Name: a two-step protocol for umpolung functionalization of ketones via enolonium species
Uploaded: 2018-08-16T13:30:00
Description: Watch this Scientific Journal Video about A Two-Step Protocol for Umpolung Functionalization of Ketones Via Enolonium Species at JoVE.com

A start-up grant from Ariel University and an ISF Individual Research Grant (1914/15) to AMS is gratefully acknowledged.

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.
<ol>
	<li>In a dry round bottomed flask equipped with a septum and a magnet for magnetic stirring, add Koser&#39;s reagent (1.5 equiv.) and flush the flask with nitrogen or argon.</li>
	<li>...

<ol>
	<li>Mizar, P., Wirth, T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Flexible+stereoselective+functionalizations+of+ketones+through+umpolung+with+hypervalent+iodine+reagents.">Flexible stereoselective functionalizations of ketones through umpolung with hypervalent iodine reagents.</a> Angewandte Chemie International Edition. 53 (23), 5993-5997 (2014).</li><li>Yoshimura, A., Zhdankin, V. V. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Advances+in+synthetic+applications+of+hypervalent+iodine+compounds.">Advances in synthetic applications of hypervalent iodine compounds.</a> Chemical Reviews. 116 (5), 3328-3435 (2016).</li><li>Zhdankin, V. V. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Hypervalent Iodine Chemistry: Preparation, Structure, and Synthetic Applications of Polyvalent Iodine Compounds. , (2013).</li><li>Wirth, T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Topics in Current Chemistry. 373, (2016).</li><li>Merritt, E. A., Olofsson, B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=&#945;-functionalization+of+carbonyl+compounds+using+hypervalent+iodine+reagents.">&#945;-functionalization of carbonyl compounds using hypervalent iodine reagents.</a> Synthesis. 4 (4), 517-538 (2011).</li><li>Arava, S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Enolonium+Species-Umpoled+Enolates.">Enolonium Species-Umpoled Enolates.</a> Angewandte Chemie International Edition. 56 (10), 2599-2603 (2017).</li><li>Parida, K. N., Maksymenko, S., Pathe, G. K., Szpilman, A. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Cross-Coupling+of+Dissimilar+Ketone+Enolates+via+Enolonium+Species+to+afford+Nonsymmetrical+1,4-Diketones.">Cross-Coupling of Dissimilar Ketone Enolates via Enolonium Species to afford Nonsymmetrical 1,4-Diketones.</a> Beilstein Journal of Organic Chemistry. 14, 992-997 (2018).</li><li>Zhdankin, V. V., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Carbon-carbon+bond+formation+in+reactions+of+PhIO&#183;HBF4-silyl+enol+ether+adduct+with+alkenes+or+silyl+enol+ethers.">Carbon-carbon bond formation in reactions of PhIO&#183;HBF4-silyl enol ether adduct with alkenes or silyl enol ethers.</a> Journal of Organic Chemistry. 54 (11), 2605-2608 (1989).</li><li>Maksymenko, S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Transition-metal-free+intermolecular+&#945;-arylation+of+ketones+via+enolonium+species.">Transition-metal-free intermolecular &#945;-arylation of ketones via enolonium species.</a> Organic Letters. 19 (23), 6312-6315 (2017).</li><li>Vita, M. V., Waser, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Azidation+of+&#946;-keto+esters+and+silyl+enol+ethers+with+a+benziodoxole+reagent.">Azidation of &#946;-keto esters and silyl enol ethers with a benziodoxole reagent.</a> Organic Letters. 15 (13), 3246-3249 (2013).</li><li>More, A., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=&#945;-N-Heteroarylation+and+&#945;-azidation+of+ketones+via+enolonium+species.">&#945;-N-Heteroarylation and &#945;-azidation of ketones via enolonium species.</a> Journal of Organic Chemistry. 83, 2442-2447 (2018).</li><li>Xie, L., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Gold-catalyzed+hydration+of+haloalkynes+to+&#945;-halomethyl+ketones.">Gold-catalyzed hydration of haloalkynes to &#945;-halomethyl ketones.</a> Journal of Organic Chemistry. 78 (18), 9190-9195 (2013).</li><li>Patonay, T., Juh&#225;sz-T&#243;th, &. #. 2. 0. 1. ;., B&#233;nyei, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Base-induced+coupling+of+&#945;-azido+ketones+with+aldehydes+&#8722;+An+easy+and+efficient+route+to+trifunctionalized+synthons+2-azido-3-hydroxy+ketones,+2-acylaziridines,+and+2-acylspiroaziridines.">Base-induced coupling of &#945;-azido ketones with aldehydes &#8722; An easy and efficient route to trifunctionalized synthons 2-azido-3-hydroxy ketones, 2-acylaziridines, and 2-acylspiroaziridines.</a> European Journal of Organic Chemistry. 2002 (2), 285-295 (2002).</li><li>Li, C., Breit, B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Rhodium-catalyzed+chemo-+and+regioselective+decarboxylative+addition+of+&#946;-ketoacids+to+allenes:+Efficient+construction+of+tertiary+and+quaternary+carbon+Centers.">Rhodium-catalyzed chemo- and regioselective decarboxylative addition of &#946;-ketoacids to allenes: Efficient construction of tertiary and quaternary carbon Centers.</a> Journal of the American Chemical Society. 136 (3), 862-865 (2014).</li></ol>

