Of piperylene

Blends of piperylenes and amylenes (mixed 2-methyl-1-butene and 2-methyl-2-butene) or UOP propylene dimers can be adjusted to produce softening points of 0—100°C and weight average molecular weights of <1200 (32,33). Careful control of the diolefin/branched olefin ratio is the key to consistent resin properties (34). 

Further study of the direct decomposition revealed that the reaction could not be quenched by the presence of piperylene thus, decomposition must result either from singlet-excited azide or from triplet azide at a rate on the order of 1011 sec"1/471... 

A. Greenberg and T. A. Stevenson, Molecular Structure and Energetics Studies of Organic Molecules (Eds. J. F. Liebman and A. Greenberg), VCH, Deerfield Beach, 1986. See also the discussion in J. F. Liebman and R. M. Pollack, in The Chemistry ofEnones, Part 1 (Eds. S. Patai and Z. Rappoport), Wiley, New York, 1989 wherein the resonance energy of crotonaldehyde was shown to be less than that of piperylene while the rotational barriers are in the reverse order. 

However, when the same reaction was performed in the presence of piperylene, 1,2-cycloaddition was observed to give naphthalene ring fused cyclobutapyrimidines 473 with high regio- and stereoselectivity <2005CPB258>. 

A related phenomenon has been observed in the benzophenone sensitized isomerization of c/y-piperylene.150 The measured quantum yield of cis to trans isomerization increased from 0.55 to 0.90 as the concentration of piperylene increased from 0.08 to lOAf. This observation can be rationalized as arising from addition of the piperylene triplet to a ground state diene molecule to give a biradical intermediate which can either cyclize to the dimer151 or dissociate to give two molecules of the more thermodynamically stable trans-isomer. This mechanism predicts that the quantum yield for the isomerization of /runs-piperylene to cw-piperylene should decrease with increasing diene concentration, an experiment that has not yet been reported. 

In the triplet counting technique the substance whose intersystem crossing yield is to be determined is used as a sensitizer for the photoisomerization of piperylene (1,3-pentadiene). Under conditions where essentially all of the sensitizer triplets are intercepted by diene, the quantum yield of isomerization is directly proportional to the intersystem crossing yield of the sensitizer. If the diene interferes with sensitizer singlets however, the yield of triplets will fall, resulting in an anomalously low value for 0,. 

Daubendiek et al.55 have extended this technique to demonstrate electronic energy transfer at the gas-solid interface. They irradiated photosensitizers, in polycrystalline form, in the presence of m-piperylene vapor and found that the extent of isomerization was comparable to values obtained for the same sensitizer in a solution of piperylene. Their results indicated an energy difference of about 1 kcal between triplet states of sensitizers in solution and on the surface of organic crystals. 

At least four applications of this technique can be cited. Quantum yields for triplet formation in benzene108 and fluorobenzene109 have been estimated by comparing the phosphorescence yields of biacetyl produced by sensitization to that produced by direct irradiation. Intersystem crossing yields of a number of organic molecules in solution have been obtained by measuring the quantum yield with which they photosensitize the cis-trans isomerization of piperylene (1,3-pentadiene) and other olefins.110 As will be discussed later, the triplet states of... 

Estimates of the triplet energy level of a molecule may be made by observing whether it can accept or transfer energy to several other molecules. Hammond and co-workers42 have shown that ds-trans interconversions of piperylene, 2-pentene, and 1 2 dichloroethylene may be affected by many photosensitizers. The stationary states of the sensitized ds-trans ratios of piperylene with various donors were found to form a coherent pattern if triplet energy transfer was assumed as the key step in the photochemical reaction. From these results they were able to infer the presence and the energies of the triplet states of acetone, phenanthrenequinone, and fluorenone, for which phosphorescence data are not available. The triplet levels were estimated as >70, 65, and 62 kcal./mole, respectively. 

The terminally bifunctional prepolymer, carboxy-terminated polyisobutylene, is prepared by the pyridine-modified ozonization of a copolymer of piperylene (2-4% ) and isobutylene. Figure 1 shows a block diagram of the process. The high molecular weight elastomer can be prepared by the conventional techniques of isobutylene-diene copoly-... 

