Diazo Compounds Room-Temperature-Stable Diazoalkenes by Diazo Transfer from Azides: Pyridine-Derived Diazoalkenes Justus Reitz, Patrick W. Antoni, Julian J. Holstein, and Max M. Hansmann* Dedicated to Guy Bertrand on the occasion of his 70th birthday Abstract: Recently, stable diazoalkenes have received significant attention as a new substance class in organic chemistry. While their previous synthetic access was exclusively limited to the activation of nitrous oxide, we here establish a much more general synthetic approach utilizing a Regitz-type diazo transfer with azides. Importantly, this approach is also applicable to weakly polarized olefins such as 2-pyridine olefins. The new pyridine diazoalkenes are not accessible by the activa- tion of nitrous oxide, allowing for a considerable extension of the scope of this only recently accessed functional group. The new diazoalkene class has proper- ties distinct from the previously reported classes, such as photochemically triggered loss of dinitrogen affording cumulenes and not C� H insertion products. Pyridine- derived diazoalkenes represent the so far least polarized stable diazoalkene class reported. Diazoalkenes (R2C=C=N2) are important intermediates in organic synthesis postulated for instance in the Seyferth– Gilbert homologation.[1] Since diazoalkenes are typically rapidly liberating dinitrogen gas to form fleeting vinylidenes,[2] there is few spectroscopic data available mostly derived from matrix isolation[3] or computations.[4] In 2021, we described the first room temperature stable diazoalkene A (Scheme 1), derived from the reaction of a mesoionic N-heterocyclic olefin (mNHO)[5] with nitrous oxide.[6] Shortly after, two additional stable diazoalkene classes based on a 1,2,3-triazole backbone (B),[7] as well as a 2-imidazole backbone (C)[8] were reported by us and the Severin group. The new diazoalkenes show unique chemistry, such as N2/CO exchange,[7,9] as precursors to access triplet vinylidenes,[10] or to generate metal-vinylidene complexes.[11] Importantly, all the reported room-temper- ature-stable diazoalkenes were synthesized by the activation of nitrous oxide, which is intrinsically a challenging molecule to activate.[12] The mechanism is proposed to be initiated by addition of the strongly polarized carbon nucleophile onto N2O to give zwitterionic alkane diazotate I,[13] which upon dehydration forms the desired diazoalkene (Scheme 1b; top). The utilization of nitrous oxide as a diazo transfer reagent, even though being highly atom-economic, has limitations: the outcome of the N2O activation seems to be dependent on the exact reaction conditions,[14] and water has to be scavenged. Most importantly, the approach is restricted to strongly polarized carbon nucleophiles which are capable of activating nitrous oxide. The synthesis and detection of diazoalkenes has been unsuccessfully targeted by several groups including Gilbert et al.[1,15] We could show that strong C� C bond polarization leads to stable diazoalkenes,[6] however, a fundamental question remains, as how much polarization is required to obtain room-temperature-stable diazoalkenes. Since pyridine derivatives are ubiquitous and electron deficient heterocycles, we were interested in such a structurally simple but yet unknown compound class D (Scheme 1). In compar- [*] J. Reitz, P. W. Antoni, Dr. J. J. Holstein, Jun.-Prof. Dr. M. M. Hansmann Fakultät für Chemie und Chemische Biologie, Technische Universi- tät Dortmund Otto-Hahn-Str.6, 44227 Dortmund (Germany) E-mail: max.hansmann@tu-dortmund.de © 2023 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. Scheme 1. a) Known room temperature stable diazoalkene classes A–C. Dipp: 2,6-diisopropylphenyl; b) Activation of N2O with only strong nucleophiles leads to diazoalkenes and c) proposed N2 transfer with azides to access novel diazoalkenes. Angewandte ChemieCommunications www.angewandte.org How to cite: Angew. Chem. Int. Ed. 2023, 62, e202301486 International Edition: doi.org/10.1002/anie.202301486 German Edition: doi.org/10.1002/ange.202301486 Angew. Chem. Int. Ed. 2023, 62, e202301486 (1 of 5) © 2023 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH http://orcid.org/0000-0003-3107-1445 https://doi.org/10.1002/anie.202301486 