Name: controlled photoredox ring-opening polymerization of o-carboxyanhydrides mediated by ni/zn complexes
Uploaded: 2017-11-21T15:00:00.000+00:00
Duration: 5 min 48 s
Description: Watch this Scientific Journal Video about Controlled Photoredox Ring-Opening Polymerization of O-Carboxyanhydrides Mediated by Ni/Zn Complexes at JoVE.com

This work was supported by start-up funding from Virginia Polytechnic Institute and State University. Q.F. acknowledges support from National Natural Science Foundation of China (21504047), Natural Science Foundation of Jiangsu Province (BK20150834), Nanjing University of Posts and Telecommunications Scientific Foundation NUPTSF (NY214179).

Caution: Please consult all relevant materials safety data sheets (MSDS) before use. Many chemicals used in the synthesis are acutely toxic and carcinogenic. Please use all appropriate safety practices when performing reaction including the use of engineering controls (fume hood and glovebox) and personal protective equipment (safety glasses, gloves, lab coat, full-length pants, closed-toe shoes, blue-light blocking safety goggles). Following procedures involve standard air-free handling techniques in a glove box. All solutions are transferred using pipette.
1. Preparation of Catalyst and Initiator
NOTE: The whole process is conducted in a glove box with a cold trap. All chemicals are dried or purified before moving into the box.13 All vials and glassware are dried and heated in the oven before moving into the box.
<ol>
	<li>Preparation of (bpy)Ni(COD) solution 
	NOTE: (bpy)Ni(COD) should be freshly prepared in situ. It should be stored in the glove box freezer (-35 &#176;C) and used within one week. All other catalysts and initiator solutions can be stored in the glove box freezer over 1 month.
	<ol>
		<li>Weigh Ni(COD)2 (3.5 mg, 12.7 &#181;mol, 1.0 eq) into a 7-mL scintillation vial.</li>
		<li>Weigh 2,2&#8242;-bipyridine (5.9 mg, 37.8 &#181;mol) into a 7-mL scintillation vial.</li>
		<li>Dissolve the 2,2&#8242;-bipyridine in 1 mL of anhydrous tetrahydrofuran (THF).</li>
		<li>Add resulting 2,2&#8242;-bipyridine solution (337 &#181;L) into the vial containing Ni(COD)2.</li>
		<li>Dilute the mixture in 1 mL of anhydrous THF.</li>
		<li>Cap the vial and place the reaction mixture at room temperature for 2 h. 
		NOTE: Ni(COD)2 is not soluble in THF, whereas (bpy)Ni(COD) is soluble in THF. There should be no precipitation in the purple solution after 2 h.</li>
		<li>Store the (bpy)Ni(COD) in the -35 &#176;C freezer.</li>
	</ol>
	</li>
	<li>Preparation of Zn(HMDS)2 solution
	<ol>
		<li>Add Zn(HMDS)2 (3.3 mg, 4 &#181;L, 8.5 &#181;mol) to a 7-mL scintillation vial.</li>
		<li>Dissolve the Zn(HMDS)2 in 1 mL of anhydrous THF.</li>
		<li>Store Zn(HMDS)2 solution in the -35 &#176;C freezer.</li>
	</ol>
	</li>
	<li>Preparation of BnOH solution
	<ol>
		<li>Add BnOH (4.0 mg, 4 &#181;L, 37.0 &#181;mol) to a 7-mL scintillation vial.</li>
		<li>Dissolve the BnOH in 4 mL of anhydrous THF.</li>
		<li>Store the BnOH solution in the -35 &#176;C freezer.</li>
	</ol>
	</li>
	<li>Preparation of Ir-1 solution
	<ol>
		<li>Turn off the glove box light. 
		Caution: It is necessary to turn off the glove box light to avoid deactivation of Ir-1 before polymerization.</li>
		<li>Add Ir-1 (2.9 mg, 2.6 &#181;mol) to a 7-mL scintillation vial.</li>
		<li>Dissolve the Ir-1 in 3 mL of anhydrous THF.</li>
		<li>Store the Ir-1 solution in the -35 &#176;C freezer.</li>
	</ol>
	</li>
</ol>
2. Photoredox ring-opening polymerization of l-1
NOTE: The whole process is conducted in a glove box with a cold trap. All OCA monomers are recrystallized in the glove box before use.13 Here we give the example of the polymerization at DP=500 ([l -1]/[(bpy)Ni(COD)]/[Zn(HMDS)2]/[BnOH]/[Ir-1] = 500/1/1/1/0.1). Polymers at different DPs can also be prepared by adjusting the monomer mass accordingly.
<ol>
	<li>Preparation of l-1 solution for polymerization
	<ol>
		<li>Add l-1 (72.2 mg, 375.7 &#181;mol) to a 7-mL scintillation vial.</li>
		<li>Dissolve the l-1 in 722 &#181;L of anhydrous THF.</li>
		<li>Add 200 &#181;L of l-1 solution into another 7-mL scintillation vial equipped with a stir bar.</li>
		<li>Add 100 &#181;L of anhydrous THF into the 7-mL scintillation vial.</li>
		<li>Cap the vial and place the l-1 solution in the cold trap.</li>
		<li>Turn off the glove box light. 
		Caution: It is necessary to turn off the glove box light to avoid deactivation of Ir-1 before polymerization.</li>
