Tethered Blatter Radical for Molecular Grafting: Synthesis of 6-Hydroxyhexyloxy, Hydroxymethyl, and Bis(hydroxymethyl) Derivatives and Their Functionalization

Synthetic access to 7-CF3-1,4-dihydrobenzo[e][1,2,4]triazin-4-yl radicals containing 4-(6-hydroxyhexyloxy)phenyl, 4-hydroxymethylphenyl or 3,5-bis(hydroxymethyl)phenyl groups at the C(3) position and their conversion to tosylates and phosphates are described. The tosylates were used to obtain disulfides and an azide with good yields. The Blatter radical containing the azido group underwent a copper(I)-catalyzed azide–alkyne cycloaddition with phenylacetylene under mild conditions, giving the [1,2,3]triazole product in 84% yield. This indicates the suitability of the azido derivative for grafting Blatter radical onto other molecular objects via the CuAAC “click” reaction. The presented derivatives are promising for accessing surfaces and macromolecules spin-labeled with the Blatter radical.

The 1,4-dihydrobenzo[e] [1,2,4]triazin-4-yls [33,51], formal derivatives of the prototypical 1,3-diphenyl derivative known as the Blatter radical [52] A (Figure 1), are exceptionally stable, π-delocalized radicals characterized by favorable redox behavior with a narrow electrochemical window (~1.2 V) [37,38] and a broad absorption in the visible range [53]. The stability of Blatter radical A is further enhanced by placing the CF 3 group in the C(7) position leading to derivative B, the so-called "super stable" radical [54]. For these reasons, 1,4-dihydrobenzo[e] [1,2,4]triazin-4-yls are promising paramagnetic structural elements of functional materials [33], and have been explored as photoconductive liquid crystals [55][56][57], sensors [58], photodetectors [59], and also in spintronics [41,60,61]. There are still relatively few studies on Blatter radicals chemisorbed on surfaces or grafted on macromolecules, mainly due to insufficiently developed access to derivatives with appropriate functional groups. For instance, Blatter radical containing two Auanchoring SMe groups (C, Figure 1) was chemisorbed on the Au(111) surface and the interactions of the resulting molecular films with the surface were investigated in detail [41]. Acetylene derivative D was prepared and reacted with an azidonorbornene derivative under the "click" reaction conditions to give a norbornene-containing monomer, which was polymerized using the ROMP method. The acetylene derivative D could also be used for chemisorption onto the Au surface. Finally, bis(triethoxysilyl) derivative E was used for grafting the Blatter radical onto mesoporous silica [62] and could also be used for the functionalization of Si and metal surfaces with native oxides.
Despite progress in functional derivatives of the Blatter radical, there is a need to broaden the range of intermediates containing active functionalities for grafting onto diverse types of surfaces and macromolecules.
Herein we describe three derivatives of the "super stable" Blatter radical containing the 6-hydroxyhexyloxy (Ia, Figure 2) or hydroxymethyl (IIa and IIIa) substituents on the C(3)-Ph ring as key intermediates to functional derivatives suitable for grafting onto low dimensional systems. We report a conversion of the alcohols Ia-IIIa to the corresponding tosylates Ib-IIIb and phosphates Ic and IIc, and transformations of the tosylates to disulfides Id and IId, and azide Ie. As a proof of concept, we demonstrate the "click" reaction of azide Ie with an alkyne.

