β,β‐Diborylated Subporphyrinato Boron(III) Complexes as Useful Synthetic Precursors

Iridium‐catalyzed borylation of B‐aryl meso‐free subporphyrinato boron(III) complexes (hereinafter referred to simply as subporphyrins) with bis(pinacolato)diboron gave 2,13‐diborylated subporphyrins regioselectively, which served as promising synthetic precursors for 2,13‐diarylated subporphyrins and doubly β‐to‐β 1,3‐butadiyne‐bridged subporphyrin dimers. 2,13‐Diarylated subporphyrins display perturbed absorption spectra, depending upon the β‐aryl substituents. Doubly 1,3‐butadiyne‐bridged syn and anti subporphyrin dimers thus prepared exhibit differently altered absorption spectra with split Soret‐like bands, which have been accounted for in terms of exciton coupling.     

Modular Synthesis of Phenanthro[9,10‐c]thiophenes by a Sequence of CH Activation,Suzuki Cross‐Coupling and Photocyclization Reactions

A total number of 15 different 3,4‐diarylthiophenes were synthesized, which bear a chlorine atom in ortho‐position of one of the aryl substituents. One aryl group was introduced by an oxidative cross‐coupling reaction, involving a C?H activation at C4(3) of the thiophene core. The other aryl group was in most cases introduced by a Suzuki cross‐coupling reaction, which succeeded the oxidative cross‐coupling step. Photocyclization reactions of the 3,4‐diarylthiophenes were performed in a solvent mixture of benzene and acetonitrile (50:50 v/v) at λ=254 nm and proceeded to the title compounds in yields of 60–82 %. The selectivity of the photocyclization was determined at the ortho‐chloro‐substituted aryl ring by the position of the chlorine substituent. At the other ring, a single regioisomer was observed for phenyl and para‐substituted phenyl groups. For 2‐naphthyl and ortho‐substituted phenyl rings a clear preference was observed in favor of a major regioisomer, while meta‐substitution in the phenyl ring led to a about 1:1 mixture of 5‐ and 7‐substituted phenanthro[9,10‐c]thiophenes. Mechanistically, the photocyclization is likely to occur as a photochemically allowed, conrotatory [(4n+2)π] process accompanied by elimination of HCl. It was shown for two phenanthro[9,10‐c]thiophene products that they can be readily brominated in positions C1 and C3 (74–77 %), which in turn allows for further functionalization at these positions, for example, in the course of halogen–metal exchange and polymerization reactions.     

Synthesis and Photochemistry of syn- and anti-9,10-Epoxy-1, 4-dihydro-1,4-ethanonaphthalene-2,3-diones

The title epoxydiketones were prepared stereoselectively, direct epoxidation of 3a-c with MCPBA produced syn isomers la-c whereas epoxidation of 4a-d followed by saponification of the spirolactone rings gave anti isomers 2a-c. The stereochemistry of 1a and 2a was established by X-ray diffraction, whereas that of the remaining epoxydiketones was determined from the correlation of visible, ¹H, and ¹³C NMR spectral data; the differences between spectra of the corresponding syn and anti isomers are explained in terms of through-space interaction and steric effects. Photolysis of syn isomers 1a-c afforded the corresponding naphthalenes 8a-c in almost quantitative yields; in contrast, irradiation of anti isomers 2a-c gave complicated mixtures. The quantum yield of disappearance of 1a was 52 times that of 2a. Reaction mechanisms are proposed to account for the product formation. The differences in the photochemical behavior of the title syn and anti isomers are rationalized in terms of stereoelectronic effects of the epoxy rings in the syn isomers.     

Carbon-13 NMR analysis of cyclobutane dimers from benzocycloalkenes

¹³C NMR Spectra for a series of 11 substituted cyclobutanes derived from photodimerization of benzocycloalkenes were recorded. Comparison of the carbon chemical shifts for the head-to-head and head-to-tail cis-syn-cis and cis-anti-cis isomers reveals shielding trends which should facilitate structural and stereochemical assignments for related compounds. The head-to-head isomers show a larger separation of cyclobutane carbon resonances than the head-to-tail isomers. The cis-syn-cis isomers relative to the cis-anti-cis isomers exhibit distinctive upfield shifts of all carbon resonances, except those of aromatic carbons ortho to alkyl-substituted aromatic carbons.     