	</ol>
	</li>
	<li>Cooling down the cold trap
	<ol>
		<li>Inside the box, put a thermometer into the cold trap.</li>
		<li>Outside of the box, add ca. 500 mL of ethanol into the dewar flask.</li>
		<li>Add liquid nitrogen into the dewar flask.</li>
		<li>Load the KGW dewar flask to the cold trap.</li>
		<li>Cool the cold trap to -50 &#176;C.</li>
		<li>Put a stir plate underneath the cold trap (see Figure 1).</li>
	</ol>
	</li>
	<li>Perform the photoredox ring-opening polymerization 
	NOTE: Before starting the polymerization, place safety goggles that block blue light within reach. All catalyst system solutions are immediately taken out from the freezer and added successively into the 7-mL scintillation vial containing 20 mg of l-1 without pause or disruption over 30 s.
	<ol>
		<li>Add (bpy)Ni(COD) solution (16.4 &#181;L, 0.208 &#181;mol) into the 7-mL scintillation vial containing l-1 (prepared in 2.1).</li>
		<li>Add Zn(HMDS)2 solution (24.4 &#181;L, 0.208 &#181;mol) into the vial.</li>
		<li>Add BnOH solution (22.5 &#181;L, 0.208 &#181;mol) into the vial.</li>
		<li>Add Ir-1 solution (24.2 &#181;L, 0.0208 &#181;mol) into the vial and cap the vial.</li>
		<li>Wear safety goggles blocking blue LED light. 
		Caution: As blue LED light with a relative high intensity is used, wear safety goggles during the whole process.</li>
		<li>Turn on the blue LED light (34 W) and the fan to dissipate the heat generated by the LED light. Direct the light towards the vial in the cold trap. (Figure 1)</li>
		<li>Turn on the stirrer.</li>
		<li>Cover the cold trap with aluminum foil.</li>
		<li>Keep the reaction temperature at -15 &#177; 5 &#176;C and add liquid nitrogen every 15-20 minutes. 
		NOTE: The maintenance of reaction temperature is important for polymerization and MW control.</li>
	</ol>
	</li>
</ol>
3. Monitor the monomer conversion by Fourier-transform infrared spectra
NOTE: Fourier-transform infrared spectra (FTIR) are recorded on an FT-IR spectrometer equipped with Diamond ATR and Transmission sampling accessory.
<ol>
	<li>At specific time points, add a small aliquot of polymer solution (20 &#181;L) into a 7-mL scintillation vial, and capped.</li>
	<li>Remove the vial out of the glove box.</li>
	<li>Immediately drop the solution (3 &#181;L) onto the FTIR-ATR diamond sampler. The solution forms a film within 10 s for the spectrum measurement.</li>
	<li>Measure the FTIR spectrum of the sample. 
	NOTE: The monomer conversion was determined by the intensity ratio between 1760 cm-1 and 1805s cm-1: conversion% = I1760/(I1760 + I1805) (Representative results in Figure 2). Generally, the polymerization takes about 1-8 h for DP ranging from 200 to 1000 (detailed kinetics are discussed in reference 13).</li>
</ol>
4. The measurement of polymer&#39;s molecular weight by gel-permeation chromatography
NOTE: Gel-permeation chromatography (GPC) experiments are performed on a system equipped with an isocratic pump with degasser, multiangle laser light scattering (MALS) detector (GaAs 30 mW laser at &#955; = 690 nm), and differential refractive index (DRI) detector with a 690-nm light source. Separations are performed using serially connected size exclusion columns (100 &#197;, 500 &#197;, 103 &#197;, and 104 &#197; columns, 5 &#181;m, 300 &#215; 4.6 mm) at 35 &#176;C using THF as the mobile phase at a flow rate of 0.35 mL/min. The polymer molecular weight (MW) and molecular weight distribution (&#208;) are determined using the Zimm model fit of MALS-DRI data. The presence of metal complex in the polymer does not affect the GPC measurement results.
<ol>
	<li>Remove a small aliquot of polymer solution (50 &#181;L) out of the glove box.</li>
	<li>Add 100 &#181;L of THF into the vial.</li>
	<li>Inject the sample into the GPC sampler.</li>
	<li>Analyze the result once the GPC run is completed. 
	NOTE: The polymer should be monodispersed with narrow &#272; (Representative results in Figure 3). The polymer (20 mg) can be dried and washed by 1 mL diethyl ether containing 1% HOAc and 1 mL methanol, which can remove 87% of Ni and 50% of Zn complexes, determined by inductively coupled plasma mass spectrometry (ICP-MS).</li>
</ol>

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The authors declare no competing financial interests. A provisional patent (U.S. Patent Application No: 62/414,016) has been filed pertaining to the results presented in this paper.

