Electronic influences on 3J(C,H) coupling constants via -S-, -S(O)- and -SO2-: their determination, calculation and comparison of detection methods

3J(C,H) coupling constants via a sulfur atom in two series of compounds, both including a sulfide, a sulfoxide and a sulfone, were detected experimentally and calculated by quantum mechanical methods. In the first series (1-3) the coupling between a hydrogen, bonded to an sp3 carbon, and an sp2 carbon is treated; the second series (4-6) deals with the coupling between a hydrogen, bonded to an sp3 carbon, and an sp3 carbon. Different pulse sequences (broadband HMBC, SelJres, 1D HSQMBC, J-HMBC-2, selective J-resolved long-range experiment and IMPEACH-MBC) proved to be useful in determining the long-range 3J(C,H) coupling constants. However, the dynamic behaviour of two of the compounds (4 and 6) led to weighted averages of the two coupling constants expected (concerning equatorial and axial positions of the corresponding hydrogens). DFT calculations proved to be useful to calculate not only the 3J(C,H) coupling constants but also the different contributions of FC, PSO, DSO and SD terms; the calculation of the Fermi contact term (FC) was found to be sufficient for the correct estimation of 3J(C,H) coupling constants.     

表面偶联甲基自由基实现低温高效甲烷氧化偶联

天然气作为一种低碳清洁能源,其储量大,价格低,被认为是最有前途的石油替代资源之一.而以天然气的主要成分——甲烷为原料来生产高价值化学品被认为是石化工业中实现天然气取代石油为原料新化工路线的技术基础,具有极为可观的社会经济价值.目前甲烷的化学利用主要采用间接转化法,即先从甲烷制合成气,再由合成气制备各种化工原料和油品.但...     

Configurational assignment in alkyl‐branched sugars via the geminal C,H coupling constants

The measurement of the magnitude and sign of ²J(C,H) couplings offers a reliable way to determine the absolute configuration at a carbon center in a fixed cyclic system. A decrease of the dihedral angle ? in the O—C_A—C_B—H fragment always leads to a change of the ²J(C_A,H_B) coupling to more negative values, independent of the type and position of substituents at the two carbon centers. The orientations of the two substituents at C‐3 of the epimeric pair 1 and 2 were determined unambiguously through the measurement of the geminal coupling constants between C‐3 and the hydrogen atoms at C‐2 and C‐4. In particular, ²J(C‐3,H‐2ax) with ?1.5 Hz, ? = 174° in 1 and ?6.6 Hz, ? = 47° in 2 , and ²J(C‐3,H‐4) with +1.5 Hz, ? = 175° in 1 and ?4.7 Hz, ? = 49° in 2 showed the greatest differences between the two epimers. Both couplings therefore allow the determination of the absolute configuration at C‐3. It should be noted, however, that the size of the coupling constants can be different for dihedral angles of nearly identical size, when there are different numbers of electronegative substituents on the two coupling pathways, i.e. no O‐substituent at C‐2, but one axial O‐substituent at C‐4. It becomes clear that it is not sufficient to measure the magnitude of ²J coupling constants only, but that the sign of the geminal coupling is needed to identify the absolute configuration at a chiral center. The coupling of C‐3 with H‐2eq is not useful for the determination of the configuration at C‐3, as the similarity of the dihedral angles ? (O—C‐3—C‐2—H‐2eq) (57° in 1 and 70° in 2 ) leads to identical coupling constants (?6.1 Hz) for both epimers. Copyright © 2003 John Wiley & Sons, Ltd.     

Direct Acylation of C(sp3)−H Bonds Enabled by Nickel and Photoredox Catalysis

Using nickel and photoredox catalysis, the direct functionalization of C(sp³)?H bonds of N‐aryl amines by acyl electrophiles is described. The method affords a diverse range of α‐amino ketones at room temperature and is amenable to late‐stage coupling of complex and biologically relevant groups. C(sp³)?H activation occurs by photoredox‐mediated oxidation to generate α‐amino radicals which are intercepted by nickel in catalytic C(sp³)?C coupling. The merger of these two modes of catalysis leverages nickel's unique properties in alkyl cross‐coupling while avoiding limitations commonly associated with transition‐metal‐mediated C(sp³)?H activation, including requirements for chelating directing groups and high reaction temperatures.     