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...

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 &#945;-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 &#945;-halogenation, oxygenation, and amination1 as well as other reactions2,3,4,5.
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 allylsilanes6,8, enolates1,6,7, 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.

<table>
	<tbody>
		<tr>
			<td>Chlorotrimethylsilane, 98+%</td>
			<td>Alfa Aesar</td>
			<td>A13651</td>
			<td>TMS-Cl</td>
		</tr>
		<tr>
			<td>Boron trifluoride diethyl etherate, 98+%</td>
			<td>Alfa Aesar</td>
			<td>A15275</td>
			<td>BF3*Et2O</td>
		</tr>
		<tr>
			<td>2-Methylindole, 98+%</td>
			<td>Alfa Aesar</td>
			<td>A10764</td>
			<td>2-Me-indole</td>
		</tr>
		<tr>
			<td>Hydroxy(tosyloxy) iodobenzene, 97%</td>
			<td>Alfa Aesar</td>
			<td>L15701</td>
			<td>Koser&#39;s reagent</td>
		</tr>
		<tr>
			<td>Acetophenone, &#62;98%</td>
			<td>Merck</td>
			<td>800028</td>
			<td></td>
		</tr>
		<tr>
			<td>n-Butyllithium solution 1.6M in hexanes</td>
			<td>Aldrich</td>
			<td>186171</td>
			<td>nBuLi</td>
		</tr>
		<tr>
			<td>BIS(ISOPROPYL)AMINE</td>
			<td>Apollo</td>
			<td>OR1090</td>
			<td>DIPA</td>
		</tr>
		<tr>
			<td>Trimethylsilyl azide, 94%</td>
			<td>Alfa Aesar</td>
			<td>L00173</td>
			<td>TMS-N3</td>
		</tr>
	</tbody>
</table>

a two-step protocol for umpolung functionalization of ketones via enolonium species

&#945;-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&#8211;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.

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 &#945;-position of ketones includes most of the common mono-cyclic and bicyclic nitrogen containi...

Watch this Scientific Journal Video about A Two-Step Protocol for Umpolung Functionalization of Ketones Via Enolonium Species at JoVE.com

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

A two step one-pot protocol for the umpolung of ketone enolates to enolonium species and addition of a nucleophile to the &#945;-position is described. Nucleophiles include chloride, azide, azoles, allyl-silanes, and aromatic compounds.

&#945;-Functionalization of ketones via umpolung of enolates by hypervalent iodine reagents is an important concept in synthetic organic chemistry. ...

