Investigations into the steric influences on the reaction mechanism of carbon dioxide insertion into metal-oxygen bonds. Carbonyl sulfide activation as a model for CO2

Darensbourg, Donald J.; Mueller, Brian L.; Bischoff, Christopher J.; Chojnacki, Stephen S.; Reibenspies, Joseph H.

doi:10.1021/ic00010a035

Cited by 56 publications

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“…This is presumably a consequence of the metal center being too electrophilic to facilitate the CO 2 insertion reaction. 6 Relevant to this latter point, we have recently demonstrated the ring-opening reaction of epoxides * To whom correspondence should be addressed. E-mail: djdarens@ mail.chem.tamu.edu.…”

Section: Introductionmentioning

confidence: 94%

Mechanistic Insight into the Initiation Step of the Coupling Reaction of Oxetane or Epoxides and CO₂ Catalyzed by (salen)CrX Complexes

Darensbourg

Moncada

2008

Inorg. Chem.

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The most active and robust current catalysts for the copolymerization of carbon dioxide and epoxides or oxetanes, (salen)CrX in conjunction with PPNX (PPN(+) = (Ph3P)2N(+)) or n-Bu4NX (X = Cl, N3, CN, NCO), are characterized both in solution by infrared spectroscopy and in the solid-state by X-ray crystallography. All anions (X) afford six-coordinate chromium(III) PPN(+) or n-Bu4N(+) salts composed of trans-(salen)CrX2(-) species. Of the X groups investigated in (salen)CrX, chloride is easily displaced by the others, that is, the reaction of (salen)CrCl with 2 equiv of N3(-), CN(-), or NCO(-) quantitatively provide (salen)Cr(N3)2(-), (salen)Cr(CN)2(-), and (salen)Cr(NCO)2(-), respectively. On the other hand, addition of less than 2 equiv of azide to (salen)CrCl leads to a Schlenk (ligand redistribution) equilibrium of the three possible anions both in solution and in the solid-state as shown by X-ray crystallography and electrospray ionization mass spectrometry. It was further demonstrated that all trans-(salen)CrX2(-) anions react with the epoxide or oxetane monomers in TCE (tetrachloroethane) solution to afford an equilibrium mixture containing (salen)CrX x monomer, with the oxetane adduct being thermodynamically more favored. The ring-opening steps of the bound cyclic ether monomers by X(-) were examined, revealing the rate of ring-opening of the epoxides (cyclohexene oxide and propylene oxide) to be much faster than of oxetane, with propylene oxide faster than cyclohexene oxide. Furthermore, both X anions in (salen)CrX2(-) were shown to be directly involved in monomer ring-opening.

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Section: Introductionmentioning

confidence: 94%

Mechanistic Insight into the Initiation Step of the Coupling Reaction of Oxetane or Epoxides and CO₂ Catalyzed by (salen)CrX Complexes

Darensbourg

Moncada

2008

Inorg. Chem.

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“…Previously, it has been well-established that COS insertion into a metal−oxygen bond in low-valent metal complexes results in the formation of a metal−sulfur bond. 5 Subsequently, the sulfur atom in the growing polymer chain predominantly attacks the less sterically crowded methylene carbon when PO inserts into the propagating center (Scheme 1). This regioselectivity is driven by the steric hindrance around the methine carbon center of the epoxide, resulting in a welldefined polythiocarbonate with 99% tail-to-head linkages.…”

Section: ■ Introductionmentioning

confidence: 99%

An Examination of the Steric and Electronic Effects in the Copolymerization of Carbonyl Sulfide and Styrene Oxide

