Synthesis and Characterization of New Heptalenes with Extended π‐Systems Attached to Them

Methyl heptalenecarboxylates of type A and B with π(1) and π(2) substituents in 1,4‐relation (Scheme 1) were synthetized starting with dimethyl 1‐methylheptalene‐4,5‐dicarboxylates 5b and 6b derived from 7‐isopropyl‐1,4‐dimethylazulene (=guaiazulene) and 1,4,6,8‐tetramethylazulene by thermal reaction with dimethyl acetylenedicarboxylate. The further general way of proceeding for the introduction of the π(1) and π(2) substituents is displayed in Scheme 3, and the thus obtained methyl heptalene‐5‐carboxylates of type A and B are listed in Table 1. The C?C bonds of the 2‐arylethenyl and 4‐arylbuta‐1,3‐dien‐1‐yl groups of π(1) and π(2) were in all cases (E)‐configured and showed s‐trans conformation at the C? C bonds (X‐ray and ¹H‐NOE evidence) in the B ‐type as well as in the A ‐type heptalenes (cf. Figs. 5–12). All B ‐type heptalenes showed a strongly enhanced heptalene band I in the wavelength region 440–490 nm in hexane/CH₂Cl₂ 9 : 1 (cf. Table 4 and Figs. 13–20). The A ‐type heptalenes showed in this region only weak absorption, recognizable as shoulders or simply tailing of the dominating heptalene bands II/III (Table 5). Absorption band I of the B ‐type heptalenes appeared almost at the same wavelength as the longest wavelength absorption band of comparable open‐chain α,ω‐diarylpolyenes (cf. Fig. 21). The cyclic double bond shift (DBS) of the A ‐ and B ‐type heptalenes could be photochemically steered in one or the other direction by selective irradiation (cf. Fig. 22).     

New Products from the Heptalene‐Forming Reaction of Azulenes and Acetylenedicarboxylates in Polar Media

A number of azulenes 1 , in particular those with π‐substituents at C(6) such as phenyl, 3,5‐dimethylphenyl, and 4‐biphenyl, have been reacted with 3 mol‐equiv. of dimethyl acetylenedicarboxylate (ADM) in MeCN at 110° (cf. Scheme 1). Main products had been, in all cases, the corresponding heptalene‐4,5‐dicarboxylates 2 . However, a whole number of side products, mainly rearranged (1+2)‐adducts with two molecules of ADM, in amounts of 0.2–9% were also isolated and characterized (cf. Scheme 2). The 2a,8a‐dihydro‐3,4‐ethenoazulene‐1,2‐dicarboxylates 14 , formed by energetically favorable ring closure from the solvent‐stabilized zwitterions 15 , resulting from bond heterolysis in the primary cycloadducts 12 (cf. Scheme 3), have been mechanistically identified as the pivotal intermediates responsible for the formation of all side product (cf. Schemes 5, 9, 12, and 13). Deuterium‐labeling experiments were in agreement with the proposed mechanisms, indicating that sigmatropic [1,5s]‐H shifts in 14 (cf. Scheme 6) as well as isoconjugate [1,4s]‐H shifts in resonance‐stabilized zwitterions of type 21 (cf. Scheme 9) are the crucial steps for side‐product formation. It is postulated that a concluding antarafacial 8e‐dyotropic rearrangement is responsible for the appearance of the 2,4a‐dihydrophenanthrene‐tetracarboxylates of type trans‐ 6 (cf. Scheme 9) in the reaction mixtures, which further rearrange thermally by a not fully understood mechanism into the isomeric tetracarboxylates 7 (cf. Schemes 10 and 11). Most surprising is the presence of a small amount (0.3–1%) of the azulene‐4,5,7,8‐tetracarboxylate 9 in the reaction mixture of azulene 1a and ADM. It is proposed that the formation of 9 is the result of a [1,5s]‐C shift in the spiro‐linked intermediates 24 , which, after prototropic shift and take‐up of a third molecule of ADM, disintegrate by a retro‐Diels‐Alder reaction into 9 and the phthalic diesters 30 (cf. Scheme 12). The UV/VIS spectra of the π‐substituted heptalene‐4,5‐dicarboxylates 2d – 2f and their double‐bond shifted (DBS) forms 2d – 2f (cf. Table 4 and Figs. 9–12) exhibit in comparison with the heptalene‐dicarboxylates 2a and 2′a , carrying a t‐Bu group at C(8), only marginal differences, which are mainly found in the relative intensity and position of heptalene bands II and III .     