Peripheral Hexabromination,Hexaphenylation, and Hexaethynylation of meso‐Aryl‐Substituted Subporphyrins

Effective peripheral fabrication methods of meso‐aryl‐substituted subporphyrins were explored for the first time. Hexabrominated subporphyrins 2 were prepared quantitatively from the bromination of subporphyrins 1 with bromine. Hexaphenylated subporphyrins 3 and hexaethynylated subporphyrins 4 and 5 were synthesized by Suzuki–Miyaura coupling and Stille coupling, respectively, in good yields. X‐ray crystal structures of 2 b , 3 b , 4 b , and 5 a revealed preservation of the bowl‐shaped bent structures with bowl depths similar to that of 1 . Hexaethynylated subporphyrins exhibit large two‐photon‐absorption cross‐sections due to effective delocalization of the conjugated network to the ethynyl substituents.     

Polymorphism in bipodal O,O′‐dimeth­yl N,N′‐(m‐phenyl­ene­di­carbonyl)bis(thio­carbamate)

The title compound, C₁₂H₁₂N₂O₄S₂, crystallizes in white and yellow polymeric forms as a result of inter­esting anti–anti and syn–anti conformational isomerism of the thio­carbon­yl and carbon­yl moieties relative to one another. This work is the first reported X‐ray crystallographic structure determination of isomers of this class of bipodal ligand. The white form, anti–anti, (I), crystallizes with the benzene ring lying about a twofold rotation axis, resulting in both of the thio­carbon­yl and carbon­yl moieties being anti relative to each other. The yellow modification crystallizes as syn–anti, (II), with one thio­carbon­yl moiety syn and the other anti relative to the respective carbon­yl groups. The individual mol­ecules of both (I) and (II) are extensively linked through inter­molecular hydrogen bonds. Inter­molecular hydrogen bonding in (II) includes a network of bifurcated N—H⋯O and N—H⋯S hydrogen bonds, while mol­ecules of (I) include bifurcated C—H⋯O hydrogen bonds.     

Influence of Catalyst Structure and Reaction Conditions on anti‐ versus syn‐Aminopalladation Pathways in Pd‐Catalyzed Alkene Carboamination Reactions of N‐Allylsulfamides

The Pd‐catalyzed coupling of N‐allylsulfamides with aryl and alkenyl triflates to afford cyclic sulfamide products is described. In contrast to other known Pd‐catalyzed alkene carboamination reactions, these transformations may be selectively induced to occur by way of either anti‐ or syn‐aminopalladation mechanistic pathways by modifying the catalyst structure and reaction conditions.     

Crystal structures of anti- and syn-9,10-di(1′-naphthyl)anthracene and isomerization in solid state

9,10-Di-(1′-naphthyl)anthracene is often used as electroluminescence materials in organic light-emitting diodes. Because of the hindered rotation about the σ-bond between naphthyl and anthracene chromophore, two possible stereoisomers can be isolated. HPLC, ¹H NMR, and ¹³C NMR spectra gave two different sets of peaks and the X-ray single crystal analysis confirmed the structures of the two isomers, anti and syn. syn was more soluble than anti in THF as well as toluene and the thermal properties of the two were quite different. Differential scanning calorimetry study and HPLC analysis showed that the isomerization between anti and syn in the solid state took place at >370 °C.     

Dipyridyl/pyridinium thieno[2,3-b]thiophenes as new atropisomeric systems. Synthesis, conformational analysis and energy minimization

Synthesis of a new class of cofacially oriented dipyridyl(pyridinium)lthieno[2,3-b]thiophenes with or without -CO₂Et and -COMe substituents at C2, and C5 positions of thieno[2,3-b]thiophene ring was readily accomplished using a double Dieckman cyclization protocol as the key step. While C2/C5 substituted dipyridylthieno[2,3-b]thiophenes exhibited syn/anti atropisomerism at least up to 70 °C with Arrhenius energy of activation (ΔG^≠) in the range of 17-18 kcal/mol, on the other hand unsubstituted dipyridylthieno[2,3-b]thiophene and its bis-N-quaternized salt were found to show free conformational rotation with an estimated ΔG^≠ of lower than 10 kcal/mol. Conformational energy minimization using AM1 protocol revealed a slight preference for the anti over syn isomers. Compared to the unsubstituted dipyridylthieno[2,3-b]thiophenes, higher energy barriers to rotation (3.7-5.1 kcal/mol) in substituted dipyridylthieno[2,3-b]thiophenes can be attributed to steric encumbrance resulting from -CO₂Et and -COMe substituents located on the non-rotating thienothiophene platform.     

