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J Am Chem Soc
J Am Chem Soc
ja
jacsat
Journal of the American Chemical Society
0002-7863
1520-5126
American Chemical Society

39164003
10.1021/jacs.4c07999
Article
An Azide-Free Synthesis of Metallodiazomethanes Using Nitrous Oxide
Ward Robert J.
Jörges Mike §
https://orcid.org/0009-0004-7275-292X
Remm Henning §
Kiliani Elias
https://orcid.org/0000-0001-9301-6930
Krischer Felix
Le Dé Quentin
https://orcid.org/0000-0001-6557-2366
Gessner Viktoria H. *
Faculty of Chemistry and Biochemistry, Ruhr-University Bochum, Universitaetsstrasse 150, Bochum 44801, Germany
* Email: viktoria.gessner@rub.de.
20 08 2024
04 09 2024
146 35 2460224608
14 06 2024
01 08 2024
31 07 2024
© 2024 The Authors. Published by American Chemical Society
2024
The Authors
https://creativecommons.org/licenses/by-nc-nd/4.0/ Permits non-commercial access and re-use, provided that author attribution and integrity are maintained; but does not permit creation of adaptations or other derivative works (https://creativecommons.org/licenses/by-nc-nd/4.0/).

Diazo compounds are valuable reagents in synthesis but usually require the use of potentially explosive or toxic starting materials. Here, we report the synthesis and isolation of alkali metal diazomethanides by the reaction of metalated ylides with nitrous oxide, resulting in a formal exchange of the phosphine ligand by dinitrogen. The reaction proceeds through a Wittig-like mechanism via a [3 + 2] cycloaddition of N2O across the ylide bond with release of phosphine oxide. The metalated diazomethanes exhibit an increased thermal stability due to the stronger binding of N2 compared to neutral diazomethanes. This is reflected in short C–N distances and red-shifted N–N vibrations and enables versatile applications such as for the preparation of transition metal diazomethanide complexes and the synthesis of 1,2,3-triazoles from nitriles, diazoacetates from carbon dioxide, or alkynes from aldehydes.

HORIZON EUROPE European Research Council 10.13039/100019180 101086951 Deutsche Forschungsgemeinschaft 10.13039/501100001659 EXC-2033 - 390677874 RESOLV document-id-old-9ja4c07999
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pmcIntroduction

Diazo compounds are important reagents in organic synthesis, used for a variety of applications such as cyclopropanations, homologations, or carbene insertions.1,2 Generally, diazomethanes are prepared in situ, often from azides, hydrazones, or amines or by modifying other diazomethanes (Figure 1). Metalated diazomethanes are also utilized, although these examples are relatively rare. The most prominent example is the Seyferth–Gilbert reagent, dimethyl(diazomethyl)phosphonate, which, after metalation to I, enables the one-carbon homologation of arylaldehydes and ketones to form arylalkynes via a Wittig-like mechanism.3,4 Due to the efficiency and reliability of this method for preparing alkynes, various modifications have been developed over the years to achieve milder conditions and the use of safer reagents. However, all modifications, including protocols for the in situ generation of the Bestmann–Ohira reagent, require the use of sulfonyl azides,5,6 which are typically toxic and potentially explosive, thus posing additional safety risks.

Figure 1 (a) Examples of preparation methods for diazo compounds, (b) important diazomethanides, and (c) the synthetic approach to diazomethanides from nitrous oxide reported in this work.

To our knowledge, no direct preparation of an α-metalated diazo compound from a nondiazo containing precursor has been reported to date. This is surprising, as diazomethanides are presumably more kinetically stable than the corresponding diazomethanes, potentially offering a more versatile approach to this chemistry than currently available methods.7 The lower liability of N2, although not thoroughly investigated, is suggested by IR spectroscopic studies on trimethylsilyldiazomethane (2075 cm–1) and its lithium salt II (1991 cm–1), which is the only structurally characterized alkali-metalated diazomethane.8−11 The lower IR stretch of the methanide indicates a weaker N–N bond and a higher degree of C–N π bonding, making the loss of N2 more difficult.