https://doi.org/10.1002/ange.202301486 http://crossmark.crossref.org/dialog/?doi=10.1002%2Fanie.202301486&domain=pdf&date_stamp=2023-04-04 ison to the five membered rings (A–C), pyridine should be significantly more electron deficient, thus leading to the least polarized C=C bond in this series and consequently towards the direction of non-polarized elusive diazoalkenes (Scheme 1). We were curious if such a diazoalkene target could be stable at room-temperature and how to syntheti- cally access it. Following the previous strategy, we attempted its synthesis by reacting the known 2-pyridine olefin 1,[16,17] with N2O but observed no reaction at room temperature (Scheme 1b). Forcing reaction conditions (90 °C for 5 days) lead to the scission of the C=C bond (see Supporting Information). The lack of reactivity is most likely due to the lower nucleophilicity of 2-pyridine olefins compared to regular NHOs[18] and mNHOs[5] hampering the initial N2O activation step. This is supported by the low field shifted 13C NMR data for the exocyclic =CH2 moiety of the pyridine olefin (δ �67–73 ppm),[16] compared to regular NHOs (δ�40– 50 ppm), or mNHOs (δ�40–45 ppm). To solve the intrinsic problem of N2O activation, we were curious if dinitrogen transfer would be feasible utilizing azides. One of the well- established ways to access diazoalkanes is the diazo transfer from azides, known as Regitz diazo transfer.[19] Already in 1953, Doering and DePuy reported that diazocyclopenta- diene can be accessed by the reaction of p-tosyl azide with cyclopentadienyl lithium.[20] We hypothesized that C-nucleo- philes such as ylidic olefins should add to azides leading to intermediate II (Scheme 1c), which was shown with 5- unprotected mNHOs but resulted in the generation of fused pyrazole heterocycles.[21] We speculated that additional base should facilitate deprotonation of II and N� N bond cleavage to lead to diazoalkenes and a sulfonamide salt byproduct. Importantly, azides should be significantly more electro- philic than nitrous oxide, increasing the applicability to weak nucleophiles and therefore to entirely new diazoalkene classes. To test this hypothesis, we treated NHO 2[22] with p-tosyl azide in the presence of KOtBu (Scheme 2a). We were delighted to isolate the known diazoalkene 3[8] in 65% yield. The deprotonated sulfonamide salt byproduct could easily be separated by precipitation. Note, 3 was previously only accessible by N2O activation in DMF over 2 days reaction time,[8] and in CH3CN afforded a dimerization product.[14] Since NHO 2 is synthesized from methylated imidazolium salt 4, we attempted the one-pot synthesis starting from 4 with 2.1 equivalents KOtBu. Indeed, 3 could be isolated in 42% yield, which represents a very convenient and rapid synthesis without the isolation of the air-sensitive olefin. We next selected triazolium salt 5 as precursor, which under the one-pot reaction conditions leads via mNHO 6[5] to diazo- alkene 7[7] in 63% yield (Scheme 2b). Other azides such as 2,4,6-triisopropylphenylsulfonyl azide, mesityl azide or p-CF3-aryl azide also afforded diazoalkene 7, but the clean isolation of the product proved more challenging. Next, we applied this strategy to pyridine derived olefin 1, which proved unreactive towards N2O at room temper- ature (Scheme 1b). Interestingly, utilizing the azide strategy, olefin 1 and p-tosyl azide afforded a yellow solid in 45% yield with a characteristic and intense IR band at 1971 cm� 1, which proved to be the unknown diazoalkene 11 (Scheme 3). 11 is stable under inert conditions at room temperature in solution or in the solid-state at least for more than 24 h. 1H NMR data of the heterocyclic core show high field shifted signals at δ=5.95, 5.88 and 5.27 ppm indicating a localized non-aromatic structure 11I over a pyridinium 11II (Scheme 3), revealing a significant contribution of a genuine R2C=C=N2 diazoalkene. Besides 11, we also observed the formation of ca. 10–20% imine byproduct 13 in which formally a CH2-fragment is lost (for X-ray verification and a mechanistic rationale, see the Supporting Information). The one-pot transformation starting from the pyridinium salt 8 also afforded the desired diazoalkene 11 in a single step. Additionally, we could perform the reaction with 1,2,3,5- collidinium salt 9 to give diazoalkene 12 (Scheme 3). We were able to obtain single crystals suitable for X-ray diffraction of 12 (Figure 1).