The synthesis of Ni^II and Al^III 10-azacorroles through coordination-induced cyclisation involving 1,2-migration

Abstract

We have synthesized Ni^II and Al^III 10-azacorroles through coordination-induced cyclisation of nitrogen-bridged bisdipyrrins. Al^III 10-azacorrole displayed contrasting optoelectronic behaviours from Ni^II 10-azacorrole, underlining the different electronic effects of the azacorrole ligand from the porphyrin.

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Porphyrinoids are macrocycles with porphyrin-like skeletons.¹ The properties of porphyrinoids are substantially affected by their skeleton and structural factors. In particular, inclusion of heteroatoms into the aromatic π-conjugation circuit would result in a dramatic change of their electronic structures. Consequently, 10-heterocorroles (X = N, O, S and Se) have regained the spotlight as novel molecular entities as electronically tuneable porphyrin analogues with a similar divalent ligand skeleton and 18π aromatic conjugation as regular porphyrins (Fig. 1).^2–4

Fig. 1 Skeletons of porphyrin and 10-heterocorrole. The bold lines indicate 18π aromatic conjugation circuits.

Recently, Bröring and co-workers reported the synthesis of β-octaalkyl-10-oxa-, 10-thia- and 10-selenacorroles from dibromodipyrrins by treatment with copper(II) salts.³ Our group independently developed the synthetic protocols of meso-diaryl-10-aza-, 10-oxa- and 10-thiacorroles.⁴ In particular, 10-azacorroles can be regarded as electron-rich porphyrin analogues owing to their low oxidation potentials. Ni^II 10-azacorrole was synthesized via palladium-catalysed amination of the Ni^II dipyrrin complex. However, the protocol was only applicable to the preparation of Ni^II complexes.

Electrocyclisation is one of powerful methods for ring construction in organic synthesis. In porphyrin chemistry, Johnson and co-workers reported in the 1960s, the synthesis of corroles by oxidative cyclisation of a,c-biladienes with Co^II or Ni^II salts.⁵ Despite this pioneering discovery, this synthetic protocol has been overshadowed for a long time because of difficulty in accessing a,c-biladienes.⁶

Herein we described the synthesis of Ni^II and Al^III 10-azacorroles from amine-bridged bisdipyrrins. Coordination of bisdipyrrins with Ni^II and Al^III induced efficient C–C bond formation between two pyrrole units to provide Ni^II and Al^III azacorroles with concomitant 1,2-migration of an α-substituent. This procedure enabled the first synthesis of Al^III 10-azacorrole and regioselective introduction of various functionalities at the 2-position of Ni^II 10-azacorroles. These two metal complexes exhibited significantly different optical and electrochemical properties.

Palladium-catalysed amination of dipyrrin 1 was performed with benzylamine (Scheme 1). Treatment of dibromodipyrrin 1a with 1.1 equiv. of benzylamine and ^tBuOK in the presence of a catalytic amount of Pd₂dba₃·CHCl₃/DPPF (5 mol%) in toluene at 80 °C provided bisdipyrrin 2a in 32% yield. The employment of Xantphos improved the yield to 45%, while DPEphos was not effective. Eventually, the reaction with 2.5 equiv. of benzylamine afforded 2a in 64% yield. Dichlorobisdipyrrin 2b was also prepared from dichlorodipyrrin 1b in 29% yield. The introduction of a phenyl group at the α-position was achieved through Migita–Kosugi–Stille coupling to afford 2c.

Scheme 1 Double amination of dihalodipyrrin and introduction of a phenyl group.

With bisdipyrrins 2 in hand, the metal coordination behaviour of 2 was investigated (Scheme 2).⁷ Treatment of 2a with 5.0 equiv. of Zn^II and Cu^II afforded the corresponding metal complexes 3Zn and 3Cu in 98 and 96% yields, respectively. The helical conformation of 3Cu was elucidated by X-ray diffraction analysis (Fig. 2a). On the other hand, heating the mixture of 2a with 5.0 equiv. of NiCl₂ furnished bromoazacorrole 4 and azacorrole 5 in 67% and 32% yields, respectively. Importantly, Ni^II bromoazacorrole 4 was obtained as a single regioisomer. X-ray diffraction analysis unambiguously determined the structure of 4, revealing that the bromine atom shifted from the α- to β-positions during cyclisation (Fig. 2b). In the case of dichlorobisdipyrrin 2b, 5 was obtained exclusively in 83% yield. Interestingly, treatment of phenylbisdipyrrin 2c with NiCl₂ provided 2-phenyl-10-azacorrole 6 in 99% yield through 1,2-migration of the phenyl group (Scheme 3).

Scheme 2 Metallation of bisdipyrrins 2a with Zn^II, Cu^II and Ni^II ions.