Our group has recently reported the synthesis of ketenyl anions from α-metalated ylides by displacement of triphenylphosphine with carbon monoxide.12−15 This reactivity can be seen as an exchange of L-type ligands at a highly reduced carbon atom.16−19 To further exploit the unique electronic situation and reactivity of the metalated carbon atom, we considered the use of other synthetically useful L-type ligands. The synthesis of metalated diazomethanes from α-metalated ylides 1 and dinitrogen appeared to be a particularly attractive, albeit challenging method for the functionalization of N2.

Results and Discussion

Synthesis and Characterization of Diazomethanides

To probe the phosphine/N2 exchange in α-metalated ylides, we selected the previously reported thiophosphinoyl (1a), phosphinoyl (1b), and sulfonyl-substituted compounds (1c) as test systems. Besides being easily accessible in gram scale, yldiide 1b is closely related to the Seyferth–Gilbert reagent3,4 and hence also synthetically attractive. Therefore, an azide-free strategy to introduce N2 from a safe and abundant resource is highly desirable. Unfortunately, treatment of the metalated ylides with dinitrogen under conditions similar to those used for their carbonylation did not result in any conversion. Likewise, elevated temperatures did not yield the desired diazomethanides, suggesting that the metalated ylide is not sufficiently nucleophilic to activate dinitrogen. Indeed, density functional theory (DFT) studies support this observation, showing the process to be highly endergonic (ΔGR = +26.3 kcal/mol for 1b to 2b; see the Supporting Information), thus ruling out the feasibility of this process under these conditions.

This led us to explore an alternative synthetic strategy. Driving reactions of ylides by coupling them with the release of phosphine oxide is well established, with the most well-known example being the Wittig reaction. Therefore, we hypothesized that the formation of diazomethanides could become possible by using nitrous oxide (N2O) as an alternative N2 transfer reagent.20−22 In fact, an analogous reaction was described by Rundel and Kästner in the 1960s for the preparation of the parent diazomethane, H2CN2, from phosphoniomethanide Ph3P = CH2. However, this method has not been used since and has not been described for anionic systems.23

To test the viability of diazomethanide formation via this strategy, 1a was treated with an atmosphere of N2O at room temperature in a THF solution (Scheme 1). To our delight, monitoring of the reaction process by 31P{1H} NMR spectroscopy showed the formation of Ph3PO (δP = 24.7 ppm)24,25 and the desired diazo salt 2a, characterized by a signal at δP = 17.2 ppm. Removal of the phosphine oxide from diazomethanide proved to be challenging. Both 2a and Ph3PO exhibit similar solubility, making them difficult to separate consistently. However, small-scale crystallization from toluene with 18-crown-6 (18-c-6) yielded single crystals of 2a, which unambiguously confirmed successful diazomethanide formation (vide infra, Figure 2). The difficulty in separating 2a and the phosphine oxide is also demonstrated by another crystal structure obtained from the reaction mixture, in which Ph3PO functioned as a coligand, coordinating to the potassium ion of the diazomethanide (see the Supporting Information).

Scheme 1 Reaction of α-Metalated Ylides 1 with Dinitrogen and Nitrous Oxide

NMR yield. All other yields are isolated yields.

Figure 2 Representation of the crystal structures of (a) the potassium complex and the naked anion 2a, (b) of the anions of 2b and 2c, and (c) and structure of the copper complex 4c. Hydrogen atoms as well as the countercation (K·[2.2.2]cryptand for 2b and 2c and K*18-c-6 for 2a) and solvent molecules as well as disordered parts omitted for clarity. Thermal ellipsoids at the 50% probability level. Selected bond lengths and angles are given in Table 1, and crystallographic details are given in the Supporting Information.

