Your browser doesn't support javascript.
loading
Mostrar: 20 | 50 | 100
Resultados 1 - 20 de 37
Filtrar
Mais filtros











Base de dados
Intervalo de ano de publicação
1.
ChemistryOpen ; 9(12): 1223, 2020 12.
Artigo em Inglês | MEDLINE | ID: mdl-33304736

RESUMO

Invited for this month's cover is the group of Stephan Enthaler at the University of Hamburg (Germany). The cover picture shows the general structure of chemical recycling processes. In more detail, the chemical recycling of end-of-life polymers (plastics) is subdivided into two linked processes. The depolymerization allows the conversion of the end-of-life polymer to monomers, which can be reused in the polymerization step for the synthesis of the polymer. By doing so advantages with respect to resource-efficiency and sustainability can be created. In our manuscript we applied the chemical recycling concept via depolymerization-polymerization for poly(lactide). Read the full text of their Communication at 10.1002/open.202000243.

2.
ChemistryOpen ; 9(12): 1224-1228, 2020 12.
Artigo em Inglês | MEDLINE | ID: mdl-33304737

RESUMO

The chemical recycling of poly(lactide) was investigated based on depolymerization and polymerization processes. Using methanol as depolymerization reagent and zinc salts as catalyst, poly(lactide) was depolymerized to methyl lactate applying microwave heating. An excellent performance was observed for zinc(II) acetate with turnover frequencies of up to 45000 h-1. In a second step the monomer methyl lactate was converted to (pre)poly(lactide) in the presence of catalytic amounts of zinc salts. Here zinc(II) triflate revealed excellent performance for the polymerization process (yield: 91 %, Mn ∼8970 g/mol). Moreover, the (pre)poly(lactide) was depolymerized to lactide, the industrial relevant molecule for accessing high molecular weight poly(lactide), using zinc(II) acetate as catalyst.

3.
ChemistryOpen ; 9(8): 818-821, 2020 08.
Artigo em Inglês | MEDLINE | ID: mdl-32789104

RESUMO

Chemical recycling processes can contribute to a resource-efficient plastic economy. Herein, a procedure for the iron-catalyzed hydrogenation of the carbonate function of end-of-life polycarbonates under simultaneous depolymerization is presented. The use of a straightforward iron pincer complex leads to high rate of depolymerization of poly(bisphenol A carbonate) and poly(propylene carbonate) yielding the monomers bisphenol A and 1,2-propanediol, respectively, as products under mild reaction conditions. Furthermore, the iron complex was able to depolymerize polycarbonates containing goods and mixture of plastics containing polycarbonates.

4.
ChemistryOpen ; 9(4): 401-404, 2020 04.
Artigo em Inglês | MEDLINE | ID: mdl-32257748

RESUMO

The chemical recycling of end-of-life polymers can add some value to a future circular economy. In this regard, the hydrogenative degradation of end-of-life PLA was investigated to produce 1,2-propanediol as product, which is a useful building block in polymer chemistry. In more detail, the commercially available Ru-MACHO-BH complex was applied as catalyst to degrade end-of-life PLA efficiently to 1,2-propanediol under mild conditions. After investigations of the reaction conditions a set of end-of-life PLA goods were subjected to degradation.

5.
ChemistryOpen ; 8(12): 1410-1412, 2019 Dec.
Artigo em Inglês | MEDLINE | ID: mdl-31867148

RESUMO

The valorization of waste to valuable chemicals can contribute to a more resource-efficient and circular chemistry. In this regard, the selective degradation of end-of-life polymers/plastics to produce useful chemical building blocks can be a promising target. We have investigated the hydrogenative depolymerization of end-of-life poly(bisphenol A carbonate). Applying catalytic amounts of the commercial available Ruthenium-MACHO-BH complex the end-of-life polycarbonate was converted to bisphenol A and methanol. Importantly, bisphenol A can be reprocessed for the manufacture of new poly-(bisphenol A carbonate) and methanol can be utilized as energy storage material.

6.
ChemistryOpen ; 8(7): 822-827, 2019 Jul.
Artigo em Inglês | MEDLINE | ID: mdl-31304075

RESUMO

The chemical recycling of end-of-life plastic waste streams can contribute to a resource-conserving and sustainable society. This matter of recycling is composed of a sequence of depolymerization and subsequent polymerization reactions. In this regard, we have studied the chemical recycling of end-of-life poly(bisphenol A carbonate) applying phenol as depolymerization reagent. In the presence of catalytic amounts of alkali metal halides as products bisphenol A and diphenyl carbonate were obtained in excellent turnover frequencies of up to 1392 h-1 and short reaction times. These depolymerization products offer the straightforward possibility to close the cycle by producing new poly(bisphenol A carbonate) and as second product phenol, which can be reused for further depolymerizations.