A New Alkylation Method for Heptalene‐4,5‐dicarboxylates and of One of Their Pseudoester Forms

Dimethyl heptalene‐4,5‐dicarboxylates

1 The locants of heptalene itself are maintained throughout the whole work. See footnote 4 in [1] for reasoning.

undergo preferentially a Michael addition reaction at C(3) with α‐lithiated alkyl phenyl sulfones at temperatures below ?50°, leading to corresponding cis‐configured 3,4‐dihydroheptalene‐4,5‐dicarboxylates (cf. Table 1, Schemes 3 and 4). The corresponding heptalenofuran‐1‐one‐type pseudoesters of dimethyl heptalene‐4,5‐dicarboxylates (Scheme 5) react with [(phenylsulfonyl)methyl]lithium almost exclusively at C(1) of the furanone group (Scheme 6). In contrast to this expected behavior, the uptake of 1‐[phenylsulfonyl)ethyl]lithium occurs at C(5) of the heptalenofuran‐1‐ones as long as they carry a Me group at C(11) (Schemes 6 and 7). The 1,4‐ as well as the 1,6‐addition products eliminate, on treatment with MeONa/MeOH in THF, benzenesulfinate, thus leading to 3‐ and 4‐alkylated dimethyl heptalene‐4,5‐dicarboxylates, respectively (Schemes 8–13). The configuration of the addition reaction of the nucleophiles to the inherently chiral heptalenes is discussed in detail (cf. Schemes 14–19) on the basis of a number of X‐ray crystal‐structure determinations as well as by studies of the temperature‐dependence of the ¹H‐NMR spectra of the addition products.     

Surprising Formation of Highly Substituted Azulenes on Thermolysis of 4,5,6,7,8‐Pentamethyl‐2H‐cyclohepta[b]furan‐2‐one and Heptalene Formation with the New Azulenes

Heating of 4,5,6,7,8‐pentamethyl‐2H‐cyclohepta[b]furan‐2‐one ( 1a ) in decalin at temperatures >170° leads to the development of a blue color, typical for azulenes. It belongs, indeed, to two formed azulenes, namely 4,5,6,7,8‐pentamethyl‐2‐(2,3,4,5,6‐pentamethylphenyl)azulene ( 4a ) and 4,5,6,7,8‐pentamethylazulene ( 5a ) (cf. Scheme 2 and Table 1). As a third product, 4,5,6,7‐tetramethyl‐2‐(2,3,4,5,6‐pentamethylphenyl)‐1H‐indene ( 6a ) is also found in the reaction mixture. Neither 4,6,8‐trimethyl‐2H‐cyclohepta[b]furan‐2‐one ( 1b ) nor 2H‐cyclohepta[b]furan‐2‐one ( 1c ) exhibit, on heating, such reactivity. However, heating of mixtures 1a / 1b or 1a / 1c results in the formation of crossed azulenes, namely 4,6,8‐trimethyl‐2‐(2,3,4,5,6‐pentamethylphenyl)azulene ( 4ba ) and 2‐(2,3,4,5,6‐pentamethylphenyl)azulene ( 4ca ), respectively (cf. Scheme 3). The formation of small amounts of 4,6,8‐trimethylazulene ( 5ba ) and azulene ( 5ca ), respectively, besides 1H‐indene 6a is also observed. The observed product types speak for an [8+2]‐cycloaddition reaction between two molecules of 1a or between 1b and 1c , respectively, with 1a , whereby 1a plays in the latter two cases the part of the two‐atom component (cf. Figs. 5 – 7 and Schemes 4 – 6). Strain release, due to the five adjacent Me groups in 1a , in the [8+2]‐cycloaddition step seems to be the driving force for these transformations (cf. Table 3), which are further promoted by the consecutive loss of two molecules of CO₂ and concomitant formation of the 10π‐electron system of the azulenes. The new azulenes react with dimethyl acetylenedicarboxylate (ADM) to form the corresponding dimethyl heptalene‐4,5‐dicarboxylates 20 , 22 , and 24 (cf. Scheme 7), which give thermally or photochemically the corresponding double‐bond‐shifted (DBS) isomers 20′ , 22′ , and 24′ , respectively. The five adjacent Me groups in 20 / 20′ and 24 / 24′ exert a certain buttressing effect, whereby their thermal DBS process is distinctly retarded in comparison to 22 / 22′ , which carry `isolated' Me groups at C(6), C(8), and C(10). This view is supported by X‐ray crystal‐structure analyses of 22 and 24 (cf. Fig. 8 and Table 5).     

