Norbornadiene Research Articles

Photolytic Studies of Norbornadiene Derivatives under High-Intensity Light Conditions.

The photoconversion of a norbornadiene (NBD) derivative was studied under high-intensity mono- and polychromatic light conditions at high concentrations. The photoisomerization quantum yield (ϕNBD→QC), proceeding from NBD to its quadricyclane (QC) isomer, was determined using a tunable OPO laser and a solar simulator light source. The solar simulator was designed to mimic the AM1.5G solar spectrum between 300 and 900 nm. Using the OPO laser, ϕNBD→QC was measured at discrete values between 310 and 350 nm in steps of 10 nm, and a variation between 0.81 and 0.96 was observed. Weighting these values of ϕNBD→QC with the spectral profile of the solar simulator, an averaged value of 0.87 ± 0.03 was obtained. Determination of ϕNBD→QC was also performed directly in the solar simulator providing a value of 0.97 ± 0.14, in good agreement with the weighted values from the OPO. Photoisomerization quantum yields were found to decrease slightly at higher concentrations. At high concentrations, we found that correcting for the presence of QC was important due to similar absorption coefficients of the NBD and QC isomers at the absorption tail. Cyclability of the forward and backward NBD/QC conversion was studied over several cycles. The NBD/QC couple exhibited excellent thermal stability, but a slight photodegradation per cycle was observed, increasing with the concentration of the sample. This result indicates that the molecules undergo some intermolecular reactions.

Synthesis of Polynorbornadiene within the Pores of Activated Carbons: Effects on EDLC and Hydrogen Adsorption Performances.

Norbornadiene (NBD) is adsorbed on activated carbon (AC), and the adsorbed NBD is polymerized within the pores of AC. Two kinds of ACs─AC-2 with only micropores of ∼2 nm and AC-4 with not only micropores but also mesopores below 4 nm─are examined to study the effects of the hybridized polynorbornadiene (PNBD) on the electric double-layer capacitor and hydrogen adsorption performance. Various measurements are performed to determine the form of the hybridized PNBD inside the pores of AC. Scanning and transmittance electron microscopy observations of the AC/PNBD hybrids confirm that PNBD is hybridized inside the pores of AC, and there is little PNBD on the surface of AC particles. The nitrogen adsorption/desorption measurement for the hybrids of AC-4 reveals that PNBD is not hybridized preferentially inside micropores rather than mesopores irrespective of the amount of PNBD. In addition, both micropore and mesopore volumes decrease at a constant rate with increasing amounts of PNBD. These results suggest that PNBD is hybridized not as a layer but as an agglomerate for both ACs, and the agglomerate delocalizes over the whole AC pores, which is supported by the results of electrochemical measurements and hydrogen adsorption behavior of the hybrids.

A mechanistic DFT study of Z-selective ring-opening metathesis polymerization by MAP catalysts

Herein we present a detailed mechanistic investigation of Z-selective ROMP of norbornadiene (NBD) with MAP-based catalyst 3a-b by DFT (M06) calculations. The studied catalytic reaction mechanism include the initiation, interconversion, and propagation stages with anti and syn NBD olefin attacks. In the initiation, we found the inversion of the metal center from the (R) configuration in the initial reactant to the (S). Further, we found that the olefin attack anti is kinetically more favorable than syn. Following this, we found that the stabilization order of the ring-opening species (ROS) is vital for the Z-selectivity due to the 3a-b-anti-1/2-cis/trans producing cis-polymers, and 3a-b-syn-1/2-trans produces trans-polymers. Furthermore, the 3a-anti-2-cis+syn pathways form cis-syndiotactic polymers that must overcome 30.8 kcal mol−1, respectively, during the propagation stage. Finally, the [2+2] cycloaddition of the propagation stages determines the reaction rate, which is essential for producing stereoregular polymers.