7.
Chimia (Aarau) ; 69(6): 327-30, 2015.
Artigo em Inglês | MEDLINE | ID: mdl-26507477

RESUMO

The formation of carbon-carbon bonds is one of the fundamental transformations in chemistry. In this regard the application of palladium-based catalysts has been extensively investigated during recent years, but nowadays research focuses on iron catalysis, due to sustainability, costs and toxicity issues; hence numerous examples for iron-catalyzed cross-coupling reactions have been established, based on the coupling of electrophiles (R(1)-X, X = halide) with nucleophiles (R(2)-MgX). Only a small number of protocols deals with the iron-catalyzed oxidative coupling of nucleophiles (R(1)-MgX + R(2)-MgX) with the aid of oxidants (1,2-dihaloethanes). However, some issues arise with these oxidants; hence more recently the potential of the industrial waste product nitrous oxide (N(2)O) was investigated, because the unproblematic side product N(2) is formed. Based on that, we demonstrate the catalytic potential of easily accessible iron complexes in the oxidative coupling of Grignard reagents. Importantly, nitrous oxide was essential to obtain yields up to >99% at mild conditions (e.g. 1 atm, ambient temperature) and low catalyst loadings (0.1 mol%) Excellent catalyst performance is realized with turnover numbers of up to 1000 and turnover frequencies of up to 12000 h(-1). Moreover, a good functional group tolerance is observed (e.g. amide, ester, nitrile, alkene, alkyne). Afterwards the reaction of different Grignard reagents revealed interesting results with respect to the selectivity of cross-coupling product formation.


Assuntos
Carbono/química , Indicadores e Reagentes/química , Ferro/química , Magnésio/química , Óxido Nitroso/química , Compostos Organometálicos/química , Aldeídos/química , Catálise , Halogênios/química , Cetonas/química , Paládio/química
8.
Angew Chem Int Ed Engl ; 54(7): 2214-8, 2015 Feb 09.
Artigo em Inglês | MEDLINE | ID: mdl-25557655

RESUMO

The Ni(II) -mediated tautomerization of the N-heterocyclic hydrosilylcarbene L(2) Si(H)(CH2 )NHC 1, where L(2) =CH(CCH2 )(CMe)(NAr)2 , Ar=2,6-iPr2 C6 H3 ; NHC=3,4,5-trimethylimidazol-2-yliden-6-yl, leads to the first N-heterocyclic silylene (NHSi)-carbene (NHC) chelate ligand in the dibromo nickel(II) complex [L(1) Si:(CH2 )(NHC)NiBr2 ] 2 (L(1) =CH(MeCNAr)2 ). Reduction of 2 with KC8 in the presence of PMe3 as an auxiliary ligand afforded, depending on the reaction time, the N-heterocyclic silyl-NHC bromo Ni(II) complex [L(2) Si(CH2 )NHCNiBr(PMe3 )] 3 and the unique Ni(0) complex [η(2) (Si-H){L(2) Si(H)(CH2 )NHC}Ni(PMe3 )2 ] 4 featuring an agostic SiH→Ni bonding interaction. When 1,2-bis(dimethylphosphino)ethane (DMPE) was employed as an exogenous ligand, the first NHSi-NHC chelate-ligand-stabilized Ni(0) complex [L(1) Si:(CH2 )NHCNi(dmpe)] 5 could be isolated. Moreover, the dicarbonyl Ni(0) complex 6, [L(1) Si:(CH2 )NHCNi(CO)2 ], is easily accessible by the reduction of 2 with K(BHEt3 ) under a CO atmosphere. The complexes were spectroscopically and structurally characterized. Furthermore, complex 2 can serve as an efficient precatalyst for Kumada-Corriu-type cross-coupling reactions.

9.
ChemSusChem ; 7(7): 2030-6, 2014 Jul.
Artigo em Inglês | MEDLINE | ID: mdl-24825826

RESUMO

The easy accessibility and high adjustability of polymers mainly accounts for the great impact of such materials on modern society. Besides this great success, an important matter is the accumulation of large amounts of end-of-life polymers, which are mainly deposited in landfills or converted by thermal recycling or down-cycling to low-quality materials. In contrast to that, the depolymerization of end-of-life polymers to monomers, which can be applied as feedstock in polymerization chemistry for high-quality polymers, is only carried out for a small fraction of waste. Polysiloxanes are extensively used in a diverse array of technological applications. Based on intrinsic properties of polymers, depolymerization is challenging and only a few high-temperature or less environment-friendly processes have been reported. In this regard, we have set up a capable low-temperature protocol for the depolymerization of poly(dimethylsiloxane) in the presence of catalytic amounts of simple iron salts in combination with different depolymerization reagents. The application of benzoyl fluoride, benzoyl chloride/potassium fluoride, or benzoic anhydride/potassium fluoride as depolymerization reagents affords difluorodimethylsilane or 1,3-difluoro-1,1,3,3-tetramethyldisilxanes as products, which are interesting building blocks for the synthesis of new polymers and allow an overall recycling of polysiloxanes.