Umpolung of ketone enolates by virtue of reversing natural activity, enables the development of unprecedented reactions to functionalized carbon neo-compounds. The key is the formation of electrophilic enolonium species as a discreet species. A main advantage of methods based on electrophilic enolonium species is that it obviates the need for older multiple step methods, allows the use of commercial starting materials and is simple to use.We recently had the idea for this method when we were able to characterize the electrophilic enolonium species for the first time. Over the last year, we have developed a series of new reactions based on the two-step protocol presented here. Today, we will describe how enolonium species made in the first step can be used in in situ coupling with allyl-silanes, aromatic compounds, chloride, azide and azoles.Keep in mind that the implications of this technique extend toward other reactions, because the type of nucleophile that may be used is much less restricted than earlier hypervalent iodine-based methods. Demonstrating the procedure will be Shlomy Arava, a graduate student in our group. In a dry round-bottomed flask equipped with a septum and a magnetic stir bar, add 1.5 equivalents of Koser's reagent and flush the flask with nitrogen or argon.Add dry dichloromethane to give a formal concentration of 0.234 millimolars per liter. Cool the resulting suspension to minus 78 degrees Celsius using a dry ice acetone bath. Then, add 1.5 equivalents of neat boron trifluoride etherate slowly.Warm the heterogeneous mixture to room temperature until it becomes a yellow solution, which typically occurs within five minutes. Following this, cool the solution to minus 78 degrees Celsius. To the cooled solution, add one equivalent of previously prepared trimethylsilyl ether solution drop-wise over two to 10 minutes to afford the enolonium species.This is the most critical step of the protocol. If the TMS-enolate ether is added too fast, dimerization products form. Add two equivalents of a previously prepared allyl trimethylsilane solution slowly to the prepared solution of enolonium species at minus 78 degrees Celsius with stirring.After five minutes, remove the cooling bath and allow the reaction mixture to reach room temperature. Once the reaction is complete, add water to the reaction mixture and transfer to a separatory funnel. Then, extract three times with dichloromethane, using two to three times the reaction volume for each extraction.Next, wash the combined organic layers twice with brine using the same volume of brine as the combined reaction volume. After transferring the combined organic layers to a flask, dry with anhydrous sodium sulfate for 30 minutes. Following this, remove the sodium sulfate by filtration.Then, remove the solvent on a rotary evaporator under reduced pressure at 40 degrees Celsius. Purify the crude product by column chromatography on silica gel using hexane and ethyl acetate as the eluant to afford the pure corresponding alpha allyl ketone. For the benzyldimethyldecylammonium chloride reaction, add two equivalents of a previously prepared chloride solution drop-wise to the prepared solution of enolonium species at minus 78 degrees Celsius with stirring.After five minutes, remove the cooling bath and allow the reaction mixture to reach room temperature. For azedation, add 2.5 equivalents of neat azedo trimethylsilane drop-wise to the prepared solution of enolonium species at minus 78 degrees Celsius with stirring. After 15 minutes, heat the reaction mixture to minus 55 degrees Celsius and stir for two to three hours until the reaction is complete by TLC.For the azole reaction, add four to five equivalents of a previously prepared azole solution drop-wise to the prepared solution of enolonium species at minus 78 degrees Celsius with stirring. After 15 minutes, heat the reaction mixture to minus 55 degrees Celsius and stir for four to eight hours until the reaction is complete by TLC. For arylation, add 1.6 equivalents of a previously prepared aromatic substrate solution drop-wise to the solution of prepared enolonium species at minus 78 degrees Celsius with stirring.After the addition of the aromatic substrate is complete, increase the temperature of the mixture to minus 55 degrees Celsius and stir for 20 minutes until the reaction is complete by TLC. A very large range of different ketones may be used successfully in the alpha functionalization reaction to give the products in good yields, as observed for the azidation. The scope of the reaction for introducing azoles in the alpha position of ketones includes most of the common monocyclic and bicyclic nitrogen-containing heterocycles.The scope of the allylation procedure includes allyl-crotyl-and prenyl-trimethylsilane. The C-aryllation procedure works for both indoles and electron-rich benzene derivatives using only 1.5 to 1.6 equivalents of the aromatic substrate. Thiophene, furane and pyrroles are also good substrates but the products are isolated in slightly lower yields and five equivalents of aromatic substrate are required.Once mastered, both steps of the protocol can be done in less than five hours for most of the reactions and overnight for the azole coupling. While attempting this procedure, it's important to remember to add the TMS-enolate slowly when making enolorium species to avoid dimerization of the enolate as a side reaction. Don't forget that working with azides can be hazardous and appropriate precautions such as limiting the scale should always be taken.Following the allylation procedure, you can add the prenyl group to make a quaternary center which is something that cannot be done using other methods. Using the arylation procedure means that you don't need to worry about using a transition metal catalyst or a halogenated aromatic starting material. Just use a aromatic substrate.After watching this video, we hope that the enolate umpolung will be a familiar concept to you and that you'll be tempted to develop new reactions of your own.

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