Long‐Term Energy Storage Systems Based on the Dihydroazulene/Vinylheptafulvene Photo‐/Thermoswitch

AbstractThe dihydroazulene/vinylheptafulvene (DHA/VHF) couple presents a photo‐/thermoswitch that has attracted interest for the development of molecular solar thermal energy storage systems. Here we present the synthesis and optical properties as well as the switching properties of DHA derivatives incorporating alkyne and norbornadiene (NBD) substituents at position C2. The corresponding VHF isomers exhibited remarkably long lifetimes. Thus, the VHF‐to‐DHA back‐reaction half‐life for a derivative with a phenylethynyl substituent was 22 days in acetonitrile at room temperature, while the half‐life of a VHF reference compound having a phenyl substituent is around 3.5 h under the same conditions. This finding is particularly attractive in the quest for long‐term energy storage systems, considering limited enhancement of molecular weight and maintenance of good quantum yield photoisomerization. Moreover, from the thermal back‐reaction of an NBD compound, used as a precursor for the DHA‐NBD dyad, we estimated the radical‐stabilizing influence of a C(Me)=C(CN)2 substituent.

Star polymers with norbornene/1-octene gradient copolymer arms synthesized by an ansa-fluorenylamidodimethyltitanium-[Ph3C][B(C6F5)4] catalyst system

Cyclic olefin copolymers (COCs), among which ethylene (E)/norbornene (NB) copolymer have been commercialized, are novel optical plastics but their brittleness narrows their application fields. In this research, star polymers with NB/1-octene (O) gradient copolymer arms and cross-linked cores were synthesized by adding norbornadiene (NBD)/E or 1,11-dodecadiene (DOD) after NB/O copolymerization using a ( t BuNSiMe 2 Flu)TiMe 2 ( I )-[Ph 3 C][B(C 6 F 5 ) 4 ] catalyst system with 2,6-di- tert -butyl-4-methylphenol-treated tri- n -octylaluminum (Oct 3 Al/BHT) as a scavenger. The obtained star polymers possessed the O-rich soft segments close to the core and the NB-rich hard segments in the outer layer. The DOD-cored star polymers had a larger number of arms (15) and a higher star polymer content (68 wt%) than the NBD/E-cored ones (6 and 58 wt%, respectively). The films of the mixtures of the star and the corresponding arm polymers were molded by a melt-pressing procedure. The tensile properties of the films of the DOD-cored star polymer mixtures were significantly improved compared to those of the arm polymers. The Young's modulus, yield stress, strength, and strain at break were controlled by the NB content, the arm length, and the gradient structure of the arms. These results clarified the novelty of the star-shaped NB/ α -olefin copolymers and would expand the application fields of COCs. • Star-shaped polymers with norbornene/1-octene gradient copolymer arms were synthesized by an “arm-first” strategy. • The star-shaped polymers showed two T g values assignable to an octene-rich soft core and a norbornene-rich hard sphere. • Transparent films were obtained from the blend of the star and the arm polymers by melt processing. • The of mechanical properties of the blend films were improved compared with those of the arm polymer films. • The thermal and the star polymers were controlled by the arm length and the norbornene content.

Pyrolysis of norbornadiene: An experimental and kinetic modeling study

Pyrolysis of norbornadiene (NBD) was investigated in a jet-stirred reactor at 760 torr and within the temperature range of 560 to 1100 K. 37 products and intermediates ranging from m/z = 26 to 178 were identified and quantified by using synchrotron radiation photoionization molecular-beam mass spectrometry accompanied by gas chromatography. The C7H8 isomers, including NBD, cycloheptatriene (CHT) and toluene (A1CH3), were differentiated. Acetylene (C2H2), cyclopentadiene (C5H6), C7H8 isomers and other aromatics are abundant during the pyrolysis process. A detailed kinetic model was developed with reasonable predictions against the measured results, involving 264 species and 1259 reactions, which is helpful to understand the kinetic and heat features of initial consumption as well as the synergistic effect of C2H2, C5H6 and A1CH3 to the formation of aromatics. Rate-of-production analysis shows that the pyrolysis of NBD experiences two stages. The first stage involves the isomerization and decomposition of NBD, yielding A1CH3, CHT, C5H6 and C2H2. Branching ratio analysis shows that C5H6 and C2H2 are dominant products in the first stage. Heat release analysis indicates that the first stage is endothermic. The second stage involves the pyrolysis and interactions of first-stage products to produce light hydrocarbons and polycyclic aromatic hydrocarbons like indene, naphthalene, acenaphthylene, fluorene and phenanthrene. Three routes dominate the formation of indene, including C5H6 + C5H5, benzyl radical + C2H2 and cycloheptatrienyl radical + C2H2, the last one of which has been scarcely considered in the previous investigation. Sensitivity analysis indicates the reactions in the first stage exhibit strong promoting effects for the consumption of NBD, while the second-stage reactions play minor roles in NBD consumption. This work would enrich the understanding of NBD consumption, PAH formation and heat release.

