Wanzlick's equilibrium in tri- and tetraaminoolefins

Abstract

The dissociation mechanism of electron-rich olefins into their parent carbenes has been a controversial topic since Wanzlick's pioneering work. Herein, we present a combined synthetic and computational study on the formation (dissociation, respectively) of hetero- and homo-carbene dimers derived from benzimidazolin-2-ylidenes (benzNHCs), imidazolidin-2-ylidenes (saNHC), and cyclic (alkyl) (amino) carbenes (CAACs) through sublimation (in vacuo) as well as in condensed phase. We quantify the effect of proton catalysis and report that even triaminoolefins dissociate to their free carbenes, yet only under proton catalysis. Accordingly, we report how the judicious choice of the base (KOtBu vs. KHMDS) and solvent (hexane/benzene vs. THF) allows N,N′-dimethylbenzimidazolin-2-ylidene to be obtained quantitatively as a metastable, kinetic product. This free carbene had been previously reported to dimerize directly to the olefin-dimer, which is the thermodynamic product.

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Electron-rich olefins serve as valuable organic materials and reagents.¹ This is arguably due to the presence of three stable redox states.^2–4 Thereby, the radical cations^5,6 show peculiar photo-chemical and conductive properties.^7–10 In their neutral, reduced redox state, they are of use as organic redox agents.^11–15 Nitrogen-containing derivatives such as enetetramines (tetraaminoolefins, respectively) 1 are dimers of N-heterocyclic carbenes (NHCs) 2.¹⁶ While the dimerization is unfavorable for unsaturated (“conventional”) NHCs,^17–19 more π-acidic carbenes dimerize.²⁰ On the contrary to carbene chemistry, where dimerization is undesired, it is required to control dissociation of their dimers in organic electronics. One approach, albeit synthetically tedious, is the installation of tethers.²¹ Alternatively, steric and electronic effects may control the dissociation/association equilibrium. However, there has been a long debate on the mechanistic intricacies surrounding potential involvement of proton catalysis. Already in the 1960s, Wanzlick proposed that free carbenes form through direct dissociation of the enetetramine (Scheme 1).^22–24 Later, Lemal and Winberg reconsidered the suggested equilibrium and emphasized the importance of proton catalysis involving transient 3 due to the requirement for the addition of catalytic amounts of proton sources, such as mineral acids.^25,26 This was also observed by Arduengo for stable thiazol-2-ylidenes which only dimerized upon the presence of thiazolium salts.²⁷ Almost 40 years later, Alder showed that 3 is integral to the formation of 1.^28,29 Murphy reported on tethered olefins, where dissociation occurred likely through proton catalysis.³⁰ Denk showed that NHCs dimerize slowly under aprotic conditions.³¹ Further, he presented evidence for carbene metathesis (“crossover”) between different enetetramines with small N-substituents (e.g.^Me1, R = Me and ^Et1, R = Et) in solution under heating.^32,33 Hahn and Lemal revealed benzimidazolin-2-ylidene formation upon heating the respective dimers in solution using in situ¹H NMR spectroscopic studies.^34–37 These results were further corroborated by computations.^38–41 To the best of our knowledge, experimental investigations during sublimation (i.e. gas phase), devoid of the serendipitous presence of protons as is the case in solution in glassware remain hitherto elusive.⁴² Furthermore, studies on the dissociation of triaminoolefins remain to be reported. Thus, we communicate herein on the sublimation of heterodimers⁴³ derived from benzNHCs, saNHCs, and CAACs as well as their protonated derivatives equivalent to Lemal's intermediate 3.⁴⁴ Complemented by computations, we quantify the effect of proton-catalysis and show how to avoid/slow down the undesired dimerization of transient carbenes (dissociation of electron-rich olefins, respectively).

Scheme 1 Acid catalysis in the equilibrium between tetraaminoolefins 1 and NHC monomers 2.

Synthesis

It has been reported that deprotonation of benzimidazolium salt ^Me5 by KOtBu gives dimer ^Me6 (Scheme 2, right).⁴⁵ To our surprise, when using KHMDS [KN(SiMe₃)₂] as a base in benzene at room temperature, we obtained instead a mixture of the dimer (44%, Fig. S1†) and the free carbene ^Me4 (56%, Scheme 2, left). The latter, unambiguously identified through a signal in the ¹³C NMR spectrum at 218 ppm, proved kinetically surprisingly stable with a half-life t_1/2 of more than one day (c = 0.4 M) and consequently required several days to dimerize with a pseudo-second order rate law (Fig. S8†). Following the deprotonation of a suspension of ^Me5 by KOtBu in C₆D₆, however, indeed corroborated the instantaneous and quantitative formation of ^Me6. These observations suggest that HOtBu (pK_a = 16.5)⁴⁶ catalyzes the dimerization, whereas less acidic HN(SiMe₃)₂ (pK_a = 25.8)⁴⁷ does not (or to a much lesser extent, respectively). Indeed, adding small amounts of HOtBu to the free carbene ^Me4 led to the immediate and quantitative formation of ^Me6. In contrast, free carbene ^Me4 could be obtained quantitatively by adding ^Me5 to KHMDS dissolved in a 1 : 1 mixture of benzene and hexane at 5 °C under moderately dilute conditions (c = 0.04 M; ≈3% dimerization after one day at room temperature; Fig. S3 and S4†). Even removing the solvent, i.e. isolation of the free carbene, led to only ≈4% dimerization (Fig. S5†). The deprotonation by lithium diisopropylamide (LDA) in benzene formed quantitatively the lithium complex (δ^carbene = 207 ppm), which converted by ≈2% to the dimer within three days (c = 0.4 M; Fig. S6 and S7†).