DFT study of core‐modified porphyrin isomers

B3LYP/6‐311+G** calculations were performed systematically on 1,2 (syn) and 1,3 (anti) tautomeric forms of oxa‐ and thia‐ core‐modified porphyrin isomers, which resulted in a total of 86 structures. The structural and energetic variation in all the isomers were analyzed. In corrphycene, hemiporphycene and porphycene the Z forms are more stable compared to the corresponding E forms in both the anti and syn oxa‐ and thiaporphyrin isomers. In contrast, in the syn isomeric forms of [3.0.1.0], [3.1.0.0] and [4.0.0.0] oxaporphyrins and in both syn and anti forms of thiaporphyrin isomers, Z forms are less stable. The HOMO and LUMO values are both negative and varied in a narrow zone, indicating no dramatic effect on the position of heteroatom substitution on the redox properties. The effect of geometric constraints due to the alteration of meso‐bridge length and the hetero atom disposition in the porphyrin core on the relative stabilities of the isomers is analyzed. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Development of a syn‐Selective Mannich Reaction of Aldehydes with Propargylic Imines by Dual Catalysis: Asymmetric Synthesis of Functionalized Propargylic Amines

Direct coupling of enolizable aldehydes with C‐alkynyl imines is realized affording the corresponding propargylic Mannich adducts of syn configuration, thus complementing previous methods that gave access to the anti‐isomers. The combination of proline and a urea Brønsted base cocatalyst is key for the reactions to proceed under very mild conditions (3–10 mol % catalyst loading, dichloromethane as solvent, ?20 °C, 1.2 molar equivalents of aldehyde) and with virtually total stereocontrol (syn/anti ratio up to 99:1; ee up to 99 %). Some possibilities of further chemical elaboration of adducts are also briefly illustrated.     

Cyclotrimerization of ‘Oxabenzonorbornadiene’: Synthesis of syn‐ and anti‐5,6,11,12,17,18‐Hexahydro‐5,18:6,11:12,17‐triepoxytrinaphthylene

An efficient synthetic route to the concave‐shaped, potentially ionophoric syn‐ and anti‐isomers of 5,6,11,12,17,18‐hexahydro‐5,18:6,11:12,17‐triepoxytrinaphthylene ( 4 ) was elaborated. Starting from ‘oxabenzonorbornadiene’ ( 5 ), the stannylated precursor 9 was prepared in three steps, followed by cyclotrimerization catalyzed by copper(I) thiophene‐2‐carboxylate (CuTC) , which afforded 4 in a syn/anti ratio of 5 : 4.     

New 2,6‐Distyryl‐Substituted BODIPY Isomers: Synthesis,Photophysical Properties,and Theoretical Calculations

A 2,6‐distyryl‐substituted boradiazaindacene (BODIPY) dye and a new series of 2,6‐p‐dimethylaminostyrene isomers containing both α‐ and β‐position styryl substituents were synthesized by reacting styrene and p‐dimethylaminostyrene with an electron‐rich diiodo‐BODIPY. The dyes were characterized by X‐ray crystallography and NMR spectroscopy and their photophysical properties were investigated and analyzed by carrying out a series of theoretical calculations. The absorption spectra contain markedly redshifted absorbance bands due to conjugation between the styryl moieties and the main BODIPY fluorophore. Very low fluorescence quantum yields and significant Stokes shifts are observed for 2,6‐distyryl‐substituted BODIPYs, relative to analogous 3,5‐distyryl‐ and 1,7‐distyryl‐substituted BODIPYs. Although the fluorescence of the compound with β‐position styryl substituents on both pyrrole moieties and one with both β‐ and α‐position substituents was completely quenched, the compound with only α‐position substituents exhibits weak emission in polar solvents, but moderately intense emission with a quantum yield of 0.49 in hexane. Protonation studies have demonstrated that these 2,6‐p‐dimethylaminostyrene isomers can be used as sensors for changes in pH. Theoretical calculations provide strong evidence that styryl rotation and the formation of non‐emissive charge‐separated S₁ states play a pivotal role in shaping the fluorescence properties of these dyes. Molecular orbital theory is used as a conceptual framework to describe the electronic structures of the BODIPY core and an analysis of the angular nodal patterns provides a reasonable explanation for why the introduction of substituents at different positions on the BODIPY core has markedly differing effects.     