Synthesis of Hydroxyl Derivatives Ia-IIIa
Radicals Ia-IIIa containing versatile hydroxyl groups were prepared using the recently discovered regioselective azaphilic addition of aryllithium to benzo[e] [1,2,4]triazines [63]. Thus, phenyllithium was reacted with benzo[e][1,2,4]triazines 1-3, and the resulting anions were oxidized with air to the corresponding radicals Ia-IIIa, which were conveniently isolated by column chromatography (SiO 2 support) in yields up to 92% (Scheme 1). It should be noted that both hydroxy (1 and 3) and acetoxy (2) derivatives were suitable starting materials for this reaction affording the corresponding radicals in comparable yields. The route to Ia involving addition of phenyllithium to compound 4, the O-benzyl protected alcohol 1 (Scheme 2), followed by Pd-catalyzed reductive debenzylation of the resulting radical If (Scheme 1) turned out to be inefficient. While the PhLi addition and formation of If worked well (yield up to 64%, Scheme 2), debenzylation of If gave only decomposition products. This presumably resulted from the more vigorous conditions needed for the removal of the O-benzyl group in the alkyl benzyl ethers than in the aryl analogues. The requisite alcohols 1 and 3 and acetate 2 were obtained in two steps following an established general procedure [57,63,64], as shown in Scheme 2. Thus, the readily available benzhydrazides 5-7 were N-arylated with 1-fluoro-2-nitro-5-trifluoromethylbenzene. The resulting hydrazides 8-10 were subsequently cyclized under reductive conditions (Sn powder/AcOH) followed by oxidation of the dihydro products with Ag 2 O or NaIO 4 giving benzo[e] [1,2,4]triazines 2-4 in good overall yields (40-75%). Interestingly, during reductive cyclization of 9 at temperatures above 100 • C, the hydroxymethyl group underwent esterification with AcOH, used as the solvent and reagent in this reaction, and acetate 2 was isolated in 45% yield. The analogous acetate was not observed in the case of cyclization of hydrazide 10 conducted at ambient temperatures.
The hydroxyhexyloxy derivative 1 was obtained in 31% overall yield by Pd-catalyzed debenzylation of 4, followed by aerial oxidation of the dihydro form. As noted above, debenzylation of 4 required longer-than-typical reaction times. The obtained hydroxy derivative 1 turned out to be sensitive to elevated temperatures: concentration of solutions of purified 1 on a rotavap at 40 • C resulted in its decomposition and formation of a foulsmelling orange oil. Handling of 1 at lower temperatures avoided this problem, and pure product was obtained.
Benzhydrazides 5-7 were obtained by hydrazinolysis of the corresponding methyl benzoates with hydrazine hydrate.

Synthesis of Tosylates and Phosphates
Reactions of alcohols Ia-IIIa with tosyl chloride gave the desired tosylates Ib-IIIb in 82-96% yield (Scheme 3). The relatively high stability of Ib allowed for isolation of the pure compound using standard silica gel chromatography. In contrast, tosylates IIb and IIIb were sensitive to chromatography conditions and were used for the next step as crude materials. Phosphorylation of alcohols Ia and IIa with diethyl chlorophosphate in the presence of DMAP and Et 3 N gave the phosphates Ic and IIc, respectively, isolated in about 85% yield (Scheme 3).

Transformation of Tosylates: Preparation of Disulfides and Azide
Disulfides Id and IId were obtained in 18% yield from tosylates Ib and IIb using a general procedure [65] involving reactions with the thiosulfate (S 2 O 3 2− ) nucleophile in DMSO, followed by oxidation of the resulting mercaptan with I 2 (Scheme 4). In contrast, the preparation of azide Ie was straightforward and more efficient. Thus, reaction of toslyate Ib with NaN 3 in DMF gave azide Ie isolated in yields up to 73% yield (Scheme 4).

Copper(I)-Catalyzed Azide-Alkyne Cycloaddition of Azide Ie
Compound Ie represents the first azido derivative of the Blatter radical, and its suitability for the Cu(I)-catalyzed cycloaddition reaction ("click") with alkynes, the CuAAC reaction, required experimental verification. Thus, the azide Ie was reacted with phenylacetylene in the presence of Cu(I), generated in situ from CuSO 4 and sodium ascorbate, according to a general literature method [66]. The "click" product, [1,2,3]triazole Ig, was isolated in a high yield of 84% (Scheme 5). This result compares to 53% yield of [1,2,3]triazole formation in an analogous CuAAC reaction of acetylene-substituted Blatter radical D ( Figure 1) with an azido derivative of norbornene [13]. Pure Ig showed no decomposition during storage for 4 years under ambient conditions, according to thin-layer chromatography analysis.