2015

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The completely alternating copolymerization of carbonyl sulfide (COS) and styrene oxide was found to occur under mild reaction conditions (0− 30°C and 1.5 MPa) in the presence of (salen)CrCl/onium salt catalyst systems to afford high molecular weight poly(monothiocarbonates) with narrow molecular weight distributions. Ring-opening of styrene oxide was shown to be 88% selective at the methylene carbon. That is the reaction is driven by steric hindrance, where ring-opening occurs preferentially at the less congested carbon center. Similar results were found upon utilizing the tetramethyltetraazaannulene (tmtaa)CrCl/ onium salt catalyst. On the other hand, upon employing a zinc−cobalt double metal cyanide (Zn−Co DMCC) catalyst, where the ligands around the active zinc site are not sterically encumbering, ring-opening of styrene oxide occurs predominantly at the methine carbon site; i.e., the reaction is electronically driven. ■ INTRODUCTIONThe catalytic copolymerization of carbon dioxide with epoxides to synthesize degradable polycarbonates has received considerable attention during the past couple of decades. 1 However, the analogous reaction of epoxides and carbonyl sulfide (COS) to form well-defined polythiocarbonates has received little consideration. 2,3 COS is an air pollutant widely released from volcanic eruptions, burning of fossil fuels, and many other chemical processes. It can cause acid rain and damage of the ozonosphere by a series of photochemical reactions. 4 Instead of being a toxic waste, COS can, on the other hand, be a good source of sulfur for synthesizing well-defined sulfur-containing polymers when it is employed as a monomer in copolymerization with epoxides. Meanwhile, as a carbon source, COS is more active than CO 2 , and the greatest difference between them is that COS is asymmetric, which makes the COS/ epoxides copolymerization more complicated and interesting. Some progress has been made in the formation of polythiocarbonates from aliphatic terminal epoxides or cyclohexene oxide, 2,3 but the selective synthesis of polythiocarbonates from epoxides with electron-withdrawing groups such as styrene oxide still remains a challenge.It was reported in our early research that when COS copolymerizes with aliphatic terminal epoxides, such as propylene oxide (PO), catalyzed by the binary (salen)CrCl/ PPNCl catalyst, the sulfur atom from COS will bind to the chromium center and form a chromium−sulfur propagating center. Previously, it has been well-established that COS insertion into a metal−oxygen bond in low-valent metal complexes results in the formation of a metal−sulfur bond. 5 Subsequently, the sulfur atom in the growing polymer chain predominantly attacks the less sterically crowded methylene carbon when PO inserts into the propagating center (Scheme 1). This regioselectivity is driven by the steric hindrance around the methine carbon center of the epoxide, resulting in a welldefined polythiocarbonate with 99% tail-to-head linkages.However, as a monomer, styrene oxide (SO) is quite special; ...

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“…The CO 2 insertion between the Mn–O bond might be a concerted reaction between the nucleophilic attack of the oxygen atom in the ligand to the carbon in CO 2 and the electrophilic attack of the central metal to the oxygen atom in CO 2 (Scheme 2). 22,31,47 Since a higher electronic density of the central metal should induce acceleration of the former process while obstructing the latter one, these conflicting effects might cancel each other out.…”

Section: Resultsmentioning

confidence: 99%

CO₂ capture by Mn(i) and Re(i) complexes with a deprotonated triethanolamine ligand

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Investigations into the steric influences on the reaction mechanism of carbon dioxide insertion into metal-oxygen bonds. Carbonyl sulfide activation as a model for CO2

Cited by 56 publications

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Mechanistic Insight into the Initiation Step of the Coupling Reaction of Oxetane or Epoxides and CO₂ Catalyzed by (salen)CrX Complexes

Mechanistic Insight into the Initiation Step of the Coupling Reaction of Oxetane or Epoxides and CO₂ Catalyzed by (salen)CrX Complexes

An Examination of the Steric and Electronic Effects in the Copolymerization of Carbonyl Sulfide and Styrene Oxide

CO₂ capture by Mn(i) and Re(i) complexes with a deprotonated triethanolamine ligand

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Investigations into the steric influences on the reaction mechanism of carbon dioxide insertion into metal-oxygen bonds. Carbonyl sulfide activation as a model for CO2

Cited by 56 publications

References 0 publications

Mechanistic Insight into the Initiation Step of the Coupling Reaction of Oxetane or Epoxides and CO2 Catalyzed by (salen)CrX Complexes

Mechanistic Insight into the Initiation Step of the Coupling Reaction of Oxetane or Epoxides and CO2 Catalyzed by (salen)CrX Complexes

An Examination of the Steric and Electronic Effects in the Copolymerization of Carbonyl Sulfide and Styrene Oxide

CO2 capture by Mn(i) and Re(i) complexes with a deprotonated triethanolamine ligand

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Mechanistic Insight into the Initiation Step of the Coupling Reaction of Oxetane or Epoxides and CO₂ Catalyzed by (salen)CrX Complexes

Mechanistic Insight into the Initiation Step of the Coupling Reaction of Oxetane or Epoxides and CO₂ Catalyzed by (salen)CrX Complexes

CO₂ capture by Mn(i) and Re(i) complexes with a deprotonated triethanolamine ligand