Heterogeneously supported active Pd(0) complex on silica mediated by PEG as efficient dimerization catalyst for the production of high energy density fuel

High energy-density hydrocarbon fuels are the key materials for aircraft to increase payload and expand flight distance. Under the catalysis of atomically dispersed Pd(0) complex, saturated hexacyclic exo-exo product with good fuel property can be synthesized via co-dimerization of norbornadiene (NBD) with quadricyclane (QC). But the fragile coordination state of homogeneous Pd(0) complex limited its application in fuel production. In this work, Pd(0) complex has been heterogeneously supported on silica support with the regulation of polyethylene glycol (PEG) to increase its stability and recyclability with limited sacrifice of the activity and dimer selectivity. The structure of silica support was tuned to achieve the proper interaction with PEG and Pd(0) complex. The chain length and concentration of PEG were adjusted to further optimize the state of Pd(0) complex on silica. The as-developed heterogeneous Pd(0) complex supported on dendritic mesoporous silica nanospheres regulated by PEG2000 exhibited superior performance for the production of high energy density fuel through selective NBD-QC co-dimerization.

A new approach exploiting thermally activated delayed fluorescence molecules to optimize solar thermal energy storage

We propose a new concept exploiting thermally activated delayed fluorescence (TADF) molecules as photosensitizers, storage units and signal transducers to harness solar thermal energy. Molecular composites based on the TADF core phenoxazine–triphenyltriazine (PXZ-TRZ) anchored with norbornadiene (NBD) were synthesized, yielding compounds PZDN and PZTN with two and four NBD units, respectively. Upon visible-light excitation, energy transfer to the triplet state of NBD occurred, followed by NBD → quadricyclane (QC) conversion, which can be monitored by changes in steady-state or time-resolved spectra. The small S1-T1 energy gap was found to be advantageous in optimizing the solar excitation wavelength. Upon tuning the molecule’s triplet state energy lower than that of NBD (61 kcal/mol), as achieved by another composite PZQN, the efficiency of the NBD → QC conversion decreased drastically. Upon catalysis, the reverse QC → NBD reaction occurred at room temperature, converting the stored chemical energy back to heat with excellent reversibility.

NORBORNADIENO: SÍNTESE E APLICAÇÕES

NORBORNADIENE: SYNTHESIS AND APPLICATIONS. Bicyclo[2.2.1]hepta-2,5-diene or norbornadiene (NBD) is a bicyclic hydrocarbon molecule used in many fields of chemistry since its first synthesis in 1950. NBD is the precursor of Aldrin, an agrochemical that was commonly used as an insecticide until the end of the XX century. Norbornadiene is capable of interconverting into its isomer, quadricyclane (QC), by the incidence of ultraviolet light. After that, the QC can release heat to return to the initial stage of NBD. To increase the energy efficiency and to enable the reaction to occur at the solar light, chemists have synthesized a wide variety of NBD derivatives. Papers in more than seventy years have studied and reported the physical and chemical properties of those compounds and their applications as rocket fuel, solar energy cell, homogeneous catalysis, and agrochemicals published between 1950 and 2021. Even computational chemistry has been used to investigate the NBD derivatives synthetic routes, elucidation, and application. In addition, this review describes other applications of norbornadiene.

Solar Energy Storage: Competition between Delocalized Charge Transfer and Localized Excited States in the Norbornadiene to Quadricyclane Photoisomerization.

We describe for the first time the full reaction coordinate regarding the photoisomerization of red-absorbing norbornadienes (NBDs) to quadricyclanes (QCs). Our studies go beyond steady-state investigations by using an arsenal of time-resolved techniques. Importantly, the red absorption of NBDs is made possible by a different charge-transfer character; adjusting its strength enables control over the photoreversibility of the rearrangement. In the case of strong charge-transfer character (a weakly electron-withdrawing ester and a strongly electron-donating dimethylaniline), photoirradiation with visible light into the delocalized charge-transfer absorption of NBD affords QC reversibly. In stark contrast, UV photoirradiation into the NBD localized excited state leads to a photoinduced degradation and cannot be back-isomerized to NBD under any circumstances. If the charge-transfer character is weak (a weakly electron-withdrawing ester and a weakly electron-donating phenyl), reversibility is seen independently of the photoirradiation light.

