What factors cause the complete substrate-controlled selectivity in Rh2(Piv)4-catalyzed cycloadditions of 1,2,3-triazoles with isocyanates or isothiocyanates

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Highlights

  • Cycloaddition mechanisms of 1,2,3-triazoles with isocyanates/isothiocyanates were studied.

  • The nucleophilic trend (Cdouble bondS > Cdouble bondN > Cdouble bondO) is the main selective origin of these reactions.

  • The presence of Rh2(Piv)4 is another selective origin via stabilizing azavinyl carbene.

Abstract

DFT studies on Rh2(Piv)4-catalyzed cycloadditions of 1,2,3-triazoles with isocyanates or isothiocyanates show that 1, 2, 3-triazoles undergo ring opening → coordination of Rh2(Piv)4 → dissociation of N2 to give Rh2(Piv)4-azavinyl carbene, where Rh2(Piv)4 exerts a stabilizing effect on azavinyl carbene. The formed Rh2(Piv)4-azavinyl carbene prefers to nucleophilically cyclize with Cdouble bondN bond of isocyanatobenzene to give the imidazolone, rather than Cdouble bondO bond of isocyanatobenzene to give the oxazolimine. In comparison, Rh2(Piv)4-azavinyl carbene preferentially cyclize with Cdouble bondS bond of isothiocyanatobenzene to give the thiazole, instead of Cdouble bondN bond of isothiocyanatobenzene to give the thioimidazolone. The nucleophilic capacity follows Cdouble bondS > Cdouble bondN > Cdouble bondO trend, which is the key selective origin of these reactions. The presence of Rh2(Piv)4 would help to form the carbene and to stabilize it via donating its d-electron into the azavinyl carbene, which is another selective origin of these reactions.

Graphical abstract

Rh2(Piv)4-azavinyl carbene nucleophilically cyclizes with isocyanatobenzene to give imidazolone, while with isothiocyanatobenzene to give thiazole. The nucleophilic capacity follows Cdouble bondS > Cdouble bondN > Cdouble bondO trend, which is main selective origin of these reactions. The presence of Rh2(Piv)4 help to form the carbene and to stabilize it, which is another selective origin.

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Introduction

The imidazolones and thiazoles are common building blocks of many bioactive molecules, such as anti-ulcer cimetidine [1], omeprazole [2], antiepileptics sodium phenytoin [3], beta-lactam antibiotics penicillins [4], antifungals drug clotrimazole [5] and antiparasitic drugs levamizole [6], and important intermediates of organic synthesis. Therefore, the constructions of imidazolones and thiazoles are very significant in organic and medicinal chemistry.

Over the past decades, various catalyzed methods involving transition metals have been developed to synthesize the imidazolone and thiazole derivatives. For instance, the imidazolone is constructed by RuCl2(PPh3)3-catalyzed cyclization between N, N′-disubstituted ureas and vicinal-diols with 73% yield [7]. p-toluenesulfonic acid(TosOH)-catalyzed cyclization of 1,3-dimethylurea with 3-hydroxybutanone provides imidazolone derivative with 38% yield [8], [9]. The imidazolone derivative is built by (η3-C3H5PdCl)2-catalyzed addition of aziridine with isocyanate in 98% yield [10]. 2-imidazolone derivatives can be achieved by silver triflate(AgOTf)-catalyzed cycloisomerization of isocyanates and propargylamines with 50–90% yield [11]. Tris(dibenzylideneacetone)dipalladium(Pd2dba3)-catalyzed cross-coupling of multicomponents generates the imidazolone derivatives with 80–90% yield [12]. Rh2(nC7H15COO)4(Rh2Oct4)-promoted N–H insertion of primary ureas and diazocarbonyls can provide imidazolone derivatives with 40–95% yield [13], [14], [15]. 5-functionalized thiazoles are synthesized by Rh2(NHCOC3F7)4-promoted addition of thiocarboxamides and carboxylates with 35–88% yield [16]. ZnBr2-catalyzed cycloaddition of aziridines and isothiocyanates affords iminothiazolidines with 70–90% yield [17]. However, these reactions usually require harsh conditions or have relatively low selectivities.

Recently, tBuCOORh2 (Rh2(Piv)4) is reported to catalyze cycloaddition of 1,2,3-triazoles with isocyanates to afford imidazolones rather than oxazolimines, while with isothiocyanates to completely provide thiazoles instead of thioimidazolones (Scheme 1) [18]. Noticeably, the difference of isocyanates and isothiocyanates only lies in one atom (oxygen or sulfur), and oxygen and sulfur are the elements in the same main group. Up to date few formation mechanism of imidazolone or thiazole are found in the literature [19], [20]. Therefore, it is very significant to obtain an insight into Rh2(Piv)4-catalyzed cycloaddition of 1,2,3-triazoles with isocyanates or isothiocyanates, and illuminate the substrate-controlled selective origin of these reactions from molecular level.

Section snippets

Computational details

All the calculations were performed using Gaussian 09 program [21]. The Kohn-Sham density functional theory was solved with B3LYP functional [22]. The LANL2DZ [23], [24] relativistic effective core potential (RECP) was used for Rh and S, and the LANL2DZ basis sets were modified, in which the secondary outer p functions of the standard LANL2DZ basis sets were replaced with optimized p functions, d-polarization functions (ζd = 0.503) were added to S, and f-polarization(ζf = 1.350) functions were

Results and discussion

Based on the experiment [18], this work is divided into the formation of Rh2(Piv)4-azavinyl carbene and the cyclization of Rh2(Piv)4-azavinyl carbene with isocyanates or isothiocyanates.

Conclusion

Rh2(Piv)4-catalyzed cycloadditions of 1,2,3-triazoles with isocyanates or isothiocyanates are systemically explored using DFT method. Several conclusions are drawn as follows.

1, 2, 3-triazoles preferentially undergo ring opening of 1, 2, 3-triazoles  coordination of Rh2(Piv)4  dissociation of N2 to form Rh2(Piv)4-azavinyl carbene(Pathway I), which is attributed to the stabilizing effect of Rh2(Piv)4 on the azavinyl carbene.

N6 and C7 atoms of isocyanatobenzene prefer to stepwise and

Acknowledgments

This work is supported by the National Natural Science Foundation of China (No. 21262004), and the Project of Guangxi Natural Science Foundation (No.2013GXNSFBA019152). The computational resources are partly provided by Multifunction Computer Center of Guangxi University.

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