Ring-expansion from tellurophenes to telluropyrans: inhibition of C–Te bond cleavages in transition metal-catalyzed reactions

Abstract

Tellurium-fused aromatic hydrocarbons have attracted extensive attention due to their unique properties as well as the synthetic challenges. However, the synthesis of organotellurium molecules falls much behind those of other chalcogen-containing compounds since the synthetic methods to build C–S and C–Se bonds always fail to extend the C–Te bonds. Herein, a PtCl₂-catalyzed ring-expansion reaction is attempted on tellurophene derivatives. However, tellurium-free vinylogous pentafulvalene analogues instead of telluropyrans are produced unexpectedly. To prevent the Pt–Te interactions and avoid the C–Te bond cleavage, bulky phosphine ligands are incorporated into the PtCl₂ catalyst. A ring-expansion cyclization towards neutral telluropyran derivatives from the corresponding tellurophene compounds has been thus developed. The mechanisms of both the ring-opening and the ring-expansion reactions are proposed and the pathways are verified by density functional theory calculations. Furthermore, the tolerance of the obtained telluropyran derivatives in metal-related reagents is further investigated and the conjugation can be facilely extended via various transition metal-catalyzed coupling reactions. Therefore, our work provides not only a synthetic methodology towards telluropyran-fused polycyclic aromatic hydrocarbons, but also an effective pathway to avoid the C–Te bond cleavages in transition metal-catalyzed coupling reactions.

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Introduction

Chalcogen-containing aromatic hydrocarbons have attracted extensive attention due to their increasing demand in both chemistry and material fields.^1–3 Among the chalcogen elements, tellurium (Te) is the heaviest non-metallic one with certain metallic properties. Therefore, Te atoms have been incorporated into organic semiconductors due to their electrical conductivity,^4,5 strong polarizability,^6,7 heavy atom effect,^8,9 and intense intermolecular interactions.¹⁰ Until now, several methodologies have been established for the synthesis of organotellurium compounds.^8,11,20 However, despite these promising advances, the synthetic approaches towards organotellurium molecules fall much behind those for S- and Se-containing compounds since the synthetic methods to build C–S and C–Se bonds always fail to extend the C–Te bond.¹² Moreover, the formed C–Te bond can be easily cleaved by Pt(II),¹³ Pd(II),¹⁴ HCl,^15–17 LiBu,^18–20 and so on (Fig. 1a) due to its lower bond energy (200 kJ mol⁻¹) in comparison with C–S (272 kJ mol⁻¹) and C–Se bonds (234 kJ mol⁻¹).¹² Furthermore, it is always unsuccessful to functionalize organotellurium compounds using the conventional methods for S- and Se-containing molecules.²¹ For example, bromination with N-bromosuccinimide has been proved to be an efficient way for thiophene and selenophene but fails for tellurophene.²² In addition, the strong coordination ability of the Te atom always poisons the transition metal-based catalyst during coupling reactions.²³ Consequently, in contrast to the abundant S- and Se-heterocyclic compounds, Te-containing cyclic systems have been much less explored and thus the synthesis thereof remains challenging.

Fig. 1 (a) Reported C–Te bond cleavages by metal-related reagents and (b) our ring-expansion reactions from tellurophenes to telluropyrans.

Transition metal-catalyzed ring-expansion and skeletal rearrangement have been extensively investigated as efficient pathways to offer direct access towards cyclic molecules which are difficult to construct by conventional synthetic methods.²⁴ Recently, we have developed a facile ring-expansion cyclization from S-fused five-membered thiophene rings to six-membered thiopyran rings catalyzed by PtCl₂.²⁵ The incorporated chalcogen atoms endow the heterocyclic systems with not only intense intramolecular charge transfer interactions but also high intermolecular charge transport mobilities.²⁶ Meanwhile, most of the Te-containing six-membered rings are reported in the form of onium salts, such as tellurorhodamine.²⁷ Neutral telluropyran derivatives and the universal synthetic methods have been rarely reported. Therefore, PtCl₂-catalyzed ring-expansion cyclization has been attempted on tellurophene derivatives in this work. Unexpectedly, no telluropyran derived product can be synthesized. Instead, Te-free vinylogous pentafulvalene derivatives are discovered by single-crystal X-ray analysis. This can be attributed to the interactions between the Te and Pt atoms and a possible mechanism of the complicated ring-opening, detelluration, and dimerization reaction is proposed. To prevent the harmful interactions, different ligands have been introduced and telluropyran derivatives can be generated (Fig. 1b). Interestingly, the incorporation of electron-deficient P(C₆F₅)₃ is the most effective to prevent the cleavage of the tellurium atoms from the heterocyclic rings and to produce telluropyran derivatives. The scope has been further investigated by tuning the aryl backbones and the alkyl/aryl substituents. Furthermore, the tolerance of the obtained telluropyran derivatives in metal-related reagents is further investigated and the conjugation can be facilely extended via various transition metal-catalyzed coupling reactions. Therefore, the reactivity of telluropyran derivatives is explored and the diversity of Te-containing heterocycles is expanded.

