A Two-Step Protocol for Umpolung Functionalization of Ketones Via Enolonium Species

Summary

A two step one-pot protocol for the umpolung of ketone enolates to enolonium species and addition of a nucleophile to the α-position is described. Nucleophiles include chloride, azide, azoles, allyl-silanes, and aromatic compounds.

Abstract

α-Functionalization of ketones via umpolung of enolates by hypervalent iodine reagents is an important concept in synthetic organic chemistry. Recently, we have developed a two-step strategy for ketone enolate umpolung that has enabled the development of methods for chlorination, azidation, and amination using azoles. In addition, we have developed C-C bond–forming arylation and allylation reactions. At the heart of these methods is the preparation of the intermediate and highly reactive enolonium species prior to addition of a reactive nucleophile. This strategy is thus reminiscent of the preparation and use of metal enolates in classical synthetic chemistry. This strategy allows the use of nucleophiles that would otherwise be incompatible with the strongly oxidizing hypervalent iodine reagents. In this paper we present a detailed protocol for chlorination, azidation, N-heteroarylation, arylation, and allylation. The products include motifs prevalent in medicinally active products. This article will greatly assist others in using these methods.

Introduction

Enolates are classical carbon nucleophiles in organic chemistry and among the most widely used. Umpolung of enolates to create electrophilic enolonium species allows valuable alternative ways to produce α-functionalized ketones as well as to enable novel reactions not possible via classical enolate chemistry. Enolonium species have been proposed as intermediates in numerous reactions, in particular reactions involving hypervalent iodine reagents. These reactions include α-halogenation, oxygenation, and amination¹ as well as other reactions^2,3,4,5.

However, the scopes of these reactions were always limited by the transient nature of the reactive enolonium species. This transiency required any nucleophile to be present in the reaction mixture during the reaction of the carbonyl enolates with the strongly oxidizing hypervalent iodine reagent. Thus, any nucleophile prone to oxidation, such as electron rich aromatic compounds (heterocycles) and alkenes, could not be used.

In the last year, we have overcome these limitations by developing conditions in which the enolonium species is formed as a discrete intermediate in one step followed by addition of the nucleophile in a second step. This protocol allows not only the classical type of functionalization such as chlorination⁶, but also the use of oxidizable carbon nucleophiles, such as allylsilanes^6,8, enolates^1,6,7, and electron rich aromatic compounds⁹, resulting in C-C bond formation. The allylation method is amenable to the formation of quaternary and tertiary centers. The ketone arylation method constitutes formal C-H functionalization of the aromatic compound without the need for a directing group⁹. Recently, we have reported the addition of azoles and azides¹⁰ as well¹¹. The detailed presentation of the protocol is expected to assist in the introduction of these methods into the day to day tool box of the synthetic organic chemist.

Protocol

1. Preparation of the Enolonium Species

Caution: Before carrying out the protocol, consult the MSDS for all reagents and solvents.

NOTE: All new reagents were used as received from the commercial source. If the boron trifluoride etherate has been stored, distill it before use.

1. In a dry round bottomed flask equipped with a septum and a magnet for magnetic stirring, add Koser's reagent (1.5 equiv.) and flush the flask with nitrogen or argon.
2. Add dry dichloromethane to give a suspension of 0.234 mol/L formal concentration.
3. Cool the suspension to -78 °C using a dry ice/acetone bath or a cold finger instrument/acetone bath.
4. Add neat BF₃OEt₂ (1.5 equiv.) slowly.
5. Warm the heterogeneous mixture to room temperature until the formation of yellow solution. Typically, this happens within 5 min.
6. Cool the solution to -78 °C.
7. To the cooled solution, add the trimethylsilyl-enolether (1 equiv., 0.313 mol/L) in dry dichloromethane dropwise over 2-10 min (depending on the scale). After the addition of silyl enol ether is complete, the formation of the enolonium species is complete.
   NOTE: The solution of enolonium species can be left at -78 °C for at least 30 min with no deterioration in yield. The enolonium species is stable during this time as indicated by reported NMR studies⁶.