4‐Styrylquinolines from cyclocondensation reactions between (2‐aminophenyl)chalcones and 1,3‐diketones: crystal structures and regiochemistry

Structures are reported for two matched sets of substituted 4‐styrylquinolines which were prepared by the formation of the heterocyclic ring in cyclocondensation reactions between 1‐(2‐aminophenyl)‐3‐arylprop‐2‐en‐1‐ones with 1,3‐dicarbonyl compounds. (E)‐3‐Acetyl‐4‐[2‐(4‐methoxyphenyl)ethenyl]‐2‐methylquinoline, C₂₁H₁₉NO₂, (I), (E)‐3‐acetyl‐4‐[2‐(4‐bromophenyl)ethenyl]‐2‐methylquinoline, C₂₀H₁₆BrNO, (II), and (E)‐3‐acetyl‐2‐methyl‐4‐{2‐[4‐(trifluoromethyl)phenyl]ethenyl}quinoline, C₂₁H₁₆F₃NO, (III), are isomorphous and in each structure the molecules are linked by a single C—H…O hydrogen bond to form C(6) chains. In (I), but not in (II) or (III), this is augmented by a C—H…π(arene) hydrogen bond to form a chain of rings; hence, (I)–(III) are not strictly isostructural. By contrast with (I)–(III), no two of ethyl (E)‐4‐[2‐(4‐methoxyphenyl)ethenyl]‐2‐methylquinoline‐3‐carboxylate, C₂₂H₂₁NO₃, (IV), ethyl (E)‐4‐[2‐(4‐bromophenyl)ethenyl]‐2‐methylquinoline‐3‐carboxylate, C₂₁H₁₈BrNO₂, (V), and ethyl (E)‐2‐methyl‐4‐{2‐[4‐(trifluoromethyl)phenyl]ethenyl}quinoline‐3‐carboxylate, C₂₂H₁₈F₃NO₂, (VI), are isomorphous. The molecules of (IV) are linked by a single C—H…O hydrogen bond to form C(13) chains, but cyclic centrosymmetric dimers are formed in both (V) and (VI). The dimer in (V) contains a C—H…π(pyridyl) hydrogen bond, while that in (VI) contains two independent C—H…O hydrogen bonds. Comparisons are made with some related structures, and both the regiochemistry and the mechanism of the heterocyclic ring formation are discussed.     

Synthesis of Racemic Δ3‐2‐Hydroxybakuchiol and Its Analogues

The first synthetic approach to (±)‐Δ³‐2‐hydroxybakuchiol (=4‐[(1E,5E)‐3‐ethenyl‐7‐hydroxy‐3,7‐dimethylocta‐1,5‐dien‐1‐yl]phenol; 14 ) and its analogues 13a – 13f was developed by 12 steps (Schemes 2 and 3). The key features of the approach are the construction of the quaternary C‐center bearing the ethenyl group by a Johnson–Claisen rearrangement (→ 6 ); and of an (E)‐alkenyl iodide via a Takai–Utimoto reaction (→ 11 ); and an arylation via a Negishi cross‐coupling reaction (→ 12e – 12f ).     

Two novel organic–inorganic hybrid materials from tetrachloridometallate(II) salts and 4‐[(E)‐2‐(pyridin‐1‐ium‐2‐yl)ethenyl]pyridinium

Two organic–inorganic hybrid compounds have been prepared by the combination of the 4‐[(E)‐2‐(pyridin‐1‐ium‐2‐yl)ethenyl]pyridinium cation with perhalometallate anions to give 4‐[(E)‐2‐(pyridin‐1‐ium‐2‐yl)ethenyl]pyridinium tetrachloridocobaltate(II), (C₁₂H₁₂N₂)[CoCl₄], (I), and 4‐[(E)‐2‐(pyridin‐1‐ium‐2‐yl)ethenyl]pyridinium tetrachloridozincate(II), (C₁₂H₁₂N₂)[ZnCl₄], (II). The compounds have been structurally characterized by single‐crystal X‐ray diffraction analysis, showing the formation of a three‐dimensional network through X—H...Cl_nM⁻ (X = C, N⁺; n = 1, 2; M = Co^II, Zn^II) hydrogen‐bonding interactions and π–π stacking interactions. The title compounds were also characterized by FT–IR spectroscopy and thermogravimetric analysis (TGA).     

