Ru-Catalyzed Heptalene Formation from Azulenes and Dimethyl Acetylenedicarboxylate

It is shown that azulenes react with dimethyl acetylenedicarboxylate (ADM) in solvents such as toluene, dioxan, or MeCN in the presence of 2 mol-% [RuH₂(PPh₃)₄] already at temperatures as low as 100° and lead to the formation of the corresponding heptalene-1,2-dicarboxylates in excellent yields (Tables 1 and 2). The Ru-catalyzed reaction of ADM with 1-(tert-butyl)-4,6,8-trimethylazulene ( 31 ) takes place even at room temperature, yielding the primary tricyclic addition product 32 and its thermal retro-Diels-Alder product dimethyl 4,6,8-trimethylazulene-1,2-dicarboxylate ( 21 ; Scheme 4). At 100° in MeCN, 32 yields 90% of 21 and only 10% of the corresponding heptalene. These observations demonstrate that [RuH₂(PPh₃)₄] catalyzes the first step of the thermal formation of heptalenes from azulenes and ADM which occurs in apolar solvents such as tetralin or decalin at temperatures > 180° (cf. Scheme 1).     

Thermal Reaction of Azulene and Some of Its Symmetrically Substituted Methyl Derivatives with Dimethyl Acetylenedicarboxylate

It is shown that azulene ( 1 ) and dimethyl acetylenedicarboxylate (ADM) in a fourfold molar excess react at 200° in decalin to yield, beside the known heptalene- ( 5 ) and azulene-1,2-dicarboxylates ( 6 ), in an amount of 1.6% tetramethyl (1RS,2RS,5SR,8RS)-tetracyclo[6.2.2.2^2,50^1,5]tetradeca-3,6,9,11,13-pentaene-3,4,9,10-tetracarboxylate(‘anti’-7) as a result of a SHOMO (azulene)/LUMO(ADM)-controlled addition of ADM to the seven-membered ring of 1 followed by a Diels-Alder reaction of the so formed tricyclic intermediate 16 (cf. Scheme 3) with a second molecule of ADM. The structure of ‘anti’-7 was confirmed by an X-ray diffraction analysis. Similarly, the thermal reaction of 5,7-dimehtylazulene ( 3 ) with excess ADM in decalin at 120° led to the formation of ca. 1% of ‘anti’- 12 , the 7,12-dimethyl derivative of‘anti’-7, beside of the corresponding heptalene- 10 and azulene-1,2-dicaboxylated (cf Scheme 2). The introduction of Me groups at C(1)and C(3)of azulene ( 1 ) and its 5,7-dimethyl derivative 3 strongly enhance the thermal formation of the corresponding tetracyclic compound. Thus, 1,3-dimethylazulene ( 2 ) in the presence of a sevenfold molar excess of ADM at 200° yielded 20% of ‘anti’- 9 beside an equal amount of dimethyl 3-mehtylazulene-1,2-dicarboxylate ( 8 ;cf. Scheme 1), and 1,3,5,7-tetramethylazulene ( 4 ) with a fourfold molar excess of ADM AT 200° gave a yield of 37% of‘anti’- 15 beside small amount of the corresponding heptalene- 13 and azulene-1,2-dicarboxylates 14 (cf.Scheme 2).     

A Detailed Investigation of the Reaction of 5,9-Diphenylbenz[a]azulene with Dialkyl Acetylenedicarboxylates Leading to Dialkyl 8,12-Diphenylbenzo[a]heptalene-6,7-dicarboxylates

