A Base-Free, Low Temperature Click and Release Reaction for the In Situ Generation of Diazomethane

: Diazomethane is a powerful reagent for numerous chemical reactions such as esterifications and the homologation of carboxylic acids. Unfortunately, the synthetic utility of diazomethane is severely limited by its toxicity and highly explosive nature. Diazald ® is typically used for ex situ synthesis, however it requires cumbersome and hazardous transfer of diazomethane from a caustic aqueous phase to the reaction medium. Herein, we present a low temperature and base-free in situ synthesis of diazomethane via a “click and release” reaction between an enamine and sulfonyl azide. Its utility is exemplified by the synthesis of diverse methyl esters in yields of up to 93%. Moreover, diazoketone synthesis from in situ generated diazomethane and acid chlorides was demonstrated for the first time. Finally, trideuteromethylation was achieved using acetone-d 6 as the deuterium source. We anticipate that this method will enable the safer use of diazomethane in organic synthesis and drug discovery programs.


Introduction
Diazomethane is a remarkably versatile reagent for the synthesis of various motifs found in drug molecules. [1]espite its reputation as one of the most difficult to handle organic reagents, diazomethane is still employed in some manufacturing processes due to its impressive selectivity profile and the fact that nitrogen gas is the only reaction byproduct. [2,3]It is, however, more commonly used in smaller scale syntheses and, in particular, in drug discovery programs for early and late stage analogue modifications.Protonation of diazomethane to the electrophilic methyldiazonium ion allows carboxylic acid esterification even in the presence of acid-sensitive groups, such as tert-butyloxycarbonyl (BOC).[6][7] Furthermore, diazomethane can also act as a nucleophile in various reactions, leading to powerful rearrangements of nearby alkyl groups.For example in the Arndt-Eistert reaction where the treatment of an acid chloride with diazomethane affords homologated carboxylic acids and derivatives. [8]Finally, from a medicinal chemistry perspective, recent literature has highlighted the utility of diazomethane for the synthesis of α-chloroketones, [9] the synthesis of Lupane analogues via 1,3-dipolar cyclisations and carbene additions with activated oxocompounds [10] and the one-pot ring-expansion and epoxidation of thieno [2,3-b]indole-2,3-diones. [11]he power and versatility of diazomethane has spurred the development of various methods for its synthesis (Figure 1).In particular, the base-catalysed cleavage of N-alkyl-N-nitroso sulfonamides (Diazald ® , N-methyl-N-nitroso-p-toluenesulfonamide) is widely used. [12,13,15]The use of Diazald ® , however, requires 6 M hydroxide and specialized glassware [14] to be able to safely transfer the synthesized diazomethane to another reaction vessel.A safer in situ batch synthesis with a water-soluble version of Diazald ® has been developed [16] but this still requires the use of 6 M hydroxide, limiting its utility for e. g. methyl esterification and diazoketone synthesis.Flow systems have been designed using Diazald ® for the safer continuous synthesis and consumption of diazomethane. [17,18]hese systems are useful, particularly in larger scales, but require expensive and complex setups.A safer but less reactive version of diazomethane, trimethylsilyldiazomethane, has also been applied to late-stage diversification. [19]Despite being less explosive, it must still be handled cautiously due to its acute toxicity. [20,21]ecently an impressive in situ batch method based on the hydrolysis of the imidazotetrazine drug molecule, temozolomide, was disclosed. [22]Despite its utility, the method requires the addition of base, heating, water and an excess of temozolomide for an efficient downstream reaction.It is, nonetheless, the first method for the in situ batch synthesis of diazomethane using weighable substrates in the literature. [23]s part of our previous work on sulfonyl azide chemistry, [24][25][26][27] we noted reports on the synthesis of sulfonyl amidines through the reaction of a sulfonyl azide with an enamine, [28][29][30][31] which reportedly gives diazomethane as a byproduct.We reasoned that this "click and release" approach for diazomethane generation would provide significant advantages compared to previous methods such as low temperature, use of organic solvents and no requirement for a base.Importantly, despite these attractive features, little attention has been paid to the leverage of these observations for the in situ generation and reaction of diazomethane.
Accordingly, we sought to develop a simpler, safer, in situ generation and reaction of diazomethane via the click and release reaction of a sulfonyl azide with an enamine (Figure S1).To achieve this, several aspects were considered: the reaction should be able to be carried out in an organic solvent (ideally green) by simply mixing the reagents together at room temperature (or lower), without the need for catalysts or harsh reagents like hydroxide bases.The generation should also be carried out in situ to avoid the dangerous buildup of diazomethane, and, most importantly, the azide and enamine should react chemoselectively even in a complex molecular setting.In order for it to be a convenient tool for chemists, the method should be as simple as possible and use solid, weighable reagents that are stable and easy to synthesize.Herein, we describe our efforts towards achieving these goals.