Platinum(II) Di-ω-alkenyl Complexes as "Slow-Release" Precatalysts for Heat-Triggered Olefin Hydrosilylation.

We describe the synthesis, characterization, and catalytic hydrosilylation activity of platinum(II) di-ω-alkenyl compounds of stoichiometry PtR2, where R = CH2SiMe2(vinyl) (1) or CH2SiMe2(allyl) (2), and their adducts with 1,5-cyclooctadiene (COD), dibenzo[a,e]cyclooctatetraene (DBCOT), and norbornadiene (NBD), which can be considered as slow-release sources of the reactive compounds 1 and 2. At loadings of 0.5 × 10-6-5 × 10-6 mol %, 1-COD is an active hydrosilylation catalyst that exhibits heat-triggered latency: no hydrosilylation activity occurs toward many olefin substrates even after several hours at 20 °C, but turnover numbers as high as 200000 are seen after 4 h at 50 °C, with excellent selectivity for formation of the anti-Markovnikov product. Activation of the PtII precatalyst occurs via three steps: slow dissociation of COD from 1-COD to form 1, rapid reaction of 1 with silane, and elimination of both ω-alkenyl ligands to form Pt0 species. The latent catalytic behavior, the high turnover number, and the high anti-Markovnikov selectivity are a result of the slow release of 1 from 1-COD at room temperature, so that the concentration of Pt0 during the initial stages of the catalysis is negligible. As a result, formation of colloidal Pt, which is known to cause side reactions, is minimized, and the amounts of side products are very small and comparable to those seen for platinum(0) carbene catalysts. The latent reaction kinetics and high turnover numbers seen for 1-COD after thermal triggering make this compound a potentially useful precatalyst for injection molding or solvent-free hydrosilylation applications.

Tunable Photoswitching in Norbornadiene (NBD)/Quadricyclane (QC) - Fullerene Hybrids.

With respect to molecular switches, initializing the quadricyclane (QC) to norbornadiene (NBD) back‐reaction by light is highly desirable. Our previous publication provided a unique solution for this purpose by utilizing covalently bound C60. In this work, the fundamental processes within these hybrids has been investigated. Variation of the linker unit connecting the NBD/QC moiety with the fullerene core is used as a tool to tune the properties of the resulting hybrids. Utilizing the Prato reaction, two unprecedented NBD/QC – fullerene hybrids having a long‐rigid and a short‐rigid linker were synthesized. Molecular dynamics simulations revealed that this results in an average QC–C60 distance of up to 14.2 Å. By comparing the NBD–QC switching of these derivatives with the already established one having a flexible linker, valuable mechanistic insights were gained. Most importantly, spatial convergence of the QC moiety and the fullerene core is inevitable for an efficient back‐reaction.

Scalable Synthesis of Norbornadienes via in situ Cracking of Dicyclopentadiene Using Continuous Flow Chemistry

AbstractThe norbornadiene (NBD)‐quadricyclane (QC) photoswitch has recently attracted attention due to its use in molecular solar thermal energy systems (MOST). Normally for device testing, several grams are needed. One way of synthesizing NBDs efficiently is through the Diels‐Alder reaction between alkynes and cyclopentadiene. However, scaling up the reaction can be troublesome in a research lab environment. Also, dicyclopentadiene needs cracking before utilization which is a time‐consuming step. Here, we developed a method where we both scale up the synthesis in a single reaction step that involves both in situ cracking of dicyclopentadiene and the direct reaction of cyclopentadiene with acetylene derivatives using a tubular coiled stainless steel flow reactor. As a proof‐of‐concept, we synthesized six different NBD compounds and scaled the synthesis to produce 87 g of a novel NBD in 9 h. The NBD is further characterized, showing promising properties for MOST applications. Our new method shows that flow chemistry is an attractive technique for the fast and efficient synthesis of large quantities of NBDs, needed to develop future real‐life devices and applications.

