Light-induced metal-free transformations of unactivated pyridotriazoles

A novel light-induced metal-free method for conversion of unactivated pyridotriazoles into pyridyl carbenes was developed, which allowed for incorporation of arylmethylpyridyl moiety into diverse molecules.

Abstract

A highly efficient and practical method for incorporation of the arylmethylpyridyl moiety into diverse molecules has been developed. This method features the transition metal-free light-induced room temperature transformation of pyridotriazoles into pyridyl carbenes, which are capable of smooth arylation, X–H insertion, and cyclopropanation reactions. The synthetic usefulness of the developed method was illustrated in a facile synthesis of biologically active molecules.

Transition metal-catalyzed denitrogenative transformations of pyridotriazoles have been recently evolving as a powerful tool for synthesis of diverse molecules possessing N-hetero-cyclic fragments.1,2 These protocols take advantage of the well-known ring-chain tautomerism of the pyridotriazole core in solution into the corresponding diazo tautomer A, which then can be trapped by a transition metal catalyst to form the reactive pyridyl metal carbene intermediate B (Scheme 1a). Since the first report on the transannulation reaction of pyridotriazoles in 2007,3 numerous effective catalytic methods including transannulation,4 X–H insertions,5 and cyclopropanation6 reactions have been developed.7 However, all these methods are not without shortcomings. The reactions proceeding at room temperature require Cl, Br, or OMe activating groups (AG) at C7,3a,b otherwise high temperatures3c–e are necessary for achieving sufficient amounts of the open form of pyridotriazole A, which would lead to the reaction products. In either case, the employment of transition metal catalysts is required. Herein, we report room temperature efficient and operationally simple light-induced metal-free arylation, X–H insertion, and cyclopropanation reactions of pyridotriazoles giving access to various pyridyl-containing synthons, which can be used for convenient synthesis of bioactive molecules.

Scheme 1. Thermal and light-induced generation of carbenes from pyridotriazoles.

In continuation of our studies on application of pyridotriazoles in the synthesis of nitrogen-containing heterocycles,3a–e we hypothesized a metal-free strategy which can be accessed by utilizing light irradiation.8 Compared with numerous methods reported for thermal processes, photochemical studies of pyridiotriazoles are relatively scarce.9 Aiming at the development of milder reaction conditions, we started our investigation by analysing the UV-vis absorption spectra of pyridotriazoles 1a–d (Fig. 1, see ESI† for more details). Among them, only pyridotriazoles 1a and 1b bearing aryl substituents at the C3 position showed appreciable absorption around 390 nm region, which is attributed to the extended conjugation in these systems. Thus, we hypothesized that upon irradiation, the excited pyridotriazole 1a could undergo a ring-chain tautomerism to deliver its diazo tautomer C (Scheme 1b), which upon denitrogenation10 would deliver reactive carbene species D.

Fig. 1. UV-vis absorption spectra of pyridotriazoles.

To test this hypothesis, we examined a metal-free arylation reaction of 3-phenyl[1,2,3]triazolo[1,5-a]pyridine 1a with boronic acid 2 (Table 1). The optimization studies indicated that performing this reaction under 390 nm irradiation in 0.1 M benzene solution with 1.5 equiv. of boronic acid in the presence of 3 equiv. K₂CO₃ at room temperature allows to produce diphenyl-2-pyridylmethane 3aa in 89% yield (entry 1). Employment of other additives (entries 2–4) or solvents (entries 5–7) led to diminished yields. Only 25% of the product was formed without K₂CO₃ (entry 8), which points on the importance of the base for formation of the reactive triphenylboroxine arylating reagent.7a The control experiments indicated no reaction under 455 nm or 427 nm LED irradiating (entries 9 and 10) or under thermal conditions in the absence of light (entry 11).11 Expectedly, the attempts on employment of pyridotriazoles 1c and 1d, which are transparent in 390 nm region under these reaction conditions failed. Although the pyridotriazole 1d showed notable absorption in 370 nm region, the attempts of its arylation under irradiation with 370 nm LED lamp failed probably due to competing side reactions of the formed unstable carbene.

Table 1. Optimization of arylation reaction parameters ^a .

Entry	Deviation from standard conditions	Yield b , %
1	None	89
2	Cs2CO3 instead of K2CO3	73
3	K3PO4 instead of K2CO3	44
4	NEt3 instead of K2CO3	40
5	PhMe instead of PhH	84
6	THF instead of PhH	0
7	CHCl3 instead of PhH	24
8	Without K2CO3	25
9	455 nm LED instead of 390 nm LED	0
10	427 nm LED instead of 390 nm LED	0
11	No light, dark, 50 to 120 °C	0

^aReaction conditions: pyridotriazole 1 (0.05 mmol), boronic acids 2 (1.5 equiv.), K₂CO₃ (3 equiv.), benzene (0.1 M), and a 40 W 390 nm LED at room temperature.

^bGC/MS yields.

