Synthesis and Purification of Iodoaziridines Involving Quantitative Selection of the Optimal Stationary Phase for Chromatography

Summary

A protocol for the diastereoselective one-pot preparation of cis-N-Ts-iodoaziridines is described. The generation of diiodomethyllithium, addition to N-Ts aldimines and cyclization of the amino gem-diiodide intermediate to iodoaziridines is demonstrated. Also included is a protocol to rapidly and quantitatively assess the most appropriate stationary phase for purification by chromatography.

Abstract

The highly diastereoselective preparation of cis-N-Ts-iodoaziridines through reaction of diiodomethyllithium with N-Ts aldimines is described. Diiodomethyllithium is prepared by the deprotonation of diiodomethane with LiHMDS, in a THF/diethyl ether mixture, at -78 °C in the dark. These conditions are essential for the stability of the LiCHI₂ reagent generated. The subsequent dropwise addition of N-Ts aldimines to the preformed diiodomethyllithium solution affords an amino-diiodide intermediate, which is not isolated. Rapid warming of the reaction mixture to 0 °C promotes cyclization to afford iodoaziridines with exclusive cis-diastereoselectivity. The addition and cyclization stages of the reaction are mediated in one reaction flask by careful temperature control.

Due to the sensitivity of the iodoaziridines to purification, assessment of suitable methods of purification is required. A protocol to assess the stability of sensitive compounds to stationary phases for column chromatography is described. This method is suitable to apply to new iodoaziridines, or other potentially sensitive novel compounds. Consequently this method may find application in range of synthetic projects. The procedure involves firstly the assessment of the reaction yield, prior to purification, by ¹H NMR spectroscopy with comparison to an internal standard. Portions of impure product mixture are then exposed to slurries of various stationary phases appropriate for chromatography, in a solvent system suitable as the eluent in flash chromatography. After stirring for 30 min to mimic chromatography, followed by filtering, the samples are analyzed by ¹H NMR spectroscopy. Calculated yields for each stationary phase are then compared to that initially obtained from the crude reaction mixture. The results obtained provide a quantitative assessment of the stability of the compound to the different stationary phases; hence the optimal can be selected. The choice of basic alumina, modified to activity IV, as a suitable stationary phase has allowed isolation of certain iodoaziridines in excellent yield and purity.

Introduction

The aim of this method is to prepare iodoaziridines that offer potential for further functionalization to aziridine derivatives. The method incorporates a protocol for the quantitative selection of the optimal stationary phase for chromatography.

Aziridines, as three-membered rings, posses inherent ring strain that makes them important building blocks in organic chemistry¹. They display a vast array of reactivity often involving aziridine ring opening^2,3, particularly as intermediates in the synthesis of functionalized amines^4,5, or the formation of other nitrogen containing heterocycles^6,7. The synthesis of a range of aziridine derivatives by functionalization of a precursor containing an intact aziridine ring has emerged as a viable strategy⁸. Functional group–metal exchange, to generate an aziridinyl anion, and reaction with electrophiles has been shown to be effective^9,10,11, and recently regio- and stereoselective deprotonation of N-protected aziridines has also been achieved^12-15. Very recently, palladium catalyzed cross-coupling methods to form aryl aziridines from functionalized aziridine precursors has been developed by Vedejs^16,17, and ourselves¹⁸.

The chemistry of heteroatom substituted aziridines opens up fascinating questions of reactivity and stability¹⁹. We have been interested in the preparation of iodoaziridines as a novel functional group that offers the potential to provide precursors to a wide range of derivatives with complementary reactivity to existing aziridine functionalization reactions. In 2012 we reported the first preparation of aryl N-Boc-iodoaziridines²⁰, and very recently reported the preparation of aryl and alkyl substituted N-Ts-iodoaziridines²¹.

The method to access iodoaziridines uses diiodomethyllithium, a reagent which has recently also been employed in the preparation of diiodoalkanes^22,23, diiodomethylsilanes^22,24, and vinyl iodides^25-27. The carbenoid-like nature of this reagent requires preparation and use at low temperatures^22,28. The techniques and conditions used for the generation of diiodomethyllithium in the preparation of iodoaziridines are described below.