Higher order (A+E) multiply sign in circle e pseudo-Jahn-Teller coupling

The pseudo-Jahn-Teller (PJT) coupling of a nondegenerate state A with a twofold degenerate state E by a degenerate vibrational mode e is studied for a general system with a C(3) main rotational axis. The PJT coupling terms up to sixth order are derived by symmetry considerations for this general (A+E) multiply sign in circle e case. The obtained expression for the 3 x 3 diabatic potential energy matrix is found to be closely related to the expression recently developed for the higher order Jahn-Teller case [A. Viel and W. Eisfeld, J. Chem. Phys. 120, 4603 (2004)]. The dynamical PJT coupling, which can arise for states of appropriate symmetry if one of the vibrational modes induces a change of the nuclear point group between D(3h), C(3v), C(3h), and C(3), is discussed. The effect of the higher order PJT coupling is tested by a two-dimensional model study based on the e bending mode of NH(3)(+). The models are analyzed by fitting the two-dimensional potential energy surfaces. The significance of the higher order terms on the nonadiabatic dynamics is demonstrated by quantum wave packet propagations.     

NMR analysis of conformationally dependent (n)J(C, H) and (n)J(C, C) in the trisaccharide α-L-Rhap-(1 → 2)[α-L-Rhap-(1 → 3)]-α-L-Rhap-OMe and a site-specifically labeled isotopologue thereof

An array of NMR spectroscopy experiments have been carried out to obtain conformationally dependent (1)H,(13)C- and (13)C,(13)C-spin-spin coupling constants in the trisaccharide α-L-Rhap-(1 → 2)[α-L-Rhap-(1 → 3)]-α-L-Rhap-OMe. The trisaccharide was synthesized with (13)C site-specific labeling at C2' and C2″, i.e. in the rhamnosyl groups in order to alleviate (1)H spectral overlap. This facilitated the measurement of a key trans-glycosidic proton-proton cross-relaxation rate using 1D (1)H,(1)H-T-ROESY experiments as well as a (3)J(C, H) coupling employing 1D (1)H,(13)C-long-range experiments, devoid of potential interference from additional J coupling. By means of both the natural abundance compound and the (13)C-labeled sample 2D (1)H,(13)C-J-HMBC and (1)H,(13)C-HSQC-HECADE NMR experiments, total line-shape analysis of (1)H NMR spectra and 1D (13)C NMR experiments were employed to extract (3)J(C, H) , (2)J(C, H), (3)J(C, C), and (1)J(C, C) coupling constants. The (13)C site-specific labeling facilitates straightforward determination of (n)J(C, C) as the splitting of the (13)C natural abundance resonances. This study resulted in eight conformationally dependent coupling constants for the trisaccharide and illustrates the use of (13)C site-specific labeling as a valuable approach that extends the 1D and 2D NMR methods in current use to attain both hetero- and homonuclear spin-spin coupling constants that subsequently can be utilized for conformational analysis.     

Insights into long-range structural effects on the stereochemistry of aldol condensations: a practical total synthesis of desoxyepothilone F.

A processable total synthesis of a potent antitumor agent, desoxyepothilone F (dEpoF, 21-hydroxy-12,13-desoxyepothilone B, 21-hydroxyepothilone D), has been accomplished. The route is highly convergent. The new technology has also been applied to a total synthesis of 12,13-desoxyepothilone (dEpoB). The crucial point of departure from previous syntheses of dEpoB and dEpoF involves presentation of the C1-C11 sector for Suzuki coupling with C3 in reduced form. Hitherto, the required S stereochemistry at C3 had been implemented via reduction of a keto function after Suzuki coupling. Whereas that chemistry worked quite well in a synthesis of dEpoB, it was not transferable to a high-yielding synthesis of dEpoF. The reduction of the keto group at C3 via a Noyori protocol after Suzuki coupling had proved to be very difficult. In our current approach, two consecutive aldol reactions are used to fashion the acyl sector. In the first aldol condensation, C6 becomes attached to C7. Following protection at C7, a two-carbon acetate equivalent is used to join C2 and C3 with very high asymmetric induction at C3. Only after this center has been implemented is the Suzuki reaction conducted. This major advance allowed us to synthesize dEpoF in a straightforward fashion. These findings found ready application in the total synthesis of dEpoB. Another part of the study involved analysis of the factors associated with aldol condensations joining C6 to C7. In the work described herein, the consequences of the status of C3 in promoting the C6-C7 aldol coupling are probed in detail. Dramatic stereochemical long-range effects uncovered during the study are described, and a working model to explain these effects has emerged.     