Fig. 2 X-ray crystal structures of (a) 3Cu and (b) 4. meso-Mesityl groups and the benzyl substituent are omitted for clarity. The thermal ellipsoids are scaled at 50% probability level.

Scheme 3 1,2-Migration of a phenyl group in metallation of bisdipyrrin 2c with Ni^II ions.

Treatment of 2a with 10 equiv. of AlCl₃ also induced the similar cyclisation effectively (Scheme 4). In this case, a mixture of regioisomeric bromoazacorroles and azacorrole 7 was obtained (Fig. S34, ESI†). Subsequent palladium-catalysed reductive dehalogenation afforded Al^III 10-azacorrole 7 in 67% yield in two steps from 2a.⁸ This result confirms that Ni^II metal ions work as neither a coupling reagent nor a catalyst in this cyclisation reaction.⁹ Notably, the present cyclisation is highly dependent on the metal ions because Zn^II and Cu^II showed no similar reactivity.

Scheme 4 Metallation of bisdipyrrins 2a with AlCl₃.

The present methodology was employed to prepare functionalized azacorroles (Scheme 5). The reaction of 4 with pinacolborane under palladium catalysis provided 8 in 52% yield. In addition, treatment of 4 with Ni(cod)₂/2,2′-bipyridyl furnished the 2,2′-linked dimer 9 in 62% yield.

Scheme 5 Reactions of Ni^II 2-bromo-10-azacorrole 5.

Fig. 3 displays the UV/vis absorption spectra of 5 and 7 in CH₂Cl₂. In contrast to Ni^II azacorrole 5, Al^III azacorrole 7 exhibits a sharp and intense Soret band and blue-shift of the lowest energy absorption bands. This tendency is different from that observed in the corresponding porphyrin metal complexes: Al^III tetraphenylporphyrin (TPP) exhibits red-shifted absorption in comparison to that of Ni^II TPP.^10,11 These differences were analysed using theoretical calculations. Oscillator strengths for 5 and 7 calculated using the TD-DFT method were in good agreement with the experimental spectra. For Ni^II and Al^III TPPs, both the Soret and Q bands consist of the HOMO → LUMO and HOMO−1 → LUMO+1 transitions. On the other hand, the lowered symmetry of azacorroles results in the additional contribution of the HOMO−3 → LUMO transition in the Soret and Q bands. The HOMO−3 of 5 is significantly different from that of 7 because of the presence of the d–σ bond between nitrogen and Ni^II (Fig. S39, ESI†). Consequently, the Soret band of 5 is split and broad.

Fig. 3 (a) UV/vis absorption spectra of 5 (red) and 7 in CH₂Cl₂. (b) The emission spectrum of 7 in CH₂Cl₂.

We then examined the electrochemistry of 5 and 7 by cyclic voltammetry, which showed two reversible oxidation and one reduction wave in both cases (Table 1 and Fig. S35, ESI†). All oxidation and reduction potentials of 7 are larger than that of 5 due to the electron-withdrawing nature of Al^III. The difference between the first oxidation and the reduction potentials (ΔE) of 7 is 2.33 V, which is smaller than that of 5 (2.19 V). This trend is opposite to that observed in the case of Ni^II and Al^III TPPs.^12,13 TPPs have the two degenerated HOMO and HOMO−1 with different symmetries, a_1u and a_2u. Central metals mainly affect the energy level of the a_2u orbital while that of the a_1u orbital remains rather unchanged. Therefore, the difference in the oxidation potentials of Ni^II and Al^III TPPs is rather small (Fig. S38, ESI†). The more positive Al^III lowers the LUMO energy more than Ni^II. Consequently, the HOMO–LUMO gap in Al^III TPP becomes smaller than that in Ni^II TPP. On the other hand, both HOMO and LUMO of azacorrole have been influenced by metallation because of their substantial molecular orbital coefficients on the inner nitrogen atoms (Fig. S37, ESI†). Therefore, 7 has lower HOMO and LUMO energies than 5 due to the more electron-deficient nature of Al^III than Ni^II.

Table 1 Results of electrochemical measurements for 5, 7, Ni^II TPP and Al^III TPP^a

Compound	E red	E ^1ox	E ^2ox	ΔE ^b
a Unit: V. b ΔE = E^1ox − Ered. c Ref. 12 (subtracted by −0.50 V). d Ref. 13.
5	−2.01	0.181	0.718	2.19
7	−1.89	0.426	0.871	2.33
Ni^II TPP^c	−1.75	0.55	—	2.30
Al^III TPP^d	−1.69	0.43	—	2.12

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Compound 7 showed fluorescence with a moderate quantum yield (Φ_f = 0.117, Fig. 3b). The emissive feature of Al^III complexes is also observed in corroles and porphyrins.¹⁴ Compound 7 is the first example of emissive 10-azacorroles.