The synthesis of 5,9-diphenylbenz[a]azulene ( 1 ) from 1,3-diphenylcyclopent[a]indene-2,8-dione ( 4 ) and cyclopropene has been re-investigated. The reduction of the decarbonylated cycloadduct 5 with LiAlH₄/AlCl₃ in Et₂O leads not only to the expected 7,10-dihydrobenz[a]azulene 6 , but also to small amounts of the cyclopropa[b]fluorenes exo- 7 and endo- 7 (cf. Scheme 2), the structures of which have been determined by X-ray crystal-structure analysis (cf. Fig. 1). The reaction of 1 with dialkyl acetylenedicarboxylates (ADR) in MeCN at 100° in the presence of 2 mol-% of catalysts such as [RuH₂(PPh₃)₄] results mainly in the formation of the expected 8,12-diphenylbenzo[a]heptalene-6,7-dicarboxylates 3 . A thorough investigation of the reaction mixture of 1 and dimethyl acetylenedicarboxylate (ADM) revealed the presence of a number of intermediates and side products (Scheme 5). Most important was the isolation and identification of the cyclobutene intermediate 9a (cf. Fig. 4), which is formed by a zwitterionic rearrangement of the primary adduct 2a of 1 and ADM and represents the direct precursor of the heptalene-diester 3a . Compounds of type 9a have so far only been postulated as necessary intermediates in the thermal reaction of azulenes and ADR to give corresponding heptalenedicarboxylates. Compound 9a is photochemically unstable and undergoes rearrangement even under the influence of normal laboratory light into a mixture of trans- 10a and cis- 10a (Scheme 8). Both diastereoisomers are also found in the original reaction mixture of 1 and ADM, but not when the reaction is performed under exclusion of light. On heating in MeCN at 100°, or better in DMF at 150°, trans- 10a and cis- 10a undergo rearrangement to the fluoranthene-1,2-dicarboxylate 11a (Scheme 9), which is also present in the original reaction mixture of 1 and ADM. The catalysts do not accelerate the reaction of 1 and ADR, but they lead to better yields of the benzo[a]heptalene-6,7-dicarboxylates 3 , especially in the reaction of 1 with diisopropyl acetylenedicarboxylate (ADiP) (cf. Tables 1 and 2).     

Thermal Reaction of Highly Alkylated Azulenes with Dimethyl Acetylenedicarboxylate: HOMO(Azulene) vs. SHOMO(Azulene) Control in the Primary Thermal Addition Step

The reaction of highly alkylated azulenes with dimethyl acetylenedicarboxylate (ADM) in decalin or tetralin at 180–200° yields, beside the expected heptalene- and azulene-1,2-dicarboxylates, tetracyclic compounds of type ‘anti’- V and tricyclic compounds of type E (cf. Schemes 2–4 and 8–11). The compounds of type ‘anti’- V represent Diels-Alder adducts of the primary tricyclic intermediates A with ADM. In some cases, the tricyclic compounds of type E also underwent a consecutive Diels-Alder reaction with ADM to yield the tetracyclic compounds of type ‘anti’- or ‘syn’- VI (cf. Schemes 2 and 8–11). The tricyclic compounds of type E , namely 4 and 8 , reversibly rearrange via [1,5]-C shifts to isomeric tricyclic structures (cf. 18 and 19 , respectively, in Scheme 6) already at temperatures > 50°. Photochemically 4 rearranges to a corresponding tetracyclic compound 20 via a di-π-methane reaction. The observed heptalene- and azulene-1,2-dicarboxylates as well as the tetracyclic compounds of type ‘anti’'- V are formed from the primary tricyclic intermediates A via rearrangement (→heptalenedicarboxylates), retro-Diels-Alder reaction (→ azulenedicarboxylates), and Diels-Alder reaction with ADM. The different reaction channels of A are dependent on the substituents. However, the main reaction channel of A is its retro-Diels-Alder reaction to the starting materials (azulene and ADM). The highly reversible Diels-Alder reaction of ADM to the five-membered ring of the azulenes is HOMO(azulene)/LUMO(ADM)-controlled, in contrast to the at 200° irreversible ADM addition to the seven-membered ring of the azulenes to yield the Diels-Alder products of type E . This competing reaction must occur on grounds of orbital-symmetry conservation under SHOMO(azulene)/LUMO(ADM) control (cf. Schemes 20–22). Several X-ray diffraction analyses of the products were performed (cf. Chapt. 4.1).     

Thermal Reaction of Azulenes with Dimethyl Acetylenedicarboxylate in Supercritical Carbon Dioxide

1,3,4,6,8-Pentamethylazulene ( 9 ), when heated at 100° in supercritical CO₂ at 150 bar in the presence of 4 equiv. of dimethyl acetylenedicarboxylate (ADM), led to the formation of 16% of a 1:1 mixture of dimethyl 3,5,6,8,10-pentamethylheptalene-1,2-dicarboxylate 12a ) and its double-bond-shifted isomer 12b as well as 4% of the corresponding azulene-1,2-dicarboxylate 13 (Scheme 4). The formation of the [1 + 2] adduct 11 (cf. Scheme 2) was not observed. Similarly, benz[a]azulene ( 25 ) yielded in supercritical CO₂ (150°/170 bar) in the presence of 4 equiv. of ADM dimethyl benzo[d]heptalene-6,7-dicarboxylate ( 29 ; 30%) and dimethyl benzo[a]cyclopent[cd]azulene-1,2-dicarboxylate ( 28 ; 22%; Scheme 5). The reaction of 5,9-diphenylbenz[a]azulene ( 26 ) and ADM in supercritical CO₂ (100°/150 bar) gave the corresponding benzo[d]heptalene-6,7-dicarboxylate 31 (22%) and dimethyl 5,9-diphenyl-4b,10-etheno-10H-benz[a]azulene-11,12-dicarboxylate( 30 ; 25%; Scheme 5).     