Results and discussion

Cleavage of tellurophene derivatives catalyzed by PtX₂

The initial investigation was carried out using easily accessible 2-alkynyl-1-tellurophenyl substituted aromatics. With the assistance of PtCl₂ (10 mol%), 2-(1-decynyl)-1-(2-tellurophenyl)naphthalene (1a) could be converted to a dimerized ring-opening product 2a (Fig. 2a). The yield was 14% if calculated from reactant 1a while it was 68% if referenced to PtCl₂. The former value increased but the latter almost remained constant with the feed ratio of PtCl₂ enhancing. No matter how much PtCl₂ was added, no Te-fused heterocyclic compound could be detected. The selection of the naphthalene skeleton was for the facile growth of the single crystals of the products. As shown in Fig. 2b, single crystal X-ray analysis reveals that compound 2a contains an octatetraene-bridged pentafulvalene. The alternating single and double bonds are delocalized and the bond lengths locate between the lengths of the conventional C–C (1.54 Å) and C=C (1.34 Å) bonds.²⁸ Moreover, two chlorine atoms can be found on the pentafulvalene backbone. To figure out the origin of the two chlorine atoms, PtBr₂ was further tested and product 2b was obtained. Similarly, two bromine atoms could be found at the same positions (Fig. 2b). Therefore, the two halogen atoms in 2a and 2b come from PtX₂ (X = Cl, Br) which serves as a reactant instead of a catalyst.

Fig. 2 (a) Ring-opening, detelluration, and dimerization reactions of the tellurophene derivatives in the presence of PtX₂ and (b) ORTEP diagrams of products 2a and 2b (CCDC 2132835 and 2132837†).

A probable mechanism of this complicated ring-opening, detelluration, and dimerization reaction is proposed and illustrated in Fig. 3. Initially, the complexation of PtX₂ with the carbon–carbon triple bond affords intermediate A, followed by trans-addition to give intermediate B. Then an ipso-cyclization occurs and the Pt atom interacts with Te to produce intermediate C. Subsequent Pt-dissociation and Te-extrusion²⁹ afford the key intermediate F which can further react with the alkyne in another molecule to yield intermediate G. After similar procedures, dimerized intermediate L is produced. Upon further dissociation of Pt, the final product M is obtained. Simultaneously, PtX₂ is consumed and thus serves as a reactant. To verify the proposed mechanism, the reaction path and the relative free energies for all stationary points were calculated at the ωB97M-V³⁰/def2-TZVPP,³¹ SMD(toluene)³²//ωB97X-D³³/6-31G(d) (LANL2DZ for Pt and Te), SMD(toluene) level. The detailed calculation methods and results are given in the ESI.† As shown in Fig. S5–S7,† all the transition states are confirmed to connect the neighboring intermediates, which proves the proposed mechanism for the cleavage of the Te atom and the formation of the Te-free vinylogous pentafulvalene derivatives.

Fig. 3 Proposed mechanism of the ring-opening, detelluration, and dimerization reaction of tellurophene derivatives in the presence of PtX₂.