The critical step within the protocol is maintaining the reaction temperature at -15 &#177; 5 &#176;C. All catalysts solutions and OCA monomers have to be stored in a glove box freezer at -35 &#176;C before the polymerization. The reaction vials have to be pre-cooled in the cold trap. During the reaction, because the LED light dissipates heat, it is necessary to monitor the reaction every 15 - 20 minutes. Once the temperature is raised up to -10 &#176;C, liquid nitrogen should be added into the dewar to cool the trap. The reason for the low temperature is the formation of the Ni(CO) complex at room temperature, which is detrimental to the controlled photoredox polymerization and affects the MW and &#272;.13
Synthesis attempts from O-carboxyanhydrides with pendent functional groups by organocatalysts have been plagued by uncontrolled polymerization including epimerization, which hampers the preparation of stereoregular high-MW polymers. This photoredox controlled ROP polymerization can successfully prepare stereoregular high-MW polymers with DP reaching 1000 for various OCA monomers, which are documented in reference 13. The copolymerization of different OCA monomers by sequential addition is also undemanding using our methods. However, for L-mandelic OCA monomers, the MW control has not been achieved at high DPs. We are currently investigating this methodology and trying to develop a new catalyst strategy for the polymerization.
In conclusion, our Ni/Zn-mediated photoredox polymerization protocol offers a strategy for achieving rapid, controlled OCA polymerization to prepare stereoregular poly(&#945;-hydroxy acids) and their copolymers bearing various functional side-chain groups. We expect that our new strategy allows for the generation of new polyesters with desirable macroscopic properties such as rigidity, elasticity, and biodegradability. This method also can be useful for new fabrication techniques such as light-cured nanoimprinting lithography.

Poly(&#945;-hydroxy acid) (PAHA) is an important class of biodegradable and biocompatible polymers with applications ranging from biomedical devices to packaging materials.1,2 Although PAHAs can be prepared directly by polycondensation of &#945;-hydroxy acids, the molecular weights (MWs) of the resulting PAHAs are generally low.3 Ring-opening polymerization (ROP) of lactones (e.g., lactide and glycolide) is an alternative synthetic approach that provides the better control on MWs and molecular weight distribution (&#272;) than polycondensation. However, the lack of side-chain functionality in PAHAs and in lactones limit the diversity of physical and chemical properties and their applications.4,5 Since 2006, 1,3-dioxolane-2,4-diones, so-called O-carboxyanhydrides (OCAs), which can be prepared with a rich variety of side-chain functionalities,6,7,8,9,10,11,12,13 have emerged as an alternative class of highly active monomers for polyester polymerization.14,15
Catalytic systems for the ROP of OCAs can be categorized into organocatalysts,8,12,16,17 organometallic catalysts12,18,19,20,21 and biocatalysts.22 Generally, the ROP of OCAs promoted by organocatalyst proceeds in a more or less uncontrolled manner, such as epimerization (i.e., lack of stereoregularity) for OCAs bearing electron-withdrawing groups,8,17 unpredictable MWs, or slow polymerization kinetics.13 To address these problems, an active Zn-alkoxide complex was developed for the ROP of OCAs.12 Well-controlled ROPs were achieved at a low degree of polymerization (DP) without epimerization. However, this Zn-alkoxide catalyst cannot efficiently produce polymers with a high degree of polymerization (DP &#8805; 300).13
We have recently reported a promising approach that greatly improves customizability and efficiency of PAHA synthesis (Figure 1).13 We merge photoredox Ni/Ir catalysts that promote OCA decarboxylation with zinc alkoxide to mediate ring-opening polymerization of OCAs. The use of low temperature (-15 &#176;C) and photoredox Ni/Ir catalysis synergistically accelerates ring-opening and decarboxylation of OCA for chain propagation while avoiding undesired side reactions, e.g., the formation of Ni-carbonyl.23,24 Upon transmetalation with Ni complex the active Zn-alkoxide is located at chain terminus for chain propagation.13
In this protocol, we add fresh prepared (bpy)Ni(COD) (bpy = 2,2&#39;-bipyridyl, COD = 1,5-cyclooctadiene), Zn(HMDS)2 (HMDS = hexamethyldisilazane),25 benzyl alcohol (BnOH) and Ir[dF(CF3)ppy]2(dtbbpy)PF6 (Ir-1, dF(CF3)ppy = 2-(2,4-difluorophenyl)-5-(trifluoromethyl)pyridine, dtbbpy = 4,4&#39;-di-tert-butyl-2,2&#39;-bipyridine) into the monomer l-1 solution26 in a glove box with a cold trap, in the presence of a blue LED light (400-500 nm) and a fan to maintain temperature (Figure 1). The temperature is kept at -15 &#176;C &#177; 5 &#176;C during the polymerization. The conversion of OCA is monitored by Fourier-transform infrared spectroscopy. The resulting polymer MWs and &#272;s is characterized by a gel permeation chromatography (GPC). The homodecoupling 1H NMR study determines whether the obtained polymer is isotactic or not. As most chemicals are highly sensitive to moisture, the detailed video protocol is intended to help new practitioners avoid pitfalls associated with the photoredox ROP of OCAs.

