Xanthates thermal elimination

The preparation of poly(2,5 dimethoxy phenylene vinylene)(DM-PPV) utilized the xanthate precursor route. Many similarities exist between the PPV xanthate thermal elimination and this PPV derivative. Previous studies had attempted to synthesis DM-PPV using the SPR method. The SPR resulted in gel formation during dialysis so that film casting was not possible. The XPR was successful in preparing precursor films that can be thermally converted to DM-PPV. The derivatives of PPV are useful to prepare a polymer with a shift in the absorbance and photoluminescence maximum. DM-PPV exhibits an 100 nm red shift as compared with PPV synthesized via the XPR. 

A number of synthetically useful elimination reactions proceed thermally, with no base or acid required. These elimination reactions proceed through a concerted retro-ene mechanism. (The mechanism is sometimes called E,.) The thermal elimination of acetic acid from alkyl acetates and the elimination of RSCOSH from alkyl xanthates (the Chugaev reaction) are retro-ene reactions. 

Dithiole-2-one (60), which can be readily transformed into its thio- or seleno-carbonyl derivatives, is a key intermediate for the synthesis of tetrathiafulvalene (Scheme 13)[31]. We first anticipated that compound 57, a Michael addition product of xanthate 54 to vinyl sulfoxide, might be an ideal intermediate for the synthesis of 60 via cyclization under Pummerer rearrangement conditions. However, although Michael addition of dithiocarbamate 53 to vinyl sulfoxide proceeded smoothly to yield compound 55, the addition reaction with xanthate 54 failed. We then turned to the alkylation approach. Xanthate 54 was alkylated smoothly with 56, which served as the synthetic equivalent of the vinyl sulfoxide, in ethanol under sonication in 90% yield [32]. Cyclization of 57 under Pummerer rearrangement conditions in the presence of trifluoroacetic acid afforded 58 in 79% yield. Sodium metaperiodate oxidation gave the unstable sulfoxide 59 which underwent thermal elimination to yield 60 in refluxing benzene in moderate yield. 

Abstract In situ spectroscopy is an important tool to characterize polymers synthesized via a precursor route. Highly conjugated polymers such as po y(p-phenylene vinylene) (PPV) and PPV derivatives are commonly prepared from a precursor polymer because the final polymers are very insoluble and intractable. Preparation in the precursor form enables the polymer materials to be cast as films. The PPV polymers are obtained from the precursor forms using a thermal elimination reaction. The exact conditions of the reaction are important as they influence the properties of the resultant polymer. The details of this thermal elimination reaction have been analyzed using thermal gravimetric analysis (TGA) coupled with infrared analysis of the evolved gas products. In situ infrared spectroscopy of the precursor films during thermal conversion to the polymers has provided further details about the elimination reaction. We have characterized PPV synthesized from a tetrahydrothiophenium monomer (sulfonium precursor route) and via the xanthate precursor route. PPV derivatives under study include poly(2,5-dimethoxy-p-phenylene vinylene) and poly(phenoxy phenylene vinylene). 

The solid state thermal elimination reaction is a very important step in the formation of the final PPV or PPV derivative. In situ infrared spectroscopy therefore plays a critical role in the ability to monitor the reaction that converts the precursor polymer to the final product. We have characterized the mechanism of this conversion reaction in the formation of PPV synthesized by both the sulfonium precursor route (SPR) and the xanthate precursor route (XPR). The polymerization reaction of PPV from the tetrahydrothiophenium monomer is shown in Figure 1. After polymerization of the precursor polymer, the material is thermally converted to the final PPV product. This SPR method involves the thermal elimination of the tetrahydrothiophenium (THT) group and HCl as shown. 

PPV can also be prepared via the xanthate precursor route as shown in Figure 2. During heating, the xanthic acid is lost and the conjugated PPV results. This route has been shown to produce a more amorphous PPV that has applications in light-emitting diodes. The differences between these two routes were investigated in the current analysis. Both of these routes (SPR and XPR) involve a thermal elimination step that is the focus of the in situ spectroscopic analysis. 

We have previously reported the synthesis of PPV and the analysis of the thermal elimination reaction in the polymer prepared from the tetrahydrothiophenium (THT) monomer via the sulfonium precursor route. The reaction is shown in Figure 1. PPV prepared via the xanthate precursor route has been recently described.The reaction is shown in Figure 2. 

Figure 4. Three-dimensional temperature-infrared frequency-intensity plot of the reaction byproducts during the thermal elimination reaction of xanthate precursor...

As with the other syn thermal eliminations, there are no intermediates prone to undergo skeletal rearrangement. The principal application of the xanthate method has been in situations in which acid-catalyzed alcohol dehydration would be accompanied by skeletal rearrangement, or in which a very sensitive olefin is being produced. 