Synthesis and Chiroptical Properties of Dimethyl 8,12-Diphenylbenzo[d]heptalene-6,7-dicarboxylate

6,10-Diphenylbenz[a]azulene ( 3 ) was reacted with dimethyl acetylenedicarboxylate (ADM) in the presence of 2 mol-% of [RuH₂(PPh₃)₄] in MeCN at 100° to yield a 7:1 mixture of dimethyl 2,6-diphenyl-9,10-benzotricyclo[6.2.2.0^1,7]dodeca-2,4,6,9,11-pentaene-11,12-dicarboxylate ( 4 ) and dimethyl 8,12-diphenylbenzo[d]heptalene-6,7-dicarboxylate ( 5 ; Scheme 2). The tricycle 4 , when heated in DMF at 150° for 1 h led to the formation of 81.5% of the heptalene-6,7-dicarboxylate 5 and 15% of the starting azulene 3 . No rearrangement of tricycle 4 was observed, when it was heated at temperatures up to 180° in pseudocumene. The heptalene-6,7-dicarboxylate 5 was easily separated into its antipodes (PM)-and (MP)- 5 on a Chiracel column (cf. Fig. 2). On heating at 150° for 1 h, (MP)- 5 showed no racemization at all. The Ru-catalyzed reaction of benz[a]azulene ( 6 ) with ADM led to the formation of dimethyl 9,10-benzotricyclo[6.2.2.0^1,7]dodeca-2,4,6,9,11-pentaene-11,12-dicarboxylate ( 7 ; Scheme 3). However, the formation of the corresponding heptalene-6,7-dicarboxylate could not be observed.     

Formation of Unusual Products from the Acid-Catalyzed Reaction of Azulenes with Dimethyl Acetylenedicarboxylate

The reaction of guaiazulene ( 4 ) and dimethyl acetylenedicarboxylate (ADM) in tetralin or toluene, catalyzed by 5 mol-% of trifluoroacetic acid (TFA) at ambient temperature, leads to the formation of the corresponding heptalene-4,5-dicarboxylate 6 and a guaiazulenyl-substituted 2,2a,4a,8b-tetrahydrocyclopent[cd]azulene derivative 7 beside the expected guaiazulenyl-substituted ethenedicarboxylates (E)- 5 and (Z)- 5 as main products (Scheme 2). The structure of 7 was unequivocally established by an X-ray crystal-structure analysis (Fig. 1). Precursor of 7 must be the 2a,4a-dihydrocyclopent[cd]azulene-3,4-dicarboxylate 9 which reacts, under TFA catalysis, with a second molecule of 4 (Scheme 3). No formation of products of type 7 has been observed in the TFA-catalyzed reaction of 4,6,8-trimethyl- and 1,4,6,8-tetramethylazulene ( 13 and 16 , respectively) and ADM (Scheme 4). On the other hand, the TFA-catalyzed reaction of azulene ( 18 ) itself and ADM at ambient temperature gives rise to a whole variety of new products (Scheme 5), the major part of which is derived from dimethyl 2a,4a-dihydrocyclopent[cd]azulene-3,4-dicarboxylate ( 25 ) as the main intermediate (Scheme 6). Nevertheless, for the formation of the 2a,4a,6,8b-tetrahydrocyclobut[a]azulene derivatives (E)- 24a and (E)- 24b , a corresponding 2a,8b-dihydro precursor 29 has to be postulated as crucial intermediate (Scheme 8).     

Double-Bond Shifts in [4n]Annulenes as a New Principle for Molecular Switches: First Results with Dimethyl Heptalene-1,2- and -4,5-dicarboxylates