<table><tbody><tr><td>Ni(COD)&lt;sub&gt;2&lt;/sub&gt;</td><td>Strem</td><td>28-0010</td><td>Stored in the glove box freezer.</td></tr><tr><td>2,2&amp;prime;-bipyridine</td><td>Strem</td><td>07-0290</td><td>Stored in the glove box freezer.</td></tr><tr><td>Zn(HMDS)&lt;sub&gt;2&lt;/sub&gt;</td><td>N/A</td><td>N/A</td><td>Synthesized following reported procedures.&lt;sup class=&quot;xref&quot;&gt;25&lt;/sup&gt; Stored in the glove box freezer.</td></tr><tr><td>Benzyl alcohol</td><td>Sigma-Aldrich</td><td>402834</td><td>Stored with 4&amp;Aring; molecular sieve</td></tr><tr><td>Ir[dF(CF&lt;sub&gt;3&lt;/sub&gt;)ppy]&lt;sub&gt;2&lt;/sub&gt;(dtbbpy)PF&lt;sub&gt;6&lt;/sub&gt;</td><td>Strem</td><td>77-0425</td><td>Stored in the glove box freezer.</td></tr><tr><td>THF</td><td>Sigma-Aldrich</td><td>34865</td><td>Dried by alumina columns and stored with 4&amp;Aring; molecular sieve in the dark bottle in the glove box.</td></tr><tr><td>Ethanol</td><td>Sigma-Aldrich</td><td>793175</td><td></td></tr><tr><td>GPC with an isocratic pump</td><td>Agilent</td><td>Agilent 1260 series</td><td></td></tr><tr><td>Dawn Heleos II Light Scatterer</td><td>Wyatt</td><td></td><td></td></tr><tr><td>Optilab rEX differential refractive index detector</td><td>Wyatt</td><td></td><td></td></tr><tr><td>Size exclusion columns</td><td>Phenomenex</td><td></td><td></td></tr><tr><td>Glass Scintillation Vials - 7 ml</td><td>VWR</td><td></td><td></td></tr><tr><td>FTIR spectrometer</td><td>Agilent</td><td></td><td></td></tr><tr><td>Stir bars</td><td>VWR</td><td>58948-091</td><td></td></tr><tr><td>Balance</td><td></td><td></td><td></td></tr><tr><td>Glove box</td><td>Mbraun</td><td>Labstar Pro</td><td></td></tr></tbody></table>

controlled photoredox ring-opening polymerization of o-carboxyanhydrides mediated by ni/zn complexes

Here, we describe an effective protocol that combines photoredox Ni/Ir catalysis with the use of a Zn-alkoxide for efficient ring-opening polymerization, allowing for the synthesis of isotactic poly(&#945;-hydroxy acids) with expected molecular weights (&#62;140 kDa) and narrow molecular weight distributions (Mw/Mn &#60; 1.1). This ring-opening polymerization is mediated by Ni and Zn complexes in the presence of an alcohol initiator and a photoredox Ir catalyst, irradiated by a blue LED (400 - 500 nm). The polymerization is performed at a low temperature (-15 &#176;C) to avoid undesired side reactions. The complete monomer consumption can be achieved within 4 - 8 hours, providing a polymer close to the expected molecular weight with narrow molecular weight distribution. The resulted number-average molecular weight shows a linear correlation with the degree of polymerization up to 1000. The homodecoupling 1H NMR study confirms that the obtained polymer is isotactic without epimerization. This polymerization reported herein offers a strategy for achieving rapid, controlled O-carboxyanhydrides polymerization to prepare stereoregular poly(&#945;-hydroxy acids) and its copolymers bearing various functional side-chain groups.