Benzo[a]heptalenes from Heptaleno[1,2‐c]furans. Part I

It is shown that heptaleno[1,2‐c]furans 1 , which are available in two steps from heptalene‐4,5‐dicarboxylates by reduction and oxidative dehydrogenation of the corresponding vicinal dimethanols 2 with MnO₂ or IBX (Scheme 4), react thermally in a Diels–Alder‐type [4+2] cycloaddition at the furan ring with a number of electron‐deficient dipolarophiles to yield the corresponding 1,4‐epoxybenzo[d]heptalenes (cf. Schemes 6, 15, 17, and 19). The thermal reaction between dimethyl acetylenedicarboxylate (ADM) and 1 leads, kinetically controlled, via a sterically less‐congested transition state (Fig. 4) to the formation of the (M*)‐configured 1,4‐dihydro‐1,4‐epoxybenzo[a]heptalenes, which undergo a cyclic double‐bond shift to the energetically more‐relaxed benzo[d]heptalenes 4 (Schemes 6 and 7). Most of the latter ones exhibit under thermal conditions epimerization at the axis of chirality, so that the (M*)‐ and (P*)‐stereoisomers are found in reaction mixtures. The (P*)‐configured forms of 4 are favored in thermal equilibration experiments, in agreement with AM1 calculations (Table 1). The relative (P*,1S*,4R*)‐ and (M*,1S*,4R*)‐configuration of the crystalline main stereoisomers of the benzo[d]heptalene‐2,3‐dicarboxylates 4a and 4f , respectively, was unequivocally established by an X‐ray crystal‐structure determination (Figs. 1 and 2). Acid‐induced rearrangement of 4 led to the formation of the corresponding 4‐hydroxybenzo[a]heptalene‐2,3‐dicarboxylates 5 in moderate‐to‐good yields (Schemes 8, 13, and 14). When the aromatization reaction is performed in the presence of trifluoroacetic acid (TFA), trifluoroacetates of type 6 and 13 (Schemes 8, 12, and 13) are also formed via deprotonation of the intermediate tropylium ions of type 7 (Scheme 11). Thermal reaction of 1 with dimethyl maleate gave the 2,3‐exo‐ and 2,3‐endo‐configured dicarboxylates 14 as mixtures of their (P*)‐ and (M*)‐epimers (Scheme 15). Treatment of these forms with lithium di(isopropyl)amide (LDA) at ?70° gave the expected benzo[a]heptalene‐2,3‐dicarboxylates 15 in good yields (Scheme 16). Fumaronitrile reacted thermally also with 1 to the corresponding 2‐exo,3‐endo‐ and 2‐endo,3‐exo‐configured adducts 17 , again as mixtures of their (P*)‐ and (M*)‐epimers (Scheme 17), which smoothly rearranged on heating in dimethoxyethane (DME) in the presence of Cs₂CO₃ to the benzo[a]heptalene‐2,3‐dicarbonitriles 18 (Scheme 18). Some cursory experiments demonstrated that hex‐3‐yne‐2,5‐dione and (E)/(Z)‐hexa‐3‐ene‐2,5‐dione undergo also the Diels–Alder‐type cycloaddition reaction with 1 (Scheme 19). The mixtures of the stereoisomers of the 2,3‐diacetyl‐1,4‐epoxytetrahydrobenzo[d]heptalenes 22 gave, on treatment with Cs₂CO₃ in DME at 80°, only mixtures of the regioisomeric inner aldol products 24 and 25 of the intermediately formed benzo[a]heptalenes 23 (Scheme 20).     

On the Formation of 2‐Sulfonylbenzo[a]heptalene‐1,3‐diols as Precursors for the Synthesis of Colchicinoids

The benzo[a]heptalene formation from 4‐[(R‐sulfonyl)acetyl]heptalene‐5‐carboxylates 15 and 5‐[(R‐sulfonyl)acetyl]heptalene‐4‐carboxylates 16 (R=Ph or morpholino) in the presence of R′SO₂CH₂Li and BuLi has been investigated (Scheme 6). Only the sulfonyl moiety linked to the C?O group at C(4) of the heptalene skeleton is found at C(3) of the formed benzo[a]heptalene‐2,4‐diols 3 in accordance with the general mechanism of their formation (Scheme 3). Intermediates that might rearrange to corresponding 2‐sulfonylbenzo[a]heptalene‐1,3‐diols lose HO^? under the reaction conditions to yield the corresponding cyclopenta[d]heptalenones of type 11 (Schemes 6 and 7). However, the presence of an additional Me group at C(α) of the lithioalkyl sulfones suppresses the loss of HO^?, and 4‐methyl‐2‐sulfonylbenzo[a]heptalene‐1,3‐diols of type 4c have been isolated and characterized for the first time (Schemes 8 and 10). A number of X‐ray crystal‐structure analyses of starting materials and of the new benzo[a]heptalenes have been performed. Finally, benzo[a]heptalene 4c has been transformed into its 1,2,3‐trimethoxy derivative 23 , a benzo[a]heptalene with the colchicinoid substitution pattern at ring A (Scheme 11).     