While silica has emerged as the material of choice for chromatography²⁹, it proved to be unsuitable for the purification of the N-Ts-iodoaziridines. Silica gel is generally the first and only solid phase material employed in flash chromatography in organic chemistry due to the availability and effective separations. However, the acidic nature of silica gel can cause the decomposition of sensitive substrates during purification, preventing isolation of the desired material. While other stationary phases or modified silica gels are available for chromatography³⁰, there was no way to assess compatibility of the target molecule to these different materials. Due to the sensitive nature of the iodoaziridines, we established a protocol to assess the stability of a compound to an array of stationary phases²¹, which is demonstrated here. This has potential for application in the synthesis of a wide range of compounds with sensitive functional groups. The following protocol provides efficient access to N-Ts iodoaziridines, allowing the diastereoselective synthesis of both alkyl and aromatic cis-iodoaziridines in high yield.

Protocol

1. Preparation of Iodoaziridines with Diiodomethyllithium

1. Flame dry a 100 ml round bottom flask containing a stirrer bar and fitted with a septum, under a stream of argon, then allow to cool to room temperature under an argon atmosphere. NOTE: Glassware dried in an oven overnight (125 °C) and cooled to room temperature in an analogous fashion is also appropriate.
2. To the flask, add 5.7 ml anhydrous THF and 2.7 ml anhydrous Et₂O via syringe, and freshly distilled hexamethyldisilazane (1.50 mmol, 315 µl) via a microsyringe.
3. Stir the resulting solution and cool to -78 °C in a dry ice/acetone bath in a suitably sized dewar to allow the flask to be well submerged. Cover the dewar with aluminum foil, to minimize exposure of the reaction vessel to light.
4. Add nBuLi (1.50 mmol, 0.60 ml, 2.5 M in hexanes) dropwise via syringe over 2-3 min to the solution at -78 °C. Allow the mixture to stir at -78 °C for a further 30 min to form a 0.17 M solution of LiHMDS. CAUTION: nBuLi solution is flammable, corrosive to the skin and pyrophoric. Excess reagent in the syringe should be quenched accordingly.
5. After 30 min, add 1 ml anhydrous THF to a flame-dried 10 ml round bottom flask via syringe, followed by diiodomethane (1.70 mmol, 135 µl) via a microsyringe and ensure they are well mixed.
6. Add the diiodomethane solution dropwise over 2 min to the solution of lithium hexamethyldisilazane at -78 °C. Leave this solution for 20 min at -78 °C.
7. During this time, weigh out N-[(E)-4-methylphenylmethylidene]-4-methylbenzenesulfonamide (137 mg, 0.50 mmol) into another flame-dried 10 ml round bottom flask and dissolve in 2.0 ml anhydrous THF.
8. After the 20 min deprotonation time, add the imine solution dropwise to the diiodomethyllithium solution over 5 min at -78 °C.
9. Immediately after the dropwise addition is complete, lift the reaction vessel out of the dry ice bath, and transfer to an ice/water bath at 0 °C. Re-cover with aluminum foil and leave for 15 min at 0 °C. NOTE: The solution should be orange in color.
10. After 15 min at 0 °C, quench the reaction by the addition of 30 ml saturated aqueous sodium bicarbonate solution. Transfer the mixture to a separating funnel and add 30 ml CH₂Cl₂. Shake the mixture and remove the lower CH₂Cl₂ layer. Repeat this extraction procedure two further times, and combine the CH₂Cl₂ layers.
11. Add sodium sulfate to the organic layers to remove any water present in the solution, then filter off the sodium sulfate and collect the filtrate in a 250 ml round bottom flask.
12. Remove the solvent under reduced pressure on a rotary evaporator to afford an impure sample of the desired iodoaziridine product.

