Redox‐Neutral Coupling between Two C(sp3)−H Bonds Enabled by 1,4‐Palladium Shift for the Synthesis of Fused Heterocycles

The intramolecular coupling of two C(sp³)?H bonds to forge a C(sp³)?C(sp³) bond is enabled by 1,4‐Pd shift from a trisubstituted aryl bromide. Contrary to most C(sp³)?C(sp³) cross‐dehydrogenative couplings, this reaction operates under redox‐neutral conditions, with the C?Br bond acting as an internal oxidant. Furthermore, it allows the coupling between two moderately acidic primary or secondary C?H bonds, which are adjacent to an oxygen or nitrogen atom on one side, and benzylic or adjacent to a carbonyl group on the other side. A variety of valuable fused heterocycles were obtained from easily accessible ortho‐bromophenol and aniline precursors. The second C?H bond cleavage was successfully replaced with carbonyl insertion to generate other types of C(sp³)‐C(sp³) bonds.     

Copper‐Mediated/Catalyzed Oxyalkylation of Alkenes with Alkylnitriles

A copper‐promoted oxyalkylation of alkenes with alkylnitriles has been developed. The protocol provides rapid access to phthalides (γ‐lactones) or isochromanones (δ‐lactones) via the formation of a C(sp³)?C(sp³) and a C(sp³)?O bond with the generation of up to two quaternary carbon atoms. Mechanistic studies suggest that this reaction is initiated by the formation of the C(sp³)?C(sp³) bond rather than the C(sp³)?O bond. Catalytic conditions were subsequently developed using carboxylic acid as an internal nucleophile.     

Base‐Catalyzed Stereoselective Vinylation of Ketones with Arylacetylenes: A New C(sp3)?C(sp2) Bond‐Forming Reaction

Alkylaryl‐ and alkylheteroarylketones, including those with condensed aromatic moieties, are readily vinylated with arylacetylenes (KOH/DMSO, 100 °C, 1 h) to give regio‐ and stereoselectively the (E)‐β‐γ‐ethylenic ketones ((E)‐3‐buten‐1‐ones) in 61–84 % yields and with approximately 100 % stereoselectivity. This vinylation represents a new C(sp³)? C(sp²) bond‐forming reaction of high synthetic potential.     

Homolytic,Heterolytic, Mesolytic ‐ As You Like It: Steering the Cleavage of a HC(sp3)−C(sp3)H Bond in Bis(1H‐2,1‐benzazaborole) Derivatives

A set of (3,3′)‐bis(1‐Ph‐2‐R‐1H‐2,1‐benzazaborole) compounds, in which R=tBu (Bab‐tBu)₂ , R=Dipp (Bab‐Dipp)₂ or R=tBu and Dipp (Bab‐Dipp)(Bab‐tBu) , was synthesized and fully characterized using ¹H, ¹¹B, ¹³C, and ¹⁵N NMR spectroscopy as well as single‐crystal X‐ray diffraction analysis. The central HC(sp³)?C(sp³)H bond with restricted rotation at the junction of both 1H‐2,1‐benzazaborole rings displayed an intriguing reactivity. It was demonstrated that this bond is easily mesolytically cleaved using alkali metals to form the respective aromatic 1Ph‐2R‐1H‐2,1‐benzazaborolyl anions M⁺(THF) _n (Bab‐tBu)^? (M=Li, Na, K) and K⁺(THF) _n (Bab‐Dipp)^? . Furthermore, the central HC(sp³)?C(sp³)H bond of bis(1H‐2,1‐benzazaborole)s is also homolytically cleaved either by heating or photochemical means, giving corresponding 1Ph‐2R‐1H‐2,1‐benzazaborolyl radicals (Bab‐tBu)^. and (Bab‐Dipp)^., which rapidly self‐terminate. Nevertheless, their formation was unambiguously established by NMR analysis of the reaction mixtures containing products of the self‐termination of the radicals after heating or irradiation. (Bab‐Dipp)^. radical was also characterized using EPR spectroscopy. Importantly, it turned out that the essentially non‐polarized HC(sp³)?C(sp³)H bond in (Bab‐tBu)₂ is also cleaved heterolytically with 2 equiv of MeLi, giving the mixture of Li⁺(SOL) _n (Bab‐tBu)^? (SOL=THF or Et₂O) and lithium methyl‐substituted borate complex Li⁺(SOL) _n (Bab‐tBu‐Me)^? in a diastereoselective fashion.     

