Thermal self-condensation

Recently, Frechet et al. [113] have reported a so called one-step synthesis of hyperbranched polyarylesters by the thermal self-condensation of 3,5-bis(trime-... 

Work reported recently by Rasmussen et al. [115] suggests that dihalodicyanoimid-azoles undergo thermal self condensation to the hyperbranched heterocycle illustrated in Scheme 10. These cyanoazacarbons with empirical formula (C5N4)n contain no hydrogen and are viewed as heterocyclic analogs of aramides. 

In industry, several dendrimers are already being synthesized on a hundred kilogram scale. The hopes for effective solubilizing systems suitable for medical applications may, however, not be justified. Phenolic ether or polyamine compounds are both normally not biodegradable so that major problems are foreseeable. An alternative is the employment of hydrolysable polyesters. One such compound already reported, is the imperfectly hyperbranched dendrimer which is easily accessible via thermal self-condensation of 3,5-bis(trimethyl-siloxyjbenzoyl chloride. This one-step procedure leads to a dendritic polyester... 

In conclusion, the self-condensation of 2-furaldehyde promoted by heat occurs with the formation of di- and trifurylic intermediates. The functionality of the growing chain increases after each oligomerization step until gelation and precipitation of the resin occurs. Thus, the process is non-linear from the onset since the condensation product 4 possesses three sites for further attack, namely the free C-5 position and the two formyl groups. It is interestering to note that while the polycondensation of 2-furfuryl alcohol is essentially linear and cross-linking is due to side reactions, the thermal resinification of 2-furaldehyde is intrinsically non-linear and gel formation occurs at earlier conversions. 

However, pMBCl 42 has a thermal stability issue and is expensive (Aldrich price 25 g for 69.90 the largest bottle). On the other hand, pMBOH 43 is stable and economically viable (Aldrich price 500 g for 84.90 the largest bottle). It was found that mono-N-alkylation of 36 proceeded well by slow addition (over 3 h) of 43 to a solution of 36 in acetonitrile in the presence of a catalytic amount of acid (p-TsOH) at 70 °C, as shown in Scheme 1.16. Slow addition of alcohol 43 minimized the self-condensation of 43 to form symmetrical ether 44, which was an equally effective alkylating agent. The product 41 was then directly crystallized from the reaction mixture by addition of water and was isolated in 90% yield and in >99% purity. A toluene solution of 41 can be used for the next reaction without isolation but the yield and optical purity of the asymmetric addition product were more robust if isolated 41 was used. In general, the more complex the reaction, the purer the starting materials the better. 

The molten material, after holding for 4 h at 78°C in a stainless steel vessel, underwent a thermal runaway reaction and 500 kg erupted through the vent line. It was later found that addition of 0.1% of rust to the hot material led to an accelerating self-condensation Friedel-Craft reaction, catalysed by iron(III) chloride, which led to formation of poly-benzyls accompanied by evolution of hydrogen chloride. 

Diels-Alder reactions are thermal reactions requiring no catalysts (120). However, over the years both acid- and metal-based homogeneous or heterogeneous catalysts have been developed (121—127). Some catalysts used in Diels-Alder catalyzed reactions of butadiene are Fe(NO)2Cl—(CH3CH2)2A1C1, Pd[P(C H5)3]4, Cu(I) exchanged silica—alumina (128,129), large pore zeolites (130), and carbon molecular sieves. An electrochemical process has also been used to catalyze the self-condensation to vinylcyclohexene (131). When the asymmetric Ni catalyst (4) was used, specificity to the enantomeric (5)-4-vinylcyclohexene (132,133) was observed (26% enantiomeric excess). 

It is worth noting that compounds 34 (R = Ph) are formed in trace amounts (in less than 1% yield) by self-condensation of thiones 33 (R = Ph) in xylene. The main products are 35 and 36 in the ratio 45 55 (82JOC1146). Their formation is due to the thermal extrusion of a sulfur atom from 33 and the subsequent substitution of a tellurium heteroatom in the six-membered ring. It was found that refluxing a solution of 34 (R = Ph) in xylene with the addition of elemental sulfur leads... 