Because of the difficulties in isolating 2a, we moved on to using phosphinoyl yldiide 1b. Fortunately, the reaction of 1b with N2O in toluene results in the precipitation of the diazo salt 2b after only 1 h reaction time. Thus, 2b could be easily isolated as colorless solid after washing with THF, toluene, and hexane, in a good yield of 71%, and unambiguously characterized by single-crystal X-ray diffraction (XRD) analysis and various spectroscopic methods. Building on this success, we next synthesized the tosyl compound 2c. The reaction of 1c with N2O was found to be significantly slower compared to those of 1a and 1b, requiring stirring overnight at room temperature to reach completion. Unfortunately, attempts to remove Ph3PO from 2c resulted in poor isolated yields (23%). To improve the yield, the reaction was carried out using the n-Bu3P-substituted yldiide, n-Bu3PCSO2(pTol) 1d, which we expected to produce a more soluble phosphine oxide. The corresponding metalated ylide could be prepared in a manner similar to that for 1b (see the SI for details). The reaction with N2O could then be conducted in diethyl ether instead of toluene and was found to be faster, being complete in 2 h. After the product was washed with diethyl ether to remove any remaining nBu3PO, the isolated yield could be improved to 42%, demonstrating that variation of the phosphine can facilitate isolation.

In the 13C{1H} NMR spectra, both diazomethanides 2b and 2c exhibit a characteristic signal at 19.8 and 45.8 ppm, respectively, corresponding to the central carbon atom.24 These signals are considerably high-field shifted compared to neutral diazoalkanes (e.g., 34.1 ppm for 2b-H and 59.3 ppm for TsC(N2)Me), suggesting a high charge accumulation at the C1 carbon atom.26 The N–N vibration gives rise to sharp signals in the IR spectrum at 1973.2 (2a), 1970.0 (2b), and 1974.8 cm–1 (2c), respectively. These bands appear red-shifted relative to 2b-H(27) (2079 cm–1) and 2c-H(28) (2102.8 cm–1), as well as most metal and main group element diazo species.24,29−31 This red shift suggests a weakening of the N–N bond and a strengthening of the C–N bond and hence an increased stability toward N2 elimination. Indeed, all three compounds remain stable at room temperature in the glovebox for weeks. No loss of N2 or any cyclization reactions were observed. To further investigate the assumed increase in thermal stability of the diazomethanides, 2b and 2c were subjected to DTA (differential thermal analysis). The DTA of 2c shows an exothermic event at 80 °C, which, however, is not associated with a significant mass loss. Regardless of whether this is due to decomposition or a phase transition, this thermal stability is higher than that reported for the neutral TsCH(N2), which decomposes at 34 °C.32 Thermal analysis of 2b shows a complex behavior with a mass loss, reasonable for N2, commencing at 63 °C. Unfortunately, we were not able to find data on the thermal stability on the protonated analogue of 2b. Nonetheless, both 2b and 2c exhibit reasonable thermal stability, which points toward a stabilization of the anionic diazo compounds compared to their protonated neutral analogues.33

Crystals of 2a–2c could be obtained from saturated toluene solutions at low temperatures with 18-crown-6 (18-c-6, 2a) or [2.2.2]cryptand (2b and 2c) for additional coordination of the potassium cation (Figure 2 and Table 1). To the best of our knowledge, these are, aside from the silyl system II (Figure 1), the only crystallographically characterized alkali metal diazomethanides reported to date.10,11 While the cryptand complexes of 2b and 2c form ligand-separated ion pairs, the [K(18-c-6)] cation in 2a features a contact with the methanide carbon atom and an additional weak interaction with a phenyl-CH bond of an adjacent molecule in the crystal structure, resulting in the formation of a coordination polymer. The C–N bond lengths in the “naked” anions in 2b and 2c amount to 1.253(3) and 1.272(5) Å, which are shorter than those reported for TsC(H)N2 (1.281 Å)32 or 3-methoxy-5-diazomethyl-pyran-4-one (1.299(2) Å).34 In contrast, the N–N bond lengths of 1.164(3) Å (2b) and 1.165(5) Å (2c) are slightly longer than those in the neutral analogues (1.137 Å for TsC(H)N2 or 1.1329(16) Å for the diazo pyrone). This confirms the stronger binding of the N2 fragment to the carbon center in diazomethanides compared to their neutral analogues, expressed by the preference of resonance structure 2 over 2′ (Figure 3). It is noteworthy that the C–N bond of 1.271(2) Å in 2a is longer than that in 2b (1.253(3) Å), and similarly, the N–N bond is shorter (1.159(2) vs 1.164(3) Å), suggesting a greater localization of the negative charge on the diazo carbon atom in 2a. This can presumably be explained by the coordination of potassium to the carbon atom in the crystal structure of 2a, whereas in 2b, the cation is fully encapsulated in the cryptand ligand, forming no contact with the anion.

