1,4‐Dioxo‐1,4‐dihydronaphthalene‐2,3‐dicarbonitrile and 1,1,2,2‐Tetracyanoethene in Heterocyclization

This survey is mainly concerned with selected reactions of 1,4‐dioxo‐1,4‐dihydronaphthalene‐2,3‐dicarbonitrile and 1,1,2,2‐tetracyanoethene, which have use or potential use in heterocyclic synthesis.     

A tetrabromo‐1,4‐ethanonaphthalene and related dibromo‐1,4‐ethenonaphthalene

(1RS,3RS,4RS,10SR)‐2,2,3,10‐Tetrabromo‐1,2,3,4‐tetrahydro‐1,4‐ethanonaphthalene, C₁₂H₁₀Br₄, (I), is the first structure to be reported with four Br atoms bound to a 1,4‐ethanonaphthalene framework and also the first which possesses three Br atoms in exo positions. Interactions between the Br atoms [three short intramolecular Br...Br distances of 3.1094 (4), 3.2669 (4) and 3.4415 (5) Å] have little effect on the C—C bond lengths but lead to significant twisting of the cage structure compared with the parent hydrocarbon, which is expected to be fully eclipsed at the two saturated C₂H₄ bridge positions. Chemically related (1SR,4RS)‐2,3‐dibromo‐1,4‐ethenonaphthalene, C₁₂H₈Br₂, (II), obtained by double dehydrobromination of (I), represents the first structure of any halogen‐substituted benzobarrelene. This cis‐dibromide shows little evidence of steric congestion at the double bond [Br...Br = 3.5276 (8) Å] as a consequence of the large C—C—Br angles [average C=C—Br angle = 126.15 (10)°].     

Catalytic Enantioselective Synthesis of 1,4‐Keto‐Alkenylboronate Esters and 1,4‐Dicarbonyls

A catalytic enantioselective method for the synthesis of 1,4‐keto‐alkenylboronate esters by a rhodium‐catalyzed conjugate addition pathway is disclosed. A variety of novel, bench‐stable alkenyl gem‐diboronate esters are synthesized. These easily accessible reagents react smoothly with a collection of cyclic α,β‐unsaturated ketones, generating a new C?C bond and stereocenter. Products are isolated in up to 99 % yield with greater than 20:1 E/Z and greater than 99:1 e.r. Mechanistic studies show the site‐selectivity of transmetalation and reactivity is ligand dependent. The utility of the approach is highlighted by gram‐scale synthesis of enantioenriched cyclic 1,4‐diketones, and stereoselective transformations of the products by hydrogenation, allylation, and isomerization.     

2,6‐Dimethyl‐1,4‐benzoquinone 4‐monooxime

Mol­ecules of the title compound, C₈H₉NO₂, are linked into sheets by a combination of C—H·N, O—H·N and O—H·O hydrogen bonds and C—H·π inter­actions. The hydrogen bonds are arranged as described by the graph‐set ring notations R₂²(7) and R₃³(5), and a C8 chain motif. There are two planar symmetry‐independent mol­ecules in the asymmetric unit, with a dihedral angle of 19.24 (5)° between their least‐squares mean planes.     

Heterocondensed 1,4‐diphosphinines

A reaction of phosphorus tribromide with a compound containing two 2,5‐dimethyl‐1‐arylpyrrolyl‐3 residues bound through a phosphorus atom gave rise to a new phosphorus‐containing heterocyclic system, 1,4‐diphosphinine. The new products thus obtained have been characterized and described. © 2002 John Wiley & Sons, Inc. Heteroatom Chem 13:46–52, 2002; DOI 10.1002/hc.1105     

Diethyl piperazine‐1,4‐diyldioxalate

The ethyl oxamate group, N–C(O)–C(O)–OEt, in the title compound, alternatively called diethyl N,N′:N,N′‐bis(ethylene)dioxamate, C₁₂H₁₈N₂O₆, can be considered as being composed of two singly bonded amide and ester functionalities. The ethyl oxamate group is not planar. The two carbonyl groups are almost perpendicular, with an oxalyl O=C—C=O torsion angle of −111.34 (17)°. The mol­ecule is located on an inversion centre. Infinite supramolecular tapes, propagating along the b axis, are formed through soft C—H⋯O interactions which form a centrosymmetric R(12) motif.     