Ring-expansion from tellurophenes to telluropyrans

To suppress the interactions between Pt(II) and Te, additive phosphine ligands were introduced simultaneously with the reactant and the catalyst. The temperature was then raised to 110 °C. As shown in Table 1, in the presence of PtCl₂ (10 mol%), the incorporation of bulky phosphine ligands (20 mol%), such as PPh₃, P(o-MePh)₃, P(t-Bu)₃·HBF₄, and P(C₆F₅)₃, successfully reduces the Pt–Te interactions and decreases the yields of by-product 2a (entries 3–6). This means the introduced ligands prevent the Pt–Te coordinate interactions to a certain degree. However, the vinylogous pentafulvalene 2a could still be detected. As discussed above, the obtained by-product consumed the PtCl₂ catalyst, which resulted in a relatively low yield of 3a. Interestingly, telluropyran derivative 3a can be isolated when using P(t-Bu)₃·HBF₄ and P(C₆F₅)₃ as ligands. However, the yields of ring-expansion product 3a are relatively low (<50%) obviously owing to the consumption of the PtCl₂ catalyst via the ring-opening pathway. To further improve the yields of telluropyran derivatives, phosphine-coordinated PtCl₂ catalysts were directly used in the absence of additional ligands. A perfluoro-substituted phosphine P(C₆F₅)₃ was utilized as the ligand and Pt[P(C₆F₅)₃]₂Cl₂ (CCDC 2132838†) was crystallized from a toluene solution containing PtCl₂ and P(C₆F₅)₃. The chemical structure of the as-prepared catalyst was verified by single-crystal analysis (Fig. S4†). Interestingly, no ring-opening by-product could be detected and the yield of 3a increased to 73% (entry 8). Under the same conditions, Pt[P(C₆F₅)₃]₂Br₂ was tested to catalyze the ring-opening reaction and a yield of 59% could be achieved (entry 9). These results suggest that the incorporation of ancillary phosphine ligands on PtX₂ can effectively weaken the Pt–Te interactions and thus reduce the reaction possibility through the ring-opening pathway. Therefore, further investigation of ring-expansion reactions from tellurophenes to telluropyrans was carried out using Pt[P(C₆F₅)₃]₂Cl₂ (10 mol%) in toluene at 110 °C.

Table 1 Optimization of the ring-expansion conditions

Entry	Catalyst	Additive	Yield (%) 2a/b : 3a
1	PtCl2	—	14 : 0
2	PtBr2	—	15 : 0
3	PtCl2	PPh3	6 : 0
4	PtCl2	P(o-MePh)3	5 : 0
5	PtCl2	P^tBu3·HBF4	6 : 18
6	PtCl2	P(C6F5)3	4 : 47
7	Pt(PPh3)2Cl2	—	0 : 0
8	Pt[P(C6F5)3]2Cl2	—	0 : 73
9	Pt[P(C6F5)3]2Br2	—	0 : 59

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The scope of this Pt(II)-catalyzed ring-expansion cyclization was further investigated by tuning the alkyne (R₁), the aryl group in the skeleton (Ar), and the substituents (R₂) in compound 1 (Table 2). Since a [1,2]-alkyl/aryl migration is involved in the cyclization mechanism which is discussed below, the scope of the substituted groups attached to the alkyne was first examined. As shown in Table 2, in addition to the linear octyl group, both cyclopropyl and t-butyl groups can migrate in this cyclization. However, the yield decreased from 73% for octyl-based 3a to 61% for cyclopropyl-based 3b, and further to 8% for t-butyl-based 3c. Moreover, when the sp³ hybrid carbon substituted at the alkynyl groups changed to an sp² hybrid carbon, telluropyrans 3d, 3e, and 3f, with phenyl, 2-thienyl, and 3-thienyl substituents, respectively, could be produced in yields over 70%. Furthermore, when electron-withdrawing fluoride and electron-donating methoxyl groups were attached at the para-position of the phenyl groups, 3g and 3h were obtained in yields of 85% and 81%, respectively, which suggests that the electron density of the substituted aryl group has little effect on this ring-expansion reaction. Secondly, the aryl groups (Ar in 1, Table 2) in the skeleton were explored. When the alkyne and tellurophene were adjusted to different ortho-positions in naphthalene, such as 1-/2-, 2-/1-, and 2-/3-positions, 3a, 3i, and 3j with conjugation extended from different directions were produced in moderate yields. Similarly, when electron-withdrawing chloride and electron-donating methoxyl groups were introduced on the phenyl groups, 3k and 3l were synthesized in yields over 70%. Thirdly, when the 5-position of tellurophene was occupied by iodide, iodo-substituted telluropyran derivative 3m was afforded in 79% yield. Moreover, when the iodide in the precursor was replaced by a triphenylamine, conjugation-extended 3n was provided (76%). The above examples suggest that this approach is a general method to convert tellurophene-containing compounds to the corresponding telluropyran derivatives.