The self condensation of a-carbonyl ketenes also constitutes a type (i) synthesis of 2//-pyran-2-oncs. Cyclophanes 627 are synthesized by the thermal decomposition of bis(4,6-dioxo-l,3-dioxanes) 628 and dimerization of the resulting bis(a-carbonyl ketenes) 629 (Scheme 141) <1999JA8270>. Similarly, thermal decomposition of 4,5-diarylfuran-2,3-diones 630 forms the intermediate a-carbonyl ketenes, which dimerize to afford 3,4,5-aryl-2//-pyran-2-ones (Scheme 142) <2003RJ0103>. 

The solvolysis reaction has certain disadvantages. First, the 2-aryl-2-chloropropanes are often too reactive to be prepared in the pure state (because of easy thermal elimination of hydrogen chloride or self-condensation) and so have to be generated in situ from the corresponding alcohol and hydrogen chloride. Second, the reaction is carried out in a strongly solvating medium, and differential steric hindrance to solvation at various... 

Phenols add intramolecularly to Michael acceptors. " Under acidic conditions, a one-pot sequence starts with initial electrophilic acetylation of the activated aromatic ring and is followed by cyclization." With an appropriate leaving group in the /f-position (OMe. or other amines such as in the unsaturated carbonyl compound (e.g., 4) is formed. Other approaches to pyroncs include the self-condensation of protected //-hydroxy acrylates,intramolecular aldol reactions followed by condensation,thermal cycli-zations of unsaturated ()-chloro esters,and an iodo-cyclization-elimination sequence w th Michael acceptors.Oxymercuration of an unsaturated alcohol is an alternative cyclization approach to tetrahydropyrans. 

Lown and Akhtar (1536) revealed a second product from this reaction, namely l,2-dibenzyl-2,5-diphenyl-1,2-dihydropyrazine (59%) (4). Lown and Akhtar(1537) also showed that the self-condensation of A -isopropylphenacylamine at room temperature gave 1,2-diisopropyl-2,5-diphenyl-1,2-dihydropyrazine (and proposed l,4-diisopropyl-2,5-diphenyl-l,4-dihydropyrazine as an intermediate). In a similar series of reactions of A -alkylphenacylamines, the thermally induced selfcondensations were followed by regiospecific 1,3-shifts to substituted carbon atoms to give in good yields l,2-dialkyl-2,5-diphenyl-l,2-dihydropyrazines at temperatures ranging from ambient to 140° (1536). 

It was assumed that an a-fluorosilyl potassium species was formed initially, and that this subsequently underwent a self-condensation reaction. The eventual product (1) displayed both nucleophilic as well as electrophilic character, which was demonstrated in various derivatization reactions. Although it contains fluorine and potassium atoms in close proximity, the compound displayed a remarkable thermal stability. Even at 80 °C, potassium fluoride elimination occurred only sluggishly. Attempted transmetalation reactions with various metal halides, though, caused an immediate elimination of metal fluoride and the formation of tetrakis(trimethylsilyl)disilene. The latter can be trapped in cycloaddition reactions [5] or, in the absence of trapping reagents, it dimerizes to a cyclotetrasilane (Scheme 2) [6]. 

The Knoevenagel reaction of aromatic and heteroaromatic aldehydes with cyanothioacetamides affords a-cyanothioacrylamides (126), which easily undergo thermal dimerization to 3,4-dihydro-2//-thio-pyrans (127). a-Keto-substituted acrylonitriles are synthesized by condensation of p-keto nitriles with aldehydes or ketones. - Benzoylacetonitrile self-condenses to afford (128), which can be transformed... 

The synthesis of 1,3,5-triazine (19) by thermal decomposition of tris(formylamino)methane is thought to proceed by self-condensation of the intermediate N-formylformimidamide.4053 406... 

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