Photochemistry of N‐(2‐Acylphenyl)‐2‐methylprop‐2‐enamides: Competition between Photocyclization and Long‐Range Hydrogen Abstraction

The photochemical reactions of various ‘N‐methacryloyl acylanilides’ (=N‐(acylphenyl)‐2‐methylprop‐2‐enamides) have been investigated. Under irradiation, the acyl‐substituted anilides 1a – 1c and 1o afforded exclusively the corresponding quinoline‐based cyclization products of type 2 (Table 1). In contrast, irradiation of the benzoyl (Bz)‐substituted anilides 1e – 1h afforded a mixture of the open‐chain amides 4e – 4h and the cyclization products 2e – 2h . Irradiation of the para‐acyl‐substituted anilides 6a – 6e and 6h afforded the corresponding quinoline‐based cyclization products of type 5 as the sole products (Table 2). The formation of the cyclization products 2a – 2c and 2o can be rationalized in terms of 6π‐electron cyclization, followed by thermal [1,5] acyl migration, and that of compounds 3p, 5a – 5e , and 5h can be explained by a 6π‐electron cyclization only. The formation of the open‐chain amides 4e – 4h probably follows a mechanism involving a 1,7‐diradical, C and a spirolactam of type D (Scheme). Long‐range ζ‐H abstraction by the excited carbonyl O‐atom of the benzoyl group on the aniline ring is expected to proceed via a nine‐membered cyclic transition state, as proposed on the basis of X‐ray crystallographic analyses (Fig. 2).     

Experimental and Theoretical Conformational Analysis of 5‐Benzylimidazolidin‐4‐one Derivatives – a ‘Playground’ for Studying Dispersion Interactions and a ‘Windshield‐Wiper’ Effect in Organocatalysis

The PF₆ salts of 5‐benzyl‐1‐isopropylidene‐ and 5‐benzyl‐1‐cinnamylidene‐3‐methylimidazolidin‐4‐ones 1 (Scheme) with various substituents in the 2‐position have been prepared, and single crystals suitable for X‐ray structure determination have been obtained of 14 such compounds, i.e., 2 – 10 and 12 – 16 (Figs. 2–5). In nine of the structures, the Ph ring of the benzyl group resides above the heterocycle, in contact with the cis‐substituent at C(2) (staggered conformation A ; Figs. 1–3); in three structures, the Ph ring lies above the iminium π‐plane (staggered conformation B ; Figs. 1 and 4); in two structures, the benzylic C? C bond has an eclipsing conformation ( C ; Figs. 1 and 5) which places the Ph ring simultaneously at a maximum distance with its neighbors, the CO group, the N?C‐π‐system, and the cis‐substituent at C(2) of the heterocycle. It is suggested by a qualitative conformational analysis (Fig. 6) that the three staggered conformations of the benzylic C? C bond are all subject to unfavorable steric interactions, so that the eclipsing conformation may be a kind of ‘escape’. State‐of‐the‐art quantum‐chemical methods, with large AO basic sets (near the limit) for the single‐point calculations, were used to compute the structures of seven of the 14 iminium ions, i.e., 3, 4 / 12, 5 – 7, 13 , and 16 (Table) in the two staggered conformations, A and B , with the benzylic Ph group above the ring and above the iminium π‐system, respectively. In all cases, the more stable computed conformer (‘isolated‐molecule’ structure) corresponds to the one present in the crystal (overlay in Fig. 7). The energy differences are small (≤2 kcal/mol) which, together with the result of a potential‐curve calculation for the rotation around the benzylic C? C bond of one of the structures, 16 (Fig. 8), suggests that the benzyl group is more or less freely rotating at ambident temperatures. The importance of intramolecular London dispersion (benzene ring in ‘contact’ with the cis‐substituent in conformation A ) for DFT and other quantum‐chemical computations is demonstrated; the benzyl‐imidazolidinones 1 appear to be ideal systems for detecting dispersion contributions between a benzene ring and alkyl or aryl CH groups. Enylidene ions of the type studied herein are the reactive intermediates of enantioselective organocatalytic conjugate additions, Diels–Alder reactions, and many other transformations involving α,β‐unsaturated carbonyl compounds. Our experimental and theoretical results are discussed in view of the performance of 5‐benzyl‐imidazolidinones as enantioselective catalysts.     

Conformational polymorphism of (E,E)‐N,N′‐bis(4‐nitrobenzylidene)benzene‐1,4‐diamine

Two polymorphs of (E,E)‐N,N′‐bis(4‐nitrobenzylidene)benzene‐1,4‐diamine, C₂₀H₁₄N₄O₄, (I), have been identified. In each case, the molecule lies across a crystallographic inversion centre. The supramolecular structure of the first polymorph, (I‐1), features stacking based on π–π interactions assisted by weak hydrogen bonds involving the nitro groups. The second polymorph, (I‐2), displays a perpendicular arrangement of molecules linked via the nitro groups, combined with weak C—H...O hydrogen bonds. Both crystal structures are compared with that of the carbon analogue (E,E)‐1,4‐bis[2‐(4‐nitrophenyl)ethenyl]benzene, (II).     