A new concept for molecular switches, based on thermal or photochemical double-bond shifts (DBS) in [4n]annulenes such as heptalenes or cyclooctatetraenes, is introduced (cf. Scheme 2). Several heptalene-1,2- and -4,5-dicarboxylates (cf. Scheme 4) with (E)-styryl and Ph groups at C(5) and C(1), or C(4) and C(2), respectively, have been investigated. Several X-ray crystal-structure analyses (cf. Figs. 1–5) showed that the (E)-styryl group occupies in the crystals an almost perfect s-trans-conformation with respect to the C?C bond of the (E)-styryl moiety and the adjacent C?C bond of the heptalene core. Supplementary ¹H-NOE measurements showed that the s-trans-conformations are also adopted in solution (cf. Schemes 6 and 9). Therefore, the DBS process in heptalenes (cf. Schemes 5 and 8) is always accompanied by a 180° torsion of the (E)-styryl group with respect to its adjacent C?C bond of the heptalene core. The UV/VIS spectra of the heptalene-1,2- and -4,5-dicarboxylates illustrated that it can indeed be differentiated between an ‘off-state’, which possesses no ‘through-conjugation’ of the π-donor substituent and the corresponding MeOCO group and an ‘on-state’ where this ‘through-conjugation’ is realized. The ‘through-conjugation’, i.e., conjugative interaction via the involved s-cis-butadiene substructure of the heptalene skeleton, is indicated by a strong enhancement of the intensities of the heptalene absorption bands I and II (cf. Tables 3–6). The most impressive examples are the heptalene-dicarboxylates 11a , representing the off-state, and 11b which stands for the on-state (cf. Fig.8).     

New Results in the Synthesis of Styrylazulene Derivatives: Application of the ‘Anil synthesis’ to the preparation of azulenes substituted with styryl groups at the seven-membered ring

The synthesis of 4,6,8-trimethyl-1-[(E)-4-R-styryl]azulenes 5 (R=H, MeO, Cl) has been performed by Wittig reaction of 4,6,8-trimethylazulene-1-carbaldehyde ( 1 ) and the corresponding 4-(R-benzyl)(triphenyl)phosphonium chlorides 4 in the presence of EtONa/EtOH in boiling toluene (see Table 1). In the same way, guaiazulene-3-carbaldehyde ( 2 ) as well as dihydrolactaroviolin ( 3 ) yielded with 4a the corresponding styrylazulenes 6 and 7 , respectively (see Table 1). It has been found that 1 and 4b yield, in competition to the Wittig reaction, alkylation products, namely 8 and 9 , respectively (cf. Scheme 1). The reaction of 4,6,8-trimethylazulene ( 10 ) with 4b in toluene showed that azulenes can, indeed, be easily alkylated with the phosphonium salt 4b . 4,6,8-Trimethylazulene-2-carbaldehyde ( 12 ) has been synthesized from the corresponding carboxylate 15 by a reduction (LiAlH₄) and dehydrogenation (MnO₂) sequence (see Scheme 2). The Swern oxidation of the intermediate 2-(hydroxymethyl)azulene 16 yielded only 1,3-dichloroazulene derivatives (cf. Scheme 2). The Wittig reaction of 12 with 4a and 4b in the presence of EtONa/EtOH in toluene yielded the expected 2-styryl derivatives 19a and 19b , respectively (see Scheme 3). Again, the yield of 19b was reduced by a competing alkylation reaction of 19b with 4b which led to the formation of the 1-benzylated product 20 (see Scheme 3). The ‘anil synthesis’ of guaiazulene ( 21 ) and the 4-R-benzanils 22 (R=H, MeO, Cl, Me₂N) proceeded smoothyl under standard conditions (powered KOH in DMF) to yield the corresponding 4-[(E)-styryl]azulene derivatives 23 (see Table 4). In minor amounts, bis(azulen-4-yl) compounds of type 24 and 25 were also formed (see Table 4). The ‘anil reaction’ of 21 and 4-NO₂C₆H₄CH=NC₆H₅ ( 22e ) in DMF yielded no corresponding styrylazulene derivative 23e . Instead, (E)-1,2-bis(7-isopropyl-1-methylazulen-4-yl)ethene ( 27 ) was formed (see Scheme 4). The reaction of 4,6,8-trimethylazulene ( 10 ) and benzanil ( 22a ) in the presence of KOH in DMF yielded the benzanil adducts 28 to 31 (cf. Scheme 5). Their direct base-catalyzed transformation into the corresponding styryl-substituted azulenes could not be realized (cf. Scheme 6). However, the transformation succeeded smoothly with KOH in boiling EtOH after N-methylation (cf. Scheme 6).     

