Oxidative [3+2]Cycloaddition of Alkynylphosphonates with Heterocyclic N-Imines: Synthesis of Pyrazolo[1,5-a]Pyridine-3-phosphonates

A series of pyrazolo[1,5-a]pyridine-3-ylphosphonates were prepared with moderate to good yields by the oxidative [3+2]cycloaddition of 2-subtituted ethynylphosphonates with in situ generated pyridinium-N-imines and their annulated analogs. 2-Aliphatic and 2-Ph acetylenes demonstrate low activity, and the corresponding pyrazolopyridines were achieved with a moderate yield in the presence of 10 mol% Fe(NO3)3·9H2O. At the same time, tetraethyl ethynylbisphosphonate, diethyl 2-TMS- and 2-OPh-ethynylphosphonates possess much greater reactivity and the corresponding pyrazolo[1,5-a]pyridines, and their annulated derivatives were obtained with good to excellent yields without any catalyst. 2-Halogenated ethynylphosphonates also readily reacted with pyridinium-N-imines, forming complex mixtures containing poor amounts of 2-halogenated pyrazolopyridines.


Results
At the beginning of our work, N-aminopyridinium tetrafluoroborate 1 and diethyl phenylethynylphosphonate 2a were taken as model substrates for screening optimal conditions. The reaction was followed using 31 P NMR to estimate the phosphonate conversion and yield of products. The commonly used K 2 CO 3 /MeCN system (Table 1, entry 1) failed to reach the full conversion of 2a, and only a 5-fold excess of salt 1 allowed for achieving the complete consumption of reagent 2a. In the next step, various additives were tried to increase the reactivity of 2a. Initially, we hypothesized that the electron-withdrawing character of the PO(OEt) 2 -group could be increased by the coordination of metal ions to the phosphonate group oxygen atom. However, strong Lewis acids such as AlCl 3 or ZnCl 2 preferably interacted with N-imines and, therefore, were not suitable for catalysis. Then, AgNO 3 was used, as it was effective in the catalytic hydration of alkynylphosphonates [44]. However, no effect was found with 10 mol% AgNO 3 (Table 1, entry 3). Next, we paid our attention to LiCl, which is known to catalyze cycloaddition reactions. The usage of 10 mol% led to notable conversion growth (Table 1, entry 4), but the increased load of LiCl resulted in lowering the conversion (Table 1, entry 5). The addition of Ni and Co salts had no significant effect on the reaction. During further experiments, redox-active additives were employed to accelerate the oxidation step of pyrazolo[1,5-a]pyridine synthesis (see discussion on the mechanism below). The application of chloranil or DDQ was unsuccessful, apparently due to the oxidation reaction with N-imines. Copper salts were previously applied for nitropyrazolo [1,5-a]pyridine synthesis from pyridinium-N-imines and nitrostyrenes [20]. However, in our case, Cu salts completely inhibited the reaction (Table 1, entries 9, 10). Fe(NO 3 ) 3 is broadly used as a catalyst in a wide scope of oxidation reactions. Recently, it has been found to be effective in the promotion of 3-acyl-1,2,4oxadiazole synthesis from alkynes and nitriles [45]. Moreover, iron (III) nitrate mediated synthesis of acylisoxazoles from terminal alkynes was reported [46]. Therefore, we decided to try Fe(NO 3 ) 3 in nonahydrate form in our reaction. With an additive level of 10 mol% in CH 3 CN, we observed an 88% conversion of 2a (  12,13). No yield changes were observed with an increase in the additive level from 10 to 20 mol%; moreover, 5 mol% Fe(NO 3 ) 3 did not lead to the full conversion of phosphonate. Heating had a negative effect on the reaction (Table 1, entry 16). Therefore, the use of Fe(NO 3 ) 3 as an additive and DMSO as a solvent were found to be the optimal conditions. and yield of products. The commonly used K2CO3/MeCN system (Table 1, entry 1) failed to reach the full conversion of 2a, and only a 5-fold excess of salt 1 allowed for achieving the complete consumption of reagent 2a. In the next step, various additives were tried to increase the reactivity of 2a. Initially, we hypothesized that the electron-withdrawing character of the PO(OEt)2-group could be increased by the coordination of metal ions to the phosphonate group