Results and Discussion
Initially, we focused on identifying suitable enamine and sulfonyl azide coupling partners capable of diazomethane generation without any additional catalysts or additives.This could then be extended to the in situ generation and reaction of diazomethane where concentrations of diazomethane are expected to be low due to its simultaneous production and consumption over time.
The design strategy for the enamine and the sulfonyl azides was inspired by previous studies on sulfonyl amidine synthesis. [28]However, we focused our attention on identifying reaction partners that were solids and contained an ionizable group.Exchange of tosyl azide with an N-methylimidazole derivative (1methyl-1H-imidazole-4-sulfonyl azide, 1 a), and the addition of a nitro group to the enamine (4-(1-(4nitrophenyl)vinyl)morpholine, (2 a)), afforded solid reagents and also allowed for removal of the sulfonyl amidine byproduct 5 by acidic aqueous work-up.Moreover, these reagents will only form diazomethane when mixed together and are stable upon storage for at least six months as monitored by 1 H-NMR (Figure S2).

RESEARCH ARTICLE
asc.wiley-vch.dereaction occurred at room temperature, without the need for a biphasic water-organic solvent system or 6 M hydroxide base. [13,16,22]ext, we optimized the conditions with a focus on reducing the equivalents of 1 a and 2 a and identifying a greener solvent.After some experimentation (Table S1), we found that 1.7 equivalents of 1 a and 2 a was sufficient to afford full conversion of 3 a and 85% yield of the desired product after an overnight reaction.This longer reaction time is potentially beneficial from a safety standpoint as it implies that diazomethane is generated slowly over time.This is consistent with a general trend towards longer reaction times with decreasing equivalents of 1 a and 2 a.These conditions were then used in a solvent screen (entries 2-7) where DMF, dioxane and acetone were found to be comparable or better than DCM.This further highlights the utility our approach as the appropriate solvent can be chosen depending on the substrate employed in the subsequent reaction.In this case, acetone was found to be the solvent of choice for esterification of 3 a (yield > 95%).Next, the reaction temperature was probed and esterification still proceeded efficiently at 0 °C (entry 8) while no reaction was observed at À 78 °C (entry 9).In the latter case, no sulfonyl amidine byproduct 5 was detected (LCMS analysis) after six hours at À 78 °C (Figure S3) indicating negligible diazomethane generation at this temperature.This ability to control diazomethane release by altering the reaction temperature offers significant flexibility and practical advantages.The reaction could even be conducted without pre-cooling of the reaction mixture (entry 11), although cooling is recommended for larger scale reactions.The use of a base was investigated (entry 12), however, no influence on the reaction time or outcome was noted.Finally, to probe substituent effects on diazomethane generation and reaction, we also conducted experiments using various enamines and sulfonyl azides (Scheme S1).Notably, we found that removing the nitro group (2 b), or replacement with an electron-donating methoxy substituent on the enamine (2 c) afforded the target ester in good yields (62% and 72% respectively), albeit slightly less than with 2 a. Replacing the sulfonyl azide 1 a with standard diazotransfer reagent tosyl azide (1 b) resulted in comparable yield (94%), although the sulfonyl amidine byproduct could not be removed by aqueous workup, resulting in a more challenging purification.Accordingly, the reagents of choice for diazomethane generation and reaction are sulfonyl azide 1 a and enamine 2 a as they provide both the highest yield and an easier purification process.

Synthesis of Diverse Methyl Esters
Having identified suitable reaction conditions (entry 11), the scope and limitations of the esterification reaction was investigated (Scheme 1).Electron-rich (3 b, 3 e and 3 f) and poor (3 c and 3 d) substrates were well-tolerated, suggesting that electronic effects have a limited influence on the reaction outcome.A modest reduction in yield was noted with ortho-nitroarene 3 d, presumably due to steric effects.The chemoselectivity of the method was demonstrated by the high yields of 4 e, 4 f and 4 g as the presence of amines, alcohols or alkenes did not impair the reaction outcome.The [a] Yield of methyl naphthoate determined by 1 H-NMR.
amino acids Fmoc-Thr-OH (3 h) and Fmoc-Trp-OH (3 i) returned the desired esters in good yields even in scales of up to 1 g.Additionally, the acid-labile Boc protecting group was well-tolerated (3 j), demonstrating the methods ability to esterify carboxylic acids in the presence of acid sensitive groups.Finally, we sought to showcase the utility of our method in late-stage modification by methylating various pharmacologically active substrates.The uricosuric probenecid (3 k), the NSAID naproxen (3 l) and the diuretic furosemide (3 m) were all methylated in good to excellent yields.The beta-lactam antibiotic ampicillin is an especially sensitive substrate and, pleasingly, it was converted into the corresponding methyl ester (4 n) in moderate yield.Here DCM was used in place of acetone to avoid aminal formation, [32,33] highlighting the importance of broad solvent compatibility.The steroid fusidic acid (3 o) could be chemoselectively esterified in 81% yield.Finally, the esterification of the dipeptide FmocÀ PheÀ AlaÀ OH (3 p) could be achieved in 69% yield with retained stereochemistry.