The conversion of OCA is monitored by Fourier-transform infrared spectroscopy, as shown in Figure 2. The peak at 1805 cm-1 is assigned as the anhydride bond stretch in OCA; the peak at 1760 cm-1 corresponds to the formation of the ester bond in the polymer. Once the monomer&#39;s peak at 1805 cm-1 completely disappears, the polymerization is finished.
The MW and &#272; of the resulting polymer is characterized by a gel permeation chromatography. Figure 3 shows controlled photoredox ring-opening polymerization of OCAs with DP ranging from 200 to 1000. Increasing the monomer feed ratio ([l-1]/[Zn(HMDS)2] ratio) leads to an increased and expected Mn of the resulting polymer. Besides, Mn of the polymers increases linearly with initial [l-1]/[Ni]/[Zn]/[Ir-1] ratio up to 1000/1/1/0.1, and all of the &#272; values are &#60; 1.1.
The NMR studies measure the stereochemistry of obtained polymer. The ROP of OCAs mediated by organocatalyst such as dimethylaminopyridine can induce epimerization on the &#945;-methine for OCAs bearing electron-withdrawing groups.8,17 The homodecoupling 1H NMR of these polymers exhibited multiple peaks in &#945;-methine region, indicating the loss of the stereoregularity in the polymerization. Using our method, the homodecoupling 1H NMR study shows single peak at &#945;-methine region (5.0 - 5.3 ppm), indicating that the obtained polymer is isotactic without epimerization (Figure 4).
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/56654/56654fig1.jpg" /> 
Figure 1. Scheme of Ni/Zn-mediated photoredox ROP of l-1. The photoredox polymerization is conducted in a glove box with cold trap, irradiated by LED light with a cooling fan to maintain the temperature. <a href="https://cloudflare.jove.com/files/ftp_upload/56654/56654fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/56654/56654fig2.jpg" /> 
Figure 2. FTIR spectra of (a) l-1 and (b) the reaction mixture during the photoredox polymerization. <a href="https://cloudflare.jove.com/files/ftp_upload/56654/56654fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/56654/56654fig3.jpg" /> 
Figure 3. (a) Plots of Mn and molecular weight distribution (Mw/Mn) of poly(l-1) versus [l-1]/[Zn(HMDS)2] ratio ([(bpy)Ni(COD)]/[Zn(HMDS)2]/[BnOH]/[Ir-1] = 1/1/1/0.1). (b) Representative gel-permeation chromatography (GPC) traces of the photoredox polymerization reaction in panel (a). <a href="https://cloudflare.jove.com/files/ftp_upload/56654/56654fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" class="xfigimg" src="/files/ftp_upload/56654/56654fig4.jpg" /> 
Figure 4. NMR spectra of poly(l-1). (a) 1H NMR spectrum; (b) 1H homodecoupling NMR spectrum. <a href="https://cloudflare.jove.com/files/ftp_upload/56654/56654fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about Controlled Photoredox Ring-Opening Polymerization of O-Carboxyanhydrides Mediated by Ni/Zn Complexes at JoVE.com

Controlled Photoredox Ring-Opening Polymerization of O-Carboxyanhydrides Mediated by Ni/Zn Complexes

A protocol for the controlled photoredox ring-opening polymerization of O-carboxyanhydrides mediated by Ni/Zn complexes is presented.

Controlled Photoredox Ring-Opening Polymerization of O-Carboxyanhydrides Mediated by Ni/Zn Complexes

Here, we describe an effective protocol that combines photoredox Ni/Ir catalysis with the use of a Zn-alkoxide for efficient ring-opening polymerization, ...

controlled-photoredox-ring-opening-polymerization-o-carboxyanhydrides

Nanyang Technological University

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8.0K Views.  Nanjing University of Posts and Telecommunications. A protocol for the controlled photoredox ring-opening polymerization of O-carboxyanhydrides mediated by Ni/Zn complexes is presented.

Video: Controlled Photoredox Ring-Opening Polymerization of O-Carboxyanhydrides Mediated by Ni/Zn Complexes

Ethylene Polymerizations Using Parallel Pressure Reactors and a Kinetic Analysis of Chain Transfer Polymerization

We demonstrate a method for high-throughput catalyst screening using a parallel pressure reactor starting from the initial synthesis of a nickel &#945;-diimine ethylene polymerization catalyst. Initial polymerizations with the catalyst lead to optimized reaction conditions, including catalyst concentration, ethylene pressure and reaction time. Using gas-uptake data for these reactions, a procedure to calculate the initial rate of propagation (kp) is presented. Using the optimized conditions, the ability of the nickel &#945;-diimine polymerization catalyst to undergo chain transfer with diethylzinc (ZnEt2) during ethylene polymerization was investigated. A procedure to assess the ability of the catalyst to undergo chain transfer (from molecular weight and 13C NMR data), calculate the degree of chain transfer, and calculate chain transfer rates (ke) is presented.

A protocol for the high-throughput analysis of polymerization catalyst, chain transfer polymerizations, polyethylene characterization, and reaction kinetic analysis is presented.