Reversible α‐Hydrogen and α‐Alkyl Elimination in PC(sp3)P Pincer Complexes of Iridium

Despite significant progress in recent years, the cleavage of unstrained C(sp³)? C(sp³) bonds remains challenging. A C? C coupling and cleavage reaction in a PC(sp³)P iridium pincer complex is mechanistically studied; the reaction proceeds via the formation of a carbene intermediate and can be described as a competition between α‐hydrogen and α‐alkyl elimination; the latter process was observed experimentally and is an unusual way of C(sp³)? C(sp³) bond scission, which has previously not been studied in detail. Mechanistic details that are based upon kinetic studies, activation parameters, and DFT calculations are also discussed. A full characterization of a C? C agostic intermediate is presented.     

Crystal and molecular structure of (r-2, c-4)-3-benzyl-2,4,5,5-tetraphenyl1,3-thiazolidine, intramolecular C–HS hydrogen bonds

The crystal and molecular structures of (r-2, c-4)-3-benzyl-2,4,5,5-tetraphenyl-1,3-thiazolidine are investigated showing the existence of C(sp²)–HS and C(sp²)–HN intramolecular contacts. The analysis of geometrical parameters shows that C–HS contacts may be treated as hydrogen bonds but C–HN do not fulfil the geometrical criteria of the existence of H-bonds. The B3LYP/6-311+G* single point calculations were performed to obtain wave functions applied later for ‘atoms in molecules’ (AIM) study. The analysis of bond critical points based on the Bader theory (AIM) supports the existence of intramolecular C–HS H-bonds.     

Conformational energies and rotational barriers in 3-methyl-1-butene and 1-butene: An ab initio and molecular mechanics study

The calculated result obtained with MM2(87) for the rotation of the isopropyl group in 3-methyl-1-butene is not in agreement with experimental data. In order to reparametrize the C_sp2-C_sp3-C_sp-C_sp3 torsional angle, 3-methyl-1-butene and 1-butene have been studied by molecular mechanics (MM2(87)) and ab initio (MP2/6-31G* and MP3/6-31G*) calculations. The reparametrization of the torsional angle gives calculated results from MM2(87) in agreement with experimental data and ab initio calculations for both 3-methyl-1-butene and 1-butene. The calculated barriers for the rotation of alkyl groups in alkylbenzenes are improved with these new parameters.     

Impact of Oxidation State on Reactivity and Selectivity Differences between Nickel(III) and Nickel(IV) Alkyl Complexes

Described is a systematic comparison of factors impacting the relative rates and selectivities of C(sp³)?C and C(sp³)?O bond‐forming reactions at high‐valent Ni as a function of oxidation state. Two Ni complexes are compared: a cationic octahedral Ni^IV complex ligated by tris(pyrazolyl)borate and a cationic octahedral Ni^III complex ligated by tris(pyrazolyl)methane. Key features of reactivity/selectivity are revealed: 1) C(sp³)?C(sp²) bond‐forming reductive elimination occurs from both centers, but the Ni^III complex reacts up to 300‐fold faster than the Ni^IV, depending on the reaction conditions. The relative reactivity is proposed to derive from ligand dissociation kinetics, which vary as a function of oxidation state and the presence/absence of visible light. 2) Upon the addition of acetate (AcO^?), the Ni^IV complex exclusively undergoes C(sp³)?OAc bond formation, while the Ni^III analogue forms the C(sp³)?C(sp²) coupled product selectively. This difference is rationalized based on the electrophilicity of the respective M?C(sp³) bonds, and thus their relative reactivity towards outer‐sphere S_N2‐type bond‐forming reactions.     

Nickel-Catalyzed C−I-Selective C(sp2)−C(sp3) Cross-Electrophile Coupling of Bromo(iodo)arenes with Alkyl Bromides

Despite several methodologies established for C(sp²)−I selective C(sp²)−C(sp³) bond formations, achieving arene-flanked quaternary carbons by cross-coupling of tertiary alkyl precursors with bromo(iodo)arenes in a C(sp²)−I selective manner is rare. Here we report a general Ni-catalyzed C(sp²)−I selective cross-electrophile coupling (XEC) reaction, in which, beyond 3° alkyl bromides (for constructing arene-flanked quaternary carbons), 2° and 1° alkyl bromides are also demonstrated to be viable coupling partners. Moreover, this mild XEC displays excellent C(sp²)−I selectivity and functional group compatibility. The practicality of this XEC is demonstrated in simplifying the routes to several medicinally relevant and synthetically challenging compounds. Extensive experiments show that the terpyridine-ligated Ni^I halide can exclusively activate alkyl bromides, forming a Ni^I−alkyl complex through a Zn reduction. Attendant density functional theory (DFT) calculations reveal two different pathways for the oxidative addition of the Ni^I−alkyl complex to the C(sp²)−I bond of bromo(iodo)arenes, explaining both the high C(sp²)−I selectivity and generality of our XEC.     