Spectroscopic Characterization of Radicals
All radicals I-III exhibit low-intensity broad absorption in the entire visible range, as shown for Ia in Figure 3. Consequently, the compounds appear dark brown in solutions and nearly black in the solid state. EPR analysis of the radicals conducted in benzene solutions revealed seven principal lines resulting from hyperfine splitting with three 14 N nuclei modulated with additional smaller splitting by 19 F and 1 H nuclei, as shown for derivative Ia in Figure 3. For some radicals, the principal lines are less resolved, presumably due to aggregation in benzene solutions (see the Supplementary Materials). Analysis demonstrated that the a N hfcc values for radicals I-III are consistent with those for other Blatter radical derivatives, and are about 7.6 G for a N(1) and about 4.5-4.9 G for a N(2) and a N(4) . Simulation of the experimental spectra indicates that the a F hfcc value is in a range of 3.2-3.6 G.

Conclusions
The "super stable" Blatter radical B was substituted at the C(3) phenyl ring with a long tether (O(CH 2 ) 6 -X, I), a short tether (CH 2 -X, II) or two anchoring groups (2 × CH 2 -X, III). The key intermediates contain the hydroxyl group (X = OH, a), which could be used for grafting in condensation (acylation) and addition (e.g., carbamination) reactions. The hydroxyl derivatives Ia-IIIa are efficiently converted to tosylates Ib-IIIb, which served as electrophilic intermediates to disulfides Id and IId (for chemisorption onto Au surfaces) and azide Ie (for the CuAAC reaction). Tosylates IIb and IIIb can additionally be exploited to attach the Blatter radical to hydroxyl-functionalized partners through the formation of benzyl-type ether linkages. Following prior work on the synthesis and supramolecular properties of cyclodextrin-xylylene hybrids [67][68][69][70], we have conducted preliminary experiments supporting the viability of such an approach, and the results will be published in due course.
The demonstration of efficient "click" reaction of azide Ie with PhC≡CH paves the way to grafting radical B onto surfaces and macromolecules functionalized with terminal ethynyl groups. This method represents a more versatile approach to paramagnetic materials, since many macromolecules substituted with the propargyl group are available.
The presented results constitute a promising approach to novel paramagnetic polymers with high radical density, e.g., for polymer electrodes with high charge storage capacity and for high-density paramagnetic surfaces for spintronic applications. This work is continued in our laboratory.

Materials and Methods
General. Reagents and solvents were purchased and used as received. THF was dried over Na metal in the presence of benzophenone and distilled before use. Column chromatography was performed on silica gel. For separation of radicals silica gel was passivated by mixing with CH 2 Cl 2 containing 2% of Et 3 N and removal of the solvent to dryness (rotovap). Reported yields refer to analytically pure samples. NMR spectra were recorded with a Bruker AVIII 500 or 600 instrument. Chemical shifts are reported relative to solvent (CDCl 3 ) residual peaks ( 1 H NMR: δ = 7.26 ppm and 13 C NMR: δ = 77.16 ppm) [71]. All 13 C NMR spectra are proton-decoupled. IR spectra were measured in KBr pellets with a FTIR NEXUS spectrometer. High-resolution mass spectrometry (HRMS) measurements were performed using SYNAPT G2-Si High-Definition Mass Spectrometry equipped with an ESI or APCI source and Quantitative Time-of-Flight (QuanTof) mass analyzer. Melting points were determined in capillaries with a MEL-TEMP II apparatus and are uncorrected. If not stated otherwise, reactions were carried out under argon atmosphere in flame-dried flasks with addition of the reactants via syringe. Subsequent manipulations were conducted in air.
EPR spectra of radicals I-III were recorded on an X-band EMX-Nano EPR spectrometer at ambient temperature using dilute and degassed solutions in distilled benzene in a concentration range of 2-5 × 10 −4 M. Additional details are shown in the SI.

