Diethyl (2-(4-Phenyl-1H-1,2,3-triazol-1-yl)benzyl) Phosphate

Here we describe a full structural elucidation of the diethyl (2-(4-phenyl-1H-1,2,3-triazol-1-yl)benzyl) phosphate. This compound is a common by-product present in the synthetic protocols to access the α-hydroxy phosphonate compounds through of a Phospha-Brook rearrangement. Thus, a complete NMR structural characterization of this rearrangement by-product was performed by 1H, 13C{1H}, 31P{1H}, COSY, HSQC, and HMBC NMR experiments. Additionally, we have demonstrated that the 1H-31P HMBC is a 2D heteroatom NMR experiment which combines the simple identification by 31P chemical shift with the detection sensitivity by 1H spectrum in a practical procedure.


Introduction
Organophosphonate compounds present a singular application in medicinal chemistry [1][2][3][4], especially in the agrochemical field [5][6][7]. Based on the Abramov and Pudovik reactions, the synthetic protocols to prepare these compounds usually employ hydrophosphoryl reagents as starting materials, which are simple and practical, with an ample substrate scope [8][9][10]. Additionally, hydrophosphoryl reagents are easily prepared by industrial sector through the oxidative coupling of hypophosphorous acids with alcohols, affording a cheaper and readily available scope of starting materials [11][12][13].
Based on the peculiar reactivity of hydrophosphoryl compounds [14,15], bioactive targets are constantly produced [16,17], mainly those containing nitrogen heterocycles [18][19][20][21]. Routinely, no full assignments are present, resulting in the absence of the precise structural data, that could be useful information for the identification of products and by-products, as well as for the recognition of biological processes [22,23].
A routine inconvenience in the synthesis of α-hydroxyphosphonate compounds is the occurrence of the Phospha-Brook rearrangement, which produces a by-product that demonstrates a similar backbone structure (Scheme 1), hampering the identification and quantification of this by-product. In this sense, a full comprehension of this by-product is important to evaluate the presence of impurities in the synthesis of the α-hydroxyphosphonates.
correlation experiment was carried out for the Phospha-Brook rearrangement to demonstrates the simplicity and additional information of this alternative 2D NMR protocol. The spectral data provided herein can be used for the identification of organophosphonate compounds, as well as pharmacological 1,2,3-triazoles [30] from copper catalyzed azidealkyne cycloaddition (CuAAC) reactions [29]. Scheme 1. Phospha-Brook rearrangement in the synthesis of the functionalized triazole 4.

Results
The α-hydroxyphosphonate compounds were obtained previously by us, through the reaction between triazole 1 and diethyl phosphite 2 in a ratio of 1:3 in the presence of Na2CO3 (10 mol%) as catalyst at 70 °C for 3.0 h under conventional heating. Using these free-solvent conditions, the α-hydroxyphosphonate containing o-functionalized 1,2,3-triazole 3 was isolated in a high yield (93%) by column chromatography (see the supporting information to access also the full assignment of product 3 for a proper data comparison), while compound 4 was obtained in 90% yield by Phospha-Brook rearrangement of product 3, after treatment with KOH (10 mol%) in DMSO at 70 °C for 3.0 h. Posteriorly, the complete NMR structural characterization of this common by-product 4 was performed by 1 H, 13 C{ 1 H}, 31 P{ 1 H}, COSY, HSQC, and HMBC NMR experiments. The sample was prepared employing 5 mg of the respective product 4 in 600 µL deuterated chloroform. The 90° pulse width was calibrated, and the resolution used in the 2D experiments was 4 K/512 (t2 × t1) data points. For the 1 H-31 P HMBC experiment, the same pulse sequence of carbon-13 was used, replacing the parameters relating to the specific nuclide.

Discussion
Initially, the α-hydroxyphosphonate containing functionalized 1,2,3-triazoles 3 was prepared (Scheme 1). In this moment, the main by-product was not detected by thin-layerchromatography, but the 31 P{ 1 H} spectrum of the crude reaction showed several small peaks. Thus, product 3 was purified and submitted to a strong base (KOH) in a DMSO solvent to favor the formation of the Phospha-Brook rearrangement (Scheme 1). Under this condition, the main by-product 4 was isolated by chromatographic column and obtained in a high yield (90%). The plausible mechanism for this reaction (Scheme 2) starts with the deprotonation of the α-hydroxyl group of compound 3 by the base A, forming the oxyanion compound B. This oxyanion of intermediate B performs a nucleophilic attack on the electrophilic phosphorus center, giving the intermediate C. After the occurrence of P-C bond cleavage, the intermediate D, a short-lived carbanion (carbanion stabilizing by the phenyl group), was generated. Finally, after protonation of the intermediate D, the desired Phospha-Brook rearrangement product 4 was formed, and the base A was regenerated for the catalytic cycle [31,32].