We demonstrate a method for high-throughput catalyst screening using a parallel pressure reactor starting from the initial synthesis of a nickel ...

Atom Transfer Radical Polymerization of Functionalized Vinyl Monomers Using Perylene as a Visible Light Photocatalyst

A standardized technique for atom transfer radical polymerization of vinyl monomers using perylene as a visible-light photocatalyst is presented. The procedure is performed under an inert atmosphere using air- and water-exclusion techniques. The outcome of the polymerization is affected by the ratios of monomer, initiator, and catalyst used as well as the reaction concentration, solvent, and nature of the light source. Temporal control over the polymerization can be exercised by turning the visible light source off and on. Low dispersities of the resultant polymers as well as the ability to chain-extend to form block copolymers suggest control over the polymerization, while chain end-group analysis provides evidence supporting an atom-transfer radical polymerization mechanism.

A method for the atom transfer radical polymerization of functionalized vinyl monomers using perylene as a visible-light photocatalyst is described.

A standardized technique for atom transfer radical polymerization of vinyl monomers using perylene as a visible-light photocatalyst is presented. The ...

Solid-phase Synthesis of [4.4] Spirocyclic Oximes

A convenient synthetic route for spirocyclic heterocycles is well sought after due to the molecule&#39;s potential use in biological systems. By means of solid-phase synthesis, regenerating Michael (REM) linker strategies, and 1,3-dipolar cycloaddition, a library of structurally similar heterocycles, both with and without a spirocyclic center, can be constructed. The main advantages of the solid-support synthesis are as follows: first, each reaction step can be driven to completion using a large excess of reagents resulting in high yields; next, the use of commercially available starting materials and reagents keep the costs low; finally, the reaction steps are easy to purify via simple filtration. The REM linker strategy is attractive because of its recyclability and traceless nature. Once a reaction scheme is completed, the linker can be reused multiple times. In a typical solid-phase synthesis, the product contains either a part of or the whole linker, which can prove undesirable. The REM linker is &#34;traceless&#34; and the point of attachment between the product and the polymer is indistinguishable. The high diastereoselectivity of the intramolecular 1,3-dipolar cycloaddition is well documented. Limited by the insolubility of the solid support, the reaction progression can only be monitored by a change in the functional groups (if any) via infrared (IR) spectroscopy. Thus, the structural identification of intermediates cannot be characterized by conventional nuclear magnetic resonance (NMR) spectroscopy. Other limitations to this method stem from the compatibilities of the polymer/linker to the desired chemical reaction scheme. Herein we report a protocol that allows for the convenient production of spirocyclic heterocycles that, with simple modifications, can be automated with high-throughput techniques.

Here we present a protocol to demonstrate an efficient method for the synthesis of spirocyclic heterocycles. The five-step process utilizes solid-phase synthesis and regenerating Michael linker strategies. Generally difficult to synthesize, we present a customizable method for the synthesis of spirocyclic molecules otherwise inaccessible to other modern approaches.

A convenient synthetic route for spirocyclic heterocycles is well sought after due to the molecule&#39;s potential use in biological systems. By means of ...

Photogeneration of N-Heterocyclic Carbenes: Application in Photoinduced Ring-Opening Metathesis Polymerization

We report a method to generate the N-heterocyclic carbene (NHC) 1,3-dimesitylimidazol-2-ylidene (IMes) under UV-irradiation at 365 nm to characterize IMes and determine the corresponding photochemical mechanism. Then, we describe a protocol to perform ring-opening metathesis polymerization (ROMP) in solution and in miniemulsion using this NHC-photogenerating system. To photogenerate IMes, a system comprising 2-isopropylthioxanthone (ITX) as the sensitizer and 1,3-dimesitylimidazolium tetraphenylborate (IMesH+BPh4-) as the protected form of NHC is employed. IMesH+BPh4- can be obtained in a single step by anion exchange between 1,3-dimesitylimidazolium chloride and sodium tetraphenylborate. A real-time steady-state photolysis setup is described, which hints that the photochemical reaction proceeds in two consecutive steps: 1) ITX triplet is photo-reduced by the borate anion and 2) subsequent proton transfer takes place from the imidazolium cation to produce the expected NHC IMes. Two separate characterization protocols are implemented. Firstly, CS2 is added to the reaction media to evidence the photogeneration of NHC through formation of the IMes-CS2 adduct. Secondly, the amount of NHC released in situ is quantified using acid-base titration. The use of this NHC photo-generating system for the ROMP of norbornene is also discussed. In solution, a photopolymerization experiment is conducted by mixing ITX, IMesH+BPh4-, [RuCl2(p-cymene)]2 and norbornene in CH2Cl2, then irradiating the solution in a UV reactor. In a dispersed medium, a monomer miniemulsion is first formed then irradiated inside an annular reactor to produce a stable poly(norbornene) latex.