Polymer Structure. Unpublished work by Bearden et al. (4) shows that there is predominantly trans-1,4 addition of piperylene in the cationic copolymerization of piperylene and isobutylene. Infrared spectroscopic examination shows only an intense 970 cm.-1 band and no band at 910 cm.-1. This indicates only 1,2- or 1,4-addition of piperylene with little or no 3,4-addition. An examination by NMR with a time-averaging computer also confirms that there is essentially no 3,4-addition and that 1,2-addition is about 10% of 1,4-addition. 

Methyl-1,4-naphthoquinone (I) was obtained by the Diels-Alder addition of piperylene to />benzoquinone.n... 

According to the results of analyses, the reaction of piperylene with element sulfur proceeds under the following scheme yielding piperylene bis- tetrasulfide ... 

Functionalization of piperylene fractions, oligodienes and oligoolefins by element sulfur has been carried out, optimum conditions to effect their interaction introducing the greatest possible quantity of sulfur have been selected. 

The effects of Lewis acids on regioselectivity are shown by the reaction of piperylene with methyl acrylate giving mainly the ortho adducts 6.203, as usual, but this preference is increased with Lewis acid catalysis. Similarly, as an... 

Another type of problem may also arise from the formation of azeotropes, w ose boiling points are close to that of isoprene. This is the case of cydopentadiene, whose separation, like that of piperylene, should normally take place by simple distillation, but wUch produces azeotropes with n-pentane (35J Q, 2butenes (38 Q and ds 2-pentene (36,9 C). 

As described in earlier chapters, the volatility of solvents is of crucial important in their applications. In 2007, the groups of Liotta, Eckert and Jessop first reported on the formation and use of piperylene sulfone (PS) as a recyclable alternative to dimethyl sulfoxide (DMSO) (Figure 9.11). DMSO, in addition to dimethylformamide (DMF) and hexamethylphosphoramide (HMPA), is a widely used dipolar, aprotic solvent. However, it is difficult to remove from products by distillation and is rarely recycled. In contrast, PS decomposes cleanly at temperatures above 100°C to give ra -l,3-pentadiene and sulfur dioxide, which reform PS at room temperature (Figure 9.11). 

Investigation of the effect of biacetyl on the decomposition of 2-hexanone in the presence of piperylene provided data for the singlet-state reactivity in type II decomposition It was assumed that product formation from the triplet state was completely quenched in the presence of 0.5 M piperylene, and the decrease caused by added biacetyl was entirely due to quenching of the excited singlet state. The value of the rate coefficient for type II decomposition from the singlet state, obtained from the Stern-Volraer plot, was foxmd to be 10 sec. This indicates that the reactivities of the , -n singlet and triplet states of 2-hexanone in reaction II are of the same order of magnitude. 

Swenton found a somewhat analogous behaviour in the photodecomposition of 2-azidobiphenyl. Direct photolysis resulted in ring closure to carbazole (71%), acetophenone sensitized photolysis produced 43% of the azo compound and less than 8% carbazole. The presence of piperylene, a triplet quencher, during photolysis reduced the formation of azo compound to 4% and enhanced the carbazole yield to 89%. 

The checkers consistently obtained a slightly higher melting point (uncor.) in the range 64.4-65.4°. The purified cyclic sulfone serves as an ideal intermediate for the preparation of extremely pure isoprene, since the latter can be regenerated nearly quantitatively at 135-140°. Other sulfones that can be prepared by this method and that are uscfid in the purification of dienes are those of butadiene, m.p. 65.5°, and 2,3-dimethyl-butadiene, m.p. 135°. The sulfone of piperylene is a liquid. ... 

Data on the photochemical stability and stabilizing action of HB s, benzotriazoles, and related compounds have been reported on several occasions (6,9,70,71,7,82,87). In order to be a good stabilizer, it is not sufficient that a compound does not decompose if dissolved for example, in a plastic, it is equally important that it does not act as a sensitizer. Investigations on the sensitizing action of HB s compared to that of benzophenone and nonchelated derivatives are, therefore, of interest. Such studies are Included by Lamola (8), A. Heller (80), and Hammond (74). Cls-trans isomerization of piperylene (1,3-pentadiene) in benzene is easily sensitized by benzophenone via triplet transfer... 

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