Esterification in Deuterated Solvents
Diazomethane is known to undergo rapid protondeuterium exchange in exchangeable deuterated media. [34,35]Given the extensive solvent compatibility of our method, we reasoned that this could be exploited to allow isotope incorporation without a priori deuterium labelling of a diazomethane precursor.This would open up a convenient new route to access deuterated compounds, an area that is of particular interest for the development of new drugs. [36]Therefore, we initiated a screening of common deuterated solvents in search of a system where proton-deuterium exchange occurred at a faster rate than the esterification reaction (Table 2).
Notably, 99% deuterium incorporation with excellent selectivity towards trideuteration (86-91%) was achieved using acetone-d 6 with (entry 1) or without (entry 4) the addition of methanol-d 4 .The higher selectivity observed using only aceone-d 6 indicates that, in this solvent, deuterium exchange occurs at a Scheme 1. Methyl ester analogues synthesized via the in situ generation of diazomethane.b] DCM as solvent.5 ----- [a] Product distribution determined by 1 H-NMR analysis [b] Isolated yields [c] 3 c was stirred in Me 2 CO-d 6 for 24 h [d] 4 q was stirred in Me 2 CO-d 6 for 24 h.
higher rate than esterification.In chloroform-d, deuterium incorporation diminished to 76% and a mixture of all deuterated products was formed (entry 2) consistent with a slower H/D exchange and/or faster methylation.Using methanol-d 4 in non-deuterated acetone afforded < 7% deuterium incorporation and only the mono deuterated product was observed, highlighting the importance of acetone-d 6 for deuterium incorporation.No deuterated products were detected in acetonitrile-d 3 , which is consistent with lower acidity and a slower exchange rate (entry 5).No direct esterification of 3 c by methanol-d 4 was observed in the absence of diazomethane-generating reagents (entry 6) and no H/D exchange was detected when non-deuterated methyl ester 4 q was incubated with acetone-d for 24 hours (entry 7).Thus, highly selective deuterium incorporation can be achieved by simply replacing acetone with its deuterated counterpart in the esterification reaction.This is a highly attractive and operationally simpler means of deuterium incorporation compared to previous approaches that rely on pre-functionalization of the diazomethane generating reagents. [17,35]

Synthesis of α-Diazoketones
To harness the utility of in situ generated diazomethane as a nucleophile for the synthesis of valuable diazoketones, additional optimization was required.Specifically, the formation of hydrogen chloride as a byproduct in the acylation of diazomethane with acid chlorides is problematic as it can react with the product to produce an α-haloketone.For this reason, an excess of diazomethane (as in the traditional Arndt-Eistert reaction) or a base [37] is required as acid scavenger.
Considering the safety issues related to the handling of diazomethane, methods employing a base as an acid scavenger are often preferred. [37]he acid chloride 6 a was chosen as the model substrate and based on results for in situ generation of diazomethane (Table 1) and literature precedent, [37] the solvent DCM and the temperature 0 °C were selected as the starting point for optimization.Initial experiments using triethylamine as a base afforded only low and variable yields of the desired diazoketone (Table S2).Switching to the more sterically hindered base DIPEA resulted in higher yields (Table 3, entry 1-2) and the replacement of azide 1 a with commercially available tosyl azide (1 b) further improved the yield to 58% (entry 3).Finally, a larger excess of the diazomethane generating reagents did not improve the yield further (entry 4).Given the highly reactive nature of acid chlorides and the complexity of the reaction mixture, the conditions from entry 3 were deemed suitable for investigating the reaction scope.
With suitable conditions at hand, a selected array of acid chlorides (6 a-e) were employed to explore the reaction scope (Scheme 2).The method was compatible with electron donating (6 a), electron withdrawing (6 b-d), and aliphatic (6 e) substituents and the corresponding α-diazoketones (7 a-e) were isolated in yields up to 60%.Notably, synthesis of a novel α-diazoketone derivative of probenecid (7 d) once again showcases the utility of the method in the late-stage functionalization of pharmaceuticals.