[23] Strikingly, the diazoalkene features a bent CCN unit (�126°) and short C1� N1 (1.273 (4) Å/1.287(4) Å) and long N1� N2 (1.171(4) Å/1.163(4) Å) bond lengths (two molecules in the asymmetric unit). This trend agrees with the recent data for the structural parameters of diazoalkenes of type A–C (Scheme 1a). The CN2 fragment is in plane with the heterocycle Scheme 2. Synthesis of reported diazoalkenes 3 and 7 by diazo transfer with p-tosyl azide. Scheme 3. Synthesis of pyridine derived diazoalkenes starting from the olefin (top) and from the salt precursor (bottom). Angewandte ChemieCommunications Angew. Chem. Int. Ed. 2023, 62, e202301486 (2 of 5) © 2023 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH 15213773, 2023, 19, D ow nloaded from https://onlinelibrary.w iley.com /doi/10.1002/anie.202301486 by T echnische U niversitaet D ortm und, W iley O nline L ibrary on [07/02/2025]. See the T erm s and C onditions (https://onlinelibrary.w iley.com /term s-and-conditions) on W iley O nline L ibrary for rules of use; O A articles are governed by the applicable C reative C om m ons L icense (aN3� C2� C1� N1: 179°) and oriented trans supporting a double bond contribution between C1 and C2, with a bond length of 1.402(4) Å/1.398(4) Å positioned between a single and double bond. In the heterocycle there is a pronounced C� C bond alternation (C2� C3�1.43 Å; C3� C4�1.37 Å) supporting the 1H NMR data of localized double bonds. In order to get insight into the electronic properties and the mechanism of the transformation, we performed 13C and 15N labeling studies (Scheme 4). Non-labeled 11 features a very weak 13C signal (CNN) positioned at δ�66 ppm which increases its intensity upon cooling to � 80 °C in d8-THF (see Figure S86). Importantly, the intense 13C signal (65.7 ppm) of 11-13C is downfield shifted compared to diazoalkenes A–C, which all appear in a narrow range of 30–35 ppm (Table 1) clearly highlighting the significant difference to the reported diazoalkenes. The 13C shift indicates a trend towards a typical 13C alkene range and supports a pronounced C=C double bond. Additionally, the labeled compound allows to derive the 13C� 13C coupling constant of 75 Hz, which is the largest reported for diazoalkenes (Table 1). It is only slightly smaller than in the olefin (80 Hz) and significantly larger than in the C� C single bond of 8-13C (47 Hz, see Supporting Information), supporting a C=C double bond and a polarization towards a genuine diazoalkene. Next, we performed the reaction of 1 with 15N labeled p-tosyl azide (50% 15N at inner and outer N-atoms, see Supporting Information). In relation to the natural abundance 15N measurement only the inner N-atom [δ(15N) - =256.6 ppm] is labeled (see Supporting Information). Therefore, the diazo transfer should mechanistically occur through attack on the outer (or inner) N-atom and not by attack on the middle N-atom of the azide. The 15N shifts as well as the IR bands are comparable to systems A–C (IR range of ca. 1940–1980 cm� 1; Table 1). The DFT optimized structural parameters agree well with the obtained solid-state structure (Figure 2a). The calculated natural charge of � 0.22 e� for the carbon atom (CNN) is the smallest compared to all model systems A–C (see Figure S113). Additionally, the Wiberg bond index for the C� CN2 bond is the highest following the calculated trend 1.35 (A), 1.37 (B), 1.39 (C) and 1.43 (11). Calculations of the E/Z isomers show a favor for the E isomer by 2.0 kcalmol� 1 in agreement with the solid-state structure (Figure 2b). Presumably, both E/Z isomers exist in solution, though low Figure 1. X-ray solid state structure of 12. Thermal ellipsoids are shown with 50% probability. Selected bond lengths and angles in [Å] and [°] (two independent molecules in the asymmetric unit): C1� C2 1.402(4)/ 1.398(4); C1� N1 1.273(4)/1.287(4); N1� N2 1.171(4)/1.163(4); C2� C3 1.425(5)/1.437(4); C3� C4 1.379(5)/1.372(4); C4� C5 1.417(4)/1.402(5); C5� C6 1.341(5)/1.368(4); C6� N3 1.377(4)/1.371(4); C2� C1� N1 125.9(3)/125.5(3); N3� C2� C1� N1 178.9(3)/178.1(3). Scheme 4. 