oxygen atom. However, strong Lewis acids such as AlCl3 or ZnCl2 preferably interacted with N-imines and, therefore, were not suitable for catalysis. Then, AgNO3 was used, as it was effective in the catalytic hydration of alkynylphosphonates [44]. However, no effect was found with 10 mol% AgNO3 (Table 1, entry 3). Next, we paid our attention to LiCl, which is known to catalyze cycloaddition reactions. The usage of 10 mol% led to notable conversion growth (Table 1, entry 4), but the increased load of LiCl resulted in lowering the conversion (Table 1, entry 5). The addition of Ni and Co salts had no significant effect on the reaction. During further experiments, redox-active additives were employed to accelerate the oxidation step of pyrazolo[1,5-a]pyridine synthesis (see discussion on the mechanism below). The application of chloranil or DDQ was unsuccessful, apparently due to the oxidation reaction with N-imines. Copper salts were previously applied for nitropyrazolo [1,5-a]pyridine synthesis from pyridinium-N-imines and nitrostyrenes [20]. However, in our case, Cu salts completely inhibited the reaction (Table 1, entries 9, 10). Fe(NO3)3 is broadly used as a catalyst in a wide scope of oxidation reactions. Recently, it has been found to be effective in the promotion of 3-acyl-1,2,4-oxadiazole synthesis from alkynes and nitriles [45]. Moreover, iron (III) nitrate mediated synthesis of acylisoxazoles from terminal alkynes was reported [46]. Therefore, we decided to try Fe(NO3)3 in nonahydrate form in our reaction. With an additive level of 10 mol% in CH3CN, we observed an 88% conversion of 2a (Table 1, entry 11). Interestingly, other iron salts, such as FeCl3 and FeSO4, were not as effective as Fe(NO3)3. Further experiments revealed that changing the solvent from CH3CN to the more polar DMSO led to the full conversion of 2a (Table 1, entries 12,13). No yield changes were observed with an increase in the additive level from 10 to 20 mol%; moreover, 5 mol% Fe(NO3)3 did not lead to the full conversion of phosphonate. Heating had a negative effect on the reaction (Table 1, entry 16). Therefore, the use of Fe(NO3)3 as an additive and DMSO as a solvent were found to be the optimal conditions. With the optimized conditions in hand, the scope of pyridinium salts and alkynylphosphonates was explored (Scheme 2). First, we examined the effect of the substituents in the pyridinium ring of N-aminopyridinium salts on the reaction with phosphonate 2a.
The cycloaddition proceeded sluggishly in case of mild donating and moderate withdrawing substituents and required increased equivalents of starting salts. 2-Me-, 4-Me-, and 4-CO 2 Me-substituted N-aminopyridinium salts showed moderate to good yields of corresponding pyrazolopyridines 3b-e. Despite the bulkiness of phenyl group, a good yield of 7-Ph-substituted pyrazolopyridine 3d was obtained with only 1 eq of salt. Regioselectivity of the cycloaddition of 3d was confirmed using X-ray analysis [47]. Strong donating groups are not favorable and significantly reduce reactivity. Thus, pyrazolopyridine 3f was obtained only in 33% yield with 5 equivalents of 4-OMe-substituted salt. At the same time, 1-amino-4-NMe 2 -pyridinium mesitylenesulfonate was completely unreactive under used conditions. Such behavior was likely associated with the low acidity of the NH 2 -group, which could not be deprotonated by K 2 CO 3 . The use of DBU or t-BuOK as stronger bases also showed no positive results, apparently due to their reaction with alkyne. Quinolinium and isoquinolinium salts facilitated the dimerization of the corresponding N-imines [48,49] and no cycloaddition products were formed. Subsequently, we investigated the impact of the R-group in 2-R-alkynylphosphonate on the reaction with salt 1. Pyrazolopyridines 3h-j were obtained in moderate yields (30-40%), indicating that both alkyl and cycloalkyl groups led to the decrease in the corresponding phosphonate reactivity. Moreover, the phosphonate bearing the bulky t-Bu-group required a large excess of starting salts to obtain the desired product. α-Hydroxyalkyl substituted acetylenes were also moderately active and formed products 3k,l.