Synthesis of Benz[a]azulenes Substituted at the Benzo Ring

It is shown that the thermal electrocyclic ring-closure reaction of 1,2-di[(E)-prop-1-enyl]benzene to yield 2,3-dimethylnaphthalene (cf. Scheme 1) [10] can successfully be applied also to the synthesis of benz[a]azulenes (cf. Schemes 2 and 3). Starting materials are methyl 4,6,8-trimethylazulen-2-yl ketone ( 6 ) and the corresponding 2-carbaldehyde 5 , which, in a Horner-Emmons reaction, are transformed into the (azulen-2-yl)-acrylates (E)- 8 and (E)- 7 , respectively. Vilsmeier formylation of these compounds, followed by the Horner-Emmons reaction leads to the formation of the bisacrylates (E,E)- 11 and (E,E)- 12 , respectively. In an alternative reaction, (E)- 8 , on treatment with dimethyl acetylenedicarboxylate (ADM) in the presence of [RuH₂(PPh₃)₄], can be transformed into the methoxycarbonyl-substituted bisacrylates (E,E)- and (E,Z)- 17 . All three bisacrylates, on heating at 180–190° in p-cymene, undergo cyclization to yield the corresponding dihydrobenz[a]azulenes 13 , 14 , and 18 , respectively, which could easily be dehydrogenated on heating in the presence of Pd/C. The new benz[a]azulenes 15 , 16 , and 19 are fully characterized.     

Thermal Reactions of Guaiazulene and Its 3-Methyl Derivative with Dimethyl Acetylenedicarboxylate

The thermal reaction of 7-isopropyl-1,3,4-trimethylazulene (3-methylguaiazulene; 2 ) with excess dimethyl acetylenedicarboxylate (ADM) in decalin at 200° leads to the formation of the corresponding heptalene- ( 5a/5b and 6a/6b ; cf. Scheme 3) and azulene-1,2-dicarboxylates ( 7 and 8 , respectively). Together with small amounts of a corresponding tetracyclic compound (‘anti’- 13 ) these compounds are obtained via rearrangement (→ 5a/5b and 6a/6b ), retro-Diels-Alder reaction (→ 7 and 8 ), and Diels-Alder reaction with ADM (→ ‘anti’- 13 ) from the two primary tricyclic intermediates ( 14 and 15 ; cf. Scheme 5) which are formed by site-selective addition of ADM to the five-membered ring of 2 . In a competing Diels-Alder reaction, ADM is also added to the seven-membered ring of 2 , leading to the formation of the tricyclic compounds 9 and 10 and of the Diels-Alder adducts ‘anti’- 11 and ‘anti’- 12 , respectively of 9 and of a third tricyclic intermediate 16 which is at 200° in thermal equilibrium with 9 and 10 (cf. Scheme 6). The heptalenedicarboxylates 5a and 5b as well as 6a and 6b are interconverting slowly already at ambient temperature (Scheme 4). The thermal reaction of guaiazulene ( 1 ) with excess ADM in decalin at 190° leads alongside with the known heptalene- ( 3a ) and azulene-1,2-dicarboxylates ( 4 ; cf. Schemes 2 and 7) to the formation of six tetracyclic compounds ‘anti’- 17 to ‘anti’- 21 as well as ‘syn’- 19 and small amounts of a 4:1 mixture of the tricyclic tetracarboxylates 22 and 23 . The structure of the tetracyclic compounds can be traced back by a retro-Diels-Alder reaction to the corresponding structures of tricyclic compounds ( 24--29 ; cf. Scheme 8) which are thermally interconverting by [1,5]-C shifts at 190°. The tricyclic tetracarboxylates 22 and 23 , which are slowly equilibrating already at ambient temperature, are formed by thermal addition of ADM to the seven-membered ring of dimethyl 5-isopropyl-3,8-dimethylazulene-1,2-dicarboxylate ( 7 ; cf. Scheme 10). Azulene 7 which is electronically deactivated by the two MeOCO groups at C(1) and C(2) shows no more thermal reactivity in the presence of ADM at the five-membered ring (cf. Scheme 11). The tricyclic tetracarboxylates 22 and 23 react with excess ADM at 200° in a slow Diels-Alder reaction to form the tetracyclic hexacarboxylates 32 , ‘anti’- 33 , and ‘anti’- 34 (cf. Schemes 10–12 as well as Scheme 13). A structural correlation of the tri- and tetracyclic compounds is only feasible if thermal equilibration via [1,5]-C shifts between all six possible tricyclic tetracarboxylates ( 22, 23 , and 35–38 ; cf. Scheme 13) is assumed. The tetracyclic hexacarboxylates 32 , ‘anti’- 33 , and ‘anti’- 34 seem to arise from the most strained tricyclic intermediates ( 36–38 ) by the Diels-Alder reaction with ADM.     

