene dianion)/dimethylidene complex [Cp′₄Lu₄(μ₃‐CH₂)₂(μ₃,η²‐O‐C?CH₂)₂] ( 2 ) in 86 % yield through the insertion of two molecules of CO into two of the four lutetium–methylidene units. In the reaction with the sterically demanding N,N‐diisopropylcarbodiimide at 60 °C, only one of the four methylidene units in 1 reacted with one molecule of the carbodiimide substrate to give the mono(ethylene diamido)/trimethylidene complex [Cp′₄Lu₄(μ₃‐CH₂)₃{iPrNC(=CH₂)NiPr}] ( 3 ) in 83 % yield. Similarly, the reaction of 1 with phenyl isothiocyanate gave the ethylene amido thiolate/trimethylidene complex [Cp′₄Lu₄(μ₃‐CH₂)₃{PhNC(S)=CH₂}] ( 4 ). In the case of phenyl isocyanate, two of the four methylidene units in 1 reacted with four molecules of the substrate at ambient temperature to give the malonodiimidate/dimethylidene complex [Cp′₄Lu₄(μ₃‐CH₂)₂{PhN=C(O)CH₂(O)C?NPh}₂] ( 5 ) in 87 % yield. In this reaction, each of the two lutetium–methylidene bonds per methylidene unit inserted one molecule of phenyl isocyanate. All the products have been fully characterized by NMR spectroscopy, X‐ray diffraction, and microelemental analyses.