Surface oxidation process of a diamond‐like carbon film analyzed by difference X‐ray photoelectron spectroscopy

Difference X‐ray photoelectron spectroscopy (D‐XPS) revealed the surface oxidation process of a diamond‐like carbon (DLC) film. Evaluation of surface functional groups on DLC solely by the C 1s spectrum is difficult because the spectrum is broad and has a secondary asymmetric lineshape. D‐XPS clarified the subtle but critical changes at the DLC surface caused by wet oxidation. The hydroxyl (C―OH) group was dominant at the oxidized surface. Further oxidized carbonyl (C?O) and carboxyl (including carboxylate) (COO) groups were also obtained; however, the oxidation of C?O to COO was suppressed to some extent because the reaction required C―C bond cleavage. Wet oxidation cleaved the aliphatic hydrogenated and non‐hydrogenated sp² carbon bonds (C―H sp² and C―C sp²) to create a pair of C―OH and hydrogenated sp³ carbon (C―H sp³) bonds. The reaction yield for C―H sp² was superior at the surface, suggesting that the DLC film was hydrogen rich at the surface. Oxidation of aromatic sp² rings or polycyclic aromatic hydrocarbons such as nanographite to phenols did not occur because of their resonance stabilization with electron delocalization. Non‐hydrogenated sp³ carbon (C―C sp³) bonds were not affected by oxidation, suggesting that these bonds are chemically inert. Copyright © 2014 John Wiley & Sons, Ltd.     

Enantioconvergent Reductive C(sp)−C(sp3) Cross-Coupling to Access Chiral α-Alkynyl Phosphonates Under Dual Nickel/Photoredox Catalysis

Transition-metal-catalyzed asymmetric carbon−carbon bond formation to forge phosphonates with an α-chiral carbon center through C(sp³)−C(sp³) and C(sp²)−C(sp³) couplings has been successful. However, the enantioselective C(sp)−C(sp³) coupling has not yet been disclosed. Reported herein is an unprecedented enantioconvergent cross-coupling of alkynyl bromides and α-bromo phosphonates to deliver chiral α-alkynyl phosphonates.     

Nichtplanare Doppelbindung in einem kristallinen Derivat von anti-Sesquinorbornen (anti-Tetracyclo[6.2.1.13,6.02,7]dodec-2(7)-en)

Non-Planar Double Bond in a Crystalline Derivative of anti-Sesquinorbornene (anti-Tetracyclo[6.2.1.1^3,6.0^2,7]dodec-2(7)-ene) The olefinic double bond of the title compound 2 has been observed by X-ray techniques to deviate from planarity by 13.2 (0.3)° (symmetric out-of-plane bending). Empirical force field calculations for the parent hydrocarbon 1 (anti-sesquinorbornene) give a much larger corresponding deviation of 35.6°. The calculated nonplanar double bond deformations in anti-sesquinorbornene 1 as well as in the syn-isomer 4 and other related olefins are a consequence of substantial widenings of the exocyclic C(sp³)–C(sp²–Csp³) angles when keeping the double bond planar. The considerable computational exaggeration of this novel out-of-plane bending mechanism is attributed to various possible force field deficiencies.     

Rhodium(I)‐Catalyzed Sequential C(sp)C(sp3) and C(sp3)C(sp3) Bond Formation through Migratory Carbene Insertion

A Rh^I‐catalyzed three‐component reaction of tert‐propargyl alcohol, diazoester, and alkyl halide has been developed. This reaction can be considered as a carbene‐involving sequential alkyl and alkynyl coupling, in which C(sp)? C(sp³) and C(sp³)? C(sp³) bonds are built successively on the carbenic carbon atom. The Rh^I‐carbene migratory insertion of an alkynyl moiety and subsequent alkylation are proposed to account for the two separate C? C bond formations. This reaction provides an efficient and tunable method for the construction of all‐carbon quaternary center.     

Rhodium(I)‐Catalyzed Sequential C(sp)?C(sp3) and C(sp3)?C(sp3) Bond Formation through Migratory Carbene Insertion

A Rh^I‐catalyzed three‐component reaction of tert‐propargyl alcohol, diazoester, and alkyl halide has been developed. This reaction can be considered as a carbene‐involving sequential alkyl and alkynyl coupling, in which C(sp) C(sp³) and C(sp³) C(sp³) bonds are built successively on the carbenic carbon atom. The Rh^I‐carbene migratory insertion of an alkynyl moiety and subsequent alkylation are proposed to account for the two separate C C bond formations. This reaction provides an efficient and tunable method for the construction of all‐carbon quaternary center.     

Alkyl Transition Metal Compounds IX. Synthesis and structure of [(Me3SiCH2)2Cr(bipy)2] I

Bis(trimethylsilylmethyyl)-bis-(2,2′-bipyridyl)chromium(III) iodide has been synthesized and its structure determined; the Cr-C(sp³) bond length is 2.107(9) Å.     