Preparation of radicals Ia-IIIa via PhLi addition to benzo[e][1,2,4]triazines 1-3. A general method.
To a solution of the appropriate benzo[e] [1,2,4]triazine derivative 1, 2, or 3 (0.792 mmol) in dry THF (10 mL), PhLi (1.9 M in dibutyl ether, 1.24 mL, 2.360 mmol) was added dropwise at −5 • C under argon and the reaction mixture was stirred at this temperature for 1 h, then for 1 h at rt under air. Water was added and the product was extracted with CH 2 Cl 2 (3×). The combined organic extracts were dried (Na 2 SO 4 ), and volatiles were removed on a rotavap. Pure product was isolated by column chromatography followed by recrystallization (MeCN).  Preparation of tosylates IIb and IIIb. A general method. To a solution of alcohol IIa or IIIa (0.79 mmol) and tosyl chloride (300 mg, 1.57 mmol) in dry THF (10 mL), 60% NaH in mineral oil (475 mg, 11.85 mmol) was added in portions over 30 min. The suspension was stirred at rt for another 30 min, and water was added dropwise to neutralize the unreacted NaH. The reaction mixture was extracted with CH 2 Cl 2 (3×). The combined organic extracts were dried (Na 2 SO 4 ) and concentrated in vacuo. The crude product was passed through a silica gel plug passivated with Et 3 N (pet ether/AcOEt 4:1) to give IIb or IIIb as green-brown solid. The product was immediately used for the next step without further

Preparation of phosphates Ic and IIc. A general method.
To a solution of alcohol Ia or IIa (0.2 mmol, 1.0 equiv.), DMAP (0.02 mmol, 0.1 equiv.) and Et 3 N (1.0 mmol, 5 equiv.) dissolved in THF (2 mL) diethyl chlorophosphate (1.0 mmol, 5 equiv.) was added slowly via syringe. During the addition, a white precipitate formed. The reaction mixture was stirred at rt for 1 h until substrate was no longer present in the reaction mixture (TLC control). The reaction was quenched with sat. NH 4 Cl solution, extracted with CH 2 Cl 2 , and the product was purified by column chromatography (pet. ether/AcOEt 3:2).   (15 mL) was stirred at 60 • C overnight. The reaction mixture was cooled to rt and extracted with AcOEt (3×). The combined organic extracts were dried (Na 2 SO 4 ) and concentrated in vacuo. The residue was dissolved in CH 2 Cl 2 (10 mL) and solid I 2 (49 mg, 0.19 mmol) was added and stirred at rt for 10 min, filtered, the solid was washed with CH 2 Cl 2 and the filtrate was evaporated. The crude product was passed through a passivated silica gel plug (pet. ether/AcOEt 5:1) and recrystallized (MeCN) to give pure product Id or IId as brown solids.

Preparation of 3-[4-(6-hydroxyhexyloxy)phenyl]-7-trifluoromethylbenzo[e][1,2,4] triazine (1). A solution of benzyloxy derivative 4 (3.00 g 6.23 mmol) in THF (35 mL) was
added to a suspension of 5% Pd/C (2.60 g) in EtOH (35 mL), and the resulting mixture was hydrogenated (50 psi) overnight. The reaction mixture was passed through a Celite pad and oxidized by exposure to air (TLC monitoring) and the solvents were removed under reduced pressure (cold bath!). Crude product was purified by column chromatography (SiO 2 , CH 2 Cl 2 /EtOAc 4:1) to give 0.76 g (31% yield) of pure product 1 as a yellow solid: Preparation of triazines 2-4. A general method. To a solution of hydrazide 8, 9 or 10 (6.96 mmol) in glacial AcOH (100 mL), Sn powder (4.54 g, 38.3 mmol) was added and stirred at rt for 2 h, and then at 115 • C for 30 min. After cooling, EtOAc and H 2 O were added, and the mixture was filtered through a Celite pad. The solution was extracted with two portions of EtOAc, and the combined organic extracts were washed with saturated aq. NaHCO 3 (3×) and dried (Na 2 SO 4 ). Solvents were removed on a rotavap, and the residue was dissolved in a CH 2 Cl 2 /MeOH mixture (1:1) and solid NaIO 4 (2.23 g, 10.44 mmol) or Ag 2 O (354 mg, 1.52 mmol) was added. The mixture was stirred at rt until complete consumption of the dihydro form. The solution was filtered, and the solvents were removed under reduced pressure. The crude product was purified on silica gel and recrystallized to give pure triazines 2-4.