Scheme 2 The rate for the formation of 6 from 5 is dependent on the base.

To further elucidate the dimerization (dissociation, respectively) mechanism, we synthesized a series of carbene heterodimers⁴⁸ with varying steric and electronic properties (Scheme 3, top). Following our previously reported route for the formation of ^Me10via^Me9,⁴³ we obtained ^Et9 (61%) and ^Et10 (86%) analytically pure. Salts ^R9 represent the triamino-equivalent of Lemal's transient intermediate 3. Conversely, the synthesis of the saNHC=benzNHC tetraaminoolefins ^R13 (Scheme 3, bottom) proved more challenging.

Scheme 3 The CAAC-derived protonated heterodimer ^Et9 and olefins ^Et10, ^Me13 do not dissociate in solution at room temperature (top), whereas saNHC-derived ^Me12 stands in equilibrium with the starting materials ^Me4 and ^Me11.

In the case of salt ^Me12, the in situ¹H NMR spectroscopic analysis in DMSO indicated that the reaction leveled out at approximately 80% conversion after 1 h, with the concomitant presence of imidazolinium- (^Me11, 5%) and benzimidazolium (^Me8, 15%) salt (Fig. S14†). In fact, after prolonged reaction time, we observed the slow formation of ^Me13. In case of ^Et13, we even did not manage to obtain useful conversion (>5%) to ^Et12. Overall, these results are consistent with the rare reports^28,49–51 of protonated enetetramines (“Lemal's intermediate”) and suggest that not only the addition of ^Me4 to ^Me11 proceeds essentially isoergic, but equally the protonation of ^Me13. Still, treatment of the mixture with KHMDS afforded, after workup, olefin ^Me13 in satisfactory purity (Fig. S16†).

Sublimation experiments

Upon heating the triaminoolefins ^Me10 and ^Et10in vacuo to 185 °C and 165 °C, respectively, yellow crystals formed at the walls of the sublimation flask (Scheme 4).

Scheme 4 Whereas sublimation was observed for the electron-rich olefins ^R10 and ^Me13 (top), dissociation and subsequent separation of volatile and non-volatile products were obtained for their protonated congeners ^R9 and ^Me12 (bottom). All experiments were conducted at 5 × 10⁻³ mbar.

The ¹H NMR spectroscopic analysis identified these crystals as the starting material without the concomitant formation of ^Me6 (Fig. S18†). In addition, no metathesis occurred when heating a mixture of ^Me10, and ^Et10 (Fig. S22†). We conclude that such triaminoolefins do not dissociate at these temperatures in the absence of proton catalysis. This seems to be also the case for enetetramine ^Me13, where only starting material was obtained in the sublimate after heating it to 120 °C. In case of the salts ^Me9, ^Et9, and ^Me12, dissociation and subsequent sublimation of homo- and heterodimers occurred at considerably lower temperatures (140 °C, 120 °C, 95 °C) than found for the olefins (Scheme 4, bottom). Accordingly, the presence of free carbene, as well as homodimers, confirms dissociation. For instance, in case of ^Me9, the sublimed material consisted of the homodimers and the free CAAC, respectively (^Me6: 30%; ^Me7: 40%), whereas the residue consisted of both benzimidazolium- (^Me8: 40%) as well as cyclic iminium (^Me14: 60%) salt. Overall, we conclude that both tetra- and triaminoolefins dissociate readily under proton catalysis on gentle heating.

