Thermal Condensation

Thermal condensation reactions which produce polyphosphates are of great industrial importance. These involve the heating of solid acid phosphate salts whereby P-O-P linkages are formed with the elimination of water (3.101). Numerous long-chain, ring and cage compounds can be produced by reactions of this kind (Chapter 5.4). 

The reverse of thermal condensation, which involves the splitting of P-O-P linkages, corresponds to hydrolysis (3.95) above. 

---

Synthesis. The first hiUy alkyl/aryl-substituted polymers were reported in 1980 via a condensation—polymeri2ation route. The method involves, first, the synthesis of organophosphine-containing alkyl or aryl substituents, followed by the ready oxidation of the phosphine to a phosphorane with leaving groups suitable for a 1,2-elimination reaction. This phosphorane is then thermally condensed to polymers in which all phosphoms atoms bear alkyl or aryl substituents. This condensation synthesis is depicted in Eigure 2 (5—7,64). 

Table 1. Thermal Condensation of Substituted Imidazoles with TrifluoroacetaJdehyde Ethyl Hemiacetal [5]...

Diphosphates (pyrophosphates) are usually prepared by thermal condensation of dihydrogen... 

When prepared by thermal condensation of the respective alkoxides and carbonates in hydrocarbon media such as decalin, these catalysts yielded living poly-e-capro-lactone. Its living nature was demonstrated by the increase of DPn with conversion and by the validity of the relation... 

Deoxy-sugars. Part XVIII. Synthesis of an Oligosaccharide by the Thermal Condensation of ap-Methyl-2-deoxy-D-ga actofuranoside, W. G. Overend, F. Shafizadeh, and M. Stacey, /. Chem. Soc., (1951)994-997. 

Chlorinated dibenzo-ip-dioxins were prepared on the gram scale for use as toxicological standards, 2,7-Dichlorodi-henzo-p-dioxin was prepared by catalytic condensation of potassium 2-bromo-4-chlorophenate in 70% yield. Thermal condensation of the potassium salt of 2,4,4 -trichloro-2 -hydroxy diphenyl ether gave a mixture of the 2,8- and 2,7-dichlorodibenzo-p-dioxins which were separated by fractional recrystallization. 2,3,7,8-T etrachlorodibenzo-p-dioxin of 99.9- -% purity was prepared by catalytic condensation of potassium 2,4,5-trichlorophenate. An isomeric mixture of hexachlorodibenzo-p-dioxins was prepared by pyrolytic condensation of sodium 2,3,4,6-tetrachlorophenate. Chlorination of pentachlorophenol (containing < 0.07% tetrachlorophenol) in trichlorobenzene gave octachlorodi-benzo-p-dioxin in 80% yield contaminated by 5-15% heptachlorodibenzo-p-dioxin. Oxidative methods were used to produce octachlorodibenzo-p-dioxin at 99.9% purity. 

Polyethercyclicpolyols possess enhanced molecular properties and characteristics and permit the preparation of enhanced drilling fluids that inhibit the formation of gas hydrates prevent shale dispersion and reduce the swelling of the formation to enhance wellbore stability, reduce fluid loss, and reduce filter-cake thickness. Drilling muds incorporating the polyethercyclicpolyols are substitutes for oil-based muds in many applications [195-197,1906,1907]. Polyethercyclicpolyols are prepared by thermally condensing a polyol, for example, glycerol to oligomers and cyclic ethers. 

A. H. Zuzich, G. C. Blytas, and H. Frank. Polyethercyclicpolyols from epihalohydrins, polyhydric alcohols, and metal hydroxides or epoxy alcohols and optionally polyhydric alcohols with thermal condensation. Patent US 5428178,1995. 

GPC traces of poly(phenylmethylsilylenes) prepared in the ultrasonication bath are shown in Fig. 1. In contrast to thermal condensation, monomodal high molecular weight polymer is formed. Oligomeric cycles (mostly cyclic pentamer), formed usually in high yield (cf. Table 1), can be very easily separated from the reaction mixture by precipitation with isopropanol. The molecular weight of polysilanes decreases and polydispersity increases with temperature. 

By analogy with B-trialkylaminoborazi ne and polyborazine derived therefrom, the first route envisioned to poly(borylaminoborazine) was the thermal condensation of molecular precursors under a convenient atmosphere. As detailed earlier the innovative idea behind this procedure is to tailor the polymeric precursor structure by increasing the distance between the two borazinic rings. For that purpose, we explored... 