Results
The α-hydroxyphosphonate compounds were obtained previously by us, through the reaction between triazole 1 and diethyl phosphite 2 in a ratio of 1:3 in the presence of Na 2 CO 3 (10 mol%) as catalyst at 70 • C for 3.0 h under conventional heating. Using these free-solvent conditions, the α-hydroxyphosphonate containing o-functionalized 1,2,3triazole 3 was isolated in a high yield (93%) by column chromatography (see the Supporting Information to access also the full assignment of product 3 for a proper data comparison), while compound 4 was obtained in 90% yield by Phospha-Brook rearrangement of product 3, after treatment with KOH (10 mol%) in DMSO at 70 • C for 3.0 h. Posteriorly, the complete NMR structural characterization of this common by-product 4 was performed by 1 H, 13 C{ 1 H}, 31 P{ 1 H}, COSY, HSQC, and HMBC NMR experiments. The sample was prepared employing 5 mg of the respective product 4 in 600 µL deuterated chloroform. The 90 • pulse width was calibrated, and the resolution used in the 2D experiments was 4 K/512 (t2 × t1) data points. For the 1 H-31 P HMBC experiment, the same pulse sequence of carbon-13 was used, replacing the parameters relating to the specific nuclide.

Discussion
Initially, the α-hydroxyphosphonate containing functionalized 1,2,3-triazoles 3 was prepared (Scheme 1). In this moment, the main by-product was not detected by thin-layerchromatography, but the 31 P{ 1 H} spectrum of the crude reaction showed several small peaks. Thus, product 3 was purified and submitted to a strong base (KOH) in a DMSO solvent to favor the formation of the Phospha-Brook rearrangement (Scheme 1). Under this condition, the main by-product 4 was isolated by chromatographic column and obtained in a high yield (90%). The plausible mechanism for this reaction (Scheme 2) starts with the deprotonation of the α-hydroxyl group of compound 3 by the base A, forming the oxyanion compound B. This oxyanion of intermediate B performs a nucleophilic attack on the electrophilic phosphorus center, giving the intermediate C. After the occurrence of P-C bond cleavage, the intermediate D, a short-lived carbanion (carbanion stabilizing by the phenyl group), was generated. Finally, after protonation of the intermediate D, the desired Phospha-Brook rearrangement product 4 was formed, and the base A was regenerated for the catalytic cycle [31,32].
Next, we performed the complete structural characterization of the Phospha-Brook product ( Table 1, compound 4) through the 1 H, 13 C{ 1 H}, 31 P{ 1 H}, COSY, HSQC, and HMBC NMR experiments. The structural profile can be a useful data for comparison with αhydroxyphosphonate [24], especially to evaluate its influence in other organic functions, as the 1,2,3-triazole heterocycle. Firstly, the 1 H spectrum demonstrates a clear signal profile, which can easily identify the aliphatic protons and the aromatic protons. In a downfield 1 H chemical shift, it is possible to detect the triazole proton in a singlet multiplicity. The aromatic ring which connects the phosphonate group with the 1,2,3-triazole ring can be identified by the standard multiplicity, but an overlapping of signals can occur due to the same backbone profile of α-hydroxyphosphonate (see the NMR spectra in Supplementary). Next, we performed the complete structural characterization of the Phospha-Brook product (Table 1, compound 4) through the 1 H, 13 C{ 1 H}, 31 P{ 1 H}, COSY, HSQC, and HMBC NMR experiments. The structural profile can be a useful data for comparison with α-hydroxyphosphonate [24], especially to evaluate its influence in other organic functions, as the 1,2,3-triazole heterocycle. Firstly, the 1 H spectrum demonstrates a clear signal profile, which can easily identify the aliphatic protons and the aromatic protons. In a downfield 1 H chemical shift, it is possible to detect the triazole proton in a singlet multiplicity. The aromatic ring which connects the phosphonate group with the 1,2,3-triazole ring can be identified by the standard multiplicity, but an overlapping of signals can occur due to the same backbone profile of α-hydroxyphosphonate (see the NMR spectra in Supplementary).
According to the 13 C{ 1 H} spectrum, eighteen peaks can be visualized, because of the presence of carbon-phosphorus scalar couplings. All aliphatic carbons demonstrate coupling constants with phosphorus nuclide due to the proximity. When the aromatic carbons are evaluated, only one 13 C-31 P coupling is perceived in carbon-2 (Table 1, entry 2). This is an important profile variation in the carbons in the aromatic ring when compared with the structural elucidation of the of α-hydroxyphosphonate 3 (see support information), which. Due to the shorter distance between the phosphonate group and the aromatic ring in compound 3, there are various 13 C-31 P scalar couplings.