Having succeeded in the Fe(NO 3 ) 3 -catalyzed alkynylphosphonates cycloaddition, bisphosphonylated acetylene 4a was further studied. In contrast to phosphonate 2a, acetylene 4a showed much greater reactivity and showed pyrazolo[1,5-a]pyridine-2,3bisphosphonates 5a-j with good to excellent yields without an Fe(NO 3 ) 3 additive (Scheme 3). However, pyridinium salts with strong donating groups were still much less reactive. Thus, 4-NMe 2 -substituted pyridinium salts did not undergo cycloaddition, and in the case of 4-OMe derivatives the corresponding pyrazolopyridine 5e was obtained with a moderate yield in the mixture with the parent 4-OMe-pyridine. Enhanced activity of alkyne 4a made it possible to obtain annulated derivatives 5g-j with relatively high yields.
TMS-substituted ethynylphosphonate 4b also possessed high reactivity toward pyridinium salts (Scheme 3). The loss of the TMS group during the process was observed and corresponding pyrazolopyridines 5k-o were obtained. Annulated derivatives 5p,q were prepared as well, but in the case of quinolinium and isoquinolinium salts only dimers of corresponding N-imines were observed. The higher activity of TMS-phosphonate 4b in contrast to 2-alkyl-and 2-phenylalkynylphosphonates could be explained by the fast removal of the TMS group of 4b in the K 2 CO 3 /DMSO medium with the formation of intermediate diethyl ethynylphosphonate.
Next, the applicability of 2-halogenated and 2-OPh alkynylphosphonates 4c-f in the cycloaddition reaction with pyridinium-N-imines was studied (Scheme 4). For all halogenated alkynes, the reaction resulted in the formation of complex reaction mixtures from which corresponding 2-Cl and 2-I-pyrazolopyridines 5r,t were isolated with a low yield. Investigation of the reaction mixtures using NMR 31 P and GC-MS for alkyne 4c showed the formation of a significant amount of dehalogenated product 5k when DMSO was used as a solvent. Various solvents were screened and the maximum yield of 5r was reached in MeCN with K 2 CO 3 as a base, while the nature of the inorganic base had no remarkable effect on the ratio of 5r:5k. When the excess of salt 1 was taken, formation of 2aminopyrazolo[1,5-a]pyridine along with products 5k and 5r was also observed. In contrast to halogenated acetylenes 4c-e, the cycloaddition reaction of OPh-substituted phosphonate 4f in THF resulted in a good yield of pyrazolopyridine 5u. The regioselectivity for product 5u was confirmed using X-ray analysis [50].
Having succeeded in the Fe(NO3)3-catalyzed alkynylphosphonates cycloaddition, bisphosphonylated acetylene 4a was further studied. In contrast to phosphonate 2a, acetylene 4a showed much greater reactivity and showed pyrazolo[1,5-a]pyridine-2,3bisphosphonates 5a-j with good to excellent yields without an Fe(NO3)3 additive (Scheme 3). However, pyridinium salts with strong donating groups were still much less reactive. Thus, 4-NMe2-substituted pyridinium salts did not undergo cycloaddition, and in the case of 4-OMe derivatives the corresponding pyrazolopyridine 5e was obtained with a moderate yield in the mixture with the parent 4-OMe-pyridine. Enhanced activity of alkyne 4a made it possible to obtain annulated derivatives 5g-j with relatively high yields.
TMS-substituted ethynylphosphonate 4b also possessed high reactivity toward pyridinium salts (Scheme 3). The loss of the TMS group during the process was observed and corresponding pyrazolopyridines 5k-o were obtained. Annulated derivatives 5p,q were prepared as well, but in the case of quinolinium and isoquinolinium salts only dimers of corresponding N-imines were observed. The higher activity of TMS-phosphonate 4b in contrast to 2-alkyl-and 2-phenylalkynylphosphonates could be explained by the fast removal of the TMS group of 4b in the K2CO3/DMSO medium with the formation of intermediate diethyl ethynylphosphonate. Scheme 3. Reaction scopes for cycloaddition of N-imines with tetraethyl acetylene bisphosphonate (isolated yields). a The substance was obtained in mixture with 4-metoxypyridine and other unidentified products, yield determined using NMR with CH2Br2 as a standard.