Table 2 Scope of the ring-expansion reaction towards telluropyran derivatives

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The chemical structures of the as-prepared telluropyran derivatives were characterized by ¹H NMR, ¹³C NMR, and high-resolution mass spectrometry (HRMS) and were found to be consistent with the proposed structures. In addition, single crystal X-ray diffraction analysis was performed to provide the structural features of the telluropyran ring. The single crystal of 3g (CCDC 2132833†) was obtained as a violet prism-shaped crystal by slow evaporation of its toluene solution at room temperature. As shown in Fig. 4, the lengths of two C–Te bonds in the telluropyran ring of 3g are 2.066 and 2.067 Å, which are close to the value in the five-membered tellurophene ring (2.08 Å).³⁴ This suggests that the lone pair electrons of the tellurium atoms are involved in the π-conjugated backbone. Moreover, the indeno[2,1-b]telluropyran backbone is coplanar with dihedral angles between two neighboring aromatic rings less than 3°. Meanwhile, the benzene ring substituted at the cyclopentadiene moiety is significantly twisted with a dihedral angle of 54° due to the steric repulsion between the hydrogen atoms. Consequently, a relatively large π–π stacking distance of 3.78 Å can be found, indicating their relatively weak intermolecular interactions.

Fig. 4 ORTEP diagrams with an ellipsoid contour probability level of 50% and selected bond lengths for 3g.

A probable mechanism for the conversion from tellurophene to telluropyran via the ring-expansion reaction was proposed. The reaction pathway and the relative free energies for all stationary points in toluene solutions were also calculated. As shown in Fig. 5, the model reaction starts with the coordination compound N which is converted to intermediate Pvia an ipso-cyclization reaction. Then the C₁–C₂ bond in the cyclopropanyl unit cleaves due to the huge ring tension. Meanwhile, the C₁ atom attacks the close Te atom with simultaneous cleavage of one C–Te bond in the tellurophene unit and intermediate R is produced. Finally, intermediate T is generated via [1,2]-alkyl migration, followed by the dissociation of PtCl₂L to give the target telluropyran derivative. The model reaction turns out to be exothermic at 28.0 kcal mol⁻¹.

Fig. 5 Calculated pathway of the ring-expansion reaction and the relative free energies (in kcal mol⁻¹) of the stationary points at the ωB97M-V/def2-TZVPP, SMD(toluene)//ωB97X-D/6-31G(d) (LANL2DZ for Pt and Te), SMD(toluene) level.

Tolerance in transition metal-catalyzed coupling reactions and conjugation extension of telluropyran derivatives

It is well known that tellurophene derivatives can be cleaved by metal-related chemicals, such as Grignard reagents³⁵ and transition metal-based catalysts.^13,14 To investigate the tolerance of the telluropyran derivatives in transition metal-catalyzed coupling reactions and extend their conjugation, halogenation of telluropyran derivatives and their further coupling reactions were carried out. In general, a tellurophene can be simply iodinated by N-iodosuccinimide (NIS)³⁶ or LiBu/I₂.³⁷ However, when telluropyran derivative 3d was treated with the above-mentioned conditions, the desired product 3o was not produced. As shown in Fig. 6a, when 3d was treated with iodine, whether LiBu was initially added or not, an oxidized product 3d·I₂ was produced. Its chemical structure was verified by two-dimensional ¹H–¹H correlation spectroscopy (2D ¹H–¹H COSY). As shown in Fig. 6b, the signals of protons a, b, and c on the telluropyran ring in 3d locate in the aromatic region at δ = 8.14 (doublet), 7.59 (triplet), and 7.75 ppm (doublet), respectively. Upon the addition of two iodides, the three proton signals shift to the higher field region at δ = 7.46, 7.01, and 7.20 ppm, respectively. This can be explained by the decreased aromaticity and deshielded effect upon the introduction of iodine on the telluropyran ring.³⁸

Fig. 6 (a) Iodation of 3d into 3d·I₂ with proton labelling and 2D ¹H–¹H COSY NMR spectra of (b) 3d and (c) 3d·I₂ in CDCl₃ with the assignment (* refers to the signal of the solvent).