Intrigued by the uncovered novel reactivity of pyridotriazoles under photo-induced conditions and inspired by the importance of triarylmethanes,12 we further explored the scope of this arylation reaction. Markedly, this C–C coupling reaction showed wide scope and high functional-group tolerance on both reaction partners (Table 2a). Thus, diversely functionalized boronic acids 2 bearing electron-rich (2b–2d), electron-deficient (2h–2j), electron-neutral (2n), halogen-containing (2e–2g and 2k–2m), and sterically encumbered (2o) substituents at the ortho-, meta- and para-positions smoothly reacted with 3-phenyl[1,2,3]triazolo[1,5-a]pyridine 1a to produce diphenyl-2-pyridylmethanes 3aa–3an in good to excellent yields. In addition, the reaction of phenylboronic acid with the para-siloxy substituent worked well to give 3ab in good yield, which upon desilylation offered access to the phenol product. Notably, the reaction also efficiently proceeded with the alkenyl boronic acid 2p providing cyclohexen-1-yl product 3ap in 70% yield. Studies on the scope of pyridotriazoles showed that pyridotriazoles 1 bearing different 4-substituted phenyl or heteroaryl groups at C3 position furnished the corresponding triarylmethane products 3ea–3ia in moderate to high yields. In addition, 5-chloro pyridotriazole, 6-bromo pyridotriazole and N-fused heterocyclic pyrazinotriazole successfully underwent arylation with different arylboronic acids to give 3ja, 3ka, 2-(diphenylmethyl) pyrazine (3la, 3le and 3lf) in good yields.

Table 2. Arylation reactions of pyridotriazoles ^{a , b} .

^aReaction conditions: pyridotriazole 1 (0.2 mmol), boronic acids 2 (1.5 equiv.), K₂CO₃ (3 equiv.), benzene (0.1 M), 40 W 390 nm LED at room temperature.

^bYield of isolated product.

^cReaction was performed in 1 mmol scale.

^dToluene (0.1 M) used as a solvent.

Next, we turned our attention to carbene X–H insertion reactions10a (Table 3). To this end, pyridotriazoles 1, under standard reaction conditions, were examined in reactions with phenols, alcohols, sulfonamides, and carboxylic acids (Table 3a). All substituted phenols tested provided the O–H insertion products 7aa–7af in moderate yields, albeit with trace to substantial amounts of the C–H insertion regioisomers.13 These reaction conditions appeared to be very general for reactions with alcohols 4g–4n. Thus, alcohols possessing various alkyl, alkenyl, thiomethyl, and even sterically hindered bicyclo groups all reacted well, providing ethers 7ag–7fm in good to high yields. Moreover, this reaction chemoselectively gave O–H insertion products with alkenols (7al and 7am), double bond moiety of which was not compromised. Likewise, aliphatic chlorine substituent was tolerated in 4n, which constitutes an additional handle for further derivatizations. In contrast to a facile O–H insertion reaction with phenols, the insertion into the N–H bond of aniline 5a was sluggish (Table 3b), which can probably be attributed to its higher pK_a value.5d However, more acidic primary and secondary sulfonamides 5b–g reacted smoothly to produce the N–H insertions products 8ab–8ag in reasonable to high yields. Phthalimide 5h provided the insertion product 8ah in reasonable yield. Furthermore, both aliphatic and aromatic carboxylic acids 6a–6c were also found to be the competent substrates for the COO–H insertion reactions producing the corresponding esters 9aa–9cc in good yields (Table 3c).

Table 3. X–H insertion reactions ^{a b} .

^aReaction conditions: pyridotriazole 1 (0.2 mmol), X–H insertion partners 4, 5 or 6 (4 equiv.), benzene (0.1 M), 40 W 390 nm LED at room temperature.

^bYield of isolated product.

^cdr 1 : 1.

In addition, it was also found that upon photoirradiation, the pyridotriazoles underwent efficient cyclopropanation with alkenes 10 (Table 4). The scope of pyridotriazoles was studied first. Pyridotriazoles 1 bearing different 4-substituted phenyl or heteroaryl groups at C3 position gave the corresponding cyclopropanes 11aa–11ia in moderate to high yields.14 3-Naphthalenyl pyridotriazole furnished product 11na in high yield. Furthermore, N-fused heterocyclic quinolinotriazole and 7-chloro pyridotriazole successfully underwent cyclopropanation with styrene to give 11ma and 11oa. The scope of the process with respect to the alkene components was examined next. Cyclopropanes 11ab–11ae were obtained from a diverse array of ortho-, meta-, para- and disubstituted styrenes in high yields. 2-Vinylpyridine, vinyl ether, acrylonitrile, vinyl ketone and ethyl acrylate efficiently participated in the reaction to give the products 11ag–11ak. Notably, a double bond of indole also participated in this reaction, providing fused product 11al in moderate yield.

Table 4. Cyclopropanation reactions ^a .

^aReaction conditions: pyridotriazole 1 (0.2 mmol), styrene 10 (3 equiv.), benzene (0.1 M), 40 W 390 nm LED at room temperature.

Synthetic usefulness of this methodology was illustrated on the facile syntheses of selected bioactive molecules. Thus, desacetyl bisacodyl (DAB) 1215 and bisacodyl 13,16 which are used as stimulant laxative drugs, were efficiently obtained via the one-pot procedures from the pyridotriazole 1b with arylboronic acid 2b (Scheme 2a). Furthermore, piperidine derivative 14,17 the synthetic precursor of antihistamine agent Bepotastine, was easily accessed by the O–H insertion reaction of 7n with pyridotriazole 1f (Scheme 2b).

Scheme 2. Synthesis of biologically active molecules. (a) Conditions A: HBr/AcOH = 1 : 1, reflux, overnight. (b) Conditions B: HBr/AcOH = 1 : 1, reflux, overnight. Then Et₃N (3 equiv.), Ac₂O (4 equiv.), DCM (0.1 M), rt, overnight.

Conclusions

We developed general and efficient arylation, X–H insertions, and cyclopropanation reactions of pyridotriazoles. This transition metal-free light-induced18 protocol, operating under mild conditions, exhibits wide functional-group tolerance efficiently producing valuable triarylmethanes and heteroatom-substituted benzylpyridine derivatives.

Conflicts of interest

There are no conflicts to declare.