Reaction of Hydroxymethyl- and Alkyl-Substituted Azulenes with Manganese Dioxide

It is shown that the 2-(hydroxymethyl)-1-methylazulenes 6 are being oxidized by activated MnO₂ in CH₂Cl₂ at room temperature to the corresponding azulene-1,2-dicarbaldehydes 7 (Scheme 2). Extension of the MnO₂ oxidation reaction to 1-methyl- and/or 3-methyl-substituted azulenes led to the formation of the corresponding azulene-1-carbaldehydes in excellent yields (Scheme 3). The reaction of unsymmetrically substituted 1,3-dimethyl-azulenes (cf. 15 in Scheme 4) with MnO₂ shows only little chemoselectivity. However, the observed ratio of the formed constitutionally isometric azulene-1-carbaldehydes is in agreement with the size of the orbital coefficients in the HOMO of the azulenes. The reaction of guaiazulene ( 18 ) with MnO₂ in dioxane/H₂O at room temperature gave mainly the expected carbaldehyde 19 . However, it was accompanied by the azulene-diones 20 and 21 (Scheme 5). The precursor of the demethylated compound 20 is the carbaldehyde 19 . Similarly, the MnO₂ reaction of 7-isopropyl-4-methyalazulene ( 22 ) as well as of 4,6,8-trimethylazulene ( 24 ) led to the formation of a mixture of the corresponding azulene-1,5-diones and azulene-1,7-diones 20 / 23 and 25 / 26 , respectively, in decent yields (Schemes 6 and 7). No MnO₂ reaction was observed with 5,7-dimethylazulene.     

An Improved and Simplified Synthesis of 4-Styrylazulenes

It is shown that 4- or 8-[(E)-styryl]-substituted azulenes can easily be prepared from 4- or 8-methylazulenes in the presence of potassium tert-butoxide (t-BuOK) with the corresponding benzaldehydes in tetrahydrofuran (THF) at −5 to 25° (see Schemes 1 and 2). 6-(tert-Butyl)-4,8-dimethylazulene ( 5 ) with both Me groups in reactive positions leads to the formation of a mixture of the mono- and distyryl-substituted azulenes 6 and 7 , respectively (Scheme 3). Vilsmeier formylation of 6 results in the formation of 3 : 2 mixture of the azulene-carbaldehydes 8a and 8b , which can be separated by chromatography on silica gel. Reduction of 8a and 8b with NaBH₄ in trifluoroacetic acid (TFA)/CH₂Cl₂ gives the 1-methyl forms 9a and 9b , respectively, in good yields (Scheme 4). The latter two azulenes are not separable on silica gel.     

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 .     

Some Unusual Products from the Thermal Reaction of Azulenes with Dimethyl Acetylenedicarboxylate

The thermal reaction of azulene-1-carbaldehydes 5 and 6 with excess dimethyl acetylenedicarboxylate (ADM) in decalin leads mainly to the formation of (1 + 1) and (1 + 2) adducts arising from the addition of ADM at the seven-membered ring of the azulenes (cf. Schemes 2 and 4). The (1 + 2) adducts are formed in a homo-Diels-Alder reaction of ADM and isomeric tricyclic carbaldehydes which are derived from the primary tricyclic carbaldehydes by reversible [1s5s]-C shifts (cf. Schemes 3 and 5). The thus formed pentacyclic carbaldehydes seem to undergo deep-seated skeletal rearrangements (cf. Scheme 7) which result finally in the formation of the formyl-tetrahydrocyclopenta[bc]acenaphthylene-tetraesters 12 and 19 , respectively. In other cases, e.g., azulene-1-carbaldehydes 7 and 8 (cf. Scheme 8), the thermal reaction with excess ADM furnishes only the already known tetracycfic (1 + 2) adducts of type anti- 26 to ‘anti’- 29 . The thermal reaction of 1,3,4,8-tetramethylazulene ( 9 ) with excess ADM in decalin resulted in the formation of two (1 + 2) and one (1 + 3) adduct in low yields (cf. Scheme 9). The latter turned out to be the 2,6-bridged barrelene derivative 32 . There are structural evidences that 32 is formed by similar pathways as the formyl-tetrahydrocyclopenta[bc]acenaphthylene-tetraesters (cf. Schemes 7 and 11). [²H₃]Me-Labelling experiments are in agreement with the proposed mechanisms (cf. Scheme 13).     

