Intermolecular reactions fragments

Rates of addition to transfer agents 11,12 are determined by the same factors that determine rates of addition to monomers Section 23). Substituents on the remote terminus of a double bond typically have only a minor influence. Thus, in most cases, the double bonds of the transfer agents have a reactivity towards propagating radicals that is comparable with that of the common monomers they resemble. With efficient fragmentation, transfer constants can be close to unity. The radicals formed by addition typically have low reactivity towards further propagation and other intermolecular reactions because of steric crowding about the radical center. 

These fragments either combine intramolecularly to form the ortho and para nitro compounds or dissociate completely and then undergo an intermolecular reaction to form the same products. The theory was not developed to include a detailed transition state and no mention was made of how the para isomer was formed. Reduction of the cation-radical could give the amine (which was observed experimentally76), but one would expect the concurrent formation of nitrogen dioxide and hence nitrite and nitrate ions however, the latter has never been... 

The changes in the IR spectra of the copolymer exposed to UV irradiation suggest the formation of coordination-bound organotin fragments due to complex intermolecular reactions of anhydride and organotin units. 

In this chapter are summarized the photochemical reactions wherein the primary chemical event is inter- or intramolecular hydrogen transfer to the excited chromophor. In intermolecular reactions hydrogen abstraction usually implies reduction or hydrodimerization of the excited molecule intramolecular hydrogen abstraction is frequently followed by either ring closure of the diradical or fragmentation to afford unsaturated molecules. 

Unsymmetrical alkenes can be prepared by mixed intermolecular reactions if one of the components, commonly acetone, is used in excess (equation 83). As the isopropyl group is a common subunit of many terpenes this method provides a valuable route for its introduction. Pattenden and Robertson used such a reaction followed by a Grob-type fragmentation in their preparation of the daucenone (42) from the readily enolized ketone (43). The bicycle (42) was used as an intermediate for the synthesis of the diterpene ( )-isoamijiol (44 equation 84). Mixed couplings are not restricted to acetone, and almost any carbonyl may be used. For example, Paquette et al. employed the aldehyde (45) in a synthesis of ( )-a-vetispirene (46 equation 85). More complex mixed couplings are also possible. For example, the aldehydes (47) and (48) are coupled efficiently to the stilbene (49), which in turn is converted to phenan-threne alkaloids such as atherosperminine and thalictuberine (equation 86). ... 

Nylon-6 will undergo re-equilibration with the cyclic monomer as well as with larger cyclics at elevated temperature. This is the reverse of the polymerization process, which occurs at 200 °C and takes place through an intermediate carboxy-terminated hydrolysis fragment that undergoes intramolecular (or intermolecular) reaction to generate the cyclic monomer s-caprolactam as shown below in Scheme 1.63. 

Cumene conversion under excess of benzene was studied over H-ZSM-11 in the adsorbed phase at 473 K by in situ C MASNMR. To follow the fate of different carbon atoms during the reaction, cumenes labelled with C-isotopes either on a-or on p-positions of the alkyl chain or in the aromatic ring have been synthesized. The primary product of cumene conversion over H-ZSM-11 was found to be n-propylbenzene. It is formed via intermolecular reaction of cumene and benzene. At long reaction times, the formation of n-propylbenzene is accompanied by complete scrambling of both cumene and n-propylbenzene alkyl chain carbon atoms and formation of toluene, ethylbenzene and butylbenzene. The rate of isomerization is higher than the rate of scrambling and fragmentation. 

The ene reaction involves an alkene fragment (the ene) that removes a hydrogen from an allylic fragment, with formation of a new carbon bond, as in 666.. s with the other reactions in this chapter, reactivity and stereochemistry in the ene reaction can be explained by frontier orbital theory. The reaction proceeds via interaction of the HOMO of the alkene (ene) and the LUMO of the allylic partner (enophile), illustrated by 667 in Figure 11.23." The intermolecular reaction usually requires very high temperatures (typically 250-... 

Tkdc followed the free radical distribution in the molten phase as well as in the pre-flame and flame zones of a burning specimen of polypropylene rod by ESR spectroscopy. Using several entrapping techniques for the radicals, he detected free radicals, bi-radicals, and non-radical fragments leaving the molten phase for the gas phase where they may be combined with traces of oxygen on the surface into peroxy radicals or recombined by cyclization as well as by intermolecular reactions. 

The additional ring which arises from an intramolecular HDA reaction may enable us to synthesize various kinds of triquinanes or other polycyclic compounds based on our proposed cycloaddition-fragmentation strategy (Scheme 3). The decrease in entropy associated with tethering the two reactive components suggests that the reaction would be significantly more facile than the intermolecular reaction. This potential rate enhancement, however, is compromised by the dramatic decrease in rate associated with intermolecular cycloadditions with substituted norbor-nadienes (Section IID, Table 7). In fact, before our studies, only one example of an attempted intramolecular HDA reaction was reported in the literature (Eq. 15). Instead of undergoing a [2 + 2 + 2] cycloaddition, an alternative intramolecular ene reaction occurs. ... 

Further evidence to exclude the triplet radical pathway includes the use of cyclopropyl substrates, which serve as a radical clock. In all cases, the reaction proceeds with no indication of ring fragmentation. The nature of the transition state of the C—H insertion step has been analyzed, via a Hammett study of the intermo-lecular C—H amination with p-substituted benzenes. A negative q value of 0.73 is obtained for the intermolecular reaction with trichloroethylsulfamate [71]. Such data indicate that there is a small, but significant, preference for electron-rich substrates, thus the resonance does contribute to the stabilization of a partial positive charge at the insertion carbon in the transition state. A kinetic isotope value of 1.9 is observed for competitive intramolecular C—H amination with a deuterated substrate (Eq. (5.21)). 

The dynamic linear combination of fragment confi ia-tions method. - Even-even intermolecular multicentric reactions. - The problem of correlation imposed barriers. -Reactivity trends of thermal cycloadditions. - Reactivity trends of singlet photochemical cycloadditions. - Miscellaneous intermolecular multicentric reactions. - tc + o addi-tion reactions. - Even-odd multicentric intermolecular reactions. - Potential energy surfaces for odd-odd multicentric intermolecular reactions. - Even-even intermolecular bicen-tric reactions. - Even-odd intermolecular bicentric reactions. 

Since the intermolecular reaction between the CS and KS radicals and the native HA macromolecule could yield various A radicals - formed for example at C(4) of the D-glucuronate/n-glucuronic acid (GlcA) unit (cf. Scheme 1) or at C(l) of GlcA unit, as well as at C(l) or C(3) of A-acetyl-D-glucosamine (GlcNAc) [18] - various biopolymer fragments are produced. 

Active species in the polymerization of cyclic acetals undergo fast isomerization. This results in chain transfer to polymer, that is, formation (by intramolecular reaction) of cyclic structures or formation (by intermolecular reaction) of branched oxonium ions, followed by exchange of the linear fragments of the chain (transacetaiization) (cf. Scheme 18). 

The nickel-catalyzed intermolecular reaction [4+2+2]-annulation of diynes with cyclobutanone as C4 fragment includes the ring expansion... 

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