13C and 15N labeling studies. Table 1: Comparison of selected spectral data of reported diazoalkenes (A–C) and pyridine diazoalkenes 11/12. Figure 2. a) Calculated (BP86/def2-TZVPP) bond parameter; bond lengths in [Å] and natural charges in red; b) calculated energy difference between trans/cis-11; c) frontier molecular orbitals (isovalues at 0.6). Angewandte ChemieCommunications Angew. Chem. Int. Ed. 2023, 62, e202301486 (3 of 5) © 2023 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH 15213773, 2023, 19, D ow nloaded from https://onlinelibrary.w iley.com /doi/10.1002/anie.202301486 by T echnische U niversitaet D ortm und, W iley O nline L ibrary on [07/02/2025]. See the T erm s and C onditions (https://onlinelibrary.w iley.com /term s-and-conditions) on W iley O nline L ibrary for rules of use; O A articles are governed by the applicable C reative C om m ons L icense temperature NMR spectroscopy does not allow their sepa- rate detection. While HOMO-1 is centered on the CN2 moiety, HOMO and LUMO are distributed over the entire π-system (Figure 2c). The frontier molecular orbitals, π-type character at C for HOMO and σ-type for HOMO-1, appear similar to the frontier molecular orbitals of the previously reported diazoalkene classes.[6,8a] Due to the low molecular weight (M�147 gmol� 1) and high N/C ratio we performed thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) of 11 and 12 (see Supporting Information). The TGA measure- ment of 11 shows a sharp mass loss of 77% around 90 °C (Figure S103). DSC measurements of 11 in high-pressure crucibles confirm onset temperatures of ca. 88–95 °C. In the DSC measurements for both 11 and 12 two scenarios were encountered: either an initial decomposition followed by a second decomposition with moderate enthalpies of decom- position (ΔHD�� 425–505 Jg � 1 and � 216–425 Jg� 1), or an instant, highly energetic decomposition with ΔHD� � 1600 Jg� 1 at ca. 90 °C (see Supporting Information). The later DSC profiles indicate new energetic materials exceed- ing by far ΔHD values of all reported diazoalkenes (A and B �300 Jg� 1; see Supporting Information) and several com- mon diazo compounds.[24] Based on the DSC results suitable precautions (explosion shield, only small quantities) should be taken handling these energetic compounds. Finally, we investigated the reactivity of the new diazoalkene class upon photochemical excitation. Irradiation of a solution of 11 with a 390 nm LED cleanly affords a ca. 1 : 1 mixture of E/Z dimerization products 15 (Scheme 5). This reactivity is in stark contrast to the reactivity of all previously reported diazoalkenes (A–C; Figure 1) which yield a range of C� H activation products, but importantly not dimers.[6–8] 15 features a characteristic 13C NMR shift of the central C-atom at δ=133 ppm, which we could unambig- uously assign by 13C labeling [in agreement with theoretical prediction (Figure S117) and X-ray data (Figure S90).[23] Interestingly, there is only a single report on a 4-substituted C4 pyridine cumulene, which is generated by KC8 reduction and discussed as pyridylidene stabilized dicarbon (py-> C2<-py). [25] The mechanism of the generation of C4 cumulene should involve the dimerization of vinylidene 14.[26] Note, dimerization of other vinylidene precursors to give cumulenes has been experimentally observed,[27] but there has been serious doubt that this process actually involves free vinylidenes but instead metalated, carbenoid species.[2a] Due to the metal free and clean photochemical access, carbenoids can be ruled out here which should give strong support for a vinylidene pathway.[26] Note, that the reactivity of the pyridine diazoalkene starts to resemble the chemistry of typical elusive diazoalkenes.[2] It is interesting to point out that if diazoalkene C (Figure 1) would hypo- thetically dimerize (NHC)C2(NHC) would be generated which is a compound studied in great detail theoretically[28] and was proven very unstable and easily decomposing into tetrazafulvalene and other products hampering its isolation.