2. Assessment of Product Stability to Stationary Phases for Chromatography

1. Dissolve the crude aziridine sample in CH₂Cl₂ (16 ml) and add 1,3,5-trimethoxybenzene (28.0 mg, 0.167 mmol) as an internal standard, ensuring this is fully dissolved. Take an aliquot (2 ml) from this mixture, remove the solvent under reduced pressure and analyze this sample by ¹H NMR spectroscopy.
2. Open the recorded ¹H NMR spectrum using standard NMR processing software. In Mestrenova, right click the spectrum and chose “integration”, then “manual” to provide the integration tool. Click and drag to cover the width of the peaks at 6.08 ppm and at 4.87 ppm to integrate the signals of the internal standard and the aziridine CHI signal respectively. Right-click on the integral for the peak at 6.08 ppm, select “edit integral” and change the “normalized” value to 3.0. NOTE: Similar steps can be applied with other software packages.
3. Use the updated value of integral for the aziridine CHI signal (4.87 ppm) to determine the yield of the iodoaziridine, here using (100/3) × (the integral of the CHI signal), which affords a calculated yield of 59%. NOTE: Given the known quantity of internal standard (0.167 mmol), and the product peak corresponding to 1 proton, the yield of iodoaziridine is calculated by the following equation:100 × (integral of product peak) × (moles of internal standard) / moles starting material.
4. Prepare slurries of the following stationary phases (25 g): silica, silica + 1% NEt₃ (triethylamine), neutral alumina, basic alumina (activity I), basic alumina (activity IV) and Florisil, each in 5% EtOAc/hexane (50 ml), in six separate 250 ml conical flasks containing stirrer bars. In another conical flask prepare a 5% EtOAc/hexane solution (50 ml), to be used as a control experiment. CAUTION: silica gel, alumina and other stationary phases employed are dangerous if inhaled, therefore should always be handled in an effective fume hood.
5. Add 2 ml aliquots of the iodoaziridine/internal standard solution to each of the conical flasks at RT. Stir the slurry mixtures for 30 min. NOTE: this represents the duration the compound may be exposed to the stationary phase during a normal flash column chromatography procedure.
6. Filter the slurry mixtures using a sintered funnel, and collect the filtrate in a 250 ml round bottom flask. Wash the residue on the sintered funnel with CH₂Cl₂ (2 × 30 ml). Repeat this filtering process for the remaining slurries. NOTE: It is appropriate to offset the commencement of each stationary phase to allow time for filtration and so maintain the same time for each of the stationary phase materials.
7. Remove the solvent from the resulting samples under reduced pressure, and analyze by ¹H NMR spectroscopy to calculate the amount of iodoaziridine recovered in each case, as described in Section 2.2.
8. Compare the yields of iodoaziridine obtained from each stationary phase tested with that obtained in Section 2.1. NOTE: The sample giving the highest yield, ideally the same as in 2.1, indicates the optimal stationary phase for chromatography. In this example, basic alumina (activity IV) was deemed the best stationary phase for purification.

3. Deactivation of Basic Alumina and Purification of the Iodoaziridine

1. Repeat Section 1 to generate the crude iodoaziridine mixture.
2. To generate basic alumina (activity IV), add 100 g of basic alumina (activity I) to a 500 ml round bottom flask and then add 10 ml water to the flask and fit with a glass stopper.
3. Shake the flask vigorously until no lumps are visible, indicating even spreading of water throughout the alumina. Allow the alumina to cool to RT. CAUTION: the adsorption of water is exothermic, so the flask may get hot and may result in a build up of pressure. Release any pressure build up frequently.
4. Purify the crude iodoaziridine by column chromatography using the basic alumina (activity IV) as the stationary phase, eluting with hexane, grading to 5% EtOAc/hexane. NOTE: high concentrations of EtOAc should not be used with basic alumina. In these cases, diethyl ether can be used instead.
5. Combine the product containing fractions and remove the solvent under reduced pressure to obtain the pure iodoaziridine.

Results

The procedure described affords cis-(±)-2-iodo-3-(4-tolyl)-1-(4-tolylsulfonyl)aziridine as a single diastereoisomer and with excellent purity (Figure 1). Prior to purification, a yield of 59% of the iodoaziridine product was calculated by ¹H NMR spectroscopy. However, this iodoaziridine was particularly challenging to purify and underwent significant decomposition on silica. Purification on basic alumina (activity IV) as determined by the stationary phase screen allowed the produ...

Discussion

A procedure for the diastereoselective preparation of cis-N-Ts-iodoaziridines is described, along with a stability study protocol to quantitatively indicate the best stationary phase for purification of potentially unstable compounds by flash column chromatography. It is envisaged that access to iodoaziridines through this approach will enable methods to access to a wide range of aziridines to be developed, by derivatization of the intact ring.

An appropriate modification to ...

Materials

Name	Company	Catalog Number	Comments
Hexamethyldisilazane	999-97-3	Alfa Aesar	Distill from KOH under argon prior to use.
n-Butyllithium	109-72-8	Sigma Aldrich	2.5 M in hexanes, titrate prior to use.
Diiodomethane	75-11-6	Alfa Aesar	Contains copper as a stabilizer.
1,3,5-Trimethoxybenzene	621-23-8	Sigma Aldrich
Silica	112945-52-5	Merck
Basic alumina	1344-28-1	Sigma Aldrich
Neutral alumina	1344-28-1	Merck
Florisil	1343-88-0	Sigma Aldrich
THF	All anhydrous solvents were dried through activated alumina purification columns.
Et2O
CH2Cl2
NMR spectrometer	Bruker AV 400	n/a
NMR processing software	MestReNova	7.0.2-8636