Cyclisation of bisdipyrrins in the presence of Ni^II and Al^III cations was intriguing in view of the reaction mechanism. Bromination of 5 with NBS afforded a mixture of regioisomeric bromoazacorroles. This fact negates that the introduction of the bromo substituent occurs after the formation of 5. The possibility of nucleophilic substitution with a bromide anion should be excluded, considering the presence of a large amount of the chloride anion from NiCl₂. In addition, the reaction of 2a with NiBr₂ afforded 4a and 5 with almost the same selectivity (60% and 36% yields) as in the case of NiCl₂. This result negates the influence of the counter anion. The addition of TEMPO in the reaction had no effect, negating a radical mechanism.¹⁵ On the basis of these findings, we propose the reaction mechanism involving electrocyclisation followed by 1,2-migration (Scheme 6).¹⁶ Electrocyclisation of 3Ni induced by electron pushing from the nitrogen atom provide the zwitterionic intermediate I. Elimination of a bromide anion yields the iminium species II, which then undergoes 1,2-migration of bromine to yield the cationic intermediate III. Deprotonation of III eventually furnishes 2-bromoazacorrole 4. On the other hand, the attack of bromide on II affords azacorrole 5 through elimination of Br₂. In the case of 2b, elimination predominantly proceeds to provide 5 because of the lower migratory ability of chloride.

Scheme 6 Proposed mechanism of the cyclisation and rearrangement process.

The role of Ni^II can be explained by the geometry of bisdipyrrin complexes. In the crystal structure of 3Cu, the distance between C(1)–C(18) was 3.52 Å (Fig. 2a). This distance was reproduced well by DFT calculations at the M06-2X/6-31G(d)+SDD level, showing the validity of the theory. The calculations also suggested that the distance between two α-carbon atoms in Ni^II bisdipyrrin complex 3Ni to be 3.35 Å (Fig. 4). This distance is within the sum of van der Waals radii of two carbon atoms, facilitating the construction of a novel carbon–carbon bond. Thus, the Ni^II metal serves as a template to bring two reacting centres closer. Considering the formation of the Al^III complex 7, the size of the metal ion should be critical. Al^III and Ni^II have similar ionic radii (0.48 and 0.49 Å), which are much smaller than those of Zn^II and Cu^II (0.60 and 0.57 Å). The mechanism was further investigated using DFT calculations (Fig. S36 and Table S2, ESI†). The transition state structure was obtained by bringing two reaction centres closer. Then the IRC calculation provided an intermediate species, which corresponds to iminium II in Scheme 4. The activation energy of the cyclisation process was calculated to be 45.0 kcal mol⁻¹.

Fig. 4 Calculated structures of 3Ni, 3Cu and 3Zn using the DFT method. The aryl and benzyl substituents are replaced with hydrogen.

In summary, we found that Ni^II and Al^III metallation of bisdipyrrins 2 efficiently provided Ni^II and Al^III 10-azacorroles. The cyclisation proceeds through electrocyclisation followed by 1,2-migration of bromo and phenyl substituents. Ni^II and Al^III 10-azacorroles exhibited significantly different optical and electrochemical properties from the corresponding porphyrin metal complexes. The present protocol would be also applicable to the chemistry of other types of 10-heterocorroles. Further investigation of the characteristics of meso-aryl-10-heterocorroles is underway in our group.

This work was supported by a Grant-in-Aid for Scientific Research on Innovative Areas “New Polymeric Materials Based on Element-Blocks” (15H00731), a Grant-in-Aid for Scientific Research (No. 24350023), and the Program for Leading Graduate Schools “Integrative Graduate Education and Research in Green Natural Sciences” from MEXT Japan. H. S. is grateful to the Mitsubishi Foundation for financial support.

Notes and references

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9. Non-cyclised complex 3Ni was isolated with other meso-substituents. See the ESI† (Scheme S1).
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16. Electrocyclisation involving 1,2-migration was observed in Nazarov cyclisation; (a) D. Lebœuf, E. Theiste, V. Gandon, S. L. Daifuku, M. L. Neidig and A. J. Frontier, Chem. – Eur. J., 2013, 19, 4842; (b) D. Lebœuf, V. Gandon, J. Ciesielski and A. J. Frontier, J. Am. Chem. Soc., 2012, 134, 6296.

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Footnote

† Electronic supplementary information (ESI) available: Synthetic procedures, analytical data of compounds, crystallographic data, and details of DFT calculations. CCDC 1059525 (3Cu) and 1059526 (4). For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5cc10247b

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