Three 2,5‐dialkoxy‐1,4‐diethynylbenzene derivatives

2,5‐Diethoxy‐1,4‐bis[(trimethylsilyl)ethynyl]benzene, C₂₀H₃₀O₂Si₂, (I), constitutes one of the first structurally characterized examples of a family of compounds, viz. the 2,5‐dialkoxy‐1,4‐bis[(trimethylsilyl)ethynyl]benzene derivatives, used in the preparation of oligo(phenyleneethynylene)s via Pd/Cu‐catalysed cross‐coupling. 2,5‐Diethoxy‐1,4‐diethynylbenzene, C₁₄H₁₄O₂, (II), results from protodesilylation of (I). 1,4‐Diethynyl‐2,5‐bis(heptyloxy)benzene, C₂₄H₃₄O₂, (III), is a long alkyloxy chain analogue of (II). The molecules of compounds (I)–(III) are located on sites with crystallographic inversion symmetry. The large substituents either in the alkynyl group or in the benzene ring have a marked effect on the packing and intermolecular interactions of adjacent molecules. All the compounds exhibit weak intermolecular interactions that are only slightly shorter than the sum of the van der Waals radii of the interacting atoms. Compound (I) displays C—H...π interactions between the methylene H atoms and the acetylenic C atom. Compound (II) shows π–π interactions between the acetylenic C atoms, complemented by C—H...π interactions between the methyl H atoms and the acetylenic C atoms. Unlike (I) or (II), compound (III) has weak nonclassical hydrogen‐bond‐type interactions between the acetylenic H atoms and the ether O atoms.     

2‐Chloro‐3‐morpholino‐1,4‐naphtho­quinone

The structure of the title compound, C₁₄H₁₂ClNO₃, (I), comprises essentially planar mol­ecules stacked parallel to the a axis. C—H?O hydrogen‐bonding interactions exist to both naphtho­quinone O atoms and the Cl atom, but not to the morpholine O atom.     

Poly(1,4‐cyclohexylenedimethylene 1,4‐cyclohexanedicarboxylate): Influence of stereochemistry of 1,4‐cyclohexylene units on the thermal properties

A series of poly(1,4‐cyclohexylenedimethylene 1,4‐cyclohexanedicarboxylate) (PCCD) samples, characterized by different cis/trans ratio of the 1,4‐cyclohexanedicarbonyl unit, have been synthesized and analyzed by thermogravimetry (TGA), calorimetry (DSC), and X‐ray diffraction (WAXD). The thermal stability results are good and are not affected by the stereochemistry of the 1,4‐cyclohexylene units. On the other hand, the thermal transitions are notably influenced by the cis/trans content. With the increment of the trans content the polymer changes from completely amorphous to semicrystalline material. T_g, T_m, and crystallinity increase. These results suggest that the trans configuration induces a better chain packing and higher symmetry, improving the crystallizability of the samples. The effect of the molecular structure on the thermal properties is analyzed by using a statistical approach. From the effective correlations found between stereochemistry of the C₆ rings and transition temperatures it is possible to extrapolate that the configuration of 1,4‐cyclohexylene ring deriving from 1,4‐cyclohexanedicarboxylic acid or dimethyl 1,4‐cyclohexanedicarboxylate results to be the main element responsible for the thermal properties. This is due to the high rigidity of the 1,4‐cyclohexanedicarbonyl unit with respect to 1,4‐cyclohexanedimethyleneoxy unit, deriving from the diol. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 619–630, 2008     

2‐(Pyrrolidin‐1‐yl)‐1,4‐naphtho­quinone and 2‐phenyl­sulfanyl‐3‐(pyrrolidin‐1‐yl)‐1,4‐naphtho­quinone

The structure of 2‐(pyrrolidin‐1‐yl)‐1,4‐naphtho­quinone, C₁₄H_12.95Cl_0.05NO₂, (I), is actually a 0.95:0.05 mixture including 2‐chloro‐3‐(pyrrolidin‐1‐yl)‐1,4‐naphtho­quinone as a minor impurity, but (I) was resolved as a single molecule containing a Cl atom with 5% occupancy at the 3‐position. Compound (I) was prepared from the fully chloro‐substituted analogue in an attempt to produce the disubstituted pyrrolidinyl derivative. 2‐Phenyl­sulfanyl‐3‐(pyrrolidin‐1‐yl)‐1,4‐naphtho­quinone, C₂₀H₁₇NO₂S, (II), was also prepared from 2‐chloro‐3‐(pyrrolidin‐1‐yl)‐1,4‐naphtho­quinone, using a strong exocyclic nucleophile. The structure of (II) differs from previous structures of 2,3‐di­chloro‐1,4‐naphtho­quinone and its derivatives in that the naphtho­quinone ring is non‐planar.     