Table 1 Selected Bond Lengths and Angles as well as IR Frequencies of the C–N–N Vibration of Diazomethanides 2 and Their Copper Complexes 4

 	2a	2b	2c	4a	4b	4c	
C–N (Å)	1.271(2)	1.253(3)	1.272(5)	1.312(6)	1.296(4)	1.303(2)	
N–N (Å)	1.159(2)	1.164(3)	1.165(5)	1.154(6)	1.141(4)	1.141(2)	
E–C–N (°)	118.72(13)	125.89(19)	112.1(3)	113.3(3)	111.63(19)	112.05(14)	
E–C (Å)	1.7324(19)	1.711(3)	1.704(4)	1.782(4)	1.769(3)	1.733(2)	
M–C (Å)	2.9028(17)	N/A	N/A	1.911(5)	1.910(2)	1.9005(17)	
C–N–N (cm–1)	1960.3	1970.0	1974.8	2018.1	2012.4	2023.4	

Figure 3 Potential canonical structures of metalated diazomethanes 2 and ketenes 3.

It is important to note that the angle around the central carbon atom in the methanides is more acute than that in the corresponding ketenyl anions. For example, the S–C–N bond angle in 2c amounts to 112.1(3)° while the S–C–C angle in the corresponding ketenyl anion, [Ts-C=C=O] (3c), was reported to be approximately 170°.13 The same holds true for the P–C–C angle in 2a (118.72(13)°) and 2b (125.9(2)°) compared to their analogue ketenyl anions, (146.2(5)° for [Ph2P(S)-C=C=O]− and 148.86(14)° for [Ph2P(O)-C=C=O]−).12,35 This trend aligns with previous calculations by Frenking and co-workers on carbones, which explained the increased angle by the stronger π acceptor properties of CO in comparison to N2.36,37 The smaller S–C–N or P–C–N angles indicate that, unlike the ketenyl anions 3, for which the ynolate structure 3′ with a C–C triple bond is also important, a similar structure (2″) is only hypothetical for the diazo systems.

Computational Studies

To gain further insights into the bonding situation in the metalated diazomethanes 2 also in comparison to the analogue ketenyl anions, we performed DFT calculations (Figure 4). In contrast to ketenyl anions, which were shown to exhibit partial C–C triple bond character (e.g., Wiberg bond index (WBI) of 2.11 in 3b) the C–N bond in 2b features a WBI of 1.65, i.e., between a single and a double bond. This corroborates with a resonance hybrid between 2 and 2′. This conclusion is supported by all calculated bond lengths and bond indices (see the Supporting Information for details). The absence of any contribution of a structure analogous to the ynolate structure 3′ results in the more pronounced bending of the diazomethanide and a steeper bending potential. This compares well to calculations performed by Hansmann et al. on a neutral diazoalkene and its ketene analogue.38 Consequently, the energy-optimized structure of 2b with an angle of 121° is 26.3 kJ/mol lower in energy than that of the linear form. In contrast, the analogous ketenyl anion featured an extremely flat potential energy surface for the bending, with the bent structure being favored by less than 2 kJ/mol.35 The highest occupied molecular orbital (HOMO) of 2b resembles the lone pair at the carbon atom reflecting the charge accumulation at this position (qC(NBO) = −0.75) as already suggested by the NMR studies. HOMO–1 is somewhat more delocalized toward the N2 unit confirming 2 and 2′ as the most important resonance hybrid.

Figure 4 (a) Comparison of the calculated structures and (b) bending potential of the ketenyl, [Ph2(O)CCO]−, and diazomethanide anion 2b and (c) display of the highest occupied molecular orbitals (HOMO) of 2b; geometries and relaxed surface scan at PBE0-D3(SMD)/ma-def2-TZVPP.