2. Functionalization of the Enolonium Species

1. Chlorination with the chloride anion
   1. To the prepared solution of enolonium species, add benzyl-dimethyl-decylammonium chloride (2.0 equiv., 1.25 mol/L) in dry dichloromethane in a drop wise manner. Add this solution at a rate such that the temperature remains below -55 °C. At the 0.5-2 mmol scale, addition over 5 min is satisfactory.
   2. Leave the reaction mixture at -78 °C for 5 min.
   3. Remove the cooling bath and allow the reaction mixture to reach room temperature.
   4. Leave the reaction at room temperature for 20 min.
   5. Add water to the reaction mixture (half the volume of dichloromethane used in the preparation of the enolonium species).
   6. Extract thrice with dichloromethane. Typically, use 2-3 times the reaction volume in each extraction at the 0.5-2 mmol small scale.
   7. Wash the combined organic layers twice with brine. Typically, use the same volume of brine as the combined reaction volume.
   8. Dry with anhydrous sodium sulfate for 30 min.
   9. Filter off the sodium sulfate (e.g., through a Celite plug).
   10. Remove the solvent on a rotary evaporator at reduced pressure and at 40 °C.
   11. Purify the crude product by column chromatography on silica gel using hexane and ethyl acetate eluents to afford, after removal of the solvents, the pure corresponding α-azido ketone.
       Note: At scales of 0.5 mmol to 2 mmol of trimethylsilyl enolate, carry out column chromatography on a glass column of 2 cm diameter using standard silica gel 60 at a height (length) of 15 cm. The volume will need to be varied for other scales.
2. Azidation with TMS-Azide
   Caution: Organic-azides in general are explosive and care should be taken in handling and preparing the products. TMS-azide is toxic. Consult MSDS before use.
   1. To the prepared solution of enolonium species at -78 °C, add neat azidotrimethylsilane (2.5 equiv.) in a dropwise fashion. Add this solution at a rate so that the temperature remains below -55 °C. At the 0.5-2 mmol scale, addition over 2-3 min is satisfactory.
   2. Stir the reaction mixture for 15 min at -78 °C.
   3. Heat the reaction mixture to -55 °C and leave at this temperature for 2 to 3 h.
   4. Add water to the reaction mixture (half the volume of dichloromethane used in the preparation of the enolonium species).
   5. Extract thrice with dichloromethane. Typically, use 2-3 times the reaction volume in each extraction at the 0.5-2 mmol small scale.
   6. Wash the combined organic layers twice with brine. Typically, use the same volume of brine as the combined reaction volume.
   7. Dry the extracts with anhydrous sodium sulfate for 30 min.
   8. Filter off the sodium sulfate.
   9. Remove the solvent on a rotary evaporator at reduced pressure and at 40 °C.
   10. Purify the crude product by column chromatography on silica gel using hexane and ethyl acetate eluents to afford, after removal of the solvents, the pure corresponding α-azido ketone.
3. Reaction with azoles
   1. To the prepared solution of enolonium species at -78 °C, add azole (4 to 5 equiv., 1 mol/L) dissolved in 5 mL of dichloromethane in a dropwise fashion. At the 0.5-2 mmol scale, addition over 5 min is satisfactory.
      Note: In the case of poorly soluble azoles such as tetrazoles, use acetonitrile at a concentration of 0.5 mol/L instead of dichloromethane. Add this solution at a rate so that the temperature remains below -55 °C.
   2. Stir the reaction mixture for 15 min at -78 °C.
   3. Heat the reaction mixture to -55 °C and leave at this temperature for 4 to 8 h.
   4. Add water to the reaction mixture (half the volume of organic solvents used in the preparation of the enolonium species).
   5. Extract thrice with dichloromethane. Typically, use 2-3 times the reaction volume in each extraction at the 0.5-2 mmol small scale.
   6. Wash the combined organic layers twice with brine. Typically, use the same volume of brine as the combined reaction volume.
   7. Dry the extracts with anhydrous sodium sulfate for 30 min.
   8. Filter off the sodium sulfate.
   9. Remove the solvent on a rotary evaporator at reduced pressure and at 40 °C.
   10. Purify the crude product by column chromatography on silica gel using hexane and ethyl acetate eluents to afford, after removal of the solvents, the pure corresponding α-azole ketone.
4. Allylation, crotylation, cinnamylation, and prenylation using allyl-silanes
   1. Add neat allyl,-, crotyl-, cinnamyl-, or prenyl-trimethylsilane (2 equiv.) slowly at -78 °C. Add this solution at a rate so that the temperature remains below -55 °C. At a 0.5-2 mmol scale, addition over 2-3 min is satisfactory.
   2. Stir the reaction mixture for 10 min at -78 °C.
   3. Allow the reaction mixture to warm slowly to room temperature by removing the cooling bath. Leave the reaction at room temperature for 20 min.
   4. Add water to the reaction mixture (half the volume of dichloromethane used in the preparation of the enolonium species).
   5. Extract thrice with dichloromethane. Typically, use 2-3 times the reaction volume in each extraction at the 0.5-2 mmol small scale.
   6. Wash the combined organic layers twice with brine. Typically, use the same volume of brine as the combined reaction volume.
   7. Dry the extracts with anhydrous sodium sulfate for 30 min.
   8. Filter off the sodium sulfate.
   9. Remove the solvent on a rotary evaporator at reduced pressure and at 40 °C.
   10. Purify the crude product by column chromatography on silica gel using hexane and ethyl acetate eluents to afford, after removal of the solvents, the pure corresponding α-allyl product.
5. Arylation
   Note: For arylation, use 3 equivalents of BF₃OEt₂ during the preparation of the enolonium species in order to avoid tosylation of the enolonium species as a major side reaction. In general, only 1.6 equivalent of the aromatic substrate is needed. However, if the aromatic substrate is a pyrane, thiophene, or pyrrole, the best results are achieved using 5 equivalents of the aromatic substrate.
   1. To the solution of prepared enolonium species add a solution of aromatic substrate in dry dichloromethane (1.6 equiv., 0.5 mol/L) in a dropwise manner. Add this solution at a rate so that the temperature remains below -55 °C. At the 0.5-2 mmol scale, addition over 5-10 min is satisfactory.
   2. After the addition of the aromatic substrate is complete, increase the temperature of the mixture to -55 °C and leave the mixture at this temperature for 20 min.
   3. Add water to the reaction mixture (half the volume of dichloromethane used in the preparation of the enolonium species).
   4. Extract thrice with dichloromethane. Typically, use 2-3 times the reaction volume in each extraction at the 0.5-2 mmol small scale.
   5. Wash the combined organic layers twice with brine. Typically, use the same volume of brine as the combined reaction volume.
   6. Dry the extracts with anhydrous sodium sulfate for 30 min.
   7. Filter off the sodium sulfate.
   8. Remove the solvent on a rotary evaporator at reduced pressure and at 40 °C.
   9. Purify the crude product by column chromatography on silica gel using hexane and ethyl acetate eluents to afford, after removal of the solvents, the pure corresponding α-arylated ketone.