Assuntos
Ferro/química , Polimerização , Siloxanas/química , Temperatura , Catálise
10.
Angew Chem Int Ed Engl ; 53(10): 2716-21, 2014 Mar 03.
Artigo em Inglês | MEDLINE | ID: mdl-24501107

RESUMO

Polymers occupy an important role in our current society. Besides their great success, an issue is the accumulation of huge amounts of end-of-life polymers. Currently, the waste management is based primarily on landfills, thermal recycling, and downcycling. Notably, only a small portion of end-of-life materials is recycled by depolymerization, which refers to the creation of synthetic precursors that can be polymerized to new polymers to close the cycle. Widely used polymers in modern times are silicones (polysiloxanes), the intrinsic properties of which make their depolymerization demanding; only a few high-temperature or less environmentally friendly processes have been reported. In this regard, we have established an efficient low-temperature protocol for the depolymerization of silicones with benzoyl fluoride in the presence of cheap zinc salts as precatalysts to yield defined products. Notably, the products can be useful synthetic precursors for the preparation of new polymers, so that an overall recycling process is feasible.

11.
J Am Chem Soc ; 135(48): 18108-20, 2013 Dec 04.
Artigo em Inglês | MEDLINE | ID: mdl-24195449

RESUMO

Reaction of FeCl2 with the chelating bis-N-heterocyclic carbene (NHC) bis-(N-Dipp-imidazole-2-ylidene)methylene (abbreviated {((Dipp)C:)2CH2}) (Dipp = 2,6-di-isopropylphenyl) affords the complex [FeCl2{((Dipp)C:)2CH2}] (1) in high yield. Reduction of complex 1 with excess KC8 with a 10-fold molar excess of PMe3 affords the Fe(II) complex [FeH{((Dipp)C:)2CH2}(PMe3)(η(2)-PMe2CH2)] (2) as a mixture of three stereoisomers. Complex 2, the first example of any iron(II) complex bearing mutually an NHC and PMe3 ligand, is likely obtained from the in situ, reductively generated 16 VE Fe(0) complex, [Fe{((Dipp)C:)2CH2}(PMe3)2] (2'), following intramolecular C-H activation of one of the phosphorus-bound CH3 groups. Complex 2 is unstable in aromatic solvents and forms, via a novel synthetic transformation involving intramolecular reductive elimination and concomitant PMe3 elimination, the Fe (0) arene complex [Fe{((Dipp)C:)2CH2}(η(6)-C6D6)] (4-d6) in C6D6. Complex 4-d6 represents the first example of an NHC stabilized iron (0) arene complex. The transformation from 2 to 4-d6 can be accelerated at higher temperature and at 60 °C forms immediately. Alternatively, the reduction of 1 in the presence of toluene or benzene affords the complexes [Fe{((Dipp)C:)2CH2}(η(6)-C7H8)] (3) and [Fe{((Dipp)C:)2CH2}(η(6)-C6H6)] (4), selectively and in good yields. DFT calculations characterizing the bonding situation in 3 and 4 reveal similar energies of the HOMO and LUMO orbitals, with the LUMO orbital of both complexes located on the Dipp rings of the bis-NHC. The HOMO orbital reflects a π-back-bonding interaction between the Fe(0) center and the chelating NHC ligand, while the HOMO-1 is associated with the arene interaction with the Fe(0) site. The calculations do not suggest any noninnocence of the coordinated arene in either complex. Moreover, the (57)Fe Mössbauer spectrum of 4 at 80K exhibits parameters (δ = 0.43 mm·s(-1); ΔEQ = 1.37 mm·s(-1)) which are consistent with a five-coordinate Fe(0) system, rendering 3 and 4 the first examples of well-defined authentic Fe(0)-η(6)-arene complexes of the type [Fe(η(6)-arene)L2] (L = η(1 or 2) neutral ligand, mono or bidentate). Some reactivitiy studies of 3 are also reported: The reaction of 3 with excess CO selectively yields the five-coordinate piano-stool complex [Fe{((Dipp)C:)2CH2}(CO)3] (6) in near quantitative yields, while the reaction of complex 3 with C6D6 under heating affords by toluene elimination 4-d6. The catalytic ability of 4 was also investigated with respect to amide reduction to amines, for a variety of substrates using Ph2SiH2 as a hydride source. In all cases good to excellent yields to the corresponding amines were obtained. The use of 4 as a precatalyst represents the first example of a well-defined Fe(0) complex to effect this catalytic process.