Therefore, to study the tolerance of the telluropyran derivatives in transition metal-catalyzed coupling reactions, compound 3m with iodide was selected and synthesized by ring-expansion of its iodinated tellurophene precursor. Generally, in the presence of a Grignard reagent, tellurophenes always decompose to dienes.³⁹ Interestingly, the as-prepared telluropyran derivatives are chemically stable enough to survive with Grignard reagents. As shown in Table 3, compound 4a could be provided by Ni(II)-catalyzed Kumada⁴⁰ coupling in 72% yield (entry 1). Moreover, similar to the Ni-containing catalyst, Pd (non- or two-valent) is one of the most important transition metal-based catalysts in coupling reactions. Under the catalyzation of Pd(II) and Pd(0), compounds 4b and 4c were obtained via Suzuki⁴¹ and Stille⁴² coupling reactions in yields of 91% and 83%, respectively (entries 2 and 3). Furthermore, in the presence of a strong base of t-BuONa, a Buchwald⁴³ coupling with diphenylamine afforded compound 4d in 75% yield (entries 4). In addition to Ni- and Pd-based catalysts, telluropyran derivatives can also tolerate Cu-based catalysts. As shown in entries 5 and 6, via Cu-catalyzed Ullmann⁴⁴ coupling and CuI-cocatalyzed Sonogashira⁴⁵ coupling reactions, compounds 4e and 4f were produced in high yields. These results suggest that, unlike tellurophenes, the obtained telluropyran derivatives are chemically stable in most of the popular transition metal-catalyzed coupling reactions and their conjugations can be facilely extended via various classic coupling reactions. The better tolerance of the telluropyran derivatives than that of the tellurophene derivatives can be explained by their distinct electronic structures. Unlike the tellurophene with 6 π-electrons, a neutral telluropyran contains 7 π-electrons. Nevertheless, the as-prepared neutral telluropyran derivatives are aromatic since the cyclopenta[b]telluropyran moiety presents the 5π + 7π electron characteristic and tends to form an isoelectronic resonance structure where the telluropyran ring is positively charged and the cyclopentadiene ring is negatively charged. Consequently, the telluropyran derivatives exhibit different reactivity and tolerance from the electron-rich tellurophene analogues. Moreover, the as-prepared telluropyran derivatives consist of the cyclopenta[b]telluropyran moiety which presents the 5π + 7π electron characteristic and is isoelectronic to the non-alternating hydrocarbon azulene. Similarly, separated electron distribution can be achieved between the frontier orbitals, which may result in a relatively low energy band gap. Therefore, the conjugation-expanded compounds are potential candidates for further phosphorescence and photovoltaic applications.

Table 3 Tolerance of iodo-substituted telluropyran derivative 3m in transition metal-catalyzed coupling reactions

Entry	Reactant	Catalyst & conditions	Product (yield)
1		Ni(dppe)Cl2
		THF
		0 °C to reflux
2		Pd(PPh3)2Cl2
		K2CO3
		THF/H2O
		Reflux
3		Pd(PPh3)4
		DMF
		120 °C
4		Pd(dba)2
		P(t-Bu)3·HBF4
		t-BuONa
		Toluene
		Reflux
5	None	Cu
		DMF
		Reflux
6		Pd(PPh3)2Cl2
		CuI
		Et3N/DMF
		r.t.

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Conclusions

In summary, a Pt(II)-catalyzed ring-expansion cyclization towards neutral telluropyran derivatives from the corresponding tellurophene compounds has been developed. Without the assistance of the phosphine ligand, the Te atom coordinates with Pt(II) and cleaves from the tellurophene ring. Consequently, Te-free vinylogous pentafulvalene analogues are produced. However, when bulky phosphine ligands are incorporated into the PtCl₂ catalyst, the Pt–Te interactions are suppressed and the cleavage of the C–Te bond is minimized. Consequently, the tellurophene derivatives are converted to the corresponding telluropyran derivatives via a ring-expansion reaction. The substrate scope has been investigated and a possible reaction mechanism for the ring-expansion reaction is proposed. The reactivity of the telluropyran derivatives is further explored and thus their conjugation can be further extended via a series of transition metal-catalyzed coupling reactions. Overall, our work provides not only a synthetic methodology towards telluropyran-fused polycyclic aromatic hydrocarbons, but also an effective pathway to avoid the C–Te bond cleavages in transition metal-catalyzed coupling reactions.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors thank Dr Lingling Li at the Instrumental Analysis Center of SJTU for single crystal analysis. This work was financially supported by the National Key Research and Development Program of China (2018YFA0209401), the National Natural Science Foundation of China (22171053, 21733003), and the Natural Science Foundation of Shanghai (21ZR1409600).

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Footnote

† Electronic supplementary information (ESI) available: Synthesis and characterization of the telluropyran derivatives and calculated geometries of all the stationary points for the ring-opening and ring-expansion reactions (PDF). CCDC 2132833, 2132835, 2132837 and 2132838. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d2qo01732f

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