Synthesis of 1,4‐Diethynyl‐ and 1,1,4,4‐Tetraethynylbutatrienes

In this paper, we report the synthesis and opto‐electronic properties of differentially substituted 1,4‐diethynyl‐ and 1,1,4,4‐tetraethynylbuta‐1,2,3‐trienes. These novel chromophores greatly extend the series of building modules for oxidative coupling, which includes 1,2‐diethynyl‐ and 1,1,2,2‐tetraethynylethenes and 1,3‐diethynylallenes (Fig. 1). A general synthesis of 1,1,4,4‐tetraethynylbutatrienes, which tolerates a significant number of peripheral substituents, starts from pentadiynols that are oxidized to the corresponding dialkynyl ketones, followed by Corey–Fuchs dibromo‐olefination, and transition metal mediated dimerization (Schemes 2 and 3). A similar protocol, including oxidation of propargyl aldehydes, dibromo‐olefination, and dimerization yields the less stable 1,4‐diethynylbutatrienes (Scheme 4). Attempts to prepare 1,1,4,4‐tetraethynylbutatrienes with four terminal electron‐donor‐substituted aryl groups failed so far, mainly due to difficulties in the dibromoolefination step (Scheme 6). cis‐trans‐Isomerization of differentially substituted 1,1,4,4‐tetraethynylbutatrienes is remarkably facile, with barriers to rotation in the range of those for peptide bond isomerization (ΔG^≠≈20 kcal mol^?1). Barriers to rotation of 1,4‐diethynylbutatrienes are higher (ΔG^≠≈25 kcal mol^?1), allowing in some cases the isolation of pure isomers. Both UV/VIS spectroscopy (Figs. 2 and 3) and electrochemical studies (Table) demonstrate that the all‐C‐cores in diethynyl‐ and tetraethynylbutatrienes have strong electron‐acceptor properties that are greatly enhanced with respect to those of diethynyl‐ and tetraethynylethenes with two C(sp)‐atoms less. Substitution with peripheral electron donor groups leads to efficient intramolecular charge‐transfer interactions, as evidenced by intense, bathochromically shifted longest‐wavelength bands in the UV/VIS spectra.     

1,4‐Dimethyl‐1,4‐diazo­nia­bicyclo­[2.2.2]octane diiodide acetonitrile solvate

In the crystal structure of the title compound, C₈H₁₈N₂²⁺·2I⁻·CH₃CN, the dication lies on a mirror plane containing the mol­ecular dication threefold axis. The structure displays C—H⋯I inter­actions between H atoms of the 1,4‐dimethyl‐1,4‐diazo­nia­bicyclo­[2.2.2]octane dication and the iodide anions. The H⋯I distances are in the range 2.96–3.18 (4) Å. The dications pack forming channels along the b axis, which contain the iodide anions and acetonitrile solvent mol­ecules.     

1,4‐Bis(triisopropyl­silyl)buta‐1,3‐diyne and 1,4‐bis­(biphenyl‐4‐yl)buta‐1,3‐diyne

We report the single crystal structures of 1,4‐bis­(triisopropyl­silyl)buta‐1,3‐diyne, C₂₂H₄₂Si₂, and 1,4‐bis­(biphenyl‐4‐yl)buta‐1,3‐diyne, C₂₈H₁₈, the packing in both of which illustrates the versatility of weak C—H⋯π supra­molecular inter­actions in dictating the overall solid‐state structures.     

Chain composition dependence of luminescence properties for copolymers of 2‐methoxy‐5‐2′‐ethyl‐hexyloxy‐1,4‐phenylenevinylene and 2,3‐diphenyl‐5‐octyl‐1,4‐phenylenevinylene

The copolymers of 2‐methoxy‐5‐2′‐ethyl‐hexyloxy‐1,4‐phenylenevinylene (MEH‐PV) and 2,3‐diphenyl‐5‐octyl‐1,4‐phenylenevinylene were prepared via the Gilch route with their chain compositions and the reactivity ratios of the monomers estimated by ¹H NMR spectroscopy. The results indicated that the copolymers tended to form an alternative copolymer as the feed ratio of the monomers closed to one‐half. When an individual copolymer solution in tetrahydrofuran was spun‐cast to form a film, the MEH‐PV units were able to attract the like units from the adjacent chains. As a result, the ultraviolet–visible absorption spectrum of the alternative copolymer in film form was broader than the spectra of those with different compositions. The photoluminescence spectra of the copolymers in film form exhibited the characteristic shoulder of poly(2‐methoxy‐5‐2′‐ethyl‐hexyloxy‐1,4‐phenylenevinylene), even though the content of MEH‐PV units was not great enough for the formation of repeat units in sequence. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 2180–2186, 2003     

Dimorphism of ethyl 1,4‐dihydro‐2,4,6‐triphenylpyridine‐3‐carboxylate

The title compound, C₂₆H₂₃NO₂, (Ia) and (Ib), shows polymorphism with crystals obtained from different solvents displaying different crystal structures. However, it is not the geometry of the single mol­ecules nor the hydrogen‐bond pattern that is different in (Ia) and (Ib), but the way in which the hydrogen‐bonded chains, running along the a‐axis direction, are arranged with respect to each other.