Thermolysis reaction

The synthesis of cZos o-borane dianions B H (1-7) relies principally on thermolysis reactions of boranes in the presence of either BH4 or amino-borane adducts. " The yields... 

Dialkyl esters of cystine (39) and lanthionine (40) undergo a surprising thermolysis reaction at between 25 C and 80 °C to afford cis and trans methyl 2-methylthiazolidine-2,4-dicarboxylates (43) in protic solvents. A two stage process is proposed for this transformation. An initial i-elimination reaction gives the thiol (41) and the enamine (42). Thiol addition to the imine tautomer of (42) is then followed by loss of ammonia and an intramolecular cyclisation to give (43) <96CC843>. 

Also the thermohydrolysis of the urea solution after the injection into the hot exhaust gas upstream of the SCR catalyst has been investigated at the diesel test rig. Urea solution was atomized about 3 m upstream of the SCR catalyst into the hot exhaust equivalent to a residence time in the pipe section of 0.1 s at 440°C. As expected for the thermolysis reaction, ammonia and isocyanic acid were found at the catalyst entrance at all temperatures (Figure 9.3). The 1 1 ratio of both components shows that only the thermolysis but not the hydrolysis is taking place in the gas phase. It can also be seen that the residence time of 0.1 s is not sufficient for the quantitative thermolysis of urea, as appreciable amounts of undecomposed urea were always found. The urea share even raises with lowering the flue gas temperature, although the residence time... 

We report here the results of our recent studies of poly(alkyl/arylphosphazenes) with particular emphasis on the following areas (1) the overall scope of, and recent improvements in, the condensation polymerization method (2) the characterization of a representative series of these polymers by dilute solution techniques (viscosity, membrane osmometry, light scattering, and size exclusion chromatography), thermal analysis (TGA and DSC), NMR spectroscopy, and X-ray diffraction (3) the preparation and preliminary thermolysis reactions of new, functionalized phosphoranimine monomers and (4) the mechanism of the polymerization reaction. 

These thermolysis reactions normally produce polymeric products, free of the cyclic analogs, in essentially quantitative yield and in sufficient purity to give satisfactory elemental analysis upon removal of the sHyl ether byproduct under vacuum. Final purification is generally achieved by precipitation of the polymer into a non-solvent such as hexane. With the exception of poly(diethylphosphazene) (2), which is insoluble in all common solvents (see below), the new polymers are readily soluble in CH CU and CHCU. In addition, the phenyl substituted compounds (3-6) are soluble in THF andvanous aromatic solvents. None of the polymers are water-soluble however, Me2PN]n (1) is soluble in a 50 50 water/THF mixture. 

The volatile products of these processes have been identified and some insight has been gained into the kinetics and mechanism of the polymer thermolysis reactions. 

On the other hand, numerous examples are already known in which monomeric metaphosphoric esters are generated by thermolysis reactions. Most worthy of mention in this context is the gas phase pyrolysis of the cyclic phosphonate 150 which leads via a retro-Diels-Alder reaction to butadiene and monomeric methyl metaphosphate (151) 108,109, no). While most of the phosphorus appears as pyrophosphate and trimeric and polymeric metaphosphate, a low percentage (<5%) of products 152 and 153 is also found on condensation of the pyrolyzate in a cold trap containing diethylaniline or N,N,N, N,-tetraethyl-m-phenylene-diamine. The... 

Summary The formation, reactivity, and cycloaddition behavior of neopentylsilenes towards suitable reaction partners is described. Especially l,l-dichloro-2-neopentylsilene. Cl2Si=CHCH2Bu (2) - easily obtained from vinyltrichlorosilane and LiBu - is a useful building block for the synthesis of SiC four membered ring compounds. These can be converted into the isomeric Diels-Alder and retro ene products upon thermolysis reactions. The mode of the silenes cycloaddition reactions ([4+2] vs [2+2] addition) can be directed by either the substitution pattern at the Si=C moiety, the choice of reaction partners or the conditions. Furthermore the products resulting from cycloaddition reactions open up a wide variety of following reactions, which possibly will lead to new organosilicon materials or pharmaceutical compounds. 

The thermolysis products of silaazetidines support the formulation of stepwise decomposition to alkene and Cl2Si=NR derivatives [3,4], The silene is not liberated from the silaazetidines when performing thermolysis reactions, but the Si-dichloro substituted silanimine (Cl2Si = N/Bu) is formed. This can be trapped by M SiOMe or Ph2C=NfBu, to give the addition products. 