13C13C spin coupling constants in phenanthrene derivatives

The synthesis of six 1-X- (X = OH, OCH₃, OCOCH₃, CH₃, CHO and CN) phenanthrene derivatives with a ¹³C label at C-1 is described. An analysis of the ¹³C¹³C spin coupling constants shows the importance of π-interaction for the coupling constant transmission. Small ¹³C¹³C spin coupling constants over 6 bonds are reported.     

The absolute sign of J coupling constants determined using the order matrix calculation

A novel methodology using the order matrix calculation to determine the absolute sign of spin-spin couplings based on the structure of organic compounds is presented. The sign of the residual dipolar coupling (RDC) depends on the sign of corresponding scalar spin-spin coupling constant and the sign of the RDC has a dramatic influence on the order matrix calculation. Therefore, the sign of the spin-spin coupling constant can be obtained by an order matrix calculation through the corresponding RDC. Six types of spin-spin coupling constants, including 2J(H,H), 1J(C,F), 2J(C,F), 3J(C,F), 2J(F,H) and 3J(F,H), were obtained simultaneously. Except for 3J(C,F) where the measured RDCs have very small magnitudes, the signs were determined unambiguously.     

NMR偶合常数的从头算级别自然杂化轨道研究

为了研究自然杂化轨道计算结果与核自旋偶合常数的相关性,本文进行了从头算级别的自然杂化轨道计算。采用STO－3G基组,利用Lowdin正交化原子轨道基组下的密度矩阵,得到了分子中各原子的杂化轨道、净电荷与^13C-H、^13C-^13C键自旋偶合常数^1Jcn和^1Jcc之间关系式。利用这些式子计算得到的结果与实验数据非常一致。     

铜试剂在有机合成中的应用研究进展

铜试剂催化的偶联反应是直接构建碳 碳键最有效的方法之一。本文综述了铜试剂催化的偶联反应构筑六种碳 碳键的方法;重点阐述了铜试剂催化构筑C(SP²)-C(SP²)的研究进展,含分子间偶联和分子内偶联。参考文献44篇。     

B(C6F5)3‐Catalyzed Tandem Friedel‐Crafts and C−H/C−O Coupling Reactions of Dialkylanilines

Tandem Friedel‐Crafts (FC) and C?H/C?O coupling reactions catalyzed by tris(pentafluorophenyl) borane (B(C₆F₅)₃) were achieved without using any other additive in the absence of solvent. This process can be used for the reactions between a series of dialkylanilines and vinyl ethers with good isolated yields of bis(4‐dialkylaminophenyl) compounds. Based on combined theoretical and experimental studies, the possible reaction mechanism was proposed. B(C₆F₅)₃ can activate the C=C and C?O bond for FC and C?H/C?O coupling reactions respectively. The FC reaction is slow, which is followed by a fast C?H/C?O coupling.     

Direct Sp3α-C-H activation and functionalization of alcohol and ether

Seven kinds of sp(3)α-C-H activation/C-C formation reactions of alcohols and ethers have been reviewed in this tutorial review, from the viewpoint of both methodology and synthetic application, towards the efficiency, chemo-, regio- and stereoselectivity, catalytic system, substrate scope and mechanistic study. Section 2 describes radical-mediated α-C-H activation and addition/elimination of ethers with unsaturated (C=C and C[triple bond]C) species. Sections 3-8 discuss the α-C-H activation and additions of alcohols and/or ethers with unsaturated (C=C, C[triple bond]C, C=O and C=N) compounds, which involve the key processes of radical mediation, carbenoid insertion, 1,5-H-migration, oxidative dehydrogenation coupling, transfer hydrogenative coupling, and metal-mediated C=C insertion into the C-H bond.     

Hyperfine coupling constants of the azafullerenes C19N, C59N, C69N, and C75N

The isomers of the nitrogen-substituted fullerenes (azafullerenes) C19N, C59N, C69N, and C75N are examined using all-electron Gaussian atomic orbital basis density functional theory, to determine the doublet radical geometries and hyperfine coupling constants. We find that the inaccuracy of previously calculated hyperfine coupling constants of C59N resulted from a poor treatment of the geometry optimization. We find that UB3LYP minimization of the radical geometry in the 6-31G basis, followed by single-point evaluation of the hyperfine constants in which an expanded basis is used on the atomic sites of interest, forms an efficient compromise between computational cost and accuracy with respect to experimental hyperfine constants. Using this approach, we assign the hyperfine signals observed in experiments on the C69N radical by calculating the hyperfine coupling constants for all five of the isomers and examine the electron spin density distribution. Finally, we present predicted hyperfine coupling constants for the isomers of C19N and C75N for use in the interpretation of future experiments.     