Synthesis of Indolones via Radical Cyclization of N‐(2‐Halogenoalkanoyl)‐Substituted Anilines

The radical reactions of N‐(2‐halogenoalkanoyl)‐substituted anilines (anilides) of type 1 have been investigated under various conditions. Treatment of compounds 1a – 1o with Bu₃SnH in the presence of (2,2′‐azobis(isobutyronitrile) (AIBN) afforded a mixture of the indolones (oxindoles) 2a – 2o and the reduction products 5a – 5o (Table 1). In contrast, the N‐unsubstituted anilides 1p – 1s, 1u , and 1v gave the corresponding reduction products exclusively (Table 1). Similar results were obtained by treatment of 1 with Ni powder (Table 2) or wth Et₃B (Table 3). Anilides with longer N‐(phenylalkyl) chains such as 6 and 7 were inert towards radical cyclization, with the exception of N‐benzyl‐2‐bromo‐N,2‐dimethylpropanamide ( 6b ), which, upon treatment with Ni powder in i‐PrOH, afforded the cyclized product 9b in low yield (Table 4). Upon irradiation, the extended anilides 6, 7, 10 , and 11 yielded the corresponding dehydrobromination products exclusively (Table 5).     

Synthesis and Characterization of Multinanometer‐Sized Expanded Dendralenes with an iso‐Poly(triacetylene) Backbone

A series of [n]dendralenes (n =3, 4, 8, 3b – d (Fig. 1)) expanded with buta‐1,3‐diynediyl moieties between the CC bonds were prepared by repetitive acetylenic scaffolding of 3‐(cyclohexylidene)penta‐1,4‐diyne building blocks (Scheme). These remarkably unstable iso‐poly(triacetylene) (iso‐PTA) oligomers were characterized by ¹H‐ and ¹³C NMR (Fig. 3 and Table 1), IR, and UV/VIS (Figs. 4 and 6 and Table 2) spectroscopy, as well as mass spectrometry (Fig. 2). The expanded [8]dendralene contains 40 C(sp)‐ and C(sp²)‐atoms in the backbone and represents the longest iso‐PTA oligomer prepared to date. HOMO‐LUMO Gap energies were determined as a function of oligomeric length (Fig. 5 and Table 3), providing insight into the degree of π‐electron delocalization in these cross‐conjugated chromophores. A continous drop in the HOMO‐LUMO gap with increasing number of monomeric repeating units provides evidence that cross‐conjugation along the oligomeric backbone is effective to some extent. The limiting HOMO‐LUMO gap energy for an infinitely long, buta‐1,3‐diynediyl‐expanded dendralene was extrapolated to about 3.3–3.5 eV. The conformational preferences of the expanded dendralenes were analyzed in semi‐empirical calculations, revealing energetic preferences for planar or slightly twisted s‐cis and ‘U‐shaped' geometries.     

Photoswitchable Tetraethynylethene‐Dihydroazulene Chromophores

The synthesis, characterization, and photophysical as well as electrochemical properties of the photochromic hybrid systems 11 – 16 and 18 , which contain photoswitchable tetraethynylethene (TEE; 3,4‐diethynylhex‐3‐ene‐1,5‐diyne) and dihydroazulene (DHA) moieties, are presented. The molecular photoswitches were synthesized by a Sonogashira cross‐coupling reaction between an appropriate TEE precursor ( 6 – 10 and 17 ) and an iodinated DHA 1 or its vinylheptafulvene (VHF) isomer ( 4 ) (Schemes 5 – 7). X‐Ray crystal structures of five DHA derivatives ( 1 , trans‐ 11a , cis‐ 11a , 12 , and 13 ) are discussed (Figs. 2 – 5). In all compounds, the cyclohexatriene moiety of the DHA chromophore adopts a clear boat conformation (Table 1). Presumably due to crystal‐packing effects, the arylated TEE moieties in the hybrid systems show substantial distortions from planarity, with the dihedral angles between the planes of the central TEE core and the adjacent aryl substituents amounting to 44°. The switching properties were investigated by electronic absorption spectroscopy. Upon light absorption, DHAs 1 , 12 – 16 , and 18 underwent retro‐electrocyclization in solution to give the corresponding VHFs (Figs. 6, 11, and 12). The reaction is thermally reversible, with half‐lives τ_1/2 between 3.9 and 5.8 h at 25° in CH₂Cl₂ (Figs. 7 and 13 and Table 3). A comparatively slower (E)→(Z) isomerization process about the central C=C bond of the TEE moiety was also observed. The N,N‐dimethylanilino‐(DMA) substituted TEE‐DHA hybrid systems trans‐ 11a and cis‐ 11a did not react to the corresponding VHFs upon irradiation (Scheme 9). Instead, only the reversible (E)→(Z) photoisomerization of the TEE core occurred (Fig. 16 and Table 4). This process was further investigated for photofatigue by electronic‐emission spectroscopy (Fig. 17). After protonation of the DMA group, the usual DHA→VHF photoreaction took place. Compound 11 represents a three‐way chromophoric molecular switch with three addressable sub‐units (TEE core, DHA/VHF moiety, and proton sensitive DMA group) that can undergo individual, reversible switching cycles (Scheme 9). A process modeling the function of an `AND' logic gate (Fig. 19) and three write/erase processes could be performed with this system. Cyclic and linear sweep‐voltammetry studies in CH₂Cl₂ (+Bu₄NPF₆) revealed the occurrence of characteristic first‐reduction steps in the TEE‐DHA hybrid systems between −1.6 and −1.8 V vs. Fc/Fc⁺ (ferrocene/ferricinium couple) (Table 5). Oxidations occur at ca. +1.10 V. After photoisomerization to the VHF derivatives, reduction steps at more positive and oxidation steps at more negative potentials were recorded. No DHA→VHF isomerization took place upon electrochemical oxidation or reduction (Fig. 20).     

