Cis-Piperylene

A short lifetime (t = 10 s) for the lowest excited triplet of nitrobenzene has also been obtained from energy-transfer experiments using cis-piperylene as quencher. Even at concentrations of 2.4 mole 1 cts-piperylene not all nitrobenzene triplets were quenched but singlet energy-transfer has been disfavored by kinetic reasoning... 

Ferrocene forms ground-state complexes with dienes such as piperylene and isoprene. Irradiation of these yields a 1.25 trans/cis piperylene ratio in the first case and 92% cyclobutanes in the second,284 both indications of a high-energy sensitizer. However, ferrocene itself quenches anthracene triplets, so that it probably has a fairly low triplet energy. Therefore it has been postulated that an excited ferrocene-diene complex dissociates into an excited diene triplet which then proceeds to decay just as though it had been formed by normal bi-molecular sensitization. 

As both, the cycloaddition of trans-piperylene and trans,trans-hexadiene implies a connection of a methylated unsaturated carbon with the more bulky part of the standard silaethene, a decrease of reactivity of trans.trans-hexadiene on sterical grouds is ruled out. On the other hand, an intermediate formation of a weak 7i-complex of trans-piperylene but not of bulkier trans.trans-hexadiene with the unsaturated silicon atom of silaethene would explain an increased reactivity of trans-piperylene (Scheme 12). In a way, the silaethene would then catalyze its own [4+2] cycloaddition with trans-piperylene but not with trans.trans-hexadiene. This mechanism can also explain the transformation of the [4+2] into a [2+2] cycloaddition by going from trans- to cis-piperylene but not fi om trans,trans- to cis.trans-hexadiene. 

In a later paper, Lewis and Saunders observed that triplet quenchers (cis-piperylene, oxygen) failed to affect the course of the direct photolysis of alkyl azides, from which it was concluded that the photolysis proceeded via a singlet azide and singlet nitrene. This was further supported by the observation that hexyl azide acted as an efficient quencher of aromatic hydrocarbon fluorescence, and that this singlet sensitization of hexyl azide led to the decomposition of the azide with an efficiency similar to that of direct photolysis Thus, although triplet sensitization leads to decomposition of alkyl azides, it appears that direct photolysis proceeds by way of an excited singlet azide without intersystem crossing to the triplet. 

A modified form of MINDO was used to discuss, the 1,3 hydrogen shift in propene and the suprafacial and antarafacial 1,5 sigmatropic hydrogen shifts in cis-piperylene the calculated activation energies were 49.2, 28.3 and 37.0 kcal/mole, respectively 135>. 

The formation of the cisoid conformation is very difficult for cis-piperylene because of the steric repulsion between the hydrogen atom and the methyl group [66] ... 

Figure 15. Stern-Volmer plot obtained from the nuclear polarizations observed in a-cleavage of methylethylketone quencher cis-piperylene (concentration C9). Filled circles, laser energy (308 nm) 3mJ half-filled circles, 7.4 mJ open circles, 18 mJ. [Adapted from ref. [103] with permission. Copyright 1993 Elsevier Sequoia.]...

Kinetic investigations presented in [247, 248] showed that use of TiCU as catalyst for industrial piperylene fraction polymerization (components content, mass % rrans-piperylene 60,8, cis-piperylene 37,2, etc.) allows to produce oligopiperylene with high yield (80-95%) with desired MM (80-2000) at comparable high process duration (nearly 1 hour). Polymerization process in toluene medium is characterized by the first orders by catalyst and monomer with constants of chain propagation, transfer on monomer and catalyst rates 1,5, 0,04 and 0,86 l/mole-min accordingly. Oligomers MMD has unimodal character, but with synthesis temperature rise products polydispersity increases. 

Figure 44 Photosensitized isomerization of cis-piperylene catalyzed by 14oAc.

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