Preparation of fused β-lactams through Weinreb amide α-anions

Enantiomerically pure β-lactams bearing a Weinreb amide functionality at the C3 position have been shown to be excellent substrates for α-alkylation reactions, generating C3 quaternary centers with predictable absolute stereochemistry. In addition, oxidative coupling of C3/C4 dianions affords entry to fused β,γ-bislactam systems.     

Thermodynamic coupling of the loop and stem in unusually stable DNA hairpins closed by CG base pairs

For certain DNA hairpin loops, a CG closing base pair has enhanced stability over other closing base pairs, which cannot be explained by the current nearest-neighbor model. We report the use of three-carbon (C3) spacers to investigate the expandability of DNA hairpin loops and the coupling between the loop and closing base pair. Inserting the C3-spacers at most positions in these model loops produced only a modest stabilization or destabilization except for insertion between the 5' end of the loop and the CG closing base pair, which gave a large destabilization. Further investigation on tetraloops and triloops with other closing base pairs established that this destabilization is specific to the unusually stable CG closing base pair. Studies with the nucleotide analogues 2-aminopurine and 2,6-diaminopurine indicated that this stabilization may be due to coupling between functional groups on the first base of the loop and the CG closing base pair. The C3-spacers provide a simple way to interrupt potential interactions and thereby probe loop/stem coupling.     

Direct C(sp2)C(sp3) Cross‐Coupling of Diaryl Zinc Reagents with Benzylic,Primary, Secondary,and Tertiary Alkyl Halides

The direct C(sp²)? C(sp³) cross‐coupling of diaryl zinc reagents with benzylic, primary, secondary, and tertiary alkyl halides proceeded in the absence of coordinating ethereal solvents at ambient temperature without the addition of a catalyst. The C(sp²)? C(sp³) cross‐coupling showed excellent functional‐group tolerance, and products were isolated in high yields, generally without the requirement for purification by chromatography. This process represents an expedient, operationally simple method for the construction of new C(sp²)? C(sp³) bonds.     

Direct C(sp2)?C(sp3) Cross‐Coupling of Diaryl Zinc Reagents with Benzylic,Primary, Secondary,and Tertiary Alkyl Halides

The direct C(sp²) C(sp³) cross‐coupling of diaryl zinc reagents with benzylic, primary, secondary, and tertiary alkyl halides proceeded in the absence of coordinating ethereal solvents at ambient temperature without the addition of a catalyst. The C(sp²) C(sp³) cross‐coupling showed excellent functional‐group tolerance, and products were isolated in high yields, generally without the requirement for purification by chromatography. This process represents an expedient, operationally simple method for the construction of new C(sp²) C(sp³) bonds.     

Synergetic catalytic effect of rGO,Pd, Fe3O4 and PPy as a magnetically separable and recyclable nanocomposite for coupling reactions in green media

In this paper, rGO/Pd–Fe₃O₄@PPy as an efficient stable nanocomposite was synthesized. To understand the synergetic effects of rGO, Pd, Fe₃O₄ and PolyPyrrole, the performance of rGO/Pd–Fe₃O₄@PPy as a heterogeneous recyclable nanocatalyst in the green synthesis of C‐C and C‐O coupling products, as well as different conditions are studied. Synthesized rGO/Pd–Fe₃O₄@PPy was characterized by FT‐IR, XRD, FE‐SEM, EDS, TGA and AFM analysis. Best results are obtained under sonication in H₂O for C‐C coupling and by ball‐milling for C‐O coupling. The benefits of this method include: green solvents and conditions, absence of external base, low reaction times with high yield and easy work‐up method.     

磁性Fe_3O_4微粒表面有机改性

在分散聚合法制备复合磁性微球过程中 ,采用硅烷偶联剂 KH- 570对磁性 Fe_3O_4微粒进行表面改性 .红外光谱 (FTIR)、光电子能谱 (XPS)分析结果表明 ,偶联剂与磁性微粒表面以化学键形式结合 .改性后 ,Fe_3O_4微粒与单体及其聚合物之间具有良好的亲和性 ,采用改性后的磁性微粒可以显著改善磁性微球的性能指标 .