A Novel Intramolecular Photocyclization of N‐(2‐Bromoalkanoyl) Derivatives of 2‐Acylanilines via 1,8‐Hydrogen Abstraction

The photochemical reactions of different N‐(2‐acylphenyl)‐2‐bromo‐2‐methylpropanamides have been investigated. Irradiation of the N‐unsubstituted anilides 1a – 1c gave the corresponding dehydrobromination, cyclization, and bromo‐migration products 2, 3 , and 4 , respectively (Table 1). Irradiation of the N‐alkyl anilides 1e – 1g afforded the corresponding deacylation and cyclization products 5 and 6 , respectively, whereas irradiation of the N‐alkyl anilides 1i – 1k , carrying 2‐benzoyl groups on the aromatic rings, afforded the unexpected tricyclic lactams 7 (besides 2, 5 , and 6 ). The formation of the cyclization products 6 could be rationalized in terms of an electrocyclic ring closure of the 6π‐electron‐conjugated enamides 2 produced by dehydrobromination of 1 , followed by thermal 1,5‐acyl migration (Path B in the Scheme). The formation of the bridged lactams 7 probably follows a mechanism involving the 1,7‐diradical 8 generated by ζ‐H‐abstraction (1,8‐H transfer) by an excited acyl O‐atom (Path A).     

Cycloaddition Reactions of 7‐Benzylidenecycloocta‐1,3,5‐triene with Ethenetetracarbonitrile and 4‐Phenyl‐3H‐1,2,4‐triazole‐3,5(4H)‐dione

An (E)/(Z) mixture (3 : 2) of 7‐benzylidenecycloocta‐1,3,5‐triene ( 5 ) is obtained when 1‐benzylcycloocta‐1,3,5,7‐tetraene ( 7 ), prepared by an improved procedure, is treated with t‐BuOK in THF. Alternatively, a ca. 9 : 1 mixture (E)/(Z)‐ 5 can be prepared in a Wittig reaction involving benzaldehyde and cycloocta‐2,4,6‐trien‐1‐ylidenetriphenylphoshorane ( 9 ). Treatment of (E)/(Z)‐ 5 88 : 12 with ethenetetracarbonitrile (TCNE) gave a complex mixture of products, from which seven mono‐adducts and two bis‐adducts were isolated (Sect. 2.2.1). Of the mono‐adducts, four are π4+π2 adducts: two ((E)‐ and (Z)‐isomers) are derived from valence tautomers of the two isomers of (E)/(Z)‐ 5 , while it is tentatively suggested that the other two (again (E)‐ and (Z)‐isomers) are formed from the intermediacy of a pentadienyl zwitterion (Sect. 2.3). The remaining three mono‐adducts, two of which are epimers, are π8+π2 adducts. It is suggested that they are derived from the intermediacy of homotropylium zwitterions (Sect. 2.3). For the two bis‐adducts, it is postulated that they are derived from an initial π2+π2 cycloaddition involving the homotropylium zwitterions followed by π4+π2 cycloaddition to the valence tautomer of each of the π2+π2 cycloadducts. With 4‐phenyl‐3H‐1,2,4‐triazole‐3,5(4H)‐dione ( 6 ), (E)/(Z)‐ 5 91 : 9 yielded two π4+π2 cycloadducts ((E)‐ and (Z)‐isomers) as well as two epimeric π8+π2 cycloadducts (Sect. 2.2.2). The intermediacy of pentadienyl (tentative suggestion) and homotropylium zwitterions accounts for the formation of the products (Sect. 2.3).     