Computational analysis

Computational investigations at the DLPNO-CCSD(T)/def2-TZVPP//BLYP-D3BJ/def2-TZVPP level of theory were performed. Consistent with the experiment, where no dissociation was observed at temperatures as high as 185 °C, the dissociation of triaminoolefin ^Me10 is predicted to occur with a high barrier of ΔG^≠ = +200 kJ mol⁻¹ (Fig. 1, top) and considerably endergonic (ΔG = +76 kJ mol⁻¹). The barrier for ethyl derivative ^Et10 is significantly lower with ΔG^≠ = +172 kJ mol⁻¹, yet still too high to form ^Et4 and ^Et7 (ΔG = +53 kJ mol⁻¹) under mild conditions. Tetraaminoolefin ^Me13 is expected to dissociate with ΔG^≠ = +146 kJ mol⁻¹ (ΔG = +55 kJ mol⁻¹), which seems consistent with dissociation starting at a (slightly) higher temperature than the sublimation temperature of 120 °C. Also here, the introduction of ethyl groups (^Et13) further facilitates dissociation (ΔG^≠ = +138 kJ mol⁻¹; ΔG = +43 kJ mol⁻¹).

Fig. 1 Calculations at the DLPNO-CCSD(T)/def2-TZVPP//BLYP-D3BJ/def2-TZVPP level of theory quantify acid catalysis for dissociation of tri- and tetraaminoolefins. Values in parentheses relate to implicit solvation in THF.

In contrast, the dissociation from the salts, representing the key elementary step of proton catalysis, requires lower activation energies (Fig. 1, bottom). Whereas the Gibbs free activation energy for ^Me9 (ΔG^≠ = +102 kJ mol⁻¹) and ^Et9 (ΔG^≠ = +98 kJ mol⁻¹) suggests dissociation at elevated temperatures, the value of ΔG^≠ = +77 kJ mol⁻¹ (ΔG = +69 kJ mol⁻¹) found for saNHC–benzNHC salts ^Me12 and ΔG^≠ = +81 kJ mol⁻¹ (ΔG = +73 kJ mol⁻¹) for ^Et12 is indicative for dissociation at milder temperatures. Modeling the dissociation in condensed phase using implicit solvation in THF confirms that the dissociation of ^Me12 (ΔG = +15 kJ mol⁻¹) and ^Et12 (ΔG = +20 kJ mol⁻¹) proceeds almost isoergic in solution with low barriers (^Me12, ΔG^≠ = +80 kJ mol⁻¹; ^Et12, ΔG^≠ = +83 kJ mol⁻¹), consistent with a reaction occurring at room temperature. Salts ^Me9 and ^Et9 are kinetically (^Me9, ΔG^≠ = +106 kJ mol⁻¹; ^Et9, ΔG^≠ = +97 kJ mol⁻¹) and thermodynamically (^Me9, ΔG = +59 kJ mol⁻¹; ^Et9, ΔG = +46 kJ mol⁻¹) more stable. These results in agreement with the experimental difficulties to isolate ^Me12 in analytically pure form (vide supra). Overall, we conclude that proton catalysis lowers the activation energy for dissociation for the methyl-substituted enetetramine by ΔΔG^≠ = 146 kJ mol⁻¹–77 kJ mol⁻¹ = 69 kJ mol⁻¹ (ΔΔG = 138 kJ mol⁻¹–81 kJ mol⁻¹ = 57 kJ mol⁻¹ for the ethyl-substituted derivative) and in case of the methyl-functionalized triaminoolefin by ΔΔG = 200 kJ mol⁻¹–102 kJ mol⁻¹ = 98 kJ mol⁻¹ (ΔΔG = 172 kJ mol⁻¹–98 kJ mol⁻¹ = 74 kJ mol⁻¹ for the ethyl-substituted derivative).

Conclusion

The dissociation of electron-rich olefins into carbenes was studied in the absence of serendipitous proton sources. To this goal, representatives of tri- and tetraaminoolefins composed of NHCs and CAACs, and their protonated congeners, alleged key intermediates in a proton-catalysis pathway, were synthesized. Based on sublimation at elevated temperatures as well as the computational analysis, we quantified proton-catalysis and showed that not only tetraaminoolefins, but as well triaminoolefins may dissociate. However, acid catalysis is crucial in case of the latter, whereas the former dissociate under heating potentially as well through a direct mechanism. Eventually, we outlined how N,N′-dimethylbenzimidazolin-2-ylidene, previously reported to dimerize instantaneously, can be isolated through deprotonation with KN(SiMe₃)₂ in a mixture of benzene and hexane. This free carbene turned out to be metastable over the course of days, whereas weaker bases such as KOtBu lead to direct dimer formation. Thus, this work serves as a guideline on how to generate transient carbenes and engineer thermodynamically and kinetically stable electron rich olefins being of use in organic electronics.

Author contributions

The project idea was conceived by D.M., who also directed all work and wrote the manuscript. J.M. performed the computations, contributed to the synthesis, and wrote the first draft. M. K. synthesized all compounds and performed the sublimation experiments. S.G. performed exploratory synthetic studies.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We thank the RRZ Erlangen for computational resources. We also thank K. Meyer for his support.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1qo01320c

‡ Both authors contributed equally.

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