13.
Chem Commun (Camb) ; 49(49): 5595-7, 2013 Jun 21.
Artigo em Inglês | MEDLINE | ID: mdl-23673422

RESUMO

The facile synthesis of the first stable selenosilanoic acid-base adduct LSi(=Se)OH(dmap) 3 (L = CH[C(Me)NAr]2, Ar = 2,6-iPr2C6H3, dmap = 4-dimethylaminopyridine), the heavier analogue of the thiosilanoic acid adduct LSi(=S)OH(dmap) 1, is reported. Both adducts 1 and 3 react readily with MesCu (Mes = 2,4,6-trimethylphenyl) to form the novel dimeric Cu(I) complexes [LSi(=E)OCu]2 (4: E = S; 5: E = Se) with unprecedented intermolecular Si=E → Cu-O-Si coordination modes. The latter are efficient pre-catalysts for the Cu(I)-mediated aziridination of styrene with PhI=N(Ts) (Ts = tosyl).


Assuntos
Cobre/química , Compostos Organometálicos/química , Compostos Organosselênicos/química , Compostos de Organossilício/química , Oxigênio/química , Silício/química , Modelos Moleculares , Estrutura Molecular
14.
J Am Chem Soc ; 135(17): 6703-13, 2013 May 01.
Artigo em Inglês | MEDLINE | ID: mdl-23570308

RESUMO

The first electron-rich N-heterocyclic silylene (NHSi)-iron(0) complexes are reported. The synthesis of the starting complex is accomplished by reaction of the electron-rich Fe(0) precursor [(dmpe)2Fe(PMe3)] 1 (dmpe =1,2-bis(dimethylphosphino)ethane) with the N-heterocyclic chlorosilylene LSiCl (L = PhC(N(t)Bu)2) 2 to give, via Me3P elimination, the corresponding iron complex [(dmpe)2Fe(←:Si(Cl)L)] 3. Reaction of in situ generated 3 with MeLi afforded [(dmpe)2Fe(←:Si(Me)L)] 4 under salt metathesis reaction, while its reaction with Li[BHEt3] yielded [(dmpe)2Fe(←:Si(H)L)] 5, a rare example of an isolable Si(II) hydride complex and the first such example for iron. All complexes were fully characterized by spectroscopic means and by single-crystal X-ray diffraction analyses. DFT calculations further characterizing the bonding situation between the Si(II) and Fe(0) centers were also carried out, whereby multiple bonding character is detected in all cases (Wiberg Bond Index >1). For the first time, the catalytic activity of a Si(II) hydride complex was investigated. Complex 5 was used as a precatalyst for the hydrosilylation of a variety of ketones in the presence of (EtO)3SiH as a hydridosilane source. In most cases excellent conversions to the corresponding alcohols were obtained after workup. The reaction pathway presumably involves a ketone-assisted 1,2-hydride transfer from the Si(II) to Fe(0) center, as a key elementary step, resulting in a betaine-like silyliumylidene intermediate. The appearance of the latter intermediate is supported by DFT calculations, and a mechanistic proposal for the catalytic process is presented.

17.
ChemSusChem ; 5(7): 1195-8, 2012 Jul.
Artigo em Inglês | MEDLINE | ID: mdl-22639421

RESUMO

Iron will: The iron-catalyzed depolymerization of a range of polyethers is studied. The products of the depolymerization reactions are chloroesters, which can be used as starting materials for new polymers. In the presence of simple iron salts extraordinary catalyst activities and selectivities are feasible at low temperature.


Assuntos
Éteres/química , Ferro/química , Polimerização , Temperatura , Catálise
19.
Chemistry ; 18(7): 1910-3, 2012 Feb 13.
Artigo em Inglês | MEDLINE | ID: mdl-22253040

RESUMO

Recycling polymers: In the present study, the efficient zinc-catalyzed depolymerization of a variety of artificial polyethers has been investigated. Chloroesters were obtained as the depolymerization products, which are suitable precursors for new polymers. By using straightforward zinc salts, extraordinary catalyst activities and selectivities were feasible (see scheme).

20.
Chem Asian J ; 7(1): 169-75, 2012 Jan 02.
Artigo em Inglês | MEDLINE | ID: mdl-21956861

RESUMO

In the present study, the zinc-catalyzed dehydration of a variety of amides with N-methyl-N-(trimethylsilyl)trifluoroacetamide (MSTFA) as a dehydration reagent into the corresponding nitriles has been examined in detail. With the straightforward and commercially available zinc(II)triflate as the precatalyst and MSTFA, an excellent system has been established to afford nitriles in excellent yields and chemoselectivities. After investigation of reaction conditions and the scope and limitations, several efforts were carried out to understand the reaction mechanism.


Assuntos
Amidas/química , Nitrilas/síntese química , Zinco/química , Catálise , Desidratação , Estrutura Molecular , Nitrilas/química
SELEÇÃO DE REFERÊNCIAS
DETALHE DA PESQUISA