Intramolecular Metal‐Free Oxidative Aryl–Aryl Coupling: An Unusual Hypervalent‐Iodine‐Mediated Rearrangement of 2‐Substituted N‐Phenylbenzamides

Hypervalent‐iodine‐mediated oxidative coupling of the two aryl groups in either 2‐acylamino‐N‐phenyl‐benzamides or 2‐hydroxy‐N‐phenylbenzamides, with concomitant insertion of the ortho‐substituted N or O atom into the tether, has been described for the first time. This unusual metal‐free rearrangement reaction involves an oxidative C(sp²)? C(sp²) aryl–aryl bond formation, cleavage of a C(sp²)? C(O) bond, and a lactamization/lactonization. Furthermore, unsymmetrical diaryl compounds can be easily obtained by removing the tether within the cyclized product.     

Copper‐Catalyzed Three‐Component Carboazidation of Alkenes with Acetonitrile and Sodium Azide

A copper‐catalyzed three‐component reaction of alkenes, acetonitrile, and sodium azide afforded γ‐azido alkyl nitriles by formation of one C(sp³)−C(sp³) bond and one C(sp³)−N bond. The transformation allows concomitant introduction of two highly versatile groups (CN and N₃) across the double bond. A sequence involving the copper‐mediated generation of a cyanomethyl radical and its subsequent addition to an alkene, and a C(sp³)−N bond formation accounted for the reaction outcome. The resulting γ‐azido alkyl nitrile can be easily converted into 1,4‐diamines, γ‐amino nitriles, γ‐azido esters, and γ‐lactams of significant synthetic value.     

Visible Light-Enabled sp3-C−H Functionalization with Chloro- and Bromoalkynes: Chemoselective Route to Vinylchlorides or Alkynes

An unprecedented direct atom-economic chemo- and regioselective hydroalkylation of chloroalkynes and an sp³-C−H alkynylation of bromoalkynes was achieved. The reaction partners are unfunctionalized ethers, alcohols, amides, and even non-activated hydrocarbons. We found that a household fluorescent bulb was able to excite a diaryl ketone, which then selectively abstracts a H-atom from an sp³-C−H bond. The product of a formal alkyne insertion into the sp³-C−H bond was obtained with chloroalkynes, providing valuable vinyl chlorides. The photo-organocatalytic hydrogen atom transfer strategy gives rise to a broad range of diversely functionalized olefins. When bromoalkynes are applied in the presence of a base, a chemoselectivity switch to an alkynylation is observed. This reaction can even be performed for the alkynylation of unactivated sp³-C−H bonds, in this case with a preference of the more substituted carbon. Accompanying quantum chemical calculations indicate a vinyl radical intermediate with pronounced linear coordination of the carbon radical center, thus enabling the formation of both diastereoisomers after H-atom abstraction, suggesting that the (Z)-diastereoisomer is preferred, which supports the experimentally observed (E/Z)-distribution.     

Construction of C(sp2)−C(sp3) Bond between Quinoxalin‐2(1H)‐ones and N‐Hydroxyphthalimide Esters via Photocatalytic Decarboxylative Coupling

A novel visible‐light‐driven decarboxylative coupling of alkyl N‐hydroxyphthalimide esters (NHP esters) with quinoxalin‐2(1H)‐ones has been developed. This C(sp²)?C(sp³) bond‐forming transformation exhibits excellent substrate generality with respect to both the coupling partners. Of note, a series of 3‐primary alkyl‐substituted quinoxalin‐2(1H)‐ones that were difficult to synthesize by previous methods could be obtained in moderate to excellent yields. Additionally, the mild conditions, easy availability of substrates, wide functional group tolerance and operational simplicity make this protocol practical in the synthesis of 3‐alkylated quinoxalin‐2(1H)‐ones.     

Fe-Catalyzed Intramolecular Cross-Dehydrogenative Arylation (CDA), Efficient Synthesis of 1-Arylnaphthalenes and 4-Arylcoumarins

Direct cross-dehydrogenative coupling of different inert C−H bonds is the most straightforward and environmentally benign method to construct C−C bonds. In this paper, we developed an iron-catalyzed intramolecular cross-dehydrogenative arylation (CDA) between benzylic C(sp³)H bond and aromatic C(sp²)H bond. From the readily available linear substrates, 1-arylnaphthalenes and 4-arylcoumarins can be quickly constructed with moderate to good yield (18 examples, up to 73 % yield) in one step. Both symmetrical and unsymmetrical substrates with different functional groups could tolerate this system well to form the anticipated products. A radical initiated dehydrogenative cyclization-dehydrogenation tandem process was proposed.