Besides the electronic structure, we were also interested in elucidating the mechanism of the formation of 2b. To probe whether any intermediate could be spectroscopically identified, in situ IR studies were performed. However, no evidence of any intermediate could be observed, only the disappearance of 1b and the appearance of 2b (see the SI for details). Therefore, different mechanisms were considered by DFT methods, showing that the lowest energetic pathway proceeds via a concerted [3 + 2] cycloaddition of N2O across the ylidic P–C bond to form the heterocyclic intermediate 2b-Int (Figure 5, pathway A). This process has a barrier of only 19.9 kcal/mol (TS1) and goes downhill by 13.1 kcal/mol (2b-Int). Subsequent [3 + 2] cycloreversion from intermediate 2b-Int leads to the diazomethanide 2b and Ph3PO, which are 55.6 kcal/mol more stable than the starting materials. This confirms the viability of the diazomethanide formation with N2O under mild reaction conditions, in contrast to the endergonic process found with dinitrogen. Notably, a stepwise process (pathway B) via initial nucleophilic attack of the metalated ylide at N2O was calculated to have a significantly higher barrier (TS3). The same observation was made for the tosyl-substituted metalated ylide (see the SI), albeit with an increased energy barrier by ΔΔG‡ = +2.8 kcal/mol, comparing the rate-determining [3 + 2] cycloaddition of the tosyl-substituted ylide to the rate-determining [3 + 2] cycloreversion of the phosphinoyl-substituted one. This observation aligns with the slower formation of 3c.

Figure 5 Calculated mechanism for the formation of diazomethanide 2b (PBE0-D3(SMD)/def2-QZVPP//PBE-D3(CPCM)/def2-TZVPP with implicit solvation in toluene and three explicit benzene molecules coordinating to potassium).

Reactivity of Isolated Diazomethanides

In the past, diazomethanides have usually been applied as in situ generated reagents. To probe the utility of the isolated reagents synthesized from N2O, we next addressed an assessment of their reactivity, focusing on isolated diazomethanides 2b and 2c (Scheme 2). To our delight, both 2b and 2c reacted cleanly with IPrCuCl (IPr = 1,3-bis(2,6-di-isopropylphenyl)-imidazol-2-ylidene) to produce the corresponding diazo copper(I) complexes 4b and 4c. This reaction can also be performed with a crude reaction mixture of 2a to produce 4a. The copper complexes were characterized by XRD analysis (Figure 2, Table 1, and SI), revealing Cu–CN2 bond lengths between 1.911(5) and 1.901(2) Å, all of which are longer than the one (1.896(2) Å) in a similar complex with (2,6-dimesitylphenyl)diazomethane reported by Hillhouse et al.30 As a result of the more covalent Cu–C bond, the N–N bond lengths shorten (e.g., 1.141(2) Å for 4c and 1.165(5) Å for 2c) while the C–N bonds are elongated relative to the potassium complexes. This corroborates well with the blue shift of the N–N stretch, which appears at 2012.4 cm–1 for 3b and 2023.4 cm–1 for 3c in the IR spectrum.

Scheme 2 Reactivity Studies of 2a–2c

[K] = K·18-c-6 except for 4c where [K] = K·(12-c-4)2. For details on the reaction conditions, see the SI.

Besides serving as precursors to transition metal complexes, the diazomethanides are valuable starting materials for organic synthesis. To further investigate the regioselectivity of the reaction with electrophiles (C versus N attack), we initially reacted 2b with carbon dioxide, yielding diazoacetate 5b in a good 75% yield. Thus, this method offers a sustainable alternative for synthesizing diazoacetates starting from two commonly occurring simple gas molecules as building blocks. 5b exhibits two characteristic signals at 164.5 and 52.8 ppm in the 13C NMR spectrum corresponding to the carboxylic carbon and diazomethyl atoms. The IR spectrum of 5b displays two strong signals at 2063.9 and 1647.4 cm–1 for the N–N and C–O stretches, respectively, indicating a significantly reduced electron delocalization into the N2 moiety and a weakened C–N bond, in agreement with the known sensitivity of diazoacetates. The charge delocalization is also reflected in the changes in the bond lengths in the crystal structure (Figure 6), such as the elongation of the C1–N1 bond from 1.253(3) Å in 2b to 1.309(2) Å in 5b. In the case of the tosyl system, the increased sensitivity prevented its isolation in crystalline form. However, NMR and IR spectroscopic studies clearly confirmed the successful formation of 5c in a 50% isolated yield.