Results

Representative results, achieved following the protocol, are given in Figure 1 and are discussed in the discussion section. Notably, a very large range of different ketones may be used successfully in the reaction to give the products in good yields as may be seen for the azidation¹¹. The scope of the reaction for introducing azoles in the α-position of ketones includes most of the common mono-cyclic and bicyclic nitrogen containi...

Discussion

The successful preparation of enolonium species from TMS-enolates is dependent on a number of factors. The major side reaction in the preparation step is the homo coupling of the starting material by reaction of a molecule of formed enolonium species with a molecule of TMS-enolate. Thus, the requirement of the reaction conditions is to avoid this dimerization by ensuring fast reaction of the Lewis acid activated hypervalent iodine reagent with added TMS-enolate relative to the rate of dimerization. This is achieved in th...

Materials

Name	Company	Catalog Number	Comments
Chlorotrimethylsilane, 98+%	Alfa Aesar	A13651	TMS-Cl
Boron trifluoride diethyl etherate, 98+%	Alfa Aesar	A15275	BF3*Et2O
2-Methylindole, 98+%	Alfa Aesar	A10764	2-Me-indole
Hydroxy(tosyloxy) iodobenzene, 97%	Alfa Aesar	L15701	Koser's reagent
Acetophenone, >98%	Merck	800028
n-Butyllithium solution 1.6M in hexanes	Aldrich	186171	nBuLi
BIS(ISOPROPYL)AMINE	Apollo	OR1090	DIPA
Trimethylsilyl azide, 94%	Alfa Aesar	L00173	TMS-N3