A ‘One-Pot’ Anellation Method for the Transformation of Heptalene-4,5-dicarboxylates into Benzo[a]heptalenes

It has been found that dimethyl heptalene-4,5-dicarboxylates, when treated with 4 mol-equiv. of lithiated N,N-dialkylamino methyl sulfones or methyl phenyl sulfone, followed by 4 mol-equiv. of BuLi in THF in the temperature range of ?78 to 20°, give rise to the formation of 3-[(N,N-dialkylamino)sulfonyl]- or 3-(phenylsul-fonyl)benzo[a]heptalene-2,4-diols of. (cf. Scheme 4, and Tables 2 and 3). Accompanying products are 2,4-bis{[(N,N-dialkylamino)sulfonyl]methyl}- or 2,4-bis[(phenylsulfonyl)methyl]-4,10a-dihydro-3H-heptaleno[1,10-bc]furan-3-carboxylates as mixtures of diastereoisomers of. cf. Scheme 4, and (Tables 2 and 3) which are the result of a Michael addition reaction of the lithiated methyl sulfones at C(3) of the heptalene-4,5-dicarboxylates, followed by (sulfonyl)methylation of the methoxycarbonyl group at C(5) and cyclization of. (cf. Scheme 5). It is assumed that the benzo[a]heptalene formation is due to (sulfonyl)methylation of both methoxycarbonyl groups of the heptalene-4,5-dicarboxylates of. (cf. Schemes 6 and 8). The resulting bis-enolates 35 are deprotonated further. The thus formed tris-anions 36 can then cyclize to corresponding tris-anions 37 of cyclopenta[d]heptalenes which, after loss of N,N-dialkylamido sulfite or phenyl sulfinate, undergo a ring-enlargement reaction by 1,2-C migration finally leading to the observed benzo[a]heptalenes of. (cf. Schemes 8 and 9). The structures of the new product types have been finally established by X-ray crystal-structure analyses (cf. Figs. 1 and 2 as well as Exper. Part).     

Cationic RhI Complexes of Azulenes and Their Catalytic Activity on the Formation of Heptalene-1,2-dicarboxylates from Dimethyl Acetylenedicarboxylate and Azulenes

[Rh¹(η⁵-azulene)(cod)]⁺BF complexes 3a–g (cod = (Z,Z)-cycloocta-1,5-diene) have been synthesized by reaction of [Rh¹(cod)]⁺BF in THF with the corresponding azulenes 1a–g (Table 1). The structure of [Rh¹(cod)(η⁵-guaiazulene)]⁺BF ( 3a ) has been determined by X-ray diffraction analysis (Fig. 1 and 2). The Rh-atom is oriented above the five-membered ring of the azulene with almost equal Rh? C distances to all five C-atoms of the ring. The (Z,Z)-cycloocta-1,5-diene ring occurs in two enantiomorphic distorted (C_2v → C₂) tub conformations in the crystals (Fig. 3). In CDCl₃ solution, the cod ligand in the complexes 3 shows a dynamic behavior on the ¹H-NMR time scale which is best explained by rotation of the cod ligand relative to the azulene ligands around an imaginary cod? Rh? azulene axis. The new complexes 3 catalyze the formation of heptalene-1,2-dicarboxylates 2 from dimethyl acetylenedicarboxylate (ADM) and the corresponding azulenes 1 just as effectively as [RuH₂(PPh₃)₄] and the analogous [RhH(PPh₃)₄] complex in MeCN solution (Table 3). On grounds of simplicity, 3 can be generated in situ, when [RhCl(cod)]₂ is applied as catalyst (Table 3).     

Synthesis of 6-Styrylheptalenes

4-Methylazulenes 3 , 15 , and 23 were transformed into 4-[(methylthio)methyl]azulene 4 , and azulene-4-carbaldehyde dimethyl dithioacetals 16 and 24 , respectively. Vilsmeier formylation of 4 and 16 , and subsequent reduction led to the 1-methyl derivatives 6 and 18 , respectively. The thermal reaction of azulenes 6 , 18 , and 24 with dimethyl acetylenedicarboxylate (ADM) in toluene afforded heptalenes with a (methylthio)methyl group or a [bis(methylthio)]methyl group at C(6). Chlorination of [(methylthio)methyl]heptalene 7 , followed by treatment with HgO and BF₃⋅OEt₂ in aqueous tetrahydrofuran (THF), led to 6-formylheptalene-dicarboxylate 12 in excellent yield. Similarly, hydrolysis of 18 and 24 by HgO and BF₃⋅OEt₂ in aqueous THF afforded the 6-formyl derivatives 21 and 27 , respectively. Wittig reaction of the 6-formyl-substituted heptalenes and phosphonium salts 13a – e in the two-phase system CH₂Cl₂/2n aqueous NaOH resulted in the formation of 6-styryl-substituted heptalenes.     