Ring Opening Metathesis Polymerization (ROMP) of Norbornenes by (Arylimido)Niobium(V)–Alkylidene Catalysts, Nb(CHSiMe<sub>3</sub>)(NAr)[OC(CF<sub>3</sub>)<sub>3</sub>](PMe<sub>3</sub>)<sub>2</sub>

Effects of arylimido ligands on the ring-opening metathesis polymerization (ROMP) of norbornene (NBE), norbornadiene (NBD), and tetracyclododecene (TCD) were investigated using a series of (arylimido)niobium(V)–alkylidene complex catalysts, Nb(CHSiMe3)(NAr)[OC(CF3)3](PMe3)2 [Ar = 2,6-Me2C6H3 (1), 2-MeC6H4 (2), 2,6-Cl2C6H3 (3)]. The catalytic activity in ROMP of NBE for 1 was higher than for 2 and 3 (toluene at 25 °C, initial NBE concentration 0.44 mmol/mL), and the activity increased at 50 °C. Further increase in the activity (TOF 236 s–1) was observed using 3 with addition of 1.0 equiv. of PMe3, and activity was higher than that for the analogous vanadium(V)–alkylidene. These complexes (1-3) could also catalyze ROMP of NBD and TCD and the activities were higher than that for the ROMP of NBE. The dichlorophenylimido analogue (3) showed the highest activity for the ROMP of TCD, whereas 1 showed higher activity than 2 and 3 for the ROMP of NBD.

The vacuum ultraviolet absorption spectrum of norbornadiene: Vibrational analysis of the singlet and triplet valence states of norbornadiene by configuration interaction and density functional calculations.

A synchrotron-based vacuum ultraviolet (VUV) absorption spectrum of norbornadiene (NBD) is reported, and the extensive vibrational structure obtained has been analyzed. The previously known 5b13s-Rydberg state has been reinterpreted by comparison with our recent high-resolution photoelectron spectral analysis of the X2B1 ionic state. Additional vibrational details in the region of this Rydberg state are observed in its VUV spectrum when compared with the photoelectron 2B1 ionic state; this is attributed to the underlying valence state structure in the VUV. Valence and Rydberg state energies have been obtained by configuration interaction and time-dependent density functional theoretical methods. Several low-lying singlet valence states, especially those that arise from ππ* excitations, conventionally termed NV1 to NV4, have been examined in detail. Their Franck-Condon (FC) and Herzberg-Teller (HT) profiles have been investigated and fitted to the VUV spectrum. Estimates of the experimental 00 band positions have been made from these fits. The anomaly of the observed UV absorption by the 1A2 state of NBD is attributed to HT effects. Generally, the HT components are less than 10% of the FC terms. The calculated 5b13s lowest Rydberg state also shows a low level of HT components. The observed electron impact spectra of NBD have been analyzed in detail in terms of triplet states.

Towards an Effective Synthesis of Difunctionalized Heptacyclo [6.6.0.02,6.03,13.04,11.05,9.010,14]tetradecane: Ligand Effects on the Cage Assembly and Selective C−H Arylation Reactions

AbstractA series of strong π‐acceptor polyfluorinated and dicationic chelating phosphines have been synthesized and evaluated in the Rh‐catalysed dimerization of norbornadiene (NBD) into its thermodynamically more stable dimer, heptacyclo[6.6.0.02,6.03,13.04,11.05,9.010,14] tetradecane (HCTD). While dicationic ligands direct the dimerization towards HCTD, by the use of neutral polyfluorinated ancillary ligands endo‐endo‐heptacyclo [8.4.0.02,12.03,8.04,6.05,9.011,13]tetradecane (BINOR−S) is selectively obtained. In addition, a selective Pd‐catalysed arylation at position C8 of the HCTD framework is achieved by the use of a picolylamide directing group previously attached at C1. Theoretical calculations have been performed to understand the origin of that regioselectivity.magnified image

Cyclodimerization of norbornadiene (NBD) into high energy-density fuel pentacyclotetradecane (PCTD) over mesoporous silica supported Co–Ni nanocatalyst

The present study focuses on the synthesis of high energy-density fuel pentacyclotetradecane (PCTD) through cyclodimerization followed by hydrogenolysis of norbornadiene (NBD) over mesoporous silica-supported Co–Ni-based nanocatalyst. Catalytic materials are synthesized by incipient wetness impregnation method using Al-MCM-41, MCM-48 and γ-alumina as supporting materials for Co–Ni metals. The textural properties of the catalyst have been investigated through XRD, TEM and N2 physisorption, and the parameters concerning the reactivity of the catalyst optimized. The reaction was performed in a single pot (pressure reactor), and the structure of the product was elucidated by using FTIR and NMR techniques. The optimum reaction parameters (catalyst, temperature, and pressure) were developed to produce a yield of ≥ 85%. The physicochemical properties such as calorific value (10754 cal./g), density (1.08 g/cc), etc. were evaluated, and these can be utilized for ramjet application. Dimerization of NBD into a high energy-density pentacyclotetradecane fuel over mesoporous aluminosilicate supported Co–Ni nanocatalyst.