We describe a protocol to photogenerate N-heterocyclic carbenes (NHCs) by UV irradiation of a 2-isopropylthioxanthone/imidazolium tetraphenylborate salt system. Methods to characterize the photoreleased NHC and elucidate the photochemical mechanism are proposed. The protocols for ring-opening metathesis photopolymerization in solution and miniemulsion illustrate the potential of this 2-component NHC photogenerating system.

We report a method to generate the N-heterocyclic carbene (NHC) 1,3-dimesitylimidazol-2-ylidene (IMes) under UV-irradiation at 365 nm to characterize IMes ...

[(DPEPhos)(bcp)Cu]PF6: A General and Broadly Applicable Copper-Based Photoredox Catalyst

Our group recently reported the use of [(DPEPhos)(bcp)Cu]PF6 as a general copper-based photoredox catalyst which proved efficient to promote the activation of a broad variety of organic halides, including unactivated ones. These can then participate in various radical transformations such as reduction and cyclization reactions, as well as in the direct arylation of several (hetero)arenes. These transformations provide a straightforward access to a range of small molecules of interest in synthetic chemistry, as well as to biologically active natural products. Altogether, [(DPEPhos)(bcp)Cu]PF6 acts as a convenient photoredox catalyst which appears to be an attractive, cheap and complementary alternative to the state-of-the-art iridium- and ruthenium-based photoredox catalysts. Here, we report a detailed protocol for the synthesis of [(DPEPhos)(bcp)Cu]PF6, as well as NMR and spectroscopic characterizations, and we illustrate its use in synthetic chemistry for the direct arylation of (hetero)arenes and radical cyclization of organic halides. In particular, the direct arylation of N-methylpyrrole with 4-iodobenzonitrile to afford&#160;4-(1-methyl-1H-pyrrol-2-yl)benzonitrile and the radical cyclization of N-benzoyl-N-[(2-iodoquinolin-3-yl)methyl]cyanamide to afford natural product luotonin A are detailed. The scope and limitations of this copper-based photoredox catalyst are also briefly discussed.

[DPEPhosbcpCu]PF6: A General and Broadly Applicable Copper-Based Photoredox Catalyst

Detailed and general protocols are presented for the synthesis of [(DPEPhos)(bcp)Cu]PF6, a general copper-based photoredox catalyst, and for its use in synthetic chemistry for the direct arylation of C-H bonds in (hetero)arenes and radical cyclization of organic halides.

Our group recently reported the use of [(DPEPhos)(bcp)Cu]PF6 as a general copper-based photoredox catalyst which proved efficient to promote the ...

Cercosporin-Photocatalyzed [4+1]- and [4+2]-Annulations of Azoalkenes Under Mild Conditions

The interest on nitrogen-containing heterocycles has expanded rapidly in the synthetic community since they are important motifs for new drugs. Traditionally, they were synthesized through thermal cycloaddition reactions, whereas today, photocatalysis is preferred due to the mild and efficient conditions. With this focus, a new photocatalytic method for the synthesis of nitrogen-containing heterocycles is highly desired. Here, we report a protocol for the biosynthesis of cercosporin, which could function as a metal-free photocatalyst. We then illustrate cercosporin-photocatalyzed protocols for the synthesis of nitrogen-containing heterocycles 1,2,3-thiadiazoles through annulation of azoalkenes with KSCN, and synthesis of 1,4,5,6-tetrahydropyridazines [4+2] through cyclodimerization of azoalkenes under mild conditions, respectively. As a result, there is a new bridge between the microbial fermentation method and organic synthesis in a mild, cost-effective, environmentally friendly and sustainable manner.

New routes for the synthesis of nitrogen-containing heterocycles utilizing cercosporin as a metal-free photocatalyst were developed.

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Depolymerizable Olefinic Polymers Based on Fused-Ring Cyclooctene Monomers

The growing consumption of synthetic polymers and the accumulation of polymer waste have led to a pressing need for new routes to sustainable materials. Achieving a closed-loop polymer economy via chemical recycling to monomer (CRM) is one such promising route. Our group recently reported a new CRM system based on polymers prepared by ring-opening metathesis polymerization (ROMP) of trans-cyclobutane fused cyclooctene (tCBCO) monomers. This system offers several key advantages, including the ease of polymerization at ambient temperatures, quantitative depolymerization to monomers under mild conditions, and a broad range of functionalities and thermomechanical properties. Here, we outline detailed protocols for the preparation of tCBCO-based monomers and their corresponding polymers, including the preparation of elastic polymer networks and compression molding of linear thermoplastic polymers. We also outline the preparation of high ring strain E-alkene tCBCO monomers and their living polymerization. Finally, the procedures for the depolymerization of linear polymers and polymer networks are also demonstrated.