Formation of 6,10‐Diphenyl‐Substituted Heptalene‐4,5‐dicarboxylates

It is shown that 4,8‐diphenylazulene ( 1 ) can be easily prepared from azulene by two consecutive phenylation reactions with PhLi, followed by dehydrogenation with chloranil. Similarly, a Me group can subsequently be introduced with MeLi at C(6) of 1 (Scheme 2). This methylation led not only to the expected main product, azulene 2 , but also to small amounts of product 3 , the structure of which has been determined by X‐ray crystal‐structure analysis (cf. Fig. 1). As expected, the latter product reacts with chloranil at 40° in Et₂O to give 2 in quantitative yields. Vilsmeier formylation of 1 and 2 led to the formation of the corresponding azulene‐1‐carbaldehydes 4 and 5 . Reduction of 4 and 5 with NaBH₄/BF₃ ? OEt₂ in diglyme/Et₂O 1 : 1 and BF₃ ? OEt₂, gave the 1‐methylazulenes 6 and 7 , respectively. In the same way was azulene 9 available from 6 via Vilsmeier formylation, followed by reduction of azulene‐1‐carbaldehyde 8 (Scheme 3). The thermal reactions of azulenes 1, 6 , and 7 with excess dimethyl acetylenedicarboxylate (ADM) in MeCN at 100° during 72 h afforded the corresponding heptalene‐4,5‐dicarboxylates 11, 12 , and 13 , respectively (Scheme 4). On the other hand, the highly substituted azulene 9 gave hardly any heptalene‐4,5‐dicarboxylate.     

Synthesis of an Isotopically Isomeric Mixture of 1,4,6,8-Tetramethyl[13C2]azulene and Its Thermal Reaction with Dimethyl Acetylenedicarboxylate

Sodium [1,3-¹³C₂]cyclopentadienide in tetrahydrofuran (THF) has been prepared from the corresponding labelled [¹³C₂]cyclopentadiene which was synthesized from ¹³CO₂ and (chloromethyl)trimethylsilane (cf. Scheme 10) according to an established procedure. It could be shown that the acetate pyrolysis of cis-cyclopentane-1,2-diyl diacetate (cis- 22 ) at 550 ± 5° under reduced pressure (60 Torr) gives five times as much cyclopentadiene as trans- 22 . The reaction of sodium [1,3-¹³C₂]cyclopentadienide with 2,4,6-trimethylpyrylium tetrafluoroborate in THF leads to the formation of the statistically expected 2:2:1 mixture of 4,6,8-trimethyl[1,3a-¹³C₂], -[2,3a-¹³C₂]-, and -[1,3-¹³C₂]azulene ( 20 ; cf. Scheme 7 and Fig. 1). Formylation and reduction of the 2:2:1 mixture [¹³C₂]- 20 results in the formation of a 1:1:1:1:1 mixture of 1,4,6,8-tetramethyl[1,3-¹³C₂]-, -[1,3a-¹³C₂]-, -[2,3a-¹³C₂]-, -[2,8a-¹³C₂]-, and -[3,8a-¹³C₂]azulene ( 5 ; cf. Scheme 8 and Fig. 2). The measured ²J(¹³C, ¹³C) values of [¹³C₂]- 20 and [¹³C₂]- 5 are listed in Tables 1 and 2. Thermal reaction of the 1:1:1:1:1 mixture [¹³C₂]- 5 with the four-fold amount of dimethyl acetylenedicarboxylate (ADM) at 200° in tetralin (cf. Scheme 2) gave 5,6,8,10-tetramethyl-[¹³C₂]heptalene-1,2-dicarboxylate ([¹³C₂]- 6a ; 22%), its double-bond-shifted (DBS) isomer [¹³C₂]- 6b (19%), and the corresponding azulene-1,2-dicarboxylate 7 (18%). The isotopically isomeric mixture of [¹³C₂]- 6a showed no ¹J(¹³C,¹³C) at C(5) (cf. Fig. 3). This finding is in agreement with the fact that the expected primary tricyclic intermediate [7,11-¹³C₂]- 8 exhibits at 200° in tetralin only cleavage of the C(1)? C(10) bond and formation of a C(7)? C(10) bond (cf. Schemes 6 and 9), but no cleavage of the C(1)? C(11) bond and formation of a C(7)? C(11) bond. The limits of detection of the applied method is ≥96% for the observed process, i.e., [1,3a-¹³C₂]- 5 + ADM→ [7,11-¹³C₂]- 8 →[1,6-¹³C₂]- 9 →[5,10a-¹³C₂]- 6a (cf. Scheme 6).