Thermolysis reactions have also been investigated as methods for preparing pyrazoles. Thermolysis of azido imines 18 led to 2-substituted-4,6-dinitro-2//-indazoles 19 <00S1474>. High temperature intramolecular cyclization of AA-diethyl-A -(4-substituted-2-... 

A kinetic study of thermolysis reactions of TV-crotyl substituted 1,2,4-triazoles was performed at temperatures in the range of 260-350 CC <00JHC1135>. Thermolysis of tetrazolo[l, 5- >]pyridazines, tetrazolo[l,5-a]pyrimidines and tetrazolo[l,5-a]pyridines allowed easy ring contraction to a facile preparation of cyanopyrazole heterocycles . 

Most significant is the formation of 92 in all thermolysis reactions of 91. This result is consistent with the sequence 91 -> diazoalkane of type 94 — carbene 52 - bridgehead olefin 53 - carbene 54 — H shift to afford 92. Formation of olefin 93 is best interpreted by H shift from the methyl group of carbene 52 (X=/-Bu, Y=Me) to the carbonic carbon, whereas 95 is formed by insertion of the carbenic center of 52 (X=Y=r-Bu) into the C-H bond of the r-Bu group. 

When no ester group is present, the thermolysis reaction may give rise to complex mixture. This is aptly explicated by the following reactions 137139). 

The last question still open addresses the alkynyl ketones. The reaction of 64 shows an example with a potential intramolecular competition and here it is possible to isomerize the propargyl substitutent on the ketone quantitatively without changing the 1-hexynyl substituent on the other side [125] (Scheme 1.28). From the publication it is not clear whether the isomerization is really a thermal reaction or occurs during the workup of the thermolysis reaction, for example by chromatography (compare the discussion above [110]). 

All solvents for these solution thermolysis reactions were freshly distilled and all reactions were done in sealed glass tubes heated in a thermostatted oven. Over a wide range of solvents (DMF, naphthalene, diphenylmethane, benzene, toluene, and decalin) there was no significant variation in either isomerization rate or product composition. Reactions were done at 125°C, 155°C and 195°C and the only limitation was that DMF could not be used as the solvent in reactions at 195°C it led to substantial substrate destruction (polymer forming reactions of substrate with DMF ). Isomer compositions were ascertained both by HPLC and by NMR. 

Aryl-5-azidopyridazin-3(2//)-ones have been used to smoothly prepare azacabolines via a thermolysis reaction involving the formation of nitrenes <2006H(68)2549>. 

Fluoro-substituted dihydrofuro[3,2-r ]pyridinones are obtained from the thermolysis reaction of cyclopropa[3]-furo[2,3-f]pyrroles, 134, along with the loss of HF (Equation 67) <2005SL1006>. Compound 134 is generated from pyrroles via reaction with a carbene to give an azomethine ylide which undergoes an electrocyclization reaction. 

Abramovitch and Takeuchi had explored the use of N-aminopyridinium ions as precursors to nitrenium ions in both photolysis and thermolysis reactions, but had not attempted to use these precursors under flash photolysis conditions. Moran and Falvey demonstrated that the LFP of 138 in CFI3CN (Scheme 57) generated a short-lived intermediate (1.5 jus) that had characteristics similar to the ions l31b-f. The transient was identified as the singlet ion 116. " Evidence from product analysis and spectrophotometric detection of the cation radical Ph2NH- as a long-lived intermediate was initially thought to indicate that there were parallel path-... 

Dihydrobenzo[c]furan (136) has been prepared in a thermolysis reaction from 5,6-dimethylene-7-oxabicyclo[2,2.1]heptene (135) in benzene at ISO C. The mechanism of Scheme 2 accounts for this result. 

The thermolysis reaction has been extended to generate 1,4-pentadienes from 3-thiabicyclo[3.1.0]hexane 3,3-dioxides (70JA6918). The reaction is again highly stereospecific,... 

Different results were reported for the photolysis of dihydro-1,2,4,5-tetrazines. 3,6-Diphenyl-l,4-dihydro-l,2,4,5-tetrazine (80) afforded 3,5-diphenyl-l,2,4-triazole (124) on photolysis (70T2619,69JOC199), while the 3,6-dimethyl-1,6-dihydro compound (125) yielded acetaldehyde azine (126) and nitrogen on photolysis, as in the thermolysis reaction (72HCA1404). 

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