As expected, borylaminoborazines behave like 2,4,6-trialkylaminoborazines. Two competitive mechanisms occurred during the thermal condensation in vacuo the formation of direct B N bonds between two borazinic rings and the formation of three-atom bridges of the N B N type between two rings (scheme 16).53... 

Many reactions in heterocyclic multistep syntheses involve thermal condensations. 

The by-product ammonium bromide is shown to volatilise in similar conditions in TG. This is a different behaviour altogether from that of melamine in whose thermal condensation melam can be isolated with difficulty only in carefully controlled experimental conditions (11.17). Indeed melem is the condensation product most easily isolated and this led some authors to doubt that melam could be an intermediate product of the condensation of melamine to melem and melon (19.22). 

A typical 1,3-dipolar cycloaddition involves the use of azomethine ylides as the reactive species. Ylides are formed in situ by thermal condensation of a-amino acids and aldehydes, which then react to form the pyrrolidine-CNT system [32]. The R substituent on the 5-membered heterocycle attached to the SWCNT can be varied by the selection of the amino acid and the aldehyde, giving access to a range of different functional groups on the nanotube sidewalls. 

Thermal condensation of a-chlorocarboxylic chlorides with amidoximes affords 1,2,4-oxadiazoles. However, in the presence of a strong base (NaH) l,2,4-oxadiazin-5-ones were isolated (Scheme 41) <87H(26)163>. 

The thermal condensation of a cyclic carbonate with potassium thiocyanate represents a practical method for the synthesis of the four-membered S-ring system (Eq. 10a). ... 

Crystalline polyammonium catena-polyphosphate, (NH4) H2P 03 + 1, has been prepared by heating urea and monoammonium orthophosphate under ammonia vapor for 16 hours,1 by ammoniation of superphosphoric acid,2 by thermal condensation of urea phosphate,3 and by heating various ammonium phosphates in a current of ammonia.4,5 The procedure given below, in which crude ammonium tetrametaphosphate is reorganized and condensed to a long-chain polymer in a stream of ammonia, is straightforward and permits the use of common laboratory equipment and supplies. 

All result from thermal condensation of dihydrogen monophosphates. 

This thermal condensation can also be brought about by exposing the vapour of benzene to the action of a wire or filament kept at red heat by an electric current. 

Scheme 16 Reaction of the main thermal condensation product of DMAc, N>N-di-methylacetoacetamide (35), with glucose to furan 36...

The thermal condensation of p-benzyloxyphenylacetic acid and of 3-methoxy-4-hydroxyphenethylamine occurs and gives, with a yield of 86% to 92%, the N-(3-methoxy-4-hydroxyphenethyl-p-benzyloxyphenyl)acetamide from this latter, by cyclization according to Bischler-Napieralski with phosphorus oxychloride in acetonitrile, followed by reduction with sodium borohydride, there is obtained with a yield of 75% to 80% the l-(p-benzyloxybenzyl)-6-methoxy-7-hydroxy-l,2,3,4-tetrahydroisoquinoline, which is methylated with... 

The Maillard reaction is actually a complex group of hundreds of possible reactions. The initial reaction sequence is similar to that for caramelizatlon, except that the sugars thermally condense with amino acids, peptides, and proteins. Condensation is followed by enolization and dehydration. Tvo of the several branches of the Maillard reaction are shown in Figure 1 (12). The major pathway... 

Substituents R and Y may be parts of other (i.e. the triazaborade-caline) ring systems 184,185). The yields range from 20 and 30% due to side reactions which are discussed below (cf. Section III.3). Attempts to synthesize such materials by thermal condensations according to Eq. (50)... 

The latter is complicated by the equilibrium between Pt(CO) (PPhs)3 and Pt(CO)2(PPh3)2 (42). Reaction (33) contrasts with the substitution of Rh6(CO)i6 at higher temperature ( 80°C) which is reported to give products such as Rh6(CO)io(PPh3)o (21, 84). This apparent discrepancy is explained in terms of initial degradation followed by thermal condensation otherwise some orthometalation of the ligand can be suspected. 

The thermal condensation of dithiooxamide with trichlorophenyl malo-nates results in thioamide 19, which with an excess of malonate gives a symmetric bis-TA 20. With other carbonyl compounds, thioamide 19 may be converted into other TAs with heterocyclic substituents, 21 and 22 (73JHC223) (Scheme 3). 

---