[29] Clearly, the photochemistry of the new pyridine diazoalkene class highlights the very different electronic nature and unique reactivity. In summary, we have reported a novel strategy to access stable diazoalkenes by dinitrogen transfer from azides. The only previously known method involved the activation of nitrous oxide, which is only applicable to strongly polarized olefins. This new approach enables the use of weakly polarized olefins, providing access to several so far unknown diazoalkenes by employing standard laboratory chemicals. The here presented pyridine-derived diazoalkenes show distinct properties such as a 13C NMR shift (CN2) of ca. 66 ppm compared to 28–35 ppm for all other previous systems and push the limits of bond polarization towards a genuine (non-polarized) diazoalkene featuring both C=C and C=N double bonds (R2C=C=N2). Remarkably, photo- chemically triggered loss of dinitrogen affords a stable C4- cumulene, while all other previous diazoalkene classes afforded C� H insertion products. Consequently, the diazo- alkene class should be a highly interesting vinylidene precursor for main group or transition metal chemistry, which is under current investigation. Considering the exceeding simplicity of the synthetic access—pyridine deriv- atives are ubiquitous and just two commercially available reagents are added in a single synthetic step—we believe that this strategy will have a large impact on the exploration of the field. We are currently exploring other heterocycles employing this method and investigating the reactivity of pyridine diazoalkenes. Acknowledgements Sebastian Henke is thanked for access to the TGA and DSC instruments, Wolf Hiller for the 15N NMR experiments as well as support by the faculty of chemistry and chemical biology. Thomas Klapötke is acknowledged for helpful discussions. The NMR spectrometer was funded by the Deutsche Forschungsgemeinschaft (DFG project 452669591). Computational resources were provided by LIDO3, the high-performance computing facility at TU Dortmund (DFG project 271512359). We thank the ERC- StG “CC-CHARGED” (101077332) and are looking for- ward to deepen the understanding of new carbon com- pounds within this project. Open Access funding enabled and organized by Projekt DEAL. Scheme 5. Irradiation of 11 leads to C4 cumulene 15. Angewandte ChemieCommunications Angew. Chem. Int. Ed. 2023, 62, e202301486 (4 of 5) © 2023 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH 15213773, 2023, 19, D ow nloaded from https://onlinelibrary.w iley.com /doi/10.1002/anie.202301486 by T echnische U niversitaet D ortm und, W iley O nline L ibrary on [07/02/2025]. See the T erm s and C onditions (https://onlinelibrary.w iley.com /term s-and-conditions) on W iley O nline L ibrary for rules of use; O A articles are governed by the applicable C reative C om m ons L icense Conflict of Interest The authors declare no conflict of interest. Data Availability Statement The data that support the findings of this study are available in the Supporting Information of this article. Keywords: Diazoalkenes · Reactive Intermediates · Diazo Compounds · Vinylidenes · Nitrogen Heterocycles [1] a) J. C. Gilbert, U. Weerasooriza, J. Org. Chem. 1982, 47, 1837–1845; b) W. Kirmse, Angew. Chem. Int. Ed. Engl. 1997, 36, 1164–1170. [2] a) P. J. Stang, Chem. Rev. 1978, 78, 383–405; b) R. Knorr, Chem. Rev. 2004, 104, 3795–3849. [3] a) J. C. Brahms, W. D. Dailey, J. Am. Chem. Soc. 1990, 112, 4046–4047; b) J. Breidung, H. Bürger, C. Kötting, R. Kopitzky, W. Sander, M. Senzlober, W. Thiel, H. Willner, Angew. Chem. Int. Ed. 1997, 36, 1983–1985. [4] M. A. Murcko, S. K. 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[26] Preliminary calculations indicate a singlet ground state for 13 in contrast to all other triplet vinylidenes derived from A–C, see ref. [10], which requires additional investigations. [27] a) D. Y. Curtin, W. H. Richardson, J. Am. Chem. Soc. 1959, 81, 4719–4728; b) G. Köbrich, W. Drischel, Angew. Chem. 1965, 77, 95. [28] S. M. N. V. T. Gorantla, S. Pan, K. C. Mondal, G. Frenking, J. Phys. Chem. A 2021, 125, 291–301. [29] D. C. Georgiou, I. Mahmood, M. A. Haghighatbin, C. F. Hogan, J. L. Dutton, Pure Appl. Chem. 2017, 89, 791–800. Manuscript received: January 30, 2023 Accepted manuscript online: March 3, 2023 Version of record online: April 4, 2023 Angewandte ChemieCommunications Angew. Chem. Int. Ed. 2023, 62, e202301486 (5 of 5) © 2023 The Authors. 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