Probing the Rotational Dynamics of meso‐(2‐Substituted)aryl Substituents in A2B‐Type Subporphyrins

A₂B‐type B‐methoxy subporphyrins 3 a – g and B‐phenyl subporphyrins 7 a – c , e , g bearing meso‐(2‐substituted)aryl substituents are synthesized, and their rotational dynamics are examined through variable‐temperature (VT) ¹H NMR spectroscopy. In these subporphyrins, the rotation of meso‐aryl substituents is hindered by a rationally installed 2‐substituent. The rotational barriers determined are considerably smaller than those reported previously for porphyrins. Comparison of the rotation activation parameters reveals a variable contribution of ΔH^≠ and ΔS^≠ in ΔG^≠. 2‐Methyl and 2‐ethyl groups of the meso‐aryl substituents in subporphyrins 3 e , 3 f , and 7 e induce larger rotational barriers than 2‐alkoxyl substituents. The rotational barriers of 3 g and 7 g are reduced by the presence of the 4‐dibenzylamino group owing to its ability to stabilize the coplanar rotation transition state electronically. The smaller rotational barriers found for B‐phenyl subporphyrins than for B‐methoxy subporphyrins indicate a negligible contribution of S_N1‐type heterolysis in the rotation of meso‐aryl substituents.     

Stereodivergent Organocatalytic Intramolecular Michael Addition/Lactonization for the Asymmetric Synthesis of Substituted Dihydrobenzofurans and Tetrahydrofurans

A stereodivergent asymmetric Lewis base catalyzed Michael addition/lactonization of enone acids into substituted dihydrobenzofuran and tetrahydrofuran derivatives is reported. Commercially available (S)‐(?)‐tetramisole hydrochloride gives products with high syn diastereoselectivity in excellent enantioselectivity (up to 99:1 d.r._syn/anti, 99 % ee_syn), whereas using a cinchona alkaloid derived catalyst gives the corresponding anti‐diastereoisomers as the major product (up to 10:90 d.r._syn/anti, 99 % ee_anti).     

Factors that regulate the conformation of m-benziporphodimethene complexes: agostic metal-arene interaction, hydrogen bonding, and η2,π coordination

Group 12 and silver(I) tetramethyl‐m‐benziporphodimethene (TMBPDM) complexes with phenyl, methylbenzoate, or nitrophenyl groups as meso substituents were synthesized and fully characterized. The dimeric silver(I) complex displays an unusual η²,π coordination from the β‐pyrrolic C?C bond to the silver ion. All of the complexes displayed a close contact between the metal ion and the inner C(22)? H(22) on the m‐phenylene ring. The downfield chemical shifts of H(22) and large coupling constants between Cd^II and H(22) strongly support the presence of an agostic interaction between the metal ion and inner C(22)–H(22). Crystal structures revealed that the syn form is the predominant conformation for TMBPDM complexes. This is distinctively different from the exclusive anti conformation observed in m‐benziporphyrin and tetraphenyl‐m‐benziporphodimethene (TPBPDM) complexes. Evidently, intramolecular hydrogen‐bonding interactions between axial chloride and methyl groups stabilize syn conformations. Unlike the merely syn conformation observed in the solid‐state structures of TMBPDM complexes that contain an axial chloride, in solution these complexes display highly solvent‐ and temperature‐dependent syn/anti ratio changes. The observation of dynamic ¹H NMR spectroscopic scrambling between syn and anti conformations from the titration of chloride ion into the solution of the TMBPDM complex suggests that axial ligand exchange is a likely pathway for the conversion between syn and anti forms. Theoretical calculations revealed that intermolecular hydrogen‐bonding interactions between the axial chloride and CHCl₃ stabilizes the anti conformation, which explains the increased ratio for the anti form when dichloromethane or chloroform was used as the solvent.     