Padmanaban, G., K. Nagesh, and S. Ramakrishnan. 2003. Segmented poly(2-methoxy-5-(2-ethyl-hexyloxy)-l,4-phenylene vinylene) via xanthate and dithiocarbamate precursors A comparative study of thermal eliminations. / Polym Sci Part A Polym Chem 41 (24) 3929-3940. 

The first decision in choosing a synthetic method for a PPV material is the way in which the material will be processed (Scheme 7.8). The precursor routes will enable the preparation of solvent-resistant and more durable thin films of PPV. This is particularly desirable if a multilayer device structure is required for the application. When choosing different precursor methods, it is important to assess the criteria of the application. Most precursor methods involve a thermal elimination step to convert the precursor polymer to the PPV material. Sul-fonium precursors require higher-temperature elimination compared to sulfinyl precursors. This makes the sulfinyl route compatible with deposition on plastic substrates. Another factor to consider in precursor methods is the nature of the elimination byproducts. Sulfonium precursors convert to PPV with elimination of acids, such as HCl or HBr, which has been shown to be detrimental to device performance. Xanthate and dithiocarbamate routes involve the elimination of amine and CO2 and CS2, respectively. 

For example, treatment with p-toluenesulfonic acid in methanol affords dimethoxyacetals which can be further hydrolyzed to aldehydes (eq 2). Aldehydes are also available in one step by oxidation of the sulfide to the sulfoxide with m-chloroperbenzoic acid and hydrolytic workup (eq 2). Vinyl ethers are produced via a thermal elimination of benzenesulfenic acid (eq 3). Alternately, the anion formed from the addition of methoxy(phenylthio)methyllithium to aldehydes can be treated with carbon disulfide and iodomethane to form a xanthate which when treated with tributylstannane effects a radical reductive elimination to form (Z)- and ( )-enol ethers (eq 4). ... 

In some cases, eliminations occur in non-ionizing solvents and without the addition of any base. In these cases the reactant itself has an internal base and a cyclic transition state leads to elimination. The symbolism for the reactions is Ei, standing for elimination, intramolecular. Only heat is required to induce the reaction, and hence these reactions are called thermal eliminations (the term pyrolysis is also sometimes used). Thioesters, xanthates, selenoxides, and N-oxides are common in these reactions. The Cope elimination involves the formation of an N-oxide and subsequent elimination via the pathway shown in Eq. 10.91, and the Chugaev elimination involves xanthate esters [ROC(S)SR]. The Chugaev elimination was shown to follow a syn elimination pathway based on the stereospecific nature of the reaction (Eqs. 10.92 and 10.93). 

For example, the sulfur-containing xanthate esters undergo elimination especially easily (Fig. 18.62). Xanthate esters can he prepared as shown in the figure by addition of alkoxide to CS2 (recall the addition of nucleophiles to carbon dioxide, p. 840). These intramolecular thermal elimination reactions provide a new route to alkenes, so update your file cards. 

There is a class of intramolecular thermal elimination reactions that provides a new route to alkenes. Esters, xanthates, and amine oxides are commonly used in this reaction. The reactions are concerted (one-step), and steiic requirements dictate that a syn elimination must occur in the reaction, as the carbonyl group cannot reach a hydrogen in an anti position (Rg. 18.61). 

Salts of O-alkyldithiocarbonatcs ( xanthates ) are hazardous as dusts, forming explosive suspensions in air. The lower-alkyl salts are claimed to be explosive in the solid state when dry [1]. Explosions reported when drying hydrated xanthate salts are probably the consequence of release of carbon disulphide to form an inflammable atmosphere of very low autoignition temperature in the oven [2], Xanthate esters are thermally unstable by a variety of eliminations and rearrangements, all distinctly exothermic and many evolving extremely flammable gases and vapours. Free xanthic acids, which may be isolated on acidification, decompose autocatalytically and perhaps explosively [3],... 

The Chugaev reaction of S methyl xanthates is a synthetically useful p5u olysiss process. The corresponding electron impact induced elimination is a prominent feature of xanthate mass spectra, (45). > The electron impact induced cleavage is at least partially cfs-stereospecific as the thermal reaction is... 

Alkenes are formed by the thermal decomposition of esters, xanthates, amine oxides, sulfoxides, and selenoxides that contain at least one (3-hydrogen atom. These elimination reactions require a cw-configuration of the eliminated group and hydrogen and proceed by a concerted process. If more than one (3-hydrogen is present, mixtures of alkenes are generally formed. Since these reactions proceed via cyclic transition states, conformational effects play an important role in determining the composition of the alkene product. 

Chugaev elimination Thermal syn elimination of xanthate esters to form alkenes. 82... 

This reaction is just a thermal internal syn E2 elimination, and a relative of path 6e. There are many possible reactants for this internal syn elimination path. Five-membered transition state examples include Y equals oxygen and Z is NR2 (amine oxides), SPh (sulfoxides), or SePh (selenoxides). Six-membered transition state examples include both Y and Z being oxygen (esters), or Y is sulfur and Z is oxygen (xanthates). 

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