η2-Alkene Complexes of [Rh(PONOP-iPr)(L)]+ Cations (L = COD, NBD, Ethene). Intramolecular Alkene-Assisted Hydrogenation and Dihydrogen Complex [Rh(PONOP-iPr)(η-H2)]+

Rhodium-alkene complexes of the pincer ligand κ3-C5H3N-2,6-(OPiPr2)2 (PONOP-iPr) have been prepared and structurally characterized: [Rh(PONOP-iPr)(η2-alkene)][BArF4] [alkene = cyclooctadiene (COD), norbornadiene (NBD), ethene; ArF = 3,5-(CF3)2C6H3]. Only one of these, alkene = COD, undergoes a reaction with H2 (1 bar), to form [Rh(PONOP-iPr)(η2-COE)][BArF4] (COE = cyclooctene), while the others show no significant reactivity. This COE complex does not undergo further hydrogenation. This difference in reactivity between COD and the other alkenes is proposed to be due to intramolecular alkene-assisted reductive elimination in the COD complex, in which the η2-bound diene can engage in bonding with its additional alkene unit. H/D exchange experiments on the ethene complex show that reductive elimination from a reversibly formed alkyl hydride intermediate is likely rate-limiting and with a high barrier. The proposed final product of alkene hydrogenation would be the dihydrogen complex [Rh(PONOP-iPr)(η2-H2)][BArF4], which has been independently synthesized and undergoes exchange with free H2 on the NMR time scale, as well as with D2 to form free HD. When the H2 addition to [Rh(PONOP-iPr)(η2-ethene)][BArF4] is interrogated using pH2 at higher pressure (3 bar), this produces the dihydrogen complex as a transient product, for which enhancements in the 1H NMR signal for the bound H2 ligand, as well as that for free H2, are observed. This is a unique example of the partially negative line-shape effect, with the enhanced signals that are observed for the dihydrogen complex being explained by the exchange processes already noted.

Ethylene and Sulfur Coordinately Modulate the Antioxidant System and ABA Accumulation in Mustard Plants under Salt Stress.

This study explored the interactive effect of ethephon (2-chloroethyl phosphonic acid; an ethylene source) and sulfur (S) in regulating the antioxidant system and ABA content and in maintaining stomatal responses, chloroplast structure, and photosynthetic performance of mustard plants (Brassica juncea L. Czern.) grown under 100 mM NaCl stress. The treatment of ethephon (200 µL L−1) and S (200 mg S kg−1 soil) together markedly improved the activity of enzymatic and non-enzymatic components of the ascorbate-glutathione (AsA-GSH) cycle, resulting in declined oxidative stress through lesser content of sodium (Na+) ion and hydrogen peroxide (H2O2) in salt-stressed plants. These changes promoted the development of chloroplast thylakoids and photosynthetic performance under salt stress. Ethephon + S also reduced abscisic acid (ABA) accumulation in guard cell, leading to maximal stomatal conductance under salt stress. The inhibition of ethylene action by norbornadiene (NBD) in salt- plus non-stressed treated plants increased ABA and H2O2 contents, and reduced stomatal opening, suggesting the involvement of ethephon and S in regulating stomatal conductance. These findings suggest that ethephon and S modulate antioxidant system and ABA accumulation in guard cells, controlling stomatal conductance, and the structure and efficiency of the photosynthetic apparatus in plants under salt stress.

Divergent Strategies for the π-Extension of Heteroaryl Halides Using Norbornadiene as an Acetylene Synthon.

Pd-catalyzed multicomponent coupling reactions of five-membered heteroaryl halides and norbornadiene (NBD) were developed. Either direct addition of (benzo)azoles or 2:1 annulation was achieved depending on the propensity of the intermediate complex to undergo palladacycle formation, determined by the nature and substitution pattern of the heteroarene. The obtained exo- and cis-diheteroaryl norbornenes underwent epimerization and retro-Diels-Alder reactions to afford the corresponding trans-isomers and π-extended heteroaromatic systems, respectively, demonstrating the versatility of NBD as an acetylene synthon.