J-modulated ADEQUATE experiments using different kinds of refocusing pulses

Owing to the recent developments concerning residual dipolar couplings (RDCs), the interest in methods for the accurate determination of coupling constants is renascenting. We intended to use the J-modulated ADEQUATE experiment by K?vér et al. for the measurement of (13)C - (13)C coupling constants at natural abundance. The use of adiabatic composite chirp pulses instead of the conventional 180 degrees pulses, which compensate for the offset dependence of (13)C 180 degrees pulses, led to irregularities of the line shapes in the indirect dimension causing deviations of the extracted coupling constants. This behaviour was attributed to coupling evolution, during the time of the adiabatic pulse (2 ms), in the J-modulation spin echo. The replacement of this pulse by different kinds of refocusing pulses indicated that a pair of BIPs (broadband inversion pulses), which behave only partially adiabatic, leads to correct line shapes and coupling constants conserving the good sensitivity obtained with adiabatic pulses.     

Application of unsymmetrical indirect covariance NMR methods to the computation of the (13)C <--> (15)N HSQC-IMPEACH and (13)C <--> (15)N HMBC-IMPEACH correlation spectra

Utilization of long-range (1)H--(15)N heteronuclear chemical shift correlation has continually grown in importance since the first applications were reported in 1995. More recently, indirect covariance NMR methods have been introduced followed by the development of unsymmetrical indirect covariance processing methods. The latter technique has been shown to allow the calculation of hyphenated 2D NMR data matrices from more readily acquired nonhyphenated 2D NMR spectra. We recently reported the use of unsymmetrical indirect covariance processing to combine (1)H--(13)C GHSQC and (1)H--(15)N GHMBC long-range spectra to yield a (13)C--(15)N HSQC-HMBC chemical shift correlation spectrum that could not be acquired in a reasonable period of time without resorting to (15)N-labeled molecules. We now report the unsymmetrical indirect covariance processing of (1)H--(13)C GHMBC and (1)H--(15)N IMPEACH spectra to afford a (13)C--(15)N HMBC-IMPEACH spectrum that has the potential to span as many as six to eight bonds. Correlations for carbon resonances long-range coupled to a protonated carbon in the (1)H--(13)C HMBC spectrum are transferred via the long-range (1)H--(15)N coupling pathway in the (1)H--(15)N IMPEACH spectrum to afford a much broader range of correlation possibilities in the (13)C--(15)N HMBC-IMPEACH correlation spectrum. The indole alkaloid vincamine is used as a model compound to illustrate the application of the method.     

Carbon-13 NMR of fluorocyclopropanes

Carbon-13 and hydrogen NMR data are presented for several cyclopropanes with one to four fluorine atoms attached to the ring. Substituents directly attached to a ring carbon are found to unshield that carbon to an extent increasing with their electronegativity, but remote substituents may have an opposite effect. Relative orientation of the substituent groups, especially if they are bulky, also has a strong influence on the ring-carbon shifts. Chemical shifts of carbons in methyl groups attached to the ring change to an extent which is additive in the effects of gem and cis substituents. One-bond coupling of ring carbons with fluorines is affected both by geminal and vicinal substituents, in a way generally related to their electronegativity. It is tentatively concluded that two-bond coupling between ring carbons and ring-attached fluorines depends in magnitude primarily on what groups are cis to the fluorine. Two-bond coupling between a carbon atom in a ring-attached methyl and the hydrogen gem to the methyl appears to be vanishingly small, but there is appreciable three-bond coupling of the methyl carbons to vicinal hydrogens.     

Regioselective Borylation of Porphyrins by C?H Bond Activation under Iridium Catalysis to Afford Useful Building Blocks for Porphyrin Assemblies

Highly regioselective and efficient borylation of a variety of porphyrins has been achieved by reaction with bis(pinacolato)diboron through C? H bond activation under iridium catalysis on the basis of the synthetic protocol developed by Miyaura, Hartwig, and Smith. A boryl group can be selectively introduced at sterically uncongested positions in the peripheral aryl groups of porphyrin substrates whose peripheral β‐positions are sterically hindered. Curiously, β substituents adjacent to the aryl group to be borylated have unexpectedly large effects on the regioselectivity, because the iridium catalyst can discriminate between subtle steric differences. Chemoselective borylation was also achieved for several functionalized porphyrins. This borylation protocol can be applied to various monomeric and oligomeric functional porphyrins, hence offering an efficient route to elaborate multiporphyrin‐based molecular constructs.     

Post‐Modification of meso–meso‐Linked Porphyrin Arrays by Iridium and Rhodium Catalyses for Tuning of Energy Gap

Skillfully attached! meso–meso‐Linked diporphyrins can be efficiently and selectively functionalized with multiple unsaturated carboxylic acid groups through iridium and rhodium catalyses. This post‐modification strategy allows fine‐tuning of energy levels of each porphyrin unit.

     

Mechanistic Insights into the One‐Pot Synthesis of Propargylamines from Terminal Alkynes and Amines in Chlorinated Solvents Catalyzed by Gold Compounds and Nanoparticles

Propargylamines can be obtained from secondary amines and terminal alkynes in chlorinated solvents by a three‐ and two‐component synthesis catalyzed by gold compounds and nanoparticles (Au‐NP) under mild conditions. The use of dichloromethane allows for the activation of two C? Cl bonds and a clean transfer of the methylene fragment to the final product. The scope of the reaction as well as the influence of different gold(III) cycloaurated complexes and salts has been investigated. The involvement of gold nanoparticles generated in situ in the process is discussed and a plausible reaction mechanism is proposed on the basis of the data obtained.