Tetramethylene

SOC calculations were recently reported for two bicyclic biradicals 10 and 11 derived from barrelene [44] and for 1,/i-alkanediyls with n = 3 - 8, for which a unique alternation of SOC with the parity of the number of intervening bonds between the radical centers was found [35]. In addition, it was shown that omis-sion of H2 eq. (7) leads to less serious errors than confining the active space to two MOs (HOMO-LUMO), and a dissection of SOC values into local hybrid orbital contribution was described [35]. 

1-Hydroxytetramethylene (12), the 1,4-biradical intermediate in the Norrish type II reaction, has been studied by 2-in-2 MCSCF calculations and all possible gauche and trans minima have been located on the singlet and triplet PE surfaces [45]. The singlet-triplet energy splitting and SOC values vary with the conformational geometry and have no correlation to the distance between the two radical 

The results also provide a rationalization of the rather surprising finding of Caldwell et al. [49], that the ISC rate constant for the Norrish type II reaction of 13, which is conformationally fixed at y = 60°, and of the flexible analogue 14 are nearly the same. In 13, rotation around one of the terminal CC bonds will lead to Tj-Sq degeneracy with SOC = 0.28 cm, which corresponds exactly to the situation in 14, which will prefer the anti conformation where near Sq-T degeneracies are found for any a and p with SOC 0.3 cm. This explains why other effects like the solvents affect the ISC rate to a larger extent than fixation of the biradical to y = 60°. 

A powerful tool for the study of bi radicals has been the use of cyclic 1,2-diazenes (azo compounds) as precursors (Eq. 11.80). Thermolysis or photolysis of a diazene generally leads to the extrusion of Nj and the production of the biradical. Photolysis with a sensitizer (see Section 16.2.3 for a discussion of sensitized photolysis) allows a direct route to the triplet biradical. This provides the most convenient way to probe the reactivity differences of singlet and triplet biradicals. 

Pathways for the thermolysis of diazene precursors to tetramethylene derivatives. 

Nonconjugated Diradicals as Reactive Intermediates in Diradicals, W. T. Borden (ed.) Wiley-Intersdence, New York, 1982. 

An important aspect of tetramethylene chemistry is that similar results are seen whether the biradical is prepared by cyclobutane thermolysis, by ethylene dimerization, or by diazene photolysis. Seeing the same product ratios from different modes of preparation is one of the most stringent tests for the existence of a common reactive intermediate. 

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Note. Both tetramethylene glycol (1 4-butanediol) and hexamethylene glycol (1 6 hexaiiediol) may be prepared more conveniently by copper-chromium oxide reduction (Section VI,6) or, for small quantities, by reduction with lithium aluminium hydride (see Section VI,10). 

Thus with R = CHj, CjHj and C H, , the compounds are called methyl carhitoli carbitol and butyl carbitol respectively. None of these compounds can be conveniently prepared in the laboratory by elementary students they are, however, readily available commercially. The preparation of one glycol, pinacol or tetramethylene glycol from acetone, has already been described (Section 111,77). 

POLYEPPiERS - PEPRAPiYDROFURAN AND OXETANE POLYPOUS] (Vol 19) Poly(tetramethylene ether) glycol... 

POLYETHERS - TETRAHYDROFURAN AND OXETANE POLYTffiRS] (Vol 19) PTMO. See Poly(tetramethylene oxide). 

Butanediol. 1,4-Butanediol [110-63-4] tetramethylene glycol, 1,4-butylene glycol, was first prepared in 1890 by acid hydrolysis of N,]S3-dinitro-l,4-butanediamine (117). Other early preparations were by reduction of succinaldehyde (118) or succinic esters (119) and by saponification of the diacetate prepared from 1,4-dihalobutanes (120). Catalytic hydrogenation of butynediol, now the principal commercial route, was first described in 1910 (121). Other processes used for commercial manufacture are described in the section on Manufacture. Physical properties of butanediol are Hsted in Table 2. 