Photochemical Reactions of N‐(2‐Halogenoalkanoyl) Derivatives of Anilines

The photochemical reactions of 2‐substituted N‐(2‐halogenoalkanoyl) derivatives 1 of anilines and 5 of cyclic amines are described. Under irradiation, 2‐bromo‐2‐methylpropananilides 1a – e undergo exclusively dehydrobromination to give N‐aryl‐2‐methylprop‐2‐enamides (=methacrylanilides) 3a – e (Scheme 1 and Table 1). On irradiation of N‐alkyl‐ and N‐phenyl‐substituted 2‐bromo‐2‐methylpropananilides 1f – m , cyclization products, i.e. 1,3‐dihydro‐2H‐indol‐2‐ones (=oxindoles) 2f – m and 3,4‐dihydroquinolin‐2(1H)‐ones (=dihydrocarbostyrils) 4f – m , are obtained, besides 3f – m . On the other hand, irradiation of N‐methyl‐substituted 2‐chloro‐2‐phenylacetanilides 1o – q and 2‐chloroacetanilide 1r gives oxindoles 2o – r as the sole product, but in low yields (Scheme 3 and Table 2). The photocyclization of the corresponding N‐phenyl derivatives 1s – v to oxindoles 2s – v proceeds smoothly. A plausible mechanism for the formation of the photoproducts is proposed (Scheme 4). Irradiation of N‐(2‐halogenoalkanoyl) derivatives of cyclic amines 5a – c yields the cyclization products, i.e. five‐membered lactams 6a , b , and/or dehydrohalogenation products 7a , c and their cyclization products 8a , c , depending on the ring size of the amines (Scheme 5 and Table 3).     

A Novel Route to 1‐Substituted 3‐(Dialkylamino)‐9‐oxo‐9H‐indeno[2,1‐c]pyridine‐4‐carbonitriles

Heptalenecarbaldehydes 1 / 1′ as well as aromatic aldehydes react with 3‐(dicyanomethylidene)‐indan‐1‐one in boiling EtOH and in the presence of secondary amines to yield 3‐(dialkylamino)‐1,2‐dihydro‐9‐oxo‐9H‐indeno[2,1‐c]pyridine‐4‐carbonitriles (Schemes 2 and 4, and Fig. 1). The 1,2‐dihydro forms can be dehydrogenated easily with KMnO₄ in acetone at 0° (Scheme 3) or chloranil (=2,3,5,6‐tetrachlorocyclohexa‐2,5‐diene‐1,4‐dione) in a ‘one‐pot’ reaction in dioxane at ambient temperature (Table 1). The structures of the indeno[2,1‐c]pyridine‐4‐carbonitriles 5′ and 6a have been verified by X‐ray crystal‐structure analyses (Fig. 2 and 4). The inherent merocyanine system of the dihydro forms results in a broad absorption band in the range of 515–530 nm in their UV/VIS spectra (Table 2 and Fig. 3). The dehydrogenated compounds 5, 5′ , and 7a – 7f exhibit their longest‐wavelength absorption maximum at ca. 380 nm (Table 2). In contrast to 5 and 5′, 7a – 7f in solution exhibit a blue‐green fluorescence with emission bands at around 460 and 480 nm (Table 4 and Fig. 5).     

Alkylation Reactions at the Benzo Moiety of 2,4‐Dimethoxy‐3‐(phenylsulfonyl)benzo[a]heptalenes – Model Compounds of Colchicinoids

Alkylation reactions of 3‐(X‐sulfonyl)benzo[a]heptalene‐2,4‐diols (X=Ph, morpholin‐4‐yl) and their dimethyl ethers were studied. The diols form with K₂CO₃/MeI in aqueous media the 1‐methylated benzoheptalenes, but in yields not surpassing 20% (Table 1). On the other hand, 2,4‐dimethoxybenzo[a]heptalenes can easily be lithiated at C(3) with BuLi and then treated with alkyl iodides to give the 3‐alkylated forms in good yield (Table 2). Surprising is the reaction with two equiv. or more of t‐BuLi since the alkylation at C(4) is accompanied by the reductive elimination of the X‐sulfonyl group at C(3) (Table 3). Most exciting is also the course of 2,4‐dimethoxy‐3‐(phenylsulfonyl)benzo[a]heptalenes in the presence of an excess of MeLi. After the expected exchange of MeO against Me at C(4) (Scheme 6), rearrangement takes place under formation of 4‐benzyl‐2‐methoxybenzo[a]heptalenes and concomitant loss of the sulfonyl group at C(3) (Table 4). In the case of X=morpholin‐4‐yl, rearrangement cannot occur. However, the intermediate benzyl anions of Type E (Scheme 8) react easily with O₂ of the air to build up corresponding benzo[a]heptalene‐4‐methanols (Table 6).