Here, we describe protocols for the preparation of trans-cyclobutane fused cyclooctenes (tCBCO),&#160;their polymerization to prepare depolymerizable olefinic polymers, and the depolymerization of these polymers under mild conditions. Additionally, protocols for the preparation of depolymerizable networks and compression molding of rigid linear plastics based on this system are described.

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Photochemical Initiation Of Radical Polymerization Reactions

Source: David C. Powers, Tamara M. Powers, Texas A&#38;M
In this video, we will carry out the photochemically initiated polymerization of styrene to generate polystyrene, which is an important commodity plastic. We will learn the fundamentals of photochemistry and use simple photochemistry to initiate radical polymerization reactions. Specifically, in this module we will examine the photochemistry of benzoyl peroxide and its role as a photo-initiator of styrene polymerization reactions. In the described experiments, we will investigate the role of wavelength, photon absorption, and excited state structure on the efficiency (measured as quantum yield) of photochemical reactions.

Source: David C. Powers, Tamara M. Powers, Texas A&#38;M
In this video, we will carry out the photochemically initiated polymerization of styrene to ...

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Inorganic Chemistry

Acid-Catalyzed Ring-Opening of Epoxides

Epoxides that are three-membered ring systems are more reactive than other cyclic and acyclic ethers. The high reactivity of epoxides originates from the strain present in the ring. This ring strain acts as a driving force for epoxides to undergo ring-opening reactions either with halogen acids or weak nucleophiles in the presence of mild acid. The acid catalyst converts the epoxide oxygen, a poor leaving group, into an oxonium ion, a better leaving group, making the reaction feasible. The reaction follows the SN2 mechanism, and the protonated oxygen, unlike other leaving groups, does not detach from the molecule. The regiochemistry of the products formed is governed by either steric or electronic effects. In the case of an asymmetrical epoxide bearing a primary and a secondary carbon, the steric effect dominates and favors the nucleophilic attack at a less-hindered carbon. However, in epoxides where one of the carbons is tertiary, the electronic effect comes into play and favors the attack at a more-hindered carbon. The stereochemistry of the products is similar to an SN2 reaction, where the nucleophile attacks anti to the leaving group. Notably, the anti-attack at a chiral carbon causes an inversion of configuration.

Epoxides that are three-membered ring systems are more reactive than other cyclic and acyclic ethers. The high reactivity of epoxides originates from the ...

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Organic Chemistry

Unit 1

Ethers, Epoxides, Sulfides

KEY TERMS AND CONCEPTS

Base-Catalyzed Ring-Opening of Epoxides

Due to their highly strained structures, epoxides can readily undergo ring-opening reactions through nucleophilic substitution, either in the presence of an acid or a base. The nucleophilic substitution reactions in the presence of acid are called acid-catalyzed ring-opening reactions, and nucleophilic substitution reactions in the presence of a base are called base-catalyzed ring-opening reactions. Epoxides undergo base-catalyzed ring-opening reactions in the presence of a strong nucleophile or a base. A variety of nucleophiles like sodium hydroxide, sodium alkoxide, sodium hydrosulfide, sodium cyanide, lithium aluminum hydride, and Grignard reagent can open the epoxide ring. The nucleophilic attack on the epoxide ring proceeds via an SN2 mechanism and involves an alkoxide intermediate. The product formed in base-catalyzed reactions shows SN2-like stereoselectivity and regioselectivity. The nucleophile attacks at the less-hindered carbon and anti to the leaving group, leading to the inversion of configuration at a chiral center. In contrast to epoxides, acyclic ethers do not undergo direct nucleophilic substitutions, as the reaction is thermodynamically unfavorable.

Due to their highly strained structures, epoxides can readily undergo ring-opening reactions through nucleophilic substitution, either in the presence of ...

Controlled Photoredox Ring-Opening Polymerization of O-Carboxyanhydrides Mediated by Ni/Zn Complexes

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Chapters in this video

Ethylene Polymerizations Using Parallel Pressure Reactors and a Kinetic Analysis of Chain Transfer Polymerization

Atom Transfer Radical Polymerization of Functionalized Vinyl Monomers Using Perylene as a Visible Light Photocatalyst

Solid-phase Synthesis of [4.4] Spirocyclic Oximes

Photogeneration of N-Heterocyclic Carbenes: Application in Photoinduced Ring-Opening Metathesis Polymerization

[(DPEPhos)(bcp)Cu]PF₆: A General and Broadly Applicable Copper-Based Photoredox Catalyst

Cercosporin-Photocatalyzed [4+1]- and [4+2]-Annulations of Azoalkenes Under Mild Conditions

Depolymerizable Olefinic Polymers Based on Fused-Ring Cyclooctene Monomers

Photochemical Initiation Of Radical Polymerization Reactions

Acid-Catalyzed Ring-Opening of Epoxides

Base-Catalyzed Ring-Opening of Epoxides