Synthesis of α‐(Fluorophenyl)pyridine by Palladium‐Catalyzed Cross‐Coupling Reaction

A series of α‐(fluoro‐substituted phenyl)pyridines have been synthesized by means of a palladium‐catalyzed cross‐coupling reaction between fluoro‐substituted phenylboronic acid and 2‐bromopyridine or its derivatives. The reactivities of the phenylboronic acids containing di‐ and tri‐fluoro substituents with α‐pyridyl bromide were investigated in different catalyst systems. Unsuccessful results were observed in the Pd/C and PPh₃ catalyst system due to phenylboronic acid containing electron‐withdrawing F atom(s). For the catalyst system of Pd(OAc)₂/PPh₃, the reactions gave moderate yields of 55% –80%, meanwhile, affording 10% –20% of dimerisation (self‐coupling) by‐products, but trace products were obtained in coupling with 2,4‐difluorophenylboronic acids because of steric hinderance. Pd(PPh₃)₄ was more reactive for boronic acids with sterically hindering F atom(s), and the coupling reactions gave good yields of 90% and 91% without any self‐coupling by‐product.     

A High‐Barrier Molecular Balance for Studying Face‐to‐Face Arene–Arene Interactions in the Solid State and in Solution

An atropisomeric molecular balance was developed to study face‐to‐face arene–arene interactions. The balance has a large central 1,4,5,8‐naphthalene diimide surface that forms intramolecular arene–arene interactions with two pendent arms. The balance adopts distinct syn and anti isomers with varying numbers of intramolecular interactions. Thus, the strength of the arene–arene interaction could be quantitatively measured by NMR spectroscopy from the anti/syn ratios. The size of the arene arms was easily varied, which allowed examination of the relationship between arene size and strength of the interaction. A nonlinear size dependence was observed in solution with larger arene arms having a disproportionately stronger arene–arene interaction. The intramolecular arene–arene interactions were also characterized in the solid state by X‐ray crystallography. These studies were facilitated by the kinetic stability of the syn and anti isomers at room temperature due to the high isomerization barrier (ΔG=27.0 kcal mol^?1). Thus, the anti isomer could be selectively isolated and crystallized in its folded conformation. The X‐ray structures confirmed that the anti isomers formed two strong intramolecular arene–arene interactions with face‐to‐face geometries. The solid‐state structure analysis also reveals that the rigid framework may contribute to the observed nonlinear size trend. The acetate linker is slightly too long, which selectively destabilizes the balances with smaller arene arms. The larger arene arms are able to compensate for the longer linker and form effective intramolecular arene–arene interactions.     

Facile synthesis of various substituted taurines,especially syn- and anti-1,2-disubstituted taurines,from nitroolefins

Taurine and substituted taurines present a group of important structural elements in many natural products. Various substituted taurines, including 1- and 2-substituted, 1,1-, syn-1,2-, and anti-1,2-disubstituted taurines, were synthesized from the corresponding nitroolefins via Michael addition with thioacetic acid, oxidation with peroxyformic acid, and the catalytic hydrogenation under the catalysis of palladium on carbon or platinum dioxide. It is a general, versatile, and salt-free method for the preparation of substituted taurines, especially for syn- and anti-1,2-disubstituted taurines and some taurines with more bulky substituents. The stereostructures of both syn- and anti-1,2-disubstituted taurines were deduced from the nitroalkyl thioacetates in the Michael addition, which were identified via the Karplus equation analysis and computational analysis, and finally confirmed by the XRD single crystal analysis. The diastereoselectivity in the Michael addition was rationalized with the Cram rule.     

The Conformational Landscape of Nicotinoids: Solving the Conformational Disparity of Anabasine

The conformational landscape of the alkaloid anabasine (neonicotine) has been investigated by using rotational spectroscopy and ab initio calculations. The results allow a detailed comparison of the structural properties of the prototype piperidinic and pyrrolidinic nicotinoids (anabasine vs. nicotine). Anabasine adopts two most stable conformations in isolation conditions, for which we determined accurate rotational and nuclear quadrupole coupling parameters. The preferred conformations are characterized by an equatorial pyridine moiety and additional N–H equatorial stereochemistry at the piperidine ring (eq‐eq; eq=equatorial). The two rings of anabasine are close to a bisecting arrangement, with the observed conformations differing by an approximately 180° rotation of the pyridine subunit, denoted either syn or anti. The preference of anabasine for the eq‐eq‐syn conformation has been established by relative intensity measurements (syn/anti～5(2)). The conformational preferences of free anabasine are directed by a weak N???H? C hydrogen bond interaction between the nitrogen lone pair at piperidine and the closest C? H bond in pyridine, with N???H distances ranging from 2.686 (syn) to 2.667 Å (anti). Supporting ab initio calculations by using MP2 and the recent M05‐2X density functional are provided, evaluating the predictive performance of both methods.