Conclusion
In summary, we have developed a method for the in situ generation and reaction of diazomethane based on the click and release reaction between a sulfonyl azide and an enamine.This approach allows for the temperature-controlled generation of diazomethane without the need for an added base, with arguably much safer conditions compared to using diazomethane itself.Additionally, the reaction can be conducted in a number of different organic solvents broadening the scope of diazomethane chemistry.A Table 3. Optimisation of diazoketone synthesis using in situ generated diazomethane.entry 1 a/1 b (equiv.) 2 a (equiv.)DIPEA (equiv.)b] Azide 1 a was used.
range of carboxylic acids could be esterified in high yields in acetone, and simply changing to its deuterated counterpart enabled facile trideuterium incorporation.Furthermore, the method was extended to achieve the first synthesis of diazoketones via in situ diazomethane generation.We envisage that this approach will provide a valuable alternative to existing diazomethane generating methods and will find widespread application in drug discovery programs for both early and late stage functionalization.

General Methods
Analytical chromatography (TLC) was performed on silica gel 60 F-254 plates and visualized with UV light.Flash column chromatography was performed using silica gel 60 (40-63 μm).Analytical HPLC/ESI-MS was performed using electrospray ionization (ESI) and a C18 column (50 × 3.0 mm, 2.6 μm particle size, 100 Å pore size) with CH 3 CN/H 2 O in 0.05% aqueous HCOOH as mobile phase at a flow rate of 1.5 ml/min.LC purity analyses were run using a gradient of 5-100% CH 3 CN/H 2 O in 0.05% aqueous HCOOH as mobile phase at a flow rate of 1.5 ml/min for 5 minutes unless otherwise stated on a C18 column.High-resolution molecular masses (HRMS) were determined on a mass spectrometer equipped with an ESI source and a time-of-flight (TOF) mass analyzer or a matrixassisted laser-desorption ionization source with a Fourier transform ion cyclotron resonance mass analyzer as indicated.Optical rotations were recorded on a Rudolph Autopol II polarimeter.

Synthesis of Sulfonyl Azide 1 a
1-Methyl-1H-imidazole-4-sulfonyl azide (1 a) was synthesized using a modified literature procedure. [24]To a solution of sodium azide (2.7 g, 1.5 equiv.) in water (70 mL) at 0 °C was added a solution of 1-methylimidazole-4-sulfonyl chloride (5.0 g 1.00 equiv.) in acetone (250 mL).The solution was left to stir at ambient temperature for 20 h.The acetone was evaporated, saturated sodium bicarbonate solution (50 mL) was added and the mixture extracted with ethyl acetate (3 × 50 mL).The combined organic layers were washed with water (50 mL), brine (50 mL), dried over Na 2 SO 4 , filtered, and concentrated under reduced pressure to afford 1 a as a white solid.

Synthesis of Enamine 2 a
4-(1-(4-nitrophenyl)vinyl)morpholine (2 a) was synthesized using a modified literature procedure. [38]4-Nitro-acetophenone (2.81 g, 17 mmol) was dissolved in 200 mL i-hexane and 100 ml dry toluene.The flask was filled with N 2 , and 8.0 mL (93.7 mmol) of morpholine was added.The flask was backfilled with N 2 three times, cooled to 0 °C and TiCl 4 (1.5 mL, 13.7 mmol) was added slowly via syringe.The mixture was refluxed for 60 min and allowed to cool to room temperature.The reaction mixture was filtered over celite and concentrated under reduced pressure to afford 2 a as a brown solid.

General Procedure A -Synthesis of Methyl Esters (4 a-4 p)
A vial was charged with carboxylic acid 3 (0.6 mmol, 1 equiv.),sulfonyl azide 1 a (169 mg, 0.9 mmol, 1.5 equiv.)and enamine 2 a (210 mg, 0.9 mmol, 1.5 equiv.).The vial was sealed with a septum and an outlet needle was added.Acetone (4 ml) was added via syringe, and the mixture was stirred at ambient temperature.After 24 h the mixture was taken up in 50 ml EtOAc and washed with 50 mL H 2 O, 3 × 50 mL 0.2 M HCl and 50 mL brine.The organic layer was dried over MgSO 4 , filtered, and concentrated under reduced pressure.The resulting crude was purified by flash column chromatography to afford the corresponding methyl ester.

General Procedure B -Synthesis of Diazoketones (7 a-7 e)
A vial was charged with sulfonyl azide 1 b (169 mg, 0.9 mmol, 1.5 equiv.)and enamine 2 a (210 mg, 0.9 mmol, 1.5 equiv.).The vial was sealed with a septum and dry DCM (3 mL) and DIPEA (0.32 mL, 1.8 mmol, 3.0 equiv.)were added at 0 °C, followed by dropwise addition of acid chloride 6 (0.6 mmol, 1 equiv.) in dry DCM (1 mL).The reaction mixture was stirred at 0 °C for 24 h after which it was concentrated under reduced pressure.The resulting crude product was purified by flash column chromatography to afford the corresponding diazoketone.

Table 1 .
Optimisation of in situ diazomethane generation via esterification of naphthoic acid (3 a).