Next, the applicability of 2-halogenated and 2-OPh alkynylphosphonates 4c-f in the cycloaddition reaction with pyridinium-N-imines was studied (Scheme 4). For all halogenated alkynes, the reaction resulted in the formation of complex reaction mixtures from which corresponding 2-Cl and 2-I-pyrazolopyridines 5r,t were isolated with a low yield. Investigation of the reaction mixtures using NMR 31 P and GC-MS for alkyne 4c showed the formation of a significant amount of dehalogenated product 5k when DMSO was used as a solvent. Various solvents were screened and the maximum yield of 5r was reached in Scheme 3. Reaction scopes for cycloaddition of N-imines with tetraethyl acetylene bisphosphonate (isolated yields). a The substance was obtained in mixture with 4-metoxypyridine and other unidentified products, yield determined using NMR with CH 2 Br 2 as a standard.
In terms of previous reports on N-imine cycloadditions [51], a plausible mechanism for the interaction of pyridinium salts with studied alkynes is shown in Scheme 5a. Ylide 6, formed by the deprotonation of salt 1a, reacts with alkynylphosphonates via concerted [3+2]cycloaddition or via Michael addition/intramolecular cyclization, yielding adduct 7. Such intermediates are typically not observable due to the fast rearrangement of dihydropyrazolopyridines 8, which lead to products 3a-l, 5a-q during oxidation. To shed some light on the role of iron (III) nitrate, we provided the reaction between 1a and 2a under an Ar atmosphere (Table 1, entries 19,20). In the case of 10 mol% additive loading, only a poor yield of 3a was observed, comparable to the value of the additive load. However, when 1 eq of Fe(NO 3 ) 3 was taken, a full conversion of 2a was accomplished with a 70% isolated yield of 3a. Additionally, Fe(NO 3 ) 3 could affect the reactivity by coordination of Fe 3+ -ion to the phosphonate group or by nitration of the triple bond (Scheme 5b). Nevertheless, no changes using 31 P NMR were found after the addition of iron nitrate to phosphonate 2a in the DMSO even after heating up to 100 • C. Therefore, it could be concluded that iron nitrate serves as a redox mediator in the oxidation of intermediate 8.
Taking into account the low reactivity of 2-alkyl and 2-Ph-acetylenephosphonates, the cycloaddition step in this case is slow and probably reversible, so large amounts of Nimines are lost to degradation. Therefore, additional amounts of N-imines are required. For acetylenes 4a-f, cycloaddition is fast and the oxidation of intermediate 8 is a limiting step. MeCN with K2CO3 as a base, while the nature of the inorganic base had no remarkable effect on the ratio of 5r:5k. When the excess of salt 1 was taken, formation of 2-aminopyrazolo[1,5-a]pyridine along with products 5k and 5r was also observed. In contrast to halogenated acetylenes 4c-e, the cycloaddition reaction of OPh-substituted phosphonate 4f in THF resulted in a good yield of pyrazolopyridine 5u. The regioselectivity for product 5u was confirmed using X-ray analysis [50]. In terms of previous reports on N-imine cycloadditions [51], a plausible mechanism for the interaction of pyridinium salts with studied alkynes is shown in Scheme 5a. Ylide 6, formed by the deprotonation of salt 1a, reacts with alkynylphosphonates via concerted [3+2]cycloaddition or via Michael addition/intramolecular cyclization, yielding adduct 7. Such intermediates are typically not observable due to the fast rearrangement of dihydropyrazolopyridines 8, which lead to products 3a-l, 5a-q during oxidation. To shed some light on the role of iron (III) nitrate, we provided the reaction between 1a and 2a under an Ar atmosphere (Table 1, entries 19,20). In the case of 10 mol% additive loading, only a poor yield of 3a was observed, comparable to the value of the additive load. However, when 1 eq of Fe(NO3)3 was taken, a full conversion of 2a was accomplished with a 70% isolated yield of 3a. Additionally, Fe(NO3)3 could affect the reactivity by coordination of Fe 3+ -ion to the phosphonate group or by nitration of the triple bond (Scheme 5b). Nevertheless, no changes using 31 P NMR were found after the addition of iron nitrate to phosphonate 2a in the DMSO even after heating up to 100 °C. Therefore, it could be concluded that iron nitrate serves as a redox mediator in the oxidation of intermediate 8.