Scheme 2.
Plausible mechanism for Phospha-Brook rearrangement.  According to the 13 C{ 1 H} spectrum, eighteen peaks can be visualized, because of the presence of carbon-phosphorus scalar couplings. All aliphatic carbons demonstrate coupling constants with phosphorus nuclide due to the proximity. When the aromatic carbons are evaluated, only one 13 C-31 P coupling is perceived in carbon-2 (Table 1, entry 2). This is an important profile variation in the carbons in the aromatic ring when compared with the structural elucidation of the of α-hydroxyphosphonate 3 (see Support Information), which. Due to the shorter distance between the phosphonate group and the aromatic ring in compound 3, there are various 13 C-31 P scalar couplings.
Although, the NMR profile can provide distinct data, a full structural elucidation is necessary to identify all carbons signals, and for this purpose 2D NMR experiments were carried out. As can be seen in Table 1, proton-carbon correlations are a valuable information to recognize all carbons. The stronger 3 J 1H-13C scalar coupling can be used to identify not only the quaternary carbons C-2 and C-7, but also to confirm the primary carbons C-3, C-4, C5, and C-6. The C-9 can be easily detected by the correlation with the proton singlet derived from 1,2,3-triazole moiety (Table 1, entry 8). It is important to note that the quaternary carbon C10 of the phenyl bound in the triazole ring overlaps on C4 (Table 1, entry 4 and 10). At the end, the aliphatic carbons' chemical shifts are easily identified, but the presence of similar carbons in the α-hydroxyphosphonate 3 emphasizes that they are not good options to differentiate between product and by-product.
An interesting alternative to check the structure of these phosphonate compounds is through the 31 P{ 1 H} spectrum. The phosphorus-31 chemical shift of the Phospha-Brook rearrangement (Supplementary, Figure S9: −6.95 ppm) demonstrates a significant change derived from the stronger diamagnetic anisotropy caused by the new bond formed between oxygen and phosphorus, even though misinterpretations could occur in the proton spectra because of the same backbone. In this sense, a simple and practical 1 H-31 P HMBC experiment was performed to highlight the advantages of this 2D long-range experiment (Figure 1), which shows these correlations are useful for the discrimination of signals around to the phosphorus element, providing also the 31 P chemical shift. To carry out the 1 H-31 P HMBC experiment, the same 1 H-13 C HMBC parameters should be optimized.

Materials and Methods
The nuclear magnetic resonance (NMR) data were collected on a Bruker Avance III HD spectrometer operating at 400.0 MHz for 1 H, 100 MHz for 13 C, and 161 MHz for 31 P. The concentration of all samples was approximately 20 mg/0.7 mL of CDCl3. NMR data were recorded at 25 °C, with chemical shifts δ reported in parts per million and coupling constants J in Hertz. The chemical shifts of the 1 H and 13 C{ 1 H} NMR experiments were referenced by TMS (tetramethylsilane) at δ = 0.0 ppm. The chemical shifts of the 31 P{ 1 H} NMR experiments were referenced by triphenyl phosphine at δ = 77.0 ppm. 2D NMR experiments COSY, HSQC, HMBC were performed using the standard Bruker pulse sequence with gradient. The relaxation delay, 90° pulse, spectral width, and number of data points for 1 H-NMR were 1 s, 9.43 µs, 5580 Hz, and 64 K, respectively. The corresponding

Materials and Methods
The nuclear magnetic resonance (NMR) data were collected on a Bruker Avance III HD spectrometer operating at 400.0 MHz for 1 H, 100 MHz for 13 C, and 161 MHz for 31 P. The concentration of all samples was approximately 20 mg/0.7 mL of CDCl 3 . NMR data were recorded at 25 • C, with chemical shifts δ reported in parts per million and coupling constants J in Hertz. The chemical shifts of the 1 H and 13 C{ 1 H} NMR experiments were referenced by TMS (tetramethylsilane) at δ = 0.0 ppm. The chemical

Conclusions
In conclusion, we have synthesized the diethyl (2-(4-phenyl-1H-1,2,3-triazol-1-yl)benzyl) phosphate and performed a full structural elucidation of its 1 H, 13 C, and 31 P NMR signals. This is an important contribution to facilitate the purity assessment of α-hydroxyphosphonate compounds, which have consolidated pharmacological applications-especially in the agrochemical field, due to the organophosphonate herbicides, because it is the major byproduct derived from the Phospha-Brook rearrangement. Thus, the full assignments are an imperative task in the development of new bioactive compounds, not only for the products, but also for the impurities formed during the synthesis. Additionally, we have demonstrated the utility of the 1 H-31 P HMBC experiment, which is a simple and practical 2D experiment that combines the simple identification by the 31 P chemical shift with the detection sensitivity by the 1 H spectrum in a practical procedure.