Figure 6 Crystal structures of (left) carboxylate 5b and triazole 7c. See the Supporting Information for details.

The reaction of carbon monoxide, CO, with diazomethanides, is known to produce the corresponding ketenyl anions.39,40 Indeed, this reaction reminiscent of a ligand exchange at carbon resulted in the clean, high-yielding formation of 3a and 3b.13,35 The reaction of the tosyl-substituted system 2c was less selective, resulting in the formation of ketenyl anion 3c and a dianionic heterocycle (see the SI for details). To our delight, 2b also served as an azide-free alternative to the Seyferth–Gilbert reagent for the formation of alkynes from the corresponding aldehydes.41 As such, the reaction of 2b with 4-bromobenzaldehyde at room temperature resulted in gas evolution and complete conversion to 4-bromophenylacetylene (6), as indicated by NMR spectroscopy and GC/MS analysis. The same reaction did not occur with 2c, presumably due to the greater stabilization of the negative charge by the tosyl group. Moreover, 2b and 2c react with acetonitrile to form 1,2,3-triazoles 7b and 7c (Figure 6). Nitriles are known to cyclize with some metal diazo salts to produce anionic 1,2,3-triazoles.42 Surprisingly, however, we could find no examples of this reaction with acetonitrile.8 The anionic triazoles 7b and 7c could be isolated as crystalline solids in good to moderate yields of 75 and 25%.

Conclusions

In conclusion, we have developed a new method for the direct preparation of metallodiazomethanes from α-metalated ylides and nitrous oxide. The reaction proceeds via a [3 + 2] cycloaddition of N2O across the ylidic P–C bond, resulting in the net introduction of N2 through phosphine oxide elimination.43 Unlike previous methods, this protocol does not require an azide reagent and allows for the isolation of diazomethanides in their crystalline form. The diazomethanide 2c exhibits a significantly increased thermal stability against N2 loss compared to neutral diazomethane, which is reflected in shorter C–N and longer N–N bonds in the solid state and a red shift of the N–N vibration. This enhanced stability of metallodiazomethanes facilitates their controlled application, which we could illustrate by the synthesis of copper complexes, the preparation of ketenyl anions via N2/CO exchange, or the synthesis of triazoles through reaction with acetonitrile. These reactions showcase the synthetic utility of diazomethanides, further highlighted by their ability to convert aldehydes to alkynes, similar to that of the Seyferth-Gilbert reagent. Since this method avoids the need for azide precursors and because diazomethanides exhibit increased stability compared to neutral diazomethanes, this novel approach may unlock previously untapped potential for organic transformations.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacs.4c07999.General experimental details; synthesis, characterization, and reactivity of the metallodiazomethanes; spectroscopic and crystallographic data; computational details (PDF)

Coordinates of the energy-optimized structures (XYZ)

Supplementary Material

ja4c07999_si_001.pdf

ja4c07999_si_002.xyz

Author Contributions

§ M.J. and H.R. contributed equally to this work.

Funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany’s Excellence Strategy – EXC-2033–390677874 – RESOLV, and the European Union (ERC, CarbFunction, 101086951). Views and opinions expressed are however those of the authors only and do not necessarily reflect those of the European Union or the European Research Council. Neither the European Union nor the granting authority can be held responsible for them.

The authors declare no competing financial interest.

Acknowledgments

We thank Dr. Tobias Kull for performing the thermal analyses.
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During the revision phase of this manuscript, Hansmann and co-workers reported on a similar reaction using a carbodiphosphorane with N2O.

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