Tetrahydrofuran is polymerized to poly(tetramethylene glycol) with fuming sulfuric acid and potassium biduoride (29). 

Perfluorosulfonyl fluorides have also been prepared by dkect fluotination, although ki general yields are lower than preparation by ECF. Perfluoromethanesulfonyl fluoride has been produced ki 15% yield from dkect fluotination of dimethyl sulfone (16). Perfluoro-2-propanesulfonyl fluoride was prepared ki 29% yield from propanesulfonyl fluoride (17). Dkect fluotination of tetramethylene sulfone leads to the kitact perfluorkiated sulfone ki 28% yield and the ting-opened product (perfluorobutanesulfonyl fluoride) ki 10% yield (eq. 10) (17). 

Nitrations can be performed in homogeneous media, using tetramethylene sulfone or nitromethane (nitroethane) as solvent. A large variety of aromatic compounds have been nitrated with nitronium salts in excellent yields in nonaqueous media. Sensitive compounds, otherwise easily hydroly2ed or oxidized by nitric acid, can be nitrated without secondary effects. Nitration of aromatic compounds is considered an irreversible reaction. However, the reversibihty of the reaction has been demonstrated in some cases, eg, 9-nitroanthracene, as well as pentamethylnitrobenzene transnitrate benzene, toluene, and mesitylene in the presence of superacids (158) (see Nitration). 

A. T. Chen, and co-workers, "Comparison of the Dynamic Properties of Polyurethane Elastomers Based on Low Unsaturation Polyoxypropylene Glycols and Poly(tetramethylene oxide) Glycols," Polyurethanes World Congress 1993, Vancouver, B.C., Canada, Oct. 10—13,1993. 

The most important tetrahydrofuran polymers are the hydroxy-terrninated polymers, that is, the a,C0-poly(tetramethylene ether) glycols used commercially to manufacture polyurethanes and polyesters (see Urethane polymers Polyesters, thermoplastic). 

A protonic acid derived from a suitable or desired anion would seem to be an ideal initiator, especially if the desired end product is a poly(tetramethylene oxide) glycol. There are, however, a number of drawbacks. The protonated THF, ie, the secondary oxonium ion, is less reactive than the propagating tertiary oxonium ion. This results in a slow initiation process. Also, in the case of several of the readily available acids, eg, CF SO H, FSO H, HCIO4, and H2SO4, there is an ion—ester equiUbrium with the counterion, which further reduces the concentration of the much more reactive ionic species. The reaction is illustrated for CF SO counterion as follows ... 

THE can be polymerized by many strongly acidic catalysts, but not all of them produce the requked bitimctional polyether glycol with a minimum of by-products. Several large-scale commercial polymerization processes are based on fluorosulfonic acid, HESO, catalysis, which meets all these requkements. The catalyst is added to THE at low temperatures and an exothermic polymerization occurs readily. The polymerization products are poly(tetramethylene ether) chains with sulfate ester groups (8). 

Many other polymerization processes have been patented, but only some of them appear to be developed or under development ia 1996. One large-scale process uses an acid montmorrillonite clay and acetic anhydride (209) another process uses strong perfiuorosulfonic acid reski catalysts (170,210). The polymerization product ia these processes is a poly(tetramethylene ether) with acetate end groups, which have to be removed by alkaline hydrolysis (211) or hydrogenolysis (212). If necessary, the product is then neutralized, eg, with phosphoric acid (213), and the salts removed by filtration. Instead of montmorrillonite clay, other acidic catalysts can be used, such as EuUer s earth or zeoHtes (214—216). 

A number of papers and patents describe polymerization processes to poly(tetramethylene ether) glycols having a narrow molecular weight distribution = 1.2—1.4). In principle, this can be achieved by having all chains grow quickly at one time, either by high temperature initiation (33)... 

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