Taking into account the low reactivity of 2-alkyl and 2-Ph-acetylenephosphonates, the cycloaddition step in this case is slow and probably reversible, so large amounts of Nimines are lost to degradation. Therefore, additional amounts of N-imines are required. For acetylenes 4a-f, cycloaddition is fast and the oxidation of intermediate 8 is a limiting step.
The TLC was carried out on Sorbfil silica plates (UV 254) with further UV light visualization. Flash column chromatography was performed on silica gel (Macherey Nagel, pore size 60 E, 230-400 mesh). Spectral and analytical studies were provided at the Chemical Service Centre of Siberian Branch of the Russian Academy of Sciences. NMR spectra were recorded on Bruker Avance-300 (300.13 MHz for 1H, 121.5 MHz for 31P) and Avance-400 (400.13 MHz for 1H and 100.62 MHz for 13C) spectrometers, using the residual proton and carbon signals of CDCl3 (δH 7.24 ppm; δC 77.16 ppm) as internal standards. The 13C NMR spectra were registered with C-H spin decoupling. Copies of spectra of newly obtained compound are presented in the Supplementary Materials. The masses of molecular ions were determined by HRMS on a DFS Thermo scientific instrument (EI, 70 eV). Melting points were determined using Kofler hot-stage microscope and are uncorrected.
XRD data were obtained on a Bruker Kappa Apex II CCD diffractometer (Mo Kα radiation and a graphite monochromator) at 296K. The structures were solved by direct methods and refined by full-matrix least-squares method against all F2 in anisotropic approximation using the SHELX2014 programs set [59]. The H atoms positions were calculated geometrically and refined with the riding model. Absorption corrections were applied empirically using SADABS programs [60]. The solvent molecule is highly disordered and, therefore, this molecule was removed using the SQUEEZE procedure in the PLATON program [61]. Solvent accessible volume was calculated as 138 Å 3 in unit cell.

General procedure for diethyl 2-phenylpyrazolo[1,5-a]pyridine-3-phosphonates synthesis.
N-aminopyridinium salt (N mmol) was dissolved in 5 mL DMSO and K 2 CO 3 (2*N mmol) was added to produce pyridinium-N-imine. The resulting mixture was stirred for 5 min and alkynylphosphonate (1 mmol) was added followed with Fe(NO 3 ) 3 *9H 2 O (10 mol%). The solution was kept with stirring overnight at room temperature under air atmosphere. Then, the mixture was diluted in 50 mL water and extracted three times with 20 mL of CH 2 Cl 2 . Extracts were combined, washed with water and dried over Na 2 SO 4 . Then, solvent was removed in vacuo and the resulting oil was purified by flash chromatography on silica with CHCl 3 /MeOH (50:1) as the eluent.
Diethyl ( General experimental procedure for synthesis of pyrazolopyridines 5a-r, 5t,u. Naminopyridinium salt (1 mmol) was dissolved in 5 mL DMSO and K 2 CO 3 (5 mmol) was added to produce pyridinium-N-imine. The resulting mixture was stirred for 5 min and alkynylphosphonate (1 mmol) was added. The solution was kept with stirring overnight at RT under air atmosphere. Then, the mixture was diluted in water and extracted three times with 20 mL of CH 2 Cl 2 . The extracts were combined and washed with water and dried over Na 2 SO 4 . Then, the solvent was removed in vacuo and the resulting oil was purified by flash chromatography on silica with CHCl 3 /MeOH (50:1). Tetraethyl

Conflicts of Interest:
The authors declare no conflict of interest.
Sample Availability: Samples of the compounds are available from the authors.